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Theoretical and Experimental Investigation of Hydrogen Production by Gasification of Biomass

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PAGE 1

1 THEORETICAL AND EXPERIMENTAL INVESTIGATION OF HYDROGEN PRODUCTION BY GASIFICATION OF BIOMASS By MADHUKAR MAHISHI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2006

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2 Copyright 2006 by MADHUKAR MAHISHI

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3 I would like to dedicate this work to my father Shri R. K. Mahishi

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4 ACKNOWLEDGMENTS First of all I would like to th ank my advisor, Professor D. Y. Goswami, for giving me an opportunity to work with him at the Sola r Energy and Energy Conve rsion Laboratory and conduct research in the areas of renewable energy and hydrogen production. His guidance, advice and encouragement to work independently greatly helped in shaping my thoughts and in molding me from a graduate student to a researcher. I would like to thank Dr. Skip Ingley, Dr. Bill Lear, Dr. S.A. Sherif and Dr. Donald Rockwood for agreeing to serve on my superv isory committee and also for their advice, comments and suggestions during the various phases of my PhD. I would like to acknowledge th e assistance and support of Dr Helena Hagelin-Weaver (Department of Chemical Engineering) during th e fabrication of the experimental set-up for biomass gasification. I would like to thank all the staff members and colleagues of the Solar Energy and Energy Conversion Laboratory at UF I would also like to acknowledge the US Department of Energy for funding our research on hydrogen production. A word of thanks goes to all my present and former room-mates (San jay Solanki, Viswanath Ur ala, Kaushal Mudaliar, Purushottam Kumar, Ashish Gupta, Sudarshan Jagannathan and others) whose enthusiasm and co-operation made graduate student life in the US a very memorable experience. Above all I would like to thank my parents for their patience, encouragement and support all through during my PhD. Finally I would lik e to thank God for giving me the opportunities and challenges which, over the years, have helped me become a better person.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES................................................................................................................ .........9 ABBREVIATIONS.................................................................................................................. .....12 ABSTRACT....................................................................................................................... ............14 CHAPTER 1 INTRODUCTION................................................................................................................. .16 Significance and Need for Hydrogen: An Overview..............................................................16 Introduction to Present Research............................................................................................19 2 HYDROGEN PRODUC TION METHODS...........................................................................24 Introduction................................................................................................................... ..........24 Hydrogen Production Methods...............................................................................................24 Steam Methane Reforming (SMR)..................................................................................24 Partial Oxidation or Autothermal Reforming of Methane..............................................25 Coal Gasification.............................................................................................................2 7 Biomass Gasification and Pyrolysis................................................................................28 Electrolysis................................................................................................................... ...29 Thermochemical Hydrogen Production..........................................................................30 Zn/ZnO cycle............................................................................................................31 UT-3 cycle................................................................................................................31 Photocatalytic Hydrogen Production...............................................................................32 Photoelectrochemical Hydrogen Production...................................................................33 Biological Hydrogen Production.....................................................................................34 Fermentation of bacteria...........................................................................................34 Biophotolysis............................................................................................................35 Summary........................................................................................................................ .........36 3 BACKGROUND AND LI TERATURE REVIEW................................................................44 Introduction................................................................................................................... ..........44 Pyrolysis...................................................................................................................... ....44 Gasification................................................................................................................... ...45 Combustion..................................................................................................................... .45 Liquefaction................................................................................................................... ..45 Lab-Scale Production of Hydrogen from Biomass.................................................................46

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6 An Overview of Biomass Gasification for Hydrogen Production..........................................47 Catalysis...................................................................................................................... ....47 Non-Metallic Oxides................................................................................................49 Commercial Nickel Reforming Catalyst..................................................................50 Additional Catalyst Formulations............................................................................51 Pretreatment Technologies..............................................................................................53 Chemical Kinetic Studies................................................................................................54 Experimental Studies on Biomass Gasification..............................................................57 Thermodynamic Studies on Gasification........................................................................58 Sorbent Enhanced Gasification.......................................................................................61 Scope of the Present Work.....................................................................................................6 4 4 THERMODYNAMIC ANALYSIS OF BIOMASS GASIFICATION..................................70 Introduction................................................................................................................... ..........70 Fundamentals................................................................................................................... .......70 Effect of Process Parameters on Equilibrium Hydrogen Yield..............................................72 Effect of Temperature......................................................................................................73 Effect of Pressure............................................................................................................7 4 Effect of Steam Biomass ratio.........................................................................................74 Effect of Equivalence Ratio.............................................................................................75 Optimum Process Parameters..........................................................................................75 Energy Analysis................................................................................................................ ......76 Effect of Temperature on Thermodynamic Efficiency...................................................78 Effect of Steam Addition on Thermodynamic Efficiency...............................................78 Effect of ER on Thermodynamic Efficiency...................................................................79 Comparison of Equilibrium Resu lts with Experimental Data................................................80 Summary and Conclusion................................................................................................81 5 ABSORPTION ENHANCED BIOMASS GASIFICATION................................................95 Introduction................................................................................................................... ..........95 Concept of Absorption Enhanced Gasification......................................................................96 Application of SEG to Biomass Gasification.........................................................................98 Case I: Base case (no sorbent).........................................................................................98 Case II: Ethanol gasification in the presence of CaO sorbent (sorbent placed in the reformer)....................................................................................................................10 0 Energy Analysis................................................................................................................ ....102 Conclusion..................................................................................................................... .......104 6 EXPERIMENTAL STUDIES ON BIOMASS GASIFICATION........................................119 Objective...................................................................................................................... .........119 Experimental Facility.......................................................................................................... ..120 Test Set-up.................................................................................................................... .120 Gasifier (Primary reactor)......................................................................................120 Secondary reactor...................................................................................................120

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7 Steam generator......................................................................................................121 Gas cooling system (heat exchanger).....................................................................121 Heaters, insulation and tubing/fittings...................................................................121 Instrumentation.......................................................................................................122 Gas Analysis Facility.....................................................................................................122 GC Calibration................................................................................................................. .....123 Test Methodology............................................................................................................... ..123 Test Results and Analysis.....................................................................................................1 25 Effect of Temperature....................................................................................................125 Effect of Sorbent............................................................................................................12 7 Conclusion..................................................................................................................... .......128 7 REGENERATION OF SPENT SORBENT.........................................................................152 Introduction................................................................................................................... ........152 The Reversible Calcination Carbonation Process.................................................................152 Combusting a Carbonaceous Fuel.................................................................................154 Using Concentrated Solar Energy.................................................................................155 Waste Heat from Gas Turbin e Exhaust or from SOFC.................................................156 Kinetics of the Reversible Calcination Carbonation Reactions....................................157 Sorbents other than Calcium oxide.......................................................................................158 Summary........................................................................................................................ .......160 8 SUMMARY, CONCLUSION and RECOMMENDATIONS.............................................163 Summary........................................................................................................................ .......163 Conclusions.................................................................................................................... .......166 Recommendations for Further Work....................................................................................166 APPENDIX LIST OF PUBLICATIONS....................................................................................168 REFERENCES..................................................................................................................... .......169 BIOGRAPHICAL SKETCH.......................................................................................................179

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8 LIST OF TABLES Table page 1-1 Summary of hydrogen production methods.......................................................................23 3-1 Feedstock composition...................................................................................................... .67 4-1 Equilibrium gas moles at di fferent gasification pressures.................................................83 5-1 Reactions in SEG for some typical fuels.........................................................................105 5-2 Comparison of energy consumption in biom ass gasification with and without sorbent.106 5-3 Thermodynamic efficiency and energies.........................................................................107 6-1 Heater ratings................................................................................................................. ..130 6-2 Ultimate and pr oximate analyses.....................................................................................131 6-3 Effect of temperature on the pr oducts of biomass gasification........................................132 6-4 Carbon conversion efficiency (no sorbent)......................................................................133 6-5 Equilibrium yields of bi omass gasification products.......................................................134 6-6 Effect of temperature on gas comp osition in the presence of sorbent.............................135 6-7 Carbon conversion efficiency (sor bent enhanced gasification).......................................136

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9 LIST OF FIGURES Figure page 2.1 Block diagram of hydrogen production by steam methane reforming .............................37 2.2 Block diagram of hydrogen production by partial oxidation of heavy oils ......................38 2.3 Principle of hydrogen production by high temperature electrolysis (HTE)......................39 2.4 Block diagram of the Zn/ZnO water sp litting thermochemical cycle for hydrogen production..................................................................................................................... .....40 2.5 UT-3 cycle reactions and flow of material........................................................................41 2.6 Photocatalytic hydrogen production..................................................................................42 2.7 Principle of photoelectroch emical hydrogen production...................................................43 3.1 Biomass gasification pilot plant.........................................................................................68 3.2 Schematic of biomass gasification set up for producing hydrogen...................................69 4.1 Variation of Gibbs energy with extent of reaction.............................................................84 4.2 Effect of temperature for P = 1 atm, = 1, ER = 0...........................................................85 4.3 Effect of SBR on equ ilibrium co mposition.......................................................................86 4.4 Effect of ER on Equ ilibrium co mposition.........................................................................87 4.5 Schematic of biomass gasifier...........................................................................................88 4.6 Efficiency Vs temperature for various (ER = 0.1)..........................................................89 4.7 Efficiency Vs temperature for various (ER = 0.2)..........................................................90 4.8 Efficiency Vs temperature for various (ER = 0.3)..........................................................91 4.9 Efficiency Vs temperature for various (ER = 0.4)..........................................................92 4.10 Comparison of equilibrium data with experi mental data o for different temperatures and residence times............................................................................................................ 93 4.11 Comparison of equilibrium data with experimental data for different and ER..............94 5.1 Concept of absorption enhanced gasification..................................................................108 5.2 Schematic of SEG (concept)............................................................................................109

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10 5.3 Flow sheet for conventional biomass gasification...........................................................110 5.4 Effect of reformer temperature on product yield.............................................................111 5.5 Effect of reformer pressure on product yield...................................................................112 5.6 Effect of steam ethanol ra tio on product yield at 700oC..................................................113 5.7 Flow sheet for ethanol gasi fication with CaO sorbent.....................................................114 5.8 Effect of temperature on product yiel d for sorbent enhanced reforming.........................115 5.9 Effect of pressure on the product yiel d for sorbent enhanced reforming........................116 5.10 Effect of steam/ethanol ratio on product yield for sorbent enhanced reforming.............117 5.11 Effect of CaO/ethanol ra tio on the product yield.............................................................118 6.1 Biomass gasification test set-up.......................................................................................137 6.2 Photograph of the test set-up............................................................................................13 8 6.3 Gas chromatograph (SRI 8610C).....................................................................................139 6.4 Set-up for GC calibration.................................................................................................1 40 6.5 Hydrogen calibration curve..............................................................................................141 6.6 CO calibration curve....................................................................................................... .142 6.7 CO2 calibration curve.......................................................................................................143 6.8 CH4 calibration curve.......................................................................................................144 6.9 Southern pine bark “as received”.....................................................................................145 6.10 Pelletized pine bark...................................................................................................... ....146 6.11 Effect of temperature on gas yields (no sorbent).............................................................147 6.12 Effect of sorbent addition at 500oC..................................................................................148 6.13 Effect of sorbent addition at 600oC..................................................................................149 6.14 Effect of sorbent addition at 700oC..................................................................................150 6.15 Tar laden condensate samples of plain biomass gasification (left) and sorbent enhanced gasification.......................................................................................................151 7.1 Equilibrium CO2 pressure as a function of temperature..................................................161

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11 7.2 Biomass gasification with calcination of used sorbent....................................................162

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12 ABBREVIATIONS ER Equivalence Ratio (actual air to biomass ratio divided by stoichiometric air to biomass ratio) Ewood chemical energy stored in biomass (wood) (kJ) H enthalpy (kJ/mol) H enthalpy change due to temperature (kJ/mol) Ho f enthalpy of formation (kJ/mol) LHV Lower Heating Value (kJ/mol) n no. of moles P pressure (atm) Qair heat supplied to air-preheater (kJ) QEG heat input to equi librium gasifier (kJ) Qsteam heat supplied to steam generator (kJ) SBR Steam Biomass Ratio (denoted by defined as moles of steam per mole of biomass) SEG Sorbent Enhanced Gasification SMR Steam Methane Reforming T Temperature (K) WGS Water Gas Shift reaction or Water Gas Shift reactor g specific Gibbs energy (kJ/kg) q heat transferred (kJ/kg) s entropy (J/kg-K) u specific internal energy (kJ/kg)

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13 v specific volume (m3/kg) w work done (kJ/kg) Greek Symbols: moles of steam per mole of biomass moles of oxygen per mole of biomass thermodynamic efficiency (%) residence time (s) extent of reaction Subscripts: gen generated sys system surr surrounding NS total number of species

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14 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy THEORETICAL AND EXPERIMENTAL INVESTIGATION OF HYDROGEN PRODUCTION BY GASIFICATION OF BIOMASS By Madhukar Mahishi December 2006 Chair: Yogi Goswami Major Department: Mechanical and Aerospace Engineering A detailed theoretical and experimental investigation of hydrogen production by thermochemical gasification of biomass wa s conducted. The thermodynamics of biomass gasification was first studied to determine the hydrogen yield at equilibri um. The gasification process is characterized by a num ber of endothermic and exotherm ic reactions. A combination of these reactions enables internal energy transfer, and therefore improved process efficiency. The maximum hydrogen yield is limited by thermodynamic equilibrium. One solution to this problem is to remove one of the co-products (CO2) that governs the equilibrium hydrogen yield. In recent times, sorbents (such as calcium oxide) have been used for CO2 removal from fossil fuel exhaust. The same principle was applied here to drive th e reactions in favor of hydrogen. In the process the sorbent gets saturated and has to be regenerated for further use. Process simulations were conducted using an ASPEN simulator with the end objective of determining the hydrogen yi eld in presence of a CO2 sorbent. Ethanol was used as the model biomass compound and calcium oxide was the representative sorbent. The simulations showed 19% increase in hydrogen yield and a bout 50% reduction in product gas CO2 while using the sorbent. The hydrogen yield in the presence of sorbent at a gasification temperature of 600oC was comparable to the hydrogen yield without the sorbent at 750oC. Hence there is a potential to

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15 reduce the gasifier operati ng temperature by about 100-150oC while still getting the same amount of hydrogen. The in-situ heat transfer (CO2 absorption is exothermic) reduced the gasifier heat duty by almost 42%. Based on the encouraging results obtained fr om simulations, experiments were conducted using Southern pine bark as the model biomass a nd calcium oxide as the re presentative sorbent. Hydrogen yield increased substantially (from 320 ml/g to 719 ml/g) by using sorbents at gasification temperature as low as 500oC. The product gas had much less tars and particulate matter as compared to conventi onal gasification. The carbon conve rsion efficiency (a way of quantifying the effectiveness of gasification) increased from a mere 23% to 63% while using sorbent. Sorbent enhanced biomass gasification has the potential to produ ce a hydrogen rich, CO2 free and possibly tar free gas that can be sent to a fuel cell or gas turbine with minimal cleaning. Hence there is a potential to reduce the equipment needed for hydrogen production. This will lead to reduced capital and opera ting costs. Hence sorbent enhan ced biomass gasification has the potential to become a cost effectiv e technology for producing renewable hydrogen.

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16 CHAPTER 1 INTRODUCTION Significance and Need for Hydrogen: An Overview Present day energy resources such as coal, o il and natural gas are being consumed at an accelerated rate with fear of depletion in the next few decades. It is reported that some of the oil rich countries would fail to meet the world energy demand in the next few decades. For example, United Arab Emirates is expected to exhaus t its oil and natural ga s reserves by 2015 and 2042 respectively [1], and fossil sources in Egypt w ould possibly be exhausted within the next two decades [2]. There is also a concern about the environmental pollution caused by the use of fossil fuels. According to a recent study the world CO2 emissions from fossil sources have increased by 24.4% from 1990 to 2004 [3]. Apart from CO2, other contaminants such as CO, NOx, and SOx are released during the combustion of fossil fuels. These contaminants cause acid rains, deplete the stratospheric ozone layer and are also known to be carcinogenic. According to an EPA study, vehicles in the US account for 65% of total oil consumption and result in 78% CO, 45% NOX and 37% Volatile Organic Compound (VOC) emi ssions [4]. Among all the air pollutants emitted by the combustion of fossil fuels, CO2 alone accounts for 99% (by we ight) of the total emissions [5]. The average surface temperat ure of earth has increased by 0.6oC over the past two centuries [6]. If this trend continues it may eventually l ead to higher sea levels and significant changes in global precipitation patterns. The tr end in the transportation sector in industrialized countries is towards more vehicles, more freight transport by road and larger and heavier passenger vehicles. Furthermore, developing countries like Chin a and India with larg e population and growing economies are expected to add to the rapid growth in vehicle usage for transportation

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17 applications [7]. This would fu rther lead to large scale emissi ons which may drastically change the global weather patterns, thus affecting mankind and environment. The world energy demand has been steadily increasing over the last few decades. According to a recent study conducted by the US Department of Energy, the world energy demand is expected to increase to 722 quads (Quadrillion BTU) by 2030 from the present demand of 421 quads (2003) [8], a 71% increase largely due to growth in developing countries. According to the same study fossil fuels will co ntinue to supply much of the increment in projected demands; however, depletion of fossil reserves is a matter of concern. Although oil would remain an important energy source, its sh are in total energy cons umption would decrease from 38% in 2003 to 33% in 2030. This is largely in response to the higher world oil prices which would be driven by rapid depletion of o il reserves in many parts of the world. Among all sectors, transportation and indus try continue to be the major oil consumers. Alternate fossil sources such as natural gas are also limite d. According to a recent study conducted by British Petroleum, the Reserves to Production ratio (R/P) of natural gas in the US is less than 10 [9]. Hence, developing alternate ener gy carriers is necessary. In recen t years, hydrogen has gained recognition as a potential substitute to fossil fuels. Some of the factors favoring hydrogen are lack of green-house gas emissions when combusted or used in a fuel cell high energy content on a mass basis as co mpared to gasoline or natural gas easy and efficient conversion to electricity using fuel cells Hydrogen is an important raw material for ch emical, petroleum and agro-based industries. The demand for hydrogen in the hydrotreating and hydrocracking of crude pe troleum is steadily increasing [10, 11]. Hydrogen is catalytically co mbined with various intermediate processing streams and is used in conjunction with catalyt ic cracking operations to convert heavy and unsaturated compounds to lighter and more stab le compounds. Large quantities of hydrogen are

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18 used to purify gases such as argon that contai n trace amounts of oxygen. This is done by catalytic combination of oxygen and hydrogen followed by remova l of the resulting wa ter. In the food and beverages industry, it is used for hydrogenation of unsaturated fatty acids in animal and vegetable oils, to produce solid fat and other f ood products. Hydrogen is also used as a carrier gas in the manufacture of semi conducting layers in integrated circuits. The pharmaceutical industry uses hydrogen to make vitamins and ot her pharmaceutical products. Hydrogen is mixed with inert gases to obtain a reducing atmosphere which is required for many applications in the metallurgical industry such as heat treating stee l and welding. It is often used in annealing stainless steel alloys, magnetic stee l alloys, sintering and for copper brazing. It is also used as a reducing agent in the float gl ass manufacturing industry. Hydrogen is consumed in the production of me thanol [12], synthesis of ammonia [13], methanol to gasoline synthesis [14] and also for hydrocarbon synthesis by Fischer Tropsch processes [15]. In recent times, the US govern ment has tightened regulations on automotive tailpipe emissions, thereby cutting down the ben zene and sulfur compounds in gasoline. Hence, more hydrogen is now needed in refineries for processing of heavy crudes and for desulphurization in order to meet the product qual ity standards. Presently, fossil fuels such as gasoline and diesel are used all over the world and have a well-established infrastructure. These fuels will continue to be in us e until a long term substi tute that is environmentally friendly and economically feasible is found. Hence the hydrogen demand for processing these fuels must be met. As a fuel, hydrogen is considered to be very clean as it releases no carbon or sulfur emissions upon combustion. The energy contained in hydrogen on a mass basis (120 MJ/kg) is much higher than coal (35 MJ/kg), gasoline ( 47 MJ/kg) and natural gas (49.9 MJ/kg) [16].

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19 However, on a volumetric basis hydrogen has lo wer energy density. Moreover chemical energy stored in hydrogen can be dire ctly converted into electricit y by a fuel cell. The conversion efficiency of a fuel cell is higher than conve ntional combustion engines, thereby making fuel cells attractive energy conversion devices (a nd hence hydrogen an attractive fuel) for transportation and stationary appl ications. Hydrogen has long been a fuel of choice for the jet propulsion and space industry. NASA has been usi ng liquid hydrogen to fuel the space shuttle’s main engine and hydrogen fuel cells provide onb oard electric power. The space crew even drinks the water produced by the fuel cell’s chemical process. The rapid developments in fuel cells have prompted many automotive companies and the US government (through the Depa rtment of Energy) to speed up research efforts on hydrogen production. In 2003, the US govern ment announced a $1.2 billion commitment over 5 years to accelerate hydrogen research to overcome obstacles in the commercial development of fuel cells [17]. Many experts predict that hydrogen will even tually power tomorrow’s industries and thereby may replace coal, oil and natural gas [ 18, 19]. However it will not happen until a strong framework of hydrogen production, st orage, transport and delivery is developed. All the steps must be technically feasible and economically viable. Introduction to Present Research Hydrogen is not found in free-stat e in nature. It is normally co mbined with other elements such as carbon, oxygen, sulfur, chlorine and so on. Hydrocarbons are a common resource, and steam reforming of hydrocarbons (methane) is a popular method of present day hydrogen production. However, producing hydrogen fr om hydrocarbons does not address the environmental concerns as the problem gets merely shifted from the automotive tailpipe to some remote location where hydrogen is produced. In or der to have environment friendly hydrogen we

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20 must be able to produce it from renewable reso urces. Table 1-1 gives a summary of the various methods used for producing hydrogen. The table lis ts the present status of technology and the cost of producing hydrogen. Of all the renewabl es, biomass is a promising resource with a good potential for hydrogen production. In fact, considering the CO2 penalty which may be imposed on fossil fuels, biomass has the potential to become cost competitive even with fossil fuels. Biomass is a resource that is abundantl y available in many parts of the world. The chemical energy stored in biomass can be converted to hydr ogen by biological or thermal methods. The current resear ch investigates the thermal pa thway of converting biomass to hydrogen. Thermo-chemical biomass gasification ha s been used for a long time for producing syngas (a mixture of CO and H2). Biomass when gasified in th e presence of a suitable medium (such as steam) produces a gas mixture rich in CO and H2 and containing other gases such as CH4, CO2 and small amounts of higher hydrocarbons. Bi omass gasification is characterized by a number of reactions that are exothermic and endothermic which suggests that heat can be transferred internally to improve the proce ss efficiency. A thermodynamic analysis would determine the necessary conditions for maximizing the process efficiency and hydrogen yield at equilibrium. A review of the literature showed us that such a thorough thermodynamic analysis has not been performed for hydrogen production from biomass. Therefore a thermodynamic analysis was conducted with the end objective of improving the process efficiency and also to determine the conditions necessary for maximi zing the hydrogen yield at equilibrium. The important variables that influence the hydrogen yi eld are gasification temperature, gasification pressure, steam to biomass ratio and equivalen ce ratio. All these parameters were varied over typical range encountered in real life gasifica tion systems. The gasifi er temperature strongly influences the hydrogen yield in product gas. In actual practi ce, the kinetics of biomass

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21 gasification reactions are fast at temperatures above 700oC. At these temperatures, lot of gases and some liquid volatiles are released. At lower temperatures, more liquids are formed and these may settle and clog the downstream equipment. At higher temperatures there is more gas in the product stream (due to reforming of all the hydr ocarbons); however this would also require a high temperature heat source. At high temperatures (above 850oC), the water gas shift reaction occurs in the reverse directi on and reduces the hydrogen yield. The gasification pressure too affects the hydrogen yield. Most biomass gasi fiers operate at atmospheric pressure. High pressure systems reduce the equilibrium hydrogen yield. Low pressure (sub-atmospheric) systems increase the hydrogen yiel d, but the increase is only ma rginal and hence the optimum pressure for hydrogen producti on is one atmosphere. Steam to biomass ratio also strongly in fluences the amount of hydrogen produced and process efficiency. In general, when more steam is supplied the hydrogen yield is higher due to reformation of hydrocarbons. Equivalence ratio, wh ich is a measure of th e amount of air supplied during biomass gasification, is an other variable that affects the amount of hydrogen produced. The hydrogen yield in biomass gasification is li mited by chemical equilibrium constraints. There is an optimum temperature, pressure, st eam to biomass ratio and equivalence ratio at which the highest hydrogen yield occurs. In or der to further enhance the hydrogen yield, the equilibrium constraint has to be removed. This is possible by removing one of the co-products of gasification (CO2) that influences the equilibrium. If we can continuously remove the CO2 as soon as it is formed, the shift reaction goes to completion and yields a hydrogen rich gas. In the past, sorbents such as calcium oxides have been used to remove CO2 from the fossil fuel exhaust. If the CO2 absorption reaction can be coupled with biomass gasification and water gas shift reactions, we can produce a gas ri ch in hydrogen with small amounts of CO, CO2, CH4

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22 and other impurities. Furthe rmore, the exothermic CO2 absorption reaction can be coupled with the endothermic biomass steam gasification reaction. This would enable in-situ heat transfer and reduce the net energy consumed by the gasifier. W ith reduced heat duty, the gasifier will become compact and this will reduce the capital cost of the system. Process simulations were carried out in ASPEN to study the effect of sorbent addi tion on hydrogen yield. The te mperature, pressure, steam to biomass ratio and sorbent to biomass ratio were varied over a wide range and the hydrogen yield was determined. An energy analys is was then carried out to determine the efficiency and energy consumption of the conve ntional and sorbent enhanced processes. An improvement in the hydrogen yield of a bout 19% and reduction in product gas CO2 of about 50.2% was observed. The gasifier heat duty was reduced by about 42%. Based on the promising results of the simulations an experimental set up was fabricated and tests were carried out. The experimental studi es showed a substantial improvement in the hydrogen yield while using sorbents. A hydroge n rich product gas was obtained by steam gasifying Southern pine bark in the presence of calcium oxi de. Thereafter, some studies on regeneration of used sorbent were carried out. The disse rtation provides a detailed theoretical and experimental investigation of hydrogen producti on by steam gasification of biomass in the presence of sorbents.

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23Table 1-1: Summary of hydrogen production methods Method Energy Efficiency H2 production Cost Scale/Current Status Major Advantage Major Disadvantage SMR 83% $ 0.75 /kg (w/out CO2 sequestration) Large/ Currently available Proven technology High Efficiency Economically favorable CO2 by-product Limited methane supply Partial Oxidation 70-80% $ 1.39 /kg (Residual oil) Large/Available for large hydrocarbons Proven technology Economically feasible Methane pipeline in place CO2 by-product Lower efficiency than SMR Autothermal reforming 71-74% $1.93 /kg Large/ Currently available Proven technology Cheaper reactor than SMR Methane pipeline in place CO2 by-product Limited methane supply Lower efficiency than SMR Coal gasification 63% $0.92/kg (w/out CO2 sequestration) Large/ Currently available Proven technology Economically favorable CO2 by-product Less H2 rich than SMR Biomass Gasification 40-50% $1.21-2.42/kg Mid-size/Currently Available Renewable No foreign imports Seasonal availability Transportation problems Biomass Pyrolysis 56% $1.21-2.19/kg Mid-size/Currently Available Renewable Easily Transportable Seasonal availability Varying H2 content of feedstocks Electrolysis 25% $2.56-2.97/kg (Nuke source) Small/ Currently available Proven technology Emission free when used with renewables Low overall efficiency High cost Current capacities still small Thermochemical 42% (850oC) $2.01/kg (Sulfur-Iodine cycle) Under research Emission free No dependence on fossil fuel sources High capital costs Severe operating conditions Highly corrosive conditions (UT-3; sulfur Iodine) Photocatalytic 10-14% (theoretical) $4.98/kg Under research Renewable No fossil dependence Costly Low efficiency Biological 24% (speculative) $5.52/kg Under research Renewable No fossil dependence Low efficiency High capital cost Efficiency is defined as the ratio of lo wer heating value of hydrogen in product gas to total energy supplied to the process Includes the efficiency of electricity production

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24 CHAPTER 2 HYDROGEN PRODUCTION METHODS Introduction Hydrogen is the most abundant element found in the universe. However as compared to fossil fuels, hydrogen does not occu r in free-state in nature. It nor mally exists in combined state with other elements. Hydrogen is bound with carbon in all hydrocarbons; it is bound with oxygen in water and is found in many other compounds such as hydrogen sulfide, hydrogen iodide, hydrochloric acid and so forth. The bound hydrogen can be separated by various methods like thermal, electrochemical, photolytic or biological methods. The ne xt few sections describe the different methods used for producing hydrogen from various sources. Some of these methods are used commercially, others are near commercial stage development and there are still others which are at research stage. Hydrogen Production Methods Steam Methane Reforming (SMR) SMR produces hydrogen in the fo llowing three steps [20]: methane is first catalytically reformed at elevated temperature and pressure to produce synthesis gas (synthesis gas or syngas is a mixture of H2 and CO) (equation 2.1) a catalytic Water Gas Shift (W GS) reaction is then carried out to combine CO and H2O to produce additional hydrogen (equation 2.2) the hydrogen product is then separated by adsorption The reforming step occurs as per the follo wing reaction (refer Figure 2.1). The reforming reaction is endothermic and so energy has to be supplied to the process. Methane is treated with high temperature steam to produce a mixture of H2, CO, CO2 and other impurities. The reaction

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25 is carried out in a reformer cont aining tubes filled with nickel catalyst at temperatures between 500oC and 950oC and a pressure of 30 atmospheres. Excess steam promotes the second step in the process the conversion of syngas to the de sired end product (hydrogen) as per the WaterGas Shift reaction 422CHHOCO2HH +206 kJ/mol (2.1) 222 COHOCOHH -41 kJ/mol (2.2) The third step of hydrogen separation is c onventionally accomplishe d by pressure swing adsorption (PSA). After removing the hydrogen, th e product gas may be treated to remove CO2 if sequestration is desired. SMR is the most widely used method for hydrogen production. High efficiency, favorable economics and proven technology char acterize the SMR process. SMR is ideal for large scale, centralized hydrogen production. A di sadvantage from an economic st andpoint is that capture of CO2 may be necessary in future resulting in a dditional capital and operating costs. Another concern is the long-term availabi lity of methane. For these re asons, SMR is considered as a transition technology [20]. SMR may play an impor tant role in helping make the switch to hydrogen, but will most likely be replaced by other technologies for long term hydrogen production. Partial Oxidation or Autoth ermal Reforming of Methane Partial oxidation (POX) and Autothermal Refo rming (ATR) are simila r alternatives to SMR. The POX process partially oxidizes methan e in a one-step reaction, while ATR combines partial oxidation and reforming reaction, catalytic ally reacting methane wi th a mixture of steam and oxygen. This differs from the steam methane reforming process which treats methane with

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26 steam only. Partial oxidation of methane produces a syngas mixture of CO and H2 as per following the reaction: 422 CH0.5OCO2HH -36 kJ/mol (2.3) A catalyst is not required but has the potential to enhance the hydrogen yield and lower the operating temperature. As the reaction is exot hermic, careful design a nd control of special reactors to facilitate heat exchange or dilution of reactan ts is necessary to prevent possible explosion. An oxygen plant is usually installed on site to supply pure oxygen feed. Pure oxygen is preferable because energy is wasted in heat ing and compressing the additional nitrogen gas if air is used. A more advanced partial oxidation process is autothermal reforming, a hybrid of partial oxidation and SMR processes. Both the partial oxidation and reforming reactions take place inside the autothermal reactor. The heat from th e exothermic partial oxid ation reaction supplies a portion of the heat required by the endothermic re forming reaction. Because a portion of the feed methane is burned within the reactor vessel as op posed to heating by an external furnace as in SMR, less energy is required in autothermal reforming. This simplifies the design of the autothermal reactor to one large vessel instead of the complex, bulky reactor design with many tubes necessary for heat exchange in SMR. At present, commercial processes for part ial oxidation using methane feedstock do not exist. This is mainly due to the lower efficien cy of the partial oxidat ion process (70-80%) as compared to more than 80% efficiency in the ca se of SMR. Commercial partial oxidation is a mature technology when using other hydrocarbon feedst ocks especially heavy residual oils (refer figure 2.2) (examples are Texaco and Shell gasifi cation processes). Small scale partial oxidation

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27 units for methane are being developed for use in fu el cell systems, but are still in the research phase [22]. Coal Gasification Coal gasification involves three steps: treatment of coal f eedstock with high temperature steam (1300oC) to produce syngas, a catalytic shift conversion, and purification of the hydrogen product. In the first step, coal is chem ically broken down by high temperature (1330oC) and high pressure steam to produce raw synthesis gas, as per the following reaction: 22 CHOCOH+ impuritiesH > 0 (2.4) The heat required for this gasification st ep comes from controlled addition of oxygen, which allows partial oxidation of a small amount of the coal feedstock. Because of this, the reaction is carried out in either an air-bl own or oxygen-blown gasifier. The oxygen-blown gasifier is generally used to minimize NOx formation and make the process more compatible for carbon dioxide sequestration. In the second step, the syngas pa sses through a shift reactor converting a portion of the carbon-monoxide to carbon-dioxide and ther eby produce additional hydrogen 222 COHOCOHH -41 kJ/mol (2.5) In the third step, the hydroge n product is purified. Physical absorption removes 99% of impurities. The majority of H2 in the shifted syngas is then removed in a Pressure Swing Adsorption (PSA) unit. In case of CO2 sequestration, a secondary absorption tower removes CO2 from the remaining shifted syngas. Coal is an at tractive energy source due to its abundance in the United States and low and traditionally stable prices. Coal gasification is an established technology used in hydrogen production today, but additional technical and economic considerations for captu re and storage of CO2 will be necessary in future.

