Improvements in reaction kinetics and stability of ilmenite as oxygen carrier by surface modification with calcium titanate in redox cycles of chemical-looping systems

•Ilmenite was modified with iron-doped calcium titanate.•Reduction kinetics of modified ilmenites were improved during redox cycles.•Surface modification of ilmenite can prevent morphological changes.•Control of ion transport can improve redox kinetics and stability of oxygen carriers.

High reactivity and long-lifetime stability of the oxygen carriers in redox cycles are crucial in chemical-looping combustion systems. In this study, to improve the redox kinetics and prevent morphological changes during redox cycles, ilmenite was modified with a calcium additive to form iron-doped calcium titanate (CTFO), which is a mixed ionic and electronic conductor, on the ilmenite surface and interior. An ilmenite-core and CTFO-shell structure was formed and the structure changed depending on the calcination temperature. Use of the modified ilmenites improved the reduction kinetics in dry H2 and wet CH4 during redox cycles. Also, it was found that morphological changes of the modified ilmenites during redox cycles were prevented, although surface iron segregation and increased porosity were observed for neat ilmenite. The formation of a CTFO phase can induce rapid oxide ion transport at the ilmenite/CTFO interface and suppress the outward diffusion of iron cations. Prevention of morphological changes can increase the lifetimes of oxygen carriers in ilmenite. The present results provide strategies for developing oxygen carriers with high reactivities and stabilities.

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