Reduction of Nitrogen Oxides by Means of Highly Turbulent Combustion and Modification of Air Composition

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This study aims at obtaining suggestions for reducing NOx and clarifying the marginal limit of the reduction in the heterogeneous combustion process in practical burners and power systems. The contents of the present study are summarized as follows :1.Production and destruction of NOx in the dilution process of fuel-rich burnt gases are investigated based on detailed chemical kinetics. It was shown that fast mixing due to strong turbulence enables a great deal reduction in NOx formation, and that NOx concentration can be halved if the temperature decreases with the volume expansion in a high-speed piston-engine motion prior to the heat release due to dilution.2.A high swirl burner was tested to investigate the mechanism of the flame stability and the NOx formation. The obtained results showed that a diffusion flame having the same low-NOx level with that in a premixed flame would be formed under a higher swirl number and a stoichiometric fuel-air ratio. Furthermore, the NOx reduction by means of burnt-gas addition in the diffusion combustion was investigated based on a detailed chemical kinetics. It was shown that the burnt-gas temperature should be reduced during recirculation process to reduce NOx, and that a lower temperature can reduce NOx a great deal.3.Exhaust-water-selective recirculation, which operates as a closed system without a supplementary water charge, was proposed for reducing NOx in diesel engine exhaust. The feasibility of the system was experimentally assessed on a single-cylinder test engine. In addition, the effects of intake-air dilution with N2 and CO2 and EGR on the exhaust emissions were predicted and discussed based on a stochastic model. The predicted results showed that cold EGR does not much affect the average temperature, but diminishes gas pockets having a higher-temperature greater than 2,500K,resulting in a great reduction of NO formation.