DeNO x mechanism caused by thermal cracking hydrocarbons in the stratified rich zone during diesel combustion

Abstract

This study concerns an experimental and theoretical investigation of the deNO _x mechanism caused by thermal cracking hydrocarbons during diesel combustion. A total gas sampling experiment using a rapid compression machine showed that NO _x can be reduced under fuel-rich and high-swirl conditions. It was found that under these conditions, a large amount of thermal cracking hydrocarbons, including unsaturated hydrocarbons such as C₂H₄, are produced during the ignition delay period, and that stratified fuel-rich combustion regions that contain these thermal cracking hydrocarbons are distributed widely throughout the combustion chamber. During the diffusion combustion phase, the CH₄ concentration surpasses that of C₂H₄ and becomes the dominant hydrocarbon species. These thermal cracking hydrocarbons are supposed to be active in NO _x reduction chemistry. To confirm the assumption, a flow reactor experiment was conducted focusing on the thermal cracking process of diesel fuel and the NO _x reduction process. The experiment showed that when a solvent was used as fuel, light hydrocarbons similar to those observed in the rapid compression experiment are formed, and that about 60 per cent of NO _x was reduced at equivalence ratios over 2.5 and a temperature of 1500 K. In addition to the above experiments, a chemical kinetic calculation using CHEMKIN III was carried out. The calculation revealed that C₂H₄ is easily decomposed during its oxidation process, forming HCCO or CHC₂, which reacts promptly with NO and that in this reaction path, C₂H₂2 formed through the thermal cracking process of C₂H₄ is an essential species to the formation of HCCO and CH₂.