Radiation effects on combustion and pollutant emissions of high-pressure opposed flow methane/air diffusion flames

Numerical simulations of one-dimensional opposed-flow laminar methane/air diffusion flames with detailed gas chemistry and global soot kinetics were conducted using the Sandia OPPDIF code to understand the interactions between radiation and chemistry. The gas-phase chemistry model was based on GRI-Mech 2.11, and Lindstedt's global soot kinetics model was used for soot formation calculations. The OPPDIF code was modified to include the effects of the radiation source term and the equations for soot mass fraction and soot particle number density based on soot kinetics. The radiation properties of the gas molecules and the soot particles, including a treatment for the self-absorption of radiation, were considered. Interactions between soot and gas-phase chemistry were considered. The effects of radiation heat loss, pressure, and injection velocity were studied for methane/air diffusion flames. The effects of radiation heat loss, thermophoresis, soot oxidation by O2 and OH radicals, and depletion of acetylene by soot formation were all found to be important in determining soot concentrations, especially for high-pressure flames. Soot oxidation by OH radical dominates soot oxidation by O2 at atmospheric pressure. However, the OH oxidation rate decreases with increasing pressure, and becomes comparable to the O2 oxidation rate, which is very low compared with the overall soot formation rate at all pressures. The neglect of radiation heat loss leads to significantly higher estimates of soot and NO emissions compared with the estimates including radiation heat loss, particularly at high pressures. The effects of radiation, including those of self-absorption are found to be significant, particularly at lower stretch rates.