NOx (Nitrogen oxides)

Nitrogen oxides refer to a series of molecules containing oxygen and nitrogen at different oxidation stages, with variable physico-chemical properties. NOx emissions have to be reduced due to their extremely detrimental effects on the environment. NOx are present in significant amounts in the flue gases of combustion units, nitric acid plants and units. This article reviews the regulation regarding NOx emission control. It then proceeds to presenting the industrial techniques in order to control such emissions according to the various NOx generating processes. Combustion units apply primary techniques (based on combustion control), secondary measures (applied between boiler and stack) as well as dry or wet ones. The selective catalytic reduction, involving the injection of NH3 into off-gases, is particularly detailed. This article finally presents the absorption of NOx in aqueous solutions as well as its reaction mechanisms and modeling principles.