Multiscale physicochemical evolution of mined coal links structure transport and reaction kinetics during thermochemical conversion

Abstract

Global reliance on coal for energy generation and chemical production necessitates physics-based frameworks that move beyond empirical bulk characterization toward predictive and sustainable utilization strategies. Coal is a heterogeneous, reactive porous medium whose structure, physicochemical properties, and reactivity evolve continuously during thermochemical conversion. Coal mining conditions and associated geological and mechanical disturbances may influence pore–fracture architecture, mineral exposure, and transport accessibility, thereby affecting subsequent conversion behavior. This review develops a unified structure–transport–reaction framework that links coal’s multiscale architecture to transport behavior, reaction regimes, and process-scale performance. Particular emphasis is placed on how thermochemical loading governs pore–fracture evolution, mineral redistribution, and interfacial functionality, thereby controlling heat and mass transfer, adsorption–desorption behavior, and heterogeneous reaction kinetics. Unlike conventional constant-property approaches, the framework highlights conversion-dependent transport properties, spatially heterogeneous reactivity, and dynamic transitions between kinetic- and diffusion-controlled regimes. Coal exhibits a hierarchical pore–fracture system spanning micropores (< 2 nm), mesopores (2–50 nm), macropores, and cleat–fracture networks, with permeability typically ranging from 10⁻²¹ to 10⁻¹⁵ m² and thermal conductivity varying between 0.15 and 0.60 W m⁻¹ K⁻¹ depending on coal rank and conversion state. Under thermochemical conditions of 800–1800 °C, continuous structural evolution modifies pore connectivity, mineral distribution, and surface chemistry, leading to significant changes in transport pathways and reaction accessibility. Major conversion routes, including combustion, gasification, liquefaction, and coal-derived carbon-material production, are critically evaluated in terms of transport limitations, reactivity evolution, ash behavior, and mechanical stability. Advances in operando characterization, image-informed pore-scale simulations, multiscale multiphysics modeling, and physics-informed data-driven approaches are assessed for their ability to bridge scales and improve predictive capability. Overall, this review establishes a mechanistic foundation for understanding mined coal as an evolving porous medium and provides a roadmap for integrating multiscale characterization, transport physics, and reactor-scale modeling to advance next-generation coal conversion technologies.