Selective Catalytic Reduction
Abbreviation: SCR
Exhaust after-treatment that injects urea into hot exhaust gas, converting NOx to N2 and water on a vanadium-titanium catalyst.
In practice
Operated continuously on Tier-III engines in NOx ECA waters. The duty engineer monitors urea injection rate, exhaust temperature window (>270 °C), and NH3 slip. Catalyst poisoning by sulphur is the major maintenance cost; some operators bypass SCR outside ECAs.
Regulatory detail & full definition
Selective Catalytic Reduction (SCR) is an exhaust after-treatment system that injects a urea solution into the hot exhaust gas, where it converts nitrogen oxides (NOx) into harmless nitrogen and water over a vanadium-titanium catalyst. It is fitted to most newbuildings operating in NOx Emission Control Areas to meet the IMO Tier III limit (3.4 g/kWh at engine speeds below 130 rpm). The catalyst has an effective window of about 250-450 °C, so an auxiliary boiler is often used to keep the gas hot at low load; the urea is carried as a 32.5% AdBlue solution and consumed at roughly 3% of fuel mass.
Authoritative source: MARPOL Annex VI (Air pollution) ↗
Related
Source: MARPOL Annex VI Reg. 13 (NOx); IMO MEPC.291(71) (NOx Technical Code)
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