development of coal gasifier operation supporting...

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Development of coal gasifieroperation supporting technique

Hiroaki WATANABEHiroaki WATANABE

Energy Engineering Research LaboratoryEnergy Engineering Research LaboratoryCentral Research Institute of Electric Power IndustryCentral Research Institute of Electric Power Industry

-- Evaluation of gasification performance and slag Evaluation of gasification performance and slag discharge characteristics using CFD technique discharge characteristics using CFD technique --

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250 MW class

around 1700 t/d

Power Output

Feed Rate

System, Spec.

Coal Gasifier

Gas Purification

Gas Turbine

Target EfficiencyLHV (HHV)

gross efficiency

net efficiency

EnvironmentalTarget

SOx

NOx

Dust

Air Blown, Dry Feed

Wet, Sulfur Recovery

1200 degC class

48% (46%)

42% (40.5%)

8 ppm (16%O2)

5 ppm (16%O2)

4 mg/Nm3 (16%O2)

Ref. Clean Coal Power R&D Co., Ltd.(http://www.ccpower.co.jp/index.html)

Specification ofSpecification ofIGCC demonstration plant in JapanIGCC demonstration plant in Japan

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BackgroundBackground

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Ø To clarify influence of oxygen concentration of

gasifying agents and air ratio on gasification

performance

Ø To discuss relationship between operation range

and variations of gasification performance

²Representative slag viscosity

²Three dimensional slag flow calculation

ObjectiveObjective

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ØØ Three dimensional timeThree dimensional time--mean conservation equationsmean conservation equations

ØØ Finite volume hybrid upwind differencingFinite volume hybrid upwind differencing

ØØ EulerianEulerian--LagrangianLagrangian manner for gasmanner for gas--particle two phase flow particle two phase flow

ØØ SIMPLEC algorithm for handling of pressure and velocity equatioSIMPLEC algorithm for handling of pressure and velocity equationsns

ØØ kk--εε turbulence modelturbulence model

ØØ Discrete transfer radiation methodDiscrete transfer radiation method

[Launder, B.E. et al. (1974)][Launder, B.E. et al. (1974)]

[Lockwood, F.C. et al. (1981)][Lockwood, F.C. et al. (1981)]

[Van [Van DoormalDoormal, J.P. et al. (1984)], J.P. et al. (1984)]

GasGas--particle two phase flow calculationparticle two phase flow calculation

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( )i pi

U Sx

ρ∂

=∂

( ) iji j j pu

i i

U U B Sx x

σρ

∂∂ = − + +∂ ∂

( )i phi i i

TU h Sx x x

ρ λ ∂ ∂ ∂= + ∂ ∂ ∂

( ) ( )i ii i i pY fY

i i

U Y DY S Rx x

ρ ρ∂ ∂

= + +∂ ∂

Equation of continuityEquation of continuity

Gas phase conservationGas phase conservation

Momentum equationMomentum equation

Energy equationEnergy equation

Chemical species equationChemical species equation

Solid phaseSolid phaseEquation of motionEquation of motion

pp D B

dUm F F

dt= +

Mass transferMass transfer

Heat transferHeat transfer

( ) ( )1 1 ln 1pdx k x xdt

= ⋅ − ⋅ − Ψ ⋅ −

( ) pi pi C M G R

i

dTm C Q Q Q Q

dt= + + +∑

Governing equations for gasGoverning equations for gas--particle two phase flowparticle two phase flow

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Coal Gasification Reactionincludes three chemical reaction processes

Ø Pyrolysis (heterogeneous reaction)

Ø Char Gasification (heterogeneous reaction)

Ø Gas phase reactions (homogeneous reaction)

Gasification reaction modelingGasification reaction modeling

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Reaction Reaction

path :path :Coal Coal ?? Volatile + CharVolatile + Char

((Equilibrium calculation based on proximate and ultimate analysisEquilibrium calculation based on proximate and ultimate analysis ))

( )i iref

dV VH T Tdt τ

= − −

DevolatilizationDevolatilization rate :rate :

