The combination of the advanced ultra-supercritical (AUSC) and oxy-fuel combustion technologies is considered the most feasible and promising choice for carbon capture and storage. The high efficiency of AUSC boilers compensates for the cost incurred in controlling CO2 emissions. In this study, a new improved model was proposed for gas-radiative properties as a polynomial function of the continuous molar ratio of H2O to CO2 and temperature, which was compiled as user-defined functions in C language. The modified radiative model was employed in the computational fluid dynamics analysis to simulate a tangentially fired 700 degrees C-USC boiler with a capacity of 300 MW under the oxy-fuel condition. The results show that the improved model has a good accuracy for gas-radiative properties compared to other classical models. In the oxy-fuel cases, the heat transfer obtained in the simulation is approximately 4-11% lower than that in the thermodynamic calculation because of the char gasification. The radiative capability of the flue gases in the oxy-fuel case is much greater than that in the air-fuel case with the increase in the molar fraction of oxygen. The heat transfer via the water wall is approximately 4-8% higher than that via air, when the molar fraction of oxygen increases from 26% to 29%. The increase in the heat absorption helps in better arrangement of the heat-exchanger surfaces in the furnace of the AUSC boiler. (C) 2017 Elsevier Ltd. All rights reserved.