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28 Biomass Gasification and Pyrolysis Biomass refers to crops or other agricultura l products including hardwood, softwood, and other plant species. It may also include municipa l solid waste or sewage, a fraction of which is burned to produce steam for the process. Biomass may be used to produce hydrogen in two ways: 1) direct gasification or 2) pyrolysis to produce liquid bio-oil for reforming. Direct biomass gasification process is similar to coal gasification. Th e process is carried out in three steps. First the bi omass is treated with high temperature steam in an oxygen-blown or air-blown gasifier to produce syngas mixture co mposed of hydrocarbon gases H2, CO, CO2, tar and water vapor. Char (carbon residue) and ash are left behind in the gasifier. Then, a portion of the char is gasified by reaction with oxyge n, steam and hydrogen while another portion is combusted to provide heat. As in the case of coal, the gasification step is followed by shift reaction and purification. Alternatively, the bioma ss can first be reformed to a liquid (bio-oil) by a process well known as pyrolysis. Pyrolysis is an endothermic process for thermal decomposition of biomass and is carried out at 450-550oC. The bio-oil produced is a liquid composed of oxygenated organics and water [23]. The bio-oil is steam reformed using a nickelcatalyst at 750-850oC, followed by shift reaction to convert CO to CO2. Following are the general reactions in biomass gasification and pyrolysis: 222nm Biomasssteam/OHCOCOCHimpuritiesH>0 (gasification) (2.6) Biomassenergybio-oilcharimpurities(pyrolysis) (2.7) 2 Bio-oilsteamCOH(reforming) (2.8) 222 COHOCOHH -41 kJ/mol (shift) (2.9) Biomass gasification technology has over the ye ars progressed from small laboratory scale models to several demonstration pilot scale plants either for producing electricity or syngas. For

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29 example BIOSYN Inc. is an oxygen-blown gasi fication process in a bubbling fluidized bed gasifier with a bed of silica or alumina which is used for making methanol. There are several commercial gasifier manufacturers in Europe a nd N. America and many of these are used for producing power or syngas [24]. Biomass reso urce has the advantage of being renewable, sulfur-free and being locally avai lable. Hence it has a great potential for the future “hydrogen economy”. However, there are many factors limiting commercial bi omass hydrogen production, chief among them being high transport cost due to low energy density of biomass high capital cost of biomass plants seasonal availability Pyrolysis is still at a relativel y early stage of research and is not as mature as gasification. However, among all the renewable resources us ed for hydrogen production, biomass is the one which has the greatest potentia l for being commercialized in the near future (Table 1-1). Electrolysis Electrolysis uses electricity to dissociate water into diatomic molecules H2 and O2. An electric potential is applied across a cell with two electrodes containing a conducting medium, generally an alkaline electrolyte solution su ch as aqueous solution of potassium hydroxide (KOH). Electrons are absorbed a nd released at the electrodes, forming hydrogen at the cathode and oxygen at the anode. Under alkaline conditio ns, this process may be described by the following reactions [25, 26]: Cathode: 2 22HO2eH2OH (2.10) Anode: 1 2 222OHOHO2e (2.11) Overall: 1 2 222HOHO (2.12)

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30 The net effect is to produce H2 and O2 by supplying only water and electricity (refer figure 1.3). The theoretical voltage for the decomposition at atmospheric pressure and 25oC is 1.23 volts (V). At this voltage, reaction rates are very slow. In practice, higher voltages are applied to increase the reaction rates. However, this resu lts in increased heat lo sses to the surroundings, decreasing the efficiency. The necessary volta ge may be lowered by using catalysts or sophisticated electrode surfaces. Increasing temp erature and pressure may also increase the efficiency at the cost of additional material needed to resist corrosion or higher pressures [27]. There are broadly two types of electrolysis technologies: (1 ) solid polymer using a proton exchange membrane (PEM) and (2) liquid elec trolyte, most commonly potassium hydroxide. A PEM electrolyzer is literally a PEM fuel ce ll operating in reverse mode. When water is introduced to the PEM electrolyzer cell, hydrogen ions (protons) are draw n into and through the membrane, where they recombine with electr ons to form hydrogen molecules. Oxygen gas remains behind in the water. As water is recirculated, oxygen accumulates in a separation tank and can then be removed from the system. Hydrog en gas is separately channeled from the cell stack and captured. Liquid elec trolyte systems typically use a caustic solution and in those systems, oxygen ions migrate through the electro lytic material, leaving hydrogen gas dissolved in the water stream. This hydrogen is readily attrac ted from water when directed into a separating chamber. Electrolysis is well suited to meet early stage fuelling needs of fuel cell vehicle market. Electrolyzers scale down reasonably well; efficiency of electrolysis reaction is independent of cell size. The US DOE has predicted an elec trolytic hydrogen producti on cost of about $2.5/kg by 2010 for hydrogen for a plant integrat ed with renewable energy [28]. Thermochemical Hydrogen Production High temperature heat (500 – 2000oC) drives a series of chemical reactions that produce hydrogen and oxygen. The chemicals used in the process are reused within each cycle. This

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31 process operates in a closed loop and consum es only water and produces hydrogen and oxygen in separate steps. The high temperature heat ne eded for the process can be supplied by nuclear reactors (up to 1000oC) or by solar energy through concen trated solar collectors (up to 2000oC). Different cycles have been identified to operate in different temperature ranges. There are more than a thousand cycles that have been proposed so far but only a few hold promise for large scale implementation [29, 30]. Two of the popular ther mochemical cycles are described below. Zn/ZnO cycle Zinc oxide is passed through a reactor heat ed by solar concentrator at about 1900oC (refer Figure 2.4). At this temperature zinc oxide dissociates into zinc and oxygen gases. Zinc is cooled, separated and reacted wi th steam (at about 300 to 400oC) to produce hydrogen and solid zinc oxide. The net products are hydrogen and o xygen with water as input. Hydrogen is later separated and purified. The zinc oxide is recycl ed into the process to produce more hydrogen. The reactions taking place are as under: 2 ZnOheatZn0.5O (2.13) 22 ZnHOZnOH (2.14) 222 HOheatH0.5O (Overall reaction) (2.15) Haueter et al have developed a solar chemical reactor for thermochemical hydrogen production based on Zn/ZnO cycle [31] UT-3 cycle The UT-3 cycle (University of Tokyo #3) was proposed by Kameyama and Yoshida in 1978 [32]. A UT-3 cycle is composed of a seri es of four thermochemical reactions. The operating temperatures are relativ ely lower than those found in ot her thermochemical cycles, the highest being 760oC. When the reactions proceed in the co rrect order all the solid reactants are

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32 regenerated, except water which is split into hydrogen and oxygen and separated from the system. The reactions taking place are as under: Reaction1:22 o760CCaBrHO(g)CaO(s)2HBr(g) (2.16) Reaction 2:222 o570CCaO(s)Br(g)CaBr0.5O(g) (2.17) Reaction 3:34222 o220CFeO(s)8HBr(g)3FeBr(s)4HO(g)Br(g) (2.18) Reaction 4: 22342 o560C3FeBr4HO(g)FeO6HBr(g)H(g) (2.19) The UT-3 cycle has been extensively studied in Japan. It may have the potential for commercial production of renewabl e hydrogen. At present investig ations are going on into the chemical kinetic aspects of the reac tions involved in the UT-3 cycle. There are other cycles too (like sulfu r-iodine cycle) which are being pursued. Thermochemical cycles are well suited for hydrogen production in conj unction with nuclear energy. The Department of Energy allocat ed $4 million research budget for select thermochemical cycles in the year 2003. Photocatalytic Hydrogen Production Photocatalyst materials (generally semiconduc tors) doped with other materials, catalyze direct water splitting using solar energy. Examples of materials that ha ve been shown to be effective in catalyzing water sp litting are oxynitirides, TaON, Ta3N5, and LaTiO2N, nickel doped indium-tantalum-oxide catalysts, and CdS/ZnS syst ems. The water splitting takes place when the catalyst is irradiated with light in the presence of an elec tron donor and acceptor, oxidizing OHions to produce O2 and reducing H+ ions to H2. The semiconductor can also be paired with catalysts to promote these oxidation and reduction reactions (r efer Figure 2.6). As a photocatalytic semiconductor material im mersed in water is exposed to light, the material absorbs photons causing valence electron s to jump to the conduction band (CB), leaving

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33 behind positively charged holes in the valence band (V B). If the conduction band is at a higher energy level than the reducti on potential of hydrogen, the elec trons in the conduction band can reduce hydrogen ions at the surface of the se miconductor to produce hydrogen gas. The valence bands are at a lower energy than the oxidation po tential of hydrogen, so the positive holes accept electrons from the hydroxide ions and oxygen gas is produced as illustrated in the figure 2.6. Effective photocatalysts are t hose in which the conduction an d valence band levels most closely match the potential for reduction and oxi dation of water. The photocatalytic hydrogen production has been demonstrated at laboratory scale [35]. Howeve r, the technology is still not feasible on commercial scale. Also, currently th e process does not have th e capability to produce hydrogen in sufficiently large quantities (like SM R). Further research will determine whether efficiency and cost of hydrogen production by phot ocatalytic water splitting will be competitive with other hydrogen production methods. Photoelectrochemical Hydrogen Production In its simplest form a phot oelectrochemical (PEC) hydrogen production cell consists of a semiconductor electrode and a metal counter elec trode immersed in an aqueous electrolyte. When light is incident on the semi-conductor elec trode, it absorbs part of the light and generates electricity. This electricity is used for electrolytically splitting water. Hence a PEC cell is a combination of a photovoltaic cell and electrolysis. Fujishima and Honda first demonstrated this concept using solar energy in 1972 [37]. The cell consists of a semiconductor photoanode which is irradiated with electromagnetic radiation. The counter electrode is a metal. Followi ng processes take place when light is incident on the semiconductor electrode. • photogeneration of charge carrier s (electron and hole pairs) he (2.20)

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34 where, is the Planck’s constant, is the frequency, h+ is the hole and eis the electron. • charge separation and migration of holes to the interface between the semiconductor and electrolyte and of the electrons to the counter electrode thr ough the external circuit. The holes are simply vacancies in the valence band due to promotion of electrons from valence band to conduction band. However, in the study of electronic behavior of materials holes are considered to be independent entities with their own mass • electrode processes: Water is oxidized to H+ ions and O2 gas by the holes at the photoanode and the H+ ions are reduced to H2 gas by electrons at the photocathode At photoanode: 1 22 2HOh2HO (2.21) At photocathode: 22H2eH (2.22) The efficiency of PEC cells for hydrogen produc tion largely depends on the efficiency of the photovoltaic cell. Due to the i nherent low efficiency of PV cells, photoelectrochemical cells are not very efficient in hydr ogen production as compared to conventional processes. Typical efficiency reached is around 5-6% [38] that too when multi band gap thin film PV cells are used. There are many issues other than low efficiency that need to be addressed such as corrosion resistance of the semiconductor material, optimiza tion of the electrolyte and cost of photovoltaic cells. This method is still under research and the success of this method largely depends on the improvements made in photovoltaic materials and their performance. Biological Hydrogen Production Biological methods for hydrogen production have b een known for over a century. Broadly there are two methods by which hydrogen can be produced: Fermentation of bacteria Fermentation by anaerobic bacteria as well as some microalgae (such as green algae) on carbo-hydrate rich substrates can produce hydrogen at 30 to 80 oC in the absence of sunlight and oxygen. The products of fermentation mainly include H2 and CO2 with small quantities of other

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35 gases such as CH4 or H2S depending on the reacti on process and substrate used. With glucose as model substrate, a maximum of four moles of H2 are produced per mole of glucose 61262322CHO2HO2CHCOOH4H2CO (2.23) The actual amount of hydrogen produced depend s on the pH value, th e hydraulic retention time as well as the gas partial pressure [39]. Biophotolysis Biophotolysis uses the same principle found in plant and algal photos ynthesis, but adapts them for the production of hydrogen instead of carbon containing biomass. Photosynthesis involves absorption of light by tw o distinct photosynthe tic systems operating in series: a water splitting and oxygen evolving system (photosyste m I or PSI) and a second photosystem (PSII) which generates the reductant used for CO2 reduction. In this coupl ed process, two photons (one per photosystem) are used for each el ectron removed from water and used in CO2 reduction or H2 formation. In green plants, only CO2 reduction takes place, as the enzymes that catalyze H2 formation, (the hydrogenase) are absent. Micr oalgae (such as cynobacteria) have hydrogenase enzyme and hence can be used to produce H2 under certain conditions [40]. The overall reaction is given by: 222solar energy2HO2HO (2.24) Although technologies for biological hydrogen pr oduction are available, they are still not mature for commercial production. There are many technical barriers and so me of them include: lack of characteriza tion of microorganisms for hydrogen production low light conversion effici ency (less than 10%) for photolytic hydrogen production low hydrogen production rate to be commercially viable hydrogen re-oxidation by the hydrogenase enzyme

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36 Due to the inherent technical problems th e cost of hydrogen produced from biological methods is still very high as compared to conventional methods such as Steam Methane Reforming. Summary A summary of the different hydr ogen production methods is provi ded in Table 1-1. It is observed that currently SMR offers the lowest hydrogen production cost. SMR is also a proven technology with very high energy efficiency. Howeve r the natural gas reserv es within the US are limited and hence SMR is considered as a tran sition phase to the “hydrogen economy”. Partial oxidation and autothermal reforming are possible alternatives to SMR, but both these methods are less efficient. Also, the cost of hydrogen pr oduction by these methods is higher than SMR. Coal gasification is cost-competitive but CO2 by-product removal is a matter of concern. Electrolysis is a proven technology but is currently expensive. Al so, capacities are very small and hence scale-up is required for bulk hydroge n production. Thermochemical water splitting process is clean (no emissions); however it is complicated by several reactions and severe operating conditions. These methods are still und er research. Biologica l and photocatalytic methods are both renewable but at the same time are expensive. The efficiency is also very low. Both these methods are also currently under research. Of all the renewables, biomass is a promis ing resource for producing environment friendly hydrogen. In fact, considering the CO2 penalty which may be imposed on fossil fuels, biomass has the potential to become cost competitive wi th fossil fuels. The dr awbacks of biomass are seasonal availability, high feedst ock and capital costs. Hydrogen can become a fully renewable energy carrier only if the raw materials and me thods used for producing it are renewable.

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37 Figure 2.1: Block diagram of hydrogen producti on by steam methane reforming (adapted from Sherif et al [21])

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38 GASIFICATIONCarbon-dioxide, hydrogen, steam and small amounts of CH4are produced as the raw gas SHIFT REACTIONCO + H2O CO2+ H2 HYDROGEN SEPARATION AIR SEPARATION CO2H2N2 Air O2Hydrocarbon Feed steam DESULFURIZATION Syngas Sulfur Figure 2.2: Block diagram of hydrogen production by partial oxidation of heavy oils (adapted from Sherif et al [21])

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39 Figure 2.3: Principle of hydrogen production by high temperature el ectrolysis (HTE)

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40 Figure 2.4: Block diagram of the Zn/ZnO wa ter splitting thermochemical cycle for hydrogen production (adapted from Weidenkaff [33])

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41 Figure 2.5 UT-3 cycle reactions and flow of material (adapted from Aochi et al [34])

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42 Figure 2.6: Photocatalytic hydrogen production (adapted from Oudenhoven et al [36])

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43 Cathode Photo-Anode Figure 2.7: Principle of phot oelectrochemical hydrogen produc tion (adapted from Fujishima et al [37])

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44 CHAPTER 3 BACKGROUND AND LITERATURE REVIEW Introduction Wood and other forms of biomass including en ergy crops and agricultural and forestry wastes are some of the main renewable ener gy sources available fo r hydrogen production. Biomass is considered the renewa ble energy source with the highest potential to contribute to the transportation energy needs of modern society for both the developed and developing economies around the world [41, 42]. Biomass can be conver ted to liquid and gaseous fuels via thermal, biological and physical processes. In the thermal technique there are four met hods suitable for the conversion of biomass: pyrolysis, gasification, liquefacti on or direct combustion and prim ary products of these processes can be gas, liquid, solid char and/or heat depending on th e conversion technology employed. Secondary higher value products may be produced by additional processing. Pyrolysis Pyrolysis is the thermal degradation (devola tization) of biomass in the absence of an oxidizing agent at temperat ures in the range 200-500oC. This leads to the formation of a mixture of liquids, gases and highly reactive carbonaceou s char, the relative proportions of which depends on the heating rate. The products can be used in a variety of ways. The char can be upgraded to activated carbon, used in the metallu rgical industry, as a domestic cooking fuel or for any suitable application. Pyrolysis gas can be used for power generation or heat, or synthesized to produce methanol. The tarry liquid (called bio-oil) can be upgraded to high grade hydrocarbon liquid fuels for combus tion engines or used directly for power generation or heat.

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45 Gasification Gasification (also called pyrolys is by partial oxidation) is a conversion process in which the goal is to maximize the gaseous product yiel d. Relatively higher te mperatures of 800-1100oC are used compared to 200-500oC in pyrolysis. The gaseous mixture produced contains H2, CO, CO2, CH4, H2O, and N2 (if air is used as the gasifying medi um) and various contaminants such as small char particles, small amounts of ash and ta rs. Air gasification produces a low heating value (LHV) gas (4-7 MJ/Nm3). The fuel gas can be burned exte rnally in a boiler for producing hot water or steam, in a gas turbine for electricity pr oduction or in an intern al combustion engine. It can also be upgraded to methanol through synthesi s. Before the fuel gas can be used in gas turbines or internal combustion engines, the cont aminants (tar, char-particles, ash) have to be removed. The hot gas from the gas turbine can be us ed to produce steam to be utilized in a steam turbine in an Integrated Gasification Combustion Cycle (IGCC). Combustion Combustion is complete oxidation of the biom ass feedstock. Combustion provides very hot gas that can be used to (1) heat a boiler and prod uce steam for process application (2) as a source of process or space heat (3) as the energy source for Rankine cycle or Stirling engines. Typically, temperatures of the order of 1200oC are encountered in combustion. Liquefaction Liquefaction is a low temperature (250-300oC), high pressure (100-200 bar) thermochemical conversion to convert bi omass into liquid phase, usually in the presence of a catalyst. The main goal here is to maximize the liquid yi eld, and the product is a higher quality liquid (in terms of heating value) than the one produced in pyrolysis. Of all the methods, biomass gasification has a ttracted the greatest interest as it offers higher efficiencies than combustion [42, 43]; ot her technologies (fast py rolysis & liquefaction)

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46 are still at a relatively early stage of development [42]. Ther mochemical biomass gasification has been identified as a possible method for produc ing renewable hydrogen [44]. Figure 3.1 shows a photograph of a pilot biomass gasification plant which uses peanut shells as feedstock for producing hydrogen. Lab-Scale Production of Hydrogen from Biomass A schematic of the experimental set-up us ed for producing hydroge n by gasification of biomass is shown in figure 3.2. Here biomass is fed continuously using a screw feeder to a fluidized-bed gasifier and steam is used as the gasifying medium. The gas coming out of the gasifier is passed through a metallic filter before being sent to a catalytic reactor. The catalytic reactor reforms tars and hi gher hydrocarbon in the product gas to produce additional hydrogen. The gas is then cooled to condense and remove the steam and then passed through a filter to get rid of as h, dust and particulate matter. The clean, dry gas coming out of the filter is then sent to a gas-chromatograph for composition analysis. Any suitable biomass can be used as a feed to the gasifier. Biomass feeds can be agricultural wastes, energy crops, municipal solid wastes, woody and tree material and so fort h. Table 3-1 gives the chemical composition, ultimate and proximate analysis and heating va lue of sawdust which is a typical biomass feedstock [44]. The C-H-O (Carbon-HydrogenOxygen) composition for any biomass is approximately the same; feedstocks differ from each other in the amount of mineral matter (alkaline material) and moisture content. For comparison the chemical composition, ultimate and proximate analysis and heating value for a grad e of coal (found in Belmont, Ohio) is also provided [16]. From the chemical composition it is seen that biomass feedstock has much less sulfur as compared to coal. This is another r eason why a biomass feedst ock is preferable over coal. However, oxygen content in biomass is highe r than coal. Typically biomass consists of about 6% hydrogen by weight. The hydrogen yi eld of plain biomass gasification can be

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47 substantially improved if we use steam as a gasify ing medium (this is expl ained in detail in the next chapter) An Overview of Biomass Gasification for Hydrogen Production Biomass gasification has been extensively studi ed over the last three decades in the United States and other countries around th e world. Different research gr oups have investigated biomass gasification with different obj ectives like optimizing syngas pr oduction, maximizing the overall gas yield, hydrogen production, product gas cleaning for trouble-free downstream operation, effective waste utilization and so on. The objective of the presen t research is to study biomass gasification from the perspective of maximizing th e hydrogen yield. A detailed literature review was conducted to know the state of the art. The following sub-areas were identified: Catalysis Pretreatment technologies Chemical kinetic studies Experimental studies on biomass gasification Thermodynamics of gasification Sorbent enhanced gasification Catalysis Biomass thermo-chemical gasification produces gases, liquids and solids. The product contains as major components H2, CO, CO2, CH4, H2O and N2, smaller amounts of hydrocarbons, inorganics (H2S, HCl, NH3, alkali metals) and particulate matter. The organic impurities range from low molecular weight hydr ocarbons such as methane to high molecular weight polynuclear aromatic hydrocarbons. The lo w molecular weight hydrocarbons can be used as fuel in gas turbine or engine applications, but are undesirable products in fuel cell applications and methanol synthesis. The high molecular weight hydrocarbons are collectively known as

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48 “tars”. Tars are undesirable in Integrated biomass Gasification Combined Cycle systems (IGCC) for a number of reasons. They can condense in exit pipes and on particulate filters leading to blockages and clogged filters. Tars also have varied impact on other downstream processes. They can clog fuel lines and injectors in intern al combustion engines. The product gas from an atmospheric pressure gasification process needs to be compressed before it is combusted in a gas turbine and tars can condense in the compressor or in the transfer lines as the product gas cools. Biomass gasification product gas requires subs tantial conditioning includ ing tar conversion or removal, before it is used in polymer electrolyte membrane (PEM) fuel cell systems that require essentially pure hydrogen. There are a number of methods to separate or reform tars from th e product gas like wet scrubbing, thermal cracking or catalytic crackin g. Wet scrubbing involves cooling the gas in order to condense the tars. This technique doe s not eliminate tars but merely transfers the problem from gas phase to condensed phase. Th ermal cracking is a hot gas conditioning option but it requires high temperatures (more than 1100oC) to achieve high conversion efficiencies. This process may also produce soot which is an unwanted impurity in the product gas stream. Catalytic steam reforming is an attractive hot ga s conditioning method. Ca talytic tar destruction has been studied for several decades and a numbe r of reviews have been written in biomass gasification hot gas clean up [4648]. Broadly three groups of catal yst materials have been used for biomass gasification systems: alkali metals non-metallic oxides, and supported metallic oxides. Alkali metals enhance biomass gasifi cation and are therefore considered primary catalysts and not tar reforming catal ysts. Alkali salts are mixed directly with the biomass as it is fed into the gasifier. The non-metallic and supporte d metallic oxide catalyst s are usually located

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49 in a separate fixed bed reactor, downstream from the gasifier, to reduce the tar content of the gasification product gas and are therefore referred to as secondary catalysts. Non-Metallic Oxides Calcined dolomites have been extensively inve stigated as biomass gasifier tar destruction catalysts. Dolomites are calcium-magnesium ore with the general formula CaMg(CO3)2. These naturally occurring catalysts are relatively inexpensive and disposable. So it is possible to use them as primary (in bed) catalysts as well as in secondary (downstr eam) reactors. Several research groups have conducted extensive studies on th e tar conversion effec tiveness of calcined dolomites and other non-metallic oxide catalysts. Simell and co-workers [49] performed a number of studies using model compounds to test the reforming effectiveness of dolomites. The catalysts were calcined at 900oC and showed high toluene conv ersion efficiencies (>97%); however catalyst activity was almost completely lost when the CO2 partial pressure was higher than equilibrium decomposition pressure of dolom ite. Simell et al also reported decomposition of benzene when it was passed over Finnish dolomite at 900oC. Aznar and co-workers [50] constructed a biom ass gasification pilot plan to study catalytic product gas conditioning. The gasifying agents used were air, steam and a mixture of steam and oxygen, and pinewood was fed into the bottom of the bubbling bed. It was found that when 20g of calcined dolomite per kg of biomass was added, the tar content in pr oduct gas decreased by a factor of 4 to 6. They also observed that th e hydrogen content of the pr oduct gas doubled and CO content reduced by a factor of two. Several other groups have also studied catalytic tar reforming with dolomites [51, 52]. All of th ese studies demonstrate that dol omite is a very effective tar reforming catalyst. High molecu lar weight hydrocarbons are effici ently removed at moderately high temperatures (800oC) with steam and oxygen mixtures as the gasifying agent; however methane concentration is not greatly affected and benzene and naphthalene are often not

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50 completely reformed. A problem with dolomites which is reported by many investigators is a decrease in mechanical strength over time, which leads to catalytic attrition. In summary, dolomites are inexpensive dispos able chemicals that can be mixed with biomass and used as primary catalysts. They ar e mainly used for reforming many high molecular weight tar compounds. Dolomites however undergo at trition over a period of time and need to be replenished. Another problem with dolomites is the waste stream they generate once they undergo attrition. Commercial Nickel Reforming Catalyst A wide variety of Ni-based reforming catalysts are commercially available because of their application in the petrochemical industry for na phtha reforming and methane reforming to make syngas. Nickel based catalysts have also proven to be very effective for hot conditioning of biomass gasification product gases. They have high activity for tar destruction; methane in the gasification product gas is reformed, and they have some water gas shift activity to adjust the H2/CO ratio of the product gas. The H2 and CO contents of the product gas increase, while hydrocarbons and methane are eliminated or subs tantially reduced for catalyst operating above approximately 740oC. The groups that were active in studying calci ned dolomite catalysts have conducted several studies involving nickel steam reforming catalysts too for hot gas conditioning. Aznar and coworkers [53] conducted several experiments with Ni-catalyst at temperatures between 750 and 850oC and found initial tar conversion efficiency to be greater th an 99%. An apparent kinetic model for tar reforming was determined for each catalyst tested based on a first order rate expression and the measured tar conversion as a function of time-on-stream. The kinetic studies gave an idea of the activation en ergy and pre-exponen tial factors obtained for the tar conversion reactions.

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51 Simell and coworkers [54] have also investig ated commercial Ni steam reforming catalyst for tar conversion using tolu ene as a model tar compound. They observed complete tar decomposition for catalyst operating at 900oC and 5 MPa. Kinoshita, Wang and Zhou [55] reported results from parametric studies on cataly tic reforming of tars pr oduced in a bench-scale gasification system. A commercial Ni-catalyst (UCG-90 B) was te sted at various temperatures (650-800oC), space times (0.6-2.0s), and steam to bi omass ratios (0-1.2) in a fluidized bed catalytic reactor. They reporte d achieving 97% tar conversion; product gas yield was higher in presence of the catalyst. Several other groups (Bangala et al [56] & Wang et al [57]) have reported high effectiveness of Ni-catalyst (>90%) in tar refo rming. However there are several factors which still limit the use of Ni-catalyst in commercial gasi fiers which need to be addressed. Some of the main limitations include sulfur, chorine and alka li metals present in th e gasification product gas which act as catalyst poisons. Coke formation on the catalyst surface can also be substantial when tar levels in the product gas are high. C oke can be removed by regenerating the catalyst; however repeated high temperature processing of nickel catalyst can lead to sintering, phase transformations and volatilization. To sum up, commercial nickel reforming catalysts have shown very high tar conversion po tential (more than 90%). However these catalysts suffer from frequent de-activation due to poisoning by su lfur, by halides and by alkaline impurities. Additional Catalyst Formulations There are several limitations of Ni reforming catalysts used for tar conversion such as deactivation by coke formation, sulfur and chlori ne poisoning and sintering. Addition of various promoters and support modifiers has been atte mpted by several groups to improve catalyst activity, lifetime, poison resistance, and resistan ce to coke formation. Rapagna et al [58] developed a catalyst with a Lanthanum additive (chemical formula LaNi0.3Fe0.7O3) that was

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52 prepared by sol-gel process. The pr epared catalyst displayed high CH4 reforming activity at 500oC resulting in 90% CH4 conversion. Garcia et al [59] ha ve prepared a number of Ni-based catalysts with different additives for optimal hydrogen production. They added Magnesium and Lanthanum as support modifiers, and Cobalt and Chromium were added to reduce coke formation. The Cobalt-promoted and Chromi um-promoted Nickel catalyst on a MgO-La2O3Al2O3 support performed best in terms of yield and life time. Sutton and co-workers [60] studied the effect of different supports using Ni-catalyst. The research group impregnated Ni on various supports including Al2O3, ZrO2, TiO2, SiO2 and a proprietary tar de struction support. High tar conversion was observed for all of the prepared catalysts. Drawing a parallel from the auto-industry, Asadullah and co-workers [61, 62] have developed a novel series of catal ysts using noble metals on oxide supports. These catalysts were prepared with Rhodium, Ruthenium, Platinum and Palladium and were tested on bench-scale fluidized-bed reactors using cel lulose as a model biomass compound. The group found more than 80% tar conversion at temp eratures as low as 550oC. Different supports we re used such as CeO2, LiO2, ZrO2, Al2O3, MgO and SiO2. It was found that Rh/CeO2 gave 100% tar conversion at 550oC. The group observed that although these cataly sts give 100% tar conversion at relatively low temperatures (500 to 600oC), they are not economically viab le. This is mainly due to the high cost associated with the noble me tal to the catalyst formulation. Several catalysts have been investigated fo r tar reforming of biomass product gases. A critical gap identified for catalytic tar reformi ng technology in biomass gasification processes is the need for extended lifetime of promising comm ercial or novel catalysts. Catalytic hot gas conditioning will not become a commercial tech nology unless adequate catalyst lifetimes can be demonstrated, even for inexpensive, disposab le catalysts like calcined dolomite. Frequent

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53 disposal of dolomite generates an additional waste stream and disposal of toxic spent Nicatalysts becomes an environmental burden. Asse ssment of catalyst lifetimes will allow biomass gasification developers to actually evaluate the cost of operati ng a biomass gasification plant. The effect of catalyst poisons li ke sulfur, chlorine and alkali metals and continued catalyst regeneration can be critically ev aluated with long term catalyst testing. Accurate catalyst cost and lifetime figures will provide important input for techno-ec onomic analysis of developing gasification technologies. Pretreatment Technologies Experimental and theoretical studies on different types of biomass have showed that pretreatment increases the volatile (gas and liquid) yield of feed stocks. Pretreatment is carried out by washing the biomass with mild acid or al kali or by impregnating them with salts before actual gasification. It is hypothe sized that during pr etreatment the biomass undergoes de-ashing (removal of mineral matter) which leads to high er gas and hence hydrogen yields. Pretreatment for gasification or pyrolysis also increases the active surface area of biomass. In some cases (especially bio-oils) the heating value of pretreated biomass is higher than the original biomass feedstock. Das and Ganesh [63] subjected sugarcane bagg ase to three different pretreatments (water leaching, mild HCl treatment and mild HF treatm ent) and found that the HF treatment reduces ash content of biomass to a ne gligible amount. The researchers also observed that the char produced in the process had a higher adsorption capacity as compared to untreated biomass. Raveendran and co-workers [64] impregnated a va riety of biomass feed stocks with chloride (KCl, ZnCl2) and carbonate (K2CO3 ZnCO3) salts and found that the gas yield increased substantially. The group later deve loped a correlation to predict the percentage change in gas yield when any biomass is subjected to potassium a nd zinc salt pretreatments. Conesa et al [65]

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54 subjected different almond shell samples to acidic and basic pretreatment followed by CoCl2 (cobalt chloride) impregnation. The samples we re then gasified and the gas composition was determined. The group found that the hydrogen yield of CoCl2 treated almond shells was higher than plain almond shell. All the research groups have hypothesized that acid, alkaline or salt pretreatment alters the mineral matter content of raw biomass. This in turn affects the product yields since the mineral matter generally tends to have a catalytic effect during the gasification process. In general, biomass pretreatment is a technique of modifying the bio-chemical ingredients of feedstock and thereby contro lling the gas and hence the hydrogen yield. In a nutshell, biomass pretreatment is a simp le and cost-effective way of influencing the product yield of any biomass gasification process. The process generally applies well to biomass with large mineral matter (Na, K, Ca, Mg, Fe, a nd P) content such as switch grass and rice husk. Chemical Kinetic Studies The development of thermo-chemical process for biomass conversion and proper equipment design requires a thorough knowledge and good understanding of several chemical and physical processes occuring in the thermal degradation proce ss. Mathematical modeling and simulation of single representative biomass particle is a very us eful tool for understanding the heat and mass transfer and chemical kinetic pr ocesses involved in bi omass gasification or pyrolysis. When a solid biomass is heated following phenomena occur: 1) heat is transferred by radiation and/or convection to the particle surface and then by conduction to inside of the particle. 2) the temperature inside the particle increases causing a) evaporation of moisture pr esent in the biomass particle b) pre-pyrolysis and pyrolysis reactions

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55 c) mass transfer from surface of biomass partic le due to formation and subsequent release of volatiles This leads to the formation of pores in solid surface. During the process, the pores of the solid enlarge and this offers many reaction sites to the volatile and gaseous products. Chemical kinetic studies predict the transient temperature pr ofile within the biomass particle as well as the yield of solid, liquid and gaseous products with time. This is done by mathematically modeling the combined effects of heat transfer and chemi cal reactions. The model is then verified with experimental results. On the expe rimental side, Thermal Gravimet ric Analysis (TGA) of a single biomass particle gives the rate of mass loss versus time and temper ature. This can be used to obtain the kinetic data (rate cons tant and activation energy) of biomass thermal degradation. The chemical kinetic data so obtained serves as a ba sis for detailed design of fixed and fluidized bed biomass reactors. Several researchers have analyzed the chem ical kinetics of biomass pyrolysis and gasification and have developed ma thematical models for the same. Kung [66] developed a basic mathematical model for pyrolysis of wood slab The model considers heat transfer due to conduction, internal heat convect ion and first order kinetics for the formation of volatiles and char. However no specific model is suggested to predict the concentr ation of the various intermediate components produced during the pyrolysis. Kansa et al [67] developed a more detailed mathematical model for the pyrolysis of wood. They incorpor ated internal force convection effects, their model used variable thermal and physi cal properties, a time-dependent surface radiant flux, a global Arrhenius pyrolysis reaction, and arbitrary boundary conditions. A comparison of their model with experimental data for maple wood showed good agreement at low surface heat fluxes, but agreement was poor fo r high fluxes. The authors concluded that for

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56 good agreement at high flux intensities, the effect of secondary pyrolysis r eactions must be taken into account. The model developed by Kansa et al was more realistic than the basic model developed by Kung. Miyanami et al further improved the model developed by Kansa et al by incorporating the heat of reacti on in the pyrolysis of solid part icles based on the volume reaction model [68]. They carried out a transient analysis of the effects of the heat of reaction on the solid biomass conversion, fluid product concentration pr ofile and temperature di stribution in the solid biomass. The results of their model had better ag reement with experimental results as compared to Kansa et al. Recently Grnli [69] deve loped a mathematical model and conducted experiments to validate the pyrol ysis of Scandinavian wood. He studied the pyrolysis of wood and developed a model that considered the eff ect of particle size on product composition. His work identified two categories of wood pyrolysis : small wood particle wh ere internal thermal resistance is negligible and chemical kinetics is the controlling mechanism, and large particles where both chemical kinetics and heat transfer need to be considered. Gronli’s work gave a better understanding of the factor s that must be taken into account while modeling biomass pyrolysis of wood particles. More recently, Jala n and Srivastava [70] de veloped a model for the pyrolysis of a single wood particle. These re searchers modeled the physical and chemical changes of a biomass particle as it undergoe s pyrolysis. This was done by considering the primary and secondary reactions. An energy balan ce equation proposed by the authors took into account the non-isothermal reaction of the bioma ss particle. Numerical schemes were employed to solve the heat transfer e quations and the equation involving chemical kinetics. The model predicted the temperature distribution within the pe llet as a function of radial distance at different times as pyrolysis progressed. The authors f ound that their model co mpared well with the experimental data from literature.

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57 In summary, chemical kinetic modeling studi es of wood pyrolysis have been conducted by several researchers over the last three decades. These models pr ovide better understanding of pyrolysis of solid biomass particles. Some of th e models have been experimentally verified. The chemical data obtained (reaction rate, rate cons tant, order of reaction, and activation energy) serve as a valuable database for the design of biomass reactors. Experimental Studies on Biomass Gasification Experimental studies on biomass gasificat ion have focused on various aspects like parametric analysis, catalytic ta r cleaning, co-gasificat ion of biomass with coal/plastic, hot gas cleaning, using multiple feed stocks, different gasi fier reactor configurations and so on. In most cases the end objective was to maximize syngas pr oduction. Turn and co-workers [44] studied the effect of gasifier temperature, steam to biomass ratio (SBR), equivalence ratio (ER) (a measure of air supplied in biomass gasification) on ga s yield (mainly H2, CO, CO2 and CH4) in fluidized-bed gasification of sawdust. They found the highest hydrogen yield to occur at a gasifier temperature of 850oC and steam biomass ratio of a bout 1. The maximum hydrogen yield was found to be 0.128 g/kg dry ash-free biomass. Narvaez and co-workers [71] have analyzed the effects of temperature, equivalence ratio and the addition of dolomite in the air gasification of pine sawdust. The group found th at maintaining an ER of 0. 3, SBR of 2.2 and gasifier temperature greater than 800oC gave good quality (maximum hea ting value) gas with minimum tar content. Herguido and co-workers [72] used different feedstocks (p ine saw dust, pine wood chips, cereal straw, and thistl es) using steam as the gasificati on medium and studied the product yield (H2, CO, CO2, and CH4 contents). The group found ma rked differences in product composition at low gasification temperatur e, but at temperatures exceeding 780oC, the gas composition was similar for all biomass feedstocks Gil and coworkers [73] have studied the effect of different gasification media (air, steam, steam and oxygen mixture) on product gas

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58 composition. They observed that using steam in pl ace of air gave a product gas with almost five times more hydrogen. Also the heating value of product gas in steam gasification (12.2–13.8 MJ/Nm3) was higher than air gasification (3.7-8.4 MJ/Nm3). Pinto and co-workers [74] have conducted experiments by co-gasifying biomass with plastic wastes and observed an increase in the hydrogen yield by about 17% when 40% (wt) of polyethylene was mixed with pinewood. One of the objectives of this rese arch was effective uti lization of plastic waste. It was found that when plain plastic was gasi fied it softened and stuck to the wa lls of the gasifier. Neither cooling nor palletizing of the waste plastic helped so lve the problem. Mixing of biomass with plastic avoided the problem of plasti c softening and effectively gasified all feedstock. In a nutshell, many researchers have carri ed out experimental studies on biomass gasification. The studies have been varied e.g. simple parametric analysis, effect of gasifying media on product yield, effect of changing feedst ock on product gas composition, co-gasification of biomass with plastic wast es, catalytic tar cleaning among others. The objective of the experimental studies in most cases was to maximize the syngas yield for power generation. Thermodynamic Studies on Gasification Biomass gasification produces a mixture of gases (mainly H2, CO, CO2 and CH4), liquids (aromatic hydrocarbons or tars) and solids (char, ash). The process parameters (temperature, pressure, steam to biomass ratio, equivalence ratio, residence time, h eating rate and so on) directly affect the product yield and compositi on. Biomass gasification also involves several reactions occurring in series and in parallel. Some of th ese reactions are as under: Steam Gasification: 1.50.7222nm RCHO0.7HOHCOCOCHtarsC(s)H119 kJ/mol (3.1) Oxidation:

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59 2RC0.5OCOH111 kJ/mol (3.2) 22RCO0.5OCOH254 kJ/mol (3.3) Boudouard: 2RCCO2COH172 kJ/mol (3.4) Water-Gas: 22RCHOCOHH +131.3 kJ/mol (3.5) 222RC2HOCO2HH0 kJ/mol (3.6) Methanation: 24RC2HCHH75 kJ/mol (3.7) Water-Gas Shift: 222RCOHOCOHH41 kJ/mol (3.8) Steam Reforming: 422RCHHOCO3HH206 kJ/mol (3.9) Some of the above reactions are exothermic and others are endothermic. Moreover the reactions occur in different react ors which operate at different te mperatures. Most of the biomass gasification systems operate at at mospheric pressure and the gasifi er operating temperature is in the range 800-850oC. In many applications the product gas needs to be cooled to lower temperatures before being sent to downstr eam equipment. There is a potential for heat integration of the various reactors so that the ne t external heat input to the biomass gasification system is reduced. This increases the thermodyna mic efficiency of the process. The hot gas coming out of the gasifier is at a sufficiently high temperature (700-800oC) and can be used to produce steam for a Rankine cycl e. The objective of the therm odynamic studies is to find the opportunities for heat integration and thereby improve the overall efficiency of the process.

