Energy Fuels 2010, 24, 1156–1163 Published on Web 01/25/2010
: DOI:10.1021/ef901085b
Three-Dimensional Simulation for an Entrained Flow Coal Slurry Gasifier Yuxin Wu,*,† Jiansheng Zhang,† Philip J. Smith,‡ Hai Zhang,† Charles Reid,‡ Junfu Lv,† and Guangxi Yue† †
Department of Thermal Engineering, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China and ‡Institute for Clean and Secure Energy, University of Utah, Salt Lake City, Utah 84112 Received September 24, 2009. Revised Manuscript Received December 14, 2009
A comprehensive three-dimensional (3D) numerical model is developed for simulation of entrained coal slurry gasifiers. In this model, a presumed probability density function (PDF) method is used to consider turbulent effects on gas-phase reactions in the gasifier. A realizable k-ε model is adopted to predict turbulence information. Coal slurry particles are tracked with the Lagrangian method. The coal slurry gasification process is divided into several simple subprocesses, which are droplet evaporation, boiling, devolatilization, and heterogeneous reactions of coal char particles. The particle-source-in-cell method is adopted to couple gas-particle interactions. With this methodology, simulations for GE and staged coal slurry gasifiers are completed. A comparison between the predictions and measured data shows that the proposed model correctly predicts the global performance of the coal gasification process. The mixing process in the staged gasifier is better than that in the GE gasifier because of the existence of secondary flow. The particle size has a negative effect on coal conversion. The effects of the coal slurry concentration and molar ratio of oxygen/carbon on the gasifier performance are also studied in this paper.
process variables (MSVP) method to simulate the gasification reactions and reactant mixing process for a 200 tons/day pilotscale gasifier test bed in Japan6-9 and gave a detailed analysis of effects of turbulent fluctuation on the coal gasification process. On the basis of commercial code CFX, Watanabe and Otaka developed a three-dimensional (3D) model to simulate a 2 tons/day entrained flow gasifier built by Central Research Institute of Electric Power Industry (CRIEPI).10 Most of these models are based on conventional methods used in coal combustion processes. Transport equations for gas phase are solved in an Eulerian framework, and coal particles are tracked in Lagrangian reference frame, so that the detailed coal gasification process is described easily. Recently, a new style of staged entrained flow coal slurry gasifier was developed in China.11 The schematic drawing is shown in Figure 1. A mixture of O2 and CO2 are injected with coal slurry into the furnace from the top section of the gasifier. When high-temperature synthesis gases are entrained into the jet, a down-fired flame is formed because of intense reactions between O2 and reactants. Because only 85% of total feeding
Introduction Entrained flow coal slurry gasifiers have been widely used in coal gasification technologies. In the past 2 decades, multidimensional numerical simulation of coal gasification has undergone significant development. Researchers at Advanced Combustion Engineering Research Center (ACERC) developed code packages PCGC-2 and PCGC-3 for coal combustion and gasification in the 1970s-1980s.1,2 Simulations were carried out for the pilot-scale entrained flow coal gasification test bed. On the basis of this work,2 Bockelie et al. at Reaction Engineering International (REI) used updated commercial software Glacier to study the gasification process in a singlestage, down-fired GE gasifier and two-stage gasifier with multiple feeding inlets.3 Performance of different devolatilization models were compared, and a slagging model was proposed to couple the multidimensional model. Choi et al. and Liu et al. at Korea Institute of Energy Research (KIER) simulated coal gasification processes in a coal slurry entrained flow gasifier with the vorticity-stream function method and the eddy-break-up model.4,5 Liu et al. and Chen et al. used an extended coal gas mixture fraction model with the multi-solids
(6) Chen, C.; 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. (7) Chen, C.; Horio, M.; Kojima, T. Use of numerical modeling in the design and scale-up of entrained flow coal gasifiers. Fuel 2001, 80, 1513–1523. (8) Liu, H.; Toshinori, K. Theoretical study of coal gasification in a 50 ton/day HYCOL entrained flow gasifier. I. Effects of coal properties and implications. Energy Fuels 2004, 18, 908–912. (9) Liu, H.; Chen, C.; Toshinori, K. Theoretical simulation of entrained flow IGCC gasifiers: Effect of mixture fraction fluctuation on reaction owing to turbulent flow. Energy Fuels 2002, 16, 1280–1286. (10) Watanabe, H.; Otaka, M. Numerical simulation of coal gasification in entrained flow coal gasifier. Fuel 2006, 85, 1935–1943. (11) Zhang, J.; Wu, Y.; Liu, Q.; Lue, J.; Yue, G. Effect of second airflow on three-dimensional velocity distribution in staged coal gasifier. J. Combust. Sci. Technol. 2007, 13 (2), 131–135 (in Chinese).
