Work on reviewing regulations to reduce emissions of air pollutants from ships was high on the agenda when the IMO Sub-Committee on Bulk Liquids and Gases (BLG) met for its 11th session in April 2007. The proposed three-tier system approach for nox emission limits is applicable to new engines. Tier I would be the current limits in marpol annex vi, Tier II would represent the best available in-engine technology, with potential reductions of NOx emissions of 15 to 25 per cent depending on engine type, and Tier III would impose more stringent limits requiring further development or the use of different after-treatment techniques.

The Working Group agreed to a 1st January 2011 implementation date for Tier II, with a possible reduction of 2 to 3.5 grams of NOx per kilowatt/hour across the current NOx curve attainable through in-engine design. The Group generally agreed that 2015/2016 was an appropriate timeframe for implementation of the Tier III NOx regulations for new engines.

Three proposals are currently under consideration:

• Option X, giving an 80 per cent reduction from Tier I levels, using Selective Catalytic Reduction (SCR) Aftertreatment or Humid Air Motor (HAM) technology (which has produced excellent emission results, but is limited in application to date), to be applicable to all marine diesel engines within 50 nautical miles from shore (worldwide);
• Option Y, giving an 83-85% per cent reduction from Tier I levels when in use, using Aftertreatment (SCR) or HAM, to be applicable to engines on large vessels only, in specific near-shore areas;
• Option Z, giving a 40-50% per cent reduction from Tier I levels, using Advanced In-Engine Modifications or Exhaust Gas Recirculation (EGR), to be applied to all marine diesel engines on a global basis

Discussing how to reduce emissions from existing engines (pre-2000), the Working Group reached a preliminary conclusion that emission modifications are technically feasible for many pre-2000 large-displacement engines. But some pre-2000 engines would not be appropriate for modification. It was noted that, for some engines, there would be significant practical difficulties due to the unavailability of parts, since some engine manufacturers are no longer in business.

Background
Regulations for the Prevention of air pollution from Ships were adopted by IMO in 1997 in a Protocol to the MARPOL Convention for the Prevention of Pollution from Ships. They entered into force in May 2005 and are included in Annex VI of the Convention. Immediately on their entry into force, however, IMO’s Marine Environment Protection Committee (MEPC) instructed the BLG Sub-Committee to begin a comprehensive review of Annex VI, taking into account advances in technology since the adoption of the regulations and the need to reduce further emissions from ships.