Experimental investigation of methane and nitric oxide interactions behind reflected shock waves

Hydrocarbon fuels remain a dominant source of energy, and their combustion in air generates nitrogen oxides (NO_x). In combustion systems employing exhaust gas recirculation, NO_x can directly interact with hydrocarbon fuels, thereby altering their combustion properties. This study investigated CH₄/NO interactions with NO as the sole oxidizer under high-temperature conditions. Shock-tube experiments were performed on CH₄/NO mixtures at near-atmospheric pressure across a temperature range of 1808–2878 K. Three Ar-diluted mixtures containing 0.5% CH₄ with NO concentrations of 0.1%, 0.3%, and 0.5% were studied. During the experiments, CO time histories were measured using a laser absorption diagnostic and compared with predictions from recent kinetic mechanisms, revealing significant discrepancies. A characteristic time was derived from the time histories to assess the performance of the mechanisms over the entire temperature range. No kinetic mechanism could reproduce the experimental data under all conditions. Two mechanisms demonstrating complementary performances in different temperature ranges were selected for a kinetic analysis. Reaction pathway analyses revealed discrepancies between the two mechanisms in the major pathways of CO formation. Furthermore, sensitivity analyses revealed that CH₃ + NO ⇄ HCN + H₂O and CH₃ + NO ⇄ H₂CN + OH were the most influential reactions in CO formation across mixtures with different NO concentrations. The rate coefficients of these reactions were modified in one mechanism using literature values to improve agreement with the present experimental data. Although the modification improved the CO predictions, comprehensive validation against other experimental data is required before adoption in future mechanisms. This study presents the first experimental investigation of CH₄ reacting directly with pure NO using a shock tube, and the reported CO time histories serve as validation benchmarks for future kinetic modeling efforts.