摘要
The kinetics of reduction with CHb>4b>, Hb>2b> and CO and oxidation with Ob>2b> of two NiO-based oxygen-carriers for chemical-looping combustion (CLC) and chemical-looping reforming (CLR) prepared by impregnation, NiO18-伪Al and NiO21-纬Al, have been determined in this work. In both solids, nickel was present as NiO and NiAlb>2b>Ob>4b> in the oxidized state and the NiO/NiAlb>2b>Ob>4b> ratio in the particles was a function of the solid conversion reached in the previous reduction process. Both nickel compounds were active for oxygen transfer although with very different reactivities. Therefore, kinetic parameters for NiO and NiAlb>2b>Ob>4b> reduction were determined for each oxygen carrier and each reacting gas. The reaction rate of NiO reduction was the same in NiO18-伪Al and NiO21-纬Al particles. A model which assumed a linear relationship between time and conversion was used to describe NiO reduction. Nevertheless, differences in reactivity were observed during NiAlb>2b>Ob>4b> reduction depending on the type of support, 伪-Alb>2b>Ob>3b> or 纬-Alb>2b>Ob>3b>, probably due to a different crystalline structure of the alumina. The changing grain size model for spherical grain geometry was used to obtain the kinetic parameters of the NiAlb>2b>Ob>4b> reduction working with both NiO-based oxygen-carriers. Chemical reaction control was assumed for NiO18-伪Al whereas diffusion through the product layer was also considered when NiO21-纬Al was used as oxygen-carrier. The oxidation reaction was very fast with both materials although small differences were observed again between NiO18-伪Al and NiO21-纬Al oxygen carriers, probably owing to the presence of a higher amount of NiAlb>2b>Ob>4b> in the solid supported on 纬-Alb>2b>Ob>3b>. The combined model for consecutive reduction of NiO and NiAlb>2b>Ob>4b> in the oxygen-carrier particles with the kinetic parameters obtained in this work predicted adequately the experimental results with both materials. These kinetic data could be used for the design of CLC and CLR systems.