DFT-based rate equation for thermochemical redox kinetics in a bubbling-fluidized bed reactor and its application to a manganese oxygen carrier in chemical looping

Abstract

Thermochemical redox reactions in a bubbling-fluidized bed reactor involve the surface→grain→particle→reactor scales from the microscope to the macroscope, and the reaction contains some physical and chemical steps. There is a requirement to develop a comprehensive and precise rate equation for the redox processes. This work established a density functional theory (DFT)-based multi-scale model to simulate the kinetic behaviors of the thermochemical reactions. The model was applied to predict the oxidation and reduction kinetics of a manganese oxygen carrier in chemical looping. The reaction mechanisms of the manganese oxygen carrier with H and O were firstly calculated by the DFT methods at the surface scale, both showing two-step reaction paths with the rate-limiting step energy barrier of 0.96 eV and 0.63 eV respectively. The reaction rate constants were 0.19/Pa/s for the oxidation and 3.50 × 10/Pa/s for the reduction at 900 °C, obtained by the transition state theory (TST). The DFT and TST results were introduced to establish a microkinetic rate equation at the grain scale, which realizes the coupling of the surface reactions and the O anion diffusion in the grain bulk. The rate equation was implemented in the mass transfer models, and the influences of the gas diffusion at the particle scale and the reactor scale were further considered, including the internal, external and interphase gas diffusion. The theoretical prediction results are validated by the experimental data from the micro-fluidized bed thermogravimetric analysis. It is demonstrated that the DFT-based model could realize an accurate prediction of the reaction kinetics of the manganese oxygen carrier in bubbling-fluidized bed reactor at a wide range of reaction temperatures and gas partial pressures. The developed DFT-based rate equation solves the theoretical problem of scale-span phenomenon for the thermochemical redox reactions, i.e. oxidation and reduction steps of an oxygen carrier in chemical looping.