Numerical Investigation on Impacts of Diluent and Oxygen Concentration on Non-premixed MILD Oxy-Combustion of Methane

The present work numerically investigated the effects of oxidizing atmosphere (O2/N2, O2/CO2 and O2/H2O) and oxygen concentration on the non-premixed MILD oxy-combustion of methane in a 15kW combustion furnace. Specifically, the flow field, reaction field, as well as the turbulence-chemical interaction were studied. Results show that under different diluents, the flow structure and flue gas entrainment are similar, but combustion reactions vary greatly. The combustion temperature T and CO and OH concentrations under different diluents are in the following order: N2> CO2> H2O. Moreover, when N2 is used as the diluent, there exists a concentrated high temperature region (> 1800K) in the furnace, and the temperature and species concentrations rise quickly as oxygen level increases. Conversely, the distributions of temperature and species are more uniform and not sensitive to the variance of oxygen level under CO2 or H2O dilution. Furthermore, comparing to CO2 or H2O dilution, the laminar flame speed and Damköhler number (Da) in the reaction zone is much larger under N2 dilution, and they will rise abruptly as the oxygen level increases. At 30% oxygen level, the in-furnace combustion is in the traditional thin reaction zone mode. Relatively, CO2 or H2O dilution can significantly reduce the laminar flame speed, and increase the chemical reaction time, thus lower Da, and keep the distributed reaction mode, i.e., MILD combustion mode. Therefore, comparing to N2, it is more favorable to establish MILD oxy-combustion under CO2 and H2O dilutions. © 2021 Chin. Soc. for Elec. Eng.