Reduction and oxidation kinetics of nickel-based oxygen-carriers for chemical-looping combustion and chemical-looping reforming

The kinetics of reduction with CH4, H2 and CO and oxidation with O2 of two NiO-based oxygen-carriers for chemical-looping combustion (CLC) and chemical-looping reforming (CLR) prepared by impregnation, NiO18-αAl and NiO21-γAl, have been determined in this work. In both solids, nickel was present as NiO and NiAl2O4 in the oxidized state and the NiO/NiAl2O4 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 NiAl2O4 reduction were determined for each oxygen carrier and each reacting gas. The reaction rate of NiO reduction was the same in NiO18-αAl and NiO21-γAl 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 NiAl2O4 reduction depending on the type of support, α-Al2O3 or γ-Al2O3, 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 NiAl2O4 reduction working with both NiO-based oxygen-carriers. Chemical reaction control was assumed for NiO18-αAl whereas diffusion through the product layer was also considered when NiO21-γAl was used as oxygen-carrier. The oxidation reaction was very fast with both materials although small differences were observed again between NiO18-αAl and NiO21-γAl oxygen carriers, probably owing to the presence of a higher amount of NiAl2O4 in the solid supported on γ-Al2O3. The combined model for consecutive reduction of NiO and NiAl2O4 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.

► The kinetics of reduction (CH4, H2, CO) and oxidation of two oxygen carriers were determined. ► Oxygen carriers were prepared by impregnation on α-Al2O3 and γ-Al2O3. ► This study includes kinetic data both for NiO and NiAl2O4. ► These data are very useful for design CLC and CLR processes.