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 GasificationMay 13-16, 2003, Park City Marriott, Park City, Utah

DOE Vision 21 Program Cost Shared Agreement DE-FC26-00NT41047

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

Carbon Conversion & Syngas Quality

See Gasification Technologies 2001:• Stiegel, Clayton and Wimer• HoltSee Clearwater 2002• Dogan

• 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

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

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

CFD Model

• Geometry• Wall Properties• Fuel & Oxidant

– Properties– Composition

• Ultimate, proximate• Ash composition

– Temperature– Particle grind– Splits

• Multiple Injectors• Quench

• 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

Inputs Outputs = Predicted Values

Gasifier CFD Model• Computer model represents

– Gasifier geometry – Operating conditions– Gasification processes

• Accuracy depends on– Input accuracy– Numerics– Representation of

physics & chemistry

Turbulence

ParticleReactions

Surface Properties

Gasification & CombustionChemistry

Radiation &Convection

Fuel Conversion

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

Hot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2

Hot GasHot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2 Tw

Ts

TiTw1

Tw2

CFD model provides• Incident heat flux

– Radiation– Convection

• Particle deposition on wall– Rate– Composition (ash,

carbon)– Burning on wall

Hot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2

Hot GasHot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2 Tw

Ts

TiTw1

Tw2

Slag model computes• Ash viscosity (ash

chemistry)

• Tcv = Temperature of critical viscosity (ash chemistry)

• Liquid slag velocity profile

• Slag surface temperature

• Liquid & frozen slag layer thickness

• Heat transfer through wall

Hot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2

Hot GasHot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2 Tw

Ts

TiTw1

Tw2TBeTA / = µ

θθ

θ∫

ρ

θd

eA ) - T(

)T,x(q

k g = T) u(x, /BnsT

Tss

2

w

Slag model computes• Info returned to

CFD model– slag surface

temperature

Hot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2

Hot GasHot GasS

Liquid Slag

xy

u (x, y)

RefractoryLining

Metal Wall

Coolant

Qradiation

Hot ParticlesSolid Slag Layer

Qconvection

TAmbient

Tw

Ts

TiTw1

Tw2 Tw

Ts

TiTw1

Tw2

Temperature at Critical Viscosity (Tcv)Correlated Fit with Measured Data

Where α = SiO2/Al2O3 and β = Fe2O3 + CaO + MgOSiO2 + Al2O3 + Fe2O3 + CaO + MgO = 100 [weight%]

22cv 860. + 8.67 - 5.74 + 5.519 - 4523 = [K] T ββαα Measured data from

Patterson et al, 2001

1000

1200

1400

1600

1800

2000

1300 1400 1500 1600 1700 1800Measured TCV (K)

Pred

icte

d TC

V (K

)

+15%

-15%

Example Results

1-stage up flow 2-stage up flow

Single Stage – Up Flow• Firing Conditions

– [Benyon, 2002], [Seggiani, 1998]– Pressure = 25 atm.– 2600 tpd dried bituminous coal

• 22% ash– Dry feed

• N2:Coal (lb) = 0.075– Oxidant

• 76% O2 , 11% H20, 10% N2, 3% Ar• O2:C (molar) = 0.46• Inlet Stoichiometry ~ 0.4

L

0.2 D

0.75 D

D

0.1 D

0.4 D

L/D = 1.75D = 3.7m Injector

Orientation

5.5--Deposition (%)

4642(4248)(4431)HHV (Btu/lb)-

-

--

1.83.2

76.51803

Seggiani

(91.5)

-

--

1.810.070.9

(1650)Benyon

~100Carbon Conversion(%)

83.1Cold-Gas Efficiency (%)

REIExit Conditions

3.0H2O (wt %)10.1N2 (wt %)

1.9H2 (wt %)6.2CO2 (wt %)

76.6CO (wt %)1740Gas Temp (K)

water jacketcooled

(…) = estimated value

Slag Model Summary

0

1

2

3

4

5

6

7

8

1000 1300 1600 1900 2200

Slag surface temperature, K

Gas

ifier h

eigh

t, m

0

2

3

4

5

6

7

8

0 100000 200000 300000 400000

Heat f lux, W/m2

0 0.005 0.01 0.015 0.02

Liquid slag thickness, m

0 0.02 0.04 0.06 0.08 0.1

Solid slag thickness, m

Slag SurfaceTemperature

Seggiani

Benyon

REI

Heat Flux to Slag

Liquid Slag Thickness

Solid Slag Thickness

Critical Viscosity ~ 1625 K

Slag Model – 2D Wall Plots

Solid Slag Thickness, m Slag Surface Temp., K Wall Heat Flux, kW/m2

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

0

100

Gas and Particle Flow FieldVolatiles Mass

FractionChar MassFraction

137 micron

Gas Temperature, K

Axial Velocitym/s

Two Stage – Up Flow• Vision 21 Firing Conditions

– Pressure = 18 atm.– 3000 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

D

11 D

1.58 D

0.33 D

0.25 D

D

Two Stage, UpflowTangentially Fired Gasifier

D

1.58 D

0.33 D

0.25 D

D

Two Stage, UpflowTangentially Fired Gasifier

Upper injectors

0.5 D

Jet centerline

0.5 D

Jet centerline

0.5 D

Jet centerline

Lower Injectors

L / D = 11 D = 1.65m

• System– 4 fuel injectors / level– Fuel Injectors ~ pipes

Gas and Particle Flow Field

Gas Composition at Elevations

H2 CO H2O CO2 O2

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

Slag Summary

ComparisonAverage Wall Temp., K 1705

Exit Temperature, K 1390Carbon Conversion, % 96.1

Exit LOI, % 19.7Deposit LOI, % 47.9Deposition, % 3.3

PFR Residence Time, s 1.797Particle Residence Time, s 0.926

Mole Fraction: CO 42.5%H2 32.3%

H2O 13.8%CO2 8.7%H2S 0.8%

COS 0.0%N2 1.6%

Exit Mass Flow, klb/hr 493HHV of Syngas, Btu/lb 4899

HHV of Syngas, Btu/SCF 247Cold-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%

• 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

Summary