Computational Fluid Dynamics Modeling of Coal Gasification in a

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Energy & Fuels 2008, 22, 1560–1569

Computational Fluid Dynamics Modeling of Coal Gasification in a Pressurized Spout-Fluid Bed Zhongyi Deng,*,† Rui Xiao,*,† Baosheng Jin,† He Huang,‡ Laihong Shen,† Qilei Song,† and Qianjun Li† Key Laboratory of Clean Coal Power Generation and Combustion Technology of Ministry of Education, Southeast UniVersity, Nanjing 210096, China, and College of Life Science and Pharmacy, Nanjing UniVersity of Technology, Nanjing 210009, China ReceiVed December 7, 2007. ReVised Manuscript ReceiVed February 23, 2008

Computational fluid dynamics (CFD) modeling, which has recently proven to be an effective means of analysis and optimization of energy-conversion processes, has been extended to coal gasification in this paper. A 3D mathematical model has been developed to simulate the coal gasification process in a pressurized spoutfluid bed. This CFD model is composed of gas-solid hydrodynamics, coal pyrolysis, char gasification, and gas phase reaction submodels. The rates of heterogeneous reactions are determined by combining Arrhenius rate and diffusion rate. The homogeneous reactions of gas phase can be treated as secondary reactions. A comparison of the calculated and experimental data shows that most gasification performance parameters can be predicted accurately. This good agreement indicates that CFD modeling can be used for complex fluidized beds coal gasification processes.

1. Introduction From the perspective of energy security and environmental sustainability, further highly effective uses for fossil fuels in energy industries are demanded. Therefore, more and more attention has been paid to clean coal technology, among which coal gasification is one of the critical technologies. The syngas produced from coal gasification can be used for Fischer-Tropsch liquid,1 synthetic natural gas (SNG), or other chemicals. Also, gas turbines based on coal gasification for power generation are considered to be more efficient and environmentally friendly technologies. Numerical simulation is an effective technology to optimize the performance of gasifiers. It also provides the best method for gasifier scale-up investigations. Since the two-phase model was first proposed by Toomey and Johnstone,2 many improvements have been developed to simulate the coal gasification process. Moreea-Taha3 described how mathematical modeling can help in understanding the combustion and gasification processes and the use of modeling as a predictive tool, such as in pollutant emission prediction. The authors used onedimensional and three-dimensional fluid dynamics models with some assumptions, such as simplified chemical reactions. De Souza-Santos4,5 developed a comprehensive mathematical model and computer program, to use as a tool for engineering design and operation optimization, by predicting the behavior of a real * Author to whom correspondence should be addressed. E-mail: ruixiao@ seu.edu.cn. † Southeast University. ‡ Nanjing University of Technology. (1) Antunes, A. Patenting trends in natural gas Fischer-Tropsch synthesis. Stud. Surf. Sci. Catal. 2007, 167, 123–128. (2) Toomey, R. D.; Johnstone, H. F. Gaseous fluidization of solid particles. Chem. Eng. Prog. 1952, 48, 220. (3) Moreea-Taha R. Modeling and simulation for coal gasification; IEA Greenhouse Gas R&D Programme: UK, 2000. (4) De Souza-Santos, M. L. Comprehensive modeling and simulation of fluidized bed boilers and gasifiers. Fuel 1989, 68, 1507–1521.

unit during steady-state operation. Watanabe and Otaka6 developed a model of a coal gasification reaction and prediction of gasification performance for an entrained flow coal gasifier. Luo7 developed a modeling of a jetting fluidized bed gasifier. Kim et al.8 proposed a mathematical model to predict gasification in an internally circulating fluidized bed reactor with draft tube, based on hydrodynamics, reaction kinetics, and empirical correlations for pyrolysis. They were not able to predict either the temperature profiles or the gas concentration inside the bed; nevertheless, their results and predictions were reasonably accurate. With the improvement of numerical methods and more advanced hardware technology, the time needed to run computational fluid dynamics (CFD) codes is decreasing. Hence, the multidimensional models for dealing with complex gas-solid hydrodynamics, heat transfer, and chemical reactions are becoming more accessible. Once the model has been validated (calculation in agreement with experimental data), CFD can be used to make sensitivity analysis as it provides the flexibility to change parameters. A series of unsteady, three-fluid CFD models were performed using FLUENT 6.0 by Cooper et al.9 to simulate particle mixing in a binary fluidized bed. Ravelli et al.10 proposed a mathematical model to simulate the combustion (5) De Souza-Santos M. L. Modeling and simulation of fluidized-bed boilers and gasifiers for carbonaceous solid. PhD Thesis, Department of Chemical Engineering and fuel Technology, University of Heffield, UK, 1987. (6) Watanabe, H.; Otaka, M. Numerical simulation of coal gasification in entrained flow coal gasifier. Fuel 2006, 85, 1935–1943. (7) Luo, C.-h. Numerical modeling of a jetting fluidized bed gasifier and the comparison with the experimental data. Fuel Process. Technol. 1998, 55, 193–218. (8) Kim, Y. J.; Lee, J. M.; Kim, S. D. Modeling of coal gasification in an internally circulating fluidized bed reactor with draught tube. Fuel 2000, 79, 69–77. (9) Cooper, S.; Coronella, C. J. CFD Simulation of particle mixing in a binary fluidized bed. Powder Technol. 2005, 151, 27–36.

