Multiscale Perspectives on Char Oxidation: Reaction Regimes, Transport Phenomena, and Modeling Challenges

This review provides a critical assessment of the current state of research on char oxidation reactivity. First, a comparative analysis of intrinsic and apparent reaction kinetics is conducted across various char particles to elucidate the influence of transport limitations on char reactivity in different regimes. Then, methodologies for calculating and determining key pore structural parameters are systematically reviewed and summarized. Finally, bed-scale char oxidation under both kinetically and diffusion-controlled regimes is critically reviewed. A linear correlation between log-(A _in) and E _in (i.e., log A _in = 0.069 E _in - 1.55) is found for intrinsic char oxidation due to compensation effects. The apparent kinetic parameters of char oxidation exhibit substantial variability across different char types. The apparent activation energy in diffusion-controlled regime is not approximately half that of kinetic-controlled regime. These two findings underscore the significant influence of diffusion limitations on char reactivity. The absence of unified formulations of pore structural parameters introduces ambiguity in selecting appropriate expressions for specific applications. Fine-tuning or coarse kinetic parameters adjustments may idealistically reduce the discrepancies between experiments and bed-scale model predictions. However, the reliance on empirical calibration may obscure some inherent mechanisms. It should bridge the gap between intrinsic kinetics (i.e., thermogravimetry analysis, TGA), apparent kinetics (i.e., fluidized bed combustion, FBC analysis), and macroscopic combustion behavior (i.e., packed-bed combustion) by incorporating the dynamic evolutions of pore structure into multiscale model. This review advances the fundamental understanding of char reactivity by integrating kinetic, structural, and transport phenomena across scales, offering a foundation for modeling char oxidation in energy conversion systems, combustion processes, and fire dynamics.