Through experiments on desulfurization, CaSO4 decomposition, and system approach using theoretical analysis, the in-furnace desulfurization in an O-2/CO2 combustion system with partial CO2 removal from recycled gas was investigated. The results revealed that the SO2 concentration increased with the CO2 removal ratio and could be much higher than conventional combustion in air. This high SO2 concentration came from the enrichment effect of gas recirculation, at a high gas recirculation ratio of 0.989. The system desulfurization efficiency also increased with the CO2 removal ratio. Under the conditions investigated, the system efficiency of in-furnace desulfurization could be as high as 88.5%. The system desulfurization efficiency for the new scheme could be 6-10 times higher than conventional combustion in air. With this new scheme, easy CO2 recovery and efficient in-furnace desulfurization could be realized simultaneously.