CFD-Based Models of Entrained-Flow Coal Gasifiers with Emphasis on Slag Deposition and Flow

Transcription

1 CFD-Based Models of Entrained-Flow Coal Gasifiers with Emphasis on Slag Deposition and Flow Mike Bockelie, Martin Denison, Zumao Chen, Temi Linjewile, Connie Senior and Adel Sarofim Reaction Engineering International Collaboration: N. Holt (EPRI), K.Hein (IVD, Germany) T.Wall, Peter Benyon, David Harris, John Kent (Black Coal CCSD, Australia) Colloquium on Black Liquor Combustion and Gasification May 13-16, 2003, Park City Marriott, Park City, Utah DOE Vision 21 Program Cost Shared Agreement DE-FC26-00NT41047

2 Outline Motivation Gasifier Model Slag Submodel One stage results Two stage results Summary

3 Carbon Conversion & Syngas Quality Fuel Switching Coal, Petcoke, Blends Fuel Feed System Dry (N2, CO2); Wet (H2O) Pre-heat Oxidant: Air vs O2 Injector Modifications L/D ratio (volume) System pressure See Gasification Technologies 2001: Stiegel, Clayton and Wimer Holt See Clearwater 2002 Dogan

4 Other Considerations Slag and Ash Management Viscosity, composition, flux mat l Carbon content: slag vs flyash Refractory Wear Heat extraction Transient Operation start-up Shutdown Switching Upsets

5 Gasifier Process Model fast running model to asses operating conditions mass & energy balance particle burnout + equilibrium chemistry heat transfer critical viscosity Includes impacts of: fuel type, recycled char ash composition oxidant conditions wet vs dry feed particle size residence time Predicts carbon conversion, unburned carbon in slag & flyash syngas temperature, composition, carryover slag flow indicator

6 CFD Model Inputs Geometry Wall Properties Fuel & Oxidant Properties Composition Ultimate, proximate Ash composition Temperature Particle grind Splits Multiple Injectors Quench Outputs = Predicted Values Carbon Conversion Flow Patterns & Velocities Gas & Surface Temperatures Gas Species Concentrations Major : CO2, CH4, H2, H2O, N2, O2 Minor: H2S, COS, NH3, HCN,. Reducing vs oxidizing Wall Heat Transfer Incident, net flux Backside cooling Particle / Droplet Trajectories and Reactions Time temperature histories Wall deposition Flyash (Unburned carbon) Slag Temperature, viscosity, thickness composition

7 Gasifier CFD Model Computer model represents Gasifier geometry Operating conditions Gasification processes Accuracy depends on Input accuracy Numerics Representation of physics & chemistry Gasification & Combustion Chemistry Turbulence Fuel Conversion Radiation & Convection Particle Reactions Surface Properties

8 Slag Flow Model Model accounts for: Wall refractory properties Back side cooling Fire side flow field Fuel ash properties Based on work by Benyon Seggiani Senior Solid Slag Layer Refractory Lining Metal Wall T Ambient T w2 T w1 T w T s T i Liquid Slag Hot Particles y x S u (x, y) Q radiation Q convection Hot Gas Coolant

9 CFD model provides Incident heat flux Radiation Convection Particle deposition on wall Rate Composition (ash, carbon) Burning on wall Solid Slag Layer Refractory Lining Metal Wall T Ambient T w2 T w1 T w T s T i Liquid Slag y x S Q radiation Hot Particles Hot Gas u (x, y) Q convection Coolant

10 Slag model computes Ash viscosity (ash chemistry) µ = A T B e / T Tcv = Temperature of critical viscosity (ash chemistry) Liquid slag velocity profile u(x, T) = ρg q s A θ - θ) Slag surface temperature Liquid & frozen slag layer thickness Heat transfer through wall 2 k (x,ts ) T T w (T s n e B / θ d θ Solid Slag Layer Refractory Lining Metal Wall T Ambient T w2 T w1 T w Coolant T s T i Liquid Slag Hot Particles y x S u (x, y) Q radiation Q convection Hot Gas

11 Slag model computes Info returned to CFD model slag surface temperature T w2 Solid Slag Layer Refractory Lining Metal Wall T w1 T w T i x T s Liquid Slag y Hot Particles T Ambient S Q radiation Hot Gas u (x, y) Q convection Coolant

