Dilution Effects on Flammability Limits and Nox Emission of Opposed-Jet Syngas Diffusion Flames

Due to the shortage of hydrocarbon fuels and the increasing concerns of greenhouse gas emissions, this research aims to investigate the dilution effects on the combustion, flammability limits and NOx emissions of opposed-jet syngas diffusion flames. The flammability limits are constructed using the strain rate, syngas composition, and dilution gas percentage as the parameters. The effects of the dilution gases on the flame structures, chemical reactions, and extinction boundaries are studied theoretically with a computational model. The roles of individual dilution effects, such as the inert effect, thermal effect, chemical effect, and radiation effect of diluents on the combustion and extinction of syngas diffusion flames are also analyzed. The characteristics of NOx emission of the opposed-jet syngas diffusion flames with dilution gases are also investigated. The NOx production rate and NOx formation mechanisms are analyzed with strain rates and diluents percentages as the parameters. The dilution effects on NOx emission and NOx formation pathways are compared. Results show that, at a low strain rate, the flame temperature is lower and the dilution percentage for flame extinction is higher with CO2 and H2O as the dilution gases. However, at a high strain rate, the flame temperature of H2O-diluted syngas flame is the highest and its dilution percentage for flame extinction is then close to that of N2-diluted flame, because the chemical effects of H2O are more conspicuous. The inversely U-shaped extinction boundaries constructed with strain rates and dilution gas percentages exhibit a cross over between CO2 and H2O-diluted syngas flames. It is attributed to the radiation effect from H2O dilution at low strain rate and thermal/diffusion effect from CO2 at high strain rate. The dilution effects on NO production rates indicate, for CO-rich syngas flame, the NO production rate for CO2-dilution is slightly higher than that with H2O-dilution. It is because the H2O-diluted syngas reduces the reaction rate of NNH+O→NH+NO, which causes a significant decrease in NO production from NNH route, despite a higher flame temperature and thermal NO production.

Project ID:PB10308-4321
External Project ID:MOST103-2221-E182-028