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60 In the past, focus on the thermodynamics of biomass gasification has been on areas not specifically addressing hydrogen pro duction. Cairns et al calculat ed the gas-phase composition in equilibrium with carbon (graphite) for a CHO (C arbon/Hydrogen/Oxygen) system for different temperatures and O/H (atomic oxygen to hydrogen) ratios [75]. Schuster et al conducted a parametric modeling study of a biomass gasificati on system. A decentralized combined heat and power station using a dual flui dized bed steam gasifier was si mulated. The group predicted net electricity to biomass efficiency of about 20% [ 76]. Kinoshita et al conducted equilibrium studies of biomass gasification with the objective of maximizing the methanol production. The theoretical methanol yields were determined and were compared with experimental results. They also determined the optimal process c onditions for methanol production based on thermodynamic equilibrium [77]. Garcia et al al so did an equilibrium study but this was for steam reforming of ethanol. They studied the effect of temperature, pressure and steam to ethanol feed ratio and determined the maximum hydroge n yield attainable at equilibrium [78]. Carapellucci [79] studied the thermodynamics and economics of biomass drying using waste heat from gas turbine exhaust and concluded th at using gas turbine exhaust for biomass drying enhances the economic feasibility of biomass fired power plants. Lede et al [80] carried out a study on using solar energy for thermochemical c onversion of biomass. The study highlights the technical and economic benefits and also lists th e difficulties of using sola r energy as a source of heat for gasification and pyrolysis of biomass. Zainal et al [81] also did an equilibrium modeling study to predict the performance of a downdraft gasifier for diffe rent biomass materials. The group investigated the effects of temperature and moisture content in biomass on the gas composition. Their equilibrium modeling results matched reasonably well with the experimental results. Alderucci et al [82] conducted a simila r equilibrium analysis of biomass gasification

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61 where steam and CO2 were the gasifying media. The product gas was assumed to be fed to a Solid Oxide Fuel Cell (SOFC); the efficiency of the fuel cell was then determined. The researchers found that CO2 gasification gave better fuel cell efficiency as compared to steam gasification. Prins et al [83] st udied the energetic and exergetic as pects of biomass gasification in the presence of steam and air. They found that the energy and exergy of product gas had sharp maxima at the point where all the carbon is consumed. They c oncluded that the choice of gasification medium should be governed mainly by the desired product gas composition. Crane et al [84] studied two alternat ives to present day gasoline powe red systems. They did this by comparing the exergy of emission of two alternat e energy conversion technologies viz. methanol fuelled spark ignition engines and hydrogen fuel led fuel cells. The authors showed that the hydrogen powered fuel cell system was better than the methanol powered spark ignition engine from both energy and exergy perspectives. Although some work has been re ported on thermodynamics of biomass gasification no one has worked specifically on optimizing the proc ess for hydrogen production. Hence the focus of the present research was to study the thermodynami cs of gasification with the end objective of maximizing or improving the hydrogen yield. Sorbent Enhanced Gasification Biomass gasification consists of many reacti ons and processes. Steam biomass gasification is endothermic whereas partial oxidation of biom ass is exothermic. One of the objectives of the present research is to identif y suitable methods that can en hance the hydrogen yield and/or improve the process efficiency. The gasifier is an important reactor where the initial thermal breakdown of biomass takes place. If the heat duty of the gasifier can be reduced by combining reactions, it can make the process more efficien t and the reactor can become compact. Detailed thermodynamic studies showed that conventio nal biomass gasification is limited by the

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62 equilibrium constraints and hence hydrogen yiel d cannot increase beyond a certain point. In order to produce more hydrogen one of the co-products of gasification (CO2) must be removed. It was found in the studies that CO2 formation limits the hydrogen yield due to equilibrium of the Water Gas Shift (WGS) reaction. In recent past, so rbents such as calcium oxide have been used to remove the CO2 from the fossil fuel exhaust stream. When the CO2 absorption reaction is coupled with the WGS reaction, the water gas shift proceeds to the ri ght and thereby more hydrogen is produced. Han and Harrison [85] studied the simulta neous water gas shift and carbon dioxide separation process for the production of hydrogen. They observed that removing CO2 as it gets formed via the non-catalytic gas solid reaction between CaO and CO2 provides the opportunity to combine reaction and separation into a single step. The resulta nt process for hydrogen production got simplified as there was no need of heat ex changer between catalyst beds as well as the absorption and stripping units for CO2 removal. The authors studied the combined shift and carbonation reactions in a laborat ory scale fixed-bed reactor us ing dolomite sorbent precursor. They studied the effects of temperature, pressu re and space velocity on the conversion of CO in the WGS reaction. They observed that more th an 99.5% of the carbon oxides got removed and the product gas was rich in hydrogen. Balasubraman ian et al [86] conducte d experimental studies on steam methane reforming in presence of a CO2 sorbent. They added calcium oxide sorbent to a commercial steam methane reforming catalyst (nickel on alumina). The combined reforming, shift and CO2 separation reactions were st udied using a laboratory scale fixed bed reactor. The effects of temperature, steam to methane ratio, sorbent to catalys t ratio and feed gas flow rate were studied. The group found that hydrogen could be produced from methane in a single step without using a shift catalyst. The product gas was rich in hydrogen (more than 90%). A

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63 reduction in operating temperature by 150-200oC was also observed. Lin et al [87] have proposed a hydrogen production technique by reducing water (s team) using hydrocarbons. The CO2 so produced was separated using a sorbent. Th e researchers named this technique as HyPrRING (Hydrogen Production by Reacti on Integrated Novel Gasifi cation). They conducted an analysis of the HyPr-RING process and concluded that it has a potential to reduce the cost of hydrogen production as compared to conventional methods. The researchers further conducted a thermodynamic analysis of coal gasification in presence of CaO as per the HyPr-RING process [88]. A mass and energy balance was carried out and the temperat ure and pressure were varied over a wide range. The product gas composed of more than 90% hydrogen at a gasification temperature of 700oC and pressure of 3.0 MPa. This ga ve a gasification efficiency of 77%. Calcium oxide has also been used for plain CO2 removal from the fossil fuel exhaust without any hydrogen co-production. Abanades et al studied the capture of CO2 from combustion gases in a fluidized bed of CaO [89]. They conducted experiments to inves tigate the potential of CaO to capture CO2 in a pilot-scale fluidized bed reac tor. The researchers found that the CO2 capture efficiency of CaO bed was very high. However th e total capture capacity of the bed was found to decay with number of carbonation (CO2 absorption) and de-carbonation (CO2 desorption) cycles. Kyaw and Kubota studied the car bonation of CaO at various temp eratures in the range 600 to 900oC at various CO2 partial pressures [90]. The authors de veloped a kinetic rate model for the absorption of CO2 by CaO. They observed that the CO2 partial pressure is an important parameter that determines the conversion of CaO to CaCO3. The authors also studied the reverse reaction (de-carbonation) and developed a ki netic model for the conversion of CaCO3 to CaO [91]. Some other group s have studied the CO2 absorption process and have identified sorbent

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64 enhanced reforming as a possible method to enha nce the hydrogen yield a nd at the same time remove the product gas CO2 during the gasification of any carbonaceous fuel [92]. The use of sorbents for simultaneous CO2 removal and hydrogen enhancement is a relatively new concept which has become popular ove r the last few years [86, 87]. It has been proposed for coal gasification. A few research groups have applie d the concept to steam methane reforming at laboratory scale. In principle, the concept of using sorbents can be applied to any carbonaceous fuel including biomass. So far no wo rk has been done in applying the concept of sorbent enhanced gasification for biomass. Scope of the Present Work Biomass gasification is a poten tial technology that holds subs tantial promise for producing renewable hydrogen. In the previ ous sections we saw several ar eas of biomass gasification and pyrolysis that have been st udied by different research groups around the world. Although hydrogen production by biomass gasifi cation has been studied in th e past, there are many areas that still need to be addressed in order to make the technology commercially feasible. There are many barriers to the commerciali zation of biomass gasification for hydrogen production. One of them is the cap ital cost and efficiency of bi omass gasification systems. The capital costs of biomass gasification/pyrolysis need to be reduced. This may be possible by combining some steps in the production process th at can significantly reduce the capital cost. For example the two step shift and PSA separation proce ss could be combined into a single step shift and integrated separation process or the gasification, reforming, shift and separation processes could be integrated into a si ngle step. Improving the process e fficiency and hydrogen yield of biomass gasification and pyrolysis is another area of concern. Th e efficiency is defined as the lower heating value of hydrogen divided by the su m of all the energy inputs into the process including the energy in the feedstock. There ar e many types of equipmen ts which operate at

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65 different temperatures. A detailed thermodynamic analysis of the biomass gasification process is necessary. The thermodynamic analysis includes a st udy of the effect of the process parameters on hydrogen yield. The process variables temperat ure, pressure, steam to biomass ratio and equivalence ratio influence the hydrogen yield. The values of these parameters at which the hydrogen yield is maximum can be determined by a thermodynamic analysis. Biomass steam gasification is endothermic and h eat energy needs to be supplied from external sources. Steam generation also requires energy. The product gas is later cooled before separating the hydrogen and this cooling process releases heat. Hence there are some pr ocesses that absorb heat and others that release heat. An energy analysis can potentially optimize the process by better heat integration of the various sub-systems. This will reduce the overall energy consumption and thereby improve the process efficiency. A thermodynamic study can also give a deeper understanding of the constraints that limit the h ydrogen yield of conventional gasification. The equilibrium of the water gas shift reaction ca n be shifted towards higher hydrogen yield by separating one of the co-products (CO2) from the exhaust stream. Sorbents such as calcium oxide have been used for removing the CO2 from the exhaust of fossil fuels. If the CO2 absorption reaction is combined with the water gas shift reac tion, the equilibrium can be shifted in favor of hydrogen. Calcium oxide has been used as a so rbent in the steam reforming of methane for producing hydrogen at the laborator y scale. Hydrogen yields of mo re than 90% (volume) have been obtained. The concept has also been pr oposed for the steam gasification of coal. In principle, it can be applied to any carbonaceous fuel. Biomass is a re newable resource that contains substantial amount of carbon (about 45% mass) and hence is a good candidate for applying the concept of sorbent enhanced gasi fication. The present research investigates renewable hydrogen production from biomass us ing sorbents. Theoretical and experimental

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66 studies have been carried out with the end obj ective of increasing th e hydrogen yield and the overall process efficiency.

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67 Table 3-1: Feedstock composition Parameter/Analysis type Description Sawdust Coal C % 48.01 80.3 H % 6.04 5.6 O % 45.43 8 N % 0.15 1.5 Ultimate Analysis (% dry basis) S % 0.05 4.6 VM % 71.04 38 FC % 17.3 44 Ash % 4.5 5 Proximate Analysis Moisture % 7.5 13 Higher Heating value MJ/kg 18.4 34.1 (sawdust data source [44], coal data source [16])

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68 Figure 3.1: Biomass gasificati on pilot plant [Courtesy NREL]

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69 Figure 3.2: Schematic of biomass gasificati on set up for producing hydrogen (adapted from Olivares et al [45])

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70 CHAPTER 4 THERMODYNAMIC ANALYSIS OF BIOMASS GASIFICATION Introduction A parametric analysis based on thermodynami cs of biomass gasification was conducted. The gas yield depends on many process variables su ch as gasification temperature, pressure, the amounts of steam and/or air added to the gasifi er. The objective of the study was to determine the operating conditions that would maximize the equilibrium hydrogen yield. An energy analysis was conducted to determine the thermod ynamic efficiency of the gasification process with the end objective of maximizing the produc t gas hydrogen. The basi c analysis lays the foundation for a novel gasification pro cess that will be described in detail in the next chapter. Fundamentals The concept of chemical reaction equ ilibrium is based on the second law of thermodynamics for reacting systems. All spontane ous reactions occur in the direction of overall increase of entropy. When system compositi on reaches a point where the total entropy is maximum, it becomes “stuck” since any further ch ange in composition would involve a decrease of entropy which cannot occur spontaneous ly. We know from thermodynamics that gensyssurrsss (4.1) For any spontaneous reaction sgen 0. Since the environment is assumed to be at a constant temperature surrsq/T (4.2) Hence for any spontaneous reaction, syssurrsurr/T0sq (4.3) In differential form, syssurrsurrT0dsq/ (4.4)

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71 The first law in differential form is given by: wduq (4.5) For a reversible process, work term is Pdv. Th e second law in differential form as applied to a system can be written as syssysTdsq (4.6) Substituting in the first law of equation (4.5) we have, sysTdsduw 0duPdvTds (4.7) We know from thermodynamics that Gibbs free energy is defined as, guPvTs (4.8) Taking the derivative we get, dgduPdvvdPTdssdT For a constant pressure and temperature case, we have dgduPdvTds (4.9) Combining equations (4.7) and (4.9) we see that for a spontaneous reaction at constant pressure and temperature, 0dg (4.10) This means that for a given temperature and pressure, a spontaneous chemical reaction will occur until the Gibbs free energy reaches a mi nimum point in composition space. Figure 4.1 shows the total Gibbs energy in relati on to the reaction coordinate. Here ‘' is defined as the extent of reaction and characterizes the degree to which a reaction has taken place. Gibbs energy is a function of temperature, pre ssure and composition (i.e. the moles of various components present e.g. H2, CO, CO2, CH4 etc). This functionality can be represented as:

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72 12NSgg(T,P,n,n,.....,n) (4.11) Here ni is the number of moles of species i. Taking total derivative of g gives NS P,nT,nP,T,nij j j1ggg dTdPdn TPndg (4.12) Since T & P are fixed for the point of minima, we have NS P,T,nij j j1g dn0 n (4.13) The number of moles of each spec ies at equilibrium adjusts itself in such a way that the total Gibbs energy is minimized. The problem of determining the chemical composition at equilibrium now reduces to a minimization problem which needs to be solved keeping in mind the elemental (C, H, O, N) and mass constrai nts (i.e. mass of reacta nts = mass of products). Various texts [93] have carried out the mathematic al treatment to cast the above problem as an optimization problem and solve it using a pers onal computer. Late Dr W. C. Reynolds of Stanford University developed an algorithm [94] to solve the above Gibbs energy minimization problem and it is now available as free software called Stanjan. The elemental composition of the various reactants at any specifi ed temperature and pressure is supplied as input and Stanjan calculates the equilibr ium yield of the product gases. In the next section various combinations of process parameters have been simulated to de termine the most favorable conditions for hydrogen production. Effect of Process Parameters on Equilibrium Hydrogen Yield Biomass can be gasified using different gasi fying media, the choice of which depends on the desired product gas composition and energy considerations. Commercial and research gasifiers generally use steam or air as the ga sifying media [44, 55, 61, 71, 72, 95, 96]. Air gasification is an exothermic process, whic h produces a low heating-value gas (LHV 5-6

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73 MJ/Nm3) rich in CO and having small amounts of H2 and higher hydrocarbons [71]. Steam gasification on the other hand is an endothermic process, which produces a medium heatingvalue gas (LHV 12-13 MJ/Nm3) rich in H2 and CO [72]. The process parameters including temperature, pressure, steam biomass ratio, equi valence ratio and residence time also influence the product-gas composition. Effect of Temperature The gasification temperature not only affects th e product yield but also governs the process energy input. High gasifica tion temperature (800-850oC) produces a gas mixture rich in H2 and CO with small amounts of CH4 and higher hydrocarbons. Figure 4.2 shows the equilibrium moles of various gases (H2, CO, CO2, CH4) and solid carbon (C(s)) at 1 atm pressure, SBR (denoted by defined in section on ‘Effect of Steam Bi omass Ratio’) of 1.0 and ER (defined in section on ‘Effect of Equivalen ce Ratio’) of 0. At low temperatures, solid carbon (C(s)) and CH4 are present in the product gas. In actual gasifiers solid carbon is carried away to the catalytic bed and is deposited on the active catalys t sites thereby de-activating the catalyst. It is necessary to ensure that the product gas is free of any solid carbon. As temperature increases, both carbon and methane are reformed. At about 1000 K both are reduced to very small amounts ( 0.04 moles) and in the process get co nverted into CO and H2. This explains the in crease in hydrogen mole numbers. At about 1030 K, the H2 yield reaches a maximum value of about 1.33 moles. At higher temperatures the H2 yield starts reducing. This is attr ibuted to the Water-Gas Shift (WGS) reaction: 222 COHOCOHH33.8kJ/mol (4.14) According to Le-Chatelier’s principle, high temp erature favors reactants in an exothermic reaction thus explaining the incr ease in CO and reduction in H2 yield at higher temperature. For

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74 the present case a gasification temperature of about 1030 K gives the highest equilibrium hydrogen yield with negligible solid carbon in the product gas. Effect of Pressure Table 4-1 shows the effect of system pressu re on equilibrium gas composition (gasification conditions T = 1100 K, = 1, ER = 0). As pressure increases equilibrium H2 and CO yields reduce. Simulations carried out to study the e ffect of reducing the pressure below 1 atm on equilibrium product yield showed that increase in H2 yield is negligible (< 0.2%) even for pressures as low as 0.1 atm. Since high pressure reduces the H2 yield, subsequent simulations were carried out at atmospheric pressure. Effect of Steam Biomass ratio SBR refers to moles of steam fed per mole of biomass. SBR, like temperature has a strong influence on both product gas composition and ener gy input. Figure 4.3 shows equilibrium yields (moles of gas) for process conditions T = 1000 K and ER = 0. At low values of SBR, solid carbon and meth ane are formed. As more steam is supplied, both of these species are reformed to CO and H2. For > 1, C(s) and CH4 moles reduce to very small values and H2 and CO2 yields increase monotonically ; CO on the other hand reduces monotonically. This trend can be attributed to the Water-Gas Sh ift reaction; si nce system is being overfed with steam (for > 1), H2O mole numbers are increasing and as per Le-Chatelier’s principle the equilibrium shifts in the forward direction. For > 1.5, the hydrogen yield increases very slowly with most of the surplus steam goi ng unreacted. This shows that operating at very high (typically more than 2 for above conditions) may not be energy efficient, as additional H2 produced may not justify the high cost of producing and supplying steam. In the next section, an energy analysis is done to find out the optimum

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75 Effect of Equivalence Ratio ER is a measure of the amount of external oxyg en (or air) supplied to the gasifier. ER is obtained by dividing the actual oxyge n (or air) to biomass molar ratio by the stoichiometric oxygen (or air) to biomass molar ratio. Oxygen (or air) is generally supplied as a gasifying and fluidizing medium. Using air in place of oxygen, though economical has the negative effect of diluting the product gas due to the presence of nitr ogen. Figure 4.4 shows the effect of ER on the equilibrium composition for the operating conditions of T = 1100 K and = 0. As more oxygen (high ER) is supplied, it is observed that the H2 and CO yields reduce and that of CO2 increases. This is due to the oxidation of H2 and CO to H2O and CO2. At low values of ER, small amounts of C(s) and CH4 are formed in the gasifier, both of whic h get oxidized as more air is supplied. Air gasification is an exothe rmic process and hence using air as a gasifying medium reduces the net energy consumption and im proves the overall thermodynamic efficiency. However supplying more air dilutes th e product gas thereby reducing the H2 yield. The optimum ER would supply enough air for the biomass to be partially oxidi zed without significant dilution of the product gas. Optimum Process Parameters One of the objectives of the pr esent analysis is to find th e process conditions that are favorable for hydrogen production (very low or no solid carbon in the product gas, high H2 yield and high efficiency). From Figur e 4.4 it is clear that the maxi mum amount of hydrogen that can be produced at equilibrium in pure air gasifica tion (no steam, T = 1100 K, and ER = 0.1) for the stated conditions is 0.7 moles. This value is sma ller than the hydrogen that can be obtained at the same temperature with steam addition (about 1.3 moles of H2 at T = 1100 K, = 1, referring to Fig 4.2). The excess H2 in the output stream is attributed to the WGS reaction, which cannot take

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76 place in pure air gasification due to the ab sence of steam, implying that for high H2 yields one should go for steam gasification. Steam not only in fluences the water-gas shift but also reforms the hydrocarbons, solid char and tars and there by produces more hydrogen. Steam gasification is an endothermic process; therefore using steam wi ll be energy intensive [97] Also most gasifiers use fluidized beds for better heat transfer. For en ergy efficiency and cost-effectiveness these beds use air (or oxygen) as a co-flu idizing medium with steam [44, 72, 96, 97]. From the above equilibrium analysis we see that as more steam is supplied the hydrogen yield increases. However, this additional hydrogen comes at the cost of extra energy that n eeds to be supplied in order to produce steam. The optimum (steam/biomass ratio) is based on the balance of these two opposing factors. ER (equiva lence ratio) affects both the gas composition and net energy input. From the earlier analysis we saw that the optimum ER, like optimum depends on the balance between partial oxidation of biomass and d ilution of the product gas. In the next section a first law analysis of the gasifi er is carried out with the obj ective of determining the optimum operating conditions for hydrogen production. Energy Analysis A schematic of a biomass gasifier with a steam generator and an air pre-heater is shown in Figure 4.5. Wood designated by CH1.5O0.7 was the model biomass compound (chemical formula based on ultimate analysis [44]). The general reaction for combined steam and air gasification is written as: 22 221.50.7222CH44COCO2H2 HO2N2CHOHO(O3.76N)nCHnCOnCOnH nHOnN (4.15) Here only the main components (H2, CO, CO2, CH4) are considered. Yields of higher hydrocarbons (C2H2, C2H4, C2H6 and so on) were found to be negligible as compared to the main constituents and hence were not considered in the analysis. The gasification temperature was

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77 varied from 900 K to 1400 K (in steps of 100 K), st eam to biomass molar ratio was varied from 0 to 5 (in steps of 1) and ER was varied in the rang e 0 to 0.4 (in steps of 0.1). These are the typical values of these variables encount ered in most commercial and rese arch gasifiers [44, 55, 61, 71, 72, 97, 98]. From equilibrium studies we know th at increasing the pressure reduces the hydrogen yield, hence the pressure was maintained at 1 atm for all further analyses. A first law analysis of the gasifier was carried out across the contro l volume (dotted) as shown in fig 4.5. An energy balance equation can be written as (assuming no heat losses and work = 0): Energy in = Energy out (4.16) 2224422 222222woodHO(v)ONEGCHCHCOCOCOCO HHHOHONNHH(H3.76H)QnHnHnH nHnHnH (4.17) Here H is the enthalpy and QEG is the heat supplied to (or rej ected by) the equilibrium gasifier. Enthalpy of each species is written in terms of enthalpy of formation and enthalpy change: o fHHH (4.18) QEG is positive for an endothermic reaction (s team gasification) where heat is to be supplied from an external source. When QEG is negative heat is liberated and this generally happens during partial oxidation of bioma ss (air gasification). A zero value for QEG is an interesting case which represents adiabatic ga sification. This would mean a self-sustaining process and can be used as a standard to comp are actual gasifiers. We assume that the steam generator provides superheated steam at 700 K a nd the air-preheater heats the air from the ambient to 350 K before entering the gasifier. The efficiency of the process was then determined for a range of temperatures, SBRs and ERs. Th e efficiency was calculated as per the following definition given by USDOE [28]: LHV of hydrogen in product gas LHV of biomass + All other energies (4.19)

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78 H2*H2 b *bEGsteamairnLHV nLHV(QQQ) (4.20) 22steamHO*HOQnH (4.21) 2*22*2airOONNQnHnH (4.22) The moles of each species at equilibrium were calculated using Stanjan. The enthalpy of formation and enthalpy change for each species are taken from standard thermodynamic tables [99]. The values for all the heat duties (QEG, Qsteam, Qair) were determined. The efficiency was then determined using the above equation for a range of temperatures, SBRs and ERs. Figures 4.6 – 4.9 show the efficiencies for the differe nt combinations. For simplicity and clarity of graphs, the efficiency values for all the temperatures (900 to 1400 K) ERs (0.1 to 0.4 in steps of 0.1) and SBR values of 1, 2 and 5 have been shown. Effect of Temperature on Thermodynamic Efficiency As gasification temperature increases, bioma ss thermally disintegrates to produce more gases and volatiles. As temper ature increases, the hydr ocarbons in the presence of steam/air get reformed to produce H2 and CO. Hence as hydrogen yield increases, the efficiency also increases. As gasification temperature further increases, more heat needs to be externally supplied to maintain the gasifi er temperature. Also at high er temperatures (>1200 K) the hydrogen yield drops. Hence, the efficiency firs t increases to a maximu m at around 1000 K (this is especially true for low ER values of 0.l or 0. 2 and SBR of 1 as shown in Figs 4.6 and 4.7) and then decreases as the te mperature is further incr eased upto about 1300-1400 K. Effect of Steam Addition on Thermodynamic Efficiency Figures 4.6 to 4.9 show the combined e ffect of adding steam and increasing the temperature for various equivalence ratios. As we have seen earlier, adding steam increases the hydrogen yield. However additional steam also demands additional energy. Therefore, there

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79 should be an optimum steam to biomass ratio whic h will justify the cost of extra steam. In this analysis, the steam biomass ratio was varied from 1 to 5. The plots for SBR values of 1, 2 and 5 are shown in the figures. At low SBR values (~ 1) the amount of hydrogen produced is relatively small. As SBR increases the efficiency increases due to higher H2 yields. However at very high SBR values (>5) the efficiency drops due to larg e amounts of external heat needed to generate the steam. This trend was observed in all four gr aphs (Figs 4.6 – 4.9). In the analysis it was found that a SBR of 2.0 gives the high est efficiency among all the cases. Effect of ER on Thermodynamic Efficiency Gasification in presence of air or oxygen partially oxidizes the biomass and thereby releases energy. However, this also dilutes the product gas (especi ally if air is used) thereby lowering the heating value of product gas. At low ER values (~0.1) energy is released due to partial oxidation of biomass. Also the hydrogen yi eld is relatively high and so the efficiency is high. Typical efficiencies were of the order 50 to 55 % for SBR in the range 2 to 3 at gasification temperatures of 900 – 1000 K. As ER increases the product gas starts getting diluted due to the presence of N2. It was observed in the previous section that the H2 yield drops beyond ER of 0.2. Hence, although the reaction is exothermic, a whole lot more biomass needs to be gasified in order to produce the same amount of hydrogen. At ER values 0.4 the efficiency starts dropping rapidly (typically ~ 44 % for ~ 1 or 2, T = 1000 K and ER = 0.4, referring to Fig. 4.9). The optimum conditions for hydrogen production occur when we have high thermodynamic efficiency, with high hydrogen yields and little or no carbon formation. From the parametric analysis of the previ ous section and the energy analysis we see that this happens for T

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80 ~ 1000 K, SBR ~ 2, ER ~ 0.1 and atmospheric pr essure. For the given biomass feedstock these conditions give a thermodynamic efficiency of 52%. Comparison of Equilibrium Results with Experimental Data Equilibrium studies are used to predict the maximum possible convers ion in any chemical reacting system. By comparing experimental re sults with equilibrium calculations one can understand the relation between thermodynamics a nd chemical kinetics of the process. In general, it was observed that experimental results deviated considerably from the equilibrium calculations. Figure 4.10 compares equilibrium a nd experimental results where two parameters, the gasification temperature and residence time ( have been varied (experimental data obtained from [96]). Of the four se ts of graphs, the first two are for temperature (700 and 800oC) and the last two are for residence times (0.4 and 1.4 s). In each set, the hatched bars are for theoretical (th) and the solid bar for experimental (ex) co mpositions (total of 8 bars for each T and each ). For both temperatures, the H2 and CO gas volumes are far from equilibrium, although the difference is less for higher temperature. The theoretical CH4 volume at equilibrium at 800oC (~ 0.01 %; not vi sible on graph) is much smaller than the experimental value. From the residence time graph, it is observed that for high residence times, the experimental values are cl oser to the equilibrium values. This is due to more time being available for reactions to take place and reach completion. Figure 4.11 shows how theoretical and experimental results compare for different (1.9 and 6.5) and ER (0.09 and 0.37) (experimental data source [44]). The experimental H2 yield is lower than the equilibrium yield for a value of 1.9. Other gas mole fractions (CO, CO2 and CH4) too differ from the equilibrium values. For very high H2 mole fraction comes close to the equilibrium value. For

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81 both low and high ERs the H2 mole fraction is away from the equilibrium value. Both equilibrium and experimental results are more sensitive to T, and than ER. During biomass gasification many complex aroma tic hydrocarbons called tars are released. These tars typically include benzene or multiple rings of benzene such as naphthalene, xylene or toluene and many complex highe r hydrocarbon chains with se veral carbon atoms [55]. Equilibrium studies were done using benzene as a possible tar compound. The results however showed negligible benzene in the product stream (about ten orders of magnitude lower than other important products such as hydroge n and carbon monoxide). This is possibly due to the infinite time being available for the reactions to occur before equilibrium is reached. This was also verified from the actual experimental data where long residence time and high temperature drastically reduced the tars in product stream [72]. Since high residence times reduce the tar yield and equilibrium studies show pr oduct yield at very long times (t ), higher hydrocarbons and tars were not included in the equilibrium modeling studies. Summary and Conclusion A thermodynamic analysis of hydrogen production from biomass was done using equilibrium modeling. The effects of process pa rameters (temperature, pressure, SBR and ER) on hydrogen yield were studi ed. It was observed that combined steam and air gasification gave much higher H2 yield than air gasificati on alone. Using air as a co-g asifying medium with steam helps reduce external energy i nput as the feedstock gets part ially oxidized. The equilibrium hydrogen yield is found to initially increase with temperature to a maximum and then gradually reduce at higher temperatures. The hydrogen yield in creases continuously with increase in SBR. Air gasification also produces hydrogen but the yield is lesser than steam gasification. The product gas in air gasification gets diluted due to the presence of n itrogen. Increasing the

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82 pressure was found to have a negative influenc e on the hydrogen yield and hence all subsequent simulations were carried out at 1 atm. The gasifier is the most cr itical component of any biomass gasification system. The gasifier was modeled as an equilibrium r eactor and a first law analysis of the gasifier was carried out to determine the maximum thermodynamic efficiency at equilibrium. The optimum operating conditions were found to be T of 1000 K, SBR of 2. ER of 0.1 and P of 1 atm which gave an efficiency of 52%. The actual energy consumption would be higher due to equipment inefficiencies and heat losses from the gasifier catalytic reactor and interconnecting tubing. Also in re al gasifiers we will not reach equilibrium conditions and hence the product gas w ill contain less H2 and CO and more CO2. Nevertheless, the above figures give an idea of the theoretical maximum efficiency for the given conditions. A comparison of the theoretical equilibrium calculations with the expe rimental results shows considerable deviations between the two. Using longer residence times, higher temperatures and higher steam input the experimental results can come clos e to equilibrium predictions. The basic studies gave an understanding of th e thermodynamics of biomass gasification. In the next chapter a novel concept of combini ng different reactions a nd thereby getting an improvement in the hydrogen yield is discussed.

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83 Table 4-1: Equilibrium gas moles at different gasification pressures P (atm) H2 CO CO2 CH4 Remark 0.1 1.303 0.746 0.253 1.61E-5 0.5 1.302 0.745 0.253 4.0E-4 Low Press. System 1 1.301 0.744 0.254 1.59E-3 10 1.09 0.633 0.286 8.13E-2 25 0.897 0.491 0.326 1.82E-1 High Pressure System

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84 Figure 4.1: Variation of Gibbs energy with extent of reaction Extent of reaction ( )Gibbs Energy constantT&P dG)T,P = 0

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85 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 90010001100120013001400 T (K)Moles (CO, CO2, H2) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14Moles (C(s),CH4) CO CO2 H2 C(s) CH4 Figure 4.2: Effect of temperature for P = 1 atm, = 1, ER = 0

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86 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.511.522.53 Moles (CO, CO2, H2) 0.000 0.010 0.020 0.030 0.040 0.050 0.060Moles (C(s), CH4) CO CO 2 H2 C ( s ) CH 4 Figure 4.3: Effect of SBR on equilibrium composition

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87 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.10.20.30.40.50.6 E R Moles (CO,CO2, H2) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16Moles (C(s), CH4) CO CO2 H 2 C ( s ) CH 4 Figure 4.4: Effect of ER on Equilibrium composition

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88 H2O (l) H2O (v) BIOMASS FEEDER CH1.5O0.7 EQUILIBRIUM GASIFIER 1 atm STEAM GEN H2, CO, CO2, H2O, CH4,N2 AIR PREHEATER Air 700 K, 1 atm 298 K, 1 atm 350 K, 1 atm 1 atm 298 K, 1 atm 298 K, 1 atm 298 K, 1 atm QEGC.V. Qsteam Qair Figure 4.5 Schematic of biomass gasifier

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89 30 35 40 45 50 55 90010001100120013001400 T (K) (%) Figure 4.6: Efficiency Vs temperature for various (ER = 0.1)

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90 30 35 40 45 50 55 90010001100120013001400 T (K) (%) Figure 4.7: Efficiency Vs temperature for various (ER = 0.2)

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91 25 30 35 40 45 50 55 90010001100120013001400 T (K) (%) Figure 4.8: Efficiency Vs temperature for various (ER = 0.3)

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92 20 25 30 35 40 45 50 90010001100120013001400 T (K) (%) Figure 4.9: Efficiency Vs temperature for various (ER = 0.4)

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93 Figure 4.10: Comparison of equilibrium data with experimental data of Corella et al [96] for different temperatures and residence times 0 10 20 30 40 50 60 70 700 (oC) 800(oC) 0.4(s) 1.4(s) T Gas (volume %) H2 th CO th CO2 th CH4th H2 ex CO ex CO2 ex CH4ex

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94 Figure 4.11: Comparison of equilibrium data with experimental data of Turn et al [44] for different and ER 0 10 20 30 40 50 60 70 1.9 6.5 0.09 0.37 ER Gas (volume %) H2 th CO th CO2 th CH4th H2 ex CO ex CO2 ex CH4 ex

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95 CHAPTER 5 ABSORPTION ENHANCED BI OMASS GASIFICATION Introduction Steam gasification of biomass produces a ga s mixture rich in hydr ogen and containing other gases such as CO, CO2, CH4 and small amounts of higher hydrocarbons. The maximum hydrogen that can be produced in conventional steam biomass gasification is limited by the thermodynamic equilibrium constraints at the speci fied gasifier temperature and pressure. The temperature option is limited by equilibrium pr oduct composition which does not favor hydrogen formation beyond 1100 K (this was obser ved in the previous chapter). At higher temperatures the biomass gets thermally dissociated, however, th is does not translate into increased hydrogen yields; hence the temperature op tion is limited. The pressure option too is limited as higher gasification pressure (above one atmosphere) reduces the hydrogen yield and lower pressure does not offer any substantial increase in the hyd rogen content. The steam to biomass ratio can be increased to give higher hydrogen yields, but this is at the cost of extra steam that needs to be supplied. As we increase the steam supply, the yield increases rapidly up to certain point but thereafter the increase is rather slow with most of the surplu s steam going unreacted. Hence in order to increase the hydrogen yi eld we need to find new tech niques which are simple, energy efficient and inexpensive. The products coming out of the biomass gasifier consist of other gases like CO, CO2, and CH4 which must be separated from H2. Hence the problem of gas separation also needs to be addressed. In recent years sorbents (such as calcium oxide) have been used for CO2 removal from the exhaust of fossil fuel plants. The sorbent absorbs CO2 and in the process releases heat which can be used for reforming the fuel. More recently, th is technique was applied to the steam reforming of methane and a hydrogen rich, CO2 free gas was obtained [86]. Th e product gas is expected to

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96 have more hydrogen with less contaminants. It ca n be used for any downstream application such as fuel cell or gas turbine with minimal cleaning. Hence there is a potenti al to reduce the number of equipment (and thereby reduce the capital costs) by using sorbents. In principle, the sorbent enhanced gasification process can be applied to any carbonaceous fu el such as coal, heavy oils, biomass, plastic or organic waste. Concept of Absorption Enhanced Gasification The concept of producing hydrogen by reforming hydrocarbons using so rbents dates back to as early as 1868 [100]. In 1967 Curra n and co-workers [101] separated CO2 at high temperature using calcined dolomite in the so-called “CO2 Acceptor Gasification Process”. More recently Harrison et al [86, 102, 103] have e xperimentally shown a novel method of improving hydrogen yield of conventional SM R and effectively separating CO2. Lin et al [87, 88, 104] have used sorbents to develop an innovative HyPr-RING (Hydrogen Production by Reaction Interaction Novel Gasification) technique for producing hydroge n by gasification of coal. The underlying concept of absorbent enhanced gasification is shown in Figure 5.1. There are two main reactors in the process. First is the gasifier/absorber. Here any carbonaceous fuel (in our case bioma ss) is supplied to the reactor to which steam is also being fed. The fuel reacts with steam to produce a gas mixture containing hydrogen, carbon monoxide, carbon dioxide and some hydrocarbons. The carb on monoxide reacts with steam to produce additional hydrogen as per the Water Gas Shift reaction. 222 COHOCOH H = -33.8 kJ/mol (5.1) The calcium oxide sorbent in the gasifier ab sorbs the carbon dioxide produced and gets converted to calcium carbonate 23 CaOCOCaCO H = -168.2 kJ/mol (5.2)

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97 During the absorption process heat is released and this can be used for the endothermic steam gasification of biomass, th ereby reducing the net external he at supply to the gasifier. The calcium carbonate is then regenerated by heating it in another reactor. The thermal energy for regeneration can be supplied either by burning ex ternal fuel or part of the biomass feedstock itself. The hydrogen produced may have small amounts of carbon monoxide, methane and tars. Hence, it is passed through a gas cleaning system so as to obtain a clean gas that is rich in H2. Through simultaneous gasification and CO2 absorption, the equilibrium of the homogenous water gas shift reaction is shifted toward H2. For any general biomass fuel the reactions taking place in sorbent enhanced gasificatio n (SEG) can be written as follows: xy22 RCHO(1y)HOCO(0.5x1y)HH0 (5.3) 222 RCOHOCOHH0 (5.4) 23 RCaOCOCaCOH0 (5.5) The overall reaction can be written as: xy232CHO(2y)HOCaOCaCO(0.5x2y)H (5.6) Equation (5.6) represents the idealized sum reacti on for sorbent enhanced gasification. Here the formation of secondary products (methane, coke & tar) is neglected. Table 5-1 gives the values of the heats of reaction for different fu els with typical reaction temperatures. Figure 5.2 shows a schematic of the Sorpti on Enhanced Gasification concept. The CO2 absorption is an exothermic reaction and the bi omass steam reforming reaction is endothermic and hence the overall reaction would consume less en ergy. The spent sorbent is regenerated in a subsequent process by supplying he at. For continuous gas production, solid fuel is gasified in presence of fresh sorbent at temperatures less than 700 C. The carbonated bed material together with the biomass coke is removed and regenerated at 800-900 C under air supply. Thus a

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98 hydrogen rich gas stream with small amounts of CO and CH4 and a CO2 rich exhaust gas stream are generated in two parallel process steps. In actual system two fluidi zed bed reactors with circulating absorbent bed material can be c oupled as shown in the set-up of figure 5.2. Application of SEG to Biomass Gasification Sensitivity studies have been carried out in order to determine the effect of process variables on the equilibri um hydrogen yield. Ethanol was used as the model biomass compound. ASPEN PLUS (version 12.1) soft ware was used to model the process flow. The choice of ethanol as a model compound was primarily due to convenience. The physical, thermodynamic and transport properties of ethanol are welldocumented and are already built into ASPEN database, hence making it convenient to carry ou t simulations (the choice of this model compound does not endorse or imply producing hydr ogen from ethanol; this is the subject of a separate study). The process vari ables studied were temperature, pressure, steam to biomass ratio and sorbent to biomass ratio. The general reaction for steam gasifi cation of ethanol is given by: 2522242CHOHHO H+COCOCHHOhigher Hydrocarbons (5.7) Experimentally it has been found that hi gher hydrocarbons (two carbon atoms containing compounds such as ethylene or acetaldehyde) and so lid carbon in the steam reforming of ethanol are negligible. Hence these were not considered in the simulations [105]. There are two cases considered here – base case (i.e. no sorbents) an d sorbent enhanced gasification each of which is explained below. Case I: Base case (no sorbent) Ethanol and water are mixed in a mixer and sent to a heater where they are heated to the desired temperature. The product is then sent to the reformer which is modeled as a Gibbs reactor which is at the desired ga sifier temperature and pressure. The products of reformer which

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99 are at thermodynamic equilibrium are then cooled be fore being sent to the water gas shift reactor where carbon monoxide reacts with steam to produce additional H2 as per the following reaction. 222 COHOCOHH33.8kJ/mol (5.8) The flow sheet for base case is shown in Figur e 5.3. The steam to biomass ratio (water to ethanol feed ratio) was varied between 3 a nd 8, the reformer temperature from 500 to 900oC (this is the temperature range for actua l gasifiers) and the ga sifier pressure from 100 kPa to 2500 kPa. The results of the sensitivity analysis are shown in figures 5.4 to 5.6. Figure 5.4 shows that the temperature has a significant eff ect on the equilibrium product yiel d. The ethanol and steam flow rates were fixed at 1 kmol/hr and 4 kmol/hr and th e reformer was at atmospheric pressure. As the reaction temperature increases the hydrogen yields also increases until it reaches a maximum at 725oC and then decreases. The increase of hydrogen yi eld is due to the react ion of ethanol with steam. As the temperature increases, the hydrocar bons (methane) are reformed and converted to hydrogen. At high temperatures the Water-Gas Shif t reaction occurs in th e reverse direction and this reduces the hydrogen yield. Figure 5.5 shows the effect of reformer pressu re on product yield. It is observed that the pressure significantly impacts the equilibrium product yield. One can conclude from the figure that the highest hydrogen yield is ob tained at atmospheric pressure a nd hence it is best to operate the reformer at one atmosphere. The effect of steam to ethanol ratio on product yield for a reformer temperature of 700oC is shown in Figure 5.6. The addition of steam increases the hydrogen yield while reducing the CH4 and CO concentrations. Although high steam to ethanol ratio gives high hydrogen yields, it will be limited by the cost of the system.

