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DOI: 10.1177/2041296710394249

2011 225: 74Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and EnergyH Lee, S Choi and M Paek

A simple process modelling for a dry-feeding entrained bed coal gasifier

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74

A simple process modelling for adry-feeding entrained bed coal gasifierH Lee1, S Choi1, and M Paek2

1Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Yuseong-Gu,

Republic of Korea2Doosan Heavy Industries and Construction Co. Ltd, Republic of Korea

The manuscript was received on 20 April 2010 and was accepted after revision for publication on 9 September 2010.

DOI: 10.1177/2041296710394249

Abstract: A dry-feeding entrained bed coal gasifier was numerically modelled by simultaneouslysolving the rate equations for chemical reactions of the solid andgas phases. This model describessimplified physical and chemical processes in the entrained bed coal gasifier. Chemical reactionprocesses for coal gasification and combustion are considered along with the simplified gas flowpassage in the reactor, so that progress of reactions at the designated spatial location is repre-sented. Gasification phenomena of coal particles were separated into devolatilization, gas-phase,and solid-phase reactions. Coal gasifier geometry was simplified to a pseudo-two-dimensional(pseudo-2D) reactor model based on the 1D plug flow concept. The dimension in the pseudo-2D model was conceptually divided by considering the recirculation effect. As a result, carbonconversion, cold gas efficiency, and temperature distribution were obtained at variable oxygento coal mass ratio, steam to coal mass ratio, and operating pressure. Operating conditions couldbe appropriately controlled by knowing the degree of reaction in the reactor.

Keywords: coal gasification, entrained bed coal gasifier, process analysis, chemical kinetics

1 INTRODUCTION

A coal integrated gasification combined cycle (IGCC)is a power cycle based on coal gasification technol-ogy. It is known that the net efficiency of the IGCCis higher than that of the conventional pulverizedcoal power plant, and white emissions, such as SOxand NOx, are sufficiently removed. The coal gasifier isprobably the most important unit among IGCC com-

ponents. The gasifier converts coal into combustiblesyngas, including CO and H2, at high-temperature andhigh-pressure conditions. Since the performance ofthe gasifier directly affects the following units of theIGCC, such as purifier of pollutant, gas turbine, andsteam turbine, operating the gasifier is an essentialtechnology. Although entrained bed coal gasifiers havebecome available through a number of demonstration

Corresponding author: Department of Mechanical Engineer-

ing, Korea Advanced Institute of Science and Technology, 373-1,

Guseong-Dong, Yuseong-Gu, Republic of Korea.

email: [email protected]

projects, information on the commercial coal gasi-fier is restricted. Therefore, an independent attemptto accumulate technical understanding is requiredbecause of the insufficiency of open information.

Previously, there has been some research on mod-elling the entrained bed coal gasifier. Govind andShah [1] developed a mathematical model to simu-late a Texaco down-flow entrained bed pilot plantgasifier. Ni and Williams [2] suggested a multivariablemodel for a Shell coal gasifier on the basis of equi-librium, mass, and energy balances. Vamvuka et al.[3] developed a one-dimensional (1D), steady-statemodel based on mass and energy balances, hetero-geneous reaction rates, and homogeneous gas-phaseequilibrium. Chenet al. [4] developed a comprehen-sive 3D computational fluid dynamics (CFD) simula-tion model. Approaches to modelling the entrainedbed coal gasifier are mainly classified into two waysbased on research objectives: one is solving the chem-ical equilibrium state of reactants at given condi-tions and the other is CFD analysis. The equilibriumstate of chemical reactants is meaningful because the

reactions move towards the equilibrium state eventu-ally. Equilibrium calculation can be used if research

Proc. IMechE Vol. 225 Part A: J. Power and Energy

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A simple process modelling for a dry-feeding entrained bed coal gasifier 75

objectives are to obtain the performance of the reac-tor. The results can be regarded as an indication of thetheoretical limits of gasification. On the other hand,fluid flow as heat transfer can be interpreted by CFDanalysis. CFD analysis provides results for the predic-tion of fluid flow in the gasifier with considerations of

reactor geometry and operating conditions.The purpose of this study is to develop a model that

could analyse chemical reaction processes in the gasi-fier as a function of oxygen to coal mass ratio, steamto coal mass ratio, and operating pressure. This articleshows a different approach compared with the previ-ous equilibrium model or CFD analysis. It is intendedthat one develop reaction models in the entrainedflow gasifiers for designing and/or scaling up, as theavailable basic experimental data on phenomena in ahigh-temperature and pressurized gasifier can be uti-lized [5]. As chemical reaction processes in terms of

coalgasification and combustionare interpreted alongwith gasifier height, the degree of reaction is obtainedat variable operating conditions. For process analy-sis, simple chemical kinetics in terms of coal reactionis considered. As a result, it is possible to investi-gate the properties distribution along with the reactor.

