Modeling of an Opposed Multiburner Gasifier with a Reduced-Order

Article pubs.acs.org/IECR

Modeling of an Opposed Multiburner Gasifier with a Reduced-Order Model Chao Li, Zhenghua Dai, Zhonghua Sun, and Fuchen Wang* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology (ECUST), Shanghai 200237, People’s Republic of China S Supporting Information *

ABSTRACT: A reduced-order model (ROM) is considered a promising solution for engineering simulation of a gasifier. In this study, a ROM of a commercial-scale opposed multiburner gasifier is developed based on a reactor network model (RNM). The RNM blocking for this gasifier is established and validated based on analysis of the gasifier flow field. The particle flow in the gasifier is characterized by the particle residence time in each reactor of the RNM. The random pore model and the “effective factor” method are employed to model the char gasification rate under high pressure and temperature. The interphase heat transfer and heat loss through the refractory wall are calculated. In model validation, the simulation results show well agreement with the industrial data. The model provides distributions of the gas temperature and compositions in the gasifier. Effects of the particle size on the particle temperature and carbon conversion are discussed quantitatively. It is observed that fine particles can be completely converted in the jet zone, while the large ones (>100 μm) are converted mainly in the impinging zone and impinging flow zone. attention. Yu et al.9 gave blocking of a GE gasifier in their early work in 1993 based on analysis of the flow field. Monaghan and Ghoniem10 established a dynamic ROM for a GE gasifier and performed a parameter study on it. Yang et al.11 employed ROM to study the performance and slag behavior in a Tsinghua oxygen-staged gasifier. Gazzani et al.12 used ROM to simulate a Shell/Prenflo entrained-flow gasifier. In this work, a ROM of an OMB gasifier is established based on the CFD simulation results of a commercial-scale OMB gasifier.8 The flow field of the gasifier is analyzed. The reactor network model (RNM) is built and validated using a Markov chain stochastic model. The motion of the particle phase is characterized by the particle mean residence time. Submodels employed in this ROM cover evaporation of the slurry water droplet, coal devolatilization, homogeneous and heterogeneous reactions, and heat transfer. The simulation results give distributions of the gas temperature and compositions and reveal the effects of the particle size on the gasification results as well.

1. INTRODUCTION Gasification is considered an important technology of coal/ petcoke utilization and hydrogen/CO production. It has many significant advantages: sulfur removal, low NOx emission, carbon capture, and a flexible configuration. As an efficient and reliable technology, entrained-flow gasification has been applied in various carbon/hydrogen-based chemical or energy systems. The opposed multiburner (OMB) gasification is developed by East China University of Science and Technology. It is widely used to produce power and other chemicals, such as methane and ammonia.1 To help design and optimize the gasifier, which is considered to be the heart equipment of the entrained-flow gasification system, many models of the gasifier has been developed, including the chemical equilibrium model,2 the simplified 1D models,3−5 and the comprehensive 3D models.6−8 The chemical equilibrium model has been integrated in process modeling and frequently used in engineering calculation. However, a major shortcoming of the gasifier chemical equilibrium model is the absence of reaction kinetics. The 1D model of the gasifier integrates the reaction kinetics in gasifier modeling. It performs reasonably well in simulating an axisymmetric gasifier, such as the GE gasifier. Compared with the chemical equilibrium and 1D models, the comprehensive 3D model based on computational fluid dynamics (CFD) contains various submodels and provides detailed results of a gasifier. The main challenge for the 3D model is the high demand of computational resources. When the 3D numerical simulation of a commercial-scale gasifier is combined with the overall gasification process modeling, the computational time of solving a 3D model will hardly be acceptable. Considered to be an affordable solution for engineering simulation, reduced-order model (ROM) has received much © 2013 American Chemical Society

2. OMB GASIFIER AND DEVELOPMENT OF THE RNM 2.1. Commercial-Scale OMB Gasifier and Blocking of the Flow Regions. The capacity of this commercial-scale OMB gasifier modeled in this work is 1500 t/day. The wall type of this oxygen-blown gasifier is refractory (hot wall). After leaving the gasifier, the syngas is quenched by water and purified by a mixer, a cyclone, and a water scrubber. The structure and size of the gasifier are illustrated in Figure 1. As shown, four three-channel burners are horizontally mounted Received: Revised: Accepted: Published: 5825

