Effects of Chemical Components and Structure on the Pore Properties of Lignite Treated by Acidification and Tetrahydrofuran

Acid-tetrahydrofuran (THF) treatment could change the soluble components and chemical structure of coal, enhancing the gas extraction performance. Gas chromatography-mass spectrometry (GC-MS) was adopted to analyze the changes in the soluble organic matter in lignite after acid-THF treatment. Based on Fourier transform infrared (FTIR) testing, parameter variations including degrees of hydrogen enrichment (<i>I</i>₁), oxygen enrichment (<i>I</i>₂), aromatization (<i>I</i>₃), and aromatic condensation (<i>I</i>₄) of coal were quantitatively analyzed. Meanwhile, the pore structure changes were analyzed through the N₂-BET experiment and FHH fractal model. The effects of lignite chemical components and structure on the pore structure were discussed. With the combined acid-THF treatment, lignite extracts are dominated by aliphatic and aromatic compounds, promoting the transformation of micropores and mesopores into macropores. The values of <i>I</i>₁, <i>I</i>₂, <i>I</i>₃, and <i>I</i>₄ of the acidified coal are smaller than those of the original coal samples. <i>I</i>₁, <i>I</i>₂, and <i>I</i>₄ further decrease with an increased THF concentration. After the combined treatment, the maximum gas adsorption amount of coal decreases from 14.1439 cm³/g to 9.6635 cm³/g and 6.5026 cm³/g. Acid-THF treatment could lead to a decrease in <i>I</i>₁, with a subsequent reduction in the specific surface area (SSA) and pore volume (PV) and an increase in the average pore size (APS). Moreover, significant solubilization of oxygen-containing functional groups results in a decrease in SSA and PV and an increase in APS with decreasing <i>I</i>₂. Relationships between <i>I</i>₄ and pore structure parameters are similar to those of <i>I</i>₁ and <i>I</i>₂, reflecting a decrease in aromatic condensation that increases the pore size.