Study on physical and chemical structure modification law and mechanism of coal based on organic acidification

Deep coal seams in China have the characteristics of low permeability, low porosity, and poor gas permeability, which makes the extraction of coalbed methane very difficult. To study the modification effect of acidic solutions on coal pore structure and mineral components, this study used three different concentrations (5%, 10%, and 15%) of organic acids (HCOOH and CH3COOH) to treat coal. Through industrial/elemental analysis, low-temperature nitrogen adsorption experiments, and x-ray fluorescence spectroscopy detection experiments, the pore size distribution, mineral elements, and oxides of the acidified coal were analyzed. The strength of the acid solution's modification effect on coal was analyzed, as well as the relationship between different mineral elements and changes in pore structure. The results show that the two acids have a pore expanding effect on coal, with an increase in average pore size; the pore size distribution is between 2 and 50 nm, belonging to mesopores; acetic acid has a stronger overall dissolution efficiency on inorganic minerals, while formic acid has a better effect on the removal of Ca and P elements, but none of them can effectively remove silicoaluminates; the content of Ca element in coal is positively correlated with the specific surface area provided by mesopores; and iron dolomite is not sensitive to the reaction with acetic acid, resulting in an opposite trend in Fe content and mesoporous specific surface area. Organic acid solution changes the pore and fracture distribution structure of coal through chemical dissolution by dissolving the mineral components inside the coal, thereby affecting its internal structural characteristics.