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NOx Reduction Method

There are two methods to reduce NOx emissions; Primary and secondary measures. 

Primary measure aim at reducing the amount of NOx formed during combustion by optimizing engine parameters with respect to emissions.
As read in previous article, the main factors influencing NOx formation are the concentrations of oxygen and nitrogen and the local temperature in the combustion process. Therefore,the primary measures focus on lowering the concentrations, peak temperature and the amount of time in which the combustion gases remain at high temperatures.

Miller cycle is employed in 4-stroke engines. It uses higher than normal pressure turbocharger. The inlet valve is closed before the piston reaches bottom dead center on the intake stroke. The charge air then expands inside the engine cylinder resulting in reduced temperature.

On the other hand, secondary measures remove NOx from the exhaust gases by downstream cleaning techniques. 

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.

The chemical reactions which occur when exhaust gases are mixed with urea solution, before they are released through the uptake are:

4NO + 4NH3 + O2 = O2 + 5N2 + 6H2O
2 NO2 + 4 NH3 +O2 = 3N2 + 6H20
NO + NO2 + 2NH3 → 2N2 + 3H2O

NOx is reduced to N2 and H2O

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.

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.


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