A process whereby ammonia (or an ammonia derivative such as urea), usually diluted with water, steam, or air, is injected into the exhaust stream of a power generating unit, and then that gas is passed over a catalyst. The ammonia reacts with NOx gases to produce molecular nitrogen (N2), a naturally occurring element that makes up 80 percent of the air we breathe, and water vapor. An SCR is “selective” in that the catalyst material promotes reactions specifically to remove NOx.

An add-on control device that reduces NOx emissions by converting them to nitrogen and water through the intervention of ammonia and a non-reacting catalyst that remains in place for continued reuse. The NOx reduction reactions occur in a narrow temperature range, generally 550-900° F. SCR is capable of NOx emission reductions of 90 percent or greater.

Selective catalytic reduction (SCR), a process where a gaseous or liquid reductant (most commonly ammonia or urea) is added to the flue or exhaust gas stream and is absorbed onto a catalyst. The reductant reacts with NOx in the fuel gas to form H2O (water vapor) and N2 (nitrogen gas).