10.1021/ef7007437 CCC: $40.75  2008 American Chemical Society Published on Web 04/25/2008

Coal Gasification in a Pressurized Spout-Fluid Bed

of refuse-derived fuel (RDF) in bubbling fluidized bed by means of the commercial code FLUENT 6.1 and the predicted and experimental data were in agreement. Frazeli et al.11 proposed a CFD model to predict methane autothermal in a catalytic microreactor. Until now, there were no reports about 3D mathematical modeling of coal gasification in a pressurized spout-fluid bed using CFD approach. The purpose of this study is to extend the CFD model to coal gasification. An outline of the coal gasification model is described. The distribution of temperature and gas composition in the gasifier is presented. The influences of gasification temperature and pressure on gasifier performance are discussed. A comparison of calculation and experimental results is also presented. 2. Mathematical Model The coal gasification process and main assumptions of this model are the following: (1) It is three-dimensional and steady-state, i.e., gas composition and temperature distribution do not change with time. This assumption was made on the basis of experimental results and observations.33 (2) The particles in the bed are spherical and uniform in size.12,13 The unreacted shrinking core model was used to treat the coal pyrolysis and the heterogeneous reactions between char and gases (O2, H2O, CO2). Therefore, it was assumed that the particles in the bed did not change with time. (3) It includes two phases: gas and solid. (4) The kinetic theory of granular flow (KTGF) was used in the transport equation to describe the particle collision and fluctuation in the bed. But, the transport equation which includes KTGF was used at room temperature. Whether granular motion was affected by the temperature still needs further study. So, we assumed that intensity of particle collision did not vary with temperature, i.e., exothermic or endothermic reaction had no impact on the fluctuation of solid velocity (us′) and granular “temperature” Θs (see section 2.1.4 Kinetic Theory of Granular Flow (KTGF)) (5) The particles were assumed to be inelastic, smooth, and monodispersed spheres. (6) The radiative heat transfer was computed apart from applying the P1j model. This model assumes the propagation of radiation energy into an orthogonal series of spherical harmonics.14 (7) Devolatilisation and drying were considered instantaneous in the feed zone. (8) Among the reactions about CH4, only the CH4 combustion reaction was considered. The methanation and carbon hydrogenation reactions to produce CH4 were ignored. The carbon hydrogenation reaction (C + H2 f CH4) and the methanation reaction (CO + H2 f CH4 + H2O) require both high pressure (2-8 MPa)36,37 and catalysts. The catalyst was not added in (10) Ravelli, S.; Perdichizzi, A.; Barigozzi, G. Description, applications and numerical modelling of bubbling fluidized bed combustion in wasteto-energy plants. Progr. Energy Combust. Sci. 2008, 34, 224–253. (11) Frazeli, A.; Behnam, M. CFD modeling of methane autothermal reforming in a catalytic microreactor. Int. J. Chem. Reactor Eng. 2007, 5, A93. (12) Chejne, F.; Hernandez, J. P. Modeling and simulation of coal gasification process in fluidized bed. Fuel 2002, 81, 1687–1702. (13) Yu, L.; Lu, J.; Zhang, X. Numerical simulation of the bubbling fluidized bed coal gasification by the kinetic theory of granular flow (KTGF). Fuel 2007, 86, 722–734. (14) Gra¨bner, M.; Ogriseck, S.; Meyer, B. Numerical simulation of coal gasification at circulating fluidized bed conditions. Fuel Process. Technol. 2007, 88, 948–958.

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the experiments, and the operating pressure (0.1-0.3 MPa) was not high enough for these reactions; therefore, the methanation and carbon hydrogenation reactions were ignored in this model. 2.1. Gas-solid Hydrodynamics. 2.1.1. Continuity Equations. ∂ (ε F ) + ∇ · (εgFgVg) ) Sgs ∂t g g ∂ (ε F ) + ∇ · (εsFsVs) ) Ssg ∂t s s where ε, F, and V are the volume fraction, the density, and the instantaneous velocity, respectively. The instantaneous velocity of particles Vs can be substituted by the solid mean velocity us. ThisformwasalsoderivedfromtheBoltzmannintegral-differential equation by Ding and Gidaspow.15 S is the source term and is set to zero only in the flow field. When the continuity equations are used in heterogeneous reaction, there is mass, momentum, and heat exchange between the gas phase and solid phase. In the present work, coal reacts with oxygen, steam, and carbon dioxide to change from the solid phase into the gas phase, so the mass source for the phase yield is Ssg ) wc