*To whom correspondence should be addressed. Telephone: (8610)62788523. Fax: (8610)-62788523. E-mail:
[email protected]. (1) Smoot, L. D.; Smith, P. J. Coal Combustion and Gasification; Plenum Press: New York, 1985. (2) Smoot, L. D.; Brown, B. W. Controlling mechanisms in gasification of pulverized coal. Fuel 1987, 66 (2), 1249–1256. (3) Bockelie, M. J.; Denison, M. K.; Chen, Z.; Linjewile, T.; Senior, C. L.; Sarofim, A. F. CFD modeling for entrained flow gasifiers in Vision 21 systems. Presented at the Pittsburgh Coal Conference, Pittsburgh, PA, Sept 2002 (http://www.reaction-eng.com). (4) Choi, Y. C.; Li, X. Y.; Park, T. J.; Kim, J. H.; Lee, J. G. Numerical study on the coal gasification characteristics in an entrained flow coal gasifier. Fuel 2001, 80, 2193–2201. (5) Liu, X.; Zhang, W.; Park, T.J. Modelling coal gasification in an entrained flow gasifier. Combust. Theory Modell. 2001, 5, 595–608. r 2010 American Chemical Society
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Energy Fuels 2010, 24, 1156–1163
: DOI:10.1021/ef901085b
Wu et al.
the gas phase are D ðF~ uj Þ ¼ Sm xj
ð1Þ
D D D ðF~ uj u~i Þ ¼ - F þ ðτhij -Fui 00 uj 00 Þ þ Fgi þ Smom xj xi xj
ð2Þ
! D D λ D ~ ~ ðF~ ui hÞ ¼ h þ Srad þ Sh Dxj Dxj cp Dxj
ð3Þ
where - represents the time average of a scalar, ∼ represents the Favre average of a scalar, F, u, and h are the density, velocity, and enthalpy of the gas mixture, respectively, and Sm, Smom, and Sh are the source terms of particle mass, momentum, and enthalpy, respectively. These terms are acquired by the particle tracking procedure. Srad is the radiation source term. In this simulation, the P1 model is used to calculate radiation heat transfer in the gasifier. τij in eq 2 is the viscous stress tensor and is expressed by 2 3 ! Du Du 2 Du i j k τij ¼ μ4 þ δij 5 ð4Þ 3Dxk Dxj Dxi
Figure 1. Schematic drawing of the staged coal slurry gasifier.
O2 is added through the top jet, the oxygen/coal ratio in the main flow is low and the temperature in the top section of the gasifier is limited. The nozzle benefits from this lowtemperature region through a prolonged lifetime. At the end of the down-fired flame, a secondary oxidizer is introduced to increase the temperature in the remaining regions of the gasifier. The additional jet flow changes the limited jet flow field and enhances the turbulent mixing in the staged gasifier.12 To better understand the characteristics of the flow field, turbulent mixing process, and coal gasification in the staged gasifier, a comprehensive 3D numerical model is developed and used to simulate a GE gasifer and staged gasifer. The flow field and coal gasification process in these two kinds of gasifiers are compared. Turbulent mixing characteristics are analyzed in this paper.
