Characterization and kinetics of reduction of CaSO4 with carbon monoxide for chemical-looping combustion

Chemical-looping combustion (CLC) of fuels via the cyclic reduction and oxidation of an oxygen carrier is a novel process for CO2 capture. CaSO4 has emerged as an alternative material with much lower cost and higher oxygen storage capacity compared to metal oxide based oxygen carrier. In principle, CaSO4 is reduced by CO and H2 (coal gasification products) generating CaS, CO2 and H2O, and then the solid product CaS is regenerated back to CaSO4 in air. Research on the reactions of CaSO4 and CaS is not only important for understanding the reaction mechanism and parameters for the new CLC technology but also of high interest in understanding the sulfur chemistry in fluidized bed combustion and gasification. This paper focuses on the reduction reaction of CaSO4 with CO to CaS and CO2 (CaSO4 + 4CO → CaS + 4CO2) by thermodynamics, characterization and kinetics analysis. Phase diagram of CaSO4–CaO–CaS indicates the operation regime of reduction–oxidation cycle by delicate control of temperature and partial pressure of reaction gases. The kinetics of the reduction reaction of a low cost natural anhydrite as oxygen carrier was investigated in an isothermal differential bed reactor where a thin layer of CaSO4 particles were exposed to a plug flow of CO balanced by N2. Prior to the kinetics modeling, extensive physical and chemical characterization analyses, such as X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM–EDX), and N2 adsorption–desorption were carried out to understand the reaction mechanism. External mass transfer and heat effects were minimized and the operation regime for intrinsic kinetics was determined. The experimental data was described with a gas–solid shrinking unreacted core model (SCM) with both chemical reaction control and product layer diffusion resistance considered.