Influence of gas composition and properties on the combustion and heat transfer characteristics of pulverized oxy-coal boiler

This paper presents a numerical study on the influences of gas composition and properties on the combustion and heat transfer characteristics of pulverized oxy-coal boiler. Previous studies were primarily focused on impacts of gas mass flow rate under different flue gas recirculation (FGR) rates while the important influences of gas composition and physical properties were overlooked. The results show that the changes in the gas composition and physical properties under different combustion modes, such as density, heat capacity and gas absorption coefficient, can significantly affect the flow, temperature, and heat transfer distributions in the furnace. Specifically, due to the differences in the heat capacities of CO2, H2O and N-2, the gas heat capacity of oxy-coal combustion is considerably higher than that of air-coal combustion. As a result, oxy-coal combustion attains furnace temperature comparable to that of air-coal combustion at remarkably lower gas mass flow rate. Moreover, because of the differences in the radiation properties of CO2, H2O and N-2, the gas absorption coefficient of oxy-coal combustion is higher than that of air-coal combustion, especially at the near-wall region, which hinders the radiation heat transfer between the high temperature flame and furnace wall. Consequently, oxy-coal combustion needs higher furnace temperature at further lower gas mass flow rate to achieve matched wall heat absorption with that of air-coal combustion. Therefore, the differences in the gas composition and physical properties under different combustion modes need to be comprehensively considered in the design of oxy-coal combustion systems or retrofit of existing air-coal combustion systems.