Comparative Study of Gasification Performance between Bituminous

Sep 27, 2012 - petroleum coke is accelerated at 5 times by catalysis effects, the carbon conversion reaches 99.2% and the consumption of oxygen...
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Comparative Study of Gasification Performance between Bituminous Coal and Petroleum Coke in the Industrial Opposed Multiburner Entrained Flow Gasifier Zhonghua Sun, Zhenghua Dai, Zhijie Zhou, Jianliang Xu, and Guangsuo Yu* Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: A comprehensive three-dimensional numerical model for simulating Shenfu bituminous coal and petroleum coke in the industrial opposed multiburner (OMB) gasifier is established. The “effectiveness factor” method is used to extrapolate the intrinsic char reactivity data to industrial gasifier conditions. With the proposed models, the predicted gasification performance of Shenfu coal is in good agreement with industrial operating data, and the predicted carbon conversion of petroleum coke is about 96.7%. Comparing with Shenfu coal, the specific solid fuel consumption of petroleum coke is lower, while the specific oxygen consumption is higher. The characteristic of char gasification in the entrained flow gasifier is investigated. In the high temperature jet flow region and impinging flow region, when the particle diameter is above 200 μm, the Thiele modulus of the char gasifying with steam for Shenfu coal and petroleum coke are 15.4 and 4.8, respectively. Therefore, pore diffusion has a significant effect on the apparent char reactivity. While for the char gasifying with CO2, the Thiele modulus is below 5 in different regions of the OMB gasifier, and the apparent char reactivity is affected by the chemical reaction and pore diffusion. When the char reactivity of petroleum coke is accelerated at 5 times by catalysis effects, the carbon conversion reaches 99.2% and the consumption of oxygen and feedstock decrease obviously. Importantly, the temperature at the dome of the gasifier decreases to 1345 °C, which prolongs the life of refractory wall and favors a long period of availability for the gasifier. coke gasification in the fixed bed gasifier and investigated the effects of ratio of oxygen petroleum coke, ratio of steam petroleum coke and gasifier load on the gasification performance, and the predicted carbon conversion of petroleum coke was about 99.5% in the fixed bed gasifier. On the basis of the two phase model, Goyal et al.8 studied the petroleum coke gasification performance in the fluidized bed gasifier, and the predicted carbon conversion is 82.4% at 1 atm which agreed with the experimental data of Winkler gasifier. Above all, there were few studies on the gasification performance of petroleum coke under industrial entrained flow gasifier conditions (pressure about 4 or 6 MPa and temperature about 1200−2000 °C). In order to explore the feasibility of petroleum coke gasification in the industrial OMB gasifier, it is necessary to understand comprehensively the petroleum coke gasification process, including gasification performance, the flow, temperature, and species distributions in the gasfieir. The numerical simulation9,10 on the gasification offers an effective technique for predicting gasification characteristics and optimizing operation parameters. In this paper, a three-dimensional steady-state numerical model is developed to simulate the entrained flow gasification performance of bituminous coal and petroleum coke. The effectiveness factor method9,11 is used to extrapolate the intrinsic char reactivity data to industrial OMB gasifier conditions. Comparing with the bituminous coal, the gasification performance of petroleum coke in the industrial OMB gasifier is studied. The characteristics of char gasification in the OMB gasifier are discussed.

1. INTRODUCTION As a byproduct of the oil refining industry, the use of high-sulfur petroleum coke is increasing due to the increasing demand for heavy oil processing. High-sulfur petroleum coke is attractive for its low price and higher heating values; on the other hand, it is also a challenging fuel for giving rise to undesirable emission characteristics.1 Gasification offers a feasible, efficient, and clean utilization of petroleum coke.2 The opposed multiburner (OMB) gasification process using coal as feedstock has been successfully applied to synthesis chemicals such as methanol and ammonia in China,3 and an industrial plant of petroleum coke gasification is in design. Recently, the gasification performance of petroleum coke in the gasifier has been studied based on experiments and simulations. Lee et al.4 studied the gasification characteristics of petroleum coke and mixture of petroleum coke and lignite in a 1 ton/day entrained-flow gasifier, and the experimental results showed that under about 1600 °C temperaure and 1 atm pressure, the carbon conversion of petroleum coke could be over 90%, but high O2/fuel ratio leaded to high CO2 content and low cold syngas efficiency about 35−55%. Shen et al.5 investigated the cogasification performance of bituminous coal and petroleum coke in a pilot-scale pressurized entrained flow gasifier, the results showed that for the pure petroleum coke the carbon conversion is 86% at the pressure of 4.2 bar and when up to pressure to 8.2 bar the carbon conversion of mixture (25% coal) increased to 96%. Marin-Sanchez and RodriguezToral6 established an equilibrium model for entrained-flow gasification of petroleum refinery residual fuels, and the predictions are in good agreement between both the simulated and published data. Nagpal et al.7 built up a 1-dimensional model for petroleum © 2012 American Chemical Society

