Characteristics of CaS–CaO Oxidation for Chemical Looping Combustion with a CaSO4‑Based Oxygen Carrier

The oxidation of CaS to a CaSO4 oxygen carrier suffers SO2 release. To capture SO2 released, an amount of CaO particles was mixed with CaS particles. Isothermal kinetics of the CaS–CaO oxidation reaction has been conducted in a thermogravimetric reactor combined with a Frourier transform infrared spectroscopy (FTIR) analyzer. The effects of the reaction temperature, O2 concentration, and molar ratio of CaO/CaS on CaSO4 generation and SO2 emission were investigated. The chemical compositions, morphology structures, and element distributions on the surfaces of solid residuals were characterized by FTIR and scanning electron microscopy–energy-dispersive X-ray spectroscopy instruments. The results show that the oxidation products of CaS species are mainly composed of CaSO4. A small amount of SO2 is released from CaS oxidation, causing losses of the oxygen carrying capacity of a CaSO4-based oxygen carrier. Using a CaO additive in the CaS oxidation process reduces SO2 liberation, improves the CaSO4 selective yield, but results in an increase in remaining CaS species in solid residues. There is a general positive correlation between the remaining percentage of CaS species and the SO2 selective yield. The SO2 liberation varies with the reaction temperature, O2 concentration, and CaO/CaS molar ratio. At 900 °C, a CaO/CaS molar ratio of 1, and 5% O2, SO2 released from CaS oxidation is completely trapped by the CaO additive.