This work aims at studying the chemical mechanisms responsible for the formation and destruction of nitrogen oxides during combustion of sewage sludge in a fluidized bed. To do so, a mathematical model has been built in order to represent the relevant phenomena occurring within the reactor and estimate the nitrous emissions as a function of the operating parameters. An important modeling effort has been done in order to predict what becomes of nitrogen initially present in the sludge. The final model allows computing the local concentration of NO, N2O, NO2, HCN, and NH3 at every location of the bed. Numerical estimations of the model are compared to experimental results obtained at industrial scale in a fluidized bed combustor (for which thermal equilibrium is reached using extra gas). This comparison is very good, allowing one to use the model for design and optimization of such reactors.