Visual Basic 6.0 code was used to write in-housecodes.

2 A DRY-FEEDING ENTRAINED BED COALGASIFIER

An entrained bed coal gasifier is a representativegasifier type used in coal IGCC plants. There is anational project for 300 MWe IGCC demonstrationplantconstruction. Korea selected a Shell-typegasifier,a dry-feeding entrained bed coal gasifier with up-flowof feedstock, which can achieve high carbon conver-sion within short residence times. The residence timeof coal particles and other gases is about 34 s [6].The operating condition of the dry-feeding entrainedbed coal gasifier is on average 1800 K temperature and4.2 MPa pressure, and ashes are extracted as moltenformat (slag) from the bottom. In this study, a sim-

plified geometry of the gasifier is described, as shownin Fig. 1.In this gasifier, mixed oxygen and steam are intro-

duced simultaneously from four burners at the lowerwall of the reactor. At the same time, pulverized coaland nitrogen as transport gas are introduced fromthe same burners in partial oxidation condition. Coaland nitrogen are injected into the gasifier throughthe centre hole of the burners, while a mixture ofoxygen and steam is blown in through surroundingholes of the burner. The pulverized coal is mixed

with limestone to lower the melting point of the ashin order to improve the slag discharge. Therefore,

four diametrically opposed burners are fed simulta-neously with a coallimestone blend transported by

Fig. 1 Rough sketch of Shell a gasifier and schematic of

a simplified gasifier model

Table 1 Basic conditions of the model gasifier for300 MWe

power generation

Operating condition Value

Average temperature 17231873 KAverage pressure 4.2 MPaFlow direction Circling up flow

Ash extraction Slagging typeFeeding type Coal Dry-feeding

Transport gas NitrogenOxidant (purity 95 vol%) Oxygen blown

Basic feedstockflowrate

Coal 2450 tonnes/day

Oxygen/coal mass ratio 0.75

Steam/coal mass ratio 0.05Nitrogen/coal mass ratio 0.112Gasifier dimension Volume 50 m3

Height 9 m

nitrogen and an oxygensteam mixture at 423 K tem-perature. Table 1 shows the basic conditions of thesubject gasifier for 300 MWe power generation. Thevalues were suggested by Doosan Heavy Industriesand Construction Company.

3 MODELLING

3.1 Gasifier geometry

The specific dimension of the gasifier is unknownbecause of a confidential affair. Therefore, the geom-etry of the gasifier is inevitably simplified. The realcoal gasifier is a 3D reactor including slag bath,complex burner geometry, and water membrane. Asreactive heat is released, the heat is not regularlytransferred to the wall because of radiation betweenheterogeneous coal particles. In addition, swirl flowmakes the phenomena very complex. Therefore, a 3Dmodel should be made for a detailed modelling of

the gasifier. However, it is inefficient to make the 3Dmodel from the point of process analysis because heat




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76 H Lee, S Choi, and M Paek

and mass transferred across the reactor radially aregenerally more dominant than that along with thereactor axially. If the distribution of the feedstocks ishomogeneous over the gasifier, 1D or 2D modelling isappropriate. This concept is based on the experimen-tal results in pilot scale [4, 7]. In this study, gasifier

geometry is simplified to a steady-state pseudo-2Dreactor model in stages. Figure 2 shows a simplifiedschematic of the pseudo-2D reactor model based onthe CFD analysis result in the Shell gasifier, as shownin Fig. 3 [8].

