NOx

NOx is the collective term for nitrogen oxides, chiefly nitric oxide and nitrogen dioxide, which are produced whenever fuel is burned at the high temperatures found in a marine diesel engine. Unlike sulphur oxides, which come from sulphur in the fuel, nitrogen oxides form mainly from the nitrogen in the air itself under combustion heat and pressure, so their production is tied to how the engine burns fuel rather than to the fuel's composition. Nitrogen oxides are harmful pollutants: they contribute to acid rain, to the formation of ground-level ozone and smog, and to respiratory health problems in coastal and port communities, and they play a role in the over-fertilisation of sensitive waters.

Because of these effects, NOx emissions from ships are regulated under MARPOL Annex VI, which sets limits on how much a marine engine may emit, expressed in relation to the engine's speed. The limits are structured in tiers that have become progressively stricter for engines built in later years. Tier I and Tier II apply globally to engines of successive vintages, while the much stricter Tier III applies to newer engines when operating in designated nitrogen-oxide emission control areas, such as parts of the North American and Baltic and North Sea regions.

Meeting the tighter NOx standards, particularly Tier III, generally requires technology beyond ordinary engine tuning. Selective catalytic reduction injects a urea-based reagent into the exhaust so that a catalyst converts the nitrogen oxides into harmless nitrogen and water. Exhaust gas recirculation returns some exhaust to the engine intake to lower combustion temperature and so reduce NOx formation. Burning cleaner fuels such as LNG also lowers NOx substantially. Managing nitrogen oxide emissions is therefore an important part of ship design and operation, working alongside the controls on sulphur oxides and, increasingly, on carbon dioxide as the industry addresses its full environmental footprint.