∑ γ R ) -S c c

gs

2.1.2. Momentum Equations. For the gas phase ∂ (ε F V ) + ∇ · (εgFgVgVg) ) -εg ∇ P + εgFgg + ∂t g g g ∇ · εgτg - βgs(Vg - us) + Sgsus where βgs is the drag coefficient between the gas phase and solid phase, g is gravity, and us is the mean velocity. In the righthand of the equations, the fifth term Sgsus describes the momentum transfer of the coal. The momentum equation for the solid phase should obtain the reverse source term and can be expressed as follows: ∂ (ε F V ) + ∇ · (εsFsVsVs) ) - εs ∇ P + εsFsg - ∇ Ps + ∂t s s s ∇ · εsτs + βgs(Vg - us) + Ssgus 2.1.3. Energy Equations. ∂ (ε F H ) + ∇ · (εgFgugHg) ) ∇ (λg ∇ Tg) + Qgs + SgsHs ∂t g g g ∂ (ε F H ) + ∇ · (εsFsusHs) ) ∇ (λs ∇ Ts) + Qsg + SsgHs ∂t s s s where H, λ, and Q are the enthalpy, thermal conductivity of the mixture, and heat exchange between the gas phase and solid phase, respectively. The third term on the right-hand side of the expression is the heat transfer in that the solid phase changed into the gas phase. The heat exchange between phases can be expressed as a function of the temperature difference and conform to the local balance condition Qsg ) - Qgs Qsg ) hsg(Ts - Tg) hsg is the heat transfer coefficient, which is relative to the Nusof the solid phase. hsg )

6kgεsεgNus dp2

(15) Ding, J.; Gidaspow, D. A bubbling fluidization model using kinetic theory of granular flow. AIChE J. 1990, 32 (1), 523–538.

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kg is the thermal conductivity of the gas, and Nus was proposed by Gunn16 Nus ) (7 - 10εg + 5εg2)(1 + 0.7Res0.2Pr1/3) + (1.33 - 2.4εg + 1.2εg2)Res0.7Pr1/3 2.1.4. Kinetic Theory of Granular Flow (KTGF). A transport equation which describes particles collision resulting in a random granular motion is defined for the solid phase:17 ∂ 2 (ε F Θ ) + ∇(εsFsΘsus) ) - (ps cI + εs cτs) : ∇ b us + ∂t s s s 3 ∇(ks∇Θs) - γ - 3βgsΘs where us′ is the fluctuating velocity of the particles and can be derived from us′ ) Vs - us ps ) εsFsΘs + 2(1 + e)εs2g0FsΘs The diffusion coefficient for granular energy ks is given by the following: ks )

150Fsds√Θsπ 2 6 1 + εsg0(1 + e) + 2Fsγs2dsg0(1 + e) 384(1 + e)g0 5

[

]

Θs π

Here g0 is the radial distribution function. g0 )

[ ( )]

εs 3 15 εs,max

1 -1 3

The dissipation of fluctuating energy due to inelastic collision takes the form:

[ ( ) ]

γ ) 3(1 - e2)εs2Fsdsg0Θs

Θs - ∇ us π

4 ds

The remaining term which needs to be considered is the interphase momentum transfer. The drag between gas phase and solid phase is thought to play an important role in the momentum exchange. If εg < 0.8, the well-known Ergun equation16 is suitable for describing the dense regime: βgs ) 150

(1 - εg)rs µg 2

εgds

+ 1.75

Fgεs|ug - us| ds

If εg > 0.8, the drag coefficient was given based on the work by Wen and Yu18 3 |ug - us| -2.65 βgs ) Cd εg 4 ds where

{

24 ( 1 + 0.15Re0.687) Re e 1000 Cd ) Re 0.44 Re > 1000 Re )

|ug - us|εgFgds µg

2.1.5. Species Transport Equations. The gas phase is assumed to be a mixture of 9 species, represented by their mass fraction as follows: O2, CO2, H2O, CH4, H2S, H2, CO, NH3, and N2.The (16) Gunn, D. J. Transfer of heat or mass to particles in fixed and fluidized beds. Int. J. Heat Mass Transfer 1978, 21, 467–76. (17) Patil, D. J.; Annaland, M. S.; Kuipers, J. A. M. Critical comparison of hydrodynamic models for gas-solid fluidized beds-Part I: bubbling gassolid fluidized beds operated with a jet. Chem. Eng. Sci. 2005, 60, 57–72. (18) Wen, C. Y.; Yu, Y. H. Mechanics of fluidization. Chem. Eng. Prog. Symp. 1966, 62, 100–113.

