Experimental research on evolution characteristics of coal pore systems under acidification–modification conditions

The pore mechanism of coal under acidic treatment is a critical factor for optimizing pore structure and enhancing effective gas extraction. This study aims to provide a comprehensive and comparative understanding of pore structure evolution across different coal ranks (Datong lignite, Ordos bituminous coal, and Zhaogu anthracite) treated with an optimized mixed-acid system (sulfuric, nitric, and acetic acids). Using low-temperature nitrogen adsorption (LTNA) and fractal dimension analysis, we quantified the changes in adsorption capacity and pore morphology. Results show that the tailored acid treatments significantly reduced the specific surface area (e.g., up to 90% reduction in lignite) and the volume of adsorption pores (< 100 nm). Conversely, the dissolution of embedded minerals facilitated the coalescence of isolated micropores into interconnected mesopores and macropores, which expanded the migration space and decreased the pore structural complexity. Consequently, the effective porosity increased, suggesting a strong potential for enhanced permeability. This study provides a novel multi-rank perspective on the microscopic physical modifications induced by mixed-acid systems, laying a robust theoretical foundation for permeability-enhancing technologies in coalbed methane recovery. The most significant contribution of this study is that, for the first time, it systematically elucidates the mechanism by which an optimized mixed acid system of sulfuric, nitric, and acetic acids differentially modifies the pore structures of lignite, bituminous coal, and anthracite from a multi-coal-rank comparative perspective: by selectively dissolving minerals, this mixed acid system drastically reduces the specific surface area (e.g., by up to 90% in lignite) and the volume of adsorption pores (100 nm), while simultaneously promoting the interconnection of isolated micropores into mesopores and macropores. This effectively reduces the complexity of the pore structure, as characterized by the fractal dimension, and ultimately significantly increases effective porosity. These findings provide a theoretical basis for the application of mixed-acid permeability enhancement technology in coalbed methane development across different coal ranks.