In recent years, chemical-looping combustion (CLC) has received growing interest as an efficient combustion technology for the capture of CO2 and the substantial reduction of NOx emissions from fossil fuel based power generation. This non-conventional technique involves the use of a solid oxygen carrier to supply oxygen, instead of air, during fuel combustion, thereby preventing the dilution of CO2 in the flue gas with N2 from air. Consequently, CO2 is delivered without any energy penalty for separation. Formation of NOx is also negligible because the fuel burns in absence of air without flame. This study deals with the development of a bimetallic Co-Ni/Al2O3 oxygen carrier suitable for fluidized bed CLC. Temperature programmed characterization shows that the addition of Co enhances the reducibility of the oxygen carrier by influencing the state of the surface minimizing the formation of nickel aluminate. Reactivity study of the prepared oxygen carriers in a CREC fluidized bed riser simulator, demonstrates that the Co-Ni/Al2O3 particles are highly reactive and stable over multiple reduction/oxidation cycles. The XRD analysis shows similar phases on both the fresh and used samples, confirming no phase transformation occurred during the CLC processes. The addition of Co also inhibits metal particle agglomeration by maintaining consistent metal dispersion during the cyclic reduction/oxidation processes, which is confirmed by pulse chemisorption analysis over repeated reduction/oxidation cycles. The SEM images of a fresh and a used carrier sample evidences similar distribution of the metal crystallites suggesting minimum agglomeration of the nickel species at high temperature CLC processes. The solid-state kinetics for both the reduction and oxidation cycles is established using a clarified Avrami-Erofeev model at non-isothermal 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, indicating the positive effect of Co on the Co-Ni/Al2O3 oxygen carrier.

Keywords: CO2 capture, NOx, CLC, Co-Ni/Al2O3 oxygen carrier, reactivity, stability, solid-state kinetics.