November 6, 2012 March 12, 2013 March 25, 2013 March 25, 2013 dx.doi.org/10.1021/ie3030505 | Ind. Eng. Chem. Res. 2013, 52, 5825−5834

Industrial & Engineering Chemistry Research

Article

enters Impinging Flow Zone #2 directly (see Figure 2c). The description of the gasifier 3D model is given in Table A1 in the Supporting Information. More details of the 3D model are published in ref 8. 2.2. Development and Validation of RNM Blocking. Figure 3 illustrates the flow network of the RNM derived from the flow field of the gasifier (see Figure 2). It shows the complicity of the flow field of an OMB gasifier. The jet zone (JZ) is a cone region that is from the outlet of each burner to the beginning of the impinging flow zone (IFZ). The recirculation flows from Recirculation Zone #1 and #2 (RZ #1 and RZ #2) are entrained into the JZ. The boundaries between the recirculation zone and IFZ are defined as the 0 m/s contours of the Y component of the gas velocity. The outlet zone (OZ) is defined as from the point where Impinging Flow Zone #2 (IFZ #2) reaches the gasifier wall to the outlet of the gasifier. Streams between different zones in Figure 3 are related to the streams shown in Figure 2. The Markov chain stochastic model has been used to calculate gas13,14 and solid residence time distribution (RTD)15 in the gasifier. In this work, it is employed to obtain the gas RTD of the RNM. Reactors in the RNM are treated as a series of states in the stochastic model. The probabilities of appearances of the tracked gas fluid elements in each state at time t + Δt are only related to their last states at time t. Transitions of their states are calculated with a discrete time step Δt. A transition matrix, pij, which is derived from flow-field analysis, is used to give the transition probability from state i to state j for a tracked fluid element. For a continuously stirred tank reactor (CSTR), the transition probability is obtained as follows:

Figure 1. Structure of a commercial-scale OMB gasifier.

around the gasifier. The center channel and outer annular channel of the burner are used to inject oxygen, while the inner annular channel is for a coal−water slurry (CWS). The velocity of oxygen at the outlet of each burner is about 120 m/s to ensure that the CWS stream can be atomized sufficiently. These four jet flows impinge in the center region of the gasifier, form an impinging flame, and generate two vertical impinging flows. Two recirculation zones are formed around these two impinging flows. Figure 2 illustrates the OMB gasifier flow field obtained by 3D simulation and blocking of the flow regions. As shown in Figure 2a, because the four burners are set around the gasifier, the flow field of the OMB gasifier is nonaxisymmetric, which means the velocity profiles of the different cross sections (see Figure 2, i-i and ii-ii) are different. It is found that Recirculation Zone #1 can extend across the burner plane (y = 0). As a result, part of the materials circulated from Recirculation Zone #1

pii = exp( −Δt /τi) pij =

qij ∑i ≠ j qij

(1 − pij )

i = 1, 2, ..., N

(1)

i , j = 1, 2, ..., N (2)

Figure 2. Flow field and blocking of the flow regions of an OMB gasifier. 5826

dx.doi.org/10.1021/ie3030505 | Ind. Eng. Chem. Res. 2013, 52, 5825−5834


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Figure 3. Flow network of the RNM.

in which τi is the mean residence time in state i. qij is the flow rate from state i to state j. For a plug-flow reactor (PFR), pij is calculated by the following equations: ⎧1 j = i + 1 pij = ⎨ ⎩ 0 other conditions

reactors in Figure 4a are related to the zones of the RNM: 1, JZ; 2, impinging zone (IZ); 3, IFZ #1; 4, IFZ #2; 5, RZ #1; 6, RZ #2; 7, OZ. The gas RTDs derived from the RNM and CFD are compared in Figure 4b. The major differences among the three cases are located in 0−4 s. Figure 4c gives the results in 0−4 s. The following criteria are employed to assess these configurations: The first one is the lag time, which is defined as the time when tracked fluid elements first arrive at the outlet of the gasifier; the other one is the average absolute deviation between the RNM and CFD results. The lag time and average absolute deviation of each case are listed in Table 1. Compared with other cases, it can be found that the configuration of case 3 is more reasonable.