From the simulation with the CFD code, FLUENT,information about scale and passage of fluid flow

Fig. 2 Conceptual divisions in the pseudo-2D reactor

model

Fig. 3 Recirculation zone based on CFD analysis [8]

in the reactor is obtained, including the recircula-tion pattern [8]. Therefore, the simplified flow pat-tern is described based on rough assumptions in thepseudo-2D model. The geometry in the coal gasifieris conceptually divided into five sections, (A) burnerzone, (B) upper recirculation zone, (C) lower recircu-

lation zone, (D) axial flow zone, and (E) dead zone,based on flow pattern as shown in Fig. 3. However,the mixed feedstocks flow as 1D plug flow in sequenceas indicated in Fig. 2 and in each designated sectionof the pseudo-2D model through 2D effective cross-section of each zone. Feedstocks are introduced intothe gasifier throughthe two burners. It is assumed thatfeedstocks are homogeneously and perfectly mixed. Ifintroduced flow reaches the central axis of the gasifierthrougheachburnerofdiameter12cm,theflowissep-arated into two flows. One is upper flow of 90 per centand the other is lower flow of 10 per cent based on gas

volume. They are recirculated in the upper and lowerparts of the gasifier, respectively. Since the path of therecirculation is known by CFD analysis, recirculationtime can be calculated by using the rate equations formodelling coal gasification. After the flow is recircu-lated in the upper and lower parts, they meet togetherat the central axis and then the flow goes to the gasi-fier exit. A uniform heat loss rate of 10 per cent in theburner zone and 3 per cent to the wall in the otherzones is assumed [9].

3.2 Coal gasification

Understanding coal behaviour is critical in designingand operating the gasifier. However, modelling coalgasification and combustion behaviour is not a simpletask in terms of fundamental and theoretical descrip-tion, and also in terms of a practical and predictivemodel of anymeaning.It is possible to model coal gasi-fication and combustion based on rough assumptionsconcerning physical and chemical processes. Volatilesare released from coal in a short period of time (in mil-lisecond order) at high temperatures around 1800 K;on the other hand, the reaction rate of solid-phasereactions is relatively slower (in second order). There-

fore, the complex reactive behaviour of coal can bedivided into three steps by the difference of reac-tion rate in terms of devolatilization, heterogeneousreaction at solid particle surfaces, and homogeneousreaction of various gaseous components. Analysis dataof design coal for this study are given in Table 2, but

Table 2 Analysis data of design coal

Ultimate analysis (wt%, mf) Proximate analysis (wt%, air dry)

C 70.65 Fixed carbon 49.25H 4.94 Volatile matter 38.76O 22.71 Moisture 2.06

N 0.66 Ash 9.93S 1.04 HHV (cal/g) 6925




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the terminology of volatile matter and fixed carbon ispoorly defined. Interrelation among ultimate analysis,proximate analysis, and the heating value is not clear.In this study, however, proximate analysis means thatthe quantity of fixed carbon is equal to that of charand volatiles are evolved as 38.76 per cent of input coal

mass except for an inherent moisture of 2 per cent incoal.

3.2.1 Devolatilization

The importance of coal devolatilization as the ini-tial step in coal conversion makes its understandingessential in the optimization of processes [10]. Volatilematter is composed of many complex substances.The composition of volatiles is not arbitrary, as itdepends on the composition of original coal. If thecomponents are assumed, they will remain on stateof CXHYOZ based on mass balance. It is difficult todistinguish components of the volatiles easily. Thereis some experimental research for devolatilization.The effects of pressure, temperature, and particle sizeon the yield of products were interpreted [1013].

As a result, the main factors that influence pyrol-ysis are heating rate and peak temperature in thereactor. However, there is no general theory to pre-dict the behaviour expected for the wide variety ofcoals and conditions of interest. Complicated struc-tures and physical and chemical phenomena of thecoal reaction are a part of the reason. In this study,the components of volatiles were assumed to be CO,

CO2, CH4, C2H6, H2, H2O, and C6H6. The mass of liq-uid hydrocarbons is represented by benzene (C6H6)[14]. The kinetics of individual product formation ismodelled using independent, parallel reactions, andis first order in the amount of product yet to beformed. Equation (1) is the first-order rate equationfor modelling devolatilization

dVi

dt =

i

kij0exp

Eij

RT

(Vij Vij) (1)

A first-order kinetics model of devolatilization has

been shown to have limitations. Nevertheless, it hasbeen found to give an adequate representation ofhydrocarbon yield. A complete devolatilization model

would describe the composition and physical state ofthe coal or char particles at all stages of devolatiliza-tion. However, the microscopic analysis is not impor-tant in the objectives predicting the process in thereactor. Therefore, a one-step mechanismwas applied.This meansthat coalchar+volatiles can be writtenafter pyrolysis finishes.