The only unknown term in these equations is the Reynolds stress ui00 uj00 , which is solved by introducing the additional turbulence model. On the basis of previous work,12 the realizable k-ε model is adopted in this simulation because of its better performance in simulations of a cold flow field of a gasifier.13 The realizable k-ε mode introduces an additional model dissipation rate equation and a realizable eddy viscosity formulation, which are given respectively by 2 3 ! D~ u D~ u 2 i j 00 00 -Fui uj ¼ μt 4 ð5Þ - kδij 5 þ 3 Dxj Dxi
Model Description and Basic Equations General Description. Entrained flow coal slurry gasifiers usually run at an elevated pressure. The coal slurry is atomized into small particles by a high-speed pure oxygen jet as it enters the gasifier. The small coal slurry particles are rapidly accelerated by the high-speed jet flow and heated by the flame in a short period. Because of the high temperature in the gasifier, both devolatilization and coal gasification reactions happen very fast. This assures a high carbon conversion rate in the entrained flow coal gasifiers. On the basis of these characteristics, some assumptions and simplifications are adopted in the simulation. In this model, steady-state transport equations are used on the basis of the assumption that industrial gasifiers run in stead state. The high-speed jet flame is a fully developed turbulent flow. Thus, a two-equation turbulence model is fit for calculating turbulence information in the gasifiers. The realizable k-ε model is used and proven to have a good performance.13 Turbulent gas-phase reactions are assumed to be controlled by the turbulent mixing process. A presumed probability density function (PDF) model is adopted to describe interactions between the turbulent fluctuations and gas-phase reactions. It is also assumed that coal slurries are atomized perfectly, so that the size distribution of coal slurry particles is the same as the size distribution of coal particles. A Langrangian particle tracking method is used to describe the detailed gasification process of the coal slurry particles. Transport Equations. On the basis of the steady-state assumption, transport equations of mass, momentum, and enthalpy of
D D ðF~ ui kÞ ¼ xi xj
"
# μt Dk μþ þ Gk þ Gb -Fε σk Dxj
D D ðFε~ ui Þ ¼ xi xj -C2 F
"
ð6Þ
# μt Dε þ FC1 Sε μþ σ ε Dxj
ε2 k pffiffiffiffiffi þ C1ε C3ε Gb ε k þ vε
ð7Þ
where μt = FCμk2/ε, the values of Cμ, σk, σε, C1, C2, C1ε, and C3ε are listed in Table 1, and Gk and Gb are calculated as !2 D~ uj Dui Duj ¼ μt Sij 2 ¼ μt þ ð8Þ Gk ¼ -Fui 00 uj 00 Dxi Dxj Dxi Gb ¼ -gi
μt DF FPrt Dxi
ð9Þ
Turbulent Reaction Model. For non-premixed flames, the instantaneous thermo-chemical state of the fluid is assumed to be related to some conserved scalar quantities known as mixture fractions. To build a relationship between the average values of the thermo-chemical state of the gas phase and the mixture fractions, a presumed β PDF function was adopted. Any average scalars, such as temperature or species mole fractions, are calculated using the presumed PDF by
(12) Wu, Y.; Zhang, J.; Yue, G.; Lu, J. Comparison of different turbulence models in computation of co-axial jet stream of Texaco gasifier. J. Combust. Sci. Technol. 2007, 58 (3), 537–543 (in Chinese). (13) Shih, T.-H.; Liou, W. W.; Shabbir, A.; Yang, Z.; Zhu, J. A new k-ε eddy viscosity model for high Reynolds number turbulent flows. Comput. Fluids 1995, 24, 227–238.
Z jhðT, Y k , :::Þ ¼ 1157
1 0
φðf , Hloss ÞPðf Þdf
ð10Þ
Energy Fuels 2010, 24, 1156–1163
: DOI:10.1021/ef901085b
Wu et al.
Table 1. Turbulent Coefficients Used in the Realizable k-ε Model σk
Cμ 1/(A0 þ ASu*k/ε)
1.0
σε
C1
1.2
maximum [0.43, η/(η þ 5)]
C2
C1ε
1.92
1.44
C3ε 1.0
A0
AS
u*
Ω h ij
φ
η
4.04
61/2 cos(φ)
(SijSij þ Ω hijΩ hij)1/2
Ωij - 3εijkωk
1/3 cos-1((61/2SijSjkSki)/(SijSij)1/2)
(2SijSij)1/2(k/ε)
where j represents the average value of a scalar and φ(f, Hloss) is the function identifying the relationship among φ, mixture fraction f, and local heat loss Hloss. The equilibrium model was adopted to identify the function φ based on the principle of minimum Gibbs energy. P(f ) is the β PDF function, which is acquired on the basis of the average mixture fraction f~ and its variance f 00 2. f~ and f 00 2 are calculated by D ~ μ ðFf Þ þ rðF~ uf~Þ ¼ r t rf~ þ Sf ð11Þ Dt σt
Table 2. A and E for Each Heterogeneous Reaction for Both Gasifiers C þ O217 C þ CO218-20 C þ H2O21 C þ H220 -2 -1
A (kg m s Pa ) 300 2224 1.3 108 2.2 108 E (J kmol-1) n 0.65 0.6
