7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

Characterization of Inhomogeneous Non-Spherical Alternative Fuel Particles for

the Cement Manufacturing Process

Solairaj Perumalsamy, Kay Schroer, Hiiseyin Yilmaz, Gerd Oeljeklaus,

Klaus GOmrer*, Thomas Klasen** Tobias Bluhm-Drenhaus, V. Scherer,t

Asian Zeki, S. Schafer

LUAT (Environmental Process Engineering and Plant Design), University of Duisburg-Essen,

45141 Essen, Germany

**InPro-Consult GmbH, 45145 Essen, Germany

SDepartment of Energy Plant Technology, Ruhr-Universitat Bochum, 44780 Bochum, Germany

40474 Dusseldorf, Germany

German Cement Works Association, 40476 Duesseldorf, Germany

solairaj.perumalsamy @stud.uni-due.de and klaus.goerer @uni-due.de

Keywords: Two-phase flow; Non-spherical particles; Lagrangian approach; Turbulence

Abstract

Solid recovered fuels (SRF) are a highly heterogeneous mixture generated from high calorific fractions of nonhaz-

ardous waste materials intended to be fired in existing cement plants and industrial furnaces. They are composed of

a variety of materials that include plastics, paper, wood and textile wastes which are offered in particle size up to

several centimetres. These materials are commonly irregular (3D and 2D) in shape and size. It is therefore difficult

to predict their aerodynamic properties, pyrolysis and burnout behaviour [1] as well as their material properties as a

mixture. In view of this, the ability to characterize SRF particles will immensely help in any modelling of particles

and their combustion behaviour in a cement plant. Bulk mixture of SRF particles were shorted out to six fraction

based on their difference in settling velocity for which pyrolysis at different temperature was measured. To determine

the aerodynamic properties of the particles which are essential for air flow and thus promote the distribution in the

calcinator, experimental work was carried out to determine the aerodynamic diameters, particle projection area,

apparent density and maximum minimum axial length of representative particles [2] which are selected manually

from each fraction. Simulations were performed in commercial CFD code, based on these measured properties

such as projection area with different degrees of non-sphericity [3], aerodynamic diameters and pyrolysis of each

fraction at different temperature. As the first stage, a model for nonspherical particles tracking in a simple nonuniform

two-phase flow is derived in this paper, in which forces, including inertia, drag, gravity, lift, a force due to pressure

gradient and virtual-mass force are taken into account in particle force balance. The drag coefficient is calculated

using the correlation of Haider and Levenspiel (1989) and Ganser (1993) and combustion behavior such as pyrolysis

and burnout of non-spherical SRF particles were simulated.

Introduction ciency and economic fuel supply is very important for

the operators of cement plants. Co-combustion of Solid

Secondary fuels have gained substantial recognition in Recovered Fuels (SRFs), which mainly consist of bio-

Europe and are now used as part of the standard fuel genic components like paper, cardboard and wood (50 %

mix in the cement industry. In the German cement in- 70 %) and of plastics with particle sizes up to several

dustry in 2007 were about 100 million GJ of thermal centimeters in existing cement plants may bring signif-

energy used to produce about 33 million tons of cement. icant economic and environmental benefits. SRFs have

Because of the high energy intensity, questions of effi-

7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

been proven to be very advantageous substitute fuel, due

to their low production cost and highly calorific value(14

-18 MJ/kg). Moreover, co-firing in existing plants nor-

mally requires low investment costs, since little extra

infrastructure in the feeding process is usually needed.

SRF utilisation results in valuable fossil fuel savings and

CO2 emissions reduction, as well as in minimization of

waste quantities to be landfilled. The share of alternative

fuels is currently averaging about 52 % of thermal input.

For technical process optimization in the context of a

project funded by the AiF research project (Project No

15407 N), the combustion behavior of SRF was investi-

gated. The focus is on the airworthy fractions prepared

from industrial and commercial waste, as it is available

as a mixture of different materials with relatively con-

stant energy value and are often used. This methodolog-

ical investigations in the laboratory scale and in a ce-

ment factory are being implemented and the developed

methodology is verified by comparing the numerical re-

sults of CFD simulations with the measured data. The

results show that a qualitative and quantitative prediction

of the temporal behavior of combustion of alternative

fuels subject to the uncertainty by the inhomogeneous

composition is possible.