3.2.2 Homogeneous reaction

There are generally two ways to model gas-phase reac-tions.One is based on a global reaction and the other is

based on a kinetic reaction, including an intermediatemechanism. It is difficult to consider the kinetics ofall intermediate reactions for one chemical reaction.In this study, a simple global reaction was consid-ered. It is appropriate to consider the global reactionbecause it has the advantage of simulating the over-

all coal reaction rate by considering many chemicalreactions. Equation (2) is the rate equation for mod-elling the homogeneous reaction. The reaction rate isproportional to the concentration of reactants in thegas-phase reactions

d[X]

dt = Ai exp

E

RTg

[X]a[Y]b (2)

3.2.3 Heterogeneous reaction

There are two kinds of representative reaction rateequations: nth-order and LangmuirHinshelwood-

type reaction rate equations [15]. In this study, thenth-order reaction rate was applied to describe theheterogeneous reaction. It is assumed that the overallchar reaction rate is proportional to the nth power ofpartial pressure of the gasifying agent and follows the

Arrhenius equation. Equation (3) is the rate equationfor modelling the heterogeneous reaction

dx

dt = A0P

nA exp

E

RTs

(1 x)

1 ln(1 x)

(3)

In modelling the solid-phase reaction, it is com-

mon to consider the chemical reaction as occurringon internal surfaces of the solid, but an estimate of thesurface area of the char is very difficult because of alarge amount of micro-pores. The random pore model

was applied to describe the gasification model for arbi-trary pore size distributions in the reacting solid [16].Itpredicts the reaction surface area at a given conversionas a function of an initial pore structure parameter.In this modelling, the pore structure parameters wereassumed as values in the literature [7,17]. If the valueof the pore structure parameter is zero, this means thatthe char is very porous. A larger value of the porestruc-

ture parameter means that the pore in the char growswith the progress of char gasification.Although many chemical reactions occur in the gasi-

fier, dominant chemical reactions were considered asshown in Table 3. The important reason why thesereactions were selected is that rate constants hadbeen specified in the open literature [7, 14, 15, 17].Since kinetics should be considered over homoge-neous and heterogeneous reactions, it is necessary toknow kinetic data such as pre-exponential factor Aand activation energyE. They are constitutive termsthat represent the rate of each chemical reaction inthe Arrhenius equation. The pre-exponential factor is

the total number of molecular collisions per unit timeand activation energy is a minimum amount of energy




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Table 3 Rate constants [7, 14, 15, 17]

Volatile component kij0(s1) E(kJ/mol) V(wt%)

Devolatilization[14]H2O 10

13 312 16.38CO 1011 238 6.54CO2 10

9 168 3.55C6H6 1013 312 12.04CH4 10

7 91 0.04C2H6 5 10

9 214 0.08H2 5 10

10 222 0.13

Chemical reaction Ai(s1) E(kJ/mol) a b c

Homogeneous reaction[14]CH4 + 1.5O2 CO + 2H2O (519.5 kJ/mol) 2.8 10

9 202.64 0.3 1.3 C2H6 + 2.5O2 2CO + 3H2O (862.4 kJ/mol) 7.31 10

9 125.6 0.1 1.65 C6H6 + 4.5O2 6CO + 3H2O (1471.3 kJ/mol) 1.35 10

9 125.6 0.1 1.85 CO + 0.5O2 CO2(283.1 kJ/mol) 2.238 10

12 167.47 1 0.25 0.5 [H2O]H2 + 0.5O2 H2O (242.0 kJ/mol) 8.8 10

12 168 0.25 0.25 CO + H2O CO2 + H2(41.2 kJ/mol) 2.75 10

10 83.8 0.5 1

Chemical reaction A0(s1) E(kJ/mol) n

Heterogeneous reaction[7, 15, 17]C(s)+ 0.5O2 CO (110.5 kJ/mol) 1.36 10

6 130 0.68 14C(s)+ H2O CO + H2(+131.3 kJ/mol) 8.55 10

4 140 0.84 3C(s)+ CO2 2CO (+172.5 kJ/mol) 6.78 10

4 163 0.73 3C(s)+ 2H2 CH4(74.6 kJ/mol) 1.8 10 138 0.5 3

CO + 0.5O2 CO2reaction has an additional exponent constantc for H2O concentration.

to be transformed into products. From the viewpointof collision theory, it can be seen that either increasingthe pre-exponential factor or decreasing the activationenergy will result in an increase of reactivity. In kineticdata with respect to the coal reaction, it is ambigu-ous to have faith in that accuracy because the datavary according to experimental conditions and coaltype. Nevertheless, available data in different litera-tures were applied to coal gasification andcombustionreactions in this study. This may cause uncertainty.Table 3 represents the rate constants applied to therate equations. Each of the rate equations is evalu-ated along with the associated kinetic data to modela reactor considering operating condition.

