Multiscale Gas–Solid Reaction Dynamics of Hematite Oxygen Carrier in Chemical Looping Combustion from Fluidized Bed Thermogravimetric Analysis

The pursuit of precise and detailed kinetic characteristics continues to drive advancements in both modeling and measurement methodologies. A multiscale kinetic model was developed to analyze the redox behaviors of the hematite oxygen carrier (OC), characterized by a fluidized bed thermogravimetric analyzer (FB-TGA). This multiscale model integrates bulk diffusion of lattice oxygen, discrete growth of product islands, particle-scale gas diffusion, and interphase gas exchange, offering a comprehensive insight into the evolution of reaction rate and mass transfer. The investigation revealed that the surface diffusion rates of gas molecules are the crucial determinant in the Fe2O3 reactivity, with rapid surface diffusion delaying the formation of an enclosed product layer. The exceptional diffusion conditions and precise weight measurement capability of FB-TGA yielded faster reaction rate constants than conventional methods, facilitating transformative insights into rate-limiting factors. External diffusion and bulk lattice oxygen diffusion were identified as rapid and insufficient in imposing limitations on the overall reaction. Conversely, intraparticle diffusion emerged as the primary rate-limiting step in kinetic experiments even involving 0.1-0.15 mm hematite OC particles, especially during the initial oxidation of Fe3O4, where the mass transfer efficiency dropped below 85%. In actual chemical looping combustion environments utilizing 0.3-0.45 mm hematite OC particles, the resistance of intraparticle diffusion significantly intensified, inhibiting gas-solid reaction rates to an extent comparable to interphase mass transfer. Based on the insights derived from the multiscale model analysis, strategies for optimizing OC particle structure and reactor design have been proposed to enhance gas-solid reactions.