NO formation mechanism of CH4/NH3 jet flames in hot co-flow under MILD-oxy condition: Effects of co-flow CO2 and H2O

Abstract

MILD-oxy combustion is an advanced technology for achieving carbon capture and low NOx emissions simultaneously. In its combustion process, the atmosphere comprises CO2 and H2O due to intense flue gas recirculation. This work numerically investigates the chemical effects of co-flow CO2 and H2O on the homogeneous fuel-NO formation mechanism of CH4/NH3 jet flames in hot co-flow under MILD-oxy condition. The results reveal that compared with H2O dilution, CO2 dilution alters the radical pool and thus lowers the overall oxidation rate, which favors achieving the MILD regime with a notably enlarged reaction zone. The increase in CO2 and H2O contents facilitates the formation of NO, wherein NH3 conversion into NO primarily occurs via four routes with HNO as the major N-containing intermediate. The addition of CO2 significantly promotes NH2 + O ↔ HNO + H and NH + CO2 ↔ HNO + CO, which enhances the oxidation of NH3 through the routes NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO. The enhancement of H2O concentration accelerates NH + OH ↔ HNO + H and NH + OH ↔ N + H2O through the formation of abundant OH radical. The conversion of NH radical to HNO and N radical is consequently facilitated and strengthens the routes NH3 → NH2 → NH → HNO → NO and NH3 → NH2 → NH → N → NO.