This paper describes a comprehensive model of a biomass combustor on a traveling grate. This general mathematical model deals with the complex multi-phase gas solid problem of biomass combustion or gasification at both the reactor and particle scale. Mass and energy balances plus proper closure equations describe the behavior of both of the phases. Key elements of the model are the detailed kinetic schemes describing the biomass degradation and the successive gas-phase secondary reactions. The model has been tested and partially validated on a semi-quantitative basis in comparison to a 12 kW biomass combustor operating in Belgium. Model results constitute a useful guideline for control purposes.