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

Abstract

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+HO2NO2+OH and NO2+HNO+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+COHNCO+H) to provide H radicals for achieving the catalytic cycle, while CH4 faces the competition between CH3 and H radicals for NO2 (CH3+NO2CH3O+NO and NO2+HNO+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+HO2NH3+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 NHi (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 NHi. 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.