Development of coal gasifier operation supporting...
Transcript of Development of coal gasifier operation supporting...
1
1
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 --
2
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
2
3
BackgroundBackground
4
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
3
5
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
6
( )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
4
7
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
8
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
5
9
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)]
10
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
6
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
12
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
7
13
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)
14
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
8
15
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
16
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)
9
17
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)
18
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
10
19
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
20
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
11
21
Ø HHV increases in higher O2 concentration and lower air ratio conditions.Ø CGE increases in lower O2 concentration and lower air ratio conditions.
Gasification performance Gasification performance –– Oxygen Oxygen vsvs Air ratioAir ratio
HHV of product gas Cold gas efficiency
22
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.
Gasification performance Gasification performance –– Oxygen Oxygen vsvs Air ratioAir ratio
Combustor temperature Heat flux on combustor wall
12
23
Gasification performance Gasification performance –– Oxygen Oxygen vsvs Air ratioAir ratio
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
24
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.
13
25
Gasification performance Gasification performance –– Oxygen Oxygen vsvs Air ratioAir ratio
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
14
27
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
28
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
15
29
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
30
Geometry Grids Boundaries
Number of grids; 35,880 InletOutlet
Cooling WallSymmetry Face
Grid and boundariesGrid and boundaries
16
31
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
32
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
17
33
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
34
Model results Model results –– solidification characteristicssolidification characteristics
Solid layerSolid layer
Air ratio = 0.47, O2 = 30 vol%
Air ratio = 0.47, O2 = 21 vol%
Air ratio = 0.43, 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.
18
35
Gasification performance Gasification performance –– Oxygen Oxygen vsvs Air ratioAir ratio
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.
36
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.