Multiscale modeling for the reduction kinetics of a perovskite oxygen carrier based on quantum chemistry and CFD–DEM

The redox of oxygen carriers in chemical looping are non-catalytic heterogeneous reactions which involve physical and chemical processes spanning across four scales: the surface atoms, grains, particles, and the reactor. Although various models are presented in the literature for every single scale, the coupling between every two adjacent scales has not been completely integrated due to the computational cost. A multiscale reaction kinetics model coupling all four scales is developed in this study, combining density-functional theory calculation for reaction mechanisms, microkinetics for grain conversion, the Fick's Law for intraparticle gas diffusion, and CFD–DEM for fluidization. Three coupling simplifications are adopted to reduce computational cost, including the partial equilibrium assumption, continuous grain distribution, and Thiele's-modulus-based effectiveness factor model. Computation is conducted for the reduction of a perovskite oxygen carrier (CaMn_0.375Ti_0.5Fe_0.125O_3−δ) by CO, which is experimentally verified on a micro-fluidized-bed thermogravimetric analyzer. The influences of parameters including the temperature, gas concentration, active site density, specific surface area, and particle diversity, are discussed, providing a comparison on the weights of every scale in the process.