12 Temperature at Critical Viscosity (T cv ) 2000 Correlated Fit with Measured Data % Predicted TCV (K) % Measured TCV (K) T 2 cv [K] = α α β β Where α = SiO 2 /Al 2 O 3 and β = Fe 2 O 3 +CaO+MgO SiO 2 + Al 2 O 3 + Fe 2 O 3 + CaO + MgO = 100 [weight%] Measured data from Patterson et al, 2001

13 Example Results 1-stage up flow 2-stage up flow

14 Single Stage Up Flow Firing Conditions [Benyon, 2002], [Seggiani, 1998] Pressure = 25 atm tpd dried bituminous coal 22% ash Dry feed N2:Coal (lb) = Oxidant 76% O2, 11% H20, 10% N2, 3% Ar O2:C (molar) = 0.46 Inlet Stoichiometry ~ 0.4 Exit Conditions Gas Temp (K) CO (wt %) CO2 (wt %) H2 (wt %) H2O (wt %) N2 (wt %) Deposition (%) Carbon Conversion(%) HHV (Btu/lb) Cold-Gas Efficiency (%) Seggiani (4431) - Benyon (1650) (4248) (91.5) REI ~ water jacket cooled 0.75 D D ( ) = estimated value L/D = 1.75 D = 3.7m 0.2 D L 0.4 D 0.1 D Injector Orientation

15 Slag Model Summary Slag Surface Temperature Heat Flux to Slag Liquid Slag Thickness Solid Slag Thickness Gasifier height, m Slag surface temperature, K Heat flux, W/m Liquid slag thickness, m Solid slag thickness, m Critical Viscosity ~ 1625 K Seggiani Benyon REI

16 Slag Model 2D Wall Plots Solid Slag Thickness, m Slag Surface Temp., K Wall Heat Flux, kw/m 2

17 Slag Thickness Surface Display curve = camera path ball on curve = camera location gas temperature at ball shown in bar display displayed = frozen slag thickness surface elevation = slag thickness surface color mm 100 0

18 Gas and Particle Flow Field Volatiles Mass Fraction 137 micron Char Mass Fraction Gas Temperature, K Axial Velocity m/s

19 Two Stage Up Flow Vision 21 Firing Conditions Pressure = 18 atm tpd Illinois #6 H2O 11%, Ash 10% Slurry: 74% solids (wt.) Slurry Distribution 39%, 39%, 22% (upper) Oxidant 95% O2, 5% N2 O2:C (molar) = 0.40 Inlet Stoichiometry ~ 0.47 System 4 fuel injectors / level Fuel Injectors ~ pipes D Upper injectors Two Stage, Upflow Tangentially Fired Gasifier 11 D Jet centerline 1.58 D Lower Injectors 0.5 D 0.25 D 0.33 D D L / D = 11 D = 1.65m

20 Gas and Particle Flow Field

21 Gas Composition at Elevations H 2 CO H 2 O CO 2 O 2

22 Slag Summary Molten Slag Thickness, m Solid Slag Thickness, m Surface Temperature, K

23 Comparison Average Wall Temp., K 1705 Exit Temperature, K 1390 Carbon Conversion, % 96.1 Exit LOI, % 19.7 Deposit LOI, % 47.9 Deposition, % 3.3 PFR Residence Time, s Particle Residence Time, s Mole Fraction: CO 42.5% H % H 2 O 13.8% CO 2 8.7% H 2 S 0.8% COS 0.0% N 2 1.6% Exit Mass Flow, klb/hr 493 HHV of Syngas, Btu/lb 4899 HHV of Syngas, Btu/SCF 247 Cold-Gas Efficiency, % 80.9 V21 Design Conditions DOE flow sheet analysis of DESTEC-style 2 stage gasifier in an IGCC plant Syngas: HHV ~ 250 Btu/SCF Temp. ~ 1300 K (1900F) Fuel Flow ~ 499 klb / hr Composition (after GCU): CO = 43.5% H2 = 32.5% H2O = 13.6% CO2 = 8.6% N2 = 0.9%

24 Summary Demonstrated CFD based gasifier models One stage and two stage designs Mechanistic based Can be used to address broad range of operational and design problems for gasifiers Comparisons to available information getting better Next? improve slag (mineral matter) model, gasification kinetics Petcoke Scavenger for model verification data pre-processor to allow defining flows to gasifier based on desired syngas at gasifier exit fuel, oxidant, recycled char, fluxing material Impact on power production and downstream equipment