Microscopic occurrence and pore-scale diffusion behavior of water-soluble sodium in coal

Abstract

The occurrence modes of water-soluble sodium (WS-Na) in coal critically govern its migration, release, and removal during cleaning, pyrolysis, and combustion, yet its microscopic distribution within pore structures and interaction mechanisms with water and the coal matrix remain elusive. To bridge this gap, we integrated scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and molecular dynamics (MD) simulations to systematically probe H₂O, Na⁺, and Cl⁻ diffusion across coals of varying porosities. Our findings reveal that Na⁺ and Cl⁻ weaken coal-water interactions—an effect exacerbated at higher porosities where ions become sequestered in larger pores distant from active surface sites. Although these ions disrupt the water hydrogen-bond network, strong ion-water electrostatics dominate, fostering the formation of larger hydrated clusters that drive saline accumulation into coal slit pores. H₂O molecules rapidly migrate to and adsorb onto the coal surface, attracting Cl⁻ to regions near hydrogen atoms of surface functional groups. Concurrently, Na⁺ preferentially localizes adjacent to oxygen-containing (e.g., –COOH, –OH) and nitrogen-containing moieties, forming ion pairs with Cl⁻ via Coulombic attraction. Notably, the diffusivities of Na⁺ and Cl⁻ increase with porosity, reflecting the relaxation of nanoconfinement effects; in contrast, the effective diffusivity of H₂O decreases with increasing porosity under saline conditions, likely due to enhanced tortuosity and prolonged surface residence time in more complex pore networks. This work establishes a mechanistic framework for alkali metal speciation and transport in coal, providing a theoretical basis for optimizing pretreatment strategies to mitigate slagging, fouling, and corrosion during thermal conversion.