The paper deals with the development of a dynamic model which allows to describe the behavior of a rotary kiln (primary combustion chamber with heterogeneous combustion) and the corresponding afterbumers system (secondary combustion chamber with homogeneous combustion). For sake of brevity, the analysis here reported will be limited to a startup procedure used, as specific application, to really check robustness and reliability of the model itself. Few comparisons with experimental data available from a commercial unit will be reported to complete the model validation.