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Secondary NOx Reduction Measures



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 by-products of 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 a secondary measure. 
The advantages are:-
  • Secondary measures reduce 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%.
  • 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.
  • cThe secondary measures do not affect the engine component and combustion process.


Scavenge Air Moisturizing & Exhaust Gas Recirculation for ME
For both scavenge air moisturizing (SAM) systems and the exhaust gas recirculation (EGR) system, the NOx reducing effect is achieved by reducing the local maximum combustion temperatures in the combustion chamber. 
                               
Scavenge Air Moisturizing:
One temperature-lowering technique, called Scavenge Air Moisturizing (SAM) by MAN and known generally as Humid Air Motor technology, is to increase the moisture content of intake air. SAM uses seawater to cool and humidify air coming out of the intake compressor, and then uses fresh water to remove the salt from the system before it can damage the engine. The system requires specially-shaped components, special materials and auxiliary machinery systems for handling the humidification water, all run by a programmable logic controller.
The SAM system has a seawater injection stage, where a surplus of seawater is injected for saturation and cooling of the hot air from the compressor. The seawater stage will provide a near 100% humidification of the scavenge air and supply all of the water for humidification.
The freshwater stages 1 and 2 will be near temperature neutral to the scavenge air and create a small freshwater production depending on the operation parameters chosen. The freshwater stages only act as cleaning stages for removal of any salt which may pass with the air from the seawater stage. A continuous accumulation of salt in the freshwater stages would eventually cause the salt content to reach an unacceptably high level. This is counteracted by cooling the saturated air with the air cooler and generating some extra
freshwater for stage 2. The extra freshwater is then sent upstream on the tank side of the SAM system. Thereby the content of salt in the freshwater stages can be controlled


Exhaust gas recirculation:
When a small percentage of exhaust gas is introduced into the combustion air, the oxygen purity of the combustion air is reduced leading to lower NOx emissions. 
                   
Primarily marine diesel operates on residual fuel that contains sulfur. Products of combustion, therefore, contain corrosive gases that require to be taken into account while designing an EGR system. Furthermore, marine diesel being turbocharged engines, the scavenge pressure is higher than the exhaust pressure.
This requires an additional exhaust blower. This system is an effective means of NOx reduction. With a 20% EGR NOx reduction is in the order of 50% with very little fuel consumption penalty.
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