Exhaust after-treatment that injects urea into hot exhaust gas, converting NOx to N2 and water on a vanadium-titanium catalyst.
Regulatory detail & full definition
Required on most newbuilds operating in NOx ECAs to meet IMO Tier III (3.4 g/kWh below 130 rpm). Operating window is 250-450 deg C — auxiliary boiler often used to maintain temperature at low load. Urea is supplied as 32.5% AdBlue solution; consumption ~3% of fuel mass.