Reduction and oxidation kinetics of Co–Ni/Al2O3 oxygen carrier involved in a chemical-looping combustion cycles

The solid-state kinetics of Co–Ni/Al2O3 oxygen carrier is studied using non-isothermal reaction data and a non-linear regression analysis. XRD analysis of the fresh samples shows that NiO is the dominant reducible phase of the oxygen carrier. Pulse chemisorption suggests a negligible nuclei growth over the repeated reduction/oxidation cycles. Mercury porosimetry confirms that the pore size of the carrier particle is slightly increased following reduction. A nucleation and nuclei growth model and an unreacted shrinking-core model are developed based on the oxygen carrier texture change during reduction/oxidation, as observed by pulse chemisorption and mercury porosimetry. Model parameters are calculated using H2-TPR and O2-TPO data. It is found that the random nucleation model describes solid phase changes adequately. The determined apparent activation energies are 45 and 44 kJ/mol for the reduction and oxidation, respectively. The established kinetic model is successfully evaluated for the reduction cycle using a CREC mini-fluidized Riser Simulator reactor operating under expected conditions of large industrial scale fluidized CLC units.