In this study, a computational fluid dynamics (CFD) model was developed to study the flow and heat transfer characteristics of a 320 MW oxy-fired boiler with dry and wet flue gas recirculation (FGR). The results show that there exist significant differences in the composition and physical properties of flue gas between the air- and oxy-combustion modes and these differences may lead to remarkable differences in the flow, temperature and heat transfer distributions of boiler. Specifically, since the specific heat of CO2 and H2O are higher than that of N2, the flue gas heat capacity of the oxy-combustion case with wet FGR is significantly higher than the air-combustion case leading to lower furnace temperature and heat absorption of furnace wall. Moreover, since the density of CO2 is higher than N2, the overall flow velocity in oxy-fired boiler is lower than that of air-fired boiler, which subsequently affects the boiler flow and temperature distributions in the furnace. The differences in the flow and heat transfer distributions between oxy-fired and air-fired boilers should be taken into consideration when designing new oxy-fuel combustion systems or retrofitting existing air combustion systems to minimize costly modifications to the boilerâ€™s heating surfaces.
Keywords oxy-fuel combustion; radiation heat transfer; flue gas recirculation; coal combustion; numerical simulation