Performance of Fe2O3/Al2O3 oxygen carrier modified by CaCO3 and CaSO4 in chemical looping combustion

Abstract

Chemical looping combustion is a promising technology for CO2 capture and storage because of its inherent sequestration of CO2. However, the accumulation of coal ash in coal-based chemical looping combustion has a certain effect on oxygen carriers. Ca is a typical active element in ash, and a fluidized bed reactor was used in this study to analyze the catalytic activity of Ca on Fe-based oxygen carriers during chemical looping combustion, and the effects of two Ca additions, CaCO3 and CaSO4, were compared. Ca was found to significantly enhance the reactivity of oxygen carriers, and the effect of CaSO4 was more obvious than that of CaCO3. With CaCO3, CO conversion reached 100% in 20 min with a Ca content of 5% in the oxygen carrier, but a further increase in the CaCO3 content was unfavorable to the reaction. The stability of oxygen carrier reactivity was evaluated through ten redox cycles, where CO conversion was measured. Results showed that the reactivity of oxygen carriers containing CaSO4 rapidly decayed after three cycles, while that with CaCO3 showed good stability. The activation energy of the reaction between Fe2O3/Al2O3 and CO was then investigated using a thermogravimetric analyzer, and the composition and apparent morphology of oxygen carriers were characterized by scanning electron microscopy and X-ray diffractometer. CaAl2O4 and CaAl4O7 were found to form when the content of the carrier Ca was over 5%, and this resulted in slagging and sintering, which adversely affected the reaction. The oxygen carrier with 5% CaCO3 exhibited the best performance.