1. 1. Simple primary reaction modelSimple primary reaction model

2. 2. Two step model [Two step model [UbhayakarUbhayakar et alet al.,., (1976)](1976)]

( )*idV k V Vdt

= − −

( ) ( )1 2 1 1 2 2exp expk k k A E RT A E RT= + = +

PyrolysisPyrolysis

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Reaction path :Reaction path :C + 1/2 OC + 1/2 O22 => CO=> CO ,,-- 9.25 MJ/kg9.25 MJ/kgC + COC + CO22 => 2CO=> 2CO ,+ 14.37 MJ/kg,+ 14.37 MJ/kgC + HC + H22O => CO + HO => CO + H22 ,+ 10.94 MJ/kg,+ 10.94 MJ/kg

Reaction rate : Reaction rate :

2.89x108

2.52x108

0.64

3

< 1533

6.78x104

1.63x108

0.73

3

> 1473

-

J/kmol-

-

K

1.40x1082.71x1081.30x108E0.840.540.68n

8.55x1043.34x1081.36x106A

3314Ψ

> 1533< 1473-Temperature range

H2OCO2O2Gasifying agent

Kinetic parameters for coal char gasificationKinetic parameters for coal char gasification

Char gasificationChar gasification

[[Kajitani, S.K. et al. (2002)]Kajitani, S.K. et al. (2002)]

( ) ( )0 exp 1 1 ln 1nox i

dx EA P x x

dt RT = − − − Ψ −

((xx: reaction rate): reaction rate) [[Bhatia, S.K. et al. (1980)]Bhatia, S.K. et al. (1980)]

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Reaction path :Reaction path :

CHCH44 + H+ H22O O ?? CO + 3 HCO + 3 H22 , , + 206 [MJ/+ 206 [MJ/kmolkmol]]

CHCH44 + 1/2 O+ 1/2 O22 ?? CO + 2 HCO + 2 H22 , , -- 35.7 [MJ/35.7 [MJ/kmolkmol]]

HH22 + 1/2 O+ 1/2 O22 ?? HH22OO , , -- 242 [MJ/242 [MJ/kmolkmol]]

CO + 1/2 OCO + 1/2 O22 ?? COCO22 ,, -- 283 [MJ/283 [MJ/kmolkmol]]

CO + HCO + H22O O ?? COCO22 + H+ H22 ,, -- 41.1 [MJ/41.1 [MJ/kmolkmol]]??Reaction rate : Reaction rate :

,min( / )tu fu oxkR C m mµ ρ φε

=

Backward reaction rate constant : Backward reaction rate constant : /b f eqk k K=

,min( )fu ch t uR R R=

[[MagnussenMagnussen, B.E. et al. (1976)], B.E. et al. (1976)]

[ ] [ ]i ix y

ch i i iR k A B= Jones, S.K. et al. (1988)Jones, S.K. et al. (1988)Westbrook, C.K. et al. (1981)Westbrook, C.K. et al. (1981)GururajanGururajan, V.S. et al. (1992), V.S. et al. (1992)

expin ii i

Ek f TRT

= −

Gas phase reactionsGas phase reactions

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11Schematic drawingSchematic drawing

Computational gridComputational grid

ReductorReductor burnerburner

Combustor burnerCombustor burner(d/D = 0.4)(d/D = 0.4)

AirAir--blown coal blown coal gasifiergasifier

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100gasPPCCE

coal char

C

C Cη = ×

+

Air ratio

Gasifier air ratio

Carbon conversion efficiency

Per pass carbon conversion efficiency

Combustor carbon conversion efficiency

Combustor air ratio

Cold gas efficiency

air

coal coal

MM A

λ =×

airg

coal coal char char

MM A M A

λ =× + ×

airc

coal coal char char

McMc A Mc A

λ =× + ×

100gasCCCE

coal char

Cc

Cc Ccη = ×

+

100gas

coal

C

Cη = ×

100gasCGE

coal

Q

Qη = ×

Definition of gasification performanceDefinition of gasification performance

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Tested coal propertyTested coal property

