An advanced zonal computational method has been used to determine the temperature and heat flux profiles and other thermal characteristics for a furnace of the 500 MW boiler fired with high-ash, medium-volatile Ekibastuz coal. The predicted temperature profiles for a 100% boiler load and different methods of fuel distribution through the burner tiers were compared with respective data obtained in experimental tests. The effects of the fuel dust fineness and excess air on the furnace temperature pattern were studied. The cases associated with the reduced boiler loads were considered as well. The lower limit for the boiler load has been established, based on the predicted results.