Autoignition studies of NH3/CH4 mixtures at high pressure

Autoignition delay times of NH3/CH4 mixtures with CH4 fractions of 0%, 5%, 10% and 50% were measured in a rapid compression machine at equivalence ratio φ = 0.5, pressures from 20 to 70 bar and temperatures from 930 to 1140 K. In addition, measurements were performed for NH3 mixtures with 10% CH4 at φ = 1.0 and 2.0. Methane shows a strong ignition-enhancing effect on NH3, which levels off at higher CH4 fractions, as the ignition delay time approaches that of pure methane. Autoignition delay times at 10% CH4 at φ = 0.5 and 1.0 are indistinguishable, while an increase of ignition delay times by factor of 1.5 was observed upon increasing φ to 2.0. The experimental data were used to evaluate six NH3 oxidation mechanisms capable of simulating NH3/CH4 mixtures. The mechanism previously used by the authors shows the best performance: generally, it predicts the measured ignition delay times to better than 30% for all conditions, except for 50% CH4 addition for which the differences increase up to 50% at the highest temperature. Sensitivity analysis based on the mechanism used indicates that under lean conditions the reaction CH4 + NH2 = CH3 + NH3 significantly promotes ignition for modest CH4 addition (5% and 10%), but becomes modestly ignition-inhibiting at 50% CH4. Sensitivity and rate-of-production analyses indicate that the ignition-enhancing effect of 50% CH4 addition is closely related to the formation and decomposition of H2O2. Flux analysis for NH3/CH4 mixtures indicates that CH4 + NH2 = CH3 + NH3 contributes substantially to the decomposition of methane early in the oxidation process, while CH3 + NO2 (+M) = CH3NO2 (+M) is a significant reservoir of NO2 at low temperature. Additionally, an anomalous pre-ignition pressure rise phenomenon, which is not reproduced by the simulations, was observed with high reproducibility for the NH3 mixture with 50% CH4 addition.