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 NO X -emission concentrations of methane/air can contribute to achieving efficient and clean natural-gas combustion. This study quantitatively analyzes the impact of N 2 and CO 2 as dilution gases on the laminar combustion characteristics and NO X emissions of methane/air under different equivalence ratios. The virtual gases FN 2 and FCO 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 NO 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 NO X -emission concentrations of methane/air under various blending conditions are discussed. The results indicate that CO 2 exhibits a more substantial reduction in laminar flame speed, flame temperature, key radical concentrations, and NO X -emission concentrations than N 2 , primarily because of its physical effects. N 2 has minimal chemical effects, but marginally increases NO 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 NO X -concentration trends with increasing hydrogen-blending ratios are observed for the three equivalence ratios (Φ = 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 NO 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. • Effects of N 2 and CO 2 on combustion characteristics of CH 4 /air are compared. • Physical and chemical effects of N 2 and CO 2 are distinguishedd. • Coupling H 2 with dilution gases enables clean and efficient combustion of CH 4. • Optimal blending ratios of dilution gases and hydrogen are discussed. [ABSTRACT FROM AUTHOR]