Table 1. Arrhenius Coefficients Related to Reactions 1-3 reaction

equations

units

1 2 3

Ks,1 ) 17.9 exp[-13750/Tp] Ks,2 ) 5.95 × 10 -5 exp[-13650/Tp] Ks,3 ) 3.92 exp[-26927/Tp]

1/Pa · s 1/Pa · s 1/Pa · s

conservation equations take the general form of the equation below for these chemical species except for N2, which is computed from the fact the sum of all mass fractions are equal to one in the gas phase ∂ (F ε Y ) + ∇ (FgεgVgYg,i) ) - ∇ · εgJg,i + ri + εgRg,i ∂t g g g,i where Jg,i, Rg,i, and Rs,i are the diffusion flux of species i in the gas phase, the net rate of production of homogeneous species i, and the heterogeneous reaction rate, respectively. In the species transport equations of gas phase, mass diffusion coefficients are used to calculate the diffusion flux of chemical species in turbulent flow using modified Fick’s law:

(

Jg,i ) - FDi,m +

)

µt ∇ Yg,i Sct

where Di,m is diffusion coefficient of the mixture (squared meters per second). All ∂/∂t terms in the equations should be equal to zero when calculating the coal gasification due to the steady-state assumption. 2.2. Coal Gasification Reactions. The following chemical processes are included in the present model: (1) coal pyrolysis and the volatile release, (2) heterogeneous char reactions, (3) homogeneous reactions of the gas phase. 2.2.1. Pyrolysis. The pyrolysis process will be completed instantaneously. The composition of volatiles released from coal is therefore prescribed and obtained by an equilibrium calculation based on the analysis of the properties of the coal.19–22 volatile f R1CO2 + R2CO + R3CH4 + R4H2 + R5H2O + R6H2S

∑R )1 i

i

2.2.2. Char Reactions. The heterogeneous reactions between char and gases (O2, H2O, CO2) can be described by different reaction mechanisms taking possible diffusion effect into account or further simplified via kinetic model.8,23 For example, Eaton et al.21 presented a char oxidation model based on measured intrinsic char kinetic rates and a pore diffusion model. Chen et al.24 assumed that the oxygen, carbon dioxide, and steam react with char on the char particle surface and the value for reaction order was 0.5. (19) Cho, H. C. A numerical study on parametric sensitivity of the flow characteristics on pulverized coal gasification. Int. J. Energy Res. 2000, 24, 511–523. (20) Petersen, I.; Werther, J. Experimental investigation and modeling of gasification of sewage sludge in the circulating fluidized bed. Chem. Eng. Process. 2005, 44, 717–736. (21) Eaton, A. M.; Smoot, L. D.; Hill, S. C.; Eatough, C. N. Components, formulations, solutions, evaluation, and application of comprehensive combustion models. Prog. Energy Combust. Sci. 1999, 25, 387–436. (22) Lee, J. L.; Kim, Y. J.; Lee, W. J.; Kim, S. D. Coal-gasification kinetics derived from pyrolysis in a fluidized reactor. Energy 1998, 23 (6), 475–488. (23) Yan, H.-m.; Heidenreich, C.; Zhang, D.-k. Mathematical modeling of a bubbling fluidized-bed coal gasifier and the significance of net flow. Fuel 1998, 77, 1067–1079. (24) Chen, C. X.; Horio, M.; Kojima, T. Numerical simulation of entrained flow coal gasifiers. Part I: modeling of coal gasification in an entrained flow gasifier. Chem. Eng. Sci. 2000, 55, 3861–3874.



In this work, we assume char particle is a spherical particle surrounded by a stagnant boundary layer through which gas species must diffuse before they react with the char. The overall char reaction rate of a particle is controlled by the rates of diffusion and kinetic. Char combustion, steam, and CO2 gasification of char are considered in the model. The reactions and rate equations are as below. Combustion 2 2 1 O f 2 - CO + - 1 CO2 ψ 2 ψ ψ Steam gasification C+

(

)

(

)

C + H2O f CO + H2

(1)

(2)

Carbon dioxide gasification (Boudouard reaction) C + CO2 f 2CO

{

(3)

Here, ψ is the mechanism factor. It could be calculated by Ross25

ψ)

2Z + 2 Z+2 2Z + 2 -

dp e 0.05 cm Z (100dp - 0.05) 0.05 cm e dp e 0.1 cm 0.095 Z+2

1.0

(

Z ) 2500 exp

Table 2. Parameters of Homogeneous Reaction Kinetics no. 4 5 6 7 8

reaction CO + 1/2O2 f CO2 H2 + 1/2O2 f H2O CH4 + 2O2 f CO2 + 2H2O CO + H2O f CO2 + H2 CO2 + H2 f CO + H2O

