Kinetics of NiO reduction by H2 and Ni oxidation at conditions relevant to chemical-looping combustion and reforming

Abstract

Twenty solid-state kinetic models, including mechanisms dominated by reaction order, geometrical contraction, diffusion, nucleation, random pore growth, as well as the more flexible Šesták–Berggren and Prout–Tompkins models, are reviewed and applied to describe the reduction of unsupported and supported NiO by H2 and the oxidation of supported Ni by air. In an effort to address the inconsistencies in the literature regarding the suitability of each mechanism to describe the kinetics of Ni-based oxygen carriers in chemical-looping, all the models are compared against experimental data from the literature and with in-house experiments at conditions relevant to chemical-looping combustion and reforming with Ni-based oxygen carriers. A statistical approach to compare models of varying fidelity is employed, involving the Akaike Information Criterion and the F-test. The effect of temperature on the selection of the best-suited model is investigated, supplemented by experimental evidence from X-ray diffraction and scanning electron microscopy analyses. This work reveals that unsupported and supported NiO reduction can be described by nucleation and nuclei growth models. The oxidation kinetics of supported Ni is well-predicted by geometrical contraction models. Calcination and high-temperature treatment during oxygen carrier synthesis are shown to significantly affect the reaction kinetics of the oxygen carrier.