One of the most promising technologies for carbon capture and storage (CCS) is oxy-fuel combustion. This study uses a commercial computational fluid dynamics (CFD) code to simulate the firing of coal and biomass under air and oxy-fuel conditions in an existing full-scale 500 MWe coal-fired utility boiler. Results are presented for conventional air-coal combustion that corresponds well against available experimental data and an in-house empirical model. Maintaining the same thermal input and exit oxygen concentration, CFD was used as a predictive tool with standard physical submodels, to examine the effects of firing under air-biomass, oxy-coal and oxy-biomass conditions. The oxy-fuel conditions were investigated at oxygen concentrations of 25% and 30% by volume for a wet flue gas recycle. The effects of firing biomass in both air and oxy-fuel conditions are predicted to have a lower total heat transfer to the tube walls, with a lower furnace exit temperature in the boiler than the coal-fired cases. This may be attributed to the effects of large biomass particles, which have a lower total surface area and therefore causes a reduction in the radiative heat transfer to the tube walls as well as an increase in carbon in ash (CIA) predictions. For oxy-coal firing, the study suggested that the optimum oxygen concentration for heat transfer to be closely matched with air-coal, lies between 25% and 30%, but for oxy-biomass firing a value greater than 30% may be needed. This study highlights the possible impact of changing the fuel and combustion atmosphere on the heat transfer characteristics of an existing power plant boiler, underlining that minor redesign may be necessary when converting to biomass firing under air and oxy-fuel conditions. (C) 2013 Elsevier Ltd. All rights reserved.