Characteristics of stoichiometric CH₄/O₂/CO₂ flame up to the pure oxygen condition

To pursue high efficiency in oxy-fuel combustion, usually a stoichiometric mixture with high oxygen content is preferred. However, the oxygen level in industrial operation is restricted by potential hazards of flame flash back, equipment erosion and combustion instability. In this study the stoichiometric CH₄/O₂/CO₂ mixtures are tested in a rapidly mixed tubular flame combustor by varying the oxygen content from 0.21 to 1.0. Based on previous studies, the combustor is optimized to successfully achieve the steady tubular combustion up to the pure oxygen condition; the adiabatic flame temperature can vary between 1750 and 3050 K; in the CH₄/O₂ tubular flame, the overall mole fraction of active radicals can reach 0.20, and the net heat release rate is much higher than that of CH₄/air flame. With three additional burners, this study systematically evaluates the influential factors such as oxygen content, inlet size, burner diameter and flow rate on the flame characteristics; furthermore, the dimensionless Damköhler number and Reynolds number are adopted to quantitatively identify different burning regimes through the Borghi diagram; the results provide a useful guide to the operation of oxy-fuel combustion up to the pure oxygen condition.