ð12Þ where σt, Cg, and Cd are 0.85, 2.86, and 2.0, respectively, and Sf is the source term of f coming from coal particles. In this simulation, the mixture fraction is defined by mfuel mfuel þ moxidizer
CðsÞ þ O2 f CO2
ðR1Þ
CðsÞ þ CO2 f 2CO
ðR2Þ
CðsÞ þ H2 O f CO þ H2
ðR3Þ
CðsÞ þ 2H2 f CH4
ðR4Þ
The total reaction rate for each reaction is determined by diffusion and the intrinsic chemical reaction rate. The final reaction rate of the ith reaction is calculated by16
ð13Þ
Ri ¼
Particle Tracking and Gasification Process. With the Lagrangian particle tracking method, particle motion is calculated by gi ðFp -Fg Þ dup, i fd ¼ ðug, i -up, i Þ þ Fp dt τp
42.5 1.62 1.42 108 1.5 108 0.4 1
Four heterogeneous reactions are used to represent the char gasification process; these are
D 00 2 μt 00 2 ε 2 00 2 þ Cg μt ðr2 f~Þ -Cd F f 00 rf ðFf Þ þ rðFvf Þ ¼ r σt Dt k
f ¼
-n
Ri, d Ri, k Ri , d þ Ri , k
ð16Þ
where Ri is the final reaction rate of the ith reaction, Ri,d is the bulk diffusion rate, and Ri,k is the apparent chemical reaction rate, which include the effects of pore diffusion and intrinsic reactivity of char. The bulk diffusion rate is calculated by
ð14Þ
The first term in the right side of eq 14 is the drag force, and the second term is the gravity force. t is the integration time; up,i is the particle velocity in the ith direction; τp is the particle aerodynamic response time; τp = Fpdp2/18μ; and gi is the gravity force in the ith direction. The drag coefficient fd is a function of the particle Reynolds number. fd = 1 when Rep < 1, and fd = 1 þ 0.15Rep0.687 when Rep > 1. Rep is defined by Rep = usdpFg/μg, and us is the slip velocity between the gas phase and the particle. Because the particle phase is very dilute in most regions of a gasifier, no particle-particle interactions are considered in this simulation. A stochastic particle-tracking model is adopted to consider the effects of turbulent fluctuation on particle dispersion.14 As the coal slurry particles enter the gasifier, water evaporation and boiling are considered at first. These two processes are predicted by the droplet evaporation and droplet boiling model.1 The coal devolatilization process is calculated by the one-step model ! dmp Ev ¼ -Av exp ðmp - mpc Þ ð15Þ dt RTp
Ri, d ¼ Ci
½ðTp þTg Þ=20:75 pi dp
ð17Þ
where Ci is the diffusion coefficient of the ith reactant, Ci = 3 10-12 s K-0.75, Tp is the absolute temperature of the particle, Tg is the absolute temperature of the bulk gas phase, dp is the particle diameter (in meters), and pi is the partial pressure of the ith reactant (in pascals). To consider the effects of the pressure on heterogeneous reactions, the apparent chemical reaction rate, Ri,k, is expressed by the nth empirical reaction model and is calculated by ! Ei pi n Ri, k ¼ Ai exp ð18Þ RTp 105 The pre-exponential A and the activation energy E for each reaction are listed in Table 2. (16) Smith, I. W. The combustion rates of coal chars: A review. Proceedings of the 19th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1982; pp 1045-1065. (17) Monson, C. R.; Germane, G. J.; Blackham, A. U.; Smoot, L. D. Char oxidation at elevated pressures. Combust. Flame 1995, 100, 669– 683. (18) Roberts, D. G.; Harris, D.J. Char gasification with O2, CO2, and H2O: Effects of pressure on intrinsic reaction kinetics. Energy Fuels 2000, 14, 483–489. (19) Liu, G.-S.; Niksa, S. Coal conversion submodels for design applications at elevated pressures. Part II. Char gasification. Prog. Energy Combust. Sci. 2004, 30, 679–717. (20) Muhlen, H. J.; Heek, K. H.; Juntgen, H. Kinetic studies of steam gasification of char in the presence of H2, CO2 and CO. Fuel 1985, 64, 944–949. (21) Fang, W. Study on reaction kinetic of steam-coal chars gasification with TGA. J. China Coal Soc. 2004, 29, 350–353.
where mp and mpc are the particle mass and the mass of char and ash in the particle. The activation energy Ev and pre-exponential Av are 3.28107 J/kmol and 2.1105 s-1, respectively.15 During the devolatilization process, the products of volatile pyrolysis are supposed to be composed of CO, CO2, H2, H2O, CH4, N2, and H2S. The composition of each species is based on the ultimate analysis of the coal. (14) Shuen, J. S.; Chen, L. D.; Feath, G. M. Evaluation of a stochastic model of particle dispersion in a turbulent round jet. AIChE J. 1983, 29, 167–170. (15) Zhang, N.; Fangui, Z.; Wenping, J. Pyrolysis kinetics analysis of Chinese typical steam coals. J. Taiyuan Univ. Technol. 2005, 36, 549–552.