Received: April 27, 2012 Revised: September 26, 2012 Published: September 27, 2012 6792

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where Po is local absolute pressure, and Xw is the mole fraction of steam. Psat is the saturation pressure of steam, which is calculated by Antoine equation:12

Besides, catalysis effects on the gasification performance of petroleum coke are investigated.

2. BITUMINOUS COAL AND PETROLEUM COKE REACTION SUBMODELS The properties of Shenfu bituminous coal and petroleum coke are shown in Table 1, and these carbon containing fuels are

Psat = 0.133289exp[18.3030 − 3816.44/(Tp − 46.13)]

2.3. Pyrolysis. The pyrolysis process is assumed as two steps: devolatilization (R-2) and volatiles simultaneous decomposition (R-3).

Table 1. Properties of Solid Fuel proximate analysis, wt%

Shenfu coal

petroleum coke

volatile matter, daf fixed carbon, daf ash, dry ultimate analysis, wt% C, daf H, daf N, daf S, daf O, daf HHVd, MJ/kg ash fusion temperature (FT), °C

36.65 63.35 8.85

10.27 89.73 0.06

83.30 4.65 0.97 0.22 10.86 30.14 1170

88.57 3.60 1.01 6.82 0.0 35.21 >1400

Coal/Petcoke → Volatiles + Coke(C(s) ·Ash)

Volatiles → α1CH4 + α2CO + α3CO2 + α4 H 2 + α5 N2 3

+ α6 H 2S ∑ α i=1

Compared to char gasification, the devolatilization rate is relatively fast and has little effect on the gasifier performance.13 So devolatilization rate for Shenfu coal and petroleum coke is determined by the Kobayashi model.14 The volatile yield for bituminous coal decreases with increasing pressure,15 while for petroleum coke has little change.16 After taking flux amount into account, the specific devolatilization products are determined by the proximate and ultimate analysis as shown in Table 3. Table 3. Devolatilization Products devolatilization products, wt% char, dry basis volatile, dry basis ash, dry basis volatile composition, mol % CO CO2 H2S N2 CH4 H2

dp, μm

0 < dp < 45

45 < dp < 70

70 < dp < 380

380 < dp < 680

davg n mass fraction, %

35 1.5 35

65 4.0 16

170 1.6 46

450 4.7 3

Shenfu coal

petroleum coke

67.73 23.42 8.85

86.31 11.14 2.55

0.2013 0.0178 0.0024 0.0120 0.0425 0.7241

0.116 0.020 0.864

2.4. Homogeneous Reactions. Simple global reactions are used to describe homogeneous reaction, including fuel gases combustion, water−gas shift and methane−steam reactions.

Table 2. Parameters of Rosin−Rammler distribution function

CO + 0.5O2 → CO2

− 283 kJ/mol

(R-4)

H 2 + 0.5O2 → H 2O

− 242 kJ/mol

(R-5)

CH4 + 0.5O2 → CO + 2H 2

CO + H 2O ⇔ CO2 + H 2 CH4 + H 2O ⇔ CO + 3H 2

2.2. Water Evaporation. In the gasifier, the atomized slurry particles are quickly heated and evaporated. The evaporation process can be described as

− 35.7 kJ/mol

(R-6)

− 41.1 kJ/mol

(R-7)

+ 206 kJ/mol

(R-8)

In the complex turbulent reacting flow, the net homogeneous reaction rate is controlled by chemical reaction rate Ri,r and turbulent mixing rate Ri,t, and the minimum of the above is taken as the limiting rate. The detailed chemical reaction rate is described as Arrhenius form according to ref 9. The turbulent mixing rate Ri,t is calculated by the eddy break-up (EBU) model17

(R-1)

The evaporation rate is governed by bulk steam partial pressure and steam saturation pressure at the particle surface. ⎛ Psat(Tp) P ⎞ dm w = − kc⎜⎜ − X w o ⎟⎟A pM w dt RT ⎠ ⎝ RTp