Experimental

To determine the aerodynamic particle properties that

are vital importance for the current flight support, and

hence for the distribution in the precalcinator, in LUAT,

a test plant has been designed in which the settling veloc-

ity of individual fractions can be determined. A scheme

of particle separator is shown in Figure 1. In order to

avoid turbulence in the filter with a combined blower-

filter unit from the stationary air is pumped through the

plant. The air flow is detected at the air inlet via an eddy

current flow meter. The SRF sample can be fed to the

separator via a lateral approach.

The test facility consists essentially of a counter and a

cross-flow. The actual classification of the particles is re-

alized in settling velocity by changing the volume flow

and the flow velocity. The battles with the respective

flow rates in the cross-flow particles are separated and

sent for analysis. The experimental procedure is divided

into two parts: 1. The air classification to separate the

SRF sample into groups and 2. the statistical determi-

nation of additional parameters to the selected particles.

These parameters include both material and geometric

properties, as well as the mass of individual particles of

that groups.

For determination of geometric properties of the parti-

cles from the six groups, each of 30 particles picked out

manually and individual particles mass, length, width,

height were measured. Furthermore, the material of the

Outlet diffuser

Counterflow

separator

SBS

By-pass valve

Stop valve

To

blowers

Gravity classifier

Flow meter

Figure 1: Particle Separator

Fuel

Dosage gas -

Reaction gas .

Cold air --

Exhaust

Figure 2: Field Tube Reactor

particles was recorded. These parameters can be com-

bined with the mean settling velocity of each group to

calculate an aerodynamic diameter as a proxy, which is

used in the CFD simulation. The modeling allows the

aerodynamic properties of particles, and thus the flight

path. The experimental work and simulation were car-

ried out in the field tube reator which is 100 mm diam-

eter and 1.47 m length tube shown in Fig (2), to study

about combustion behaviour of different fractions and

burnout.

7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

Table 1: Settling velocity and aerodynamic properties of different fraction

SRF Weight

wt %

Fraction 1 13

Fraction 2 32

Fraction 3 21

Fraction 4 9

Fraction 5 7

Fraction 6 Rest

Settling velocity Aerodynamic diameter

m/s mm

0.81 16

2.13 25

3.13 32

3.80 35

4.46 38

>4.46

Re Cw Apparent density

8.426 2.849 628.45

34.348 0.699 864.55

70.392 0.341 958.79

97.869 0.245 1080.07

126.523 0.19 1338.9

SRF

Plastics

Foils

Paper

wood

Fibers

Weight %

19

26

11.8

4.4

9.7

Table 2: Proximate and Ultimate analysis of SRF

proximate analysis, (wt %) Ultimate analysis,(wt %)

Fixed carbon Volatile C H O N

1.0 99 75.2 10.4 12.4 0.5

5.0 95 78.4 12.9 4.35 0.6

20 80 45.7 6.8 44.5 0.8

16.4 83.6 49.0 6.3 43.8 0.2

17.9 82.1 55.1 6.8 28.8 2.3

1 Forces on the particle

1.1 Particle force balance

The equation governing particle velocity i in a nonuni-

form flow field u was derived by Maxey and Riley

(1983).

inertia

inertia

67al (u V+ V(pp pf)g

drag gravity

Du 1 d

2 t ,dt

pressure gradient virtual mass

Jo 7 _(t V ) V I

67a_/t_ \V Pf 2/ )

Baset history term

In the above equation, we neglected the Faxen correc-

tion term, which becomes ,inilik.IIh only in the event

of large curvature in the velocity profile. pf and p are

the density and viscosity of the surrounding fluid; pp and

V represent the density and volume of the particles; and

i = [u uz] describes the fluid undisturbed veloc-

ity vector at the point occupied by the particles centre

of mass, with respect to the inertial frame. In Eq (1)

the derivative d / dt is used to denote a time derivative

LHV as received

(MJ/Kg)

