On 26 September 1997, the Conference of Parties to the International
Convention for the Prevention of Pollution from Ships, 1973, as modified
by the Protocol of 1978 relating thereto (MARPOL
73/78), adopted, by Conference resolution
2, the Technical Code on Control of Emission of Nitrogen Oxides
from Marine Diesel Engines. Under the provisions of Annex VI - Regulations for the Prevention
of Air Pollution from Ships, of MARPOL 73/78,
and subsequent to the entry into force of Annex
VI, each marine diesel engine to which regulation 13 of that annex applies,
must comply with the provisions of this Code.
As general background information, the precursors to the
formation of nitrogen oxides during the combustion process are nitrogen
and oxygen. Together these compounds comprise 99% of the engine intake
air. Oxygen will be consumed during the combustion process, with the
amount of excess oxygen available being a function of the air/fuel
ratio which the engine is operating under. The nitrogen remains largely
unreacted in the combustion process, however a small percentage will
be oxidized to form various oxides of nitrogen. The nitrogen oxides
(NOx) which can be formed include NO and NO2,
while the amounts are primarily a function of flame or combustion
temperature and, if present, the amount of organic nitrogen available
from the fuel. It is also a function of the time the nitrogen and
the excess oxygen are exposed to the high temperatures associated
with the diesel engine's combustion process. In other words, the higher
the combustion temperature (e.g., high peak pressure, high compression
ratio high rate of fuel delivery, etc.), the greater the amount of
NOx formation. A low-speed diesel engine, in general, tends
to have more NOx formation than a high-speed engine. NOx has
an adverse effect on the environment causing acidification, formation
of ozone, nutrient enrichment, and contributes to adverse health effects
globally.
The purpose of this Code is to establish mandatory procedures
for the testing, survey and certification of marine diesel engines
which will enable engine manufacturers, shipowners and Administrations
to ensure that all applicable marine diesel engines comply with the
relevant limiting emission values of NOx as specified within regulation 13 of Annex VI to MARPOL
73/78. The difficulties of establishing with precision, the actual
weighted average NOx emission of marine diesel engines
in service on vessels have been recognized in formulating a simple,
practical set of requirements in which the means to ensure compliance
with the allowable NOx emissions, are defined.
Administrations are encouraged to assess the emissions performance
of propulsion and auxiliary diesel engines on a test-bed where accurate
tests can be carried out under properly controlled conditions. Establishing
compliance with regulation 13 of
Annex VI at this initial stage is an essential feature of this Code.
Subsequent testing on board the ship may inevitably be limited in
scope and accuracy and its purpose should be to infer or deduce the
emission performance and to confirm that engines are installed, operated
and maintained in accordance with the manufacturer's specifications
and that any adjustments or modifications do not detract from the
emissions performance established by initial testing and certification
by the manufacturer.