(R-3)

=1

prepared as slurry with water for gasification in the industrial OMB gasifier. Due to low ash content of petroleum coke, 2% (wt) flux agents of total solid carbonaceous feeds (dry basis) are added for petroleum coke gasification to discharge the liquid slag continuously and remove harmful elements such as nickel and vanadium. Under high temperature and high pressure in the gasifier, there are complicated physical and chemical processes occurring to fuels particles: slurry atomization, water evaporation, pyrolysis, and homogeneous and heterogeneous char reactions. 2.1. Slurry Atomization. The burner of OMB gasifier is a three channel airblast atomizer. The slurry (annular channel) is fed into gasifier and then accelerated by high speed about 120 m/s pure oxygen (the first and third channel) to atomize fully into droplets. Due to fast droplet evaporation in the high temperature gasifier, the droplet size distributions are considered to be consistent with particle size distributions at the grinding mill, and particle size distributions are measured by Mastersizer2000 analyzer and fitted by the Rosin−Rammler expression. The Rosin−Rammler distribution function is Yd = exp[(−dp/davg)n], and its parameters are shown in Table 2.

Slurry → Coal/Petcoke + H 2O(g)

(R-2)

R i ,t = υi′,rMi ,rCrρ (1) 6793

⎛ Y Yi ,p ⎞ ε i ,r ⎟⎟ min⎜⎜ , k ⎝ υi′,rMi ,r υi′,pMi, p ⎠

(2)

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Table 4. Char Combustion Kinetic Parameters Shenfu coal

Table 7. Governing Equations for Gas−Particle Phase in the OMB Gasifier

petroleum coke

constants

Ai

Ei (kJ/mol)

k1 k2 k3

1.9 × 104 atm−1 s−1 3.3 × 104 atm−1 s−1 5.8 × 107 s−1

35 130 180

Ai

Ei (kJ/mol)

1.0 × 104 atm−1 s−1 1.7 × 104 atm−1 s−1 3.1 × 107 s−1

35 130 180

gas phase mass ∂ (ρ ̅ uĩ ) = Sm ∂xi

∂uj̃ ⎤ ∂(ρu′i u′ j ) ∂ ∂P ̅ ∂ ⎡⎢ ∂uĩ ⎥− (ρ ̅ uĩ uj̃ ) = − μt + + ρgi + Sui + ∂xj ∂xi ⎥⎦ ∂xj ∂xi ∂xj ⎢⎣ ∂xj

Table 5. LH Model Kinetic Parameters of Shenfu Char rate constant

Ai

Ei (kJ/mol)

k4 k5 k6 k7 k8 k9 k0

3.78 × 104 atm−1 s−1 293 atm−1 s−1 192 atm−1 s−1 1.33 × 107 atm−1 s−1 4 × 103 atm−1 s−1 3 atm−1 s−1 4.1 × 104 s−1

178 145 122 226 160 70 176

∂ ∂ ⎛⎜ λ ∂h ⎞⎟ + Sh (ρuih ) = ∂xi ∂xj ⎜⎝ c p ∂xj ⎟⎠

μ ⎞ ∂Y ∂ ∂ ⎛ (ρuiYi ) = ⎜ρ ̅ Di , m + t ⎟ i + SYi + R Yi ∂xi ∂xj ⎝ Sct ⎠ ∂xj

Ai, MPa−n s−1

Ei, kJ/mol

n25

Ψ25

2.75 × 104 6.06 × 106

198 240

0.49 0.49

10 10

C + O2 → 2C(O)

(k1)

C(O) + O2 → CO2 + C(O)

(k2)

C(O) → CO

(k3)

(4)

particle phase mass dm p dmc dm w dm v = + + (5) dt dt dt dt momentum du p 1 1 mp = πd p2ρC D|u ̅ − u p|(u ̅ − u p) + πd p3(ρp − ρ)g (6) dt 8 6 24 0.687 CD = (1.0 + 0.15Re ) (7) Re heat dTp λNu mpCp = Ap (T − Tp) + A pεa(Q R − σBTp 4) + m w hw + m v hv dp dt

−

∑ mc,iQ i

(8)

i

C + CO2 ↔ C(O) + CO

(k4)

C(O) ↔ CO

(k5)

C + H 2O ↔ H 2 + CO

(k6)

