Chemical looping combustion of lignite with the CaSO4–CoO mixed oxygen carrier

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Chemical looping combustion (CLC) has well developed as a novel combustion technology for simultaneous completion of the coal combustion and CO2 capture with a low energy penalty. Among all the oxygen carriers available, CaSO4 has gained great attention as a promising oxygen carrier (OC) in CLC due to its high oxygen capacity and low price. But further application of CaSO4 OC also suffers the problems of low reactivity and even deactivation due to the sulfur loss via the side reactions of CaSO4, which should be well addressed. In this research, the CaSO4–CoO mixed OC was prepared firstly, and experiments based on thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG-FTIR) were conducted to evaluate the reaction characteristics and evolution of the gaseous products during the reaction of the prepared CaSO4–CoO mixed OC with lignite (abbreviated as YN). Both the higher reaction rate of the prepared mixed OC with YN coal and the elevated CO2 concentration fully reflected the enhanced reactivity of the prepared mixed OC for YN coal conversion. Furthermore, the micromorphology of the solid reaction products was analyzed by the field emission scanning electron microscopy spectrometry (FESEM). Good sintering resistance of the prepared CaSO4–CoO mixed OC during its reaction with YN was verified, which was ascribed to the temporary inert support role played by the CaSO4 substrate. In order to further study the desulfurization ability of CoO in the mixed OC, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermodynamic simulation were used for in-depth analysis. The gaseous sulfur species released by the CaSO4 side reactions were mainly fixed as solid CoS, Co9S8 and CaS, with the total content higher than 99.8%. And the sulfided OC could be completely regenerated to its original state at the oxidation stage according to the X-ray diffraction (XRD) result. Overall, the prepared CaSO4–CoO mixed OC not only has the enhanced reactivity and good sintering resistance, but also owns the potential to control sulfur released from the CaSO4 side reactions, which has broad application prospect to simultaneously achieve decarbonization and desulfurization in the CLC process.