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Table 3. Main Properties of YiMa Coal for the GE Gasifier and ShenFu Coal for the Staged Gasifier item
coal analysis for a GE gasifier
proximate analysis (%) ultimate analysis (%) particle size (μm) distribution (%)
V 21.98 C 65.32 30 50
FC 63.24 H 4.1 60 15
A 14.78 O 18.39 110 30
coal analysis for the staged gasifier
HHV (MJ/kg) 29.10 N S 0.78 0.43 175 5
V 33.39 C 74.39 25 45
FC 57.97 H 4.36 58 25
A 6.74 O 13.08 110 25
HHV (MJ/kg) 26.16 N S 0.92 0.51 175 5
Temperature Profile, Flow Field, and Mixing Process of the GE and Staged Gasifiers. In the GE entrained flow coal slurry gasifier, the coal slurry is atomized into small coal slurry particles by pure oxygen. High-temperature synthesis gas around the oxygen-slurry jet is entrained into the jet flow and reacts with pure oxygen rapidly. Coal slurry particles are heated by the surrounding high-temperature gas phase. Volatile yields leave very fast and initiate the gas reactions with oxygen. These exothermic processes help to form a stable flame in the down-flow region of the nozzle. Figure 4 shows the predicted temperature profile and flame shape in the GE gasifier. Below the nozzle, there is a narrow flame region along the centerline. In this region, temperature changes intensely; there is only a small gap between contours of the temperature of 1000 and 2500 K. Beyond the flame region, the temperature in the whole gasifier changes smoothly and follows the characteristics of the flow field. For example, the contour surface of T = 1600 K appears as a “V” shape, which draws the outline of the jet flow region and the backflow direction. In comparison to the axisymmetric distribution of the temperature in a GE gasifier, the temperature profile in the staged gasifier is different and shown in Figure 5. For the staged gasifier, the secondary nozzle is introduced horizontally in the x-z plane at a height of x = 1.86 m. Because only 85% of total oxygen is injected from the main nozzle, the main flame region is smaller and the temperature in the dome of the furnace is lower. The lifetime of the coal slurry nozzle is prolonged because of a lower temperature operation condition. As the secondary oxygen is introduced, two small horizontal flames are formed in the x-z plane. The flame curves up slightly under the influence of back flow. Two impinging flames are merged at the end of the main flame. Then, the temperature in this region is increased, and the coal conversion process is accelerated. In the x-y plane, the temperature profile is similar to the profile in a GE gasifier, except that the dome temperature is lower. The secondary nozzle changes the temperature profile as well as the flow field in the gasifier. This effect is shown in Figure 6. Figure 6a shows the vector of velocity in the GE gasifier. The high-speed jet flow goes straight downward until the boundary of the jet reaches the wall at 2/3 the height of the furnace, where the backflow region begins near the wall until it develops back to the dome region. In the remaining 1/3 of the furnace, the plug flow is formed and particles and gas are well-mixed. Panels b and c of Figure 6 show the vector profiles of the staged gasifier in the x-y and x-z planes, respectively. There is a big difference between the vector profiles in these two planes. In the x-y plane, the merged secondary jet flow expands horizontally. The radial velocity at the height where the secondary nozzle introduced is much larger than the velocity at the same location in the GE gasifier, as shown in Figure 7. Because more lateral gas flow is entrained into the backflow, the backflow region in
Figure 2. Boundary conditions and meshes adopted in the simulation.
During the coal reaction, it is assumed that turbulent fluctuation has little effect on the reaction rate. Both Tg and pi in eqs 17 and 18 are represented by the local average value.