A (m3/K · mol · s)

Es (kJ/kmol)

ref

3.09 × 104 8.83 × 108 2.552 × 1014 2.978 × 1012 6.245 × 1014

9.976 × 104 9.976 × 104 9.304 × 105 3.69 × 105 3.983 × 105

17 17 17 18 18

The homogeneous reactions of the gas phase that we consider are as below: ri ) Kg,iCACB

( )

Eg,i RT In order to give an immediate idea of the model, Figure 1 shows gas/solid mixing, the main chemical reactions, and the heat transfer in the gasifier.28,29 Kg,i ) Ag,i exp -

3. Numerical Considerations CFD software provides a user-defined function (UDF), with which the software can be used in various applications. In this work, a UDF was used to program the chemical reaction rate equations of coal gasification. The kinetics parameters of coal gasification reaction are shown in Tables 1 and 2.

dp g 0.1 cm -6249 Tp

)

The diffusion rate can be derived:13 Kdif )

ShDgswc RTpdp

Here, Sh is the Sherwood number and is written as Sh ) 2 + 0.654Re0.5Sc1/3 udpFp Re ) µ µ Sc ) FgDg The char reaction rate is obtained as a mixture of kinetic and diffusion controlled mass transfer rate:13 rc ) Kc )

6VcKcPXi dp 1 1 1 + Ks,i Kdif

Arrhenius coefficients Ks,i are shown in Table 1. 2.2.3. Gas Phase Reactions. As is known in spouted fluidized beds, where turbulent flow is very strong, combustible gases (CO, H2, CH4) and oxygen mix well. Therefore, the reactions between combustible gases and oxygen are controlled by kinetic and treated as secondary reactions. Besides combustion reactions, water-gas shift and reverse water-gas shift reactions (reactions 7 and 8 in Table 2, respectively) were also taken into consideration. The parameters of homogeneous reactions and their kinetics are shown in the Table 2.26,27 (25) Smoot, L. D.; Pratt, D. T. Combustion & gasification of pulVerized coal; Fu, W., et al., translators; Tsinghua University Press: Beijing, 1983; pp 203-245, in Chinese.

Figure 1. Schematics of a reactor with heat and mass fluxes and the structure of the numerical model.


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the total volume fraction of solids was patched as 0.42.10 To prevent the spacing between particles from decreasing to zero, the maximum particle packing was εs,max ) 0.6.22

Table 3. Coal Analyses and Properties proximate analysis (ad)

value

moisture/% volatile matter/% fixed carbon/% ash/%

1.16 26.93 46.01 25.90

ultimate analysis (ad) Cad/% Had/% Oad/% Nad/% Sad/% Mad/% Aad/% LHV/kJ/kg mean particle size (mm)

4. Experimental Details

57.86 3.77 8.7 1.11 1.50 1.16 25.9 23220 0.8

Table 4. Operating Conditions no.

1

2

3

4

5

6

coal feed (kg/h) 4.2 4.6 5.87 5.79 5.79 5.79 7.8 7.5 9.4 8.2 10.19 11.53 air supply (Nm3/h) steam supply (kg/h) 1.15 1.24 1.6 1.78 1.86 1.93 pressure (MPa) 0.1 0.2 0.3 0.3 0.3 0.3 air and steam 534 549 557 525 533 550 temperature at entrance (°C) temperature of 860 860 893 884 898 912 reactor (°C)

k-ε turbulence models are selected. The drag coefficient between the gas phase and solid phase that we used were proposed by Gidaspow.30 The restitution coefficient between the solid particles is 0.9. The heat transfer coefficient between the gas phase afnd solid phase we used was proposed by Gunn.16 For the boundary conditions, we select the velocity-inlet condition for the inlet of the reactor and the outflow condition for the outlet of the reactor. At the walls, a zero gradient condition was used for the turbulent kinetic energy. The noslip wall condition was used for the gas phase and solid phase.15 The value of all the compositions into the gasifier can be obtained from Table 4. The simulations were carried out with the finite volume method (FVM), in which the interphase slip algorithm (IPSA) of Spalding31 was used to solve velocitypressure coupled differential equations. For the evaluation of the convective terms, the second order QUICK scheme was used. The time step was set as 2 × 10-4 s. The simulation grid is shown in Figure 2. In the calculation domain, the number of grids was 22 230. Among these grids, the minimum volume was 8.366 445 × 10-10 m3, and the maximum volume was 1.434 600 × 10-6 m3. It is divided into two partssthe upper cylinder and the bottom inverted cone. The upper cylinder was divided with hexahedron structure grid and the bottom inverted cone was divided with tetrahedral grid. The bed was initially filled with particles 0.3 m high, where (26) Hurt, R. H.; Calo, J. M. Semi-global intrinsic for char combustion modeling. Combust. Flame 2001, 125 (3), 1138–1149. (27) Watkinson, A P.; Lucas, J. P.; Lim, C. J. A prediction of performance of commercial coal gasifiers. Fuel 1991, 70, 519–527. (28) Zhong, W. Flow behaviors of a large spout-fluid bed at high pressure and temperature by 3D simulation with kinetic theory of granular flow. Powder Technol. 2007, 175, 90–103. (29) Zhong, W.; Zhang, M. Experimental investigation of particle mixing behavior in a large spout-fluid bed. Chem. Eng. Process. 2007, 46, 990– 995. (30) Gidaspow, D.; Bezburuah, R.; Ding, J. Hydrodynamics of Circulating Fluidized Beds, kinetic theory approach. In Fluidization VII, Proceedings of the 7th Engineering Foundation Conference on Fluidization, Brisbane, Australia, 1992; pp 75-82. (31) Spalding, D. B.; Markatos, N. C. Computer Simulation of MultiPhase Flows, CFDU. Imperial College: UK, 1983; pp 36-50.

