Effects of CO2 and H2O on oxy-fuel combustion characteristics and structural evolutions of Zhundong coal pellet at fast heating rate

•A concentrating photothermal reactor at heating rate of 4800 °C/min was conducted.•Combustion characteristics in O2/N2, O2/CO2 and O2/H2O at fast heating rate were studied.•Faster consumption rates of functional groups and higher condensed char in O2/H2O were evidenced.•Second-order function relationship expressions between R and I(Gr+Vl+Vr)/ID were observed.