Chemical effects of hydrogen addition on low-temperature oxidation of premixed laminar methane/air flames

Abstract

Hydrogen addition exhibits its chemical incentive on hydrocarbon flames via its reactivity in elementary reactions and the heat enthalpy release in self-oxidation. In the study, a one-step reaction kinetics was provided to fictitious hydrogen addition for separating the hydrogen reactivity effect on radical formation from the heat release effect on flame temperature change. The numerical investigation was conducted by adding the fictitious hydrogen into premixed flame modeling. The structures of the flame preheat zone were compared in terms of the profiles of flame temperature, the build-up of active radical pool and the consumption rates of methane and hydrogen addition. Comparative analyses showed that the reactivity and the heat release of hydrogen addition could both lead to the early boosting of flame oxidation, but showed different dependences on hydrogen enrichment rate. Reaction path analyses were conducted to explore the kinetic reasons for the above observations. The results suggested that OH + H2 = H + H2O was an apparent reaction route to show the reactivity of hydrogen addition, whereas the enhanced triangular reaction loops between OH, H and HO2 were the key factors for strengthening the low-temperature oxidation of hydrogen-enriched methane/air flames. Besides, the effects of the heat release and the reactivity of hydrogen addition on the early build-up of flame kernel were quantified.