The experimental verification was carried out in a 0.1 MW unit operating at elevated pressure. Figure 3 shows a schematic diagram of the system. The whole system consisted of an air/steam supply section, an air/steam preheating section, a spout-fluid bed gasifier, a coal feeding section, a gas cooling, cleanup, sampling, and burning section, and a temperature and pressure controlling section. The air from the compressor was first preheated by a low temperature air heater. Meanwhile, the steam from a boiler was also heated by a superheater. Then these two gases were mixed and further heated by a high-temperature heater. The gasifying agent was diverted into two streams: the first half went directly to the spouting nozzle forming a high-velocity jet, while the second half entered the reactor through the distributor. A Chinese bituminous coal, Xuzhou coal, was used as the feedstock for gasification. The chemical characteristics of this coal, including proximate and ultimate analyses and mean particle size are shown in Table 3. A more detailed description of the experimental setup can be found elsewhere.32–34

5. Results and Discussion The product gas from the experiments was a dry gas due to the fact that most of the moisture in the product gas was condensated by a gas cooler. However, the result of simulation was not a dry gas because it had moisture content inside the gas. In order to compare the simulation results with experiment results, the simulation results were corrected to remove moisture and the removed molar fraction of moisture was converted into the molar fraction of gas mixture. The simulation results of the outlet molar fraction of gas composition are expressed by the area average as below Xi )

1 SA

∫ X dS i

A

Six different cases were designed to study the effect of two main parameters including temperature and pressure. Cases 1-3 were to study the effect of the pressure on gasification performance. Cases 4-6 were used to study the effect of the temperature on gasification performance. Table 4 lists the main conditions that are applied to these cases. 5.1. Comparison of Calculation and Experimental Results. Experimental verification was carried out in the abovementioned pressurized spouted fluidized bed. The experimental tests were conducted close to the calculation cases. Table 5 shows the comparison of prediction and experimental results. The errors between calculation and experiment are also presented. It can be observed that the prediction results are in good agreement with the experimental data. Most of the calculation errors are within the range of 10%. The average calculation error is 7.3%, and the mean square deviation is 6.3%. The maximum error is 25% in the prediction of methane, probably due to underestimating the methane formation in the coal devolatilization process. The sensitivity of this CFD model with pressure (32) Xiao, R.; Zhang, M.; Jin, B.; Huang, Y.; Zhou, H. High-temperature air/steam-blown gasification of coal in a pressurized spout-fluid bed. Energy Fuels 2006, 20 (2), 715–720. (33) Rui, R.; Jin, B.; Xiong, Y.; Duan, Y.; Zhong, Z.; Chen, X.; Huang, Y.; Zhang, M. Air blown partial gasification of coal in a pilot plant pressurized spout-fluid bed reactor. Fuel 2007, 86, 1631–1640. (34) Rui, X.; Jin, B.; Xiong, Y.; Duan, Y.; Zhong, Z.; Chen, X.; Huang, Y.; Zhang, M. Fluidized Bed Partial Gasification of Coal: Comparison of a Laboratory and Pilot Scale Reactors. Korean J. Chem. Eng. 2007, 24 (1), 175–180.



Figure 2. Picture of the simulation grid.

