Stability limits of methane/oxygen mixtures diluted by n₂ and CO₂ under various oxygen contents

For safe utility of methane, including underground mining, ordinary air combustion and oxygen enhanced combustion, stability limits of methane mixtures diluted by nitrogen and carbon dioxide were experimentally investigated by using a swirl type tubular flame burner. The oxygen mole fraction (β) in the oxidizer was varied to determine the minimum β to sustain the methane combustion, and then gradually increased to 1.0 (pure oxygen). In the tests of β less than 0.4, the premixed mode combustion (PM), in which a pre-mixture was tangential injected into a cylindrical burner, was adopted to examine the flame structure and stability limits. In the case of β above 0.4, to avoid flame flash back, the rapidly mixed mode (RM) was conducted, in which the fuel and oxidizer were individually injected. The results show that RM can obtain the same stability limit as that of PM. The flammable range of N₂ diluted mixture was much larger than that of CO₂ dilution. The minimum β to sustain the combustion was 0.11 and 0.2 for N₂ and CO₂ diluted mixture, respectively. For the lean stability limit of N₂ diluted mixture, the fuel volume percentage in the overall reactants was maintained around 5% under various oxygen mole fractions; that for CO₂ diluted combustion gradually decreased from 8% to 6%. The differences between two diluents were also evaluated.