A comprehensive model for steady state combustion of biomass char in a fast fluidized bed is presented, although the model is capable to deal any fluidization regime. The air needed for combustion is staged as bottom, primary and secondary air. The combustor is divided into two sectors: a dense (bottom) zone and a transport (upper) zone. The model consists of sub-models for bed hydrodynamics, conversion and conservation. The hydrodynamic description of the bottom bed is based on the modified two phase theory while the freeboard is based on a. simplified core-annulus approach. Biomass char is modelled as partially volatile fuel. Char composition is a model parameter and is defined by weight fractions of C, H and O. The model includes the release of volatiles from biomass char and their subsequent combustion. The combustion reactor also serves as a sink for spent organic solvent. Char mass balance and energy balances are solved. globally in each zone. The sensitivity analysis shows that the model is not very sensitive to changes in operating parameters. The model is robust to deal with solid, liquid and gaseous fuel combustion. (C) 2008 Elsevier B.V. All rights reserved.