34.6

39

17.6

18.3

22.6

following the moving particle, so that

dui ( ui ,Oui

dt a +vO)

is the time derivative of the velocity component ui. the

derivative D / Dt is used by contrast to denote the time

derivatives following a fluid element, and

Dui (ui aui\

D Kt-+ UajxIj (3)

is the fluid acceleration as observed at the instantaneous

centre of the particle. The different terms in Eq (3) rep-

resent in order, the force needed to accelerate the par-

ticles, viscous stokes drag, gravity, a pressure gradient

force accounting for the acceleration of the displaced

fluid, virtual mass force, and basset history term. The

order-of-magnitude estimates presented by Lazaro and

Lasheras (1989) indicate that, for small heavy particles

in the dilute regime, drag and inertia effects dominate

over those of the pressure field, the cirtual mass, and

the particles history. According to this, only the terms

describing inertia drag and gravity are retained in the

study of accumulation and dispersion of heavy particles

in the forced mixing layers (Martin Meiburg, 1994; Raju

Meiburg, 1995).

In this study, since our final purpose is to track the

nonspherical SRF particle in SRF fired cement kiln, and

the PVC particles used for experiment is a light particle,

the virtual mass force is retained, as well as the force

arising due to pressure gradient in the fluid. Addition-

ally, since we are tracking nonspherical particles while

Sphericity

0.008

0.2

0.42

0.64

0.8

7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

not sphere, a lift force should also be taken in to account,

as stated above. Therefore the governing equation for

evaluation of the particles velocity then becomes

ldv

PPV d

gT

FD+V(pp-pf)J+FPC+FPVM+PFL, (4)

where PD, FpP, PVM and FL represent drag, force

due to pressure gradient in the fluid, virtual-mass force

and lift force respectively. The forces FpG and FVM

are determined in the same way as shown in Eq (4);

while FD and FL governed by particles orientation (e.g.

Blaser, 2002; Hoerner, 1965), will be introduced below.

In this model, except Faxen correction term and

Basset history term, some other terms, such as ther-

mophoretic force and Brownian force are also neglected,

as well as Saffman lift thanks to the negligible rotation

with respect to the major axis of particles, i.e. z', axis.

It might be acceptable to exclude these forces in the par-

ticle force balance of the gas-solid flow in a SRF fired

cement kiln.

1.2 Drag force

The drag force is commonly expressed over the entire

Reynolds number spectra as

FD CDPf Sef f 1 V(u V), (5)

2

where Seff is the particle area normal to the direc-

tion of the drag force. It changes with the incidence an-

gle (ai) between relative velocity (i v),and particle

major axis direction is determined by

Sefti = ia (cos2 + (43 / i) sin ) (6)

where CD is the drag coefficient. Chhabra, Agarwal,

and Sinha (1999) collected about 1900 individual exper-

imental data of drag coefficient on nonspherical parti-

cles from 19 independent studies, covering wide range

of particle shapes: cyclinders, needles, cones, prisms,

discs, cubes, and so on, and excompassing wide ranges

of physical and kinematics conditions as: sphericity of

0.09-1 and Reynolds number of 10 4- 5.105; examined

critically the performance of five commonly used meth-

ods with those data, including Haider and Levenspiel

(1989) and Ganser (1993); and found that the best meth-

tod was that proposed by Ganser (1993): the resulting

overall mean error is about 16 %. The method of Ganser

(1993) is therefore used here,

24

-24 [1 + 0.1118(RepK1K2)0 6567

RepK1 K

0.4305

1+ 3305/(RepK1K2)'

(7)

where coefficient K1 = (dn /(3d) + 2 /( ;-" '))1; co-

efficient K2 = 10' ' '' ; d, is the equal pro-

jected area circle diameter d, = Re is the

particle Reynolds number, defined on the basis of the

relative velocity between particle and surrounding fluids

and equal-volume sphere diameter; e is particle spheric-

ity, and can be calculated as e = s/S, s is the surface of

a sphere having the same volume as the particle and S is

the actual surface area of the nonspherical particle.

1.3 Lift force

A lift force can be directly related to a drag force; how-

ever, it is still difficult to assign a specific value to the

constant of proportionality. Here, a lift formation is

used, in which the particle major axis (z') is also used

to characterize the force.