(3)

The probability of a fluid element transiting from state i to state j for the first time after n steps, denoted as f ij(n), can be obtained: ⎧0 ⎪ ⎪ pij fij (n) = ⎨ ⎪ (n) ⎪ pij − ⎩

n=0 n=1 n−1

∑ fij (m) pjj(n− m)

3. SUBMODEL DESCRIPTIONS 3.1. Assumptions and Parameters Used in the Simulation. Coal gasification in an OMB gasifier combines several complex processes, including multiphase flow, droplet atomization and evaporation, coal particle devolatilization, volatile combustion, char combustion and gasification, homogeneous reaction, heat transfer, and slag deposition. In order to reduce the time consumed in model solving, the following assumptions are made in this study: (1) the particle mean residence time is employed to characterize the particle flow; (2) atomization of the CWS droplet is neglected in this work (the particle size distribution of pulverized coal at the outlet of coal miller is used directly; Table A2 in the Supporting Information gives the particle size distribution of coal); (3) interparticle radiation and slag deposition are not calculated in the simulation; (4) the temperature of the gasifier wall is set to constant as 1500 K, which is obtained from industrial data; (5) Shenfu bituminous coal is chosen as the feedstock. The parameters of the RNM are derived from the CFD results and listed in Table A3 in the Supporting Information. The proximate and ultimate analyses of coal are listed in Table 2. The operating conditions of the gasifier are listed in Table 3. 3.2. Evaporation. The evaporation rate of the CWS droplet is calculated in different conditions: (1) When the particle temperature is below the boiling point Tb, the evaporation rate is determined by the partial pressure of steam in bulk gas and the saturation steam pressure on the particle surface:

n = 2, 3, ...

m=1

(4)

where is the probability of a fluid element transiting from i to j after n steps. It is expressed as the following equation: p(n) ij

n

pij(n) =

n−1

∑ fij (m) pjj(n− m) = ∑ fij (m) pjj(n− m) + fij (n) m=1

m=1

n = 1, 2, ...

(5)

The gasifier outlet is defined as the “trapping state”, d, which is the only outlet of the system. All tracked fluid elements will leave the gasifier after entering into this state. Thus, the probability of a fluid element reaching the outlet of the gasifier between nΔt and (n + 1)Δt is adopted: Ei(nΔt )Δt = fid (n)

(6)

where Ei is the density function of the fluid element residence time distribution. After eq 4 is introduced to eq 6, Ei is obtained as follows: ⎧0 n=0 ⎪ Δ / p t n=1 ⎪ ⎪ id Ei(nΔt ) = ⎨ ⎡ (n) n−1 ⎪ ⎢ pid − ∑m = 1 fid (m) ⎤⎥ /Δt n = 2, 3, ... ⎪⎢ ⎥⎦ Δt ⎪⎣ ⎩ (7)

Figure 4 gives a comparison of the RNM gas RTD calculated by the Markov chain stochastic model and the gas RTD derived from 3D simulation results. Three configurations of the RNM shown in Figure 4a are tested in this study. The numbers of the

revap = 5827

⎛ Psat(Tp) dmw P ⎞⎟ A pM w = −kc⎜⎜ − Xw dt RT∞ ⎟⎠ ⎝ RTp

(8)



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Figure 4. Comparison of the RNM gas RTD and 3D simulation results.

Table 1. Lag Time and Average Absolute Deviation of Each Case lag time (s) average absolute deviation

case 1

case 2

case 3

CFD

0.04 4.36 × 10−2

1.76 5.33 × 10−2

1.01 3.09 × 10−2

0.71

Table 2. Analysis Results of Shenfu Coal proximate analysis (wt %, dry basis) FC

Vm

57.74

ash

HHVd (kJ/kg)

33.41 8.85 ultimate analysis (wt %, dry basis)

29741

C

H

O

N

S

75.93

4.24

9.90

0.88

0.20

Table 3. Operating Conditions of the Gasifier variable

value

units

pressure CWS flow rate CWS concn CWS density oxygen oxygen concn

5.8 72.0 60.5 1176 33960 99.8

MPa m3/h wt % kg/m3 N·m3/h %

Figure 5. Cell meshing and solution process diagram of a PFR.

