Selective catalytic reduction (SCR) is an industrial process for controlling nitrogen oxides (NOx) after combustion, in which ammonia is mixed with the flue gas in the presence of a vanadium catalyst on a titanium oxide support. Alternatively, tungsten-based catalysts can also be used. Total oxidation usually occurs after the SCR stage.

Nitrogen oxides are produced during combustion in stationary engines and are contained in the exhaust gases of power plants and industrial facilities. These pollutants consist of nitric oxide and nitrogen dioxide (also known as nitrous gases) and are considered harmful to health. In higher concentrations, they also act as strong respiratory poisons.

Selective catalytic reduction, or SCR, involves several chemical reactions in which nitrogen oxides are converted into diatomic nitrogen and water using ammonia as a reducing agent. As such, these reaction products can be released into the environment without concern. Anhydrous ammonia, aqueous ammonia, or a urea solution can be used as a reducing agent. The optimal process temperature is in the range of 320 to 500 °C.

The advantages of SCR lie in its high efficiency in removing nitrogen oxides (NOx) and in the low ammonia slip. Another advantage is the relatively simple integration of an SCR system into existing plants.