The process of selective catalytic reduction (SCR) is based on reduction of NOx with NH3 in the presence of excess O2 and an appropriate catalyst. Thus transforming it into harmless substances such as water and nitrogen. Ammonia, in the form of liquid ammonium hydroxide, is vaporized, diluted with air and injected directly into the stream of gases to be treated through a distributor.

In the case of combustibles with a high percentage of sulfur, during combustion SO2 is also produced which can be catalytically oxidized to SO3. The oxidation of SO3 can react with water and the unreacted ammonia to form sulfuric acid and ammonium sulfate.

Sulfate salts can deposit and accumulate on the catalyst resulting in its deactivation if the catalyst temperature is not high enough. In addition, the sulfuric acid formed can cause corrosion problems downstream in the plant. Therefore, depending on the required operating conditions, a highly selective DeNOx catalytic system for reducing NOx with NH3 in presence of O2 should be available, in order to avoid any undesirable secondary reactions.