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 (NOx Technical
Code). Following the entry into force, on 19 May 2005, of MARPOL Annex VI – Regulations for
the Prevention of Air Pollution from Ships, each marine diesel engine
to which regulation 13 of that Annex applies must comply with the
provisions of this Code. MEPC 53 in July 2005 agreed to the revision
of MARPOL Annex VI and the NOx Technical
Code. That review was concluded at MEPC 58 in October 2008 and this
version of the NOx Technical Code, hereunder referred to
as the Code, is an outcome of that process.
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 under which the engine is operating. 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) that can be formed include nitric oxide (NO) and
nitrogen dioxide (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, NOx formation
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 slow-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 tropospheric ozone,
nutrient enrichment and contributes to adverse health effects globally.
The purpose of this Code is to provide mandatory procedures
for the testing, survey and certification of marine diesel engines
that 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. The difficulties
of establishing with precision, the actual weighted average NOx emission
of marine diesel engines in service on ships 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 marine 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 shall 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.