This paper presents a new development in the study of a rotary kiln incinerator. The modelling of the furnace has been divided into two parts. On the one hand, a model describing the physico-chemical processes which occur within the burning bed of municipal solid waste (assumed to be a mixture of wood, cardboard and PVC) has been set. This model mainly relies on the assumptions of plug flow and macroscopic pyrolysis kinetics of burning waste. On the other hand, C.F.D. has been used to describe the processes occurring within the gaseous phase of the kiln and of the post combustion chamber (turbulence, combustion, radiation). A data processing tool has been built to automate the data exchanges between the two parts of the model. Some results of the overall model are shown in two different situation (working with and without extra burner).