SCR (Selective Catalytic Reduction)

Selective catalytic reduction injects a urea or ammonia reductant into the exhaust ahead of a catalyst, converting NOx to nitrogen and water and cutting NOx by roughly 80 to 95 percent. It is the most common route to MARPOL Annex VI Tier III for engines on ships built from 2016 operating in NOx-ECAs, and it is integral to ammonia and many dual-fuel engine packages. The catalyst needs a minimum exhaust temperature to work, so low-load operation and high-sulphur fuel can foul or deactivate it; low-temperature operation can also form N2O.