•Different combustion characteristics in two coal-fired boilers of same design were assessed.•CFD for individual windboxes determined the air flow distribution between burners.•Swirl strength of burners had a linear relationship with the gap of air register vanes.•Predicted temperature and heat transfer rates reasonably agreed well with the operating data.•Detailed data for heat flux and flow path can be used for combustion optimization. In a large coal-fired power plant, it is common for the multiple boilers to be of an identical design in order to reduce construction and operating costs. Despite the design and operating conditions being the same, the boilers often exhibit different combustion characteristics after being in operation for an extended period. Using computational fluid dynamics (CFD), this study diagnosed the combustion in two identical coal-fired boilers with a capacity of 500 MWe, but which exhibited different trends in their temperature and heat distribution. Based on a survey on the detailed operation parameters, individual windboxes with different damper settings were analyzed to determine the air flow distribution between the burners and the overfire air ports. Then, the swirl burners were investigated in an attempt to identify a correlation with the swirl strength for a range of vane angles and air flow rates. The results were introduced into the CFD simulations for the boilers, for which we adopted advanced models for the coal devolatilization and char conversion. The predicted temperatures and heat transfer rates were in reasonable agreement with the measured data. The main reason for the higher gas temperatures and heat transfer rates in boiler A was found to be the higher air flow rate in the burner zone. Individual burners produced different gas flow paths and, therefore, the low-O2 pockets downstream could be traced to particular burners. Those burners with narrower vane gaps produced stronger swirls in the secondary air, which led to wider-angled flames closer to the burners, together with strong internal recirculation zones. This increased the heat flux on the adjacent wall. This study shows that CFD simulations for existing boilers require careful consideration of the operating parameters that may lead to significant differences in the combustion characteristics.