By simultaneous calculation of the rate equationsafter devolatilization occurs, the gas component perunit time is calculated and then gas temperature iscalculated based on the gas component at that time.

The heat of reaction, produced or absorbed, from eachchemical reaction and the heat capacity of mixed gasareused.The initial temperature of feedstocks is 423 K.Heat capacity coefficients are from the literature [18].It is assumed that the temperature of coal particles ishigher than that of gases [14]. The velocity of the mix-ture is obtained by considering mass flowrate of themixture, average density of the mixture, and an effec-tive cross-section wherereaction occurs. Equations (4)and (5) indicate the relations. They are based on theideal gas equation of state

mix=

i

Xi PRg,i Tg

(4)

V = mmix1

mix

1

Aeff(5)

Mass andheat balance equationsare obtained alongwith spatial location in the reactor. The phase is com-pletely mixed after pyrolysis and each compartment

is treated as a stirred tank reactor in the system.The number of moles of each species changes; how-ever, total mass is conserved. As chemical reactionsprogress, heat is produced and absorbed becausethere are both exothermic and endothermic reactions.Figure 4 shows a schematic diagram of the mass andheat balances. Equations (6) and (7) show the massbalance and equation (8) shows the heat balance.Based on the assumptions, mass and heat balanceequations are set up for each control volume as shownby (refer to Fig. 4)

Gas phase

(Wg,i)n+1 (Wg,i)n

=

j

(i,j rj)prod

j

(i,j rj)react (6)

Solid phase

(Ws)n+1 (Ws)n =

k

(s,k rk) (7)

(Wg,mixCP,mixTg+ WsCP,sTs)n+1

(Wg,mixCP,mixTg+ WsCP,sTs)n

=j

(Hj)rj+k

(Hk)rk Hloss,wall,

whereT0 = 423 K (8)




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Fig. 4 Schematic diagram of mass and heat balances in

nth control volume

The above equations mean a total heat balance onboth solid and gas phases in each control volume. Thespecific heat of the gaseous mixture is

CP,mix(Tg) =a +b Tg+ c T2

g+d T3g

a =

i

CPa,i Yi

b =

i

CPb,i Yi

c=

i

CPc,i Yi

d=

i

CPd,i Yi

(9)

3.3 Major uncertainty

3.3.1 Rate constant

The kinetic data were adopted from different refer-ences [7], [14], [15],and[17]. These are the incoherentrateconstants incorporated from different experimen-tal conditions and coal types. This is a serious problemfor application of chemical kinetics because the rate

constants are sensitive to the experimental environ-ment. Therefore, it is one of the reasons to cause the

uncertainty. Since each time different kinetic data aremeasured in experiment, it is impossible to reflectaccurate kinetic characteristics for the coal reaction.However, if the equilibrium state is not considered,the rate constants are inevitably required. The quan-tification of the rate constants should be seriously

debated.

3.3.2 Heat transfer

The heat transfer mechanism is very complex inthe gasifier. The most important thing is the heattransfer among coal particles. Coal particles emitstrong light during the combustion process. Mod-elling the interactive heat transfer is very difficult.Since the gasifier operates at high temperature around1800 K, radiation is the dominant mechanism. Irreg-ular heat transfer complexifies the coal gasification

and combustion phenomena. However, simple anduniform heat loss to the wall was considered inthis study. It is one of the reasons to cause theuncertainty.

3.3.3 Turbulence and swirl

Tangential firing burners make strong swirl flow in thegasifier and turbulence complexifies coal gasificationand combustion phenomena. However, the turbu-lence was assumed to be neglected in this study. Itmaycause the uncertainty. If the turbulence is addition-ally modelled, it will affect the mixing of the chemical

species and other phenomena.

