Over the past years we have gained experience in employing varying types of coal fired in the same boiler. Consequently, we developed evaluation criteria regarding the operation of these coals. In the present study we evaluated fouling propensity of two bituminous South African coals Billiton-Prime and Anglo-Kromdraai (AKD). From an experimental work carried out in a 50 kW test facility where we obtained emissivity and thermal resistance of the two coal ashes. We also developed a new method for evaluating fouling in full-scale utility boilers that is based on adhering of an ash particle depending on its temperature and velocity vector at the boundary layer of the water-walls. The flow and temperature fields were determined by computational fluid dynamic (CFD) simulations. We used the emissivity and thermal resistance determined from the test furnace in the CFD calculations to evaluate fouling in the combustion chamber of a 575 MW tangential-fired utility boiler when the two bituminous coals were fired. Mapping of the fouling locations that are mostly to occur were determined and this behavior fit the experience from the power station and data obtained for these two coals.