Enhancement Mechanism of Minerals on Lignite-Water Molecule Interactions

Five coal samples were prepared by deashing Shengli lignite in distinct phases, which consisted of residual ash from spontaneous combustion. The effects of removal and introduction of inherent minerals on the water reabsorption performance of coal samples were systematically investigated in three aspects: pore structure, oxygen-containing functional groups, and lignite materials. Low-field nuclear magnetic resonance spectroscopy was employed to investigate the changes in the water molecular adsorption tendency of coal samples with the variation in the mineral content. The study elucidates that the hygroscopic performance of the coal samples is significantly reduced due to the massive removal of inherent minerals. However, the pore structure of the coal samples after HCl/HF washing becomes more developed, and the oxygen-containing functional groups on the surface are more exposed, leading to an increase in the equilibrium adsorbed moisture content (EMC) of the coal samples. The binding force between coal samples and water molecules is reduced by the removal of the inherent minerals, which weakens the interaction forces between lignite and water molecules. The oxygen-containing functional groups on the surface of lignite interact with the residual ash from spontaneous combustion to enhance the binding force between lignite and surface water molecules, thus leading to the improved tendency of lignite to adsorb water molecules. The formation of intermediate complexes between minerals and oxygen-containing functional groups, in particular, carboxyl functional groups, on the surface of lignite enhances the acting force of polar sites, which improves the interaction of lignite-water molecules.