30.86MJ/kgHHV1.52wt%N0.45wt%S7.19wt%O5.23wt%H

75.08wt%C8.94wt%ash1.60wt%moisture

Coal MC(wet)

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Problem descriptionProblem description

XO2

λ

21, 30, 40vol%Oxygen

concentration

0.39, 0.41, 0.43, 0.47-Air ratio

51.159.263.40.47

61.970.274.50.43

67.175.679.90.41

71.880.084.60.39

O2 40 vol%O2 30 vol%O2 21 vol%Air ratio

Char feeding rate kg/hChar feeding rate kg/h

Coal feeding rate = Coal feeding rate = 100100 kg/hkg/h

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ØCalculation results of gas temperature distribution, per pass carbon conversion and product gas composition are in good agreement with the experimental data.

Comparison of calc. and exp. ResultsComparison of calc. and exp. Results(air ratio = 0.47, X(air ratio = 0.47, XOO22 = 21 = 21 volvol%)%)

Gas temperaturePer pass carbon conversionand product gas composition

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Gasification performance Gasification performance –– Varying air ratio at 21% OVarying air ratio at 21% O22

Temperature H2 CO CO2 H2O

(0.39, 0.41, 0.43, 0.47)(0.39, 0.41, 0.43, 0.47)

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Ø Both combustor and reductor temperature rise, as air ratio increases.

Ø Both carbon conversion in combustor and reductorare improved, as air ratio increases. So per pass carbon conversion is improved, as air ratio increases.

Gasification performance Gasification performance –– Varying air ratio at 21% OVarying air ratio at 21% O22(0.39, 0.41, 0.43, 0.47)(0.39, 0.41, 0.43, 0.47)

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Gasification performance Gasification performance –– Varying OVarying O22 concentrationconcentration

Temperature H2 CO CO2 H2O

(21, 30, 40 (21, 30, 40 volvol%) at air ratio 0.39%) at air ratio 0.39

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Ø Combustor temperature rises and reductortemperature drops, as air ratio increases.

Ø Carbon conversion in combustor is improved but in reductor decreases, as air ratio increases. Totally, per pass carbon conversion is improved, as air ratio increases.

Gasification performance Gasification performance –– Varying OVarying O22 concentrationconcentration(21, 30, 40 (21, 30, 40 volvol%) at air ratio 0.39%) at air ratio 0.39

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Gasification performance Gasification performance –– Oxygen Oxygen vsvs Air ratioAir ratio

Ø Total carbon conversion is 100%.Ø PPCC and char production rate are used for an assessment of gasifier’s capacity.Ø For instance, if ASU facility is included, air ratio can be reduced.

Per pass carbon conversion Char production rate

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Ø HHV increases in higher O2 concentration and lower air ratio conditions.Ø CGE increases in lower O2 concentration and lower air ratio conditions.


HHV of product gas Cold gas efficiency

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Ø Combustor temperature and heat flux on the combustor wall rises in higher O2 concentration and higher air ratio conditions.

Ø Slag properties are obtained from these data.


Combustor temperature Heat flux on combustor wall

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Ø (Representative) Molten slag temperature and slag viscosity canbe obtained from ash feeding rate and heat generated in the combustor (using slag viscosity model).

Slag temperature Slag viscosity

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Viscosity model for molten slagViscosity model for molten slag

Ø T-shift model is employed in slag viscosity estimation from a comparison of the model results with the experimental data.

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Ø Operating range in which stable operation can be done was obtained from slag viscosity data (using slag viscosity model and prefixed critical viscosity).

Ø Evaluation for high efficient and stable operation can be done (using representative slag property) .

Slag viscosity and cold gas efficiency

26unstable dischargeunstable discharge

Stable dischargeStable discharge

Molten slag flowMolten slag flow

Discharge of molten slagDischarge of molten slag

CombustorCombustor

Slag holeSlag hole

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Ø Gas-liquid two phase flow modelØ Gas phase … Combustor gas layerØ Liquid phase … Molten slag layerØ Molten slag viscosity is estimated by the T-shift model.