3.3.4 Minor species

Syngas at the gasifier exit is introduced into the follow-ing clean-up system. The first purifier is a desulphur-izer, so it is important to predict the quantity of theCOS and H2S species for operation of the sequentialpurifier system. As these are minor species, the con-centration is very low. In this study, sulphur was notconsidered, so the concentration of the minor speciescannot be predicted. Since nitrogen is assumed as

inert, the concentration of poisonous gas such as NH3cannot be predicted.

3.3.5 Volatile yield

The yield of volatiles and their composition dependnot only on the volatile matter content of raw coalbut also on the operating conditions during pyroly-sis [13, 19, 20]. A variety of gases have been foundin the volatiles with the proportions of differentgases dependent on coal chemistry and the con-ditions of devolatilization. Although devolatilizationkinetics is being strongly influenced by physical pro-

cesses during coal decomposition, the devolatiliza-tion process is simplified as a one-step mechanism.




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The composition of volatiles was assumed to beconstant for seven species. This will cause theuncertainty.

3.3.6 Coal particle size

The shape of all particles is different. Most particlesare generally pulverized to 50300m before they areintroduced into the gasifier. The reactivity is differentbecause of the difference in particle size. As the chemi-cal reactions progress, the size of the particles willbecome smaller and only ash will remain. In this study,all particles were assumed to a uniform and sphericalcoal particle size of 100m.

4 RESULT

As a result of modelling the pseudo-2D model, carbonconversion, cold gas efficiency, and gas temperaturealong with the gasifier were calculated

Carbon conversion (per cent)

=

1

Carbon in gasification residue

Carbon in feedstock

100

Cold gas effciency(per cent)

=

Heating value inproduct gas CO, H2, CH4

Heating value in feedstock

100

The results shown in Figs 5 to 13 are values at theaxial flow zone 6 m in the gasifier. The gasifier heighton the abscissa is the height after the coal passed bythe burner zone, which explains why a non-zero value

was recorded at zero height.

0 1 2 3 4 5 650

60

70

80

90

100

CarbonConverion(%)

Gasifier Height (m)

0.3

0.5

0.7

0.9

Fig. 5 Effect of oxygen to coal ratio on carbon conver-

sion

0 1 2 3 4 5 6

35

40

45

50

55

60

65

70

75

0.3

0.5

0.7

0.9ColdGasE

fficiency(%)

Gasifier Height (m)

Fig. 6 Effect of oxygen to coal ratioon cold gas efficiency

0 1 2 3 4 5 6

1600

1700

1800

1900

2000

2100

2200

2300

2400 0.3

0.5

0.7

0.9

Temperature(K)

Gasifier Height (m)

Fig. 7 Effect of oxygen to coal ratio on gas temperature

0 1 2 3 4 5 65560

65

70

75

80

85

90

95

100

0.01

0.05

0.1

0.15

0.2

CarbonConversion(%)

Gasifier Height (m)

Fig. 8 Effect of steam to coal ratio on carbon conversion

4.1 Effect of oxygen to coal ratio

The oxygen to coal mass ratio is one of the mostimportant variables to influence gasifier performancebecause the oxygen-blown gasifier strongly dependson the concentration of oxygen. The heat required for

char + CO2 and char + H2O gasification reactions issupplied by the combustion of volatiles. In this study,




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0 1 2 3 4 5 630

35

40

45

50

55

60

65

70

0.01

0.05

0.1

0.15

0.2ColdGasEfficiency(%)

Gasifier Height (m)

Fig. 9 Effect of steam to coal ratio on cold gas efficiency

0 1 2 3 4 5 6

1600

1700

1800

1900

2000

2100

0.01

0.05

0.1

0.15

0.2

Temperature(K)

Gasifier Height (m)

Fig. 10 Effect of steam to coal ratio on gas temperature

0 1 2 3 4 5 630

40

50

60

70

80

90

100

2MPa

3MPa

4MPaCarbonConversion(%)

Gasifier Height (m)

Fig. 11 Effect of operating pressure on carbon conver-

sion

the oxygen to coal ratio varies from 0.3 to 0.9 as aparameter at 4.2 MPa pressure andwith a steam to coalratio of 0.05.

