Selection and Application of Binders for CaSO4 Oxygen Carrier in Chemical-Looping Combustion

CaSO4 has recently been recognized as a promising oxygen carrier in chemical-looping combustion (CLC) for CO2 capture. Attrition and decomposition of CaSO4 at high temperature limits its application in CLC processes. In this paper, the screening and application of suitable binders for CaSO4 carrier to enhance its mechanical strength are presented. Four types of commercial powder and sols (Al2O3 powder, Al2O3 colloidal sol, ZrO2 colloidal sol, and SiO2 colloidal sol) as binders were first screened by a dropping test and the silicon sol was selected as a suitable binder in the subsequent experiments. The attrition test was then performed and results showed that the presence of SiO2 led to a:a increase in the mechanical strength and the optimal weight ratio of added SiO2 was above 13.5 wt %. Through balancing the mechanical strength and oxygen capacity of CaSO4 based carriers, ratios of 8, 10.5, 13.5, and 16 wt % of added SiO2 were chosen for thermogravimetric analysis (TGA) and the results showed that the addition of SiO2 as binder enhanced the reduction reaction rate of CaSO4 with CH4, CO, and H-2. Regardless of the fuel gas used, the maximum reaction rate was dependent on the content of SiO2 and the optimum content of SiO2 was 13.5 wt %. The addition of SiO2 did not alter the reaction route and the main reduced solid product of CaSO4 was calcium sulfide (CaS). These results were also confirmed by X-ray diffraction (XRD) test. Based on the results, the use of SiO2 sol as binder of CaSO4 carrier is recommended and SiO2-supported CaSO4, a novel oxygen carrier, could have significant potential to be used in practical application of CLC system.