Marine vessel engines produce Nitrogen Oxides (NOx) during the combustion of fossil fuel which adds more than 14% to the global NOx production. NOx are toxic acidic gases which results in smog formation, ground level ozone and acid rain. Upon reacting with NH3 (Ammonia), moisture and other components in the atmosphere, NOx forms nitric acid, toxic organic nitrates and other particulate material which causes respiratory and heart diseases and may lead to premature death. Therefore, concentration of NOx emissions needs to be regulated in heavily traversed marine areas.
Certain Emission Control Areas (ECA) are designated, which includes the Baltic Sea, North Sea, Japan, Puerto Rico, US Virgin Islands, US Caribbean ECA, US and Canadian coasts, where strict regulations regarding NOx emissions are applicable. Depending on the maximum operating speed of the engine, the International Maritime Organization (IMO) has set emission standards referred to as Tier III, which will be applicable to marine vessels operating in NOx ECAs. However outside ECA, Tier II will be applicable. Also exemption may be provided to recreational vessels <25m and vessels with total propulsion power <750kW.
Table 1. IMO NOx Emission Standards
| Tier | Year | NOx Limit (g/kW-hr) | ||
| N<130 rpm | 130 ≤ N < 2000 rpm | N ≥ 2000 rpm | ||
| Tier I | 2000 | 17.0 | 45*n-0.2 | 9.8 |
| Tier II | 2011 | 14.4 | 44*n-0.23 | 7.7 |
| Tier III | 2016 | 3.4 | 9*n-0.2 | 1.96 |
For maintaining Tier III limits, dedicated NOx emission control technologies are required. The best commercially available control technology is Selective Catalytic Reduction (SCR) technology which reduces the levels of NOx in the exhaust by converting it into nitrogen and water with the help of a catalyst and a high purity urea solution (AUS40). The SCR technology is highly advantageous as it effectively reduces NOx emissions upto 99%, is fuel efficient and does not require additional cooling of the system. Also, it is very cost effective and does not require major redesign of the engine.
The Tier III regulations require that the NOx emissions in 2016 must reduce by 80% than in 2010. However, at the 65th session of the IMO’s Marine Environment Protection Committee (MEPC 65), held on May 2013, it was voted to postpone the implementation of the new Tier III regulations by five years (from 2016 to 2012). This decision, though condemned by environmental groups and NGOs, is expected to be adopted in the next MEPC 66 session to be held on 2014.
