Performance evaluation for dual circulating fluidized-bed steam gasifier of biomass using quasi-equilibrium three-stage gasification model

The effects of gasification temperature (T-G) and steam to fuel ratio (gamma) on product gas composition and yield were experimentally investigated for steam gasification of pine woodchips in a bench-scale circulating fluidized-bed (CFB) gasifier with external heat supplier. To evaluate process performance in a dual circulating fluidized-bed (DFB) with heat carrier (silica sand), a quasi-equilibrium three-stage gasification (qETG) model was developed and validated with experimental data of biomass steam gasification. The model was divided into three stages including biomass pyrolysis, char-gas reactions, and gas-phase reactions. Carbonic and methane formation ratios were considered at the pyrolysis stage under the assumption of spontaneous decomposition. At the second and third stages, char-gas and gas-phase equilibrium reactions were corrected by two empirical equations concerning the steam participation ratio and the non-equilibrium factor, respectively. Using the qETG model, parametric study on T-G and gamma was performed to predict final gas composition, carbon conversion, char residue, gas yield, lower heating value, additional fuel ratio, solid circulation ratio, heat recovery and H-2 to CO molar ratio. Focusing on the solid circulation ratio and H-2/CO molar ratio, several effective operating conditions were suggested from the contour of performance criteria. (C) 2011 Elsevier Ltd. All rights reserved.