Selective Catalytic Reduction (SCR)

Selective catalytic reduction injects a urea solution (or ammonia) into the exhaust ahead of a vanadium- or zeolite-based catalyst, where it reduces NOx to nitrogen and water, typically cutting NOx by 80 to 95 percent. It is the leading route to IMO NOx Tier III for ships operating in emission control areas. The catalyst needs exhaust above about 300 C, so on low-load two-strokes the reactor sits before the turbine (high-pressure SCR) to keep temperature up.