It has long been known that NOx emissions from diesel engines can be reduced by reducing the peak temperatures of combustion. Dilution of the cylinder charge with inert gases is one method of lowering the peak cylinder temperatures. Nitrogen is an obvious diluent; however until recently its use was limited to stationary laboratory engines. Compact, high productivity, air separation membranes have recently been developed. These membrane modules provide means for generating nitrogen-enriched air (NEA) at the point of use, for example, under the hood of a diesel truck. NEA offers an attractive, clean alternative to dilution with exhaust gases.
NEA is generated by feeding the cooled, turbocharged air to the bore side of a hollow fiber membrane device. A pressure differential across the wall of the hollow fiber causes oxygen to permeate preferentially through the polymeric wall.
Thus as air flows along the length of the hollow fibers, it becomes slightly depleted of oxygen and enriched in nitrogen. The resulting NEA is fed to the intake manifold of the engine at only slightly lower pressure than the turbocharged air. The oxygen-enriched co-product (OEA) is simply vented to the atmosphere. The effectiveness of the NEA for NOx emissions reduction is also related to the composition of the NEA. Only slight enrichment is needed and NEA compositions in the 80% to 82 % nitrogen range prove to be very effective in NOx reduction.
Developments of the NEA technology have progressed beyond the laboratory engine scale. NEA is now being studied on a number of commercial engine platforms with good success. NOx emission reductions as high as 50% are being achieved on diesel engines supplied with membrane generated NEA. Results from some of these tests are presented.
What Primary methods can be adopted to reduce NOx emissions? Manufacturers have explored many different methods of reducing NOx by primary control methods. Some of the more publicised are listed as follows:
? Delaying fuel injection. Retarded fuel injection timing retards the combustion process. Nitric oxide (NO) formation occurs later and with lower concentrations.
? Reducing the amount of scavenge air, hence reducing the quantity of excess O2 available for conversion to NOx.
? Common Rail control - Common Rail fuel injection has proven to be a very effective way in combating smoke problems as well as a NOx reduction technique. There are two main ingredients, one being the freedom to choose injection pressure and timing totally independently of engine load, the other adding an element of computerised control making it possible to consider several engine parameters and then automatically optimise the injection and therefore combustion in each load situation ? Injecting water into the combustion chamber (Direct Water Injection, DWI) Greater heat capacity is utilised to reduce high peak temperatures as the water evaporates immediately upon injection. Rapid evaporation of the injected water also helps to create a homogeneous fuel-air mixture.
? Emulsified Fuel or Fuel-Water Emulsions (FEW) is favoured by some manufacturers claiming clear reduction in NOx emissions at low cost with no significant design changes and with no adverse effect on the reliability of the engine. Other manufacturers claim that fuelwater emulsions in a conventional injection system causes considerable problems.
? Injection of very fine water mist after the turbocharger using special nozzles (Combustion Air Saturation System, CASS). The fine water droplets evaporate fast and further heat is introduced in the air cooler (now acting as an air heater) and humidifies the combustion air.
? Re-circulating part of the exhaust gas (EGR) - this is one method of adding dilutants to the intake air, reducing burned gas temperature for any given mass of fuel and oxygen. It has been successfully employed in the automotive industry where good quality fuel is used but marine diesel engine manufacturers claim that even when the fuel has insignificant amounts of sulphur, the practical application of EGR causes unacceptable operational problems.
? Water Cooled Rest Gas (WaCoReG) - Developed for slow speed engines, this system utilises the same mechanism as an EGR system, i.e. introducing some 'rest gas' into the combustion space. In an engine with 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.
? Humid Air Motor (HAM). Hot compressed air from the turbocharger is led to a humidification tower and exposed to a large surface area and flushed with hot water. The water can be heated by a heat exchanger connected to the jacket cooling system or using an exhaust gas boiler. The principle is the same as that described under Combustion Air Saturation System, CASS previously. One manufacturer claims considerable success in service in reducing NOx emissions with the added claim of increasing the indicated power of the engine at certain loads therefore reducing fuel consumption hence proportionally reducing CO2 emissions The actual degree of NOx reduction varies from 10% to over 60%, depending on the engine type and which of the above reduction methods are adopted.
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