Structures and fractal characteristics of pores in low volatile bituminous deformed coals by low-temperature N2 adsorption after different solvents treatments

Abstract

Coal is a porous medium with fractal characteristics. With the removal of soluble fractions under solvent treatment, the structures and fractal characteristics of pores are bound to produce changes. Exampled by low volatile bituminous coals with different degrees of deformation from Huoerxinhe and Changcun coal mines in Qinshui basin, North China, the paper investigated on the changes of pore structure parameters and minerals compositions of coals before and after different solvents treatments, including tetrahydrofuran (THF), hydrochloric acid (HCl) and chlorine dioxide (ClO2). And Insight into the relationships between fractal dimensions and pore structure parameters and coal compositions were provided. Results show that: The shapes of N2 adsorption/desorption isotherms rarely change but their adsorption quantities differ sharply after different solvents treatments for coal with same deformation, indicating no change in pore type but a change in the number of pore in the same pore size. Compared to raw coals, with the removal of soluble fractions in coals pore volume (PV) and specific surface area (SSA) tend to ascend after THF extraction; changes in PV and SSA depend jointly on the dissolution of carbonate minerals and the swelling of clay minerals after HCl treatment; and the varieties of PV and SSA, to a great extent, are closely related to the swelling of clay minerals after ClO2 treatment. Pore structure fractal dimension (DA) and surface fractal dimension (DB) are obtained based on Frenkel-Halsey-Hill model. DA is mainly affected by the proportion of transitional pores and mesopores on PV; therefore, DA can be used to describe pore volumetric roughness of these transitional pores and mesopores. Meanwhile, DB is mainly influenced by the proportion of micropores on SSA; therefore, DB can be utilized to represent pore surface roughness of these micropores. As coal deformation increases, DA grows for raw coals and coals after extraction with THF; therefore, PV has a significant migration to the pores of small apertures causing the growing contribution of micropores to PV. However, DA declines for coals treated by HCl and ClO2, potentially because the welling of clay minerals blocks the pores of small apertures and increase the proportion of the pores of large apertures on PV. DB shows no clear trends with increasing coal deformation. A “reversed U-shaped” curve relationship is observed between moisture in coal and two fractal dimensions, which is due to the effect of water molecule interfacial tension. Ash correlates negatively with DA and positively with DB because of the various influences of minerals on pore homogeneity in pores of different apertures.