Selective catalytic reduction

Selective catalytic reduction (SCR) means converting nitrogen oxides, also referred to as NO
_x with the aid of a catalyst into diatomic nitrogen (N
₂), and water (H
₂O). A reductant, typically anhydrous ammonia (NH
₃), aqueous ammonia (NH
₄OH), or a urea (CO(NH
₂)
₂) solution, is added to a stream of flue or exhaust gas and is reacted onto a catalyst. As the reaction drives toward completion, nitrogen (N
₂), and carbon dioxide (CO
₂), in the case of urea use, are produced.

Selective catalytic reduction of NO
_x using ammonia as the reducing agent was patented in the United States by the Engelhard Corporation in 1957. Development of SCR technology continued in Japan and the US in the early 1960s with research focusing on less expensive and more durable catalyst agents. The first large-scale SCR was installed by the IHI Corporation in 1978.^[1]

Commercial selective catalytic reduction systems are typically found on large utility boilers, industrial boilers, and municipal solid waste boilers and have been shown to reduce NO
_x by 70-95%.^[1] More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and automobiles.

SCR systems are now the preferred method for meeting Tier 4 Final and EURO 6 diesel emissions standards for heavy trucks, cars and light commercial vehicles. As a result, emissions of NOx, particulates, and hydrocarbons have been reduced by as much as 95% when compared with pre-emissions engines.^[2]