Microstructural Stabilization and Redox Reactions Involving Structural Phase Transformation of Calcium Ferrite Oxygen Carriers in Chemical Looping Combustion

Abstract

Chemical looping combustion has the potential to reduce the energy penalty associated with carbon dioxide (CO₂) separation during the combustion of hydrocarbon fuels. Calcium ferrite oxygen carriers are promising for practical applications due to their ability to be synthesized from inexpensive and eco-friendly materials. In this study, the calcium ferrite oxygen carrier achieved complete combustion and exhibited high stability while effectively controlling its structural phase transformation. The structural phase transformations were influenced by the molar ratio of Fe/(Ca + Fe) and the oxygen partial pressure of the reducing gas, which determined the extent of the reduction reaction. Thermogravimetric measurements of the cyclic redox reaction were conducted under two conditions: (a) lower p(O₂) conditions using H₂ as the reducing gas and (b) higher p(O₂) conditions using humidified methane (CH₄) as the reducing gas. Under lower p(O₂) conditions, Ca and Fe were clearly separated after the reduction reaction, indicating the formation of Fe domains. The diameter of the Fe domains depended on the Ca ratio, with Ca suppressing the growth of these domains, which may contribute to maintaining redox reactivity. Under higher p(O₂) conditions, the microstructural changes were also dependent on the Ca ratio. The microstructure of Fe₂O₃/CaFe₂O₄ remained dynamically stable, although Fe₂O₃ exhibited the formation of large hollows. The migration of Fe ions during the structural phase transformation likely controlled the microstructural changes. Product gas analysis using a fluidized bed reactor revealed complete combustion of humidified CH₄ in the presence of CaFe₂O₄, Fe₂O₃, or both. At an optimal static bed height, the CO₂ yield reached 100%. These results indicate that Fe₂O₃/CaFe₂O₄ is a promising oxygen carrier and offer valuable insights into the development of high-performance oxygen carriers utilizing structural phase transformations.