The incineration of municipal waste under existing legislative and operational boundary conditions is a very complex process. An optimisation in the emissions is possible by a splitting up of the combustion air into primary and secondary air. The mixing of the secondary air into the furnace is the most important feature in order to reach uniform oxygen, temperature and velocity distributions. The physical and chemical processes, which are occurring in the fixed bed of the furnace (e.g. the heterogeneous combustion of the solid waste, the drying of the waste or the turbulent combustion of the gas species), are very complicated because of the large range of the waste composition. At the Institute of Environmental Process Engineering and Plant Design simple mathematical sub-models were developed for the heterogeneous combustion of the solid waste. The thermal input is defined as the integral of the function “heat release” over the grate. The heat release profile along the grate is a function of the axial distance of the waste input and the partition of volatile matter in relation to the sensitive heat at the waste surface. Volatiles emitted from the waste surface are CO and CxHy. The gas products CO2, CO, H2O and CxHy released from the packed bed are calculated in the same way as the heat generation. The O2 concentration is described by an opposite profile over the grate. 3D-simulations of the gaseous phase using FLUENT® were made in a complete furnace and burnout chamber respectively the radial part of a large MSW incinerator for several cases. Temperature, species and velocity distributions could be improved by a so-called “Prism” . That’s an additional secondary injection port in the area between the furnace and the first path. Under specific conditions experimental data is available for different cross sections and could be compared to the simulation results. Simulated and experimental data were very similar for the reference cases.