DIESEL DE-NOX CATALYSTS
The diesel engine exhaust always has high amount of excess oxygen. Conversion of NOx to N2 requires a reducing atmosphere. In the diesel engines due to oxidizing atmosphere in the exhaust, a NOxreduction catalyst different than the conventional 3-Way catalyst is required. For reducing NOx in the oxygen rich atmosphere, the reducing agents also termed as ‘reductants’ are necessary. The reductants can be supplied either from the engine itself or added by external sources in the exhaust. Hydrocarbons or ammonia are the two most frequently used reductants. As discussed earlier, the main strategies employed for NOx reduction in oxygen rich atmosphere are:
- NOx Storage – Reduction (NSR) Catalysts
- Selective Catalytic Reduction (SCR)
Low temperature plasma/catalyst systems are also being developed for application to diesel engines.
NOx Storage-Reduction (NSR) Catalysts
The NOx storage-reduction catalyst system or ‘NOx Trap’ was first developed for application to gasoline direct injection, lean-burn DISC spark ignited engines. It has been discussed in Module 5. In the diesel engines, diesel derived hydrocarbons are used as reductants. The principle of operation and basic features of Diesel NSR catalysts are the same as for the lean burn SI engines. The first step is to absorb NOx (NO converted to NO2 on the catalyst itself) on rare earth metal oxides and the second step is release of NOx in presence of hydrocarbons for reduction to N2.
For significant reduction in NOx, typically 2 to 5:1 HC/NOx molar ratios are required. Normally, engine out hydrocarbon emissions are quite low in the diesel engines. In the diesel NSR system, hydrocarbons are added to the exhaust gas by;
- post injection of fuel in the cylinder after the main fuel injection event
- adding secondary fuel into the exhaust system.
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