3.3. Devolatilization. The components of volatile matter are determined using Merrick’s method.16 The reaction of coal devolatilization is given as

where Psat is the saturation steam pressure at particle temperature Tp, Xw is the mole fraction of steam in bulk gas, and kc is the mass-transfer coefficient. (2) When the particle temperature reaches the boiling point Tb, the evaporation rate can be expressed as revap

coal → αChar + βCH4 + γCO + δCO2 + ξH 2 + φH 2O + ζN2 + χH 2S

dm w 1 = = − [hA p(Tg − Tp) + A pεpσ(Tg 4 − Tp 4)] dt hfg

(R-1) 16

The maximum yield of volatile matter is modified as VMp = VM − 0.36VM 2

(9)

in which hfg means the latent heat of water vaporization and h represents the heat-transfer coefficient.

(10)

where VM is the volatile matter content (daf basis) of proximate analysis. 5828



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3.4. Homogeneous Reactions. Homogeneous reactions in the gasifier include gas combustion reactions R-2−R-4, water−gas shift reaction R-5, and methane transformation reaction R-6. The reaction rates and parameters are listed in Table A4 in the Supporting Information.

Figure 6. Cell meshing and the recirculation flow distribution of the JZ.

H2 +

1 O2 → H 2O 2

(R-2)

CO +

1 O2 → CO2 2

(R-3)

CH4 + 2O2 → CO2 + 2H 2O

(R-4)

CO + H 2O ↔ H 2 + CO2

(R-5)

CH4 + H 2O ↔ 3H 2 + CO

(R-6)

3.5. Heterogeneous Reactions. Char combustion reaction R-7 and gasification reactions R-8 and R-9 are considered the rate-determining steps in the gasifier. Their reaction rates depend on many variables including the coal type, char properties, thermal history, and partial pressure of the gasification agent.20−24 C + 0.5O2 → CO

(R-7)

C + CO2 → 2CO

(R-8)

C + H 2O → H 2 + CO

(R-9)

The “effective factor” model is employed in this study to obtain the reaction rates. The random pore model (RPM) is used to fit the experimental data gained by using a pressured thermogravimetric analyzer:8 dx = ηAi Pin exp(−Ei /RT )(1 − x) 1 − ψ ln(1 − x) dt (12)

where the carbon conversion of char particle x is defined as x = 1 − mc /mc0

(13)

The reaction rate for the pore-diffusion- and chemical-reactioncontrolled region is obtained by combining eqs 12 and 13: rpore =

dmc, i dt

= −mc, i0ηAi Pin exp(−Ei /RT )(1 − x)

Figure 7. Solution process diagram of a CSTR.

1 − ψ ln(1 − x)

Many models17,18 have been developed to calculate the rate of coal devolatilization. In this study, the general model of devolatilization developed by Fu et al.19 is used, in which the coal devolatilization rate is obtained as follows: rdecomp =

⎛ E ⎞ dm v ⎟⎟ = −(mmv − m v )A 0 exp⎜⎜ − dt ⎝ RTp ⎠

(14)

In eq 14, Ai is the frequency factor, Pi represents the partial pressure of the gas reactant, Ei is the activation energy, and ψ is the parameter of the RPM. The effective factor η is introduced to extrapolate the reactivity data to the high-temperature region, where the reaction rate is dominated by pore diffusion. The following equation for calculation of the effective factor η is adopted:25

(11)

η = fc

mmv is the maximum yield of volatile matter, and mv is the yield of volatile matter.

1⎡ 1 1 ⎤ − ⎢ ⎥ ϕ ⎣ tanh(3ϕ) 3ϕ ⎦

(15)

Table 4. Comparison of Industrial Data and the ROM Results

industrial data simulation results

H2, vol %

CO, vol %

CO2, vol %

carbon conversion, %

temperature, °C

coal consumption, kg × 1000N/m− 3 (CO + H2)

oxygen consumption N·m3 × 1000 N/m3(CO + H2)

32.98 33.81

49.00 50.39

17.02 15.21

98.0 98.3

1224 1230

568 560

376 371

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Figure 8. Temperature distribution in the gasifier.

