The paper presents a computational fluid dynamics (CFD) analysis of a 33 MW straw-fired grate boiler. Combustion on the grate plays a key-role in the analysis of these boilers and in this work a stand-alone code was used to provide inlet conditions for the CFD analysis. Model predictions were compared with available gas temperature and species concentration measurements showing good agreement. Combustion of biomass in grate-based boilers is often associated with high emission levels and relatively high amounts of unburnt carbon in the fly ash. Based on the CFD analysis, it is suggested that poor mixing in the furnace is a key issue leading to these problems. (C) 2004 Elsevier Ltd. All rights reserved.