Extinction characteristics of ammonia/air counterflow premixed flames at various pressures

Ammonia has promising features as a carbon-free fuel without greenhouse gas emission. However, due to its low combustion intensity, the combustion characteristics of ammonia have been rarely investigated. The design of ammonia based industrial applications requires the development of effective turbulent ammonia combustion models. Thus, a study of the flame stretch effect in ammonia/air premixed combustion is necessary. The objective of this research was to study the ammonia flame extinction stretch rate both experimentally and numerically and to investigate the effects of pressure on its extinction characteristics. Experiments were conducted using a counterflow flame burner. Numerical simulations were run on CHEMKIN-PRO, using the PREMIX opposed flow model, for various detailed chemistry mechanisms. The effects of pressure on the extinction stretch rate of ammonia/air premixed flame were compared with that of methane/air flame, and the effects of pressure on the detailed reaction paths were clarified. It was found that the extinction stretch rate of ammonia/air flame is low compared with that of methane/air flame, as could be expected from its low laminar burning velocity. However, the increase of extinction stretch rate with pressure was found to be greater in the case of ammonia/air flame. From detailed chemistry analysis, it was found that the different dependence on pressure of the reaction path of the two fuels could explain this difference. Indeed, the heat release process and flame strength are affected by a greater dependence on pressure of the reactions contributing the most to heat released in the case of methane/air flame. For methane/air flame, CH3 is consumed in the main heat releasing reactions, and they experience competition by the third body recombination, 2CH(3)+MC2H6+M at high pressure. In the case of ammonia/air flame, the heat release process is mainly related to NH3+OH reversible arrow NH2+H2O, NH2+OH reversible arrow NH+H2O and NH2+NO reversible arrow N-2+H2O, which remain preponderant when pressure increases. Thus, the decrease of characteristic reaction time with pressure was found to be greater in the case of ammonia/air flame, explaining a larger increase in extinction stretch rate when pressure increases.