Effect of Ammonia on Laminar Combustion Characteristics of Methane-Air Flames at Elevated Pressures

In this paper, laminar combustion characteristics of methane/ammonia/air flames are numerically investigated using the Chemkin/Premix code. The initial temperature is set as 298 K; the initial pressures are set as 1, 2, 5, 10, and 20 atm; and the equivalence ratios are set as 0.8-1.6. Laminar burning velocity (LBV); adiabatic flame temperature (AFT); net heat release rate (NHRR); and the mole fractions of H, NH₂, NO, NO₂, and HCN at stoichiometric ratio are studied with ammonia (NH₃) addition. Meanwhile, temperature sensitivity and rate of production (ROP) are analyzed. The results show that with the increase of the initial pressures, LBV decreases and AFT and NHRR increase. With the increase of ammonia doping ratios, LBV, AFT, and NHRR decrease. From temperature sensitivity analyses, the main reactions that promote temperature rise are R39 (H + O₂ < = > O + OH), R100 (OH + CH₃ < = > CH₂(S) + H₂O), R102 (OH + CO < = > H + CO₂), and R122 (HO₂ + CH₃ < = > OH + CH₃O). The main reactions that inhibit temperature rise are R53 (H + CH₃(+M) < = > CH₄(+M)), R36 (H + O₂ + H₂O < = > HO₂ + H₂O), and R46 (H + HO₂ < = > O₂ + H₂). For the rate of production of the free radical pool, the trends of H and NO are consuming first and then producing, and the trends of NH₂, NO₂, and HCN are the opposite. The pathway from methane to carbon dioxide is CH₄ → CH₃ → CH₃O → CH₂O → HCO → CO → CO₂, and the pathway from ammonia to nitrogen is NH₃ → NH₂ → NH/HNO → NO/NO₂ → N₂.