Sludge incineration in fluidized bed is a very complex process, producing gaseous pollutants (carbon monoxide (CO) and nitrogen oxides (NOx)). The legislative norms need to be respected despite of variations of sludge in composition and in quantity. The NOx formation, due to lots of chemical reactions from the sludge nitrogen, is partly unknown. This paper deals with the design of a dynamic model of NOx emissions for a fluidized bed sludge combustor to be used in control strategy. The model is validated with industrial data, and the simulation results validate the simplification hypotheses, but need the reconstruction of sludge composition.