A generalized model is developed for combustion of solid fuel in a fixed bed on a grate. The model treats a bed consisting of thermally large fuel particles of optional shapes (spheres, finite cylinders, and parallelepipeds). In the special case of countercurrent combustion, the model shows how the fuel undergoes drying, devolatilization, and char combustion simultaneously in a conversion front that progresses with constant rate from the surface of the bed to the grate. The progress of the temperature front down through the bed is caused by the combustion of char and some volatiles on, or close to, the surface of the fuel particles before the remaining gas between the particles is ignited. The results show the movement of a reaction wave through the bed. The width of this combustion front is aaout 0.05m in agreement with measurements in similar cases. The processes of drying, devolatilization, and char combustion within the front emphasize the importance of a rather detailed description of the conversion of the thermally large fuel particles used here, and prove that a continuous treatment (such as when the bed is represented by a porous medium) does not give a correct picture of the conversion front for most of the particle sizes used in fixed-bed combustion of biofuel. Propagation rate, temperature profile, and width of the conversion front obtained by the model agree well with experimental data.