Reactivity of a CaSO₄-oxygen carrier in chemical-looping combustion of methane in a fixed bed reactor

Abstract

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO₂. A reactivity study of CaSO₄ oxygen carrier in CLC of methane was conducted in a laboratory scale fixed bed reactor. The oxygen carrier particles were exposed in six cycles of alternating reduction methane and oxidation air. A majority of CH₄ reacted with CaSO₄ to form CO₂ and H₂O. The oxidation was incomplete, possibly due to the CaSO₄ product layer. The reactivity of CaSO₄ oxygen carrier increased for the initial cycles but slightly decreased after four cycles. The product gas yields of CO₂, CH₄, and CO with cycles were analyzed. Carbon deposition during the reduction period was confirmed with the combustible gas (CO+H₂) in the product gas and slight CO₂ formed during the early stage of oxidation. The mechanism of carbon deposition and effect was also discussed. SO₂ release behavior during reduction and oxidation was investigated, and the possible formation mechanism and mitigation method was discussed. The oxygen carrier conversion after the reduction decreased gradually in the cyclic test while it could not restore its oxygen capacity after the oxidation. The mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO₄ oxygen carrier. XRD analysis illustrated the phase change of CaSO₄ oxygen carrier. CaS was the main reduction product, while a slight amount of CaO also formed in the cyclic test. ESEM analysis demonstrated the surface change of particles during the cyclic test. The reacted particles tested in the fixed bed reactor were not uniform in porosity. EDS analysis demonstrated the transfer of oxygen from CaSO₄ to fuel gas while leaving CaS as the dominant reduced product. The results show that CaSO₄ oxygen carrier may be an interesting candidate for oxygen carrier in CLC.