In order to explore the differences in the radiation heat transfer characteristics between oxy-fired and airfired boilers

and to provide guidance to the design of new oxy-fired boilers or retrofit of existing air-fired boilers

a numerical study was conducted to investigate the boiler flow and heat transfer distributions under different combustion modes. The necessity of using the improved weighted-sum-gray-gas(WSGG) model to calculate the gas absorption coefficient of oxy-fired boilers is discussed at first. Then

the Johansson WSGG model is incorporated into the CFD model of coal-fired boilers so that the model is suitable for the simulation of radiation heat transfer in oxy-fired boilers. On this basis

the flow and heat transfer distributions of a 330 MW subcritical unit oxy-fired boiler with dry and wet flue gas recirculation(FGR) are numerically studied and compared with the air combustion mode. The results show that

under conditions with the same coal

inlet gas and oxygen mass flow rates

the differences in the composition and physical properties of flue gas lead to remarkable differences in the flow

temperature and boiler heat transfer distributions between different combustion modes. Specifically

the specific heat of CO_2 and H_2O that are rich in oxy-fired flue gas is higher than that of N_2

leading to considerably lower overall temperature and heat absorption of oxy-fired boiler with wet FGR than those of air-fired boiler. Moreover

since the density of CO_2 is higher than N_2

the overall flow velocity in oxy-fired boiler is lower than that in air-fired boiler

which subsequently affects the boiler temperature and heat transfer distributions. The differences in the flow field and heat transfer between oxy-fired and air-fired boilers should be taken into consideration when designing new oxy-combustion systems or retrofitting existing air combustion systems to ensure proper boiler heat transfer distribution and to minimize modifications to the boiler's heating surfaces.