Development of coal gasifier operation supporting technique - Evaluation of gasification performance and slag discharge characteristics using CFD technique - Hiroaki WATANABE Energy Engineering Research Laboratory Central Research Institute of Electric Power Industry 1 Specification of IGCC demonstration plant in Japan Power Output Feed Rate System, Spec. Target Efficiency LHV (HHV) Environmental Target Coal Gasifier Gas Purification Gas Turbine gross efficiency net efficiency SOx NOx Dust 2 250 MW class around 1700 t/d 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/nm 3 (16%O2) Ref. Clean Coal Power R&D Co., Ltd. (http://www.ccpower.co.jp/index.html) 1
Background 3 To clarify influence of oxygen concentration of gasifying agents and air ratio on gasification performance Objective To discuss relationship between operation range and variations of gasification performance Representative slag viscosity Three dimensional slag flow calculation 4 2
Gas-particle two phase flow calculation Three dimensional time-mean mean conservation equations Finite volume hybrid upwind differencing Eulerian-Lagrangian manner for gas-particle two phase flow SIMPLEC algorithm for handling of pressure and velocity equations k-ε turbulence model [Launder, B.E. et al. (1974)] Discrete transfer radiation method [Lockwood, F.C. et al. (1981)] [Van Doormal, J.P. et al. (1984)] 5 Governing equations for gas-particle two phase flow Gas phase conservation Equation of continuity ( ρui) = Sp x i Momentum equation x i σ UU = B + + S ij ( ρ ) Energy equation i j j pu xi T ( ρuh i ) = λ + S xi xi xi Chemical species equation x i x ( ρuy i i) = ( ρdyi ) + SpY + R i fyi i ph Solid phase 6 Equation of motion du p mp = FD + FB dt Mass transfer dx Heat transfer ( 1 ) 1 ln( 1 ) k x x p dt = Ψ p ( ) i dt mc = Q + Q + Q + Q dt i pi C M G R 3
Gasification reaction modeling Coal Gasification Reaction includes three chemical reaction processes Pyrolysis (heterogeneous reaction) Char Gasification (heterogeneous reaction) Gas phase reactions (homogeneous reaction) 7 Reaction path Coal :? Volatile + Char (Equilibrium calculation based on proximate and ultimate analysis ) Devolatilization rate : 1. Simple primary reaction model dvi = H T dt ( Tref ) Vi τ 2. Two step model [Ubhayakar et al., (1976)] dv i = k V dt * ( V) Pyrolysis ( ) exp( ) k = k + k = A exp E RT + A E RT 1 2 1 1 2 2 8 4
Reaction path : C + 1/2 O2 => CO,- 9.25 MJ/kg C + CO2 => 2CO,+ 14.37 MJ/kg C + H2O => CO + H2 Reaction rate : Gasifying agent Char gasification O 2,+ 10.94 MJ/kg dx n E = AP exp 1 x 1 ln 1 x 0 ox Ψ i dt RT ( ) ( ) Kinetic parameters for coal char gasification CO 2 (x: reaction rate) [Bhatia, S.K. et al. (1980)] [Kajitani, S.K. et al. (2002)] H 2 O Temperature range K - < 1473 > 1473 < 1533 > 1533 Ψ n E A - - J/kmol - 14 0.68 1.30x10 8 1.36x10 6 3 0.54 2.71x10 8 3.34x10 9 8 3 0.73 1.63x10 8 6.78x10 4 3 0.64 2.52x10 8 2.89x10 8 3 0.84 1.40x10 8 8.55x10 4 Reaction path : CH4 + H2O? CO + 3 H2 CH4 + 1/2 O2? CO + 2 H2 10, + 206 [MJ/kmol], - 35.7 [MJ/kmol] H2 + 1/2 O2? H2O, - 242 [MJ/kmol] CO + 1/2 O2? CO2, - 283 [MJ/kmol] CO + H2O CO2 + H2,? - 41.1 [MJ/kmol] Reaction rate : R = min( R R ) xi [ ] [ ] fu ch, tu yi = n i i Jones, S.K. et al. (1988) ch i i i k = ft exp i i Westbrook, C.K. et al. (1981) RT Gururajan, V.S. et al. (1992) R k A B Gas phase reactions k Rtu = Cµ ρ min( mfu, mox / φ) ε [Magnussen, B.E. et al. (1976)] Backward reaction rate constant : k = k / K E b f eq 5
Air-blown coal gasifier Reductor burner Schematic drawing Combustor burner (d/d = 0.4) 11 Computational grid Definition of gasification performance Combustor air ratio Mcair λ c = Mc A + Mc A coal coal char char Combustor carbon conversion efficiency η = Ccgas 100 CCCE Cc + Cc coal M air Gasifier air ratio λ g = M A + M A Per pass carbon conversion efficiency η = Cgas 100 PPCCE C + C coal char coal coal char char char 12 M Air ratio air λ = Mcoal Acoal Cgas Carbon conversion efficiency η = 100 Ccoal Qgas Cold gas efficiency η = 100 CGE Q coal 6
Tested coal property moisture ash C H O S N HHV (wet) wt% wt% wt% wt% wt% wt% wt% MJ/kg Coal MC 1.60 8.94 75.08 5.23 7.19 0.45 1.52 30.86 13 Problem description Air ratio λ - 0.39, 0.41, 0.43, 0.47 Oxygen concentration X O2 vol% 21, 30, 40 Coal feeding rate = 100 kg/h Char feeding rate kg/h Air ratio O 2 21 vol% O 2 30 vol% O 2 40 vol% 0.39 84.6 80.0 71.8 0.41 79.9 75.6 67.1 0.43 74.5 70.2 61.9 0.47 63.4 59.2 51.1 14 7
Comparison of calc. and exp. Results (air ratio = 0.47, X O 2 = 21 vol%) Gas temperature Per pass carbon conversion and product gas composition Calculation results of gas temperature distribution, per pass carbon conversion and product gas composition are in good agreement with the experimental data. 15 Gasification performance Varying air ratio at 21% O 2 (0.39, 0.41, 0.43, 0.47) Temperature H 2 CO CO 2 H 2 O 16 8
