Abstract: A method of reducing the nitrogen oxide level in the flue gases issuing from combustion units by introduction of reducing agents into contact with gases containing nitrogen oxides in first and second reducing stages, is provided. The first reducing stage is a non-catalytic stage (e.g. at temperatures over 800.degree. C.), while the second stage is a catalytic stage (e.g. at temperatures of about 300-400.degree. C.). A steam generation boiler with improved nitrogen reduction facilities is also provided. The amount of nitrogen oxides in the hot gases is reduced in the combination of the first and second reducing stages while producing steam in a steam generation boiler system, thus resulting in gases essentially free from nitrogen oxides while eliminating the possibility of NH.sub.3 (or other reducing agent) slip in the exhausted flue gases. Heat transfers in a convection section as used to establish stabilized temperature conditions for catalytic reduction.

Abstract: A method of reducing the nitrogen oxide level in the flue gases issuing from combustion units by introduction of reducing agents into contact with gases containing nitrogen oxides in first and second reducing stages, is provided. The first reducing stage is a non-catalytic stage (e.g. at temperatures over 800.degree. C.), while the second stage is a catalytic stage (e.g. at temperatures of about 300.degree.-400.degree. C.). A steam generation boiler with improved nitrogen reduction facilities is also provided. The amount of nitrogen oxides in the hot gases is reduced in the combination of the first and second reducing stages while producing steam in a steam generation boiler system, thus resulting in gases essentially free from nitrogen oxides while eliminating the possibility of NH.sub.3 (or other reducing agent) slip in the exhausted flue gases. Heat transfers in a convection section as used to establish stabilized temperature conditions for catalytic reduction.