A computational comparison of NH₃/O₂ and CH₄/O₂ non-premixed laminar flames

The structure of non-premixed, laminar, counterflow NH₃/O₂ flames was studied and compared with the structure of CH₄/O₂ flames. A commercially available computational tool was utilized through the introduction of ammonia and methane chemistry in order to compute the flow fields of strained flames. The tool was validated by comparing with previously published results for CH₄ flames and by employing two different mechanisms for NH₃ oxidation kinetics. It was shown that NH₃ flames achieve lower maximum temperature and narrower high-temperature area compared to CH₄ flames, which was attributed to much less heat release from the NH₃ oxidization process. This is due to the fact that NH₃ oxidation proceeds through a chemical path drastically different than the one of CH₄ and is completed with the formation of N₂ as an equilibrium product, without substantial formation of nitrogen oxides. The structure of the CH₄ and NH₃ flames were compared for mildly strained flames and it was shown that, despite its much slower kinetics, ammonia can sustain near-equilibrium flames, even for relatively small values of the Damköhler number.