Considering the lift force is orthogonal to the relative

velocity (u v) and lies in the plane defined by the par-

ticle major axis direction (z') and the relative velocity

and that the lift must be invariant under a 180 rotation

of the particle major axis z' and vanishes if a, < 0 or i7,

we express the lift forces as

FL 2 CLpf Sfj 1 -8-C-J', (8)

where SeJf2 is the particle area normal to the direc-

tion of the lift force and is calculated by

Seff2 = 7a 2(sin2 i + (4 / Cr)2cos2 ai), (9)

Where CL is the lift coefficient. In this study, it is

determined in the way that the ratio of lift to drag meets

the relationship (Hoerner, 1965)

I = Ssin 2 aos ai (10)

IFD

1.4 Initial heating of biomass particles

During the first stage, a simple heat balance (without any

mass transfer) is used to relate the particle temperature to

the convective heat transfer and the absorption/emission

of radiation at the particle surface

cdT

mpQ-t = hAp(T, -Tp)+epApa(T -TR ), (11)

in which Cp, Tp, h, Ap, To, ~p, a, and OR repre-

sent particle heat capacity, particle temperature, convec-

tive heat transfer coefficient, particle surface area, lo-

cal fluid temperature at particle position, particle emis-

sivity, Stefan-Boltzmann ,and radiation temperature, re-

spectively. The heat transfer coefficient, h, is calculated

by Nu hdp / k o 2 + 0.6Re/ 2Pr /3 where koo

Pr are thermal conductivity and Prandtl number of the

fluid, respectively. The radiation temperature OR is de-

fined by (, )1 /4 where I is the radiation intensity. The

surface area of a cylindrical particle is calculated by,

Ap Trd / e where e is the particle shape factor.

1.5 Modelling species conservation and gas

phase reactions

The mixing and transport of chemical species are mod-

eled by solving the conservation equations describing

convection, diffusion and reaction sources for each com-

ponent species. The standard model usually applied for

the volatiles combustion is the Eddy-Break Up (EBU)

model of Magnussen and Hjertager (1977). It is based

on the assumption that chemical reactions occur rapidly

compared to mixing. The limiting step is therefore the

mixing process, which is governed by the rate of dissi-

pation of the fuel- and oxygen-containing eddies. The

reactions taken into consideration for the combustion of

coal or biomass volatiles are usually global with a maxi-

mum of two reaction steps, as presented in Eq (12). The

volatiles are expressed as a hydrocarbon CHyOzNa,,

where coefficients x, y, z and a derive from the ultimate

analysis of the fuel and the enthalpy of formation of the

volatiles is based on their heating value.

C1HON x y

C H OzNo + 2 4

) 02 -> xCO+

y a

2HO N2

1

CO + 1 02 -- C02 (13)

1.6 Devolatilization of biomass particles

When the temperature of the particle reaches the va-

porization temperature, devolatilization begins to release

volatiles. The homogeneous combustion of the volatiles

takes place once they are released from biomass parti-

cles. A single-rate kinetic devolatilization model is used

to predict the volatiles yield rate, which assumes that the

rate of devolatilization is dependent on the amount of

volatiles remaining in the particle via a first-order reac-

tion

dm k(mp (1 fo)mpo (14)

dt

where k, fvo, and mpo denote the kinetic rate, the mass

fraction of the volatiles that are initially present in the

particle, and the initial particle mass, respectively. The

kinetic rate k is defined by the input of an Arrhenius-

type, pre-exponential factor and an activation energy ex-

pression

k = Al E/RT, (15)

7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

where A1 and E are taken to be 10E+06s-

and 74E+07J/Kmol, respectively, for the biomass

(Kaer,2001). The particle diameter changes during de-

volatilization according to the swelling coefficient, C,,

d~,

dp +(C'.

dp, 0

1) mp, mp

fvormp,

where dp,o and C,, are the particle diameter at the start

of devolatilization and the swelling coefficient, respec-

tively. The particle temperature during devolatilization

is updated by including contributions from convection,

radiation and the heat consumed during devolatilization:

CdT

dt

hAp(T, Tp) + dHf,+

_pApa(O4 T4)

where Hfg is the latent heat of the particle.