Simulation of GE and Staged Gasifiers Simulation of a 500 tons/day GE gasifier located in Huainan, China, was conducted in this work. The concentration of the coal slurry for the gasifier is 62%. Properties of the coal are listed in Table 3. The gasifier is operated at an elevated pressure of 4.2 MPa. The ratio of the mass flow rate of oxygen to coal under normal conditions is 0.874. On the basis of a simple 1D heat-transfer model between the slagging layer wall and atmosphere, the heat loss of a gasifier is calculated. A constant heat flux boundary condition is adopted for the walls of the gasifier. The simulation domain is one-quarter of the gasifier, which is divided into a set of 16 32 130 body-fitted hexahedral meshes. The boundary conditions and meshes of the calculation domain are shown in Figure 2. The coal slurry is represented by 9000 particle tracks in this simulation. The distributions of the coal slurry particle size are given in Table 3. The simulation for the staged gasifier is similar to the simulation for the GE gasifier. The coal properties and size distribution of coal particles are listed in Table 3. The concentration of the coal slurry for the gasifier is 59.5%. The gasifier is operated at an elevated pressure of 4.0 MPa. The ratio of the mass flow rate of oxygen/coal under normal conditions is 0.93. The simulation domain is half of the staged gasifier, which is shown in Figure 3. All of simulation work mentioned above is based on the commercial computational fluid dynamics (CFD) software Fluent. Results and Discussion Gasification Performance for the GE and Staged Gasifiers. With the proposed model, the global performance of two types of coal slurry entrained flow gasifiers is predicted and shown in Table 4. Comparisons between the model prediction and measured data are listed for each gasifier. Because of the difficulty of measurement, the major species concentration for the staged gasifier is on a dry basis. For both gasifiers, there is good agreement between prediction and measured data. 1159
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: DOI:10.1021/ef901085b
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Figure 3. Boundary conditions and meshes adopted in the simulation. Table 4. Model Prediction and Measured Data for the GE and Staged Gasifiers GE gasifier, measured data GE gasifier, prediction staged gasifier, measured data staged gasifier, prediction
T (K)
[CO] (%)
[CO2] (%)
[H2O] (%)
[H2] (%)
CC (%)
1623 1578 1523 1511
32.4 35.9 45.3 48.9
16.1 13.4 19.4 19.6
25.3 25.8
25.7 24.4 34.7 30.8
95 94.6 96 96.1
Figure 4. Temperature profile in a GE gasifier. Figure 6. Vector profiles of velocity in the GE and staged gasifiers: (a) GE gasifier, (b) x-y plane of the staged gasifier, and (c) x-z plane of the staged gasifier.
Figure 7. Comparison of the radial velocity profile at the x-y plane at x = 1.86 m.
Figure 5. Temperature profile in the staged gasifier at the (a) x-y and (b) x-z planes.
the x-y plane shrinks and the main jet has a larger expansion rate. In the x-z plane, the backflow is split by the secondary jet. Three flames impinge on the centerline. The mixing process between the coal particles and gas phase is enhanced, as shown by Figure 6c. Figure 8a shows the distribution of the temperature and the main species concentrations in the axial direction in the GE gasifier; each point is an average value over a crosssection at that axial location. In the near-nozzle region, the
temperature increases sharply and reaches a maximum of 1850 K at a distance of 0.6 m from the nozzle. It then decreases smoothly and remains steady at around 1570 K. Oxygen is consumed over a short distance, and all main species increase sharply. CO and CO2 have an obvious change and reach steady state soon, while H2O and H2 continue to experience small changes even at the end of the gasifier. Figure 8b shows scalar profiles along the axis; therefore, it provides the information of the temperature 1160
Energy Fuels 2010, 24, 1156–1163
: DOI:10.1021/ef901085b
Wu et al.
Figure 8. Temperature and main species concentration profile in the axial direction in the GE gasifier, (a) averaging over the cross-section at an axial location and (b) at the center line.
Figure 9. Distribution of the temperature and main species concentrations along the axial direction in the staged gasifier, (a) averaging over the cross-section at a certain height and (b) at the center line.
