Super-adiabatic flame temperatures in premixed methane flames: A comparison between oxy-fuel and conventional air combustion

Abstract

As an alternative to usual combustion processes with air as oxidant, oxy-fuel processes are attractive in high temperature thermal or thermochemical processes, novel power plant concepts or in gasification processes. Especially for production of basic chemicals or synthetic fuels, high-purity synthetic gas without nitrogen dilution is required for further industrial processing.In case of oxy-fuel combustion, the absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. In the present work, freely propagating methane-oxygen-nitrogen flames were numerically calculated using a 1D model from lean to rich conditions in order to investigate the appearance of super-adiabatic flame temperatures (SAFT). The calculations were performed for equivalence ratios of 0.5<Φ<3.0 with an increment of 0.1. Additionally, the nitrogen content of the oxidizer was varied from air to pure oxy-fuel conditions. Furthermore, the influence of preheat temperature and pressure on SAFT were analyzed for oxy-fuel flames.The SAFT phenomenon was identified based on temperature and species profiles. Additionally, a detailed analysis of convection, diffusion and chemical source of both, temperature and species, were performed in order to investigate the role of physical transport processes on SAFT.The results showed that the maximum flame temperature exceeds the equilibrium temperature for equivalence ratios Φ>0.9. Two different regimes were identified, where SAFT phenomenon appears. The first regime was found in slightly rich conditions (1.0<Φ<2.0), whereas the second regime occurred in ultra-rich regime (Φ>2.0). A first maximum of temperature difference is observed at an equivalence ratio of Φ=1.5. Here, an exaggeration of approximately 120–180K, depending on the applied reaction mechanism, was found for standard conditions. The first maximum at Φ=1.5 correlates with the maximum concentration of the H-radical, which plays a key role in the first SAFT regime. A minimum temperature difference of 50K was identified at an equivalence ratio of Φ=2.1. While increasing the equivalence ratio further, the maximum flame temperature exceeded the equilibrium up to almost 400K at Φ=3.0 in the second SAFT regime.An increased preheating temperature enhanced the occurrence of SAFT in the first regime and degraded it in the second regime. Elevated pressure leads to the opposite effects with decreased SAFT in the first and increased SAFT in the second regime.