A numerical model of a 75 t/h gas‐fired boiler burning blast furnace gas (BFG) and coke oven gas (COG) was developed to study the effect of O2/CO2 conditions on the combustion characteristics, verified by comparing with the results measured in field experiments. The results showed that the ignition distance of COG was shorter than that of BFG, and COG reached the stable combustion temperature of 1000 K earlier in the condition of O2/CO2 combustion with 31% O2 than in the condition of air combustion. Under different O2/CO2 conditions, the overall temperature of gas‐fired boiler decreased. The maximum temperature of air combustion, O2/CO2 combustion with 26% O2, and O2/CO2 combustion with 31% O2 were 1867, 1678, and 1834 K, respectively. Under the condition of O2/CO2 combustion, the radiation heat transfer of mixed gas was enhanced, and the overall heat flux was obviously improved. Under the O2/CO2 condition, the overall temperature was lower than that of air combustion, but the wall heat flux increased. The NOx production decreased after the change to O2/CO2 combustion, and the NOx generation at the furnace arch decreased from 70.28 mg/m3 to 6.94 mg/m3 under the condition of O2/CO2 combustion with 26% O2. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.