Multicycle Study on Chemical Looping Combustion with a CaSO₄-CaO Mixed Oxygen Carrier

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_SO2/n_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.