C(O) ↔ CO

(k7)

After taking four simple reactions (k4−k7) into account, the overall char gasifying rate with CO2 and H2O are

For char combustion at high temperature and high pressure, a three-step semiglobal mechanism18 (k1−k3) is used, which could describe the major trends in reaction order, activation energy, and the CO/CO2 ratio from 600 to 2000 K. The steady-state expression for overall char oxidation rate is

rCO2 = fx η

rH2O = fx η

k1k 2PO2 2 + k1k 3PO2 k1PO2 + k 3/2

(3)

species

where υ′i,r and υ′i,p are stoichiometric coefficients for reactant and product respectively. Yi,r and Yi,p are mass fraction of the reactant and product, respectively. Cr is an empirical constant equal to 4.0. The water−gas shift and methane−steam reactions are assumed to proceed to chemical equilibrium, and the forward reaction rate and equilibrium constant are used to calculate the chemical reaction rate. 2.5. Heterogeneous Reactions. 2.5.1. Char Combustion. Heterogeneous reactions of char with oxygen, carbon dioxide, and steam are rate-determining steps in the bituminous coal or petroleum coke gasification processes, and it is important to adopt more reliable model to describe the heterogeneous reactions.

rO2 =

(2)

enthalpy

Table 6. Char Reactivity Data of Petroleum Coke char−CO2 char−steam

(1)

momentum

k4PCO2 1 + γk5/k 0PCO2 + γk6/k 0PCO

(4)

k 7PH2O 1 + k 8/k 0PH2O + k 9/k 0PH2

(5)

where Pi is the partial pressure of the gasifying agent (unit: atm), and η is the effectiveness factor. The rate constant k4 and k7 are simulated by thermogravimetric analyzer (TGA) experimental data of Shenfu char, and the other inhibition reaction parameters as shown in Table 5 are obtained from ref 24. Where k5 and k8 represent the forward rate constant for k4 and k6, respectively. Thus k6 and k9 represent the reverse rate constant for k4 and k6, respectively, and k0 represents the desorption rates of steam. The random pore model factor f x is implemented into the gasification rate to account for the loss of surface area due to carbon conversion (x): f x = (1 − x)[1 − ψ ln(1−x)]1/2, and the structural parameter ψ is 12 for Shenfu

(3)

where the rate constants ki = Ai exp(−Ei/RTp). As shown in Table 4, the rate coefficient A3 for different carbon containing fuels is calculated by log10(A3) = 12.22 − 0.0535Cdaf,19 and other parameters for char oxidation are obtained from ref 18. The char combustion product is considered as CO for the high temperature above 1800 K.20 2.5.2. Char Gasification. Taking the saturation of adsorbed surface complexes21 and the inhibition by the produced gas into account,22,23 a simplified Langmuir−Hinshelwood (LH) reaction model is used to model Shenfu char gasification at high pressures. 6794

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char. The term γ is equal to 10, which represents the ratio of desorption rates of steam and CO2 gasification. Due to rare reports on the char gasification of petroleum coke under mixed gasifying agent, the nth order model is used to simulate char gasification of petroleum coke at high pressure.

ϕH O = 2

nCO2 = (6)

n H 2O =

(9b)

K5PCO2,S K5PCO2,S 1 2 K5PCO2,S − ln(1 + K5PCO2,S) 1 + K5PCO2,S K8PH2O,S K8PH2O,S 1 2 K8PH2O,S − ln(1 + K8PH2O,S) (1 + K8PH2O,S) (10b)

where the parameter K5 = γk5/k0 and K8 = k8/k0. For the nth order model, the modified Thiele modulus Φ is calculated as ϕ=

dp 6 n−1 (n + 1) Ai exp(− Ei /RTP)·(1 − x) 1 − ψ ln(1 − x) ·ρp RTPPi 2 McDeff

(11)

where the effective diffusivity Deff, based on the polymodal model, can be calculated by the following equation:

2

(8)

Deff = εa 2Da + εe 2De + (1 − εa − εe)2 Di 2 + 2εa(1 − εa − εe) (1/Da + 1/Di ) 2 + 2εe(1 − εa − εe) + 2εaεe (1/De + 1/Di ) 2 (1/Da + 1/De)

For the LH model, the modified Thiele modulus Φ is calculated as dp

νk4ρP RTP

6

2McDeff,CO2 1 + K5PCO2,S

K5PCO2,S

[K5PCO2,S − ln(1 + K5PCO2,S)]−0.5

(9a)

