A method is presented to predict the conversion of biomass in a fluidized bed gasifier. The model calculates the yields of CO, H-2, CO2, N-2, H2O, CH4, tar (represented by one single lump), and char, from fuel properties, reactor geometry and some kinetic data. The equilibrium approach is taken as a frame for the gas-phase calculation, corrected by kinetic models to estimate the deviation of the conversion processes from equilibrium. The yields of char, methane, and other gas species are estimated using devolatilization data from literature. The secondary conversion of methane and tar, as well as the approach to equilibrium of the water-gas-shift reaction, are taken into account by simple kinetic models. Char conversion is calculated accounting for chemical reaction, attrition and elutriation. The model is compared with measurements from a 100 kW(th) bubbling fluidized bed gasifier, operating with different gasification agents. A sensitivity analysis is conducted to establish the applicability of the model and to underline its advantages compared to existing quasi-equilibrium models. (c) 2012 Elsevier Ltd. All rights reserved.