1.7 Char combustion

The combustion of biomass char is quite complicated

since it is affected not only by the composition of the

biomass fuel but also by the shape and size of the fuel

particles. Biomass char is believed to be more reac-

tive than coal char. Different measures in modeling of

biomass char combustion can be found in literature, for

instance, using diffusion-limited surface reaction rate

model modified by aspect ratio-dependent enhancement

factors (in the range of 1-1.6 for the aspect ratio from

1 to 11) due to the non-sphericity of biomass particles

(e.g., Gera et al. 2002; Yin et al. 2004), and using

Smiths-intrinsic model modified by a constant enhance-

ment factor of 4 in order to represent the high burning

rate of the biomass char particles (e.g., Ma et al. 2007,

2009). In this study, due to the lack of non-spherical

properties of the straw particles and the low char content

in the straw, the diffusion-limited surface reaction rate

model is used in all the simulations, without considering

any enhancement factor.

2 Results and discussion

2.1 Particle track and devolatilization

A Lagrangian particle model (Crowe, 1979) is used to

calculate particle aerodynamics and turbulent dispersion

for the SRF particles in the field tube reactor. The con-

tinuous distribution of discrete particle size, velocity and

material property such as apparent density is represented

by discrete number of particle streams (typically several

thousands). Each size is characterized by an initial posi-

tion, particle size, mass, velocity, temperature and num-

ber flow rate. The flight history of each stream through

the field tube reactor is determined by gravity, the aero-

dynamic forces such as drag, lift and pressure forces and

turbulent dispersion. The model compute the drag force

Table 3: Settling velocity of different fractions m/s

Experimental Simulation

Fraction 1 0.81 0.89

Fraction 2 2.13 2.14

Fraction 3 3.13 3.18

Fraction 4

Fraction 5

Fraction 6

3.8

4.46

>4.46

for the nonspherical particles (Heider and Levenspiel,

1988) such as wood, plastic and fibers which may be

tumble and rotate as they progress in the tube reactor.

A stochastic particle dispersion model (Milojevis, 1990)

accounts for the effects of turbulent fluctuations in the

gas velocity on the instantaneous motion of the particle.

As fuel particles are heated by radiation and convection,

then dry and burn at nearly constant volume and break

up after completion of char combustion. Weight loss and

density variations have a significant effect on the gravita-

tionaly and aerodynamic forces on each particle. Mois-

ture, volatiles, and char products that evolve from the

particles during flight are used in the solution of overall

material and energy balance. The motion of the three

Figure 3: Lagrangian particle track for different fraction

((a) Fr 1, (b) Fr 2, (c) Fr 3)

different fractions are shown in Fig (3). Quite clearly,

it is possible to see that for the fraction 1, dispersion

is a quicker process, by which the particles already in

the quarl section have reached a higher dispersion due

7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

to their aerodynamic forces such as drag and lift forces.

Due to the misalignment of the flow streamlines, and the

S0.00953

0.00763

0.00572

0.00381

10.00191

0.00000

(a)

0.00011

0.00009

0.00007

0.00005

0.00002

0.00000

(b)

'I

H 0.00028

0.00023

0.00017

0.00011

0.00006

0.00000

(c)

Figure 4: Mass fractions of volatiles for different frac-

tions ((a) Fr 1, (b) Fr 2, (c) Fr 3)