Table 8. Particle Deposition Submodel

(12)

Table 10. Properties of Shenfu Ash

sticky particle sticky wall nonsticky wall

2McDeff,H2O 1 + K8PH2O,S

(7)

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

2

K8PH2O,S

(10a)

where the char reactivity data as shown in Table 6 are obtained from TGA experimental data. Due to pressure-dependence of the char gasification,22 the reaction order n are adopted to the Malekshahian’s experimental data, and the random pore structure parameter Ψ equals 10.25 With increasing temperature, the rate-determining step of char gasification changes from a chemical reaction control to a pore diffusion hindered. The effectiveness factor η is introduced to extrapolate reactivity data to high-temperature in regime II. The modified Thiele modulus Φ and correction function fc, conducted by Hong et al.,26 are used to calculate the effectiveness factor.

ϕCO =

6

νk 7ρP RTP

[K8PH2O,S − ln(1 + K8PH2O,S)]−0.5

dx = ηAi Pi nexp(−Ei /RT )· (1 − x) 1 − ψ ln(1 − x) dt

1⎡ 1 1 ⎤ − η = fc ⎢ ⎥ ϕ ⎣ tanh(3ϕ) 3ϕ ⎦

dp

nonsticky particle

Ee ≥ 0

Ee < 0

Ee ≥ 0

Ee < 0

trap reflect

reflect reflect

trap

trap

Table 9. Operating Conditions of Shenfu Coal and Petroleum Coke Gasification in the OMB Gasifier projects

Shenfu coal

petroleum coke

gasifier pressure, MPa coal-slurry concentration, wt % coal-slurry feed rate of one burner, kg/s oxygen feed rate of one burner, Nm3/h oxygen concentration, mol %

5.9 60.5 5.99 8490 99.85

6.2 62 4.87 8490 99.85

ash component

wt%

Fe2O3 SiO2 Al2O3 CaO MgO Na2O K2O TiO2 SO3 MnO apparent contact angle temperature of critical viscosity, Tcv slag viscosity

14.17 36.78 17.43 25.47 1.29 1.13 1.36 0.57 0.29 0.32 70° 1235 °C Browing model

Figure 1. Boundary conditions and computational grids for simulation. 6795

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3. GOVERNING EQUATIONS AND SOLUTION METHODS The governing equations for gas and particle phase are shown in Table 7. The realizable k−ε turbulence model is used to calculate the gas flow field, and based on the lagrangian methods, the stochastic particle-tracking model is adopted to simulate the particle motion without considering the particle interactions.27 The particle mass transport due to water evaporation, coal pyrolysis, char combustion, and gasification are involved. The convective heat transfer is evaluated by the Ranz−Marshall method correlation, and radiative heat transfer is simulated by P1 model. The gas absorption coefficient εa is a function of local species concentrations, path length, and total pressure, which is evaluated by the weighted-sum-of-gray-gases model (WSGGM). When the particles impact on the wall, the particle capture criterion is determined by deposition submodel as shown in Table 8, which depends on the stickiness and the rebound energy of the particles.28 The particle or wall is sticky when the particle temperature (Tp) or the wall temperature (Tw) is above the ash temperature of critical viscosity Tcv29 and the particle conversion is above a critical carbon conversion about 0.88 for the bituminous coal.30 The rebound energy Ee, which is developed for predicting for the molten slag deposition,31 is formulated as

where Da, De, and Di denotes the effective diffusivity for macro-, meso-, and micropores, respectively, which take the bulk and Knudsen diffusion into account. For bulk diffusion limitation, the reaction rate is dmci dt

=−

νShMcDi A pPi RTmd p

(13)

where v is stoichiometric coefficient and Sh is the Sherwood number. Tm is the arithmetic mean value of particle temperature and gas temperature. Table 11. Validation of Shenfu Coal and Petroleum Coke Gasification Shenfu coal mole fraction, % (dry basis) CO H2 CO2 carbon conversion, % outlet temperature specific solid fuel consumption, kg (dry)/1000 N m3 CO + H2 specific oxygen consumption, N m3/1000 N m3 CO + H2 cold gas efficiency, %

petroleum coke

simulation data

operating data

simulation data

45.66 36.97 18.21 98.0 1236 575

46.77 35.10 17.62 98.0 1220 571

53.08 31.44 13.09 96.7 1324 521

367

364

387

E-GAS32 reported data 48.6 33.2 15.4 99 1310

Ee = 0.25(ξmax )2 (1 − cos α) − 0.12(ξmax )2.3 73.19

69.3

(1 − cos α)0.63 + 2/3(ξmax ) − 1

71−74

(14)

