Abstract: Numerical models were established for bed material concentration and heat transfer in the furnace of a 1 000 t/h circulating fluidized bed (CFB) boiler according to the gas-solid two-phase flow and heat transfer characteristics to study the variation behavior of the material concentration and heat transfer respectively in conventional air, 21%φ(O₂)/79%φ(CO₂) and 30%φ(O₂)/70%φ(CO₂) atmosphere, and subsequently the way to adjust the bed material concentration was proposed by optimizing the particle size of the bed material, thus further improving the in-furnace heat transfer effectiveness. Results show that when the combustion atmosphere is switched from air to 21%φ(O₂)/79%φ(CO₂), the material concentration distribution and heat transfer coefficient would keep almost unchanged along the height of the furnace; when the combustion atmosphere is switched from air to 30%φ(O₂)/70%φ(CO₂), the bed material concentration would decrease in the dilute phase due to the reduction of fluidized velocity, which would further lead to the decrease of the heat transfer coefficient. It is available to operate existing CFB boilers in 30%φ(O₂)/70%φ(CO₂) atmosphere by only optimizing the particle size of bed material without adding extra heating surfaces.