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100 Case II: Ethanol gasification in the presen ce of CaO sorbent (sorbent placed in the reformer) Figure 5.7 shows the flowsheet of sorbent en hanced biomass gasification. Adding CaO to the steam reforming of ethanol can be considered with the following reaction 23 CaO(s)CO(g)CaCO(s)H = -168 kJ/mol (5.9) The removal of CO2 from the gaseous phase will di splace the equilibrium of the gas mixture to a higher hydrogen yield and a lower CO concentration. The products are the same as in the base case plus CaO and CaCO3. The flow sheet of the simulation is shown in Fig. 5.7. Again ethanol and water are mixed together a nd are sent to the heater (HEATER1) at 700oC. The mixture enters the Gibbs reacto r (REFORMER) which in this case includes solid CaO. The reformer output is sent to the separator (SEP) for separation of gases from solids. The gases including H2, CO, CO2, CH4 and steam are cooled to 300oC in the heat recovery heat exchanger (HT-RECOV) and enter the Water-Gas Shift (W GS) reactor. The solids are sent to the regeneration reactor (REGENERA) in which CaCO3 decomposes to CO2 and CaO at 850oC. The effect of temperature on the product mola r flow rates is shown in figure 5.8. At temperatures lower than 750oC the hydrogen production is greatly enhanced by the separation of CO2. Above this temperature the molar flows are similar to the previous case. The maximum hydrogen is produced at 650oC (which is almost 100oC lesser than the base case). It is also observed that the maximum hydrogen produced in th e sorbent enhanced gasification case (5.24 mol/mol of ethanol) is almost 12% more than the corresponding figure for the base case (4.68 mol/mol of ethanol). The am ount of carbon oxides (CO and CO2) produced is less than the base case due to absorption by CaO. It is observed that the sorbent absorption is effective up to 800oC, thereafter, the hydrogen yield drops and is similar to the base case. This is probably due to the

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101 reverse reaction (calcination) of CaCO3 which is favored at those temperatures. When CO2 is no longer absorbed, the hydrogen yield starts dropping. The effect of pressure on the product molar flow rates is shown in figure 5.9. Similar to the base case, operating the reactor at high pressures is not desirable due to the decrease in hydrogen yield. The steam to ethanol ratio was changed from 3 to 8. This ratio greatly enhances the steam reforming of ethanol in the pres ence of CaO. Figure 5.10 shows th e results of varying this ratio at a reformer temperature of 700oC and atmospheric pressure. The results are similar to the previous case with hydrogen yield being consiste ntly higher than the ba se case (by about 10%). Finally, the effect of addi ng sorbent on the product yiel d is shown in figure 5.11. CaO/ethanol ratio of zero corresponds to the ba se case (no sorbent). As the amount of CaO is increased, it absorbs the carbon di oxide and gets converted to CaCO3. The carbon dioxide goes on getting absorbed as it is produced until only a small amount (corresponding to equilibrium) remains. Hence after a certai n CaO/ethanol ratio the CaCO3 reaches saturation. The amount of hydrogen too goes on increasing as the CO2 is getting absorbed. Be yond a certain point the increase in hydrogen is not significant. Also the surplus unused CaO shows up in the product stream. Adding excess CaO is energy inefficient as it simply gets heated and cooled and does not help in the CO2 removal. Hence, it is essential to add only the necessary amount of CaO as needed for removing the CO2 from the product gas. For the given conditions adding sorbent more than a CaO/ethanol ratio of 4 does not gi ve any substantial improvement in the hydrogen yield.

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102 Energy Analysis A simple energy analysis has been carried out to study the effect of sorbent addition on hydrogen yield of ethanol steam reforming. In or der to have a common basis for comparison, the process conditions for both cases were kept the same and are given below: Ethanol flow rate : 1 kmol/hr Steam flow rate : 4 kmol/hr Reformer pressure : 1 atm Reformer temperature : 700oC WGS reactor temperature : 300oC Regenerator Temperature : 850oC CaO flow rate : 3 kmol/hr The enthalpy of various streams was calcula ted. The first law of thermodynamics was applied to each of the reactors and heat exchan gers and the heat-duty was then calculated. The heat duty of the different reactors and the product gas distribution for the tw o cases considered is given in Table 5-2. The thermodynamic efficiency is then calculated as per the following definition: Energy Output Energy Supplied (5.10) Lower Heating Value of Product gas LHV of biomass + Total heat supplied (5.11) Here the total heat supplied includes 1) heat input to the heater (o r vaporizer) that is used to produce steam and ethanol, 2) heat input to the reformer 3) heat input to the regenerator (applicable to the sorbent case only)

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103 The Lower heating value of product gas is calculated as follows: 2244HHCOCOCHCHLHV of gas = n*LHVn*LHVn*LHV (5.12) Table 5-3 gives the values of the output and inpu t energy and the thermodynamics efficiency for the two cases. A simple thermodynamic analysis shows that the efficiency of the sorbent enhanced gasification is higher th an conventional gasification. In actual biomass gasification systems ther e will be many reactors and heat transfer equipment. The heater (Figure 5.3) may be fired by any fuel (bioma ss or natural gas) and the flue gases coming out can be used for making steam or hot water. The heat rej ected by the cooler and the WGS reactor (Figures 5.3 and 5.7) could ac t as a low temperature heat source and may be used to heat the incoming biomass feed. The regene rator may be fired by any fuel (natural gas or biomass). In this case the flue gases could ac t as a high temperature heat source (since the regenerator operates at high temper ature) and could be used to produce superheated steam for the gasifier or reformer. The calcium oxide cooling reactor (CaO-cool, Figure 5.7) can also be used to heat some other fluid stream within the system. An actual biomass gasification system therefore will have many sources of waste heat that can be used internally. In light of the heat integra tion possibilities identified above the overall system efficiency will be different from the efficiency values obtained above. The objective of the present study was to introduce the concept of sorbent enha nced biomass gasification and hence a simple energy analysis was conducted. In order to ge t more realistic data an optimization of the complete system taking into account all the avai lable waste heat sources and thermal integration needs to be conducted.

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104 Conclusion Sorbent Enhanced Gasification (SEG) is a novel technology for producing a hydrogen rich gas from carbonaceous fuels like biomass. The concept has been used for CO2 removal from fossil fuel exhaust. We have applied this con cept to enhance the hydrogen yield of conventional biomass gasification. The potenti al advantages of SEG are: higher hydrogen yields lower operating temperature as compared to conventional gasification lower heat requirements of the reformer/gasifie r due to in-situ heat supply (hence a smaller gasifier will be required; this also means a reduction in capital cost) it is a novel technique for pr oducing a hydrogen rich and CO2 free gas; the product gas will need minimal cleaning and hence many downstr eam equipments such as Water Gas Shift reactor and PSA (Pressure Swing Adsorptio n) unit used conventionally may not be required in the sorbent case. Hence there is a possibility of si gnificantly reducing the capital cost of hydrogen production from biomass a pure CO2 stream is produced and CO2 can be either used for suitable applications or sequestered for appropriate disposal In the next chapter experimental studies conducte d to validate the concep t of sorbent enhanced biomass gasification are discussed.

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105 Table 5-1 Reactions in SE G for some typical fuels Fuel Type of Reaction Reactions T(oC) H (kJ/mol) Reforming 422CHHOCO3H 650 +224.6 Shift 222COHOCOH 650 -36.7 Absorption 23CaOCOCaCO 650 -171 Overall 4232CH2HOCaOCaCO4H 650 +16.9 Methane CH4 (x = 4, y = 0) Regeneration 23CaOCOCaCO 850 +168 Reforming 1.50.722CHO0.3HOCO1.05H 700 +98.1 Shift 222COHOCOH 700 -35.5 Absorption 23CaOCOCaCO 700 -169.5 Overall 1.50.7232CHO1.3HOCaOCaCO2.05H 700 -106.9 Wood CH1.5O0.7 (x = 1.5, y = 0.7) Regeneration 23CaOCOCaCO 850 +168

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106 Table 5-2: Comparison of energy consumption in biomass gasification with and without sorbent Case I (Base case) Case II (SEG) 4.79 5.69 0.097 0.089 1.53 0.762 0.001 0.053 Output (kmol/hr) H2 CO CO2 CH4 H2O 1.84 1.195 +108.6 +108.6 +65.9 +38 -33.9 -27.9 -15.8 -6.7 NA +61.2 NA -12.4 Heat Duty (kW) HEATER1 REFORMER COOLER 1/ HT-RECOV WGS REGENERATOR CO2-COOL CAO-COOL NA -34.9

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107 Table 5-3: Thermodynamic efficiency and energies Case I (Base case) Case II (SEG) LHV of product gas (kJ/hr) 1176.3 x 103 1435.9 x 103 LHV of biomass (kJ/hr) 1242 x 103 1242 x 103 Total heat supplied (kJ/hr) 628.1 x 103 748.2 x 103 Efficiency as given by 5.11 62.9 % 72.1 %

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108 H2 rich, CO2free gas Q1Carbonaceous fuel (biomass) Q2CO2for sequestration Q3H2to storage CaO GAS CLEANING steam CO2 CaCO3(REGENERATOR) Regenerated GASIFIER/ ABSORBER Saturated Figure 5.1: Concept of abso rption enhanced gasification

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109 Steam High H2, low CO, prod gas GasifierRegenerator CO2 Biomass Spent Sorbent Supplemental fuel Air Regenerated Sorbent Figure 5.2: Schematic of SEG (concept)

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110 MIXER1 HEATER1 REFORMER 1-WATER 2-ETOH-I 3-MIX REFORM-I REFORM-O COOLER1 WGS-IN WGS-OUT WGSEthanol Gasification without CO2 Absorption4 kmole/hr 1 kmole/hr 300 C, 1 bar 700 C, 1 bar Figure 5.3: Flow sheet for c onventional biomass gasification

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111 0 1 2 3 4 5 6 500600700800900 T(oC)Moles/mole of EtOH 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Moles CH4/mole of EtOH CO2 CO H2 CH4 Figure 5.4: Effect of reformer temperature on product yield

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112 0 1 2 3 4 5 6 0510152025P (atm)Moles/mole of EtOH 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Moles CH4/mole of EtOH . CO2 CO H2 CH4 Figure 5.5: Effect of reformer pressure on product yield

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113 0 1 2 3 4 5 6 345678 Steam/EtOH ratioMoles/mole of EtOH .0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035Moles CH4/mole of EtOH CO2 CO H2 CH4 Figure 5.6: Effect of steam etha nol ratio on product yield at 700oC

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114 WATER ETHANOL MIXTURE REFORM-I GASES WGS-IN WGS-OUT CAO-RECY REFORM-O CACO3 REG-OUT CO2 CAO CAO-OUT CO2-OUT MIXER1 HEATER1 HT-RECOV WGS REFORMER SEP REGENERA SOL-SEP CAO-COOL CO2-COOLEthanol Gasification with CO2 Absorption Figure 5.7: Flow sheet for ethano l gasification with CaO sorbent

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115 0 1 2 3 4 5 6 500600700800900 T(oC)Moles/mole of EtOH .0.00 0.05 0.10 0.15 0.20 0.25Moles CH4/mole of EtOH CO2 CO H2 CaO CaCO3 CH4 Figure 5.8: Effect of temperature on pr oduct yield for sorbent enhanced reforming

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116 0 1 2 3 4 5 6 05101520P (atm)Moles/mole of EtOH .0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Moles of CO,CH4 /mole of EtOH CO2 H2 CH4 CO Figure 5.9: Effect of pre ssure on the product yield for sorbent enhanced reforming

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117 0 1 2 3 4 5 6 345678 Steam/EtOH ratioMoles/mole of EtOH .0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10Moles CH4/mole of EtOH CO2 CO H2 CH4 Figure 5.10: Effect of steam/ethanol ratio on product yield for sorbent enhanced reforming

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118 0 1 2 3 4 5 6 0123456 CaO/EtOH ratioMoles/mole of EtOH 0.0 0.2 0.4 0.6 0.8 1.0 1.2Moles/mole of EtOH H2 CaO CaCO3 CO2 CO Figure 5.11: Effect of CaO/et hanol ratio on the product yield

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119 CHAPTER 6 EXPERIMENTAL STUDIES ON BIOMASS GASIFICATION Objective The overall objective of experimental study was to understand the effect of adding calcium oxide sorbent on the hydrogen yield during the steam gasification of biomass. Specific objectives were to: 1) experimentally determine the total gas yield, hydrogen yield and yields of other important constituents (carbon monoxide, carbon dioxide and methane) by gasifying plain biomass (Southern pine bark) in the presence of steam 2) to determine possible increase in hydrogen yield and overall gas yi eld by adding calcium oxide sorbent during biomass gasification. 3) determine the carbon conversion efficiency (fract ion of carbon in biomass that is converted into carbon containing gases) of biomass gasification with and without sorbents The effects of the following variables were studied: a) Temperature b) Presence of sorbent Output variables studied were: a) Gas composition b) Total gas yield (ml/g of biomass gasified) c) Yields of hydrogen, carbon monoxide, carbon dioxide and methane (in ml/g biomass) d) Carbon conversion efficiency

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120 Experimental Facility Test Set-up Figure 6.1 shows a sketch of the test set-up fo r biomass gasification. It consists of the following components: a) gasifier (primary reactor) b) secondary reactor c) steam generator d) gas cooling system (heat exchanger) e) heaters, insulati on and tubing/fittings f) instrumentation g) gas analysis facility (GC) Gasifier (Primary reactor) This is a fixed-bed batch type reactor made of ” SS316 tubing. The gasifier can hold about 6 gm of biomass feedstock. So uthern pine bark in the form of pellets (2 to 5 mm in size) is introduced from the top and is supported approxima tely at the center of the gasifier by quartz wool packing. A radiant ceramic heater placed con centrically around the gasifier heats the biomass bed. A K-type thermocouple is used to measure the temperature of the bed. Secondary reactor This reactor is identical in construction to th e primary reactor except that it is used to hold the calcium oxide sorbent. Quartz wool packing is used as a support a nd a K-type thermocouple is used to measure the temperature of the so rbent bed. A radiant ceramic heater placed concentrically around the reacto r tube heats this reactor.

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121 Steam generator This consists of a ” SS 316 tubi ng and is divided into two parts. The first part is a straight portion of the tube around which a rope heater is wound helically which acts as a boiler. The boiler generates superheated steam at about 150oC and 1 atm. Thereafter the steam enters another ” tube which is bent in the form of a he lical coil. The helical coiled tube is placed concentrically inside a radiant ceramic heater The helical coil acts as a superheater and generates superheated steam at a temperature of 500oC and 1 atm. To enhance the heat transfer coefficient on the steam side, a stee l wire (0.02” thick) in the form of a spring was inserted inside the ” helical coil. Gas cooling system (heat exchanger) The steam and gas mixture coming out of the ca talytic reactor is cooled by a concentric tube heat exchanger. Ethylene glycol solution flows through the outer tu be while steam and gas mixture flows through the inner tube. The ethylene gl ycol solution is later cooled by a refrigerant circulating in a liquid bath (Polyscience make). The cooled gas is then passed through a vacuum trap, kept in an ice bath, to collect any moisture left uncondensed. The gas is then sent to a gas chromatograph (GC) for online analysis before collecting it in a sample bag. Heaters, insulation and tubing/fittings The ratings of the various heaters used in the set-up and their maximum achievable temperatures are as given in Table 6-1. Swagel ok fittings (SS316) were used to connect the reactors, steam generator and the intermediate tu bing. All the tubing material used was of SS310 or SS316. Silica alumina insulation (Thermal Ceramics Inc make) was wrapped around the intermediate tubing and fittings to minimize the heat losses.

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122 Instrumentation a) Temperature measurement: K-type thermocoupl es were used to measure the temperatures of 1) steam at boiler exit 2) steam at superheater exit 3) gasifier bed 4) secondary reactor bed 5) boiler tube surface 6) superheater tube surface The four heaters were controlled by four TICs (Temperature Indicators and Controllers, Omega make, model CN9000A). The contro llers were set within 1% range. b) Pressure measurement: The pressure of th e gas was measured by a Bourdon gauge after it came out of the gasifier but before it entered the secondary reactor. The measurement was done to ensure that there is no pressure bu ild up while gas is flowing over the packed sorbent bed. c) Flow measurement: The gas flow rate was m easured using a Soap Bubble Film Flowmeter. Gas Analysis Facility The biomass-generated gas coming out of the te st set-up was analyzed on-line by passing it through the sampling loop of a Gas Chromatogr aph The GC (SRI make) has two columns (Molecular Sieve and Hayesep-D) and is equipp ed with a Thermal Conductivity Detector. The Molecular Sieve column separates hydrogen, Ar, O2, N2, CH4 and CO whereas the Hayesep D column separates CO2 and higher hydrocarbons (C1-C6). Helium was used as the carrier gas for all the components. A desktop comp uter installed with PeakSimple software (version 3.29) was used as a chromatography analysis station and storage device.

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123 GC Calibration The GC was calibrated for four important gase s, which are found in the highest proportion (H2, CO, CO2 and CH4). Argon gas was used as the internal standard. Gas sta ndards of each of the four calibration gases were purchased in the fo rm of lecture bottles. Different concentrations of the calibration gas and the internal standard were accurately injected into the GC using mass flow controllers. Following was the calibration procedure: 1) The calibration standard (eg H2) and Argon were passed throu gh two different mass flow controllers kept in parallel su ch that a total of 100 ml/min was supplied to the GC (Figure 6.4) 2) Different concentrations (from 0% to 100 % H2 in steps of 20% with the balance being Ar) of gases were passed through the sampling l oop and chromatograms were obtained for each case. The concentrations covered the expected range of the gas. 3) The area counts of the calibration standard and Argon were noted and graphs of area count versus concentration were plotted. The graphs were then curve fitted; figures 6.5 to 6.8 show the calibration curves for the four gases. Test Methodology In all the experiments ‘Southern pine bark’ was used as the model biomass compound. The biomass is popularly used as mulch and is spread around trees to prevent erosion and to enrich the soil. It was bought from a local store. Typically five grams of pine bark in the form of pellets (2-5 mm in size) were used for each test r un. The pellets were made by breaking the “as received” pine bark and grindi ng it. Figure 6.9 shows the bioma ss “as received” and figure 6.10 shows the pelletized form. The feedstock compos ition was determined by proximate and ultimate analyses and is give n in Table 6-2.

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124 The biomass and sorbent were accurately we ighed in an electronic balance (Denver instruments make, least count: 0.0001 g) and then fed into the gasifier and secondary reactor respectively with quartz wool p acking that acted as a support. Silver goop (a high temperature heat resistant paste) was used on all fittings a nd tubes in order to prev ent the seizure at high temperature. Water was supplied to the gasifier by means of a peristaltic pump (Cole Parmer). A fixed water flow rate was maintained for each experiment. This was based on practical considerations and these included: maintaining a uniform flow of steam around the biomass bed and maintaining reasonable steam temperature. High flow rate would blow the biomass bed and low flow rate could not generate sufficiently high temperature as the flow was laminar. Based on these considerations a water flow rate of 5 g/min was maintained for each experiment. The operation sequence while starti ng the test was as follows: 1) the GC carrier gas (Helium) cylinde r was opened and the GC was switched ON 2) the condenser was turned ON and th e coolant temperature was set at 15oC 3) the rope heater, superheater and heater for catalytic reactor were then turned ON 4) after the three heaters had reached the set-point, the pump was turned ON 5) finally the gasifier heater was turned ON The stopping sequence was as follows: 1) all heaters were first turned OFF 2) pump was turned OFF 3) condenser was turned OFF after all the steam had condensed 4) the GC was turned OFF and the Helium gas cylinder was closed

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125 Test Results and Analysis Effect of Temperature The pine bark was steam gasified at different temperatures from 500 to 700oC. No sorbent was used in this case. The baseline data of total gas yield and yiel ds of hydrogen, CO, CO2 and methane were obtained. Table 6-3 and Figure 6. 11 show the effect of temperature on the products of biomass gasification. It was found that the total ga s yield increased monotonically with the temperature (Table 6-3). This was exp ected, as higher temperatures favor conversion of higher hydrocarbons and tars into gas. The hydr ogen yield was also found to increase steadily with the increase in temperature. The total gas yield of 11 11 ml/g of biomass at 700oC compared well with the results of other groups. Turn et al obtaine d a total gas yield of about 1000 m l/g while gasifying sawdust in presence of steam and air at 750oC [44]. Herguido et al obtained a gas yield of approximately 1050 ml/g while steam gasifying straw at 700oC [72]. Walawender et al steam gasified cellulose at various temperatures in the range 600 to 800oC and they found a total gas yield of about 1250 ml/g at 680oC [106]. The carbon conversion efficiency is defined as the ratio of moles of carbon in the product gas per mole of carbon in the biomass. pro woodM X = x 100 M Where, Mpro: total moles of carbon in the product gas (CO, CO2, CH4) Mwood: moles of carbon in the biomass Carbon conversion efficiency, in a way repres ents the effectivene ss of the gasification process in converting the solid biomass into gas. It was found that the carbon conversion

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126 efficiency increased from a mere 23% at 500oC to approximately 40% at 700oC. Walawender et al had obtained a carbon convers ion efficiency of 32% at 600oC [106]. The carbon conversion efficiency increases with temper ature as the tars and higher hydr ocarbons get converted into gas. The hydrogen in the product gas also increases with temperature. This trend was also observed by other research groups [97, 107, 108]. Hovela nd et al conducted e xperiments by gasifying cellulose in the presence of st eam and found that the hydrogen yield continuously increased from 250 ml/g at 600oC to 600 ml/g at 750oC [108]. The CO2 yield was also found to be high and this was attributed to the presence of steam, which oxidizes all the hydrocarbons and tars. For hydrocarbons such as methane: 4222CH2HOCO4H (6.1) Higher hydrocarbons also get oxidized in presence of steam: nm222CH2nHOnCO(2nm/2)H (6.2) In addition to the above reactions, steam also converts char into additional hydrogen and carbon dioxide as per the water gas reaction: 222C2HOCO2H (6.3) Also there is the water-g as shift reaction, which converts the CO into CO2 and produces H2 in the presence of steam 222COHOCOH (6.4) Equilibrium calculations were done to understand how closely actual results meet thermodynamic results. The equilibrium yields of important gases were determined. The actual yields were different from th e equilibrium yields; however the difference between the two reduced with increasing temperature (as we ha d observed in Chapter 4). The theoretical equilibrium hydrogen (and CO2) yield is high because all the hydrocarbons and tars are broken

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127 down and converted into these gases (as per reacti ons 6.1 to 6.4) as th ere is no limitation on time for reactions to occur. In act ual gasification however, the reac tions do not reach completion and hence the actual H2 and CO2 yields are lower than pr edicted by equilibrium. The methane and carbon monoxide mo lecules get converted into H2 and CO2 in the presence of steam and hence their equilibrium yi elds are lower than actual. Higher temperatures favor hydrocarbon (methane) conversion into additional H2 and CO2 and hence at 700oC the equilibrium methane is reduced to zero. Effect of Sorbent Experiments were carried out by gasifying the biomass in the presence of calcium oxide sorbent. Table 6-6 summarizes the effect of sorbent addition on gas yields at different temperatures. The biomass was placed in the gasi fier reactor and calcium oxide was placed in the secondary reactor. Calcium oxide sorbent was in the form of powder (particle size 0.045 mm). The sorbent in the powder form offers substantial surface area for the reactions to occur and it is hypothesized that the ga s yield will be high. Figures 6.12 to 6.14 show the individual gas yi elds at different gasification temperatures. In all the experiments the CaO sorbent to biomass molar ratio was maintained at 1. The temperature was varied in the same range as the base case (500-700oC). In general, it was observed that the total gas yield was higher than the base case. The carbon conversion efficiency (Table 6-7) was also found to be higher than the base case. The hi gh gas yield is attributed to the cracking of tars and complex hydrocarbons in the presence of calcium oxide sorbent which provided active surface area for steam reform ing reactions to occur. The breakdown of hydrocarbon molecules releases th e hydrogen atoms, which would ot herwise be locked with the carbon atoms. This was verified by the quality of tar-laden water that was obtained from two

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128 tests conducted at 600oC. From the figure 6.15 it was observed that the tar-laden sample of plain biomass gasification (dark colored) had far more particulates and suspended matter as compared to the sorbent enhanced gasification. The tar laden water for the sorbent enhanced case was found to be relatively clear and free of particulate matter. The sorbent was found to be very effectiv e at temperatures in the range 500-600oC. At 500oC the hydrogen yield was found to increase subs tantially from 320 ml/g for the base case to 719 ml/g for the sorbent enhanced case. At higher temperature, (700oC) the total gas and hydrogen yields were found to appro ach the base case. This might be due to the fact that the carbonation reaction (CO2 absorption) reaction becomes less dominant at temperatures above 700oC. The CO2 in the product gas is absorbed by the ca lcium oxide sorbent. The calcium oxide sorbent gets converted into calcium carbonate. The degree of conversion depends on the thermodynamic conditions such as pressure, temperature and partial pressure of CO2 in the gas mixture and on chemical kinetic parameters su ch as surface area and pore volume of sorbent. Conclusion 1) Biomass steam gasification in the presence of calcium oxide gives s ubstantially higher gas yield as compared to plain biomass gasifi cation. In the experiments conducted it was observed that the tota l gas yield at 500oC increased from 550 ml/g to 1360 ml/g. 2) The hydrogen yield at 500oC also increased substantially while using sorbent from 320 ml/g (base case) to 719 ml/g (sorbent case). Th is is attributed to the catalytic action of calcium oxide sorbent in whose presence the tars and complex hydrocarbons, which normally remain uncracked got reformed by r eacting with steam to yield more hydrogen.

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129 3) The carbon conversion efficien cy (fraction of carbon in bi omass converted into carbon containing gases – a measure of effectiveness of gasification) also improved considerably while using sorbent (from a mere 23% to more than 63% at 500oC). 4) The high gas and hydrogen yield trend is obs erved until the gasifi cation temperature of 700oC after which the sorbent case gave almost similar yield as the base case. This is attributed to the ca rbonation reaction (CO2 absorption) which becomes less dominant at temperatures above 700oC. 5) The hydrogen yield at 500oC using sorbent (719 ml/g) is co mparable to the hydrogen yield at 700oC for plain biomass gasification (712 ml/g). Hence there is a pot ential to reduce the gasifier operating temperature by 150-200oC. This was proved on a lab scale gasifier. In practice even if it is possible to ope rate the commercial gasifier at 100oC lower than conventional and get the same amount of hydr ogen it will result in substantial savings in the heat input to the gasifier. 6) The product gas, while using sorben t, is rich in hydrogen, free of CO2 and is relatively clean of any particulates and ta rs. If we can set the operating c onditions such that we get an almost pure hydrogen gas stream with very l ittle contaminants, it may be possible to eliminate all or most of the gas cleaning e quipment used downstream of the gasifier in conventional biomass gasification systems such as High Temperature Water Gas Shift (HTWGS), Low Temperature Water Gas Shift ( LT-WGS) and Pressure Swing Adsorption (PSA) unit. With no WGS reactor s there is no need of any catalyst and hence there will be savings in running cost. In place of these three reactors there will be a single reactor used for regenerating the used sorbent. Hence, there is a potential to reduce the capital cost of biomass gasification plants for hydrogen production.

