No abstract

9Nitrogen dioxide (NO 2 ) is an important impurity in coal-bed methane (CBM) and a dominant 10 component of NO x pollution in practical engines. Its promoting effect on methane ignition has been 11 studied in the current experimental and kinetic study. Ignition delay times of NO 2 /CH 4 /O 2 /Ar 12 mixtures, with blending ratios of NO 2 :CH 4 of 30:70, 50:50 and 70:30 for stoichiometric methane 13 mixtures were measured in a shock tube. Experiments cover a range of pressures (1.2 -10.0 atm) 14 and temperatures (933 -1961 K). Under all tested pressures, NO 2 addition promotes the reactivity 15 of methane and reduces the global activation energy at all pressures, and these effects are most 16 significant for the mixtures with highest NO 2 concentrations, at the highest pressures and at the 17 lowest temperatures. To simulate the experimental measurements, five literature NO x sub-18 mechanisms were integrated with AramcoMech 1.3. The simulations demonstrate that, for the 19 mixtures with low levels of NO x concentrations , the five models agree well with the experimental 20 ignition delay times. For the mixtures with high NO x content, however, all five models are unable to 21 reproduce the measured data, and the level of disagreement increases with increasing NO 2 22concentration. An updated mechanism is proposed, based on modifications made as a result of 23 2 sensitivity and reaction flux analyses performed to quantitatively determine the chemical reasons 24 for NO 2 promoting methane ignition. The results indicate that, NO 2 addition perturbs the branching 25 ratio of key reaction pathways by affecting the structure of the free radical pool at the initial ignition 26 stage of methane oxidation. A new reaction cycle via the following sequence of reactions ĊH 3 + 27 NO 2 <=> CH 3 Ȯ + NO, CH 3 Ȯ + M <=> CH 2 O + Ḣ + M, NO 2 + Ḣ <=> NO + ȮH, and CH 4 + ȮH 28 <=> ĊH 3 + H 2 O is proposed to explain the observed effect of NO 2 addition on the promotion of 29 methane ignition. 30 31

Ignition delay times of N 2 O/CH 4 /O 2 /Ar mixtures with varying N 2 O mixing ratios (N 2 O/CH 4 mole blending ratio = 0:100, 30:70, 50:50, and 70:30) were measured behind reflected shock waves at pressures of 1.2−16 atm, equivalence ratios of 0.5−2.0, and temperatures of 1220−2336 K. At currently investigated conditions, the reactivity of methane is significantly promoted by N 2 O addition, resulting in an obvious reduction of the ignition delay time, and this effect becomes more pronounced at the fuel-rich condition and high pressure. However, N 2 O addition only results in a slight reduction in the global activation energy. To eliminate the effect of different hydrocarbon mechanisms, a widely accepted kinetic mechanism, Aramco Mech 1.3, is used to combine with three available NO x submodels (Gersen et al., Konnov, and Mathieu et al.) and simulate the measured ignition delay times. Generally, the three assembled models give a similar prediction performance and agree well with the experimental data for a low level of N 2 O addition, but they exhibit a discrepancy for a high level of N 2 O addition. Sensitivity analysis and radical pool analysis are conducted to interpret the kinetic effect of N 2 O addition on methane ignition chemistry. Results indicate that the promoting effect of N 2 O addition on methane ignition is mainly attributed to the contribution of the following three reactions: N

Rate coefficients of H atom abstraction and H atom addition reactions of 3-hexene by the hydroxyl radicals were determined using both conventional transition-state theory and canonical variational transition-state theory, with the potential energy surface (PES) evaluated at the CCSD(T)/CBS//BHandHLYP/6-311G(d,p) level and quantum mechanical effect corrected by the compounded methods including one-dimensional Wigner method, multidimensional zero-curvature tunneling method, and small-curvature tunneling method. Results reveal that accounting for approximate 70% of the overall H atom abstractions occur in the allylic site via both direct and indirect channels. The indirect channel containing two van der Waals prereactive complexes exhibits two times larger rate coefficient relative to the direct one. The OH addition reaction also contains two van der Waals complexes, and its submerged barrier results in a negative temperature coefficient behavior at low temperatures. In contrast, The OH addition pathway dominates only at temperatures below 450 K whereas the H atom abstraction reactions dominate overwhelmingly at temperature over 1000 K. All of the rate coefficients calculated with an uncertainty of a factor of 5 were fitted in a quasi-Arrhenius formula. Analyses on the PES, minimum reaction path and activation free Gibbs energy were also performed in this study.

No abstract