Abstract: Selective elimination of nitrogen oxides contained in the exhaust gases from furnaces.With the addition of ammonia to the exhaust gases and acceleration of the reaction by a catalyst, the nitrogen oxides are reduced to molecular nitrogen and water. The procedure is such that the catalyst is traversed in one zone by the exhaust gases and in a zone separate therefrom by the fresh air being fed to the furnace as combustion air, while the position of the zones traversed by the exhaust gases and by the fresh air is changed continuously or step-wise such that the zone first traversed by the exhaust gases is then traversed by fresh air, and vice versa.The ammonia is mixed with the fresh air before it enters the catalyst, bound to the surface of the catalytically active material, and carried along into the zone traversed by the exhaust gases, where the reduction of the nitrogen oxide then takes place.