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.