The Tier Ⅲ compatibility certificate was awarded by DNV GL for a MAN 8L21/31 four-stroke engine which had been retrofitted with a selective catalytic reduction (SCR) system. The system allows the engine, previously Tier Ⅱ compliant, to operate at the very low levels of Nox emissions demanded by the IMO Tier Ⅲ rules. The vessel in question, the Petunia Seaways, regularly sails on fixed routes across the North Sea. In September 2012, one of its 8L21/31 auxiliary engines was retrofitted with a SCR system which, says MAN Diesel, has proven daily that freight ships can now meet the strict emission levels laid down by IMO Tier Ⅲ where Nox emissions have to be reduced in certain areas by 75% compared to current limits. The SCR injects urea into the exhaust gases and passes them through a catalytic converter at a temperature of 300° to 400℃. The subsequent chemical reaction reduces Nox by over 80％. The SCR system in its entirety consists of a catalytic converter with several control units and a surveillance and control unit.
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