Figure 2. Velocity and temperature distributions of petroleum coke gasification. 6796

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Figure 3. Gas Mole Fraction Distributions of Petroleum Coke Gasification

Figure 4. Axial Distributions of Temperature and Char Concentration Distributions.

where α is contact angle. Without considering slag solidification, the dimensionless maximum spread diameter ξmax is modified as

(15)

The standard scheme is used for pressure discretization, and the first-order upwind scheme is used for other terms (turbulent kinetic energy, dissipation rate, and P1 model). The convergence criteria for continuity and energy is 10−4 and 10−6, respectively, and for the momentum and species, it is 10−3. The current char reaction model and particle deposition submodel are added to Fluent via user defined functions (UDFs).

where the Weber number We = ρνrel2dp/σ, and the relative Reynolds number Re = ρνrelD/μ1, σ, and μl are the surface tension and viscosity of molten slag, respectively. The governing equations for the conservation of mass, momentum, energy, and turbulence are solved using the finitevolume method (FVM), on Fluent 6.2. The velocity correction is realized to satisfy continuity through the SIMPLE algorithm which couples velocity and pressure. To evaluate the convective terms and species, the second-order upwind scheme is used.

4. COMPARISON OF GASIFICATION PERFORMANCE BETWEEN BITUMINOUS COAL AND PETROLEUM COKE IN THE OMB GASIFIER Numerical simulation for the industrial OMB entrained flow gasifier was performed. The operating conditions of Shenfu coal and petroleum coke gasification in the OMB gasifier are shown in the Table 9, and the computational grids and boundary conditions for simulation is shown in Figure 1. Taking the

ξmax =

We + 12 3(1 − cos α) +

4We Re

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The heat flux from fluid cell to wall boundary is computed as

symmetry of gasifier geometry and computational cost into account, the one-quarter of gasifier is adopted as the calculation domain. The number of grids is determined as 406 164 by the grid-dependent test. The liquid slag layer is thick at the wall zones of the gasifier cone, where particles are trapped. And at other wall regions the particle capture criterion is determined by the deposition submodel. The ash composition, contact angle, and slag viscosity of Shenfu coal is in accordance with the data of Ni et al.31 as shown in Table 10. For petroleum coke, the adding flux is supposed to be Shenfu ash, and the same boundary condition with Shenfu coal is adopted.

Q wall = (Tg − Tw )/(∑ δi /λi)

(16)

where δi and λi represent the thickness and thermal conductivity of steel metal, refractory, solid slag layer, and ash deposition layer, respectively.9 The temperature of outside wall Tw is 190 °C for Shenfu coal and 220 °C for petroleum coke. 4.1. Gasification Performance of Bituminous Coal and Petroleum Coke. With the proposed model, the predicted performances of Shenfu coal and petroleum coke

Figure 5. Effects of gas diffusion on the char gasification in the different region of the OMB gasifier. 6798

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gasification in the industrial OMB gasifier are shown in Table 11. The simulation results of Shenfu coal agree well with the industrial operating data, and the predicted carbon conversion of petroleum coke is about 96.7%, which is lower than the reported data of the E-GAS gasifier operated by Global Energy Inc. at the Wabash River plant.32

Owing to higher carbon content of petroleum coke, the specific solid fuel consumption of petroleum coke is lower than Shenfu coal, while the specific oxygen consumption is higher than Shenfu coal due to the high operating temperature of gasifier and low carbon conversion of the petroleum coke. 4.2. Flow Field, Temperature, and Composition Distributions. The predicted velocity, temperature, and gas mole fraction distributions of petroleum coke gasification in the OMB gasifier are shown in Figures 2 and 3. In the jet flow and impinging flow regions, the entrained hydrogen, carbon monoxide, and devolatilization products combust with oxygen to form high temperature regions about 2200 °C. In the impinging-jet flow region, the endothermic reactions, including the char−CO2/steam reaction and the reverse water−shift reaction, reduce the gas temperature dramatically. At the dome of the OMB gasifier, the temperature is 1428 °C as shown in Figure 2, which decreases the refractory life. As shown in Figure 3, the volume fractions of CO and H2 increase along the central line at the axial direction, while CO2 and H2O decrease. The flow pattern of bituminous coal and petroleum coke gasification in the OMB gasifier are similar. Due to the lower char reactivity of petroleum coke, the predicted temperature

