SCR meaning in Chemistry ?

Selective catalytic reduction (SCR) is a means of 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 an 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.

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%. More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and even automobiles.

SCR systems are now the preferred method for meeting Tier 4 Final and EURO 6 diesel emissions standards for heavy trucks, and also for cars and light commercial vehicles. In many cases, emissions of NOx and PM (particulate matter) have been reduced by upwards of 90% when compared with vehicles of the early 1990s.