Ø Solidification modelØ Define a liquid phase fraction as a function of temperature

Ø Vary a drag coefficient in solidification layerØ Take a latent heat release into account

Modeling of molten slag flowModeling of molten slag flow

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GasGas--liquid two phase flow calculationliquid two phase flow calculation

Slag viscosity : empirical model [Browning et al., (2003)] Slag viscosity : empirical model [Browning et al., (2003)]

14788log 10.931

s sT T T Tη

= − − −

Solidification : solidification model [Solidification : solidification model [BennonBennon et al., (1987)]et al., (1987)]

( ) 0i i iα ρ∇ ⋅ =u

( ) ( ){ } ( )Ti i i i i i s g

iPα ρ αµ α β ∇ ⋅ − ∇ ⋅ ∇ + ∇ = − ∇ + − u u u u u u

ii = = gg (gas), (gas), ll (liquid)(liquid)

( ) 0Lρ∇⋅ =u L l l s sf f= +u u u

( ) ( ) ( ) ( )L L L L sP Kρ µ µ∇ ⋅ − ∇ ⋅ ∇ =−∇ − −u u u u u

( ){ }230 1l lK K f f= − ( ) ( )l s l sf T T T T= − −

Modeling of molten slag flowModeling of molten slag flow

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Heat from Combustor

Cooling

Cooling Tube

SlagTap

Analysis Area

Com

bust

or W

all

Cen

tral

Axi

s of

Gas

ifie

r Gas Layer

Molten Slag Layer

Heating Boundary

Cooling Boundary

Solidification Layer

Schematic drawing of slag holeSchematic drawing of slag hole

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Geometry Grids Boundaries

Number of grids; 35,880 InletOutlet

Cooling WallSymmetry Face

Grid and boundariesGrid and boundaries

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CombustorCombustor

Molten slag flowMolten slag flow

Slag holeSlag hole

Slag hole gateSlag hole gateSlag hole inner wallSlag hole inner wall

Slag flow characteristicsSlag flow characteristicsØØ VelocityVelocity

ØØ Slag temperature (viscosity)Slag temperature (viscosity)

ØØ Slag liquid & solid layer thicknessSlag liquid & solid layer thickness

Slag flow vectors & temperatureSlag flow vectors & temperature

Model resultsModel results

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Model results Model results –– Air ratio = 0.47, OAir ratio = 0.47, O22 = 21 = 21 volvol%%

Ø Slag flows toward the gate.Ø Slag is cooled down by the bottom boundary (cooling water).Ø Slag viscosity rises, as slag temperature drops.

Temperature Ts Kand velocity vectors

Slag viscosity µs Pa*s

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Model results Model results –– Air ratio = 0.47, OAir ratio = 0.47, O22 = 21 = 21 volvol%%

Ø Slag solid layer develops on the bottom of combustor.Ø Highest point of slag surface is located at 90 deg. from the gate.Ø Slag overflow might be observed at the points.

Solid layer on the bottom Slag surface height ys m

overflow location

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Model results Model results –– solidification characteristicssolidification characteristics

Solid layerSolid layer

Air ratio = 0.47, O2 = 30 vol%



Solid layerSolid layer

Ø Thickness of solid layer develops thicker, as temperature drops.

Ø Total thickness of slag layer develops thicker, as the thickness of solid layer develops.

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Slag overflow region and cold gas efficiency

Ø Operating range in which it is possible to avoid slag overflow was obtained by 3-D slag flow calculation.

Ø Evaluation for high efficient and stable operation can be done.

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SummarySummaryØ Influence of air ratio and oxygen concentration in gasifying agent on gasification performance and slag discharge was investigated by 3-D gas-particle reacting flow calculation. Representative slag viscosity was obtained by the calculation in order to discuss slag discharge characteristics.

Ø Slag behavior such as slag overflow over inner wall, which is caused by slag solidification, can be predicted by 3-D gas-liquid-solid free surface calculation in detail.

Ø Presented technique is useful to assess gasification performance and slag discharge.

development of coal gasifier operation supporting...

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