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 (I1), oxygen enrichment (I2), aromatization (I3), and aromatic condensation (I4) 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 I1, I2, I3, and I4 of the acidified coal are smaller than those of the original coal samples. I1, I2, and I4 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 I1, 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 I2. Relationships between I4 and pore structure parameters are similar to those of I1 and I2, reflecting a decrease in aromatic condensation that increases the pore size.