Figure 9. Distributions of the gas compositions in the gasifier.

where the fc is defined as the correlation function and ϕ is the

The reaction rate at bulk-diffusion-dominated region can be obtained as follows:

25

modified Thiele module number:

rbulk =

2

⎤0.5(1 − n) ⎡ 1/2 ⎥ fc = ⎢1 + 2ϕ2 + 1/(2ϕ2) ⎦ ⎣

ϕ=

×

(16)

dmc, i dt

=−

αShMcDi A pPi RTmd p

(18)

The parameters of the heterogeneous char reactions are given in Table A5 in the Supporting Information. 3.6. Heat Transfer. Heat transfer between the particle and gas phases is considered to be the sum of convection and radiation. The total heat flux is calculated by the following equation:

dp 6 n−1 (n + 1) Ai exp(−Ei /RTp)(1 − x) 1 − ψ ln(1 − x) ρp RTpPi 2 McDeff

Q trans = πd p2Np[h(Tp − Tg) + εpσ(Tp 4 − Tg 4)]

(17) 5830

(19)



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Figure 10. Mean particle temperature distributions in different zones of the gasifier: □, 20 μm; ○, 40 μm; Δ, 60 μm; +, 200 μm; ×, 500 μm.

80 μm; ☆, 100 μm;

respectively. The simulation performed on a dual-core computer with 2.8 GHz CPU could converge in 15 min.

The heat loss through refractory is obtained as follows: Q lost = A w h(Tw − Tg)

▽,

(20)

5. MODEL VALIDATION A set of industrial data derived from a commercial-scale OMB gasifier is used to validate the model. A comparison of industrial data and the ROM results is given in Table 4. It can be found that the results of the ROM show well agreement with industrial data. The relative deviations of carbon conversion and gas temperature are less than 0.5%, while those of the coal and oxygen consumptions are less than 2%. The relative deviations of mole fractions of the gas components are relatively higher. On the basis of the validation, some detailed results are discussed in the next section.

where Aw is the area of refractory of the gasifier.

4. NUMERICAL SOLUTION OF THE ROM 4.1. Solution of the PFR. The PFR is meshed into a number of cells along the flow direction for the numerical solution. Figure 5 illustrates the cell meshing and the solution process diagram of a PFR. Recirculation flows are added to the first cell or distributed to a certain number of cells depending on the flow field. For instance, Figure 6 gives the cell meshing of the JZ and the recirculation flow distribution. According to the CFD results, the recirculation flow to the JZ is divided and added to each cell of the JZ. The fourth-order Runge− Kutta method is employed to solve the governing equations in the PFR. 4.2. Solution of the CSTR. In this study, the IZ, RZ, and OZ of the RNM are solved as CSTRs. Figure 7 shows the solution process of a CSTR. The tolerances for mass and energy balances are set to 10−4 kg (and 10−3 for the relative residual error) and 1 K, respectively. 4.3. Solution of the ROM. On the basis of the solution of each reactor, the ROM is solved by using a sequential method. The numerical solution of the ROM is implemented by a FORTRAN code to ensure accessibility to other simulation software. The tolerances for mass flow and temperature are set to 10−4kg (and 10−2 for the relative residual error) and 1 K,

6. RESULTS AND DISCUSSION 6.1. Distributions of the Gas Temperature and Compositions. Figure 8 illustrates the gas-temperature distribution in the gasifier. It can be found that the highest gas temperature around the IZ is 2374 K, which is generated by intensive combustion of volatile matter and surrounding combustible gases. The high gradient of temperature in the same area indicates high reaction rates as well. In addition, the temperatures of the regions (IFZ #1 and RZ #1) above the burner plane (Y < 0) are higher than those of the regions (IFZ #2 and RZ #2) below the burner plane (Y > 0). This trend qualitatively agrees with the industrial data. The main reasons for this result are considered to be follows: In IFZ #1, there is no exit for gas or solid, while there is a large part of gas 5831



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Figure 11. Mean carbon conversion distributions in different zones of the gasifier: □, 20 μm; ○, 40 μm; Δ, 60 μm; +, 200 μm; ×, 500 μm.

