The presence of NOx in marine engine’s exhaust emission is due to high combustion temperature
which reacts with nitrogen in the air supplied for combustion.
Following are the methods to reduce NOx emission from ship:
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. Generally saline sea water is utilized in this method by heating it with jacket water
and turbo charger heat, and the left over brine is disposed back to the sea. This method can
achieve reduction of NOx by 70-80%.
2. Exhaust Gas Re circulation (EGR): As the name suggests, 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 this method, 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. This method has a drawback of increasing the specific fuel oil combustion with
reduction in NOx by only 20-45%.
4. High Scavenge Pressure and Compression Ratio: With high scavenge pressure and
compression ratio, 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. The only disadvantage of SCR is
its expansive installation and operating cost.
6. Two Stage Turbocharger: ABB’s latest two stage turbocharger can reduce the exhaust
temperature in the intercoolers and also the NOx content in the emitted exhaust. Read more
about 2 stage turbochargers here.
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 like Green Ultra long stroke engine from MAN (GME series) with reduced mean
piston speed gives more time for excess air and proper combustion to lessen NOx formation
which reacts with nitrogen in the air supplied for combustion.
Following are the methods to reduce NOx emission from ship:
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. Generally saline sea water is utilized in this method by heating it with jacket water
and turbo charger heat, and the left over brine is disposed back to the sea. This method can
achieve reduction of NOx by 70-80%.
2. Exhaust Gas Re circulation (EGR): As the name suggests, 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 this method, 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. This method has a drawback of increasing the specific fuel oil combustion with
reduction in NOx by only 20-45%.
4. High Scavenge Pressure and Compression Ratio: With high scavenge pressure and
compression ratio, 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. The only disadvantage of SCR is
its expansive installation and operating cost.
6. Two Stage Turbocharger: ABB’s latest two stage turbocharger can reduce the exhaust
temperature in the intercoolers and also the NOx content in the emitted exhaust. Read more
about 2 stage turbochargers here.
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 like Green Ultra long stroke engine from MAN (GME series) with reduced mean
piston speed gives more time for excess air and proper combustion to lessen NOx formation
0 comments:
Post a Comment