Chemical and radiation effects on flame extinction and NOx formation in oxy-methane combustion diluted with CO2

Abstract

Oxy-methane counterflow diffusion flames diluted with CO2 were investigated to clarify impact of radiation heat loss and chemical effects of additional CO2 to oxidizer stream on flame extinction and NOx formation caused by air infiltration. Flame stability maps were presented with a functional dependency of critical diluents mole fraction upon global strain rate at various oxidizer stream temperatures in CH4–O2/N2, CH4–O2/CO2, and CH4–O2/CO2/N2 counterflow diffusion flames. The effects of radiation heat loss on the critical diluent mole fractions for flame extinction are not significant even at low strain rate in CH4–O2/N2 flame whereas those are significant at low strain rate and are negligible at high strain rate (>200s−1) in CH4–O2/CO2 and CH4–O2/CO2/N2 flames. Chemical effects of additional CO2 to oxidizer stream on the critical diluent mole fractions for flame extinction were appreciable in CH4–O2/CO2 and CH4–O2/CO2/N2 flames. A scaling analysis based on asymptotic solution of stretched flame extinction was applied. A specific radical index, which could reflect the OH population in main reaction zone via controlling the mixture composition in the oxidizer stream, was identified to quantify the chemical kinetic contribution to flame extinction. A good correlation of predicted extinction limits to those calculated numerically were obtained via the ratio between radical indices and oxidizer Lewis numbers for the target and baseline flames. This offered an effective approach to estimate extinction strain rate of oxy-methane diffusion flames permitting air infiltration when the baseline flame was taken to CH4–O2/N2 flame. Further study was conducted to investigate NOx formation in air infiltration environments. NOx formation could be suppressed by radiation heat loss and chemical effects of additional CO2 in oxy-methane combustion permitting air infiltration. The effect of air infiltration on NOx emission is addressed, and chemical effects of CO2 on NOx emission are discussed.