NOx EMISSION AND ITS REDUCTION MEASURES

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PRIMARY NOx REDUCTION MEASURE:- 
These are measures aimed at reducing the amount of NOx formed during combustion by optimizing engine parameters with respect to emission. Some of the methods are listed below –
1) Water addition:-
a) Direct water injection:- Greater heat capacity is utilized to reduce high peak temperatures as the water evaporates immediately upon injection. Rapid evaporation of injected water also helps to create a homogeneous fuel-air mixture.
b) Emulsified fuel or fuel – water emulsion :- It is favoured by some manufacturer claiming clear reduction in NOx emission at low cost with no significant design changes.
2) Altered fuel injection:-
a) Delayed fuel injection:- Retarded fuel injection timing retards the combustion process. Nitrogen oxide formation occurs later and with lower concentration.
b) Common rail control :- It has proven to be a very effective way in combating NOx reduction techniques.
c) NOx optimized fuel spray pattern:- Different fuel nozzle types and models have a significant impact on NOx formation, and the intensity of the fuel injection also has an influence.
3) Combustion air treatment:-
a) Exhaust gas circulation :- This is one method of adding dilutants to intake air, reducing burned gas temperature for any given mass of fuel and oxygen. But it is more practical for engines burning cleaner fuel such as low sulphur and low ash fuel.
b) Humidifying the scavenge air:- Injection of very fine water after the turbocharger using special nozzles. The fine water droplets evaporate fast and further heat is introduced in the air cooler and humidifies the combustion air. Drawback is too much humidity can be harmful to the cylinder condition.
c) Reducing the amount of scavenge air, and hence reducing the amount of excess oxygen available for conversion to nitrogen oxide
d) Water cooled rest gas:- In an electronically controlled exhaust valve timing it is quite easy to leave some of the exhaust gas in the cylinder. This obviously has a negative impact on engine performance, however this can be dramatically reduced by cooling the rest gas with a water spray, in which case the rest gas accounts for some of the NOx reduction and the water spray for the rest.
SECONDARY NOx REDUCTION MEASURE:-
These are measures designed to remove NOx from the exhaust gas by downstream cleaning technique. The most common secondary measures are –
1) SCR ( Selective catalytic reduction) :- In this system the exhaust gas is mixed with ammonia before passing through a layer of special catalyst at a temperature between 300 deg Celsius to 450 deg Celsius. The NOx is reduced to gaseous and harmless byproducts water and nitrogen.
2) SNCR( Selective non catalytic reduction) :- In case of SNCR, the reaction between ammonia and NOx takes place in a gaseous phase in a temperature window of between 800 and 900 deg Celsius.
The most effective method of NOx reduction is secondary measure. The advantages are:-
1) Secondary measures reduces Nox in case of SCR of up to 80- 95% and in case of SNCR 30-50%. Whereas primary measures average Nox reduction is up to 25- 35%.
2) Possible side effects of primary measures are lower overall energy efficiency, increased carbon mono-oxide and soot formation and hydrocarbon emission, corrosion due to reducing atmosphere, increase in unburnt carbon in fly ash. So, this directly affects the combustion process and some measures can also damage the engine components.
     The secondary measures do not affect the engine component and combustion process.

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