An experimental and kinetic modeling study on the low and intermediate temperatures oxidation of NH₃/O₂/Ar, NH₃/H₂/O₂/Ar, NH₃/CO/O₂/Ar, and NH₃/CH₄/O₂/Ar mixtures in a jet-stirred reactor

The oxidation of neat NH₃ and co-oxidation of NH₃ with H₂/CO/CH₄ are investigated both experimentally and numerically. Experiments were carried out in a fused silica jet-stirred reactor with a fixed residence time of 1.5 s. Oxidation products of NH₃/O₂/Ar, NH₃/H₂/O₂/Ar, NH₃/CO/O₂/Ar, and NH₃/CH₄/O₂/Ar mixtures were measured at various temperatures from 950 K to 1400 K, and equivalence ratios of 0.5∼2.0. A detailed kinetic model was developed and validated by the present experimental data and literature data. The experimental results indicate that NH₃ reactivity is promoted by H₂ addition below 950 K and by CO/CH₄ addition below 1175 K. Kinetic analysis shows that HO₂ radical is an important intermediate to trigger the oxidation of NH₃. HO₂ radicals can be converted into OH radicals through the catalytic cycle reactions of NO+HO₂[dbnd]NO₂+OH and NO₂+H[dbnd]NO+OH, realizing the consumption of NH₃. Then NH₂ is oxidized by HO₂ radicals and NO₂ through the pathway of NH₂→H₂NO→HNO→NO→NO₂ for producing more active radicals. H₂ can provide a large amount of HO₂ and H radicals, and CO relies on the interaction of NH₂ and CO (NH₂+CO[dbnd]HNCO+H) to provide H radicals for achieving the catalytic cycle, while CH₄ faces the competition between CH₃ and H radicals for NO₂ (CH₃+NO₂[dbnd]CH₃O+NO and NO₂+H[dbnd]NO+OH), resulting in the ability of H₂ to lower the NH₃ reactivity onset much stronger than that of CO and CH₄. With the increase in temperature, the HO₂ radicals tend to inhibit the oxidation of NH₃ by the chain-termination reaction of NH₂+HO₂[dbnd]NH₃+O₂. The oxidation of NH₃ is gradually dominated by the independent interaction of fuels and H/OH/O radicals. H/OH/O radicals promote the conversion of NH_i (i = 0, 1, 2) to NO. As a result of the stronger ability to carry H atoms, the most active NO chemistry is found in the co-oxidation of NH₃/CH₄, while the least active NO chemistry is discovered in the oxidation of neat NH₃ for the strong reduction effect of NH_i. In stoichiometric conditions, the formation of NO mainly depends on NH radicals, and the formation and reduction of NO by N and NH₂ radicals are almost equivalent. In lean conditions, as the importance of NH radicals decrease, the production of NO is mainly dependent on the conversion of NH₂ to NO through HNO intermediate.