While sulphur oxides (SOx) are formed from Sulphur in the fuel, the formation of nitrogen oxides (NOx) results from nitrogen in the air during the combustion process i.e. almost irrespective of the fuel being used. The main determining factor for the NOx formation is the maximum peak combustion temperature – which is quite high in marine diesel engines. Due to the adverse environmental impact and the potential health hazards caused by NOx emissions, this has become an issue of major global concern.
NOx emissions are regulated through the revised MARPOL Annex VI 2008, which puts a limit on the specific emission from engines as a function of the rpm. The limits are imposed on engines larger than 130 kW, in three tiers based on the year of construction and operation area:
Recent engine designs will most likely comply with the Tier II requirements. Measures suggested to reduce NOx emissions from existing and new engines include the following:
In addition, one possible solution is to use natural gas such as LNG as fuel, as this will reduce the NOx emissions by some 90%.
|Adding water||Simple and inexpensive||Increased fuel consumtion |
Does not comply with Tier III
|LNG||Cheaper fuel |
Effective NOx reduction
|Retrofit difficult |
Fuel availability uncertain
Infrastructure currently limited
|SCR||Effective NOx reduction||Complex system and operation |
Requires ammonia normally in the form of urea
Requires engine modifications and additional arrangements