Figure 9b shows the scalar profiles along the centerline in the staged gasifier. In comparison to Figure 8b, the main flame region in the staged gasifier is much smaller. The flame temperature reaches a peak of 2700 K and then drops. There is no steady high-temperature region, as in Figure 8b. This means that the main flame thickness is much smaller for the staged gasifier. As the temperature profile changes, the distribution of the main species concentrations are also changed, which is shown in Figure 9b. Figures 8 and 9 show the relationship between the temperature and the main species concentrations. H2 and H2O are more sensitive to the temperature than CO and CO2. As a result, both CO and CO2 come to a steady state very early, while H2 and H2O still experience changes even at the gasifier outlet. Effects of the [O]/[C] Ratio and Coal Slurry Concentration on Gasification Performance. Intensive research has been focused on the effects of the [O]/[C] ratio (global molar ratio of elemental oxygen to elemental carbon in both feed oxygen and coal) and the coal slurry concentration22-24 for the
and main species in the flame. At the beginning of the jet, the temperature does not change rapidly because of the evaporation of coal slurry particles and low initial temperature of the jet. As the turbulent mixing process develops, combustion expands to the center of the jet and temperature increases to an extremely high value immediately. The H2O and CO2 concentrations also jump at the same point. After 0.15 m, the CO and H2O concentrations begin to increase. The first jump of the CO and H2 concentrations is due to the assumption that they are a part of the main species of coal pyrolysis products. The second jump of these two lines shows the beginning point where char reactions take place rapidly. In the flame region, there is a long distance where the temperature reaches 3000 K and the oxygen concentration drops to 0 smoothly. When oxygen is supplied in a staged way, the temperature profile changes substantially. Figure 9 shows the temperature profile and the main species concentrations in the axial direction in the staged gasifier. The cross-section averaged values of temperature, H2O, and H2 have an obvious inflection at the height where the secondary nozzles are located. At this height, because the rest of the oxygen is added to the gasifier, the temperature comes to its maximum value of 1750 K. In comparison to Figure 8a, the average temperature of the staged gasifier in the upper section is lower; however, there is no significant difference in the profiles of the CO, CO2, and H2 concentrations between the two gasifiers.
(22) Chen, C.; Horio, M.; Kojima, T. Numerical simulation of entrained flow coal gasifiers. Part II: Effects of operating conditions on gasifier performance. Chem. Eng. Sci. 2000, 55, 3875–3883. (23) Ni, Q.; Williams, A. A simulation study on the performance of an entrained flow coal gasifier. Fuel 1995, 74 (1), 102–110. (24) Vamvuka, D.; Woodburn, E. T.; Senior, P. R. Modeling of an entrained flow coal gasifier. 1. Development of the model and general predictions. Fuel 1995, 74 (10), 1452–1460.
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Table 5. Predicted Main Performance of a GE Gasifier at Different Coal Slurry Concentrations with Fixed [O]/[C] coal slurry concentration (%)
56
59
60.5
62
63.5
65
68
[CO] (%) [H2] (%) [CO2] (%) [H2O] (%) T (K) carbon conversion (%) cold gas efficiency (%)
29.99 25.44 15.29 28.78 1424 85.29 69.66
32.56 24.8 14.7 27.39 1479 89.89 69.60
34.93 25.24 13.39 25.99 1533 94.23 71.36
35.88 24.44 13.39 25.83 1573 94.64 71.37
37.15 24.4 13.1 24.91 1556 96.1 71.71
39.08 24.55 12.14 23.79 1634 97.04 73.42
41.95 23.92 11.17 22.52 1733 98.64 73.57
Table 6. Model Predictions at Different [O]/[C] Atom Ratios with a Fixed Coal Slurry Concentration of 0.62 for the GE Gasifier [O]/[C]
0.9
0.95
0.96
0.997
1.05
1.1
[CO] (%) [H2] (%) [CO2] (%) [H2O] (%) T (K) carbon conversion (%) cold gas efficiency (%)
35.65 26.84 13.49 23.36 1450 84.97 70.57
35.78 25.49 13.44 24.77 1517 90.45 70.92
35.69 25.28 13.53 24.97 1520 91.0 70.80
35.88 24.44 13.39 25.83 1573 94.64 71.37
35.84 23.59 13.45 26.68 1613 96.34 71.55
35.67 21.54 13.66 28.73 1720 98.58 69.35
Figure 10. Particle tracks in the (a) GE and (b) staged gasifiers.