Figure 3. Schematic of the experimental setup. Table 5. Comparison of Calculation and Experimental Results no. CO H2 CH4 CO2

case 1 case 2 case 3 case 4 case 5 case 6 experiment/% prediction/% calculation error/% experiment/% prediction/% calculation error/% experiment/% prediction/% calculation error/% experiment/% prediction/% calculation error/%

8.32 8.75 5.16 9.76 10.5 7.58 2.4 3 25 15.23 13.97 8.27

10.05 10.44 3.89 11.97 12.03 0.5 3.08 3.63 17.8 15.24 14.2 6.82

11.37 11.2 1.5 12.97 12.81 1.23 4 4.27 6.75 16.18 14.6 9.76

11.25 11.95 6.22 13 13.7 5.38 3.45 3.98 15.36 15.69 14.06 10.38

11.18 11 1.63 11.7 11.48 1.88 3.17 3.34 5.36 17.37 14.53 16.35

11.15 10.98 1.52 10.85 10.91 0.55 2.66 2.71 1.88 16.84 14.36 14.7

and temperature is also perfect. Hence, it can be claimed that the present CFD model can be extended to predict the performance of the pressurized spouted fluidized bed coal gasification processes. 5.2. Distribution of Temperature in the Gasifier. Figure 4 shows the cross section (Y ) 0) and longitudinal section (Z ) 0.04) temperature distributions in the six cases. The distributions of temperature had a similar trend in the different cases. The temperature was higher at the bottom of the reactor than that in the upper section of the reactor. The highest temperature

zone was in the outlet of the spouting nozzle. The oxygen was supplied into the reactor and consumed along the reactor height due to the combustion reaction. Reactions 1, 4, 5, and 6 are exothermic reactions. The heat of combustion was released, and the temperature of the reactor bottom increased rapidly. The central jet has the highest oxygen concentration, so that the temperature in the jet was much higher than other zone. When the oxygen is depleted, heterogeneous reactions 2 and 3 are dominant. Heterogeneous reactions are endothermic reactions; therefore, temperature decreases along the gasifier height. 5.3.in5Distribution of Gas inin the Gasifier. Figure shows thesection distributions of As gasshown compositions of 5,these cases the cross (Y )Composition 0). Figure the concentrations of O2 decreased very quickly along the height of the reactor, while the concentrations of CO2 increased and reached a maximum at the bottom of the reactor. This indicates that the rates of the combustion reactions (reactions 1, 4, 5, and 6) are very fast and oxygen is consumed very close to the outlet of the nozzle. After the oxygen is depleted, the gasification reactions 2, 3, 7, and 8 become dominant. H2 and CO are produced while the molar fraction of CO2 is decreased. In the freeboard, the changes of molar fraction of the gas compositions were not as obvious as the dense bed. The possible reason is that the gasification reaction rates slow down due to the lower solids concentration and temperature.


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Figure 4. Distributions of gas temperature of the different cases.

5.4. Effect of the Pressure. The results of cases 1-3 reveal the effect of pressure on gasification as shown in Figure 6. From the calculations, the operating pressure does have influence on gasification. When the operating pressure increased from atmospheric pressure to 0.3 MPa, the outlet molar fractions of CO and H2 increased from 8.75% and 10.5% to 11.2% and 12.81%, respectively. The molar fraction of CH4 increased slightly from 3% to 4%, while the molar fraction of CO2 remained almost constant with pressure. The results indicate that the gas quality (combustible fractions and caloric value) improves at a higher operating pressure. Two possible reasons are responsible for this change. One is that the gasification rate is enhanced directly by pressure due to the increase in the partial pressure of reactants. The other is related to the fact that the fluidization in the reactor becomes better at elevated pressure (On the one hand, the voidage increases with elevated pressure. On the other hand, when the pressure increases, the distribution of radial profiles of particle velocities becomes more and more irregular, which will allow for more horizontal movement of particles. This makes fluidization in the reactor better).28,35 That is, bubbles become smaller, gases and solids mix better, and a longer residence time of the gasifying agent in the dense bed is achieved, which favors the gasification reactions in the gasifier. All of these make the quality of coal gas under pressurized conditions better than that of atmospheric pressure conditions. Hence, it can be concluded that the high pressure condition is better for coal gasification. 5.5. Effect of the Bed Temperature. The bed temperature is one of the most important operating parameters affecting the performance of coal gasifiers, since the main gasification reactions are endothermic. Cases 4-6 were designed to study the effect of the temperature on gasification performance. In (35) Zhong, W.; Zhang, M. Experimental and model investigations on gas mixing behaviors of spout-fluid Beds. Int. J. Chem. Reactor Eng. 2007, 5, A32. (36) Lee, S. H.; Lee, J. G. Hydrogasification characteristics of bituminous coals in an entrained-flow hydrogasifier. Fuel 2006, 85, 803–806. (37) Mısırlıog˘lu, Z.; Canel, M. Hydrogasification of chars under high pressures. Energy ConVers. Manage. 2007, 48, 52–58.

