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.
Requirements
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:
- Tier I (globally)
- Engines installed on vessels constructed on or after 1 January 2000
- Engines larger than 5,000 kW and with a per cylinder displacement at or above 90 litres, installed on vessels constructed on or after 1 January 1990 but prior to 1 January 2000. (This requirement is subject to the availability of an approved engine upgrade kit).
- Tier II (globally)
- Engines installed on vessels constructed on or after 1 January 2011.
- Engines installed on vessels constructed on or after 1 January 2011.
- Tier III (sailing in NOx ECAs)
- Engines installed on vessels constructed on or after 1 January 2016 when operating within NOx-controlled ECAs.
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:
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Water can be added as direct water injection, emulsified water in the fuel, humidity in the scavenge air, etc. This is an effective way of reducing NOx emissions, but there is some concern that it may affect the thermal efficiency of the engine and cause fuel consumption to increase. This measure will only reduce NOx emissions to Tier II levels.
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Exhaust gas recirculation (EGR), i.e. feeding exhaust gas into the combustion process, is regarded as a quite promising method and engine makers currently have a strong focus on developing their EGR systems to ensure compliance with the strict Tier III requirements.
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Selective Catalytic Reactors (SCR) have already been installed on a number of ships and seem to work well, although certain operational issues have to be resolved, such as minimum exhaust temperature requirements to achieve the optimum effect of the catalyst. The SCR efficiency is also known to be reduced at low engine loads. Sulphur in the fuel also has a tendency to pollute the catalyst material, so the SCR should ideally be installed after a SOx reducing step, if not using fuel with low sulphur content.
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%.
| Solution | Pros | Cons |
| 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 |
| EGR | Known technology | Requires engine modifications and additional arrangements |
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