Oxy-fuel combustion is considered to be a promising technology for the abatement of CO2. To study the chemical effects of CO2 on the combustion temperature, we have adopted counter-flow jets of methane versus heated air and an O-2/CO2 mixture within a wide range of O-2/CO2 volume ratios. An artificial material X, which is defined as such that it has exactly the same thermochemical, transport, and radiation properties as the real CO2 and is chemically inert, was adopted to separate the physical and chemical effects of CO2. The comparisons among air combustion, the oxy-X combustion and oxy-fuel combustion show that the chemical effects of CO2 reduce the combustion temperature in oxy-fuel combustion, and they are comparable to the physical effects of CO2 in an atmosphere of 36% O-2/64% CO2. The heat release rates of R99(OH + CO double left right arrow H + CO2) and R153(CH2(s)+ CO2 CO + CH2O) are the most heavily impacted by CO2 in the range of the mole fraction of O-2 from 20% to 28%. When the mole fraction of O-2 increases from 28% to 32%, the effects of CO2 on elementary reactions offset each other. The heat release rates of R84(OH + H-2 double left right arrow H + H2O) and R38(H + O-2 double left right arrow O + OH) are the most heavily influenced by CO2 in the mole fraction of O-2 from 32% to 36%. The heat release rates of R287(OH + HO2 double left right arrow O-2 + H2O) and R84 are significantly impacted by CO2 in the range of the mole fraction of O-2 from 36% to 50%. The key reasons to the combustion temperature during oxy-fuel combustion are that R38 and R35(H + O-2 + H2O double left right arrow HO2 + H2O) in the mole fraction of O-2 from 20% to 24% and from 26% to 50%, respectively. (C) 2015 Elsevier Ltd. All rights reserved.