Figure 5 shows the effect of oxygen to coal ratio oncarbon conversion. As the concentration of oxygen

0 1 2 3 4 5 6

15

20

25

30

3540

45

50

55

60

65

70

2MPa

3MPa

4MPaColdGasEff

iciency(%)

Gasifier Height (m)

Fig. 12 Effect of operating pressure on cold gas effi-

ciency

0 1 2 3 4 5 6

1800

1900

2000

2100

2200

2300

2400 2MPa

3MPa

4MPa

Temperature(K)

Gasifier Height (m)

Fig. 13 Effect of operating pressure on gas temperature

increases, carbon conversion increases rapidly andreaches about 100 per cent at the gasifier exit. As theconcentration of oxygen decreases, on the other hand,the rate of increase of carbon conversion is lower andthe value at the gasifier exit is also small. If it oper-ates at an oxygen to coal ratio of 0.9, a length of 5 mis enough. If it operates at an oxygen to coal ratio

of 0.7, a reactor length of 6 m is appropriate, but allcarbon components are not converted into gases. If itoperates at an oxygen to coal ratio of 0.3, carbon con-version is very low because there is not enough oxygento burn carbon. Therefore, as the concentration of oxy-gen increases, enough carbon conversion is obtainedand reactor length shorter than 6 m is sufficient to getthe maximum efficiency at a given oxygen to coal ratio.

Figure 6 shows the effect of oxygen to coal ratioon cold gas efficiency. As the concentration of oxygendecreases, cold gas efficiency is higher during the earlystages of combustion and gasification because incom-plete combustion takes place. However, the rate of

increase does not last because there is not enough heatto make endothermic gasification reactions, such as




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char + CO2 andchar+ H2O, active. Maximum cold gasefficiency is shown in the vicinity of an oxygen to coalratio of 0.5. Meanwhile, as the concentration of oxy-gen increases, cold gas efficiency decreases because ofre-combustion of fuel gas, such as CO, H2, and CH4.Cold gas efficiency is competing with carbon conver-

sion on deciding the oxygen to coal ratio. Therefore,the criteria for operating objectives are needed.

Figure 7 shows the effect of oxygen to coal ratio ongas temperature. As the oxygen to coal ratio increases,the total mass flowrate is higher. Therefore, gases passthrough the burner zone fast before the combustionof volatiles ends. As a result, the initial temperature inFig. 7 is smaller as the oxygen to coal ratio increases,and combustion of fuel gases lasts by oxidation reac-tion of excess oxygen. Therefore, peak temperature ishigher as the oxygen to coal ratio increases. In thiscase, the maximum cooling point may be high along

with the reactor.

4.2 Effect of steam to coal ratio

The purpose of steam input is to protect the refractoryfrom rising temperature and to raise hydrogen yield.In this study, the steam to coal ratio varies from 0.01to 0.2 as a parameter at 4.2 MPa pressure and with anoxygen to coal ratio of 0.75. A small amount of steamis introduced because this is the dry-feeding-type coalgasifier.

Figure 8 shows the effect of steam to coal ratio

on carbon conversion. As the concentration of steamincreases, carbon conversion efficiency increasesslowly during the early stages of reaction. However, iteventually reaches about 98 per cent at the gasifier exitin all cases. As the concentration of steam increases,the rate of gasification and combustion is low becausethe overall temperature is low in the reactor. Rateequations are Arrhenius type and are a function oftemperature.

Figure 9 shows the effect of steam to coal ratioon cold gas efficiency. As the concentration of steamincreases, cold gas efficiency decreases. Gasificationreactions are endothermic reactions, so they require

sufficient heat from the combustion of volatiles. As thesteam to coal ratio increases, however, temperature

will be low because steam has a relatively high heatcapacity. Figure 10 shows the situation. As a result, theconcentration of fuel gas produced by the gasificationreaction is low except for hydrogen because the highconcentration of steam results in a high concentrationof hydrogen.

4.3 Effect of pressure

Pressure is an important variable that affects the

gasifier performance. Especially, pressure influencesthe reactivity of heterogeneous reactions. As pressure

increases, the partial pressure of gasifying agents willincrease, so the reactivity of the solid-phase reactionsalso increases. In this study, the pressure varies from 2to 4 MPa as a parameter with oxygen to coal ratio 0.75and steam to coal ratio 0.05.

Figures 11 and 12 show the effect of pressure on car-

bon conversionand coldgas efficiency, respectively. Asoperating pressure increases, both carbon conversionand cold gas efficiency increase because the reactivityof char + H2O and char+ CO2reactions increases. It isnot easy to operate the gasifier in the pressurized con-dition than in the atmospheric condition. However, itis clear that high efficiencies can be obtained in thepressurized gasifier.

