This paper presents a new method for the determination of NOx formation from industrial boilers. The flow and temperature fields within the furnace, obtained through CFD codes, are approximated by a network of ideal reactors. This approach allows using a very detailed and comprehensive kinetic model, without losing the detailed thermal and fluidynamic information. The meaning of this work is to move a further step in the direction of a better description of both kinetic and fluidynamic aspects during combustion. The prediction of pollutant byproducts, whose amount is in terms of ppms or even ppbs, necessarily requires the use of a detailed description of the process chemistry. Moreover, in the case of NOx formation, the air and fuel staging, as applied to more recent plants, implies the interactions between nitric oxides and hydrocarbon radicals. A detailed description of these reaction paths is extremely important to correctly reproduce the final emissions. The comparison of the obtained predictions with the experimental industrial results shows a quite satisfactory agreement. qualitatively and quantitatively, in different operating conditions and plant configurations.