A generalised model is developed for combustion of solid fuel in a fixed bed on a grate. The model can be applied to co-current and countercurrent combustion and treats a bed consisting of thermally large fuel particles of optional shape (spheres, finite cylinders and parallelepipeds) of any solid fuel. The result of model calculations agrees well with the measurements available in the literature, and the validity of the model is also shown to be satisfactory for the investigation of the differences between co- and counter-current combustion in a fuel bed, simulated by a fuel batch. The results show how the different phases, drying, devolatilisation and char combustion interact during conversion. (C) 2002 Elsevier Science Ltd. All rights reserved.