Meanwhile, Fig. 13 shows the effect of pressureon gas temperature. Although the rate of increase oftemperature is higher as pressure increases duringthe early stages of gasification and combustion, peak

temperature is lower. This is because the reactivityof endothermic gasification reactions becomes activefast after oxygen is consumed because of the combus-tion of volatiles. Therefore, one can know that peaktemperature is inversely proportional to the operatingpressure.

5 CONCLUSION

A dry-feeding entrained bed coal gasifier was mod-elled for process analysis in the reactor. By consider-ing chemical kinetics over gasification processes, the

degree of reaction was obtained at variable operat-ing conditions. Results from the numerical simula-tions of the process in the model gasifier are carbonconversion, cold gas efficiency, and gas temperaturedistributions. The main conclusions are as follows.

1. As the concentration of oxygen increases, enoughcarbon conversion was obtained and reactor lengthshorter than 6 m was sufficient to get maximumefficiency at a given oxygen to coal ratio.

2. As the oxygen to coal ratio increases, combustionof fuel gases lasted by oxidation reaction of excessoxygen. Therefore, peak temperature was higher as

oxygen to coal ratio increases. In this case, the max-imum cooling point may be high along with thereactor.

3. Although the rate of increase of temperature ishigher as pressure increases during the early stagesof reaction, peak temperature is lower. This isbecause high pressure results in high reactivityof endothermic gasification reactions. As pres-sure increases, the partial pressure of the gasifyingagents, CO2 and H2O, will increase. As a result,carbon conversion and cold gas efficiency alsoincrease.

This model has uncertainties as a simplified model.It relies on rate constants that are difficult to measure




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at high temperature and pressure. There are few datain the open literature on the process analysis of thecommercial gasifier for benchmarking this model sothat validation of the model is not easy. Therefore,model prediction may be best used to evaluate trendsin the process of the dry-feeding entrained bed coal

gasifier rather than to provide specific values of theseresults.

ACKNOWLEDGEMENTS

This work was supported by the Ministry of Knowl-edge Economy, New and Renewable Energy Center(2006-N-C012-P-01-0-000), Doosan Heavy Industries& Construction Company Limited, and Brain Korea(BK) 21.

Authors 2011

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APPENDIX

Notationa global reaction component for speciesX

Aeff effective cross-section (m2)

Ai pre-exponential factor for the rateequation of homogeneous reaction (s1)

A0 pre-exponential factor for the rateequation of heterogeneous reaction (s1)

b global reaction component for speciesYCP,mix specific heat of gas mixture (kJ/kmol K)CP,s specific heat of solid (kJ/kmol K)E activation energy (kJ/mol)Eij activation energy of reaction forming

producti(kJ/mol)Hj heat generated by thejth homogeneousreaction (kJ/s)

Hk heat generated or absorbed by thekthheterogeneous reaction (kJ/s)

Hloss,wall heat loss through reactor wall (kJ/s)ki first-order rate constant of reaction

forming producti(s1)kij0 pre-exponential factor inki(s

1)mmix mass flowrate of mixed gas (kg/s)n pressure order ofnth-order equationPA partial pressure of gasifying agent (Pa)R gas constant (kJ/mol K)

rj reaction rate ofjth gas-involved reaction(kmol/s)




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rk reaction rate ofkth solid-phase reaction(kmol/s)

t times (s)T absolute temperature (K)Tg temperature of gas (K)Ts temperature of solid particle (K)

V velocity of mixed gas (m/s)Vi mass of volatile speciesi(kg)Vij amount of productigenerated up to

timet, fraction of the original coalweight

Vij value ofViatt= (ultimate yield),approximated by measurements at longreaction times, fraction of the originalcoal weight

Wg,i flowrate of gas componenti(kmol/s)Wg,mix flowrate of gas mixture (kmol/s)Ws flowrate of solid (kmol/s)

x carbon conversionX reactantXXi mass fraction of speciesi

Y reactantYYi mole fraction of speciesi

i,j stoichiometric parameter for gascomponentiin thejth reaction

s,k stoichiometric parameter for the solid inthekth reaction

mix density of mixed gas (kg/m3)

particle structure parameter


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