▽,

80 μm; ☆, 100 μm;

than that of R-8. With decreasing gas temperature and increasing carbon conversion, the rates of the gasification reactions slow down and the gas compositions change slowly in the IFZ, RZ, and OZ. As another consequence of the split ratio of the particle phase mentioned above, the distributions of the gas compositions in IFZ #1 and RZ #1 show differences from those in IFZ #2 and RZ #2. 6.2. Particle Temperature and Carbon Conversion. The effect of the particle size on the particle temperature is investigated in this study. The mean particle temperature is shown in Figure 10. Figure 10a illustrates the mean temperature distributions of particles with different diameters in the JZ, where the X position is the radial distance, as shown in Figure 2b. It can be observed that the particle diameter has a significant effect on the particle temperature. With decreasing particle size, the particle temperature increases sharply in the JZ. Parts b and c of Figure 10 give the particle temperature distributions in IFZ #1 and IFZ #2. Because the reaction rates of evaporation and devolatilization of the large particles are relatively slow, the temperature of the largest particles (500 μm) rises much slower than that of tiny particles. It is found that the largest particles (500 μm) are heated in these two regions. Figure 10d shows particle temperatures at the outlet of the IZ, RZ, and OZ, which are considered as CSTRs. The temperatures of the particles with different diameters show negligible differences at the outlet of all CSTR zones except the IZ.

and solid entering the OZ and leaving the gasifier at the end of IFZ #2. Also, the wall surface of RZ #1 is smaller than that of RZ #2. Therefore, the heat loss in IFZ #2 and RZ #2 is significantly higher than that in IFZ #1 and RZ #1. Another reason considered is the particle phase split ratio of IFZ #1 to IFZ #2. In this study, the split ratio of the particle phase derived from the 3D result is 0.61. More particles entering IFZ #2 means more heat consumptions caused by gasification reactions, and consequently lower temperature. Figure 9 shows the distributions of gas compositions in the gasifier. At the beginning of the JZ, the mole fraction of H2 increases significantly because of the entrainment of recirculation flows. However, as a result of the high reaction rate of R-3, CO is consumed immediately and the mole fraction of CO2 increases constantly. With increasing of gas temperature at the end of the JZ, the combustion rate of H2 is enhanced greatly and the mole fraction of H2 drops sharply as a consequence. As a result of droplet evaporation and H2 combustion, the mole fraction of H2O increases at the end of the JZ. Because most of the oxygen is exhausted in the JZ, the gas compositions in the other zones are dominated by the gasification reactions R-8 and R-9. Because of the high temperature in the IZ, the rates of gasification reactions in this region are extremely high. Large amounts of H2 and CO are generated, while CO2 and H2O are consumed significantly in the IZ. Compared with H2O, the mole fraction of CO2 drops more slowly because the reaction rate of R-9 is 5−10 times higher 5832



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Keq equilibrium constant mw mass flow of steam in the gas phase, kg/s Mw molecular weight of water, g/mol mi mass of a particle, kg mv yield of volatile matter of a particle, kg mmv maximum yield of volatile matter of a particle, kg mc char mass of a particle, kg mc0 initial char mass of a particle, kg Mc molecular weight of carbon, g/mol n reaction order Np particle number P pressure, MPa revap evaporation rate of a droplet, kg/s rdecomp devolatilization rate of a coal particle, kg·s−1 rpore heterogeneous reaction rate at the pore-diffusion- and reaction-kinetic-controlled region, kg/s rbulk heterogeneous reaction rate at the bulk-diffusioncontrolled region, kg/s R universal gas constant, J/kmol·K S source term Sh Sherwood number Tg temperature of the gas, K Tp temperature of the particle, K Tm average temperature of the gas and particle phases, K Vmp modified maximum yield of volatile matter

At the outlet of the IZ, the temperatures of large particles (>200 μm) are obviously low. Figure 11 shows the mean carbon conversion distributions of particles in the gasifier. As illustrated in Figure 11a, carbon conversions of the particles with diameters of less than 100 μm rise significantly in the JZ. The fine particles (20 μm) have been converted completely before entering into the IZ. Parts b and c of Figure 11 give carbon conversion distributions in the IFZ where gasification of large particles mainly happens. As shown in Figure 11d, by the promotion effect of high temperature and mixing rate to heat transfer and char reactivity, carbon conversions of the particles (≤60 μm) reach 100% after leaving the IZ. At the outlets of the RZ and OZ, all particles are converted completely except the large ones (>200 μm).

