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Chemical looping combustion (CLC) is a carbon capture technology which enables CO2 capture with low net efficiency penalty. Calcium sulfate (CaSO4) is an optional oxygen carrier for commercial use, but its usage is limited due to sulfur dioxide (SO2) emission. This study approaches this issue by adding CaO species into the CaSO4 oxygen carrier to inhibit the release of SO2 from CaSO4 oxygen carrier. In this study, the cyclical tests of a CaSO4-based oxygen carrier under alternating reducing and oxidizing conditions were performed at 900 °C and 800 °C respectively in a tubular furnace reactor at atmospheric pressure. The effects of reducing gas concentration and molar ratio of CaO/CaSO4 on the performance of CaSO4-CaO oxygen carrier were studied in terms of CO2 yields, Environmental factors of SO2 and COS, molar ratios of gas sulfides to CO2 generated in fuel reactor, and molar ratios of SO2 and COS to total carbon inlet. The use of CaO additive increased the yields of CO2 obviously. The release of COS in the fuel reactor and SO2 in the air reactor decreased, but while the overall release of SO2 in the fuel reactor increased. However, for per mole CO2 generation, less gas sulfides released from the fuel reactor. High concentrations of CO were beneficial for CO2 production and a low SO2 environmental factor, and meanwhile, the molar ratios of SO2 released to inlet CO n S O 2 {{\text{n}}_{{\text{S}}{{\text{O}}_2}}} / n CO {{\text{n}}_{{\text{CO}}}} decreased. However, it led to a drop in CO2 yield and an increase in COS environmental factor. As a whole, the use of CaO additive and higher CO concentration both accelerated the parallel CaSO4 reductions in fuel reactor, especially the selectivity of CaSO4 reduction to CaS. [ABSTRACT FROM AUTHOR]