entrained flow coal gasifier. In this work, a systematic simulation study was carried out to explore the effects of the [O]/[C] ratio and coal slurry concentration for the GE gasifier. The effects of the coal slurry concentration on gasification performance (particularly carbon conversion, cold gas efficiency, and mole fraction of the main species) are shown in Table 5. As the coal slurry concentration increases, the temperature, carbon conversion, cold gas effiency, and CO mole fraction increase, while the CO2 and H2O mole fractions exhibit the opposite trend. Increasing the coal slurry concentration improves the cold gas efficiency and efficient gas component. For the H2 concentration, there is only a slight decrease when the coal slurry concentration increases. The reason is that, as the temperature increases, the water shift reaction moves to the direction in which H2 production is restrained. The molar ratio of oxygen/carbon in feed coal and oxygen is another important operating parameter. The effects of [O]/[C] on predicted gasification performance is shown in Table 6. When more oxygen is fed into the gasifier, the temperature and CO2 and H2O mole concentrations increase rapidly. Because a higher temperature accelerates char reactions, a higher carbon conversion rate is achieved. However, the H2 mole concentration decreases, because more O2 is added to the gasifier, leading to a higher operating temperature, which suppresses H2 generation. It is interesting that the CO mole concentration does not change with the [O]/[C] ratio. On one hand, a higher temperature makes the water shift reaction move to the direction in which more CO is generated. On the other hand, the additional oxygen restrains the production of CO. These two factors balance when the [O]/[C] ratio changes. For cold gas efficiency, there is a maximum value when the [O]/[C] ratio is around 1.05. At this point, an increase of carbon conversion and a decrease of efficient gas composition balance and create an optimization point. Particle Tracks and Effects of the Particle Size on the Coal Gasification Process. Coal particle tracks in two different types of gasifier are shown in Figure 10. There are no significant differences between panels a and b of Figure 10. When the coal particles enter the gasifier, they follow the
Table 7. Effect of the Particle Size on Coal Conversion and Residence Time in the GE Gasifier particle size (μm)
30
60
110
175
coal conversion (%) residence time (s)
100 11.4
98.1 10.6
87.6 11.7
72.2 12.4
main jet flow at first. As the jet develops, turbulent fluctuations of the gas flow field make the particles depart from the average gas flow. Although the average flow field is smooth, most coal particles are entrained into the backflow and merge into the main jet again because of the effects of turbulent fluctuations. This recirculation inside the gasifier prolongs the coal particle residence time. It plays an essential role in improving the coal conversion and gasification performance for the entrained flow coal gasifier. Smoot and Smith and Chen et al.1,22 found that particle size has a strong effect on the coal gasification process. Small particles have a shorter response time to gas velocity and gas temperature. As a result, those small particles are heated more rapidly and burned out faster. In this simulation, the gasification process for different particles was analyzed and compared. Residence time and coal conversion of different particles are listed in Table 7. Although particle residence time does not change with the particle size, it is apparent that small particles have a higher coal conversion because of a higher heterogeneous reaction rate and shorter burnout time. Conclusion A comprehensive 3D numerical model was proposed and used to simulate GE and staged gasifiers. A comparison of the global gasification performance between predictions and measured data proves that the model can be used to study the performance and mechanisms of the coal slurry gasification process. As the oxidizer feeding system changes, the temperature and flow field in the gasifier vary. For the staged gasifier, introducing the secondary jet strengthens turbulent mixing processes in the gasifier. A lower oxygen/coal ratio in the main jet decreases the jet flame region sharply. This makes the temperature in the top section of the staged gasifier drop 1162
Energy Fuels 2010, 24, 1156–1163
: DOI:10.1021/ef901085b
Wu et al.
and protects the jet nozzle. However, the secondary flow of oxygen increases the gas temperature in the low section. As the coal slurry concentration increases, both the coal conversion and cold gas efficiency increase. The CO concentration increases sharply, while the H2 concentration decreases a little, as a result of a higher temperature. As [O]/[C] increases, both the temperature and coal conversion increase. However, the cold gas efficiency increases at first and then drops, because the efficient gas component decreases when more oxygen is introduced.
P = pressure (Pa) R = thermal constant (J kg-1 K-1) or reaction rate S = source terms of the dispersed phase t = time (s) T = temperature (K) u = velocity (m/s) ui00 uj00 = Reynolds stress x, y, and z = coordinate in x, y, and z direction F = density (kg/m3) μ = kinetic viscosity (kg m-1 s-1) λ = conduction coefficient (W m-1 K-1) τp = particle aerodynamic response time (s) τij = stress tensor ε = turbulent kinetic energy dissipation rate j = scalar
Acknowledgment. This paper is based on work supported by the National Basic Research Program of China (973 Program, 2010CB227000).
Subscripts
Nomenclature A = pre-exponential index Cp = heat capacity at constant pressure (J kg-1 K-1) Ci = diffusion coefficient of the ith reaction dp = particle size (m) E = activate energy (J/mol) f = mixture fraction g = gravity acceleration (m/s2) h = enthalpy (J) Hloss = heat loss k = turbulent kinetic energy m = mass (kg)
d = diffusion f = mixture fraction g = gas h = enthalpy i = the ith reaction k = kinetic reaction m = mass mom = momentum p = particle rad = radiation v = volatile
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