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

本研究旨在获得降低NOx的建议，并澄清实际燃烧器和电力系统中非均相燃烧过程中降低NOx的边际界限。本文的研究内容概括如下：1.基于详细的化学动力学，研究了富燃燃气稀释过程中NOx的生成和分解。结果表明，由于强湍流的快速混合可以大大减少NOx的生成，在高速活塞发动机运动中，在稀释放热之前，如果温度随着体积膨胀而降低，NOx浓度可以减半。2.对火焰稳定和NOx生成的机理进行了实验研究。结果表明，在较高的旋流数和化学计量比下，可以形成与预混火焰相同的低NOx扩散火焰。在详细化学动力学的基础上，研究了在扩散燃烧中添加燃气降低NOx的反应机理。研究表明，降低柴油机排气中NOx的方法是降低再循环过程中的燃气温度，较低的温度可以很好地降低NOx。3.提出了一种无补水的选择排水再循环降低柴油机排气中NOx的方法。在一台单缸试验发动机上对该系统的可行性进行了试验评估。此外，基于随机模型，预测和讨论了进气中添加氮气和二氧化碳的稀释以及废气再循环对尾气排放的影响。预测结果表明，冷EGR对平均温度影响不大，但减少了高于2500K的高温气穴，从而大大减少了NO的生成。

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.