• Oxy-fuel combustion of CH4 and acid gas in a diffusion flame was experimentally characterized, revealing three distinct reaction zones: initial reaction, oxidation, and complex reaction sections.
• Competitive oxidation between CH4 and H2S occurs, with H2S preferentially oxidized, leading to the formation of SO2, H2, and CO under varying equivalence ratios and CH4/AG ratios.
• Lower equivalence ratios and higher CH4/AG ratios enhance temperature, oxidation of H2S and CH4, and CO2 reactivity, promoting conversion to H2, CO, and SO2.
• The formation of COS and CS2 is influenced by interactions of CH4 and CO2 with sulfur species; COS dominates over CS2 due to inhibition by H2, and both can be further oxidized or consumed in complex reactions.