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Subject: Marine Engines and NOx Emissions
(Posted on Jan 10, 2013 at 10:14AM by Media Manager)

Marine Engines and NOx Emissions

Apart from a very few exceptions where power cables from land sources are connected and used on board vessels in port, ships are self sufficient regarding energy supply. A general overview of potential combustion (and emission) sources and their use on board ships is presented in Table C.1. In terms of number and emission magnitude, Main (ME) and Auxiliary (AE) diesel engines dominate by far, followed by turbine machinery (steam and gas turbines).

from boilers, emergency diesel engines and waste incinerators are relatively very small and can be considered negligible (excluded hereafter). Rather than size, ME and AE engines are normally sub-divided according to their engine speed at the crankshaft as: high speed, medium speed and slow speed7. Slow and medium speed engines are more abundant than high speed engines for main engines. For auxiliary engines, high and medium speed engines dominate. Old steam turbine systems, which use steam to drive turbines geared to the propeller shaft, have a relatively low efficiency and consequently are being replaced by diesel engines.

For the world fleet in general, the total installed main engine power consists of 63% as slow speed diesel, 31% as medium speed diesel and 6% as others e.g. gas and steam turbines (IMO, 2000). Engine types by number for the world fleet (ship size > 100 GRT) are reported as 65,7% (slow speed diesel), 32,2% (medium speed diesel) and 2,1% (other) (Davies et al., 2000). In a 1990 emission study for the Mediterranean Sea including ca. 7300 vessels (Lloyds Register Engineering Services, 1999), the number of engines were reported as 47,3% (slow speed diesel),50,6% (medium speed diesel), and 2,1% (steam turbine) while based on installed power, the proportions were 65,8% (slow speed diesel), 26,2% (medium speed diesel), and 8,0% (steam turbine).

In contrast to CO2 and SO2 emissions, which are fuel dependent emissions, emissions of NOx are particularly dependent on the combustion process (engine type). For slow speed engines a longer period at higher temperatures occurs which gives improved of combustion efficiency but greater thermal fixation of nitrogen in the combustion air to NOx. Thus the new maximum allowable NOx emission limits for marine diesel engines (IMO Technical NOx Code, 1997) are directly related to the rated speed of the engine.


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