Heterogeneous reaction kinetics of a perovskite oxygen carrier for chemical looping combustion coupled with oxygen uncoupling

Abstract

The determination of more exact heterogeneous kinetics of oxygen carriers is a crucial task for modelling, used to predict performance and design of demonstrating pilot units. Thermogravimetric analysis (TGA) is the most widely used method to measure the kinetics. However, the mass and heat transfer limitations observed inside the TGA specially for oxygen uncoupling effect will lead to underestimation of the kinetics. Micro-fluidized bed thermogravimetric analysis method (MFB-TGA) solve this challenge to measure the fast heterogeneous kinetics precisely, based on real-time mass measurement of oxygen carrier in a fluidizing state with similar mass and heat transfer characteristics as in a CLC reactor. In this study the oxygen uncoupling kinetics and redox reactions kinetics of a newly developed perovskite oxygen carrier material (CaMn0.5Ti0.375Fe0.125O3-δ) is established using a MFB-TGA method. The oxygen uncoupling kinetics measured by MFB-TGA is ~4 times faster than that measured by regular TGA. The oxidization reaction of CaMn0.5Ti0.375Fe0.125O3-δ occurs only in the initial stage controlled by chemical reaction, and the time required for full oxidation is ~5 s, while the reduction kinetics consists of a fast reaction stage and a slow reaction stage, controlled by chemical reaction and diffusion through the product layer, respectively. Both the oxygen uncoupling kinetic parameters and redox kinetic parameters are obtained by analyzing the MFB-TGA data based on the simplified K-L model for the fluidized bed. The morphology characterizations of the fresh and tested CaMn0.5Ti0.375Fe0.125O3-δ particles were investigated by SEM-EDS. The comparison of heterogeneous reaction kinetics with the mainstream OCMs were done.