Fast redox kinetics of a perovskite oxygen carrier measured using micro-fluidized bed thermogravimetric analysis

Abstract

Redox kinetics of oxygen carrier in chemical looping combustion (CLC) is important for reactor design and its oxidation enthalpy is important in order to establish auto thermal demonstration. Most published redox kinetics of oxygen carrier has been measured by thermogravimetric analysis (TGA) which can include additional diffusion limitations and thus underestimate the overall kinetics. In this study, the redox kinetics of a new perovskite oxygen carrier (CaMn_0.375_Ti_0.5_Fe_0.125_O3-δ) was measured by a novel micro-fluidized bed thermogravimetric analysis (MFB-TGA) method which can achieve real-time weight measurement of oxygen carrier in a fluidizing state with similar mass and heat transfer characteristics as in a CLC reactor. The experimental data from MFB-TGA were analyzed with a reactor model. The redox kinetics was described by a two-stage model of gas-solid reaction. The effect of temperature, O2 concentrations and reducing gas type (H2 and CH4) on the redox kinetics in MFB-TGA was investigated and compared with other oxygen carriers such as natural manganese ore and ilmenite. It is observed that the oxidation of both manganese ore and ilmenite can be divided into two stages, a fast initial stage followed by a second slower stage, resulting in slower total oxidation rates. A very interesting finding is that there is only the fast initial stage for the oxygen carrier of CaMn0.375Ti0.5Fe0.125O3-δ, and the full oxidation of CaMn_0.375_Ti_0.5_Fe_0.125_O3-δ can be finished within ∼4 s which is ∼7.5 and ∼30 times faster than that of manganese ore and ilmenite. The reduction kinetics of CaMn_0.375_Ti_0.5_Fe_0.125_O3-δ by H2 is also ∼5 and ∼2.2 times faster than that of manganese ore and ilmenite, respectively. The kinetic parameters of three oxygen carriers were compared and the redox mechanism of CaMn_0.375_Ti_0.5_Fe_0.125_O3-δ was discussed.