Table 12. Catalysis Effects on the Petroleum Coke Gasification Performance

CO, vol % H2, vol % CO2, vol % H2O, vol % T, °C CC% specific oxygen consumption, N m3/1000 N m3 (CO + H2) specific petroleum coke consumption, kg (dry)/1000 N m3 (CO + H2) cold gas efficiency, %

base case

2 times

43.66 25.86 10.77 17.74 1324 96.7 387

44.76 26.05 10.64 16.57 1313 98.6 378

44.99 26.26 10.77 15.99 1310 99.2 375

45.21 26.45 10.84 15.53 1306 99.7 373

521

510

506

503

69.3

70.8

71.4

71.8

5 times 8 times

Figure 6. Catalysis Effects on the Axial Distributions of Gasification Performance 6799

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refractory wall and favors long periods of running for the gasifier. Because the char reactivity with steam is faster than carbon dioxide, the axial mole fraction of H2O decreases and the axial mole fraction of CO and H2 increase as the increase of char reactivity, while the axial mole fraction of CO2 changes slightly.

distribution and char concentration on the axial line are higher than Shenfu coal as shown in the Figure 4. 4.3. Characteristic of Char Gasification in the Entrained Flow Gasifier. The parameters such as particle temperature, species mole fraction, and carbon conversion at different regions in the OMB gasifier, including the jet flow region, impinging flow region, impinging-jet flow region, are obtained, which are used for studying the characteristic of char gasification in the entrained flow gasifier. As shown in Figure 5, the left bar in a graph represents the effectiveness factor of Shenfu coal, the right is the effectiveness factor of petroleum coke, and the dotted line represents the apparent char reactivity Rapp at the pore diffusion limitation (PDL) or boundary diffusion limitation (BDL). As shown in the Figure 5, as the particle diameter increases from 50 to 500 μm, the effectiveness factor and apparent char reactivity decrease for both Shenfu coal and petroleum coke. In different regions, the effectiveness factor of Shenfu coal is about 0.08−0.86, which agrees with the reported data about 0.2−1 for bituminous coal gasification in the entrained flow gasifier.24 The char reactivity of petroleum coke is lower than Shenfu coal, and the effectiveness factor of petroleum coke, about 0.1−1, is a little higher than that of Shenfu coal. In the high temperature region such as the jet flow and impinging flow regions, when the particle diameter is above 200 μm, the Thiele modulus of Shenfu char gasifying with the steam is greater than 15.6 and 4.8, respectively, and the Thiele modulus of the petroleum coke char gasifying with the steam is 5.5 and 5.6, respectively; therefore, the pore diffusion has a significant effect on the apparent char reactivity. For the char gasifying with CO2, the Thiele modulus of Shenfu coal and petroleum coke are below 5 in different regions of the OMB gasifier and the apparent char reactivity is affected by the chemical reaction and pore diffusion. 4.4. Catalysis Effects on the Petroleum Coke Gasification Performance. Salvador et al.33 studied a large number of high sulfur petroleum cokes combustion with air by temperature-ramped thermogravimetric analysis and found that the apparent char reaction rate was elevated by the catalysis effect of the ash and active metal (vanadium). Zhou et al.34 also found a significant synergistic effect between calcium hydroxide and iron species on the petroleum coke gasifying with CO2, and the char reactivity was accelerated 6−10 times with different catalysis loading. For the petroleum coke gasification, the adding flux is also rich in metallic compounds of iron and calcium. The char reactivity with CO2 and steam are both increased by 2−8 times, and catalysis effects on the petroleum coke gasification performance are investigated. The operating conditions of petroleum coke gasification as shown in Table 9 is the base case, and catalysis effects on the petroleum coke gasification performance are shown in Table 12. As the char reactivity accelerates to 5 times, the carbon conversion increases to 99.2% and the temperature at the outlet of the gasifier decreases to 1310 °C. The specific oxygen and petroleum coke consumption decrease to 375 N m3/1000 N m3 (CO + H2) and 506 kg (dry)/1000 N m3 (CO + H2), respectively. The cold gas efficiency increases to 71.4%, which is in accord with the reported data of E-GAS gaisfier.32 Catalysis effects on the axial distributions of petroleum coke gasification performance are shown in Figure 6. As the increase of char reactivity, the axial temperature, and char concentration decrease obviously. More important, the gas temperature at the top of the gasifier decreases from 1421 to 1345 °C as the char reactivity increases from 1 to 5 times, which prolongs the

