NUMERICAL SIMULATION OF COAL GASIFICATION IN A CIRCULATING FLUIDIZED BED GASIFIER

Sharma, Vikrant; Agarwal, Vijay K.

doi:10.1590/0104-6632.20190363s20180423

Vikrant Sharma

Indian Institute of Technology Roorkee, Department of Chemical Engineering, Roorkee, Uttarakhand, India. E-mail: vsharma@ch.iitr.ac.in

http://orcid.org/0000-0002-8628-7529

Vijay K. Agarwal

Indian Institute of Technology Roorkee, Department of Chemical Engineering, Roorkee, Uttarakhand, India. E-mail: vsharma@ch.iitr.ac.in

http://orcid.org/0000-0003-4328-4528

* Corresponding author: Vikrant Sharma - E-mail: vsharma@ch.iitr.ac.in

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Figure 1
Geometry of the CFB loop.

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Figure 2
Grid independence study.

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Figure 3
Mesh of the CFB loop.

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Figure 4
Transient variation of (a) outlet gas temperature and (b) outlet mass flow rate.

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Figure 5
Axial distribution of mean static pressure along the riser axis.

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Figure 6
Axial distribution of mean solid volume fraction along the riser axis.

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Figure 7
Radial distribution of mean solid volume fraction at various riser heights.

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Figure 8
Radial distribution of mean solid axial velocity at various riser heights.

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Figure 9
Radial distribution of mean gas axial velocity at various riser heights.

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Figure 10
Axial profile of mean gas and solid temperature.

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Figure 11
Contours of instantaneous species mole fraction at 10 sec in BFB gasifier.

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Figure 12
Contours of instantaneous species mole fraction at 10 sec in the CFB gasifier.

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Table 1
Heterogeneous and homogeneous reaction kinetics.

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Table 2
Basic characteristics of Xuzhou bituminous coal.

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Table 3
Simulation setup.

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Table 4
Comparison between experiment and simulation.

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Table 5
Composition and temperature of gas and solid at the outlet.

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S.No.	Reaction	A	E	n
Heterogeneous Reactions
1	Volatiles → 0.07CO + 0.01CO₂ + 0.03CH₄ + 0.03H₂ + 0.01H₂O + 0.013NH₃ + 0.0031H₂S + 0.056Tar	A₁ = 2x10⁵ s^-1 A₂ = 1.3x10⁷ s^-1	E₁ = 104.6x10³ J/mol E₂ = 167.4x10³ J/mol	0
2	Tar + O₂ → 0.05CO + 0.01CO₂ + 0.003H₂O	2x10⁵ s^-1	104.6x10³ J/mol	0
3	Moisture → H₂O	1.1x10⁵ s^-1	88700.8 J/mol
4	C + H₂O → CO + H₂	5.95x10^-5 Pa^-1s^-1	113486.1 J/mol	0
5	C + O₂ → CO₂	1.5x10⁶ s^-1	108730.5 J/mol	0
6	C + ½ O₂ → CO	0.046 atm^-1s^-1	110.5x10³ J/mol	0
7	C + CO₂ → 2CO	3.92 Pa^-1s^-1	223871.08 J/mol	0
8	C + 2H₂ → CH₄	8.36x10^-4 atm^-1s^-1	67160.5 J/mol	0
Homogeneous Reactions
9	CH₄ + H₂O → CO + 2H₂	3.1x10⁵ m³/mol.s	124710 J/mol	0
10	CO + 3H₂ → CH₄ + H₂O	1.1698x10¹⁰ (m³/mol)²s^-1	3.9824x10⁵ J/mol	0
11	CH₄ + 2O₂ → CO₂ + 2H₂O	3.552x10¹¹ K.m³/mol.s	1.3053x10⁵ J/mol	-1
12	CH₄ + 3/2 O₂ → CO + 2H₂O	5.102x10¹¹ m³/mol.s	2x105 J/mol	0
13	CO + H₂O → CO₂ + H₂	2.78 m³/mol.s	1.2567x10⁴ J/mol	0
14	CO₂ + H₂ → CO + H₂O	104905.7 m³/mol.s	4.5448x10⁴ J/mol	0
15	CO + ½ O₂ → CO₂	2.2x10⁹ (m³/mol)^0.25s^-1	1.67x10⁵ J/mol	0
16	H₂ + ½ O₂ → H₂O	5.159x10¹⁰ K^1.5(m³/mol)^1.5s^-1	2.8517x10⁴ J/mol	-1.5

Proximate Analysis (wt%)				Ultimate Analysis (Dry ash free, wt%)
Char	Volatiles	Moisture	Ash	C	H	O	N	S
45.06	29.24	6.62	19.08	80.76	5.01	12.13	1.36	0.74