this work, because the coal gasification process is an autothermic unit, the air/coal ratio was used to change the gasifier temperature. The air/coal ratios were 0.24, 0.298, and 0.337 and the bed temperature was 884, 898, and 912 °C, respectively. The calculation results of outlet gas composition are shown in Table 5 and Figure 7. Figure 7 also demonstrates the influence of the bed temperature on the outlet gas composition. In principle, the increase of bed temperature should enhance formation of CO, CH4, and H2, but as shown in Figure 7, the molar fractions of H2 and CH4 decrease with increasing reaction temperature. With fixed coal feeding, the increase in bed temperature means that more air will be required, and some of the air will react with the produced CO, CH4, and H2. Therefore, the molar fractions of H2 and CH4 decrease. It also can be observed that the change of molar fraction of CO and CO2 are not so evident. With an increase in the air feed rate, the molar fraction of CO decreases while CO2 increases. However, with an increase in reactor temperature, the rate of Boudouard reaction (C + CO2 f 2CO) which consumes CO2 and produces CO will become faster. This leads to the change of molar fractions of CO and CO2. 6. Conclusions Coal gasification is one of the key technologies among current advanced clean coal technologies. Numerical simulation is an effective technology for scale-up and optimizing the performance of gasifiers. With the improvement of numerical methods and more advanced hardware technology, CFD modeling has recently been proven to be an effective means of analysis and optimization of energy-conversion processes which has been extended to coal gasification in this work. Three-dimensional CFD modeling of coal gasification in a pressurized spout-fluid bed has been developed. The distributions of temperature and gas composition in the gasifier were presented. The effects of pressure and temperature on the gasification performance were also predicted. The model shows that the central jet near the outlet of nozzle is the highest temperature zone, due to the higher oxygen concentration and strong exothermic combustion thereafter. The


Figure 5. (Continued)



Deng et al.

Figure 5. Distributions of O2, H2, CO, CH4, and CO2 in different cases.

gradual temperature decrease along the gasifier were attributed to endothermic gasification reactions. Oxygen was consumed

very rapidly as soon as it entered the gasifier, while the concentrations of CO2 reached a maximum at the bottom of



modeling is a powerful method to give insight into the behavior of coal gasifiers. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 50606006 and No. 90610016) and Hi-tech Research and Development Program of China (863 Program, 2006AA020101). Mr. Josh Fackler from the University of Kentucky who polished the paper is greatly acknowledged.

Nomenclature

Figure 6. Outlet gas composition variation with the operating pressure.

Figure 7. Outlet gas composition variation with the reaction temperature.

the reactor. The concentrations of gas composition variation in the freeboard were not as obvious as those in the dense bed. The operating pressure and the bed temperature had an obvious effect on the coal gasification performance. The high pressure increased coal gasification performance partly due to the fact that the fluidization in the reactor becomes better. The bed temperature is one of the most important operating parameters affecting the performance of coal gasification, since the main gasification reactions are endothermic. On one hand, a higher temperature can accelerate the chemical reaction rate. On the other hand, more air will be introduced to the gasifier to keep the higher bed temperature that will lead to combust a part of combustible gases (CO, H2, CH4) produced by coal prolysis or gasification. Experimental verification was carried out in a 0.1 MW thermal input pressurized spouted fluidized bed. Comparison of the calculation with the experimental data shows that most of the calculation errors are within the range of 10%. The overall deviation is 6.3%. This good agreement indicates that the CFD

A ) frequency factor Ci ) molar concentration (kmol/m3) dp ) diameter (m) Dm,i ) diffusion coefficient of the mixture (m2/s) Dgs ) binary mass diffusion coefficient (m2/s) E ) activation energy (kJ/kmol) hsg ) heat transfer coefficient (W/m2 · K) Hi ) enthalpy (J/kg) Jg,i ) diffusion flux (kg/m2 · s) Kg,i ) reaction rate constant Kdif ) gas diffusion rate (1/m2 · s) Kg,i ) kinetic rate constant P ) pressure (Pa) Qsg ) intensity of heat exchange between the gas and solid phases (W/m2) Rg,i ) net rate of production of homogeneous species i Nu ) Nusselt number Sh ) Sherwood number Sc ) Schmidt number Pr ) Prandtl number Re ) Reynolds number rc ) heterogeneous reaction rate ri ) homogeneous reaction rate S ) source term T ) temperature (K) V ) instantaneous velocity (m/s) VC ) volume (m3) SA ) area (m2) wc ) carbon molecular weight (kg/kmol) Xi ) molar fraction u ) the mean velocity (m/s) R ) universal gas constant (J/kmol · K) Yi ) the mass fraction Greek Symbols ε ) volume fraction F ) density (kg/m3) λ ) thermal conductivity of mixture (W/m · K) τg ) stress tensor (Pa) γ ) dissipation of fluctuating energy (W/m3) β ) drag coefficient (kg/m3 · s) µ ) viscosity (kg/m · s) ψ ) mechanism factor Subscripts s ) solid phase g ) gas phase i ) the ith species EF7007437

Computational Fluid Dynamics Modeling of Coal Gasification in a

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