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130 Table 6-1 Heater ratings Heater type Rating Rated temperature Gasifier heater 1400 W, 220V 982oC Catalytic reactor heater 870 W, 110V 982oC Heater for Superheater 1700 W, 110V 982oC Boiler 500 W, 110V 482oC

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131 Table 6-2: Ultimate and proximate analyses Ultimate analysis Proximate analysis C 51.13 % Fixed Carbon 26.94 % H 6.10 % Volatiles 63.21 % N 0.14 % Moisture 9.22 % S 0.04 % Ash 0.63 % O 41.96 %

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132 Table 6-3: Effect of temperature on the products of biomass gasification T 500oC 600oC 700oC Biomass (g) 5.0010 5.0300 5.0400 Steam flow (g/min) 5.0000 5.0000 5.0000 Volume of gas (ml) 2750 4400 5600 Total gas Yield (ml/g biomass) 549.9 874.8 1111.1 H2 (ml/g) 320.3 573.0 712.2 CH4 (ml/g) 34.1 28.0 15.6 CO (ml/g) 66.4 79.1 90 CO2 (ml/g) 108.8 181.5 288.9

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133 Table 6-4: Carbon conversion efficiency (no sorbent) T 500oC 600oC 700oC C conversion 22.9 % 30.3 % 40.3 %

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134 Table 6-5: Equilibrium yields of biomass gasification products T 500oC 600oC 700oC H2 (ml/g) 1250 1325 1400 CH4 (ml/g) 0.01 0.003 0 CO (ml/g) 11.9 17.4 27.9 CO2 (ml/g) 910 1025 1200

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135 Table 6-6: Effect of temperature on gas composition in the presence of sorbent T 500oC 600oC 700oC Biomass(g) 5.000 5.0067 5.0176 Steam flow (g/min) 5.0000 5.0000 5.0000 Sorbent (g) 11.200 11.210 11.207 Volume of gas (ml) 6800 7100 6200 Total gas Yield (ml/g biomass) 1360.0 1418.1 1235.7 H2 (ml/g) 719.4 852.3 773.5 CH4 (ml/g) 66.6 38.3 43.2 CO (ml/g) 106.1 78 53.1 CO2 (ml/g) 432.5 384.3 316.3

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136 Table 6-7: Carbon conversion efficiency (sorbent enhanced gasification) T 500oC 600oC 700oC C conversion 63.5% 56% 49%

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137 WaterSteam Primary reactor Steam Generator Secondary reactor Heat Exchanger Eth glycol To C.T. bath Hot Gases Eth glycol From C. T. bath Dry clean gas to GC Peristaltic pump Heaters Gas Analysis Facility T1: Thermocouple (biomass bed) T2: Thermocouple (sorbent bed) P: Pressure T1 T2 P Figure 6.1: Biomass ga sification test set-up

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138 Figure 6.2: Photograph of the test set-up Steam generator Primary Reactor Secondary Reactor GC

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139 Figure 6.3: Gas chromatograph (SRI 8610C)

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140 Gas Chromatograph Mass Flow Controller Tee Valve ” SS tubing Cylinder Figure 6.4: Set-up for GC calibration

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141 H2 Calibration Curvey = 0.3968x 11.006 R2 = 0.9786 0 5 10 15 20 25 30 35 020406080100% H2 in mixtureH2 Area count H2 Calibration Curve Linear (H2 Calibration Curve) Figure 6.5: Hydrogen calibration curve

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142 CO Calibration curvey = 8.5995x R2 = 0.9985 0 100 200 300 400 500 600 700 800 900 020406080100% CO in mixtureCO Area cou n CO Absolute Linear (CO Absolute) Figure 6.6: CO calibration curve

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143 CO2 Calibration curvey = 12.547x R2 = 0.9995 0 200 400 600 800 1000 1200 1400 020406080100 % CO2 in mixtureCO2 Area count CO2 Absolute Qty Linear (CO2 Absolute Qty) Figure 6.7: CO2 calibration curve

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144 CH4 Calibration curvey = 7.4534x R2 = 0.999 0 100 200 300 400 500 600 700 800 020406080100% CH4 in mixtureCH4 Area Count CH4 Absolute qty Linear (CH4 Absolute qty) Figure 6.8: CH4 calibration curve

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145 Figure 6.9: Southern pine bark “as received”

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146 Figure 6.10: Pelletized pine bark

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147 Gas yield plain biomass no sorbent0 100 200 300 400 500 600 700 800 900 H2CH4COCO2CnHmother GasYield (ml/g) 500 C 600 C 700 C Figure 6.11: Effect of temperatur e on gas yields (no sorbent)

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148 Gas yield without & with sorbent at 500 C0 100 200 300 400 500 600 700 800 H2CH4COCO2CnHmotherGasYield (ml/g) Without CaO With CaO Figure 6.12: Effect of sorbent addition at 500oC

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149 Gas yield without & with sorbent at 600 C0.0 100.0 200.0 300.0 400.0 500.0 600.0 700.0 800.0 900.0 H2CH4COCO2CnHmotherGasYield (ml/g) Without CaO With CaO Figure 6.13: Effect of sorbent addition at 600oC

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150 Gas yield without & with sorbent at 700 C0 100 200 300 400 500 600 700 800 900 H2CH4COCO2CnHmother GasYield (ml/g) Without CaO With CaO Figure 6.14: Effect of sorbent addition at 700oC

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151 Figure 6.15: Tar laden condensate samples of plain biomass gasification (left) and sorbent enhanced gasification

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152 CHAPTER 7 REGENERATION OF SPENT SORBENT Introduction In the previous chapter the concept of sorb ent enhanced biomass gasification was proved experimentally. We saw that the calcium oxide absorbs CO2 in the product gas and thereby shifts the reactions in favor of hydrogen. After some time the calcium oxide gets saturated and is converted to calcium carbonate. For successful a pplication of the technology the carbonate must be fully converted back to its original oxide fo rm and must be reusable over many cycles. The process of converting the used sorbent (calcium carbonate) back to its oxide form is called regeneration. In recent past, re generation has been studied for CO2 (and in some cases for SO2 removal) in the gasification and combustion of ca rbonaceous fuels [86, 88, 89, 109]. In order for the sorbent enhanced biomass gasification for hydr ogen production to be t echnically feasible and commercially viable, the sorbent must be easily regenerated and must be usable over several alternate calcination car bonation cycles [110, 111]. The Reversible Calcination Carbonation Process The process of converting calcium carbonate to calcium oxide is well-known in the cement manufacturing industry as calci nation. The calcinati on reaction is endothermic and typically occurs at 850oC. 32 CaCOCaOCOH = +178 kJ/mol (7.1) The reverse reaction is called car bonation and is favored at lowe r temperatures (typically 600 – 700oC) 23 CaOCOCaCOH = -178 kJ/mol (7.2)

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153 Figure 7.1 shows the characteri stics of the reversible calc ination-carbonation reaction. From figure 7.1 it is observed that the calcinati on reaction is favored at high temperatures and low pressures (below and to the right of the equilibrium curve) wher eas carbonation reaction is favored at low temperatures and high pressures (region above and to the left of the equilibrium curve). This shows that calcination can be acco mplished at a lower temperature by reducing the CO2 partial pressure. This can be achieved e ither by creating vacuum or adding another gas stream (such as steam or nitrogen) into the reaction vessel. The calcination reaction has served as the basis of production of lime from limestone. Lime and cement manufacturing are energy intensive processes that occur at temperatures in the range 1200-1300 K [112]. The carbonation reaction a lthough has no commercial implication, has still been studied by a number of researchers inte rested in the kinetics of gas solid reactions. Dedmen and Owen [113] st udied the reaction of CO2 with calcined limestone over the temperature range of 100 to 600oC. They reported that the reaction occurred in two stages: a rapid initial stage where most of the CO2 was absorbed. This was followed by a much slower diffusion-controlled stage where CO2 molecules diffused through the carbonate layer. Similar behavior was observed in the i ndependent works carried out by Barker [114] and Bhatia and Perlmutter [115]. The heat required for the endothermic calcina tion reaction can be supplied in different ways. The most economical way would be to combus t a conventional fuel su ch as natural gas or coal and the energy released by the exothermic process can be supplied to the endothermic calcination reaction. Alternately part of the biomass feedstock can al so be combusted. If there is a waste heat stream available (such gas turbines or a solid oxide fuel cell exhaust) it can also be used. Another alternative would be to use c oncentrated solar energy. Figure 7.2 shows a

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154 schematic of a biomass gasification system with cal cination of used sorbent. The set-up consists of two main reactors: gasifier reactor and calciner. Biomass is steam gasified in the gasifier reactor which has a fixed bed (or for large units a fluidized bed) of calcium oxide sorbent. The CO2 absorption process releases heat which can be used in-situ for the endothermic steam gasification of biomass. A hydrogen rich, CO2 free product gas with very little tars is produced. This hydrogen rich gas is cleaned of any particulat e matter or residual tars and later sent to a gas turbine or fuel cell. The sorbent after some time ge ts saturated. It is regenerated by heating in the calciner. The CO2 released can be collected for safe storage and disposal. Some ways of supplying heat to the calc iner and their implicati ons are discussed below: Combusting a Carbonaceous Fuel The calciner or regenerator can be supplie d heat energy by com busting any carbonaceous fuel. A stoichiometric mixture of fuel and oxygen can be directly supplied to the calciner and the CO2 so produced can be collected along with the CO2 that is being desorbed by the carbonate. The CO2 produced from the two sources can be sequestered, compressed and stored for appropriate disposal. Alternately, natural gas can be supplied to the calciner along with air. The gas mixture coming out of the calciner will consists of CO2 and N2 (By using air in place of oxygen also has the advantage that CO2 partial pressure will be less, due to presence of inert nitrogen and that way the calcination can be carried out at a lower temperature). The gas mixture can then be supplied with a dditional natural gas (or methane) and water (or steam) to produce ammonium bicarbonate whic h is popularly used as a fertilizer. The following reaction shows how CO2 can be converted into a useful byproduct: 422243 3CH4N14HO5CO8NHHCOH = 5.44 kJ/mol (7.3)

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155 Ammonium bicarbonate is solid at room temperature and is popul arly used as a fertilizer in the agro and farming industries. Here five molecules of CO2 are consumed and are converted into solid form. Lee et al [116] conducted a t echnical and economic st udy at Oak Ridge National Laboratory to integrate CO2 sequestration from fossil fuel plants with co-production of ammonium bicarbonate. As pe r their study they found the fe rtilizer co-production to be important from economic point of view. The au thors did some preliminary calculations and concluded that the methane supplied for CO2 removal justifies the econo mic benefits obtained by producing fertilizers like ammonium bicarbonate. The authors have suggested further detailed investigation into the te chnical and economic aspects of this concept. Alternately, instead of using methane, part of the hydrogen produced in the process can also be used with the CO2 and N2 to produce ammonium bicarbonate as per the following reaction: 222243 2CON3H2HO2NHHCOH = -86.18 kJ/mol (7.4) Fiaschi et al [117] have developed a novel system for the capture of CO2 produced from fossil sources and convert them later into useful fertilizers. Heat can also be supplied by combusting part of the solid biomass feedstock with very little or no penalty on the carbondioxide emissions. Using Concentrated Solar Energy Solar energy can also be used to driv e the endothermic calcination reaction. The advantages of using sola r energy are three fold no discharge of pollutants gaseous product stream is not cont aminated (with combustion products) a one time investment with no running cost Zedtwitz et al [118] have proposed the use of solar energy for the thermal gasification of coal. The authors envisaged a solar concentrating plant (consisting of a sola r tower or solar tower

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156 reflector system) to direct the sunlight onto a reactor located on the ground. The concentrating system was capable of achieving he at flux intensities of about 5000 kW/m2. Such radiation fluxes can attain temperatures up to 1500 K which is more than the typical calcination temperature of 1100-1200 K. Haueter et al [31] ha ve developed a solar chemical reactor for the thermal decomposition of zinc oxide which w ill be used for thermochemical hydrogen production by water splitting. Th e prototype reactor was of 10 kW capacity and could generate solar radiation heat flux of 3500 kW/m2 and temperatures up to 2000 K. For successful application of the solar technology it has to be ec onomically viable. Similar reactors can be built for calcination of used sorbents in biomass plants. The economics of using solar reactor technology has to be carefully studied. Some government subsidies a nd incentives like tax credits for totally CO2 free hydrogen production can make the concept of using solar energy for regeneration with hydrogen production from biomass economically feasible. An important feature of using concentrated solar energy for so rbent regeneration is that the process becomes fully renewable. On the other hand, if CO2 is sequestered it will be a process with negative carbon potential i.e. carbon dioxide although was absorbed during photosynthesis, it was not released into the environment. Waste Heat from Gas Turbin e Exhaust or from SOFC Gas turbine exhaust stream can also be us ed for supplying heat to the endothermic calcination process. The gas turbine can be fired from the hydrogen rich product gas. The exhaust stream coming out of the gas turbine can first be used for heating the calciner at 850900oC and thereafter can also be used for produc ing steam to run a steam turbine at 500-600oC. Alternately, the product gas from the biomass gasi fication system can be used directly to run a solid oxide fuel cell. The exhaust of the SOFC can then be used to heat the calciner at 850-900oC

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157 and thereafter once again be used for generati ng steam in a Heat Recovery Steam Generator (HRSG) to run a steam turbine at 500-600oC. Kinetics of the Reversible Calcination Carbonation Reactions The reversible non-catalytic gas solid reaction between CaO(s) and CO2(g) has been studied by many researchers. Silaban and Ha rrison [109] conducted multi-cycle tests by calcining CaCO3 at 750 – 900oC at pressures of 1-15 atm in the presence of N2. The reverse carbonation reaction was carried out in the same pressure rang e but at a slightly lower temperature (550-750oC). The researchers characterized the samples by determining the surface area and pore volume. They observed that the surface area and pore volume of the “as-received” and carbonated samples (CaCO3) were quite small as compared to those of calcium oxide. At the end of first carbonation it was found that almost 30% of the calcium oxide sorbent had not reacted. The surface characterization suggested that pore closure might have prevented CO2 gas molecules from reaching the unreacted calcium ox ide core. In the second calcination the surface area reduced even further. The authors hypothesize d that sintering might have taken place at high temperature. At the end of the second carbonatio n the authors found that more than 40% of the original calcium oxide sorbent remained unreacted due to sintering and pore closure. The authors observed that carbonation of plain CaO occurs in two steps: rapid initial step where most of the calcium oxide is converted to calcium carbonate. This is followed by a much slower diffusion controlled step where CO2 molecules diffuse thr ough the carbonate layer. The authors concluded that pore plugging leads to inco mplete carbonation and sintering inhi bits effective regeneration. To overcome the problem of pore closure, Gupta and Fan [119] studied the pore size distribution of calcium carbonate precursors from four diff erent sources. The authors hypothesized that incomplete conve rsion of calcium oxide to carbona te can be attributed to the

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158 micro pores which are susceptibl e to pore blockage and plugging. They suggested synthesizing a new calcium oxide sorbent from a carbonate prec ursor which has large sized pores which they called meso pores. Accordingly they synthesized sorbents from pre-cursors in the mesoporous range. Multi cycle tests conducted on mesoporous sorbents showed remarkable improvement (conversion of CaO to CaCO3 increased by about 35% above conve ntional). This was attributed to the high porosity and pore volum e which was big enough for the CaCO3 product layer not to plug the pores. Kuramoto et al [120] also conducted multi cycle tests and found sintering to occur during the high temperature calcination pr ocess. Sintering causes reduction in the surface area and ultimately leads to incomplete regenera tion. To counter this problem, the authors hydrated (added water) the samples before car bonating them. The resear chers hypothesized that the water or steam molecules would fill the pore vo lume and thereby reduce sintering; tests were carried out at atmospheric and hi gh pressures. It was observed that the hydrated sorbent samples had higher conversion (>85%) as compared to the regular sorb ent. However, these are only preliminary results, further detailed investigation is necessary. Sorbents other than Calcium oxide Many research groups that used pure calcium oxide sorbent observed sintering and pore closure to occur during the re versible calcination carbonation reactions. When these groups conducted multi cycle tests they also found the conversion to decrease and hypothesized that sintering and pore closure ultimately lead to inco mplete regeneration. One way of addressing this problem is to use a stabilizing material which does not take part in the carbonation process. Compounds such as MgO are inert at high temp erature and do not participate in the CO2 absorption/desorption process. They can serve as excellent stabilizing materials. Hence the sorbent can maintain its porosity over severa l cycles. Bandi et al [121] tested dolomite (50%Mgo, 50%CaO) and huntite (75% MgO, 25% CaO) in a Thermal Gravimetric Analyzer

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159 (TGA). The carbonation wa s carried out at 830oC and calcination was carried out at 500oC. The researchers measured the absorption capacity ov er several cycles and found that dolomite suffered loss in absorption capacity of about 30% after 15 cycles. The performance of dolomite was found to be much better than pure calcite. They attributed this to th e structural stabilizing effect of MgO which prevented the pores from cl osing. Huntite too was found to maintain high levels of CO2 absorption capacity (>85%) even after 45 cycles. Kato et al [122] tested many silicates and among them found Lithium orthosilicate (Li2SiO4) to maintain high level of CO2 absorption capacity 242232LiSiOCOLiCOSiO (7.5) This sorbent was originally developed at Tosh iba (Japan) in 2001 and the research group is presently investigating the long term stability of the sorben t. The researchers believe Li2SiO4 to be a promising material for CO2 absorption. In recent times, sodium based sorbents have been tested in the laboratory for CO2 absorption from fossil fuel plants. The advantage of sodium based sorbents is the low operating temperature (CO2 capture can take place at temperatures as low as 60-70oC and regeneration takes place at 120-200oC). Liang et al [123] conducted multicycle tests using sodium carbonate sorbent. The group found that as much as 90% of the CO2 could be captured at appropriate reaction conditions. Based on the sorbent durability and energy consumption during regeneration, the researchers found the c oncept of sodium based sorbent for CO2 removal to be better than the amine-scrubbing process for CO2 removal which is presently followed. The choice of appropriate so rbent depends on many factors such as calcination and carbonation temperatures, energy consumed, sorb ent durability over multiple cycles, sorbent

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160 availability and the capital and maintenance costs. The choice of the correct sorbent will affect the economic viability of the process. Summary The calcination reaction has been studied in the past for the production of lime from limestone. More recently, the reversible calci nation-carbonation reaction has received renewed interest from the scientific community due to the possibility of CO2 removal from fossil fuel exhaust. Although many research groups have been actively working for the past several years, till date no single sorbent has been found that ca n be effectively regenerated and re-used over many cycles. The main concerns that need to be addressed are sint ering and pore closure. Sintering occurs at high temper atures and reduces the surface area of sorbent; pore pluggage blocks the CO2 molecules from reaching the core and hence lot of CaO remains unreacted (as much as 40%). Many efforts are being pursued to address these problems. Some of them include modifying the surface morphology to reduce plugging, hydration of sorbent and adding stabilizing materials like magne sium compounds or silicates to the sorbent. However, no one method can ensure effective regeneration. A furt her detailed study of th e chemical kinetics and surface characterization is needed to understand the mechanism of regeneration and address the concerns.

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161 Figure 7.1: Equilibrium CO2 pressure as a function of temperat ure (adapted from Silaban et al [109]) 0.0001 0.001 0.01 0.1 1 10 5005506006507007508008509009501000 Temperature (oC)Pressure (atm)CaO (s)+ CO2 (g) = CaCO3(s) Carbonation favored in this region CaCO3(s) = CaO(s)+ CO2 (g) Calcination favored in this region

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162 Steam Boiler Gas Cleaning WaterQ1Q2 Condenser CO2+ H2O CO2for sequestration H2to fuel cell/ gas turbine H2rich, CO2free gas GASIFIER CALCINER H2O Biomass Figure 7.2: Biomass gasification with calcination of used sorbent

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163 CHAPTER 8 SUMMARY, CONCLUSION AND RECOMMENDATIONS Summary The concept of sorbent enha nced gasification has been applied to biomass for producing hydrogen. First a thermodynamic analysis was carr ied out and later simula tions were done to study the effect of adding sorbents on hydrogen yield. Later on experiments were conducted to verify the concept of using sorbents and finally regeneration of spent sorbent was studied. The basic thermodynamic analysis of biomass ga sification was carried out to determine the equilibrium hydrogen yield. The following conc lusions were drawn from the thermodynamic studies: 1) The highest hydrogen yield was observed at gasification temperatures of around 1000 – 1100 K; steam gasification gave higher hydrogen yields than ai r gasification. This was due to the reformation of tars, char and highe r hydrocarbons. Using air as a co-gasifying medium partially oxidizes the biomass and hence reduces the net energy consumption; however, it dilutes the product gas. The highest hydrogen yield was found when the gasifier was operated at atmospheric pressure. 2) The highest obtainable hydrogen yield wa s limited by thermodynamic equilibrium. To get more hydrogen, the equilibrium constraints must be removed. This is possible by constantly removing one of the co-products of gasification (CO2) thereby driving the reactions in favor of hydrogen. The basic studies laid the foundation of the concept of using sorbents for gasifying biomass. The concept was theoretically invest igated by carrying out simulations in ASPEN process simulator. Calcium oxide was used as the model sorbent and ethanol was the model biomass compound. The effects of important proce ss variables such as temperature, pressure,

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164 steam to biomass ratio and sorbent to biomass ratio on hydrogen yield were studied. The output variables included gas yields (H2, CO, CO2 and CH4), reformer heat duty and thermodynamic efficiency. Based on the simulations th e following conclusions were drawn: 3) The hydrogen yield while using the sorb ent increased by 19% in comparison to conventional biomass gasification 4) The CO2 content of the product gas was reduced by almost 50% due to absorption by CaO 5) The reformer heat duty was reduced by almost 42% due to in-situ heat transfer 6) The gasifier operating temper ature was lowered by about 100-150oC in the presence of the sorbent; in other words the same hydrogen yi eld was obtained by runni ng the gasifier at about 100 – 150oC lower than the conventional gasification temperature 7) Sorbent enhanced gasification is a novel technique for producing a hydrogen rich and CO2 free gas; the product gas will need mini mal cleaning and hence many downstream equipment such as Water Gas Shift reactor and PSA (Pressure Swing Adsorption) unit used conventionally may not be needed in the sorb ent case. Hence there is a possibility of significantly reducing the capital cost of hydrogen production from biomass 8) A pure CO2 stream is produced which can be either used for suitable applications or sequestered for appropriate disposal Based on the encouraging results from the si mulations, experimental studies on sorbent enhanced biomass gasification were carried out us ing Southern pine bark and calcium oxide. The temperature of the gasifier was varied from 500 to 700oC. The yield of individual gases and the total yield were determined. Based on these st udies, following conclusions can be drawn: 9) The hydrogen yield at 500oC increased substantially from 320 ml/g without the sorbent to 719 ml/g while using sorbent.

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165 10) The hydrogen yield at 500oC with sorbents was more than the hydrogen yield at 700oC for plain gasification. Hence there is a possibility of reducing th e operating temperature of the gasifier (this confirmed the obser vation made in the simulation). 11) CO2 absorption is an exothermic reaction whil e biomass steam gasification is endothermic. The two reactions can be coupled together so that there is in -situ energy transfer. Therefore the external heat duty of the gasifier can be significantly reduced. Hence a smaller and more compact gasifier can be used in place of the conventional. This would reduce the capital cost of the gasifier wh ich is a significant part of the overall cost of biomass plant. 12) The overall gas yield increased from 550 ml/g to 1360 ml/g at 500oC. It was also observed that the tars in the product gas were less when the sorbent was used. Based on these observations it was concluded that the sorbent had a catalytic effect in reforming the tars to additional gas. 13) The carbon conversion efficiency (which in a way quantifies th e effectiveness of gasification) improved considerably from 23% to 63% at 500oC while using the sorbent. 14) The high overall gas and hydrogen yields were observed until a gasifi cation temperature of 700oC after which the sorbent case gave almost the same yield as the case without the sorbent. This is due to the f act that the carbona tion reaction (CO2 absorption) is effective in the temperature range 500 700oC. Beyond this temperature CO2 is no longer absorbed (this was also observed in theoretical simulations). 15) The product gas while using the so rbent is rich in hydrogen, is CO2 free and is relatively clean of any particulates and tars. If we can set the operatin g conditions such that we get almost pure hydrogen gas with very little contam inants, it may be possible to eliminate all or most of the downstream equipment used in conventional bioma ss gasification systems

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166 such as High Temperature Water Gas Shif t reactor (HT-WGS), Low Temperature Water Gas Shift (LT-WGS) reactor and Pressure Swing Adsorption (PSA) unit. With no WGS reactors there is no need of any catalyst and he nce there will be savings in the running cost. In place of these reactors ther e will be a single reactor used for regenerating the sorbent. Hence there is a potential to reduce the capital cost of biomass gasification plants for hydrogen production. Conclusions 1) Conventional biomass gasification is constrained by equilibrium of water gas shift reaction. This constraint can be removed by using a CO2 sorbent such as calcium oxide. 2) The hydrogen yield is enhanced as the water gas shift reactio n goes to completion in the absence of CO2; CO and CO2 in the product gas reduce drastically 3) In-situ heat transfer reduces the reformer (gasifier) duty there by making it compact 4) Product gas is rich in hydrogen with small amounts of impuriti es and can be sent to the downstream equipment with minimum gas conditioning 5) Less gas cleaning and conditioning equipment imp lies reduced capital cost and hence there is a potential to become cost comp etitive to conventional gasification Recommendations for Further Work Sorbent enhanced biomass gasification is a novel technique for producing renewable hydrogen. The concept has been proved theoreti cally and experimentally with remarkable improvement in hydrogen yield. For successful a pplication of this technology the sorbent must be regenerated for further use. Many research gr oups are currently investigating the multi-cycle performance of calcium oxide sorbent. Sinterin g and pore pluggage have been identified as the key problems. Further work must focus on addr essing these problems. Some techniques include modification of sorbent surface to create me sopores so as to cr eate enough surface area to

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167 prevent plugging. Other research groups are de veloping alternat e sorbents by adding stabilizing materials such as MgO and silicates which resist pore closure and sinterin g at high temperature. Sorbent regeneration by itself is an area of intense research and deep study involving different sub-areas like chemical kinetics, su rface chemistry, thermodynamics, catalysis and material characterization. Future research must concentrate on a nd integrate these sub-areas with the end objective of developing a sorbent that can be effectively used over multiple calcinationcarbonation cycles without any attrition or loss of capacity.

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168 APPENDIX LIST OF PUBLICATIONS Journals Mahishi MR, Sadrameli SM, Vijayaraghavan S, Goswami DY. A novel approach to enhance the hydrogen yield of biomass gasification using CO2 sorbent, Accepted for publication by the ASME Journal of Engineer ing for Gas Turbines and Power. Mahishi MR, Goswami DY. An experimental st udy of hydrogen production by gasification of biomass in the presence of a CO2 sorbent, Accepted for publicati on by the International Journal of Hydrogen Energy, Elsevier publications. Mahishi MR, Goswami DY. Thermodynamic optimizati on of biomass gasifier for hydrogen production under review with International Journal of Hydrogen Energy, Elsevier publications. Conference Proceedings Mahishi MR, Vijayaraghavan S, Deshpande DA, Goswami DY. A thermodynamic analysis of hydrogen production by gasification of biomass. Proceedings of the 2005ISES Solar World Congress, August 6-12, 2005, Orlando, FL. Mahishi MR, Sadrameli MS, Vijayaraghavan S, Go swami DY. Hydrogen production from ethanol: A thermodynamic analysis of a nove l sorbent enhanced gasification process. American Society of Mechanical Engineers, Adv anced Energy Systems (publication) AES vol 45, pp 455-463, 2005.

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169 REFERENCES [1] Kazim A, Veziroglu TN. Utilizat ion of solar-hydrogen energy in the UAE to maintain its share in the world energy market for the 21st century. Renewable Energy 2001; 24: 259-274. [2] Abdallah MAH, Asfour SS, Veziroglu TN. Solar hydrogen system for Egypt. International Journal of Hydr ogen Energy 1999; 24(6): 505-517. [3] Jefferson M. Sustainable energy developm ent: performance and prospects. Renewable Energy 2006; 31(5): 571-582. [4] Fuelcells.org. Benefits of Fuel Cells in Transportation. Internet resource found at http://www.fuelcells.org/bas ics/benefits_transp.html last accessed November 2006 [5] Spath PL, Mann MK. Life cycle assessment of hydrogen production via natural gas steam reforming. In: National Renewable En ergy Laboratory, Golden, CO, NREL/TP570-27637, 2001. [6] Barnett TO, Adam JC, Lettenmaier DP. Poten tial impacts of a warming climate on water availability in snow-dominated re gions. Nature 2005; 438: 303-309. [7] Laura C. IEA World Energy Outlook 2004; Implications for energy and CO2 emissions (accessed from www.iea.org ) last accessed November 2006. [8] International Energy Outlook 2006 report. June 2006. Energy Information Administration Office of Inte grated Analysis and Forecasting: US Department of Energy, Washington DC ( www.eia.doe.gov/oiaf/ieo/index.html ) last accessed Novermber 2006. [9] Quantifying Energy: BP Statisti cal review of World Energy. June 2006. An annual report prepared by the British Petroleum o il company (www.bp.com) last accessed November 2006. [10] Pena MA, Gomez JP, Fierro JLG. New catalytic routes fo r syngas and hydrogen production. Applied Catalysis A: General 1996; 144: 7-57. [11] Prins R, De Beer VHJ, Somorjai GA. Stru cture and function of the catalyst and the promoter in Co-Mo hydrodesulphurization catalysts. Catalysis Reviews – Science and Engineering 1993; 35(1):141. [12] Hindermann JP, Hutchings GJ, Kiennemann A. Mechanistic aspects of the formation of hydrocarbons and alcohols from CO hydroge nation. Catalysis Reviews – Science and Engineering 1993; 35(1): 1-127. [13] Nielsen A. Ammonia synthesi s: Exploratory and applied research. Catalysis Reviews 1980; 23 (1 & 2): 17-51.

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179 BIOGRAPHICAL SKETCH Madhukar was born and raised in Bombay, India. He completed his b achelor’s degree in Mechanical engineering from th e University of Bombay in Summer 1996 and later completed his master’s degree in Mechanical engineering (with specialization in Thermal science and Fluid dynamics) from the Indian Institute of Technol ogy, Bombay, in Spring 1999. He worked for a well-known automotive company in India (Tata Auto Comp Systems Limited) as a Design Engineer for three years where he was in ch arge of thermal design of automotive cooling systems for cars, trucks and co mmercial vehicles. Madhukar joined the MAE Department at UF in Spring 2003 and was admitted to the direct PhD program. Upon graduation he plans to pursue research either in industry or academia in the ar eas of thermal design, energy conversion and fuel cells.


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THEORETICAL AND EXPERIMENTAL INVESTIGATION OF HYDROGEN
PRODUCTION BY GASIFICATION OF BIOMASS





















By

MADHUKAR MAHISHI


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2006

































Copyright 2006

by

MADHUKAR MAHISHI

































I would like to dedicate this work to my father Shri R. K. Mahishi









ACKNOWLEDGMENTS

First of all I would like to thank my advisor, Professor D. Y. Goswami, for giving me an

opportunity to work with him at the Solar Energy and Energy Conversion Laboratory and

conduct research in the areas of renewable energy and hydrogen production. His guidance,

advice and encouragement to work independently greatly helped in shaping my thoughts and in

molding me from a graduate student to a researcher.

I would like to thank Dr. Skip Ingley, Dr. Bill Lear, Dr. S.A. Sherif and Dr. Donald

Rockwood for agreeing to serve on my supervisory committee and also for their advice,

comments and suggestions during the various phases of my PhD.

I would like to acknowledge the assistance and support of Dr Helena Hagelin-Weaver

(Department of Chemical Engineering) during the fabrication of the experimental set-up for

biomass gasification. I would like to thank all the staff members and colleagues of the Solar

Energy and Energy Conversion Laboratory at UF. I would also like to acknowledge the US

Department of Energy for funding our research on hydrogen production. A word of thanks goes

to all my present and former room-mates (Sanjay Solanki, Viswanath Urala, Kaushal Mudaliar,

Purushottam Kumar, Ashish Gupta, Sudarshan Jagannathan and others) whose enthusiasm and

co-operation made graduate student life in the US a very memorable experience.

Above all I would like to thank my parents for their patience, encouragement and support

all through during my PhD. Finally I would like to thank God for giving me the opportunities

and challenges which, over the years, have helped me become a better person.









TABLE OF CONTENTS

page

A C K N O W L E D G M E N T S ..............................................................................................................4

L IS T O F T A B L E S ................................................................................. 8

LIST OF FIGURES .................................. .. ..... ..... ................. .9

ABBREVIATIONS .................................... ... .. .... ...... .................12

A B S T R A C T ......... ....................... ............................................................ 14

CHAPTER

1 INTRODUCTION ............... .......................................................... 16

Significance and Need for Hydrogen: An Overview.........................................................16
Introduction to Present Research .......................................................................... 19

2 HYDROGEN PRODUCTION METHODS................................................................ 24

In tro d u ctio n ......................................................... ................... ................ 2 4
H hydrogen Production M ethods.................................................................. ..... ..................24
Steam M ethane R eform ing (SM R )...................................................................... ...... 24
Partial Oxidation or Autothermal Reforming of Methane ............................................25
C o al G asification ............................................................................................... .... 2 7
Biom ass G asification and Pyrolysis ........................................... ......................... 28
E lectroly sis ........................................................ ..........................29
Therm ochemical Hydrogen Production ........................................ ....... ............... 30
Z n/Z n O cy cle ...................................................................... 3 1
U T-3 cycle.......................................................... ........... 31
Photocatalytic H ydrogen Production......................................... .......................... 32
Photoelectrochemical Hydrogen Production .............. ............................................33
B biological H ydrogen Production.......................................................... ............... 34
Ferm entation of bacteria................................................. .............................. 34
Biophotolysis...................................35
S u m m ary ................... ...................3...................6..........

3 BACKGROUND AND LITERATURE REVIEW ..................................... ...............44

In tro d u ctio n ................... ...................4...................4..........
P y ro ly sis ................................................................................4 4
G asificatio n .............................................................................. 4 5
C o m b u stio n ........................................................................................4 5
Liquefaction ............. ..................... ............................ 45
Lab-Scale Production of Hydrogen from Biomass ........................ ...... .... ......... 46









An Overview of Biomass Gasification for Hydrogen Production....................................47
C ata ly sis .............................................................4 7
N on-M metallic Oxides .................................................... ..... ........... .. .. ...... ... 49
Com m ercial N ickel Reform ing Catalyst............................................................... 50
A additional Catalyst Form ulations ........................................ ........................ 51
Pretreatm ent Technologies ........................................... .................. ............... 53
C hem ical K inetic Studies ..................................................................... ............... 54
Experimental Studies on Biomass Gasification ................................... .................57
Therm dynamic Studies on Gasification ............................................. ............... 58
Sorbent Enhanced G asification ............................................... ............................ 61
Scope of the P resent W ork ...................................................................... .. .......................64

4 THERMODYNAMIC ANALYSIS OF BIOMASS GASIFICATION..............................70

In tro d u ctio n ................... ...................7...................0..........
Fundam entals ............................................ ................. .. ........................ ......... 70
Effect of Process Parameters on Equilibrium Hydrogen Yield............................................72
Effect of Tem perature ................................................. .... .. .............. .. 73
Effect of Pressure ............................... .................. ........... 74
E effect of Steam B iom ass ratio .............................................................. .....................74
E effect of E quivalence R atio............................................................................ ........ 75
O ptim um Process Param eters......................................................................... .......... 75
E n erg y A n aly sis ............................. ................................................................................... 7 6
Effect of Temperature on Thermodynamic Efficiency ................................................78
Effect of Steam Addition on Thermodynamic Efficiency.............................................78
Effect of ER on Thermodynamic Efficiency...... .................... .............79
Comparison of Equilibrium Results with Experimental Data.........................................80
Sum m ary and C onclu sion ........................................................................ .................. 8 1

5 ABSORPTION ENHANCED BIOMASS GASIFICATION .............................................95

Introdu action .................................................... .................................. 9 5
Concept of Absorption Enhanced Gasification ........................................... .....................96
Application of SEG to Biom ass Gasification...................................................................... 98
C ase I: B ase case (no sorbent)............................................................................... .... 98
Case II: Ethanol gasification in the presence of CaO sorbent sorbentt placed in the
re fo rm e r) .............................................................................1 0 0
Energy A analysis .............................................. 102
C o n clu sio n ................... ...................1...................0.........4

6 EXPERIMENTAL STUDIES ON BIOMASS GASIFICATION............................119

O b j e ctiv e ................... ...................1.............................9
Experimental Facility........... ........ .......... ......... .... ............... .. 120
T est Set-up ..................................................................................................120
G asifier (Prim ary reactor) ............................................... ............ ............... 120
Secondary reactor............ .......................................................... ........ ........ 120



6









Steam generator .................. .................. ..................... ... .......... 121
Gas cooling system (heat exchanger)...............................................................121
Heaters, insulation and tubing/fittings ....................................... ............... 121
In strum entation ................................................................... ......... .... .. ..... 12 2
Gas Analysis Facility.............. ... .................................122
G C Calibration ............................................123
Test M methodology ................ ................. ................. ............. .. ............. 123
T est R results and A naly sis ........................................................................... ... ....... .. 125
E effect of T em perature ................................................................. ..................... 12 5
Effect of Sorbent .................................... .................. .. ........ ....... ..... 127
C o n clu sio n ................... ...................1...................2.........8

7 REGENERATION OF SPENT SORBENT......................................................................152

Introdu action ......................................................... .................. ................. 152
The Reversible Calcination Carbonation Process...................................... ...............152
Com busting a Carbonaceous Fuel ....................................................... .... ........... 154
U sing C concentrated Solar E energy ...................................................... .....................155
Waste Heat from Gas Turbine Exhaust or from SOFC ..............................................156
Kinetics of the Reversible Calcination Carbonation Reactions ..................................157
Sorbents other than C alcium oxide............................................................ ............... 158
S u m m ary ................... ...................1...................6.........0

8 SUMMARY, CONCLUSION and RECOMMENDATIONS ..........................................163

S u m m ary ................... ...................1...................6.........3
Conclusions.................. ...... ............................... ...................166
R ecom m endations for Further W ork ........................................................................ ...... 166

APPENDIX LIST OF PUBLICATIONS .............................................................................168

R E F E R E N C E S .........................................................................169

B IO G R A PH IC A L SK E T C H ......................................................................... .. ...................... 179


















7









LIST OF TABLES


Table page

1-1 Summary of hydrogen production methods ................... .......... .................23

3-1 F eedstock com position ............................................................................. ....................67

4-1 Equilibrium gas moles at different gasification pressures .......................................83

5-1 Reactions in SEG for som e typical fuels .............................................. ............... 105

5-2 Comparison of energy consumption in biomass gasification with and without sorbent .106

5-3 Thermodynamic efficiency and energies..................................................................... 107

6-1 H eater ratings ..................................... .................. ............... .......... 130

6-2 U ltim ate and proxim ate analyses ....................................................................... 131

6-3 Effect of temperature on the products of biomass gasification.............. ... ...............132

6-4 Carbon conversion efficiency (no sorbent)................................................................... 133

6-5 Equilibrium yields of biom ass gasification products...................................................... 134

6-6 Effect of temperature on gas composition in the presence of sorbent .............................135

6-7 Carbon conversion efficiency sorbentt enhanced gasification) ................................. 136









LIST OF FIGURES


Figure page

2.1 Block diagram of hydrogen production by steam methane reforming ............................37

2.2 Block diagram of hydrogen production by partial oxidation of heavy oils ....................38

2.3 Principle of hydrogen production by high temperature electrolysis (HTE) ......................39

2.4 Block diagram of the Zn/ZnO water splitting thermochemical cycle for hydrogen
p ro du action .................................................................................4 0

2.5 UT-3 cycle reactions and flow of material .......... ..........................................41

2.6 Photocatalytic hydrogen production ............................................................................ 42

2.7 Principle of photoelectrochemical hydrogen production...........................................43

3.1 B iom ass gasification pilot plant............................................... .............................. 68

3.2 Schematic of biomass gasification set up for producing hydrogen ..............................69

4.1 Variation of Gibbs energy with extent of reaction ................... ...............................84

4.2 Effect of temperature for P = 1 atm, 3 = 1, ER = 0 ................................. ............... 85

4.3 Effect of SBR on equilibrium composition ............................................ ............... 86

4.4 Effect of ER on Equilibrium composition .............................................. ............... 87

4.5 Schem atic of biom ass gasifier ....................... ......... ......... .................................. 88

4.6 Efficiency Vs temperature for various 3 (ER = 0.1)............................... ............... 89

4.7 Efficiency Vs temperature for various 3 (ER = 0.2)............................... ............... 90

4.8 Efficiency Vs temperature for various 3 (ER = 0.3)............................... ............... 91

4.9 Efficiency Vs temperature for various 3 (ER = 0.4)............................... ............... 92

4.10 Comparison of equilibrium data with experimental data o for different temperatures
and residence times ................................................................. .........93

4.11 Comparison of equilibrium data with experimental data for different 3 and ER..............94

5.1 Concept of absorption enhanced gasification ....................... ..................................... 108

5.2 Schem atic of SEG (concept) ........................................................................ 109









5.3 Flow sheet for conventional biomass gasification.....................................110

5.4 Effect of reformer temperature on product yield ........ .......... ......................111

5.5 Effect of reformer pressure on product yield........ ........................ .............. 112

5.6 Effect of steam ethanol ratio on product yield at 700C.............................................113

5.7 Flow sheet for ethanol gasification with CaO sorbent.......................... .....................114

5.8 Effect of temperature on product yield for sorbent enhanced reforming .........................115

5.9 Effect of pressure on the product yield for sorbent enhanced reforming ........................116

5.10 Effect of steam/ethanol ratio on product yield for sorbent enhanced reforming ............117

5.11 Effect of CaO/ethanol ratio on the product yield......... ........ ...................... 118

6.1 B iom ass gasification test set-up...................... .... ................................. ............... 137

6.2 Photograph of the test set-up...................... .. .. ......... .......................... ............... 138

6.3 Gas chrom atograph (SRI 8610C)............................. ............................. ............... 139

6.4 Set-up for G C calibration ........................................................................ ...................140

6.5 H hydrogen calibration curve........................................... ....................................... 141

6.6 CO calibration curve .................. .................. ................. .......... .............. .. 142

6.7 CO 2 calibration curve ................ ............ ............ .................... 143

6.8 C H 4 calibration curve............ .... .......................................... ................ .......... ....... 144

6.9 Southern pine bark "as received". .................... ......... .. ......... .............................. 145

6.10 Pelletized pine bark ........ ..................... .... .................. ........ ........... 146

6.11 Effect of temperature on gas yields (no sorbent) .............................. ................147

6.12 Effect of sorbent addition at 500 C......................................... ........... ... ................. 148

6.13 Effect of sorbent addition at 600 C......................................... ........... ... ................. 149

6.14 Effect of sorbent addition at 700 C......................................... ........... ... ................. 150

6.15 Tar laden condensate samples of plain biomass gasification (left) and sorbent
enhanced gasification ............ ... .... ....... .................... ........ ............ ...... 151

7.1 Equilibrium CO2 pressure as a function of temperature .........................................161









7.2 Biomass gasification with calcination of used sorbent................. ............................ 162









ABBREVIATIONS

ER Equivalence Ratio (actual air to biomass ratio divided by

stoichiometric air to biomass ratio)

Ewood chemical energy stored in biomass (wood) (kJ)

H enthalpy (kJ/mol)

AH enthalpy change due to temperature (kJ/mol)

Hf enthalpy of formation (kJ/mol)

LHV Lower Heating Value (kJ/mol)

n no. of moles

P pressure (atm)

Qair heat supplied to air-preheater (kJ)

QEG heat input to equilibrium gasifier (kJ)

Qsteam heat supplied to steam generator (kJ)

SBR Steam Biomass Ratio (denoted by P; defined as moles of steam per mole of

biomass)

SEG Sorbent Enhanced Gasification

SMR Steam Methane Reforming

T Temperature (K)

WGS Water Gas Shift reaction or Water Gas Shift reactor

g specific Gibbs energy (kJ/kg)

q heat transferred (kJ/kg)

s entropy (J/kg-K)

u specific internal energy (kJ/kg)









specific volume (m3/kg)

work done (kJ/kg)


Greek Symbols:

P moles of steam per mole of biomass

Y moles of oxygen per mole of biomass

r thermodynamic efficiency (%)

T residence time (s)

Extent of reaction



Subscripts:

gen generated

sys system

surr surrounding

NS total number of species









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

THEORETICAL AND EXPERIMENTAL INVESTIGATION OF HYDROGEN
PRODUCTION BY GASIFICATION OF BIOMASS

By

Madhukar Mahishi

December 2006

Chair: Yogi Goswami
Major Department: Mechanical and Aerospace Engineering

A detailed theoretical and experimental investigation of hydrogen production by

thermochemical gasification of biomass was conducted. The thermodynamics of biomass

gasification was first studied to determine the hydrogen yield at equilibrium. The gasification

process is characterized by a number of endothermic and exothermic reactions. A combination of

these reactions enables internal energy transfer, and therefore improved process efficiency. The

maximum hydrogen yield is limited by thermodynamic equilibrium. One solution to this problem

is to remove one of the co-products (C02) that governs the equilibrium hydrogen yield. In recent

times, sorbents (such as calcium oxide) have been used for CO2 removal from fossil fuel exhaust.