Solver	Pressure-based, Transient
Models	Laminar, Multiphase (Eulerian, 2 Phases), Energy, Species Transport (Volumetric reactions, Full multicomponent diffusion), Discrete Phase (Interaction with continuous phase, No tracking)
Materials and Phases	Fluid (Primary phase, Mixture of H₂O, O₂, N₂, CO, CO₂, CH₄, H₂, NH₃, H₂S and TAR (C₆H₆), Finite rate reactions - Homogeneous, Ideal gas), Coal (Secondary phase, Mixture of Char (C), Volatiles (coal-hv-volatiles), Moisture (H₂O(l)) and Ash (SiC), No reactions, Granular, Granular Temperature Model = Phase property, Diameter = 1x10^-3 m, Density = 1400 kg/m³ (UDF), Granular Viscosity = Gidaspow, Granular Bulk Viscosity = Lun et al., Frictional Viscosity = Schaeffer, Heat capacity = 1260 J/kg.K, Thermal Conductivity = 0.0454 W/m.K, Mass Diffusivity = 10-10 m²/s for all components)
Phase Interaction	Drag = EMMS (Zhang et al., 2008): Wen-Yu (Drag Correction (UDF)), Heat = Gunn, Reactions - Heterogeneous (UDF)
Boundary Conditions	Inlet = Velocity Inlet [Base Case - (Primary phase = 1.16 m/s (Base Case), Composition = (79% N₂, 21% O₂), Temperature = 1073 K, Secondary phase = 0.0 m/s)] Outlet = Pressure Outlet (Gauge Pressure = 0.0) Coal Inlet = Mass Flow Inlet [Base Case - (Primary phase = 0.0 kg/s, Secondary phase = 0.34 kg/s, Volume fraction = 0.552, Composition = (45.06% Char, 29.24% Volatiles, 6.62% Moisture, 19.08% Ash))] Wall = Primary phase = No Slip, Secondary phase = Partial Slip (Johnson and Jackson, 1987) (Specularity Coefficient = 0.6)
Pressure Velocity Coupling	Phase-Coupled SIMPLE
Spatial Discretization	First order upwind
Temporal Discretization	First order implicit
Gradient	Least square cell based
Time Step	0.0001 s

	A/C = 2.65; S/C = 0.0			A/C = 2.65; S/C = 0.15
	Experiment	Simulation	Error (%)	Experiment	Simulation	Error (%)
CO (mol%)	14.12	14.19	0.47	15.66	14.44	7.77
CO₂ (mol%)	11.04	11.92	7.98	11.19	11.98	7.05
CH₄ (mol%)	5.78	5.94	2.63	5.20	4.90	5.63
H₂ (mol%)	12.47	12.76	2.37	13.33	12.71	4.68
Bed Temperature (ºC)	960.32	890.24	7.30

	BFB	CFB
H₂O (mol%)	8.76	1.62
O₂ (mol%)	0.0	0.06
N₂ (mol%)	31.44	40.48
CO (mol%)	14.44	35.48
CO₂ (mol%)	11.98	0.46
CH₄ (mol%)	4.90	0.42
H₂ (mol%)	12.71	15.75
NH₃ (mol%)	2.81	1.05
H₂S (mol%)	0.68	0.25
Tar (mol%)	12.26	4.41
Gas Temperature (K)	951.63	1405.31
Char (wt%)		0.03
Volatiles (wt%)		~ 0.0
Moisture (wt%)		~ 0.0
Ash (wt%)		0.97
Solid Temperature (K)		1390.32

\{\begin{matrix} A = 0.05204 + \frac{1.9743}{[1 + {(\frac{α_{g}}{0.53027})}^{50.25633}]} \\ B = 0.14294 - \frac{0.15047}{[1 + e^{\frac{- α_{g} - 0.50084}{0.0047}}]} \{1 - \frac{1}{[1 + e^{\frac{- α_{g} - 0.60902}{0.02736}}]}\} \\ C = 0.56038 - \frac{0.58092}{[1 + {(\frac{α_{g}}{0.54322})}^{34.39893}]} \end{matrix}

\{\begin{matrix} A = {(429.90532 - 434.69389 α_{g})}^{- 0.5868} \\ B = \frac{α_{g} - 1.02901}{- 1.58454 + 7.56707 (α_{g} - 1.02901) - 19.64414 {(α_{g} - 1.02901)}^{2}} \\ C = {(0.05377 - 0.05394 α_{g})}^{0.163018} \end{matrix}

\{\begin{matrix} A = 0.36777 + \frac{5.13409}{[1 + e^{\frac{- α_{g} - 0.995945}{0.00447}}]} \{1 - \frac{1}{[1 + e^{\frac{- α_{g} - 0.999915}{0.00044}}]}\} \\ B = 0.0175 + 0.71397 e^{[- 0.5 {(\frac{α_{g} - 0.9986}{0.00186})}^{2}]} \\ C = 0.32769 - 0.25491 e^{[- 0.5 {(\frac{α_{g} - 0.99863}{0.00969})}^{2}]} \end{matrix}

μ_{s} = \frac{4}{5} α_{s} ρ_{s} d_{s} g_{0} (1 + e_{s s}) \sqrt{\frac{θ_{s}}{π}} + \frac{10 ρ_{s} d_{s} \sqrt{θ_{s} π}}{96 α_{s} (1 + e_{s s}) g_{0}} {[1 + \frac{4}{5} g_{0} α_{s} (1 + e_{s s})]}^{2} + \frac{p_{s} \sin φ}{2 \sqrt{I_{2 D}}}

\frac{3}{2} [\frac{\partial (ρ_{s} α_{s} θ_{s})}{\partial t} + \nabla . (ρ_{s} α_{s} θ_{s} u_{s})] = (- p_{s} I + τ_{s}) : \nabla u_{s} + \nabla . (k_{θ s} \nabla θ_{s}) - γ_{θ s} - 3 K_{g s} θ_{s}

k_{θ s} = \frac{150 ρ_{s} d_{s} \sqrt{θ_{s} π}}{384 (1 + e_{s s}) g_{0}} {[1 + \frac{6}{5} α_{s} g_{0} (1 + e_{s s})]}^{2} + 2 ρ_{s} α_{s}^{2} d_{s} (1 + e_{s s}) g_{0} \sqrt{\frac{θ_{s}}{π}}

[1] * Corresponding author: Vikrant Sharma - E-mail: vsharma@ch.iitr.ac.in

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NUMERICAL SIMULATION OF COAL GASIFICATION IN A CIRCULATING FLUIDIZED BED GASIFIER

Abstract