Reactivity and stability of Co-Ni/Al₂O₃ oxygen carrier in multicycle CLC

This study deals with the development of a bimetallic Co-Ni/Al ₂O₃ oxygen carrier suitable for a fluidized bed chemical-looping combustion process. Temperature programmed characterization shows that the addition of Co enhances the reducibility of the oxygen carrier by influencing the metal-support interactions helping the formation of reducible nickel species. Reactive characterization of the prepared oxygen carriers in a CREC fluidized riser simulator, using multiple reduction/oxidation cycles, demonstrates that the Co-Ni/Al₂O₃ particles display excellent reactivity and stability. The addition of Co in the bimetallic Co-Ni/Al₂O₃ influences the state of the surface minimizing the formation of nickel aluminate. The addition of Co also inhibits metal particle agglomeration by maintaining consistent metal dispersion during the cyclic oxidation/reduction processes. A solid-state kinetics for both reduction and oxidation cycles is established using a clarified Avrami-Erofeev model at nonisothermal conditions. This random nucleation model describes solid phase changes adequately. The activation energy for Co-Ni/Al₂O₃ reduction is found to be significantly lower than the activation energy for the unpromoted Ni/Al₂O₃ sample, with this observation confirming the positive influence of adding Co on the Ni-Al₂O ₃ oxygen carrier.