By NAM, DONG | In September 1997, the Protocol of 1997 to MARPOL 73/78 was adopted to introduce the new Annex VI - Air pollution from ships. When the Protocol enters into force, the requirements of the NOx will be applied to each diesel engine with a power output of more than 130 kW which is installed on a ship, or which undergoes major conversion, on or after 1 January 2000. Annex VI deals with a wide range of air pollution control matters including regulations on halons, Hydrochlorofluorocarbons (HCFCs) and other ozone depleting substances, Nitrogen oxides (NOx), Sulphur oxides (SOx), Volatile organic compounds (VOCs), shipboard incinerators and fuel oil quality. However, the main focus has so far been on reducing the NOx.
The NOx Technical Code introduces a new concept of engine family, engine group, parent engine and the technical file to be determined before issuing the Engine International Air Pollution Prevention Certificate (EIAPP Certificate) and the International Air pollution Certificate (IAPP Certificate). Because the new Annex VI has not yet come into force, guidelines have been introduced to issue a Statement of Compliance (SOC Certificate).
NOx formation builds up by reaction between nitrogen and oxygen in the combustion air (thermal NOx), by reaction between exhaust gas hydrocarbon and combustion air oxygen (prompt NOx) and by reaction between nitrogen bindings in fuel (fuel NOx).
Thermal NOx is decisive for total emission and all the reducing methods are targeted to reduce that component. NOx emission can be reduced by primary methods such as retard injection, fuel nozzle modification, change of compression ratio, water direct injection, water emulsification, exhaust gas recirculation (EGR) and secondary method such as selective catalytic reduction (SCR).