Gasification performance Varying air ratio at 21% O 2 (0.39, 0.41, 0.43, 0.47) Both combustor and reductor temperature rise, as air ratio increases. Both carbon conversion in combustor and reductor are improved, as air ratio increases. So per pass carbon conversion is improved, as air ratio increases. 17 Gasification performance Varying O concentration 2 (21, 30, 40 vol%) at air ratio 0.39 Temperature H 2 CO CO 2 H 2 O 18 9
Gasification performance Varying O concentration 2 (21, 30, 40 vol%) at air ratio 0.39 Combustor temperature rises and reductor temperature drops, as air ratio increases. 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. 19 Gasification performance Oxygen vs Air ratio Per pass carbon conversion Char production rate Total carbon conversion is 100%. PPCC and char production rate are used for an assessment of gasifier s capacity. For instance, if ASU facility is included, air ratio can be reduced. 20 10
Gasification performance Oxygen vs Air ratio HHV of product gas Cold gas efficiency HHV increases in higher O 2 concentration and lower air ratio conditions. CGE increases in lower O 2 concentration and lower air ratio conditions. 21 Gasification performance Oxygen vs Air ratio Combustor temperature Heat flux on combustor wall Combustor temperature and heat flux on the combustor wall rises in higher O 2 concentration and higher air ratio conditions. Slag properties are obtained from these data. 22 11
Gasification performance Oxygen vs Air ratio Slag temperature Slag viscosity (Representative) Molten slag temperature and slag viscosity can be obtained from ash feeding rate and heat generated in the combustor (using slag viscosity model). 23 Viscosity model for molten slag T-shift model is employed in slag viscosity estimation from a comparison of the model results with the experimental data. 24 12
Gasification performance Oxygen vs Air ratio Operating range in which stable operation can be done was obtained from slag viscosity data (using slag viscosity model and prefixed critical viscosity). Slag viscosity and cold gas efficiency Evaluation for high efficient and stable operation can be done (using representative slag property). 25 Discharge of molten slag Combustor Slag hole Stable discharge Molten slag flow 26 unstable discharge 13
Modeling of molten slag flow Gas-liquid two phase flow model Gas phase Combustor gas layer Liquid phase Molten slag layer Molten slag viscosity is estimated by the T-shift model. Solidification model Define a liquid phase fraction as a function of temperature Vary a drag coefficient in solidification layer Take a latent heat release into account 27 Modeling of molten slag flow Gas-liquid two phase flow calculation ( αρu ) 0 = i i i ( ) ( ) Slag viscosity : empirical model [Browning et al., (2003)] η 14788 log = 10.931 T Ts T Ts i = g (gas), l (liquid) T { } P ( ) αρ uu αµ + = α + β u u u u i i i i i i s g i Solidification : solidification model [Bennon et al., (1987)] ( ρ ) 0 = u = fu + fu u L L l l s s ( ρ ulul) ( µ ul) P ( µ K) ( ul us) = { ( ) 2 } 3 0 l 1 l f = ( T T ) ( T T ) = K K f f l s l s 28 14
Schematic drawing of slag hole Central Axis of Gasifier Heat from Combustor Slag Tap Analysis Area Cooling Tube Cooling Combustor Wall Gas Layer Solidification Layer Molten Slag Layer Heating Boundary Cooling Boundary 29 Grid and boundaries Geometry Grids Boundaries Number of grids; 35,880 Inlet Outlet Cooling Wall Symmetry Face 30 15
Model results Slag hole inner wall Slag hole gate Combustor Slag hole Slag flow vectors & temperature Molten slag flow Slag flow characteristics 31 Velocity Slag temperature (viscosity) Slag liquid & solid layer thickness Model results Air ratio = 0.47, O 2 = 21 vol% Temperature T s K and velocity vectors Slag viscosity µ s Pa*s Slag flows toward the gate. Slag is cooled down by the bottom boundary (cooling water). Slag viscosity rises, as slag temperature drops. 32 16
Model results Air ratio = 0.47, O 2 = 21 vol% overflow location Solid layer on the bottom Slag surface height y s m Slag solid layer develops on the bottom of combustor. Highest point of slag surface is located at 90 deg. from the gate. Slag overflow might be observed at the points. 33 Model results solidification characteristics Solid layer Solid layer Air ratio = 0.47, O 2 = 30 vol% Air ratio = 0.47, O 2 = 21 vol% Air ratio = 0.43, O 2 = 30 vol% 34 Thickness of solid layer develops thicker, as temperature drops. Total thickness of slag layer develops thicker, as the thickness of solid layer develops. 17
Gasification performance Oxygen vs Air ratio Operating range in which it is possible to avoid slag overflow was obtained by 3-D slag flow calculation. Evaluation for high efficient and stable operation can be done. Slag overflow region and cold gas efficiency 35 Summary 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. 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. Presented technique is useful to assess gasification performance and slag discharge. 36 18