Novel core shell structure to preclude phase segregation of iron-based oxygen carriers for chemical looping combustion

Iron-based oxygen carriers as one of the most commonly used oxygen carriers in chemical looping combustion, shows poor cycling stability. In this work, spinel-structured, Fe2O3@MgO core-shell structure, and Fe2O3/ CuO@MgO core-shell structure oxygen carriers were prepared using compression molding. The effects of deep reduction and oxidative regeneration processes on the cyclic stability of oxygen carriers with different structures were investigated using thermal gravimetric analyzer. The mechanisms for the changes in the cyclic stability were explored through a series of characterization analyses. The results showed that the reduction process of spinel oxygen carriers generated Fe-Mg solid solution, which was unfavorable to deep reduction, and phase separation occurred during 5 cycles. Due to the migration of Fe3+ ions to the surface, surface was seriously sintered, deteriorating the cyclic stability of spinel oxygen carriers. For Fe2O3@MgO oxygen carrier, the deep reduction can be realized, but the interdiffusion phenomenon of Fe2+ and Mg2+ occurred after 5 reductions, and the generation of (MgO)x(FeO)1_x solid solution impeded the deep reduction. The addition of CuO in the core of oxygen carriers to form Fe2O3/CuO@MgO could inhibit the interdiffusion of Fe2+ and Mg2+, and maintained the oxygen carrying capacity up to more than 95 % in 10 cycles. Meanwhile, CuO synergized with Fe2O3 to enhance the reduction and oxidation reactivity of the oxygen carrier. On this basis, the ion diffusion mechanism during the deep reduction and oxidation of iron-based oxygen carriers with different structures was proposed. This study provides guidance for the modification of iron-based oxygen carriers with superior cycling stability.