The same principle was applied here to drive the reactions in favor of hydrogen. In the process

the sorbent gets saturated and has to be regenerated for further use.

Process simulations were conducted using an ASPEN simulator with the end objective of

determining the hydrogen yield in presence of a CO2 sorbent. Ethanol was used as the model

biomass compound and calcium oxide was the representative sorbent. The simulations showed

19% increase in hydrogen yield and about 50% reduction in product gas CO2 while using the

sorbent. The hydrogen yield in the presence of sorbent at a gasification temperature of 600C

was comparable to the hydrogen yield without the sorbent at 750C. Hence there is a potential to









reduce the gasifier operating temperature by about 100-150C while still getting the same

amount of hydrogen. The in-situ heat transfer (CO2 absorption is exothermic) reduced the

gasifier heat duty by almost 42%.

Based on the encouraging results obtained from simulations, experiments were conducted

using Southern pine bark as the model biomass and calcium oxide as the representative sorbent.

Hydrogen yield increased substantially (from 320 ml/g to 719 ml/g) by using sorbents at

gasification temperature as low as 500C. The product gas had much less tars and particulate

matter as compared to conventional gasification. The carbon conversion efficiency (a way of

quantifying the effectiveness of gasification) increased from a mere 23% to 63% while using

sorbent.

Sorbent enhanced biomass gasification has the potential to produce a hydrogen rich, CO2

free and possibly tar free gas that can be sent to a fuel cell or gas turbine with minimal cleaning.

Hence there is a potential to reduce the equipment needed for hydrogen production. This will

lead to reduced capital and operating costs. Hence sorbent enhanced biomass gasification has the

potential to become a cost effective technology for producing renewable hydrogen.














CHAPTER 1
INTRODUCTION

Significance and Need for Hydrogen: An Overview

Present day energy resources such as coal, oil and natural gas are being consumed at an

accelerated rate with fear of depletion in the next few decades. It is reported that some of the oil

rich countries would fail to meet the world energy demand in the next few decades. For example,

United Arab Emirates is expected to exhaust its oil and natural gas reserves by 2015 and 2042

respectively [1], and fossil sources in Egypt would possibly be exhausted within the next two

decades [2]. There is also a concern about the environmental pollution caused by the use of fossil

fuels. According to a recent study the world CO2 emissions from fossil sources have increased by

24.4% from 1990 to 2004 [3]. Apart from C02, other contaminants such as CO, NOx, and SOx

are released during the combustion of fossil fuels. These contaminants cause acid rains, deplete

the stratospheric ozone layer and are also known to be carcinogenic. According to an EPA study,

vehicles in the US account for 65% of total oil consumption and result in 78% CO, 45% NOX

and 37% Volatile Organic Compound (VOC) emissions [4]. Among all the air pollutants emitted

by the combustion of fossil fuels, CO2 alone accounts for 99% (by weight) of the total emissions

[5]. The average surface temperature of earth has increased by 0.6C over the past two centuries

[6]. If this trend continues it may eventually lead to higher sea levels and significant changes in

global precipitation patterns. The trend in the transportation sector in industrialized countries is

towards more vehicles, more freight transport by road and larger and heavier passenger vehicles.

Furthermore, developing countries like China and India with large population and growing

economies are expected to add to the rapid growth in vehicle usage for transportation









applications [7]. This would further lead to large scale emissions which may drastically change

the global weather patterns, thus affecting mankind and environment.

The world energy demand has been steadily increasing over the last few decades.

According to a recent study conducted by the US Department of Energy, the world energy

demand is expected to increase to 722 quads (Quadrillion BTU) by 2030 from the present

demand of 421 quads (2003) [8], a 71% increase largely due to growth in developing countries.

According to the same study fossil fuels will continue to supply much of the increment in

projected demands; however, depletion of fossil reserves is a matter of concern. Although oil

would remain an important energy source, its share in total energy consumption would decrease

from 38% in 2003 to 33% in 2030. This is largely in response to the higher world oil prices

which would be driven by rapid depletion of oil reserves in many parts of the world. Among all

sectors, transportation and industry continue to be the major oil consumers. Alternate fossil

sources such as natural gas are also limited. According to a recent study conducted by British

Petroleum, the Reserves to Production ratio (R/P) of natural gas in the US is less than 10 [9].

Hence, developing alternate energy carriers is necessary. In recent years, hydrogen has gained

recognition as a potential substitute to fossil fuels. Some of the factors favoring hydrogen are

* lack of green-house gas emissions when combusted or used in a fuel cell

* high energy content on a mass basis as compared to gasoline or natural gas

* easy and efficient conversion to electricity using fuel cells

Hydrogen is an important raw material for chemical, petroleum and agro-based industries.

The demand for hydrogen in the hydrotreating and hydrocracking of crude petroleum is steadily

increasing [10, 11]. Hydrogen is catalytically combined with various intermediate processing

streams and is used in conjunction with catalytic cracking operations to convert heavy and

unsaturated compounds to lighter and more stable compounds. Large quantities of hydrogen are









used to purify gases such as argon that contain trace amounts of oxygen. This is done by catalytic

combination of oxygen and hydrogen followed by removal of the resulting water. In the food and

beverages industry, it is used for hydrogenation of unsaturated fatty acids in animal and

vegetable oils, to produce solid fat and other food products. Hydrogen is also used as a carrier

gas in the manufacture of semi conducting layers in integrated circuits. The pharmaceutical

industry uses hydrogen to make vitamins and other pharmaceutical products. Hydrogen is mixed

with inert gases to obtain a reducing atmosphere which is required for many applications in the

metallurgical industry such as heat treating steel and welding. It is often used in annealing

stainless steel alloys, magnetic steel alloys, sintering and for copper brazing. It is also used as a

reducing agent in the float glass manufacturing industry.

Hydrogen is consumed in the production of methanol [12], synthesis of ammonia [13],

methanol to gasoline synthesis [14] and also for hydrocarbon synthesis by Fischer Tropsch

processes [15]. In recent times, the US government has tightened regulations on automotive

tailpipe emissions, thereby cutting down the benzene and sulfur compounds in gasoline. Hence,

more hydrogen is now needed in refineries for processing of heavy crudes and for

desulphurization in order to meet the product quality standards. Presently, fossil fuels such as

gasoline and diesel are used all over the world and have a well-established infrastructure. These

fuels will continue to be in use until a long term substitute that is environmentally friendly and

economically feasible is found. Hence the hydrogen demand for processing these fuels must be

met.

As a fuel, hydrogen is considered to be very clean as it releases no carbon or sulfur

emissions upon combustion. The energy contained in hydrogen on a mass basis (120 MJ/kg) is

much higher than coal (35 MJ/kg), gasoline (47 MJ/kg) and natural gas (49.9 MJ/kg) [16].









However, on a volumetric basis hydrogen has lower energy density. Moreover chemical energy

stored in hydrogen can be directly converted into electricity by a fuel cell. The conversion

efficiency of a fuel cell is higher than conventional combustion engines, thereby making fuel

cells attractive energy conversion devices (and hence hydrogen an attractive fuel) for

transportation and stationary applications. Hydrogen has long been a fuel of choice for the jet

propulsion and space industry. NASA has been using liquid hydrogen to fuel the space shuttle's

main engine and hydrogen fuel cells provide onboard electric power. The space crew even drinks

the water produced by the fuel cell's chemical process.

The rapid developments in fuel cells have prompted many automotive companies and the

US government (through the Department of Energy) to speed up research efforts on hydrogen

production. In 2003, the US government announced a $1.2 billion commitment over 5 years to

accelerate hydrogen research to overcome obstacles in the commercial development of fuel cells

[17].

Many experts predict that hydrogen will eventually power tomorrow's industries and

thereby may replace coal, oil and natural gas [18, 19]. However it will not happen until a strong

framework of hydrogen production, storage, transport and delivery is developed. All the steps

must be technically feasible and economically viable.

Introduction to Present Research

Hydrogen is not found in free-state in nature. It is normally combined with other elements

such as carbon, oxygen, sulfur, chlorine and so on. Hydrocarbons are a common resource, and

steam reforming of hydrocarbons (methane) is a popular method of present day hydrogen

production. However, producing hydrogen from hydrocarbons does not address the

environmental concerns as the problem gets merely shifted from the automotive tailpipe to some

remote location where hydrogen is produced. In order to have environment friendly hydrogen we









must be able to produce it from renewable resources. Table 1-1 gives a summary of the various

methods used for producing hydrogen. The table lists the present status of technology and the

cost of producing hydrogen. Of all the renewables, biomass is a promising resource with a good

potential for hydrogen production. In fact, considering the CO2 penalty which may be imposed

on fossil fuels, biomass has the potential to become cost competitive even with fossil fuels.

Biomass is a resource that is abundantly available in many parts of the world.

The chemical energy stored in biomass can be converted to hydrogen by biological or

thermal methods. The current research investigates the thermal pathway of converting biomass to

hydrogen. Thermo-chemical biomass gasification has been used for a long time for producing

syngas (a mixture of CO and H2). Biomass when gasified in the presence of a suitable medium

(such as steam) produces a gas mixture rich in CO and H2 and containing other gases such as

CH4, CO2 and small amounts of higher hydrocarbons. Biomass gasification is characterized by a

number of reactions that are exothermic and endothermic which suggests that heat can be

transferred internally to improve the process efficiency. A thermodynamic analysis would

determine the necessary conditions for maximizing the process efficiency and hydrogen yield at

equilibrium. A review of the literature showed us that such a thorough thermodynamic analysis

has not been performed for hydrogen production from biomass. Therefore a thermodynamic

analysis was conducted with the end objective of improving the process efficiency and also to

determine the conditions necessary for maximizing the hydrogen yield at equilibrium. The

important variables that influence the hydrogen yield are gasification temperature, gasification

pressure, steam to biomass ratio and equivalence ratio. All these parameters were varied over

typical range encountered in real life gasification systems. The gasifier temperature strongly

influences the hydrogen yield in product gas. In actual practice, the kinetics of biomass









gasification reactions are fast at temperatures above 700C. At these temperatures, lot of gases

and some liquid volatiles are released. At lower temperatures, more liquids are formed and these

may settle and clog the downstream equipment. At higher temperatures there is more gas in the

product stream (due to reforming of all the hydrocarbons); however this would also require a

high temperature heat source. At high temperatures (above 850C), the water gas shift reaction

occurs in the reverse direction and reduces the hydrogen yield. The gasification pressure too

affects the hydrogen yield. Most biomass gasifiers operate at atmospheric pressure. High

pressure systems reduce the equilibrium hydrogen yield. Low pressure (sub-atmospheric)

systems increase the hydrogen yield, but the increase is only marginal and hence the optimum

pressure for hydrogen production is one atmosphere.

Steam to biomass ratio also strongly influences the amount of hydrogen produced and

process efficiency. In general, when more steam is supplied the hydrogen yield is higher due to

reformation of hydrocarbons. Equivalence ratio, which is a measure of the amount of air supplied

during biomass gasification, is another variable that affects the amount of hydrogen produced.

The hydrogen yield in biomass gasification is limited by chemical equilibrium constraints.

There is an optimum temperature, pressure, steam to biomass ratio and equivalence ratio at

which the highest hydrogen yield occurs. In order to further enhance the hydrogen yield, the

equilibrium constraint has to be removed. This is possible by removing one of the co-products of

gasification (C02) that influences the equilibrium. If we can continuously remove the CO2 as

soon as it is formed, the shift reaction goes to completion and yields a hydrogen rich gas.

In the past, sorbents such as calcium oxides have been used to remove CO2 from the fossil

fuel exhaust. If the CO2 absorption reaction can be coupled with biomass gasification and water

gas shift reactions, we can produce a gas rich in hydrogen with small amounts of CO, CO2, CH4









and other impurities. Furthermore, the exothermic CO2 absorption reaction can be coupled with

the endothermic biomass steam gasification reaction. This would enable in-situ heat transfer and

reduce the net energy consumed by the gasifier. With reduced heat duty, the gasifier will become

compact and this will reduce the capital cost of the system. Process simulations were carried out

in ASPEN to study the effect of sorbent addition on hydrogen yield. The temperature, pressure,

steam to biomass ratio and sorbent to biomass ratio were varied over a wide range and the

hydrogen yield was determined. An energy analysis was then carried out to determine the

efficiency and energy consumption of the conventional and sorbent enhanced processes. An

improvement in the hydrogen yield of about 19% and reduction in product gas CO2 of about

50.2% was observed. The gasifier heat duty was reduced by about 42%.

Based on the promising results of the simulations an experimental setup was fabricated and

tests were carried out. The experimental studies showed a substantial improvement in the

hydrogen yield while using sorbents. A hydrogen rich product gas was obtained by steam

gasifying Southern pine bark in the presence of calcium oxide. Thereafter, some studies on

regeneration of used sorbent were carried out. The dissertation provides a detailed theoretical and

experimental investigation of hydrogen production by steam gasification of biomass in the

presence of sorbents.









Table 1-1: Summary of hydrogen production methods


Method Energy
Efficiency'


SMR


Partial
Oxidation

Autothermal
reforming

Coal
gasification

Biomass
t Gasification
Biomass
Pyrolysis

Electrolysis


Thermo-
chemical


Photocatalytic

Biological


83%


70-80%


71-74%


63%


40-50%

56%


25%2


42%
(850C)


10-14%
(theoretical)
24%
(speculative)


H2 production
Cost
$ 0.75 /kg
(w/out CO2
sequestration)
$ 1.39/kg
(Residual oil)

$1.93 /kg


$0.92/kg
(w/out CO2
sequestration)
$1.21-2.42/kg

$1.21-2.19/kg


$2.56-2.97/kg
(Nuke source)

$2.01/kg
(Sulfur-Iodine
cycle)

$4.98/kg

$5.52/kg


Scale/Current Status Major Advantage Major Disadvantage


Large/ Currently
available

Large/Available for
large hydrocarbons

Large/ Currently
available

Large/ Currently
available

Mid-size/Currently
Available
Mid-size/Currently
Available

Small/ Currently
available

Under research



Under research

Under research


'Efficiency is defined as the ratio of lower heating value of hydrogen in
2Includes the efficiency of electricity production


Proven technology
High Efficiency
Economically favorable
Proven technology
Economically feasible
Methane pipeline in place
Proven technology
Cheaper reactor than SMR
Methane pipeline in place
Proven technology
Economically favorable

Renewable
No foreign imports
Renewable
Easily Transportable

Proven technology
Emission free when used
with renewables
Emission free
No dependence on fossil
fuel sources

Renewable
No fossil dependence
Renewable
No fossil dependence


CO2 by-product
Limited methane supply

CO2 by-product
Lower efficiency than SMR

CO2 by-product
Limited methane supply
Lower efficiency than SMR
CO2 by-product
Less H2 rich than SMR

Seasonal availability
Transportation problems
Seasonal availability
Varying H2 content of
feedstocks
Low overall efficiency
High cost
Current capacities still small
High capital costs
Severe operating conditions
Highly corrosive conditions
(UT-3; sulfur Iodine)
Costly
Low efficiency
Low efficiency
High capital cost


product gas to total energy supplied to the process














CHAPTER 2
HYDROGEN PRODUCTION METHODS

Introduction

Hydrogen is the most abundant element found in the universe. However as compared to

fossil fuels, hydrogen does not occur in free-state in nature. It normally exists in combined state

with other elements. Hydrogen is bound with carbon in all hydrocarbons; it is bound with oxygen

in water and is found in many other compounds such as hydrogen sulfide, hydrogen iodide,

hydrochloric acid and so forth. The bound hydrogen can be separated by various methods like

thermal, electrochemical, photolytic or biological methods. The next few sections describe the

different methods used for producing hydrogen from various sources. Some of these methods are

used commercially, others are near commercial stage development and there are still others

which are at research stage.

Hydrogen Production Methods

Steam Methane Reforming (SMR)

SMR produces hydrogen in the following three steps [20]:

* methane is first catalytically reformed at elevated temperature and pressure to produce
synthesis gas (synthesis gas or syngas is a mixture of H2 and CO) (equation 2.1)

* a catalytic Water Gas Shift (WGS) reaction is then carried out to combine CO and H20 to
produce additional hydrogen (equation 2.2)

* the hydrogen product is then separated by adsorption

The reforming step occurs as per the following reaction (refer Figure 2.1). The reforming

reaction is endothermic and so energy has to be supplied to the process. Methane is treated with

high temperature steam to produce a mixture of H2, CO, CO2 and other impurities. The reaction









is carried out in a reformer containing tubes filled with nickel catalyst at temperatures between

500C and 950C and a pressure of 30 atmospheres. Excess steam promotes the second step in

the process the conversion of syngas to the desired end product (hydrogen) as per the Water-

Gas Shift reaction

CH4+H20->CO+2H2 AH= +206kJ/mol (2.1)

CO+H20->CO2+H2 AH = -41kJ/mol (2.2)

The third step of hydrogen separation is conventionally accomplished by pressure swing

adsorption (PSA). After removing the hydrogen, the product gas may be treated to remove CO2

if sequestration is desired.

SMR is the most widely used method for hydrogen production. High efficiency, favorable

economics and proven technology characterize the SMR process. SMR is ideal for large scale,

centralized hydrogen production. A disadvantage from an economic standpoint is that capture of

CO2 may be necessary in future resulting in additional capital and operating costs. Another

concern is the long-term availability of methane. For these reasons, SMR is considered as a

transition technology [20]. SMR may play an important role in helping make the switch to

hydrogen, but will most likely be replaced by other technologies for long term hydrogen

production.

Partial Oxidation or Autothermal Reforming of Methane

Partial oxidation (POX) and Autothermal Reforming (ATR) are similar alternatives to

SMR. The POX process partially oxidizes methane in a one-step reaction, while ATR combines

partial oxidation and reforming reaction, catalytically reacting methane with a mixture of steam

and oxygen. This differs from the steam methane reforming process which treats methane with









steam only. Partial oxidation of methane produces a syngas mixture of CO and H2 as per

following the reaction:

CH4 + 0.502 CO +2H2 AH= -36kJ/mol (2.3)

A catalyst is not required but has the potential to enhance the hydrogen yield and lower the

operating temperature. As the reaction is exothermic, careful design and control of special

reactors to facilitate heat exchange or dilution of reactants is necessary to prevent possible

explosion. An oxygen plant is usually installed on site to supply pure oxygen feed. Pure oxygen

is preferable because energy is wasted in heating and compressing the additional nitrogen gas if

air is used.

A more advanced partial oxidation process is autothermal reforming, a hybrid of partial

oxidation and SMR processes. Both the partial oxidation and reforming reactions take place

inside the autothermal reactor. The heat from the exothermic partial oxidation reaction supplies a

portion of the heat required by the endothermic reforming reaction. Because a portion of the feed

methane is burned within the reactor vessel as opposed to heating by an external furnace as in

SMR, less energy is required in autothermal reforming. This simplifies the design of the

autothermal reactor to one large vessel instead of the complex, bulky reactor design with many

tubes necessary for heat exchange in SMR.

At present, commercial processes for partial oxidation using methane feedstock do not

exist. This is mainly due to the lower efficiency of the partial oxidation process (70-80%) as

compared to more than 80% efficiency in the case of SMR. Commercial partial oxidation is a

mature technology when using other hydrocarbon feedstocks especially heavy residual oils (refer

figure 2.2) (examples are Texaco and Shell gasification processes). Small scale partial oxidation









units for methane are being developed for use in fuel cell systems, but are still in the research

phase [22].

Coal Gasification

Coal gasification involves three steps: treatment of coal feedstock with high temperature

steam (1300C) to produce syngas, a catalytic shift conversion, and purification of the hydrogen

product. In the first step, coal is chemically broken down by high temperature (1330C) and high

pressure steam to produce raw synthesis gas, as per the following reaction:

C +H20 > CO + H2 + impurities AH > 0 (2.4)

The heat required for this gasification step comes from controlled addition of oxygen,

which allows partial oxidation of a small amount of the coal feedstock. Because of this, the

reaction is carried out in either an air-blown or oxygen-blown gasifier. The oxygen-blown

gasifier is generally used to minimize NOx formation and make the process more compatible for

carbon dioxide sequestration. In the second step, the syngas passes through a shift reactor

converting a portion of the carbon-monoxide to carbon-dioxide and thereby produce additional

hydrogen

CO+H20 + CO2+ H2 AH = -41kJ/mol (2.5)

In the third step, the hydrogen product is purified. Physical absorption removes 99% of

impurities. The majority of H2 in the shifted syngas is then removed in a Pressure Swing

Adsorption (PSA) unit. In case of CO2 sequestration, a secondary absorption tower removes CO2

from the remaining shifted syngas. Coal is an attractive energy source due to its abundance in the

United States and low and traditionally stable prices. Coal gasification is an established

technology used in hydrogen production today, but additional technical and economic

considerations for capture and storage of CO2 will be necessary in future.









Biomass Gasification and Pyrolysis

Biomass refers to crops or other agricultural products including hardwood, softwood, and

other plant species. It may also include municipal solid waste or sewage, a fraction of which is

burned to produce steam for the process. Biomass may be used to produce hydrogen in two

ways: 1) direct gasification or 2) pyrolysis to produce liquid bio-oil for reforming.

Direct biomass gasification process is similar to coal gasification. The process is carried

out in three steps. First the biomass is treated with high temperature steam in an oxygen-blown

or air-blown gasifier to produce syngas mixture composed of hydrocarbon gases H2, CO, CO2,

tar and water vapor. Char (carbon residue) and ash are left behind in the gasifier. Then, a portion

of the char is gasified by reaction with oxygen, steam and hydrogen while another portion is

combusted to provide heat. As in the case of coal, the gasification step is followed by shift

reaction and purification. Alternatively, the biomass can first be reformed to a liquid (bio-oil) by

a process well known as pyrolysis. Pyrolysis is an endothermic process for thermal

decomposition of biomass and is carried out at 450-5500C. The bio-oil produced is a liquid

composed of oxygenated organic and water [23]. The bio-oil is steam reformed using a nickel-

catalyst at 750-8500C, followed by shift reaction to convert CO to CO2. Following are the

general reactions in biomass gasification and pyrolysis:

Biomass + steam /02 H2 + CO + CO2+ CJnH + impurities AH>0 (gasification) (2.6)

Biomass + energy -> bio-oil + char + impurities pyrolysiss) (2.7)

Bio-oil + steam CO + H2 (reforming) (2.8)

CO+H20 4 CO2+H2 AH = -41 kJ/mol (shift) (2.9)

Biomass gasification technology has over the years progressed from small laboratory scale

models to several demonstration pilot scale plants either for producing electricity or syngas. For









example BIOSYN Inc. is an oxygen-blown gasification process in a bubbling fluidized bed

gasifier with a bed of silica or alumina which is used for making methanol. There are several

commercial gasifier manufacturers in Europe and N. America and many of these are used for

producing power or syngas [24]. Biomass resource has the advantage of being renewable,

sulfur-free and being locally available. Hence it has a great potential for the future "hydrogen

economy". However, there are many factors limiting commercial biomass hydrogen production,

chief among them being

* high transport cost due to low energy density of biomass

* high capital cost of biomass plants

* seasonal availability

Pyrolysis is still at a relatively early stage of research and is not as mature as gasification.

However, among all the renewable resources used for hydrogen production, biomass is the one

which has the greatest potential for being commercialized in the near future (Table 1-1).

Electrolysis

Electrolysis uses electricity to dissociate water into diatomic molecules H2 and 02. An

electric potential is applied across a cell with two electrodes containing a conducting medium,

generally an alkaline electrolyte solution such as aqueous solution of potassium hydroxide

(KOH). Electrons are absorbed and released at the electrodes, forming hydrogen at the cathode

and oxygen at the anode. Under alkaline conditions, this process may be described by the

following reactions [25, 26]:

Cathode: 2H20 + 2e H2 + 20H (2.10)

Anode: 20H Y 0,2 +H20+2e (2.11)

Overall: H20 ,-H2 + X 02 (2.12)









The net effect is to produce H2 and 02 by supplying only water and electricity (refer figure

1.3). The theoretical voltage for the decomposition at atmospheric pressure and 25C is 1.23

volts (V). At this voltage, reaction rates are very slow. In practice, higher voltages are applied to

increase the reaction rates. However, this results in increased heat losses to the surroundings,

decreasing the efficiency. The necessary voltage may be lowered by using catalysts or

sophisticated electrode surfaces. Increasing temperature and pressure may also increase the

efficiency at the cost of additional material needed to resist corrosion or higher pressures [27].

There are broadly two types of electrolysis technologies: (1) solid polymer using a proton

exchange membrane (PEM) and (2) liquid electrolyte, most commonly potassium hydroxide. A

PEM electrolyzer is literally a PEM fuel cell operating in reverse mode. When water is

introduced to the PEM electrolyzer cell, hydrogen ions (protons) are drawn into and through the

membrane, where they recombine with electrons to form hydrogen molecules. Oxygen gas

remains behind in the water. As water is recirculated, oxygen accumulates in a separation tank

and can then be removed from the system. Hydrogen gas is separately channeled from the cell

stack and captured. Liquid electrolyte systems typically use a caustic solution and in those

systems, oxygen ions migrate through the electrolytic material, leaving hydrogen gas dissolved

in the water stream. This hydrogen is readily attracted from water when directed into a separating

chamber. Electrolysis is well suited to meet early stage fuelling needs of fuel cell vehicle market.

Electrolyzers scale down reasonably well; efficiency of electrolysis reaction is independent of

cell size. The US DOE has predicted an electrolytic hydrogen production cost of about $2.5/kg

by 2010 for hydrogen for a plant integrated with renewable energy [28].

Thermochemical Hydrogen Production

High temperature heat (500 2000C) drives a series of chemical reactions that produce

hydrogen and oxygen. The chemicals used in the process are reused within each cycle. This









process operates in a closed loop and consumes only water and produces hydrogen and oxygen

in separate steps. The high temperature heat needed for the process can be supplied by nuclear

reactors (up to 10000C) or by solar energy through concentrated solar collectors (up to 20000C).

Different cycles have been identified to operate in different temperature ranges. There are more

than a thousand cycles that have been proposed so far but only a few hold promise for large scale

implementation [29, 30]. Two of the popular thermochemical cycles are described below.

Zn/ZnO cycle

Zinc oxide is passed through a reactor heated by solar concentrator at about 1900C (refer

Figure 2.4). At this temperature zinc oxide dissociates into zinc and oxygen gases. Zinc is

cooled, separated and reacted with steam (at about 300 to 4000C) to produce hydrogen and solid

zinc oxide. The net products are hydrogen and oxygen with water as input. Hydrogen is later

separated and purified. The zinc oxide is recycled into the process to produce more hydrogen.

The reactions taking place are as under:

ZnO + heat Zn + 0.502 (2.13)

Zn + H20->ZnO + H2 (2.14)

H20 + heat H2 + 0.502 (Overall reaction) (2.15)

Haueter et al have developed a solar chemical reactor for thermochemical hydrogen production

based on Zn/ZnO cycle [31]

UT-3 cycle

The UT-3 cycle (University of Tokyo #3) was proposed by Kameyama and Yoshida in

1978 [32]. A UT-3 cycle is composed of a series of four thermochemical reactions. The

operating temperatures are relatively lower than those found in other thermochemical cycles, the

highest being 760C. When the reactions proceed in the correct order all the solid reactants are









regenerated, except water which is split into hydrogen and oxygen and separated from the

system. The reactions taking place are as under:

Reaction: CaBr2 + H20(g) 7600C >CaO(s) + 2HBr(g) (2.16)

Reaction 2: CaO(s) + Br2(g) 57C >CaBr2 + 0.502(g) (2.17)

Reaction 3: Fe304(s) + 8HBr(g) 220 > 3FeBr2(s) + 4H20(g)+ Br2(g) (2.18)

Reaction 4: 3FeBr2 + 4H20(g) 560C >Fe304 + 6HBr(g)+ H2(g) (2.19)

The UT-3 cycle has been extensively studied in Japan. It may have the potential for

commercial production of renewable hydrogen. At present investigations are going on into the

chemical kinetic aspects of the reactions involved in the UT-3 cycle.

There are other cycles too (like sulfur-iodine cycle) which are being pursued.

Thermochemical cycles are well suited for hydrogen production in conjunction with nuclear

energy. The Department of Energy allocated $4 million research budget for select

thermochemical cycles in the year 2003.

Photocatalytic Hydrogen Production

Photocatalyst materials (generally semiconductors) doped with other materials, catalyze

direct water splitting using solar energy. Examples of materials that have been shown to be

effective in catalyzing water splitting are oxynitirides, TaON, Ta3N5, and LaTiO2N, nickel doped

indium-tantalum-oxide catalysts, and CdS/ZnS systems. The water splitting takes place when the

catalyst is irradiated with light in the presence of an electron donor and acceptor, oxidizing OH-

ions to produce 02 and reducing H+ ions to H2. The semiconductor can also be paired with

catalysts to promote these oxidation and reduction reactions (refer Figure 2.6).

As a photocatalytic semiconductor material immersed in water is exposed to light, the

material absorbs photons causing valence electrons to jump to the conduction band (CB), leaving









behind positively charged holes in the valence band (VB). If the conduction band is at a higher

energy level than the reduction potential of hydrogen, the electrons in the conduction band can

reduce hydrogen ions at the surface of the semiconductor to produce hydrogen gas. The valence

bands are at a lower energy than the oxidation potential of hydrogen, so the positive holes accept

electrons from the hydroxide ions and oxygen gas is produced as illustrated in the figure 2.6.

Effective photocatalysts are those in which the conduction and valence band levels most

closely match the potential for reduction and oxidation of water. The photocatalytic hydrogen

production has been demonstrated at laboratory scale [35]. However, the technology is still not

feasible on commercial scale. Also, currently the process does not have the capability to produce

hydrogen in sufficiently large quantities (like SMR). Further research will determine whether

efficiency and cost of hydrogen production by photocatalytic water splitting will be competitive

with other hydrogen production methods.

Photoelectrochemical Hydrogen Production

In its simplest form a photoelectrochemical (PEC) hydrogen production cell consists of a

semiconductor electrode and a metal counter electrode immersed in an aqueous electrolyte.

When light is incident on the semi-conductor electrode, it absorbs part of the light and generates

electricity. This electricity is used for electrolytically splitting water. Hence a PEC cell is a

combination of a photovoltaic cell and electrolysis. Fujishima and Honda first demonstrated this

concept using solar energy in 1972 [37].

The cell consists of a semiconductor photoanode which is irradiated with electromagnetic

radiation. The counter electrode is a metal. Following processes take place when light is incident

on the semiconductor electrode.

S photogeneration of charge carriers (electron and hole pairs)
hv h + e (2.20)









where, h is the Planck's constant, v is the frequency, h' is the hole and e- is the electron.

* charge separation and migration of holes to the interface between the semiconductor and
electrolyte and of the electrons to the counter electrode through the external circuit. The
holes are simply vacancies in the valence band due to promotion of electrons from valence
band to conduction band. However, in the study of electronic behavior of materials holes
are considered to be independent entities with their own mass

S electrode processes: Water is oxidized to H+ ions and 02 gas by the holes at the photoanode
and the H ions are reduced to H2 gas by electrons at the photocathode

At photoanode: H20 + h 2H + 02 (2.21)

At photocathode: 2H + 2e H2 (2.22)

The efficiency of PEC cells for hydrogen production largely depends on the efficiency of

the photovoltaic cell. Due to the inherent low efficiency of PV cells, photoelectrochemical cells

are not very efficient in hydrogen production as compared to conventional processes. Typical

efficiency reached is around 5-6% [38] that too when multi band gap thin film PV cells are used.

There are many issues other than low efficiency that need to be addressed such as corrosion

resistance of the semiconductor material, optimization of the electrolyte and cost of photovoltaic

cells. This method is still under research and the success of this method largely depends on the

improvements made in photovoltaic materials and their performance.

Biological Hydrogen Production

Biological methods for hydrogen production have been known for over a century. Broadly there

are two methods by which hydrogen can be produced:

Fermentation of bacteria

Fermentation by anaerobic bacteria as well as some microalgae (such as green algae) on

carbo-hydrate rich substrates can produce hydrogen at 30 to 80 C in the absence of sunlight and

oxygen. The products of fermentation mainly include H2 and CO2 with small quantities of other









gases such as CH4 or H2S depending on the reaction process and substrate used. With glucose as

model substrate, a maximum of four moles of H2 are produced per mole of glucose

C6H1206 + 2H20 2CH3COOH + 4H2 + 2CO2 (2.23)

The actual amount of hydrogen produced depends on the pH value, the hydraulic retention

time as well as the gas partial pressure [39].

Biophotolysis

Biophotolysis uses the same principle found in plant and algal photosynthesis, but adapts

them for the production of hydrogen instead of carbon containing biomass. Photosynthesis

involves absorption of light by two distinct photosynthetic systems operating in series: a water

splitting and oxygen evolving system (photosystem I or PSI) and a second photosystem (PSII)

which, generates the reductant used for CO2 reduction. In this coupled process, two photons

(one per photosystem) are used for each electron removed from water and used in CO2 reduction

or H2 formation. In green plants, only CO2 reduction takes place, as the enzymes that catalyze H2

formation, (the hydrogenase) are absent. Microalgae (such as cynobacteria) have hydrogenase

enzyme and hence can be used to produce H2 under certain conditions [40]. The overall reaction

is given by:

220 sar ) 2H2+02 (2.24)

Although technologies for biological hydrogen production are available, they are still not

mature for commercial production. There are many technical barriers and some of them include:

* lack of characterization of microorganisms for hydrogen production

* low light conversion efficiency (less than 10%) for photolytic hydrogen production

* low hydrogen production rate to be commercially viable

* hydrogen re-oxidation by the hydrogenase enzyme









Due to the inherent technical problems the cost of hydrogen produced from biological

methods is still very high as compared to conventional methods such as Steam Methane

Reforming.

