Reduction Kinetics of La Modified NiO/La-γAl2O3 Oxygen Carrier for Chemical-Looping Combustion

Abstract

La modified Ni/La-γAl2O3 oxygen carrier reduction kinetics is investigated using temperature programmed reduction (TPR) and a parameter nonlinear regression analysis. TPR profile study and XRD analysis of the completely oxidized samples show that NiO is the prevalent phase of the oxygen carrier. Hydrogen pulse chemisorption demonstrates that the nickel crystallite sizes remain unchanged over repeated reduction/oxidation cycles. A nucleation and nuclei growth model and an unreacted shrinking core model are developed based on the oxygen carrier characterization. Model discrimination is conducted based on SSQ, goodness of fittings, and minimum cross-correlation coefficients. On the basis of these statistical indicators, it is established that the random nucleation model describes the reduction of the oxygen carrier adequately. The estimated value of the activation energy for the La modified Ni/La-γAl2O3 sample is found to be 73.4 ± 2.6 kJ/mol, with this being significantly lower than the activation energy for the unmodified Ni/γAl2O3 sample (104.5 ± 3 kJ/mol). This suggests that the unmodified oxygen carrier requires higher activation energy, with this reflecting an increased difficulty of nickel phase reduction due to a strong interaction between nickel and alumina. The nucleation model, as established using TPR, is successfully validated for the reduction cycle using methane as a fuel gas in a CREC minifluidized riser simulator reactor operating under the expected operating conditions for large industrial scale chemical-looping combustion (CLC) units.