Fundamental insights into chemical looping combustion (CLC): a materials characterization approach to understanding mechanisms and size effects in oxygen carrier performance

This work aims to develop fundamental insights about the underlying surface and bulk chemical processes instrumental to the efficiency of chemical looping combustion (CLC). CLC, which uses a solid-state oxygen carrier (e.g., metal oxides) to drive hydrocarbon combustion, is a promising combustion alternative that minimizes byproduct formation and facilities capture of CO2. In this work, we compare the performance of different transition metal oxides, namely iron, copper, cobalt, manganese, and nickel oxides, as oxygen carriers in CLC using CH4 as the reducing agent. Experiments used a continuous flow reactor across temperatures ranging from 500 to 800 oC and feed flowrates from 12.5 to 250 h-1. In addition to monitoring size-, temperature- and flow rate-dependent performance trends for CH4 conversion to CO2, microscopic and spectroscopic techniques were used to investigate the solid-state mechanism of oxygen carrier reduction and the coupled surface chemical and bulk material processes influencing performance. Bulk (XRD) and surface (XPS) analysis reveal that oxygen carrier reduction can be generally represented by two models, the unreacted shrinking core model (USCM) and the nuclei growth model (NNGM). The reduction of some metal oxides can also proceed via a two-stage solid-state mechanism; for example, hematite reduction to magnetite follows USCM, while the subsequent reductions of magnetite to wustite and wustite to iron metal follow NNGM. Furthermore, our results reveal that minimizing the particle size promotes oxygen carrier performance, but only for metal oxides reduced according to the USCM, where metal oxide reduction initiates on the particle surface. In contrast, no benefit of decreasing particle size was observed for materials reduced according to the NNGM because the reaction initiates in the particle bulk, such that a more critical determinant of reactivity may be the available oxygen carrier volume rather than surface area. Beyond these fundamental insights, cycling experiments were also performed to provide more practical information about the effect of oxygen carrier particle size on their long-term performance in CLC applications.