Synergistic effects of dilution gases and hydrogen on methane/air laminar combustion characteristics and NOX--emission concentrations

Studying the synergistic effects of dilution gases and hydrogen on the laminar combustion characteristics and NOX-emission X-emission concentrations of methane/air can contribute to achieving efficient and clean natural-gas combustion. This study quantitatively analyzes the impact of N2 2 and CO2 2 as dilution gases on the laminar combustion characteristics and NOX X emissions of methane/air under different equivalence ratios. The virtual gases FN2 2 and FCO2 2 are introduced to distinguish between the physical and chemical effects of the dilution gases. Subsequently, the effect of hydrogen addition on the laminar combustion characteristics and NOX X emissions of a methane/5% dilution gas/air mixture is investigated. Finally, the synergistic effects of the dilution gases and hydrogen on the laminar flame speed and NOX-emission X-emission concentrations of methane/air under various blending conditions are discussed. The results indicate that CO2 2 exhibits a more substantial reduction in laminar flame speed, flame temperature, key radical concentrations, and NOX-emission X-emission concentrations than N2, 2 , primarily because of its physical effects. N2 2 has minimal chemical effects, but marginally increases NOX X emissions. The addition of hydrogen increases the laminar flame speed and key radical concentrations of the methane/dilution gas/air mixture. however, significant differences in the NOX-concentration X-concentration trends with increasing hydrogen-blending ratios are observed for the three equivalence ratios (Phi = 0.7,1.0,1.4). Selecting appropriate blending ratios of dilution gases and hydrogen can simultaneously enhance the laminar flame speed of methane/air while reducing the NOX-emission X-emission concentrations. This study provides valuable insights into the optimization of the blending ratio of hydrogen-enriched natural-gas engines coupled with exhaust-gas recirculation.