How to reduce NOx & SOx emission from vessels?
Methods to reduce NOx emission from vessels:
1. Humid Air Method: In this method, water vapour is mixed in the combustion air before supplying it to the cylinder. Air from the T/C blower is passed through a cell that humidifies and chills the hot air taking moisture from the cooling water until air saturation is achieved.
2. Exhaust Gas Re circulation (EGR): Some amount of engine exhaust gases are send back to the scavenge space to mix up with the air to be supplied to cylinder for combustion. This reduces the oxygen content of the air and hence reduces formation of NOx.
3. Water Injection and Water emulsion: In water injection, water is added to reduce the temperature of combustion leading to low NOx emission. In water emulsion, fuel is blended with water and in water injection a separate fresh water injector is mounted in the cylinder head which injects water.
4. High Scavenge Pressure and Compression Ratio: In this method large amount of air can be introduced inside the cylinder to lower combustion temperature and NOx emission.
5. Selective Catalytic Reduction: The SCR is the most efficient method to reduce NOx emissions from ships (up to 90-95% of reduction). In this method, low sulphur fuel oil is used and exhaust temperature is maintained above 300 deg C. The exhaust gas is mixed by water solution of urea and then it is passed through catalytic reactor.
6. Two Stage Turbocharger: This method can reduce the exhaust temperature in the intercoolers and also the NOx content in the emitted exhaust.
7. Engine Component Modification: It is better to design an engine which has a property to reduce the NOx formation during combustion process rather than investing on expensive secondary measures. Integration of slide valve type fuel injector with almost zero sack volume eliminates any chance of fuel dripping and after burning, leading to cylinder temperature and NOx formation.
New designs with reduced mean piston speed gives more time for excess air and proper combustion to lessen NOx formation.
Reducing SOx Emission
Following are the methods and technologies used to reduce SOx emission from marine engines:
1. Use of Low sulphur fuel oil: The most commonly used method to comply with Annex VI of MARPOL while entering emission controlled Area or ECA.
2. Exhaust Gas Scrubber Technology: The exhaust gas from the engine is passed through the scrubber tower where a liquid is showered over it. Fresh water blended with caustic soda (NaOH) is used as a scrubbing liquid.
3. Cylinder Lubrication: Good quality cylinder lubrication along with efficient control systems can neutralise the sulphur in the fuel and reduce SOx emissions from the engine.
