Combustion of solid fuels on a grate is widely used. Mostly, the combustion behaviour is explained by the classical theory of Rogers. However, that theory cannot explain the combustion process when primary air preheating is applied. Solid fuel grate combustion is studied by experiments in a pot furnace. Experiments with and without primary air heating are described. These are compared with conclusions learnt from real plant experiments. It was found that the pot furnace experiments have a limited value in explaining the combustion behaviour of solid fuels on a grate. In order to be able to explain the results from practice an quantitative extension of Rogers' theory for the case with air preheating is presented.