Summary

A summary of the different hydrogen production methods is provided in Table 1-1. It is

observed that currently SMR offers the lowest hydrogen production cost. SMR is also a proven

technology with very high energy efficiency. However the natural gas reserves within the US are

limited and hence SMR is considered as a transition phase to the "hydrogen economy". Partial

oxidation and autothermal reforming are possible alternatives to SMR, but both these methods

are less efficient. Also, the cost of hydrogen production by these methods is higher than SMR.

Coal gasification is cost-competitive but CO2 by-product removal is a matter of concern.

Electrolysis is a proven technology but is currently expensive. Also, capacities are very small

and hence scale-up is required for bulk hydrogen production. Thermochemical water splitting

process is clean (no emissions); however it is complicated by several reactions and severe

operating conditions. These methods are still under research. Biological and photocatalytic

methods are both renewable but at the same time are expensive. The efficiency is also very low.

Both these methods are also currently under research.

Of all the renewables, biomass is a promising resource for producing environment friendly

hydrogen. In fact, considering the CO2 penalty which may be imposed on fossil fuels, biomass

has the potential to become cost competitive with fossil fuels. The drawbacks of biomass are

seasonal availability, high feedstock and capital costs. Hydrogen can become a fully renewable

energy carrier only if the raw materials and methods used for producing it are renewable.












steam


REFORMER

Fuel is catalytically converted to
syngas. Some carbon dioxide and
unreacted fuel remains


FEED
FEED


SHIFT REACTION
HEAT
RECOVERY CO + H,0-CO, + H,






H2
HYDROGEN
SEPARATION


Figure 2.1: Block diagram of hydrogen production by steam methane reforming (adapted from
Sherif et al [21])















GASIFICATION


Carbon-dioxide, hydrogen, steam and small
amounts of CH4 are produced as the raw gas


Syngas SHIFT REACTION


CO+ H20O-CO2 +H2


steam


Sulfur


Hydrocarbon Feed


Figure 2.2: Block diagram of hydrogen production by partial oxidation of heavy oils (adapted
from Sherif et al [21])


Air N2














Pure Oxygen


Porous Cathode


Porous Anode


Gas-Tight
Electrolyte
Figure 2.3:Principle of hydrogen production by high temperature electrolysis (HTE)


(-)


Hydrogen
in Steam














Steam
























Figure 2.4: Block diagram of the Zn/ZnO water splitting thermochemical cycle for hydrogen
production (adapted from Weidenkaff [33])




































CaOt 8rl Fe34+BHBr
--CaBr2 +1 2 0I --3FeBr2 4H2O* Br2



Figure 2.5 UT-3 cycle reactions and flow of material (adapted from Aochi et al [34])

















H2

hv


Catalyst 2
VB

02 + 2H+


20H-


Figure 2.6:Photocatalytic hydrogen production (adapted from Oudenhoven et al [36])












2H'+ 2e-- H2


H20 + 2h--N
2H + 0.502


Figure 2.7: Principle of photoelectrochemical hydrogen production (adapted from Fujishima
et al [37])









CHAPTER 3
BACKGROUND AND LITERATURE REVIEW

Introduction

Wood and other forms of biomass including energy crops and agricultural and forestry

wastes are some of the main renewable energy sources available for hydrogen production.

Biomass is considered the renewable energy source with the highest potential to contribute to the

transportation energy needs of modern society for both the developed and developing economies

around the world [41, 42]. Biomass can be converted to liquid and gaseous fuels via thermal,

biological and physical processes.

In the thermal technique there are four methods suitable for the conversion of biomass:

pyrolysis, gasification, liquefaction or direct combustion and primary products of these processes

can be gas, liquid, solid char and/or heat depending on the conversion technology employed.

Secondary higher value products may be produced by additional processing.

Pyrolysis

Pyrolysis is the thermal degradation (devolatization) of biomass in the absence of an

oxidizing agent at temperatures in the range 200-500C. This leads to the formation of a mixture

of liquids, gases and highly reactive carbonaceous char, the relative proportions of which

depends on the heating rate. The products can be used in a variety of ways. The char can be

upgraded to activated carbon, used in the metallurgical industry, as a domestic cooking fuel or

for any suitable application. Pyrolysis gas can be used for power generation or heat, or

synthesized to produce methanol. The tarry liquid (called bio-oil) can be upgraded to high grade

hydrocarbon liquid fuels for combustion engines or used directly for power generation or heat.









Gasification

Gasification (also called pyrolysis by partial oxidation) is a conversion process in which

the goal is to maximize the gaseous product yield. Relatively higher temperatures of 800-1100C

are used compared to 200-5000C in pyrolysis. The gaseous mixture produced contains H2, CO,

CO2, CH4, H20, and N2 (if air is used as the gasifying medium) and various contaminants such as

small char particles, small amounts of ash and tars. Air gasification produces a low heating value

(LHV) gas (4-7 MJ/Nm3). The fuel gas can be burned externally in a boiler for producing hot

water or steam, in a gas turbine for electricity production or in an internal combustion engine. It

can also be upgraded to methanol through synthesis. Before the fuel gas can be used in gas

turbines or internal combustion engines, the contaminants (tar, char-particles, ash) have to be

removed. The hot gas from the gas turbine can be used to produce steam to be utilized in a steam

turbine in an Integrated Gasification Combustion Cycle (IGCC).

Combustion

Combustion is complete oxidation of the biomass feedstock. Combustion provides very hot

gas that can be used to (1) heat a boiler and produce steam for process application (2) as a source

of process or space heat (3) as the energy source for Rankine cycle or Stirling engines. Typically,

temperatures of the order of 1200C are encountered in combustion.

Liquefaction

Liquefaction is a low temperature (250-300C), high pressure (100-200 bar) thermo-

chemical conversion to convert biomass into liquid phase, usually in the presence of a catalyst.

The main goal here is to maximize the liquid yield, and the product is a higher quality liquid (in

terms of heating value) than the one produced in pyrolysis.

Of all the methods, biomass gasification has attracted the greatest interest as it offers

higher efficiencies than combustion [42, 43]; other technologies (fast pyrolysis & liquefaction)









are still at a relatively early stage of development [42]. Thermochemical biomass gasification has

been identified as a possible method for producing renewable hydrogen [44]. Figure 3.1 shows a

photograph of a pilot biomass gasification plant which uses peanut shells as feedstock for

producing hydrogen.

Lab-Scale Production of Hydrogen from Biomass

A schematic of the experimental set-up used for producing hydrogen by gasification of

biomass is shown in figure 3.2. Here biomass is fed continuously using a screw feeder to a

fluidized-bed gasifier and steam is used as the gasifying medium. The gas coming out of the

gasifier is passed through a metallic filter before being sent to a catalytic reactor.

The catalytic reactor reforms tars and higher hydrocarbon in the product gas to produce

additional hydrogen. The gas is then cooled to condense and remove the steam and then passed

through a filter to get rid of ash, dust and particulate matter. The clean, dry gas coming out of the

filter is then sent to a gas-chromatograph for composition analysis. Any suitable biomass can be

used as a feed to the gasifier. Biomass feeds can be agricultural wastes, energy crops, municipal

solid wastes, woody and tree material and so forth. Table 3-1 gives the chemical composition,

ultimate and proximate analysis and heating value of sawdust which is a typical biomass

feedstock [44]. The C-H-O (Carbon-Hydrogen-Oxygen) composition for any biomass is

approximately the same; feedstocks differ from each other in the amount of mineral matter

(alkaline material) and moisture content. For comparison the chemical composition, ultimate and

proximate analysis and heating value for a grade of coal (found in Belmont, Ohio) is also

provided [16]. From the chemical composition it is seen that biomass feedstock has much less

sulfur as compared to coal. This is another reason why a biomass feedstock is preferable over

coal. However, oxygen content in biomass is higher than coal. Typically biomass consists of

about 6% hydrogen by weight. The hydrogen yield of plain biomass gasification can be









substantially improved if we use steam as a gasifying medium (this is explained in detail in the

next chapter)

An Overview of Biomass Gasification for Hydrogen Production

Biomass gasification has been extensively studied over the last three decades in the United

States and other countries around the world. Different research groups have investigated biomass

gasification with different objectives like optimizing syngas production, maximizing the overall

gas yield, hydrogen production, product gas cleaning for trouble-free downstream operation,

effective waste utilization and so on. The objective of the present research is to study biomass

gasification from the perspective of maximizing the hydrogen yield. A detailed literature review

was conducted to know the state of the art. The following sub-areas were identified:

* Catalysis

* Pretreatment technologies

* Chemical kinetic studies

* Experimental studies on biomass gasification

* Thermodynamics of gasification

* Sorbent enhanced gasification

Catalysis

Biomass thermo-chemical gasification produces gases, liquids and solids. The product

contains as major components H2, CO, CO2, CH4, H20 and N2, smaller amounts of

hydrocarbons, inorganics (H2S, HC1, NH3, alkali metals) and particulate matter. The organic

impurities range from low molecular weight hydrocarbons such as methane to high molecular

weight polynuclear aromatic hydrocarbons. The low molecular weight hydrocarbons can be used

as fuel in gas turbine or engine applications, but are undesirable products in fuel cell applications

and methanol synthesis. The high molecular weight hydrocarbons are collectively known as









"tars". Tars are undesirable in Integrated biomass Gasification Combined Cycle systems (IGCC)

for a number of reasons. They can condense in exit pipes and on particulate filters leading to

blockages and clogged filters. Tars also have varied impact on other downstream processes.

They can clog fuel lines and injectors in internal combustion engines. The product gas from an

atmospheric pressure gasification process needs to be compressed before it is combusted in a gas

turbine and tars can condense in the compressor or in the transfer lines as the product gas cools.

Biomass gasification product gas requires substantial conditioning including tar conversion or

removal, before it is used in polymer electrolyte membrane (PEM) fuel cell systems that require

essentially pure hydrogen.

There are a number of methods to separate or reform tars from the product gas like wet

scrubbing, thermal cracking or catalytic cracking. Wet scrubbing involves cooling the gas in

order to condense the tars. This technique does not eliminate tars but merely transfers the

problem from gas phase to condensed phase. Thermal cracking is a hot gas conditioning option

but it requires high temperatures (more than 1100C) to achieve high conversion efficiencies.

This process may also produce soot which is an unwanted impurity in the product gas stream.

Catalytic steam reforming is an attractive hot gas conditioning method. Catalytic tar destruction

has been studied for several decades and a number of reviews have been written in biomass

gasification hot gas clean up [46-48]. Broadly three groups of catalyst materials have been used

for biomass gasification systems: alkali metals, non-metallic oxides, and supported metallic

oxides. Alkali metals enhance biomass gasification and are therefore considered primary

catalysts and not tar reforming catalysts. Alkali salts are mixed directly with the biomass as it is

fed into the gasifier. The non-metallic and supported metallic oxide catalysts are usually located









in a separate fixed bed reactor, downstream from the gasifier, to reduce the tar content of the

gasification product gas and are therefore referred to as secondary catalysts.

Non-Metallic Oxides

Calcined dolomites have been extensively investigated as biomass gasifier tar destruction

catalysts. Dolomites are calcium-magnesium ore with the general formula CaMg(CO3)2. These

naturally occurring catalysts are relatively inexpensive and disposable. So it is possible to use

them as primary (in bed) catalysts as well as in secondary (downstream) reactors. Several

research groups have conducted extensive studies on the tar conversion effectiveness of calcined

dolomites and other non-metallic oxide catalysts. Simell and co-workers [49] performed a

number of studies using model compounds to test the reforming effectiveness of dolomites. The

catalysts were calcined at 9000C and showed high toluene conversion efficiencies (>97%);

however catalyst activity was almost completely lost when the CO2 partial pressure was higher

than equilibrium decomposition pressure of dolomite. Simell et al also reported decomposition of

benzene when it was passed over Finnish dolomite at 9000C.

Aznar and co-workers [50] constructed a biomass gasification pilot plan to study catalytic

product gas conditioning. The gasifying agents used were air, steam and a mixture of steam and

oxygen, and pinewood was fed into the bottom of the bubbling bed. It was found that when 20g

of calcined dolomite per kg of biomass was added, the tar content in product gas decreased by a

factor of 4 to 6. They also observed that the hydrogen content of the product gas doubled and CO

content reduced by a factor of two. Several other groups have also studied catalytic tar reforming

with dolomites [51, 52]. All of these studies demonstrate that dolomite is a very effective tar

reforming catalyst. High molecular weight hydrocarbons are efficiently removed at moderately

high temperatures (800C) with steam and oxygen mixtures as the gasifying agent; however

methane concentration is not greatly affected and benzene and naphthalene are often not









completely reformed. A problem with dolomites which is reported by many investigators is a

decrease in mechanical strength over time, which leads to catalytic attrition.

In summary, dolomites are inexpensive disposable chemicals that can be mixed with

biomass and used as primary catalysts. They are mainly used for reforming many high molecular

weight tar compounds. Dolomites however undergo attrition over a period of time and need to be

replenished. Another problem with dolomites is the waste stream they generate once they

undergo attrition.

Commercial Nickel Reforming Catalyst

A wide variety of Ni-based reforming catalysts are commercially available because of their

application in the petrochemical industry for naphtha reforming and methane reforming to make

syngas. Nickel based catalysts have also proven to be very effective for hot conditioning of

biomass gasification product gases. They have high activity for tar destruction; methane in the

gasification product gas is reformed, and they have some water gas shift activity to adjust the

H2/CO ratio of the product gas. The H2 and CO contents of the product gas increase, while

hydrocarbons and methane are eliminated or substantially reduced for catalyst operating above

approximately 740C.

The groups that were active in studying calcined dolomite catalysts have conducted several

studies involving nickel steam reforming catalysts too for hot gas conditioning. Aznar and

coworkers [53] conducted several experiments with Ni-catalyst at temperatures between 750 and

850C and found initial tar conversion efficiency to be greater than 99%. An apparent kinetic

model for tar reforming was determined for each catalyst tested based on a first order rate

expression and the measured tar conversion as a function of time-on-stream. The kinetic studies

gave an idea of the activation energy and pre-exponential factors obtained for the tar conversion

reactions.









Simell and coworkers [54] have also investigated commercial Ni steam reforming catalyst

for tar conversion using toluene as a model tar compound. They observed complete tar

decomposition for catalyst operating at 9000C and 5 MPa. Kinoshita, Wang and Zhou [55]

reported results from parametric studies on catalytic reforming of tars produced in a bench-scale

gasification system. A commercial Ni-catalyst (UCG-90 B) was tested at various temperatures

(650-800C), space times (0.6-2.0s), and steam to biomass ratios (0-1.2) in a fluidized bed

catalytic reactor. They reported achieving 97% tar conversion; product gas yield was higher in

presence of the catalyst.

Several other groups (Bangala et al [56] & Wang et al [57]) have reported high

effectiveness of Ni-catalyst (>90%) in tar reforming. However there are several factors which

still limit the use of Ni-catalyst in commercial gasifiers which need to be addressed. Some of the

main limitations include sulfur, chorine and alkali metals present in the gasification product gas

which act as catalyst poisons. Coke formation on the catalyst surface can also be substantial

when tar levels in the product gas are high. Coke can be removed by regenerating the catalyst;

however repeated high temperature processing of nickel catalyst can lead to sintering, phase

transformations and volatilization. To sum up, commercial nickel reforming catalysts have

shown very high tar conversion potential (more than 90%). However these catalysts suffer from

frequent de-activation due to poisoning by sulfur, by halides and by alkaline impurities.

Additional Catalyst Formulations

There are several limitations of Ni reforming catalysts used for tar conversion such as

deactivation by coke formation, sulfur and chlorine poisoning and sintering. Addition of various

promoters and support modifiers has been attempted by several groups to improve catalyst

activity, lifetime, poison resistance, and resistance to coke formation. Rapagna et al [58]

developed a catalyst with a Lanthanum additive (chemical formula LaNio.3Feo.703) that was









prepared by sol-gel process. The prepared catalyst displayed high CH4 reforming activity at

500C resulting in 90% CH4 conversion. Garcia et al [59] have prepared a number of Ni-based

catalysts with different additives for optimal hydrogen production. They added Magnesium and

Lanthanum as support modifiers, and Cobalt and Chromium were added to reduce coke

formation. The Cobalt-promoted and Chromium-promoted Nickel catalyst on a MgO-La2O3-a-

A1203 support performed best in terms of yield and life time. Sutton and co-workers [60] studied

the effect of different supports using Ni-catalyst. The research group impregnated Ni on various

supports including A1203, ZrO2, TiO2, Si02 and a proprietary tar destruction support. High tar

conversion was observed for all of the prepared catalysts.

Drawing a parallel from the auto-industry, Asadullah and co-workers [61, 62] have

developed a novel series of catalysts using noble metals on oxide supports. These catalysts were

prepared with Rhodium, Ruthenium, Platinum and Palladium and were tested on bench-scale

fluidized-bed reactors using cellulose as a model biomass compound. The group found more than

80% tar conversion at temperatures as low as 550C. Different supports were used such as CeO2,

LiO2, ZrO2, A1203, MgO and Si02. It was found that Rh/CeO2 gave 100% tar conversion at

550C. The group observed that although these catalysts give 100% tar conversion at relatively

low temperatures (500 to 6000C), they are not economically viable. This is mainly due to the

high cost associated with the noble metal to the catalyst formulation.

Several catalysts have been investigated for tar reforming of biomass product gases. A

critical gap identified for catalytic tar reforming technology in biomass gasification processes is

the need for extended lifetime of promising commercial or novel catalysts. Catalytic hot gas

conditioning will not become a commercial technology unless adequate catalyst lifetimes can be

demonstrated, even for inexpensive, disposable catalysts like calcined dolomite. Frequent









disposal of dolomite generates an additional waste stream and disposal of toxic spent Ni-

catalysts becomes an environmental burden. Assessment of catalyst lifetimes will allow biomass

gasification developers to actually evaluate the cost of operating a biomass gasification plant.

The effect of catalyst poisons like sulfur, chlorine and alkali metals and continued catalyst

regeneration can be critically evaluated with long term catalyst testing. Accurate catalyst cost

and lifetime figures will provide important input for techno-economic analysis of developing

gasification technologies.

Pretreatment Technologies

Experimental and theoretical studies on different types of biomass have showed that

pretreatment increases the volatile (gas and liquid) yield of feed stocks. Pretreatment is carried

out by washing the biomass with mild acid or alkali or by impregnating them with salts before

actual gasification. It is hypothesized that during pretreatment the biomass undergoes de-ashing

(removal of mineral matter) which leads to higher gas and hence hydrogen yields. Pretreatment

for gasification or pyrolysis also increases the active surface area of biomass. In some cases

(especially bio-oils) the heating value of pretreated biomass is higher than the original biomass

feedstock.

Das and Ganesh [63] subjected sugarcane baggase to three different pretreatments (water

leaching, mild HC1 treatment and mild HF treatment) and found that the HF treatment reduces

ash content of biomass to a negligible amount. The researchers also observed that the char

produced in the process had a higher adsorption capacity as compared to untreated biomass.

Raveendran and co-workers [64] impregnated a variety of biomass feed stocks with chloride

(KC1, ZnCl2) and carbonate (K2CO3 ZnCO3) salts and found that the gas yield increased

substantially. The group later developed a correlation to predict the percentage change in gas

yield when any biomass is subjected to potassium and zinc salt pretreatments. Conesa et al [65]









subjected different almond shell samples to acidic and basic pretreatment followed by CoC12

(cobalt chloride) impregnation. The samples were then gasified and the gas composition was

determined. The group found that the hydrogen yield of CoC12 treated almond shells was higher

than plain almond shell. All the research groups have hypothesized that acid, alkaline or salt

pretreatment alters the mineral matter content of raw biomass. This in turn affects the product

yields since the mineral matter generally tends to have a catalytic effect during the gasification

process. In general, biomass pretreatment is a technique of modifying the bio-chemical

ingredients of feedstock and thereby controlling the gas and hence the hydrogen yield.

In a nutshell, biomass pretreatment is a simple and cost-effective way of influencing the

product yield of any biomass gasification process. The process generally applies well to biomass

with large mineral matter (Na, K, Ca, Mg, Fe, and P) content such as switch grass and rice husk.

Chemical Kinetic Studies

The development of thermo-chemical process for biomass conversion and proper

equipment design requires a thorough knowledge and good understanding of several chemical

and physical processes occurring in the thermal degradation process. Mathematical modeling and

simulation of single representative biomass particle is a very useful tool for understanding the

heat and mass transfer and chemical kinetic processes involved in biomass gasification or

pyrolysis. When a solid biomass is heated following phenomena occur:

1) heat is transferred by radiation and/or convection to the particle surface and then by

conduction to inside of the particle.

2) the temperature inside the particle increases causing

a) evaporation of moisture present in the biomass particle

b) pre-pyrolysis and pyrolysis reactions









c) mass transfer from surface of biomass particle due to formation and subsequent release of

volatiles

This leads to the formation of pores in solid surface. During the process, the pores of the

solid enlarge and this offers many reaction sites to the volatile and gaseous products. Chemical

kinetic studies predict the transient temperature profile within the biomass particle as well as the

yield of solid, liquid and gaseous products with time. This is done by mathematically modeling

the combined effects of heat transfer and chemical reactions. The model is then verified with

experimental results. On the experimental side, Thermal Gravimetric Analysis (TGA) of a single

biomass particle gives the rate of mass loss versus time and temperature. This can be used to

obtain the kinetic data (rate constant and activation energy) of biomass thermal degradation. The

chemical kinetic data so obtained serves as a basis for detailed design of fixed and fluidized bed

biomass reactors.

Several researchers have analyzed the chemical kinetics of biomass pyrolysis and

gasification and have developed mathematical models for the same. Kung [66] developed a basic

mathematical model for pyrolysis of wood slab. The model considers heat transfer due to

conduction, internal heat convection and first order kinetics for the formation of volatiles and

char. However no specific model is suggested to predict the concentration of the various

intermediate components produced during the pyrolysis. Kansa et al [67] developed a more

detailed mathematical model for the pyrolysis of wood. They incorporated internal force

convection effects, their model used variable thermal and physical properties, a time-dependent

surface radiant flux, a global Arrhenius pyrolysis reaction, and arbitrary boundary conditions. A

comparison of their model with experimental data for maple wood showed good agreement at

low surface heat fluxes, but agreement was poor for high fluxes. The authors concluded that for









good agreement at high flux intensities, the effect of secondary pyrolysis reactions must be taken

into account. The model developed by Kansa et al was more realistic than the basic model

developed by Kung. Miyanami et al further improved the model developed by Kansa et al by

incorporating the heat of reaction in the pyrolysis of solid particles based on the volume reaction

model [68]. They carried out a transient analysis of the effects of the heat of reaction on the solid

biomass conversion, fluid product concentration profile and temperature distribution in the solid

biomass. The results of their model had better agreement with experimental results as compared

to Kansa et al. Recently GrOnli [69] developed a mathematical model and conducted

experiments to validate the pyrolysis of Scandinavian wood. He studied the pyrolysis of wood

and developed a model that considered the effect of particle size on product composition. His

work identified two categories of wood pyrolysis: small wood particle where internal thermal

resistance is negligible and chemical kinetics is the controlling mechanism, and large particles

where both chemical kinetics and heat transfer need to be considered. Gronli's work gave a

better understanding of the factors that must be taken into account while modeling biomass

pyrolysis of wood particles. More recently, Jalan and Srivastava [70] developed a model for the

pyrolysis of a single wood particle. These researchers modeled the physical and chemical

changes of a biomass particle as it undergoes pyrolysis. This was done by considering the

primary and secondary reactions. An energy balance equation proposed by the authors took into

account the non-isothermal reaction of the biomass particle. Numerical schemes were employed

to solve the heat transfer equations and the equation involving chemical kinetics. The model

predicted the temperature distribution within the pellet as a function of radial distance at different

times as pyrolysis progressed. The authors found that their model compared well with the

experimental data from literature.









In summary, chemical kinetic modeling studies of wood pyrolysis have been conducted by

several researchers over the last three decades. These models provide better understanding of

pyrolysis of solid biomass particles. Some of the models have been experimentally verified. The

chemical data obtained (reaction rate, rate constant, order of reaction, and activation energy)

serve as a valuable database for the design of biomass reactors.

Experimental Studies on Biomass Gasification

Experimental studies on biomass gasification have focused on various aspects like

parametric analysis, catalytic tar cleaning, co-gasification of biomass with coal/plastic, hot gas

cleaning, using multiple feed stocks, different gasifier reactor configurations and so on. In most

cases the end objective was to maximize syngas production. Turn and co-workers [44] studied

the effect of gasifier temperature, steam to biomass ratio (SBR), equivalence ratio (ER) (a

measure of air supplied in biomass gasification) on gas yield (mainly H2, CO, CO2 and CH4) in

fluidized-bed gasification of sawdust. They found the highest hydrogen yield to occur at a

gasifier temperature of 850C and steam biomass ratio of about 1. The maximum hydrogen yield

was found to be 0.128 g/kg dry ash-free biomass. Narvaez and co-workers [71] have analyzed

the effects of temperature, equivalence ratio and the addition of dolomite in the air gasification of

pine sawdust. The group found that maintaining an ER of 0.3, SBR of 2.2 and gasifier

temperature greater than 800C gave good quality (maximum heating value) gas with minimum

tar content. Herguido and co-workers [72] used different feedstocks (pine saw dust, pine wood

chips, cereal straw, and thistles) using steam as the gasification medium and studied the product

yield (H2, CO, C02, and CH4 contents). The group found marked differences in product

composition at low gasification temperature, but at temperatures exceeding 780C, the gas

composition was similar for all biomass feedstocks. Gil and co- workers [73] have studied the

effect of different gasification media (air, steam, steam and oxygen mixture) on product gas









composition. They observed that using steam in place of air gave a product gas with almost five

times more hydrogen. Also the heating value of product gas in steam gasification (12.2-13.8

MJ/Nm3) was higher than air gasification (3.7-8.4 MJ/Nm3). Pinto and co-workers [74] have

conducted experiments by co-gasifying biomass with plastic wastes and observed an increase in

the hydrogen yield by about 17% when 40% (wt) of polyethylene was mixed with pinewood.

One of the objectives of this research was effective utilization of plastic waste. It was found that

when plain plastic was gasified it softened and stuck to the walls of the gasifier. Neither cooling

nor palletizing of the waste plastic helped solve the problem. Mixing of biomass with plastic

avoided the problem of plastic softening and effectively gasified all feedstock.

In a nutshell, many researchers have carried out experimental studies on biomass

gasification. The studies have been varied e.g. simple parametric analysis, effect of gasifying

media on product yield, effect of changing feedstock on product gas composition, co-gasification

of biomass with plastic wastes, catalytic tar cleaning among others. The objective of the

experimental studies in most cases was to maximize the syngas yield for power generation.

Thermodynamic Studies on Gasification

Biomass gasification produces a mixture of gases (mainly H2, CO, CO2 and CH4), liquids

(aromatic hydrocarbons or tars) and solids (char, ash). The process parameters (temperature,

pressure, steam to biomass ratio, equivalence ratio, residence time, heating rate and so on)

directly affect the product yield and composition. Biomass gasification also involves several

reactions occurring in series and in parallel. Some of these reactions are as under:

Steam Gasification:

CH150o7+0.7H20->H2+CO+CO2+CnHL+tars+C(s) AHR=+119 kJ/mol (3.1)

Oxidation:









C+ 0.502 CO AHR=-111 kJ/mol (3.2)

CO + 0.502 -CO2 AHR=-254 kJ/mol (3.3)

Boudouard:

C+CO2-> 2CO AHR=+172 kJ/mol (3.4)

Water-Gas:

C+H20->CO+H2 AHR= +131.3 kJ/mol (3.5)

C+2H20- CO2+2H2 AHR>0 kJ/mol (3.6)

Methanation:

C+2H2->CH4 AHR=-75 kJ/mol (3.7)

Water-Gas Shift:

CO+H20->CO2+H2 AHR=-41 kJ/mol (3.8)

Steam Reforming:

CH4+H20->CO+3H2 AHR=+206 kJ/mol (3.9)

Some of the above reactions are exothermic and others are endothermic. Moreover the

reactions occur in different reactors which operate at different temperatures. Most of the biomass

gasification systems operate at atmospheric pressure and the gasifier operating temperature is in

the range 800-850C. In many applications the product gas needs to be cooled to lower

temperatures before being sent to downstream equipment. There is a potential for heat

integration of the various reactors so that the net external heat input to the biomass gasification

system is reduced. This increases the thermodynamic efficiency of the process. The hot gas

coming out of the gasifier is at a sufficiently high temperature (700-800C) and can be used to

produce steam for a Rankine cycle. The objective of the thermodynamic studies is to find the

opportunities for heat integration and thereby improve the overall efficiency of the process.









In the past, focus on the thermodynamics of biomass gasification has been on areas not

specifically addressing hydrogen production. Cairns et al calculated the gas-phase composition in

equilibrium with carbon (graphite) for a CHO (Carbon/Hydrogen/Oxygen) system for different

temperatures and O/H (atomic oxygen to hydrogen) ratios [75]. Schuster et al conducted a

parametric modeling study of a biomass gasification system. A decentralized combined heat and

power station using a dual fluidized bed steam gasifier was simulated. The group predicted net

electricity to biomass efficiency of about 20% [76]. Kinoshita et al conducted equilibrium studies

of biomass gasification with the objective of maximizing the methanol production. The

theoretical methanol yields were determined and were compared with experimental results. They

also determined the optimal process conditions for methanol production based on

thermodynamic equilibrium [77]. Garcia et al also did an equilibrium study but this was for

steam reforming of ethanol. They studied the effect of temperature, pressure and steam to ethanol

feed ratio and determined the maximum hydrogen yield attainable at equilibrium [78].

Carapellucci [79] studied the thermodynamics and economics of biomass drying using waste

heat from gas turbine exhaust and concluded that using gas turbine exhaust for biomass drying

enhances the economic feasibility ofbiomass fired power plants. Lede et al [80] carried out a

study on using solar energy for thermochemical conversion of biomass. The study highlights the

technical and economic benefits and also lists the difficulties of using solar energy as a source of

heat for gasification and pyrolysis of biomass. Zainal et al [81] also did an equilibrium modeling

study to predict the performance of a downdraft gasifier for different biomass materials. The

group investigated the effects of temperature and moisture content in biomass on the gas

composition. Their equilibrium modeling results matched reasonably well with the experimental

results. Alderucci et al [82] conducted a similar equilibrium analysis of biomass gasification









where steam and CO2 were the gasifying media. The product gas was assumed to be fed to a

Solid Oxide Fuel Cell (SOFC); the efficiency of the fuel cell was then determined. The

researchers found that CO2 gasification gave better fuel cell efficiency as compared to steam

gasification. Prins et al [83] studied the energetic and exergetic aspects of biomass gasification in

the presence of steam and air. They found that the energy and exergy of product gas had sharp

maxima at the point where all the carbon is consumed. They concluded that the choice of

gasification medium should be governed mainly by the desired product gas composition. Crane

et al [84] studied two alternatives to present day gasoline powered systems. They did this by

comparing the exergy of emission of two alternate energy conversion technologies viz. methanol

fuelled spark ignition engines and hydrogen fuelled fuel cells. The authors showed that the

hydrogen powered fuel cell system was better than the methanol powered spark ignition engine

from both energy and exergy perspectives.

Although some work has been reported on thermodynamics of biomass gasification no one

has worked specifically on optimizing the process for hydrogen production. Hence the focus of

the present research was to study the thermodynamics of gasification with the end objective of

maximizing or improving the hydrogen yield.

Sorbent Enhanced Gasification

Biomass gasification consists of many reactions and processes. Steam biomass gasification

is endothermic whereas partial oxidation of biomass is exothermic. One of the objectives of the

present research is to identify suitable methods that can enhance the hydrogen yield and/or

improve the process efficiency. The gasifier is an important reactor where the initial thermal

breakdown of biomass takes place. If the heat duty of the gasifier can be reduced by combining

reactions, it can make the process more efficient and the reactor can become compact. Detailed

thermodynamic studies showed that conventional biomass gasification is limited by the









equilibrium constraints and hence hydrogen yield cannot increase beyond a certain point. In

order to produce more hydrogen one of the co-products of gasification (C02) must be removed.

It was found in the studies that CO2 formation limits the hydrogen yield due to equilibrium of the

Water Gas Shift (WGS) reaction. In recent past, sorbents such as calcium oxide have been used

to remove the CO2 from the fossil fuel exhaust stream. When the CO2 absorption reaction is

coupled with the WGS reaction, the water gas shift proceeds to the right and thereby more

hydrogen is produced.

Han and Harrison [85] studied the simultaneous water gas shift and carbon dioxide

separation process for the production of hydrogen. They observed that removing CO2 as it gets

formed via the non-catalytic gas solid reaction between CaO and CO2 provides the opportunity to

combine reaction and separation into a single step. The resultant process for hydrogen production

got simplified as there was no need of heat exchanger between catalyst beds as well as the

absorption and stripping units for CO2 removal. The authors studied the combined shift and

carbonation reactions in a laboratory scale fixed-bed reactor using dolomite sorbent precursor.

They studied the effects of temperature, pressure and space velocity on the conversion of CO in

the WGS reaction. They observed that more than 99.5% of the carbon oxides got removed and

the product gas was rich in hydrogen. Balasubramanian et al [86] conducted experimental studies

on steam methane reforming in presence of a CO2 sorbent. They added calcium oxide sorbent to

a commercial steam methane reforming catalyst (nickel on alumina). The combined reforming,

shift and CO2 separation reactions were studied using a laboratory scale fixed bed reactor. The

effects of temperature, steam to methane ratio, sorbent to catalyst ratio and feed gas flow rate

were studied. The group found that hydrogen could be produced from methane in a single step

without using a shift catalyst. The product gas was rich in hydrogen (more than 90%). A









reduction in operating temperature by 150-200C was also observed. Lin et al [87] have

proposed a hydrogen production technique by reducing water (steam) using hydrocarbons. The

CO2 so produced was separated using a sorbent. The researchers named this technique as HyPr-

RING (Hydrogen Production by Reaction Integrated Novel Gasification). They conducted an

analysis of the HyPr-RING process and concluded that it has a potential to reduce the cost of

hydrogen production as compared to conventional methods. The researchers further conducted a

thermodynamic analysis of coal gasification in presence of CaO as per the HyPr-RING process

[88]. A mass and energy balance was carried out and the temperature and pressure were varied

over a wide range. The product gas composed of more than 90% hydrogen at a gasification

temperature of 700C and pressure of 3.0 MPa. This gave a gasification efficiency of 77%.

Calcium oxide has also been used for plain CO2 removal from the fossil fuel exhaust without any

hydrogen co-production. Abanades et al studied the capture of CO2 from combustion gases in a

fluidized bed of CaO [89]. They conducted experiments to investigate the potential of CaO to

capture CO2 in a pilot-scale fluidized bed reactor. The researchers found that the CO2 capture

efficiency of CaO bed was very high. However the total capture capacity of the bed was found to

decay with number of carbonation (CO2 absorption) and de-carbonation (CO2 desorption) cycles.

Kyaw and Kubota studied the carbonation of CaO at various temperatures in the range 600 to

900C at various CO2 partial pressures [90]. The authors developed a kinetic rate model for the

absorption of CO2 by CaO. They observed that the CO2 partial pressure is an important

parameter that determines the conversion of CaO to CaCO3. The authors also studied the reverse

reaction (de-carbonation) and developed a kinetic model for the conversion of CaCO3 to CaO

[91]. Some other groups have studied the CO2 absorption process and have identified sorbent









enhanced reforming as a possible method to enhance the hydrogen yield and at the same time

remove the product gas CO2 during the gasification of any carbonaceous fuel [92].

The use of sorbents for simultaneous CO2 removal and hydrogen enhancement is a

relatively new concept which has become popular over the last few years [86, 87]. It has been

proposed for coal gasification. A few research groups have applied the concept to steam methane

reforming at laboratory scale. In principle, the concept of using sorbents can be applied to any

carbonaceous fuel including biomass. So far no work has been done in applying the concept of

sorbent enhanced gasification for biomass.

Scope of the Present Work

Biomass gasification is a potential technology that holds substantial promise for producing

renewable hydrogen. In the previous sections we saw several areas of biomass gasification and

pyrolysis that have been studied by different research groups around the world. Although

hydrogen production by biomass gasification has been studied in the past, there are many areas

that still need to be addressed in order to make the technology commercially feasible.

