Experimental study on changes in components and pore characteristics of acidified coal treated by organic solvents.

• Coal chemical components and structures affected by acidification and THF are studied. • Effect of successive treatments of acidification and THF on pore fractal features is analyzed. • Relationship between SSA, average pore size and fractal parameters are discussed. • Successive treatments of acidification and THF could well improve pore development. Chemical solvents could change coal components and effectively enhance conductivity of coal reservoir, which is beneficial to efficient methane extraction. Relationship between coal chemical structure parameters and main functional groups variations affected by acidification and tetrahydrofuran (THF) is investigated through infrared spectroscopy tests and curve peak fitting. Meanwhile, variations in fractal dimensionality and pore structure characteristics are studied by adopting low-temperature nitrogen adsorption tests and Frenkel-Halsey-Hill (FHH) fractal model. Main results are: (1) After extracting raw coal and acidified coal using THF with varying concentrations, absorbance of aromatic hydrocarbon –CH surface and aromatic methyl bond become weaker. While the proportion of substituted aromatic hydrocarbons increases in acidified coal than in raw coal. Acidification treatment could reduce strength of C-O-C absorption peak. Proportion of CH 2 and CH 3 in raw coal decreases with increasing THF concentration, indicating that THF could react with aliphatic structure to reduce CH 2 and CH 3 content. Extraction with THF dissolves aromatic structure, lowering C = C content in coal to different degrees. (2) Acidification pretreatment could produce more micropores in coal samples. Micropore contents of acidified coal after THF extraction are all lower than coal sample S, while transition pores and mesopores contents are both higher than those in coal sample S. (3) Fractal dimension D 1 of raw coal indicates a relatively linear relationship with specific surface area, while the D 1 of acidified coal is less correlated with specific surface area. The factor D 2 shows a linear relationship with average pore size. After treatment of THF, average pore size of raw coal increases with D 2 , while that of acidified coal varies in different degrees with uneven distribution of pore sizes. [ABSTRACT FROM AUTHOR]