Bluff-body MILD combustion regime for semicoke and bituminous coal mixtures with effect of oxidizer O2 concentration in O2/N2 and oxy-fuel (O2/CO2) atmospheres

•Homo-/heterogeneous Damkhöler numbers are adopted to characterize the MILD regime.•A low O2 content and a CO2 environment promote the homo-MILD regime while hetero-MILD regime is inhibited.•An ideal hetero-MILD combustion can be realized by increasing O2 content.•Reduced effect of char-O2 reaction share and promoted effect of char-CO2 reaction share with increasing XO2 are weakened.•The surface behavior of char and NOx emission are affected by XO2 and CO2 dilution.