Structure transition from oxygen-enhanced to oxy-fuel methane non-premixed flames near extinction

Abstract

As an alternative to usual combustion processes with air as oxidant, oxy-fuel or oxygen-enhanced 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. Because of the increased reactivity, most combustors are designed in non-premixed configuration.In the present work, the extinction limits of non-premixed CH4/N2/O2 flames were numerically calculated using the CHEMKIN Pro opposed flow code together with the GRI 3.0 mechanism. The inlet compositions were varied from high nitrogen dilution to pure oxy-fuel conditions. Additionally, the stoichiometric mixture fraction was varied by shifting the (inert) N2-fraction stepwise from the oxidizer to the fuel side. Therewith, the equilibrium temperature of the mixture is constant for a fixed N2-dilution.The results show, that as expected, the extinction occurs at higher strain rates with increasing the flame temperatures by reducing the N2-diluation. Additionally, the extinction limits strongly increase for a fixed N2-dilution with increasing the stoichiometric mixture fraction. The shift of the N2 from the oxidizer to the fuel side leads to a movement of the stagnation plane towards the fuel side and hence, a significant change of the temperature field and the flame structure. Thus, the radical chain branching occurs at higher temperature level, which strengthen the flame resistance against strain. The increase of the stoichiometric mixture leads to a shift of the flame towards the fuel side and crosses at high values the stagnation plane.