Partially Premixed Combustion in a Low Load Operated Power Plant Engine
Authored by: K. Shrestha, O. Kaario, M. Imperato2, T. Sarjovaara, M. Larmi, L. Liavåg, C. Wik
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Stationary power generation is part of the mandate of Industrial Combustion, be it large coal fired steam generators, gas turbines, or diesel engines. This paper presents research on a strategy for reducing the NOx and soot from diesel engine. Aspects of this technique, called Partially Premixed Combustion, were investigated both experimentally and by CFD simulation. The potential reductions of the targeted emissions are impressive.
Partially Premixed Combustion, NOx, soot, multiple injection, ignition delay, Exhaust Gas Recirculation
Diesel engines are used extensively in large-scale power plant generators due to their excellent performance and thermal efficiency. On the other hand, diesel engines are associated with numerous environmental issues such as soot and NOx emissions. In recent years, several strategies have been introduced to eradicate these environmental problems. Among them, Partially Premixed Combustion (PPC) has been one of the most promising and reliable strategies. PPC is a compression-ignited combustion process in which ignition delay is controlled to enhance better homogeneity of the air-fuel mixture. In PPC, ignition delay control is typically achieved with Exhaust Gas Recirculation (EGR), resulting in both low NOx and soot emissions.
A single-cylinder research engine was used in the present study. The computational model was validated against experimental data for both normal diesel combustion conditions and PPC conditions. A detailed study was conducted for the effects of injection pressure and nozzle-hole size for PPC combustion at low engine load conditions. In addition, the fuel injection direction in combination with the injection timing was analysed to reveal optimum operation points for PPC combustion. Two piston top shapes were used in order to get better homogeneity of the air-fuel mixture prior to combustion and to reduce the amount of fuel vapour from the periphery of the combustion chamber. The final part of the present study implemented EGR to control the temperature for prolonged ignition delay. The results suggest that PPC combustion significantly aids in reducing emissions. It was found out that NOx was reduced by 60-80% and soot by 70-80% as a result of optimized injection settings. Furthermore, the advantage of a deeper piston bowl was taken into account in order to reduce reactions close to the periphery of the combustion chamber, which subsequently increased combustion efficiency.
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