To reveal the combustion mechanism of coal mine methane with different oxygen concentration, a series of combustion experiments on the CH₄/O₂/N₂ blends were conducted at a cylindrical chamber. The influences of oxygen concentration on the flame structure, combustion pressure and flame instability of methane blends with different concentrations were analyzed. The results show that the reducing O₂ concentration deepens the depth of spherical flame inversion due to the enhancement of buoyancy effect, which contributes additionally to the suppression of combustion pressure by reducing the quenching expansion waves. The increasing O₂ concentration enhances the intensity of Darrieus-Landau instability, and reducing the critical conditions for flame front impacted by the buoyancy effect. Since the wavelength of spherical flame inversion is greater than the critical wavelength, which implies that the inversion will continue to grow until Thermal-Diffusion stability becomes significant. Moreover, the dimensionless maximum combustion pressure pmax/(ρb∙Sl2)  and flame thickness R/lf  exist a tight correlation, which indicates that the combustion pressures of CH₄/O₂/N₂ blends largely depends on flame thickness. A linear relationship of Di  vs. R/lf  was established, but the inverted spherical flame deviates from this linear prediction due to the weakening of the quenching expansion wave. According to the chemical pathway analysis, the O₂-consuming and producing the reaction pathways are mainly dominated by CH₃O →  CH₂O/HO₂ and HO₂ →  H₂O₂/O₂, respectively. Also, the O₂ consumed by the pathways CH₃O P-O2  HO₂/CH₂O is obtained from HO₂ →  H₂O₂/O₂, which is extremely sensitive to O₂ concentration, and causes the deflagration index Di  of CH4/O2/N2 blend is more sensitive to the O₂ concentration at the higher CH₄ concentration.
 

Coal mine methane; Spherical flame; Combustion pressure; Flame instability; Chemical kinetics