A numerical model has been developed to study the incineration of solid waste within a fixed-bed reactor. The model simplifies the flow to a one-dimensional succession of perfectly stirred reactors (PSRs) and includes a detailed chemical model. The calculations use the PSR model included in CHEMKIN II. The model predictions or residual nitrogen oxide are in good agreement with the experiments and have permitted validation of the model. The model has been used to investigate the mechanisms of NO formation during the combustion process of solid waste and to determine the principal reactive zones into the setup. NO is formed rapidly as gaseous fuel enters the reactor and occurs in three different steps. The principal intermediates are NCO, NH, and HNO. The rate of formation is dependant on the local oxygen content. A second region of NO production can be identified later in the trajectory of the fuel. Here the NO already formed will react with reducing species such as NH2, NH, NCO, CH3, and HCCO to produce N-2.