direction of the aerodynamic forces upon the particles,

they break off from the streamlines, and tumble in a dis-

orderly fashion through the test rig. For fraction 2 and

3 which is having relatively higher mass than fraction

1 carried along the fluid stream and dispersion is little

bit slower due to higher gravitational forces than lift and

drag force. Almost end of the field tube reactor particle

movements for different fractions are similar because of

their weight loss during the devolatilization. Due to their

difference in mass and aerodynamic forces, particle set-

tling velocity vary for each fraction. Settling velocity of

different fraction materials were compared with experi-

mental data which shows good agreement with simula-

tion results shown in table (3) for fraction up to 3. For

fraction 4-6 we can clearly see that the predicted set-

tling velocity deviate from the experimental data due to

the presence of inert metal wastes which are mixed with

SRF particles. Due to their difference in settling veloc-

ity of the particles, residence time within the field tube

reactor reduced. Thus promote different particle heat-

ing and devolatilization behaviour for different fractions

as shown in Fig(4), and also different material such as

plastics, foils, paper and wood has different combustion

behavior. Mass fraction of the volatiles released from

SRF clearly shows the trent that aerodynamic and grav-

itational forces affecting the heating and devolatiliza-

tion within the filed tube reactor. Since the particle vol-

ume is not changed neither during devolatilization due

I400

350.4

300.9

251.3

S201.8

on. /

Figure 5: Track of particles colored by particle density

(kg/m3) ((a) Fr 1, (b) Fr 4)

to C,, = 1 nor in diffusion-limited char combustion.

Therefore burnout fraction will be reflected from history

of particle density as shown in Fig (5). As discussed

above the particle track is affected by two factors: the

rate at which the particle loses its weight (i.e., burning

rate) and the different aerodynamic forces.

3 Conclusion

From the numerical study in a 1.47 m-long field tube re-

actor, the following conclusions could be drawn on SRF

combustion simulation. It is very important to take into

account the particle non-sphericity in order to achieve

a better understanding on SRF combustion. The par-

ticle non-sphericity affects greatly both its motion and

its combustion. These two aspects are interacting with

each other very closely in SRF combustion. If the mo-

tion of a particle is not predicted accurately, it is less

likely to predict the particle combustion correctly. On

the other hand, if the particle burnout is not predicted

correctly, the aerodynamic forces on the particle may

not be sufficient to sustain the weight of the particle and

thus the fate of the particle could be changed totally. To

accurately model the motion of SRF particles, it is es-

sential to include all the important forces in the parti-

cle force balance. In this paper, a drag correlation for

non-spherical particles, an additional lift due to particle

non-sphericity, lift force and virtual-mass force due to

relatively light particles are taken into account as well

as gravity and a pressure-gradient force. Since the drag

Nomenclature

Roman symbols

C,

0D

CL

Csw

d,

T,

To,

FD

FL

Surface area of the particle, (m2)

Heat capacity of particle, (J/(KgK))

Drag coefficient

Coefficients in evaluating lift force

Swelling coefficient

Equal-volume sphere diameter, (m)

Particle temperature, (K)

Local fluid temperature, (K)

Drag force, (N)

Lift force, (N)

FpG Force due to the pressure gradient

in the fluid,(N)

FVM Virtual mass force,(N)

h Convective heat transfer coefficient, (W/(mr2K))

mi Particle mass, (kg)

mpo Initial particle mass, (kg)

Re, Particle Reynolds number, (kg)

Seff 1 Projected area of particle normal

to drag force, (m2)

Seff2 Projected area of particle normal to

lift force, (mn2)

flo Mass fraction of volatiles that are

initially present in the particle (-)

Greek symbols

particle major axis z'

Ep particle emissivity

P viscosity of the surrounding fluid, (kg/ms)

pf density of the surrounding fluid, (kg/m3)

pp particle bulk density, (kg/m3)

a Stefan-Boltzmann constant,

(5.67 10 W/m2K 4)

7th International Conference on Multiphase Flow,

ICMF 2010, Tampa, FL, May 30 -June 4, 2010

and the lift are both shape factor and particle orientation-

dependent, the coupled particle rotation is also resolved.

To better model the combustion of SRF, it is primary to

take into account the actual particle surface area avail-

able and the average oxygen mass flux at particle sur-

face, both of which are shape factor-dependent. In or-

der to model these SRF particles experimental work were

carried out to characterize SRF particle as a mixture for

better understanding of aerodynamic properties and set-

tling velocity of SRF particles and these properties were

used for the simulation which shows good agreement

with simulation results.

Acknowledgements

The authors would like to thank the AIF for the financial

support of the project (15407 N)

1000

997

994

991.1

988.1

985.1

particle sphericity (= s/S)

Subscripts

max maximum

ref reference

Superscipts

9 Gas

p Particle

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