Mechanism of solid state diffusion on the performance evolution of iron-based oxygen carrier at different operating conditions for chemical looping process

Abstract

In the oxidation half cycle of chemical looping, the outward diffusion of Fe cations occurred for Fe-based oxygen carriers. The focus of this study is to investigate into the influence of operating conditions on Fe segregation, and then reveal the role of operating conditions in performance evolution of Fe2O3/Al2O3 in the presence of solid state diffusion. The chemical looping cycles were performed at different operating conditions (temperature, oxygen concentration, reduction degree of oxygen carrier). This work evaluated the performance evolution of Fe2O3/Al2O3 in redox cycles and indicated that the generally accepted theory of particle sintering deactivation in gas–solid reactions was incomplete, which ignored the solid state diffusion in oxygen carrier particles. Contrary to the traditional view, the effect of reaction temperature on the performance evolution of oxygen carrier is conditional, depending on the dominating mechanism of solid state diffusion in redox cycles. The results of the experiment confirmed that the surface enrichment of active components enabled more active components to participate in gas–solid reactions on the surface of material particles, which leaded to a higher deactivation rate at low temperature: 700 °C (68.33 %) > 800 °C (54.41 %) > 900 °C (29.73 %). Reducing the oxygen concentration in oxidation half cycle and reduction degree can effectively inhibit the solid state diffusion of Fe2O3/Al2O3. When the oxygen concentration is reduced to 1 %, the Fe2O3/Al2O3 has the lowest deactivation rate, which is only 9.92 %. This study provided more experimental and theoretical results for better understanding the performance evolution of iron-based oxygen carriers.