This paper describes the activities carried out for the development and the use of CRFD codes and related procedures for the design of industrial furnaces. The objective is to predict the emission of pollutants, such as nitrogen oxides, in combustion flue gases. This prediction may be correctly accomplished using a detailed kinetic mechanism, which cannot be directly implemented on CRFD codes due to actual computational limits (both in terms of memory and CPU time consumption). A new approach has been developed: analysing the 3D CRFD flow fields, an “equivalent” chemical reactors network model is extracted with corresponding residence time distributions and overall reactor properties, and the detailed kinetic calculation is performed on this simpler scheme.