7. CONCLUSIONS A ROM of a commercial-scale OMB gasifier is developed in this work. The RNM of the gasifier is established based on a verified 3D model. Distributions of the gas temperature and compositions in the gasifier are calculated. The effects of the particle size on the particle temperature and carbon conversion are discussed as well. The following conclusions have been obtained: (1) Using particle mean residence time to characterize particle flow in each reactor of the RNM is verified to be feasible. (2) The highest gas temperature around the IZ is 2374 K. The high gradients of gas temperature and compositions around the IZ indicate the high reaction rates near the IZ. (3) Heating, combustion, and gasification of fine particles (20, 40, and 60 μm) mainly happens in the JZ and IZ, while most of the particles with medium diameter (80 and 100 μm) are heated and converted in the IZ and the beginning of the IFZ. The temperature and carbon conversion of large particles (>200 μm) increase slowly in the IFZ.

■

Greek Letters

ε emissivity of the particle α stoichiometric coefficient of the reaction σ Stefan−Boltzmann constant, W/m2·K4 ψ parameter of the RPM ρ density, kg/m3

Subscripts

ASSOCIATED CONTENT

■

S Supporting Information *

Tables A1−A5. This material is available free of charge via the Internet at http://pubs.acs.org.

■

i index of particle, gas component, or Markov state j index of reaction or Markov state p particle phase g gas phase

REFERENCES

(1) ICCT. Applications of OMB Gasification Technology, 2012, http://klcg.ecust.edu.cn. (2) Dai, Z. H.; Gong, X.; Guo, X. L.; Liu, H. F.; Wang, F. C.; Yu, Z. H. Pilot-Trial and Modeling of a New Type of Pressurized EntrainedFlow Pulverized Coal Gasification Technology. Fuel 2008, 87, 2304− 2313. (3) Wen, C. Y.; Chaung, T. Z. Entrainment Coal Gasification Modeling. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 684−695. (4) Vamvuka, D.; Woodburn, E. T.; Senior, P. R. Modelling of an Entrained Flow Coal Gasifier. 1. Development of the Model and General Predictions. Fuel 1995, 74, 1452−1460. (5) Kasule, J. S.; Turton, R.; Bhattacharyya, D.; Zitney, S. E. Mathematical Modeling of a Single-Stage, Downward-Firing, Entrained-Flow Gasifier. Ind. Eng. Chem. Res. 2012, 51, 6429−6440. (6) Watanabe, H.; Otaka, M. Numerical Simulation of Coal Gasification in Entrained Flow Coal Gasifier. Fuel 2006, 85, 1935− 1943. (7) Wu, Y. X.; Zhang, J. S.; Smith, P. J.; Zhang, H.; Reid, C.; Lv, J. F.; Yue, G. X. Three-Dimensional Simulation for an Entrained Flow Coal Slurry Gasifier. Energy Fuels 2010, 24, 1156−1163. (8) Sun, Z. H.; Dai, Z. H.; Zhou, Z. J.; Guo, Q. H.; Yu, G. S. Numerical Simulation of Industrial Opposed Multiburner Coal−Water Slurry Entrained Flow Gasifier. Ind. Eng. Chem. Res. 2012, 51, 2560− 2569. (9) Yu, Z. H.; Wang, S. C. D.; Yu, F. C.; Xiao, J. G.; Gong, K. J.; Three-Region, X. Model for Gasification Process in Coal-Slurry Gasifier. J. Fuel Chem. Technol. (Chinese) 1993, 21, 90−95.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-21-6425 0784. Fax: +86-21-6425 1312. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is financially supported by the National Key State Basic Research Development Program of China (973 Program, 2010CB227000) and the Fundamental Research Funds for the Central Universities (WB1014037).

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NOMENCLATURE A frequency factor Ap surface area of the particle, m2 Aw area of the refractory wall, m2 Cp specific heat capacity, J/kg·K dp particle diameter, m Deff effective diffusion coefficient, m2/s Di diffusion coefficient of species i in the gas mixture, m2/s E activation energy, kJ/mol 5833



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Modeling of an Opposed Multiburner Gasifier with a Reduced-Order

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