There are many barriers to the commercialization of biomass gasification for hydrogen

production. One of them is the capital cost and efficiency of biomass gasification systems. The

capital costs of biomass gasification/pyrolysis need to be reduced. This may be possible by

combining some steps in the production process that can significantly reduce the capital cost. For

example the two step shift and PSA separation process could be combined into a single step shift

and integrated separation process or the gasification, reforming, shift and separation processes

could be integrated into a single step. Improving the process efficiency and hydrogen yield of

biomass gasification and pyrolysis is another area of concern. The efficiency is defined as the

lower heating value of hydrogen divided by the sum of all the energy inputs into the process

including the energy in the feedstock. There are many types of equipment which operate at









different temperatures. A detailed thermodynamic analysis of the biomass gasification process is

necessary. The thermodynamic analysis includes a study of the effect of the process parameters

on hydrogen yield. The process variables temperature, pressure, steam to biomass ratio and

equivalence ratio influence the hydrogen yield. The values of these parameters at which the

hydrogen yield is maximum can be determined by a thermodynamic analysis. Biomass steam

gasification is endothermic and heat energy needs to be supplied from external sources. Steam

generation also requires energy. The product gas is later cooled before separating the hydrogen

and this cooling process releases heat. Hence there are some processes that absorb heat and

others that release heat. An energy analysis can potentially optimize the process by better heat

integration of the various sub-systems. This will reduce the overall energy consumption and

thereby improve the process efficiency. A thermodynamic study can also give a deeper

understanding of the constraints that limit the hydrogen yield of conventional gasification. The

equilibrium of the water gas shift reaction can be shifted towards higher hydrogen yield by

separating one of the co-products (CO2) from the exhaust stream. Sorbents such as calcium oxide

have been used for removing the CO2 from the exhaust of fossil fuels. If the CO2 absorption

reaction is combined with the water gas shift reaction, the equilibrium can be shifted in favor of

hydrogen. Calcium oxide has been used as a sorbent in the steam reforming of methane for

producing hydrogen at the laboratory scale. Hydrogen yields of more than 90% (volume) have

been obtained. The concept has also been proposed for the steam gasification of coal. In

principle, it can be applied to any carbonaceous fuel. Biomass is a renewable resource that

contains substantial amount of carbon (about 45% mass) and hence is a good candidate for

applying the concept of sorbent enhanced gasification. The present research investigates

renewable hydrogen production from biomass using sorbents. Theoretical and experimental









studies have been carried out with the end objective of increasing the hydrogen yield and the

overall process efficiency.









Table 3-1: Feedstock composition
Parameter/Analysis type Description Sawdust
Ultimate Analysis C% 48.01
(% dry basis) H % 6.04
0% 45.43
N% 0.15
S % 0.05
Proximate Analysis VM % 71.04
FC % 17.3
Ash % 4.5
Moisture % 7.5
Higher Heating value MJ/kg 18.4
(sawdust data source [44], coal data source [16])


Coal
80.3
5.6
8
1.5
4.6
38
44
5
13
34.1





















































Figure 3.1: Biomass gasification pilot plant [Courtesy NREL]





















68


. w ,















NJ --I
Na


1. Gasifier
2. Metallic fiber
3. Cataytic bed
4. Waste feeder
5. Water pump
6. Condeser
7. Gas filter
8, Flowmeter
9. Gas sampling
10. Condensate


NJ


Figure 3.2: Schematic of biomass gasification set up for producing hydrogen (adapted from
Olivares et al [45])









CHAPTER 4
THERMODYNAMIC ANALYSIS OF BIOMASS GASIFICATION

Introduction

A parametric analysis based on thermodynamics of biomass gasification was conducted.

The gas yield depends on many process variables such as gasification temperature, pressure, the

amounts of steam and/or air added to the gasifier. The objective of the study was to determine

the operating conditions that would maximize the equilibrium hydrogen yield. An energy

analysis was conducted to determine the thermodynamic efficiency of the gasification process

with the end objective of maximizing the product gas hydrogen. The basic analysis lays the

foundation for a novel gasification process that will be described in detail in the next chapter.

Fundamentals

The concept of chemical reaction equilibrium is based on the second law of

thermodynamics for reacting systems. All spontaneous reactions occur in the direction of overall

increase of entropy. When system composition reaches a point where the total entropy is

maximum, it becomes "stuck" since any further change in composition would involve a decrease

of entropy which cannot occur spontaneously. We know from thermodynamics that

Sgen = ASsys + ASs (4.1)

For any spontaneous reaction sgen > 0. Since the environment is assumed to be at a constant

temperature

As- = q/T (4.2)

Hence for any spontaneous reaction,

Ass,+ q,/T,> 0 (4.3)

In differential form,

dss,+ 86q./T.> 0 (4.4)









The first law in differential form is given by:

6q 6w = du (4.5)

For a reversible process, work term is Pdv. The second law in differential form as applied to a

system can be written as

6qsys < Tdssy, (4.6)

Substituting in the first law of equation (4.5) we have,

Tdss, 2 du + 6w

S> du+Pdv-Tds (4.7)

We know from thermodynamics that Gibbs free energy is defined as,

g =u+Pv-Ts (4.8)

Taking the derivative we get,

dg = du + Pdv + vdP Tds sdT

For a constant pressure and temperature case, we have

dg = du + Pdv- Tds (4.9)

Combining equations (4.7) and (4.9) we see that for a spontaneous reaction at constant pressure

and temperature,

0 2 dg (4.10)

This means that for a given temperature and pressure, a spontaneous chemical reaction will

occur until the Gibbs free energy reaches a minimum point in composition space. Figure 4.1

shows the total Gibbs energy in relation to the reaction coordinate. Here '' is defined as the

extent of reaction and characterizes the degree to which a reaction has taken place.

Gibbs energy is a function of temperature, pressure and composition (i.e. the moles of various

components present e.g. H2, CO, C02, CH4 etc). This functionality can be represented as:









g = g(T,P,nl,nm,..... ,nNs) (4.11)

Here ni is the number of moles of species i. Taking total derivative of g gives

dg= ndT+ T,ndP+ +P,T,.dn, (4.12)
5 T lapj an^ )

Since T & P are fixed for the point of minima, we have


SP, T,mdn = 0 (4.13)


The number of moles of each species at equilibrium adjusts itself in such a way that the

total Gibbs energy is minimized. The problem of determining the chemical composition at

equilibrium now reduces to a minimization problem which needs to be solved keeping in mind

the elemental (C, H, O, N) and mass constraints (i.e. mass of reactants = mass of products).

Various texts [93] have carried out the mathematical treatment to cast the above problem as an

optimization problem and solve it using a personal computer. Late Dr W. C. Reynolds of

Stanford University developed an algorithm [94] to solve the above Gibbs energy minimization

problem and it is now available as free software called Stanjan. The elemental composition of the

various reactants at any specified temperature and pressure is supplied as input and Stanjan

calculates the equilibrium yield of the product gases. In the next section various combinations of

process parameters have been simulated to determine the most favorable conditions for hydrogen

production.

Effect of Process Parameters on Equilibrium Hydrogen Yield

Biomass can be gasified using different gasifying media, the choice of which depends on

the desired product gas composition and energy considerations. Commercial and research

gasifiers generally use steam or air as the gasifying media [44, 55, 61, 71, 72, 95, 96]. Air

gasification is an exothermic process, which produces a low heating-value gas (LHV 5-6









MJ/Nm3) rich in CO and having small amounts of H2 and higher hydrocarbons [71]. Steam

gasification on the other hand is an endothermic process, which produces a medium heating-

value gas (LHV 12-13 MJ/Nm3) rich in H2 and CO [72]. The process parameters including

temperature, pressure, steam biomass ratio, equivalence ratio and residence time also influence

the product-gas composition.

Effect of Temperature

The gasification temperature not only affects the product yield but also governs the process

energy input. High gasification temperature (800-850C) produces a gas mixture rich in H2 and

CO with small amounts of CH4 and higher hydrocarbons. Figure 4.2 shows the equilibrium

moles of various gases (H2, CO, CO2, CH4) and solid carbon (C(s)) at 1 atm pressure, SBR

(denoted by 3, defined in section on 'Effect of Steam Biomass Ratio') of 1.0 and ER (defined in

section on 'Effect of Equivalence Ratio') of 0. At low temperatures, solid carbon (C(s)) and CH4

are present in the product gas. In actual gasifiers solid carbon is carried away to the catalytic bed

and is deposited on the active catalyst sites thereby de-activating the catalyst. It is necessary to

ensure that the product gas is free of any solid carbon. As temperature increases, both carbon and

methane are reformed. At about 1000 K both are reduced to very small amounts (< 0.04 moles)

and in the process get converted into CO and H2. This explains the increase in hydrogen mole

numbers. At about 1030 K, the H2 yield reaches a maximum value of about 1.33 moles. At

higher temperatures the H2 yield starts reducing. This is attributed to the Water-Gas Shift (WGS)

reaction:

CO+H20 4 CO2 +H2 AH= -33.8kJ/mol (4.14)

According to Le-Chatelier's principle, high temperature favors reactants in an exothermic

reaction thus explaining the increase in CO and reduction in H2 yield at higher temperature. For









the present case a gasification temperature of about 1030 K gives the highest equilibrium

hydrogen yield with negligible solid carbon in the product gas.

Effect of Pressure

Table 4-1 shows the effect of system pressure on equilibrium gas composition (gasification

conditions T = 1100 K, 3 = 1, ER = 0). As pressure increases equilibrium H2 and CO yields

reduce. Simulations carried out to study the effect of reducing the pressure below 1 atm on

equilibrium product yield showed that increase in H2 yield is negligible (< 0.2%) even for

pressures as low as 0.1 atm. Since high pressure reduces the H2 yield, subsequent simulations

were carried out at atmospheric pressure.

Effect of Steam Biomass ratio

SBR refers to moles of steam fed per mole of biomass. SBR, like temperature has a strong

influence on both product gas composition and energy input. Figure 4.3 shows equilibrium yields

(moles of gas) for process conditions T = 1000 K and ER = 0.

At low values of SBR, solid carbon and methane are formed. As more steam is supplied,

both of these species are reformed to CO and H2. For 3 > 1, C(s) and CH4 moles reduce to very

small values and H2 and CO2 yields increase monotonically; CO on the other hand reduces

monotonically. This trend can be attributed to the Water-Gas Shift reaction; since system is

being overfed with steam (for 3 > 1), H20 mole numbers are increasing and as per Le-Chatelier's

principle the equilibrium shifts in the forward direction. For 3 > 1.5, the hydrogen yield increases

very slowly with most of the surplus steam going unreacted. This shows that operating at very

high P (typically more than 2 for above conditions) may not be energy efficient, as additional H2

produced may not justify the high cost of producing and supplying steam. In the next section, an

energy analysis is done to find out the optimum P.









Effect of Equivalence Ratio

ER is a measure of the amount of external oxygen (or air) supplied to the gasifier. ER is

obtained by dividing the actual oxygen (or air) to biomass molar ratio by the stoichiometric

oxygen (or air) to biomass molar ratio. Oxygen (or air) is generally supplied as a gasifying and

fluidizing medium. Using air in place of oxygen, though economical, has the negative effect of

diluting the product gas due to the presence of nitrogen. Figure 4.4 shows the effect of ER on the

equilibrium composition for the operating conditions of T = 1100 K and 3 = 0. As more oxygen

(high ER) is supplied, it is observed that the H2 and CO yields reduce and that of CO2 increases.

This is due to the oxidation of H2 and CO to H20 and CO2. At low values of ER, small amounts

of C(s) and CH4 are formed in the gasifier, both of which get oxidized as more air is supplied.

Air gasification is an exothermic process and hence using air as a gasifying medium

reduces the net energy consumption and improves the overall thermodynamic efficiency.

However supplying more air dilutes the product gas thereby reducing the H2 yield. The optimum

ER would supply enough air for the biomass to be partially oxidized without significant dilution

of the product gas.

Optimum Process Parameters

One of the objectives of the present analysis is to find the process conditions that are

favorable for hydrogen production (very low or no solid carbon in the product gas, high H2 yield

and high efficiency). From Figure 4.4 it is clear that the maximum amount of hydrogen that can

be produced at equilibrium in pure air gasification (no steam, T = 1100 K, and ER = 0.1) for the

stated conditions is 0.7 moles. This value is smaller than the hydrogen that can be obtained at the

same temperature with steam addition (about 1.3 moles of H2 at T = 1100 K, 3 = 1, referring to

Fig 4.2). The excess H2 in the output stream is attributed to the WGS reaction, which cannot take









place in pure air gasification due to the absence of steam, implying that for high H2 yields one

should go for steam gasification. Steam not only influences the water-gas shift but also reforms

the hydrocarbons, solid char and tars and thereby produces more hydrogen. Steam gasification is

an endothermic process; therefore using steam will be energy intensive [97]. Also most gasifiers

use fluidized beds for better heat transfer. For energy efficiency and cost-effectiveness these beds

use air (or oxygen) as a co-fluidizing medium with steam [44, 72, 96, 97]. From the above

equilibrium analysis we see that as more steam is supplied the hydrogen yield increases.

However, this additional hydrogen comes at the cost of extra energy that needs to be supplied in

order to produce steam. The optimum 3 (steam/biomass ratio) is based on the balance of these

two opposing factors. ER (equivalence ratio) affects both the gas composition and net energy

input. From the earlier analysis we saw that the optimum ER, like optimum 3 depends on the

balance between partial oxidation of biomass and dilution of the product gas. In the next section

a first law analysis of the gasifier is carried out with the objective of determining the optimum

operating conditions for hydrogen production.

Energy Analysis

A schematic of a biomass gasifier with a steam generator and an air pre-heater is shown in

Figure 4.5. Wood designated by CH1.500.7 was the model biomass compound (chemical formula

based on ultimate analysis [44]). The general reaction for combined steam and air gasification is

written as:

CHi150o7 + PH20 + Y(O2 + 3.76N2) ncH4CH4 + ncoCO + nco2CO2 + nH2H2
(4.15)
+nH2oH20 + nN2N2

Here only the main components (H2, CO, CO2, CH4) are considered. Yields of higher

hydrocarbons (C2H2, C2H4, C2H6 and so on) were found to be negligible as compared to the main

constituents and hence were not considered in the analysis. The gasification temperature was









varied from 900 K to 1400 K (in steps of 100 K), steam to biomass molar ratio was varied from 0

to 5 (in steps of 1) and ER was varied in the range 0 to 0.4 (in steps of 0.1). These are the typical

values of these variables encountered in most commercial and research gasifiers [44, 55, 61, 71,

72, 97, 98]. From equilibrium studies we know that increasing the pressure reduces the hydrogen

yield, hence the pressure was maintained at 1 atm for all further analyses. A first law analysis of

the gasifier was carried out across the control volume (dotted) as shown in fig 4.5.

An energy balance equation can be written as (assuming no heat losses and work = 0):

Energy in = Energy out (4.16)

Hwood + PHH2o(v) + y(Ho2 + 3.76HN2) + QEG -> nCH4HCH4 + ncoHco + nco2Hco2
(4.17)
+nH2 H2 + nH2oHH2O + nN2HN2

Here H is the enthalpy and QEG is the heat supplied to (or rejected by) the equilibrium gasifier.

Enthalpy of each species is written in terms of enthalpy of formation and enthalpy change:

H = Hf + AH (4.18)

QEG is positive for an endothermic reaction (steam gasification) where heat is to be

supplied from an external source. When QEG is negative heat is liberated and this generally

happens during partial oxidation of biomass (air gasification). A zero value for QEG is an

interesting case which represents adiabatic gasification. This would mean a self-sustaining

process and can be used as a standard to compare actual gasifiers. We assume that the steam

generator provides superheated steam at 700 K and the air-preheater heats the air from the

ambient to 350 K before entering the gasifier. The efficiency of the process was then determined

for a range of temperatures, SBRs and ERs. The efficiency was calculated as per the following

definition given by USDOE [28]:

LHV of hydrogen in product gas
LHV of biomass + All other energies(4.19)
LHV of biomass + All other energies









niH2 LHVH2
r = (4.20)
nb* LHVb + (QEG + Qsteam + Qair)

Qsteam = nH2O AHH2z (4.21)

Qar = no2 AHo2+ nN2 AHN2 (4.22)

The moles of each species at equilibrium were calculated using Stanjan. The enthalpy of

formation and enthalpy change for each species are taken from standard thermodynamic tables

[99]. The values for all the heat duties (QEG, Qsteam, Qair) were determined. The efficiency was

then determined using the above equation for a range of temperatures, SBRs and ERs. Figures

4.6 4.9 show the efficiencies for the different combinations. For simplicity and clarity of

graphs, the efficiency values for all the temperatures (900 to 1400 K), ERs (0.1 to 0.4 in steps of

0.1) and SBR values of 1, 2 and 5 have been shown.

Effect of Temperature on Thermodynamic Efficiency

As gasification temperature increases, biomass thermally disintegrates to produce more

gases and volatiles. As temperature increases, the hydrocarbons in the presence of steam/air get

reformed to produce H2 and CO. Hence as hydrogen yield increases, the efficiency also

increases. As gasification temperature further increases, more heat needs to be externally

supplied to maintain the gasifier temperature. Also at higher temperatures (>1200 K) the

hydrogen yield drops. Hence, the efficiency first increases to a maximum at around 1000 K (this

is especially true for low ER values of 0.1 or 0.2 and SBR of 1 as shown in Figs 4.6 and 4.7) and

then decreases as the temperature is further increased upto about 1300-1400 K.

Effect of Steam Addition on Thermodynamic Efficiency

Figures 4.6 to 4.9 show the combined effect of adding steam and increasing the

temperature for various equivalence ratios. As we have seen earlier, adding steam increases the

hydrogen yield. However additional steam also demands additional energy. Therefore, there









should be an optimum steam to biomass ratio which will justify the cost of extra steam. In this

analysis, the steam biomass ratio was varied from 1 to 5. The plots for SBR values of 1, 2 and 5

are shown in the figures. At low SBR values (-1) the amount of hydrogen produced is relatively

small. As SBR increases the efficiency increases due to higher H2 yields. However at very high

SBR values (>5) the efficiency drops due to large amounts of external heat needed to generate

the steam. This trend was observed in all four graphs (Figs 4.6 4.9). In the analysis it was

found that a SBR of 2.0 gives the highest efficiency among all the cases.

Effect of ER on Thermodynamic Efficiency

Gasification in presence of air or oxygen partially oxidizes the biomass and thereby

releases energy. However, this also dilutes the product gas (especially if air is used) thereby

lowering the heating value of product gas. At low ER values (-0.1) energy is released due to

partial oxidation of biomass. Also the hydrogen yield is relatively high and so the efficiency is

high. Typical efficiencies were of the order 50 to 55 % for SBR in the range 2 to 3 at gasification

temperatures of 900 1000 K.

As ER increases the product gas starts getting diluted due to the presence of N2. It was

observed in the previous section that the H2 yield drops beyond ER of 0.2. Hence, although the

reaction is exothermic, a whole lot more biomass needs to be gasified in order to produce the

same amount of hydrogen. At ER values > 0.4 the efficiency starts dropping rapidly (typically r

~ 44 % for P ~ 1 or 2, T = 1000 K and ER = 0.4, referring to Fig. 4.9).

The optimum conditions for hydrogen production occur when we have high

thermodynamic efficiency, with high hydrogen yields and little or no carbon formation. From the

parametric analysis of the previous section and the energy analysis we see that this happens for T









~ 1000 K, SBR 2, ER 0.1 and atmospheric pressure. For the given biomass feedstock these

conditions give a thermodynamic efficiency of 52%.

Comparison of Equilibrium Results with Experimental Data

Equilibrium studies are used to predict the maximum possible conversion in any chemical

reacting system. By comparing experimental results with equilibrium calculations one can

understand the relation between thermodynamics and chemical kinetics of the process. In

general, it was observed that experimental results deviated considerably from the equilibrium

calculations. Figure 4.10 compares equilibrium and experimental results where two parameters,

the gasification temperature and residence time (c), have been varied (experimental data obtained

from [96]). Of the four sets of graphs, the first two are for temperature (700 and 800C) and the

last two are for residence times (0.4 and 1.4 s). In each set, the hatched bars are for theoretical

(th) and the solid bar for experimental (ex) compositions (total of 8 bars for each T and each C).

For both temperatures, the H2 and CO gas volumes are far from equilibrium, although the

difference is less for higher temperature.

The theoretical CH4 volume at equilibrium at 800C (~ 0.01 %; not visible on graph) is

much smaller than the experimental value. From the residence time graph, it is observed that for

high residence times, the experimental values are closer to the equilibrium values. This is due to

more time being available for reactions to take place and reach completion. Figure 4.11 shows

how theoretical and experimental results compare for different 3 (1.9 and 6.5) and ER (0.09 and

0.37) (experimental data source [44]). The experimental H2 yield is lower than the equilibrium

yield for a 3 value of 1.9. Other gas mole fractions (CO, CO2 and CH4) too differ from the

equilibrium values. For very high 3, H2 mole fraction comes close to the equilibrium value. For









both low and high ERs the H2 mole fraction is away from the equilibrium value. Both

equilibrium and experimental results are more sensitive to T, c and 3 than ER.

During biomass gasification many complex aromatic hydrocarbons called tars are released.

These tars typically include benzene or multiple rings of benzene such as naphthalene, xylene or

toluene and many complex higher hydrocarbon chains with several carbon atoms [55].

Equilibrium studies were done using benzene as a possible tar compound. The results however

showed negligible benzene in the product stream (about ten orders of magnitude lower than other

important products such as hydrogen and carbon monoxide). This is possibly due to the infinite

time being available for the reactions to occur before equilibrium is reached. This was also

verified from the actual experimental data where long residence time and high temperature

drastically reduced the tars in product stream [72]. Since high residence times reduce the tar

yield and equilibrium studies show product yield at very long times (t -* oo), higher

hydrocarbons and tars were not included in the equilibrium modeling studies.

Summary and Conclusion

A thermodynamic analysis of hydrogen production from biomass was done using

equilibrium modeling. The effects of process parameters (temperature, pressure, SBR and ER)

on hydrogen yield were studied. It was observed that combined steam and air gasification gave

much higher H2 yield than air gasification alone. Using air as a co-gasifying medium with steam

helps reduce external energy input as the feedstock gets partially oxidized. The equilibrium

hydrogen yield is found to initially increase with temperature to a maximum and then gradually

reduce at higher temperatures. The hydrogen yield increases continuously with increase in SBR.

Air gasification also produces hydrogen but the yield is lesser than steam gasification. The

product gas in air gasification gets diluted due to the presence of nitrogen. Increasing the









pressure was found to have a negative influence on the hydrogen yield and hence all subsequent

simulations were carried out at 1 atm. The gasifier is the most critical component of any biomass

gasification system. The gasifier was modeled as an equilibrium reactor and a first law analysis

of the gasifier was carried out to determine the maximum thermodynamic efficiency at

equilibrium. The optimum operating conditions were found to be T of 1000 K, SBR of 2. ER of

0.1 and P of 1 atm which gave an efficiency of 52%. The actual energy consumption would be

higher due to equipment inefficiencies and heat losses from the gasifier, catalytic reactor and

interconnecting tubing. Also in real gasifiers we will not reach equilibrium conditions and hence

the product gas will contain less H2 and CO and more CO2. Nevertheless, the above figures give

an idea of the theoretical maximum efficiency for the given conditions. A comparison of the

theoretical equilibrium calculations with the experimental results shows considerable deviations

between the two. Using longer residence times, higher temperatures and higher steam input the

experimental results can come close to equilibrium predictions.

The basic studies gave an understanding of the thermodynamics of biomass gasification. In

the next chapter a novel concept of combining different reactions and thereby getting an

improvement in the hydrogen yield is discussed.









Table 4-1: Equilibrium gas moles at different gasification pressures
P (atm) H2 CO CO2 CH4 Remark
0.1 1.303 0.746 0.253 1.61E-5 Low Press.
0.5 1.302 0.745 0.253 4.0E-4 System
1 1.301 0.744 0.254 1.59E-3 High Pressure
10 1.09 0.633 0.286 8.13E-2 System
25 0.897 0.491 0.326 1.82E-1














constant T & P


dG)T,p = 0

Extent of reaction ( )


Figure 4.1: Variation of Gibbs energy with extent of reaction












1.4 0.14


1.2 0.12


1.0 0.10


0.8 0.08


S0.6 0.06


0.4 0.04


0.2 0.02


0.0 0.00
900 1000 1100 1200 1300 1400
T(K)

CO C02 -- H2 --C(s) -*-CH4




Figure 4.2:Effect of temperature for P = 1 atm, 3 = 1, ER = 0












1.8 0.060

1.6
0.050
1.4

1.2 0.040

1.00.030

0.8

0.6 0 020

0.4
0.010
0.2

0.0 0.000
0.5 1 1.5 2 2.5 3

C- CD --- 02 -A- H2 -X- C(s) -*-



Figure 4.3:Effect of SBR on equilibrium composition














1.0 0.16

0."9 0.14

08
0.12
0.7
0.6 0.10


0.5 0.08
0.4



0.04
043 0.o4


0.2


0.0 0.00

0.1 0.2 0.3 0.4 0.5 0.6
IR
-- -co0 -- o02 -A- 2 --- C(s) -- CIH


Figure 4.4: Effect of ER on Equilibrium composition











298 K,
CH1.50o.7 BIOMASS 1 atm

298 K, FEEDER I -1 -
SI H2, CO, C02,
Satm H20, CH, N
STEAM GEN EQUILIBRIUM I
H2Ov | GASIFIER
H20 (1) H20 (v 1 tm I atm
(1)^r T^ / /n(\\J/


! 6 N / IU" I
1 atm 1 atm


C.V.



QEG


Steam I

Air AIR

298 K, PREHEATER
'I 350 K
1 atm 1 atm

Qair


Figure 4.5 Schematic ofbiomass gasifier


I^


I
I
I
















50



45



40



35



30
900 1000 1100 1200 1300
T(K)

-A- P= 1 -a f 2 -*- f=5



Figure 4.6: Efficiency Vs temperature for various (ER = 0.1)












55



50



45



40



35 -



30
900 1000 1100 1200 1300 1400
T(O

-A- = 1 -- f3=2 .-- j3=5


Figure 4.7: Efficiency Vs temperature for various P (ER= 0.2)




























900 1000 110 111200 1300 1400
T(IQ


Figure 4.8:Efiency Vs temperature for various (ER= 0.3)

Figure 4.8:Efficiency Vs temperature for various J3 (ER = 0.3)


























900 1000 1100 1200 1300 1400


Fy Vs te atu f=1 2 --va s 3=5



Figure 4.9: Efficiency Vs temperature for various 3 (ER = 0.4)












-H2th CCOth ECO2th [mIIICH4th
SH2ex CO ex CO2ex --CH4ex


700 (oC) 800s() 0.4(s) 1.4(s)
T


Figure 4.10: Comparison of equilibrium data with experimental data of Corella et al [96] for
different temperatures and residence times










H2 th Cth h C COth h [C11111 CH4 th
H2 ex CO ex CO2 ex =WCH4 ex

/60 I -


19 65 0.09 037
P R



Figure 4.11: Comparison of equilibrium data with experimental data of Turn et al [44] for
different 3 and ER









CHAPTER 5
ABSORPTION ENHANCED BIOMASS GASIFICATION

Introduction

Steam gasification of biomass produces a gas mixture rich in hydrogen and containing

other gases such as CO, CO2, CH4 and small amounts of higher hydrocarbons. The maximum

hydrogen that can be produced in conventional steam biomass gasification is limited by the

thermodynamic equilibrium constraints at the specified gasifier temperature and pressure. The

temperature option is limited by equilibrium product composition which does not favor hydrogen

formation beyond 1100 K (this was observed in the previous chapter). At higher temperatures the

biomass gets thermally dissociated, however, this does not translate into increased hydrogen

yields; hence the temperature option is limited. The pressure option too is limited as higher

gasification pressure (above one atmosphere) reduces the hydrogen yield and lower pressure

does not offer any substantial increase in the hydrogen content. The steam to biomass ratio can

be increased to give higher hydrogen yields, but this is at the cost of extra steam that needs to be

supplied. As we increase the steam supply, the yield increases rapidly up to certain point but

thereafter the increase is rather slow with most of the surplus steam going unreacted. Hence in

order to increase the hydrogen yield we need to find new techniques which are simple, energy

efficient and inexpensive. The products coming out of the biomass gasifier consist of other gases

like CO, C02, and CH4 which must be separated from H2. Hence the problem of gas separation

also needs to be addressed.

In recent years sorbents (such as calcium oxide) have been used for CO2 removal from the

exhaust of fossil fuel plants. The sorbent absorbs CO2 and in the process releases heat which can

be used for reforming the fuel. More recently, this technique was applied to the steam reforming

of methane and a hydrogen rich, CO2 free gas was obtained [86]. The product gas is expected to









have more hydrogen with less contaminants. It can be used for any downstream application such

as fuel cell or gas turbine with minimal cleaning. Hence there is a potential to reduce the number

of equipment (and thereby reduce the capital costs) by using sorbents. In principle, the sorbent

enhanced gasification process can be applied to any carbonaceous fuel such as coal, heavy oils,

biomass, plastic or organic waste.

Concept of Absorption Enhanced Gasification

The concept of producing hydrogen by reforming hydrocarbons using sorbents dates back

to as early as 1868 [100]. In 1967 Curran and co-workers [101] separated CO2 at high

temperature using calcined dolomite in the so-called "CO2 Acceptor Gasification Process". More

recently Harrison et al [86, 102, 103] have experimentally shown a novel method of improving

hydrogen yield of conventional SMR and effectively separating CO2. Lin et al [87, 88, 104] have

used sorbents to develop an innovative HyPr-RING (Hydrogen Production by Reaction

Interaction Novel Gasification) technique for producing hydrogen by gasification of coal. The

underlying concept of absorbent enhanced gasification is shown in Figure 5.1.

There are two main reactors in the process. First is the gasifier/absorber. Here any

carbonaceous fuel (in our case biomass) is supplied to the reactor to which steam is also being

fed. The fuel reacts with steam to produce a gas mixture containing hydrogen, carbon monoxide,

carbon dioxide and some hydrocarbons. The carbon monoxide reacts with steam to produce

additional hydrogen as per the Water Gas Shift reaction.

CO + H20 CO2 + H2 AH = -33.8 kJ/mol (5.1)

The calcium oxide sorbent in the gasifier absorbs the carbon dioxide produced and gets

converted to calcium carbonate

CaO + CO2 CaCO3 AH = -168.2 kJ/mol (5.2)









During the absorption process heat is released and this can be used for the endothermic

steam gasification of biomass, thereby reducing the net external heat supply to the gasifier. The

calcium carbonate is then regenerated by heating it in another reactor. The thermal energy for

regeneration can be supplied either by burning external fuel or part of the biomass feedstock

itself. The hydrogen produced may have small amounts of carbon monoxide, methane and tars.

Hence, it is passed through a gas cleaning system so as to obtain a clean gas that is rich in H2.

Through simultaneous gasification and CO2 absorption, the equilibrium of the homogenous

water gas shift reaction is shifted toward H2. For any general biomass fuel the reactions taking

place in sorbent enhanced gasification (SEG) can be written as follows:

CHO + (1- y)H20 CO + (0.5x +1- y)H2 AHR>0 (5.3)

CO + H20 CO2 + H2 AHR<0 (5.4)

CaO + CO2 CaCO3 AHR <0 (5.5)

The overall reaction can be written as:

CHxOy + (2 y)H20 + CaO > CaCO3 + (0.5x + 2 -y)H2 (5.6)

Equation (5.6) represents the idealized sum reaction for sorbent enhanced gasification. Here the

formation of secondary products (methane, coke & tar) is neglected. Table 5-1 gives the values

of the heats of reaction for different fuels with typical reaction temperatures.

Figure 5.2 shows a schematic of the Sorption Enhanced Gasification concept. The CO2

absorption is an exothermic reaction and the biomass steam reforming reaction is endothermic

and hence the overall reaction would consume less energy. The spent sorbent is regenerated in a

subsequent process by supplying heat. For continuous gas production, solid fuel is gasified in

presence of fresh sorbent at temperatures less than 700C. The carbonated bed material together

with the biomass coke is removed and regenerated at 800-900C under air supply. Thus a









hydrogen rich gas stream with small amounts of CO and CH4 and a CO2 rich exhaust gas stream

are generated in two parallel process steps. In actual system two fluidized bed reactors with

circulating absorbent bed material can be coupled as shown in the set-up of figure 5.2.

Application of SEG to Biomass Gasification

Sensitivity studies have been carried out in order to determine the effect of process

variables on the equilibrium hydrogen yield. Ethanol was used as the model biomass compound.

ASPEN PLUS (version 12.1) software was used to model the process flow. The choice of

ethanol as a model compound was primarily due to convenience. The physical, thermodynamic

and transport properties of ethanol are well-documented and are already built into ASPEN

database, hence making it convenient to carry out simulations (the choice of this model

compound does not endorse or imply producing hydrogen from ethanol; this is the subject of a

separate study). The process variables studied were temperature, pressure, steam to biomass ratio

and sorbent to biomass ratio. The general reaction for steam gasification of ethanol is given by:

C2H5OH + H20 -> H2+CO + CO2 + CH4 + H20 + higher Hydrocarbons (5.7)

Experimentally it has been found that higher hydrocarbons (two carbon atoms containing

compounds such as ethylene or acetaldehyde) and solid carbon in the steam reforming of ethanol

are negligible. Hence these were not considered in the simulations [105]. There are two cases

considered here base case (i.e. no sorbents) and sorbent enhanced gasification each of which is

explained below.

Case I: Base case (no sorbent)

Ethanol and water are mixed in a mixer and sent to a heater where they are heated to the

desired temperature. The product is then sent to the reformer which is modeled as a Gibbs

reactor which is at the desired gasifier temperature and pressure. The products of reformer which









are at thermodynamic equilibrium are then cooled before being sent to the water gas shift reactor

where carbon monoxide reacts with steam to produce additional H2 as per the following reaction.

CO + H20 CO2 + H2 AH = -33.8kJ /mol (5.8)

The flow sheet for base case is shown in Figure 5.3. The steam to biomass ratio (water to

ethanol feed ratio) was varied between 3 and 8, the reformer temperature from 500 to 9000C (this

is the temperature range for actual gasifiers) and the gasifier pressure from 100 kPa to 2500 kPa.

The results of the sensitivity analysis are shown in figures 5.4 to 5.6. Figure 5.4 shows that the

temperature has a significant effect on the equilibrium product yield. The ethanol and steam flow

rates were fixed at 1 kmol/hr and 4 kmol/hr and the reformer was at atmospheric pressure. As the

reaction temperature increases the hydrogen yields also increases until it reaches a maximum at

725C and then decreases. The increase of hydrogen yield is due to the reaction of ethanol with

steam. As the temperature increases, the hydrocarbons (methane) are reformed and converted to

hydrogen. At high temperatures the Water-Gas Shift reaction occurs in the reverse direction and

this reduces the hydrogen yield.

Figure 5.5 shows the effect of reformer pressure on product yield. It is observed that the

pressure significantly impacts the equilibrium product yield. One can conclude from the figure

that the highest hydrogen yield is obtained at atmospheric pressure and hence it is best to operate

the reformer at one atmosphere. The effect of steam to ethanol ratio on product yield for a

reformer temperature of 700C is shown in Figure 5.6. The addition of steam increases the

hydrogen yield while reducing the CH4 and CO concentrations. Although high steam to ethanol

ratio gives high hydrogen yields, it will be limited by the cost of the system.









Case II: Ethanol gasification in the presence of CaO sorbent sorbentt placed in the
reformer)

Figure 5.7 shows the flowsheet of sorbent enhanced biomass gasification. Adding CaO to

the steam reforming of ethanol can be considered with the following reaction

CaO(s) + C02(g) CaCO(s) AH = -168 kJ/mol (5.9)

The removal of CO2 from the gaseous phase will displace the equilibrium of the gas

mixture to a higher hydrogen yield and a lower CO concentration. The products are the same as

in the base case plus CaO and CaCO3. The flow sheet of the simulation is shown in Fig. 5.7.

Again ethanol and water are mixed together and are sent to the heater (HEATER1) at 700C. The

mixture enters the Gibbs reactor (REFORMER) which in this case includes solid CaO. The

reformer output is sent to the separator (SEP) for separation of gases from solids. The gases

including H2, CO, C02, CH4 and steam are cooled to 300C in the heat recovery heat exchanger

(HT-RECOV) and enter the Water-Gas Shift (WGS) reactor. The solids are sent to the

regeneration reactor (REGENERA) in which CaCO3 decomposes to CO2 and CaO at 8500C.

The effect of temperature on the product molar flow rates is shown in figure 5.8. At

temperatures lower than 750C the hydrogen production is greatly enhanced by the separation of

CO2. Above this temperature the molar flows are similar to the previous case. The maximum

hydrogen is produced at 650C (which is almost 100C lesser than the base case). It is also

observed that the maximum hydrogen produced in the sorbent enhanced gasification case (5.24

mol/mol of ethanol) is almost 12% more than the corresponding figure for the base case (4.68

mol/mol of ethanol). The amount of carbon oxides (CO and CO2) produced is less than the base

case due to absorption by CaO. It is observed that the sorbent absorption is effective up to 8000C,

thereafter, the hydrogen yield drops and is similar to the base case. This is probably due to the