Multicycle Study on Chemical-Looping Combustion of Simulated Coal Gas with a CaSO4 Oxygen Carrier in a Fluidized Bed Reactor

Abstract

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas and solid fuel with efficient use of energy and inherent separation of CO2. In this study, the cyclic test of a CaSO4-based oxygen carrier (natural anhydrite) in alternating reducing simulated coal gas and oxidizing conditions was performed at 950 °C in a fluidized bed reactor at atmospheric pressure. A high concentration of CO2 was obtained in the reduction. The H2 and CO conversions and CO2 yield increased initially and final decreased significantly. The release of SO2 and H2S during the cyclic test was found to be responsible for the decrease of reactivity of a CaSO4 oxygen carrier. The oxygen carrier conversion after the reduction reaction decreased gradually in the cyclic test. Through the comparison of mass-based reaction rates as a function of mass conversion at typical cycles, it was also evident that the reactivity of a CaSO4 oxygen carrier increased for the initial cycles but finally decreased after around 15 cycles. The mass conversion rate of a CaSO4 oxygen carrier was considerably lower than that of metal oxides. X-ray diffraction analysis revealed that the presence and intensity of the reduction sulfur species was in accordance with the results of gas conversion. The content of CaO was higher than expected, suggesting the formation of SO2 and H2S during the cycles. Surface morphology analysis demonstrates that the natural anhydrite particle surface varied from impervious to porous after the cyclic test. It was also observed that the small grains on the surface of the oxygen carrier sintered in the cyclic tests. Energy-dispersive spectrum analysis also demonstrated the decrease of oxygen intensity after reduction, and CaO became the main component after the 20th oxidation. Pore structure analysis suggested that the particles agglomerated or sintered in the cyclic tests. The possible method for sulfur mitigation is proposed. Finally, some basic consideration on the design criteria of a CLC system for solid fuels using a CaSO4 oxygen carrier is discussed by the references and provides direction for future work.