Abstract
The brownmillerite-structured Ca2Fe2O5 oxygen carrier has shown its great potential in chemical looping processes, due to it can be completed regeneration in a H2O or CO2 atmosphere without the air reactor. However, the low reaction reactivity of Ca2Fe2O5 restricts its application. In this study, Ca2Fe2O5 oxygen carrier was prepared by sol-gel method. The reduction and oxidation kinetics of Ca2Fe2O5 were evaluated in H2 and CO2 atmospheres, respectively. The reduction of Ca2Fe2O5 in H2 atmosphere in good agreement with the random nucleation and growth model with Ea and A of 53.82 kJ/mol and 2.65 s−1, respectively. The dimension of the model increases with conversion (A1.5 → A2 → A3 → A4). The oxidation of reduced Ca2Fe2O5 in CO2 atmosphere can be described by the zero-order contraction model with Ea and A of 11.66 kJ/mol and 0.05 s−1, respectively. The kinetics analysis showed that both the reduction of H2 and the oxidation of CO2 are one-step reactions, as evidenced by the fact that only Fe0 and Fe3+ phases were detected in semi-in situ XRD analysis. It was inferred that the release and recovery of lattice oxygen is from inside to outside for Ca2Fe2O5 oxygen carrier in the redox process. By reducing the migration energy barrier of lattice oxygen between bulk and surface would be an effective means to improve the reactivity of Ca2Fe2O5 oxygen carriers.
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