DeNOx 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.