5. CONCLUSIONS A comprehensive three-dimensional numerical model for simulating Shenfu bituminous coal and petroleum coke in the industrial OMB gasifier is established. The solid fuel gasification process is divided into several submodels, including water evaporation, pyrolysis, and homogeneous and heterogeneous char reactions. The “effectiveness factor” method is used to extrapolate the intrinsic char reactivity data to industrial gasifier conditions. With the proposed models, the predicted gasification performance of Shenfu coal is in good agreement with industrial operating data, and the predicted carbon conversion of petroleum coke is 96.7%. Comparing with Shenfu coal, the specific solid fuel consumption of petroleum coke is lower, while the specific oxygen consumption is higher. Besides the distributions of flow field, temperature and gas composition in the gasifier are investigated. The characteristic of char gasification in the entrained flow gasifier is investigated. In the high temperature jet flow region and impinging flow region, when the particle diameter is above 200 μm, the Thiele moduli of the char gasifying with steam for Shenfu coal and petroleum coke are 15.4 and 4.8, respectively. Therefore, the pore diffusion has a significant effect on the apparent char reactivity. While for the char gasifying with CO2, the Thiele moduli are below 5 in different regions of the OMB gasifier, and the apparent char reactivity is affected by the chemical reaction and pore diffusion. When the char reactivity of petroleum coke is accelerated at 5 times by catalysis effects, the carbon conversion reaches 99.2%, and the consumption of oxygen and feedstock decrease obviously. Importantly, the temperature at the dome of the gasifier decreases to 1345 °C, which prolongs the life of refractory wall and favors for a long period of availability of the gasifier.

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AUTHOR INFORMATION

Corresponding Author

*To whom correspondence should be addressed. Tel.: +86-216425 2974. Fax: +86-21-6425 1312. E-mail address: gsyu@ ecust.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Key State Basic Research Development Program of China (973 Program, Grant 2010CB227006), the National Nature Science Foundation of China (Grant 21176078).

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NOMENCLATURE A = frequency factor Ap = surface area of the particle, m2 CD = drag coefficient Cr = constant of eddy break-up model dp = particle diameter, m Di,m = diffusion coefficient of species i in the gas mixture, m2/s Deff = effective diffusivity, m2/s dx.doi.org/10.1021/ef301189c | Energy Fuels 2012, 26, 6792−6802

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Da, De, Di = effective diffusivity for macro-, meso- and micropores, respectively, m2/s E = activation energy, kJ/mol Ee = rebound energy H = enthalpy, J/kg hw, hv = latent heat of evaporation and heat of pyrolysis, respectively, J/kg k = turbulent kinetic energy, m2/s2 kc = mass-transfer coefficient, m/s Mw, Mc = molecule of water and carbon, g/mol mw, mv, mc = particle mass transport due to slurry water evaporation, coal pyrolysis, char gasification, respectively, kg mp = particle mass, kg n = reaction order Nu = Nusselt number P = pressure, Pa R = molar gas constant, 8314 J/(kmol K) Re = relative Reynolds number Sh = Sherwood number Sm, Sui, Sh, SYi = the source terms of particle mass, momentum, enthalpy, and species, respectively Sct = turbulent Schmidt number T, Tp = gas and particle temperature, K t = time, s u, up = gas and particle velocity, m s−1 We = Weber number xi, xj = coordinate of directions, m Yi = mass fraction of specie i Greek Letters

α = contact angle of liquid slag, deg ρ, ρp = gas and particle density, kg/m3 μ = dynamic viscosity, kg/(m s) μt = turbulent viscosity, kg/(m s) ε = turbulence dissipation rate, 1/(m2 s3) εa, εp = radiative absorption coefficients of gas and particle, 1/m εs = particle radiative scatter coefficient, 1/m εa, εe, and εi = macro-, meso-, and microporosity, respectively σB = Stefan−Bolzmann constant, 5.67 × 10−8 W/(m2 K4) λ = thermal conductivity, W/(m K) η = effectiveness factor Φ = Thiele number Ψ = random pore model constant ξmax = dimensionless maximum spread diameter

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