Effect of CO2 on the explosion limit parameters and kinetic characteristics of ammonia-hydrogen-air mixtures

To explore the effect of CO₂ on the explosion limit parameters and kinetic characteristics of the NH₃-H₂-air mixtures, the standard combustible gases explosion limits device was utilized to test and analyze the explosion limit, critical oxygen concentration, and explosion triangle. Based on the CHEMKIN software, the suppression mechanism of CO₂ on NH₃-H₂-air mixtures explosion with 0.4 R (hydrogen ratio) and a near-inerting critical concentration was simulated and analyzed. The results indicated that increasing F (volume fraction of CO₂) led to a decrease in the upper explosion limit (UEL) of NH₃-H₂-air mixtures and an increase in the lower explosion limit (LEL). When F increased to 36%, the explosion of the NH₃-H₂-air mixtures with 0.4 R was completely suppressed. As R increased, the UEL and LEL decreased and increased linearly, respectively, the critical explosion-suppression concentration of CO₂ increased, the critical oxygen concentration decreased, and the explosive zone area also increased. In addition, based on the analysis of the changing characteristics of the UEL and LEL, prediction models for the UEL and LEL of NH₃-H₂-air mixtures with different hydrogen ratios under the effect of CO₂ were provided. Reaction kinetics analysis found that less CO₂ had a more pronounced impact on ·H and ·OH. However, when F increased to 20–30%, the maximum mole fraction of ·O decreased the most. At this point, the sensitivity of·NH₂ towards decreasing oxygen concentrations became comparable to that of ·H and ·OH. When F increased to a near-critical explosion-suppression concentration of 30%, ·H, ·OH, ·O, and ·NH₂ were in a critical state of disappearing, and the chain initiation of the explosion was destroyed. Sensitivity analysis indicated that ·H + O₂(+M)<=>HO₂(+M) and ·NH₂+·NH=N₂H₃ performed a more significant inerting effect on ·H and ·NH₂, which ultimately led to the termination of the explosive chain reactions.