Carbonate Mineral Dissolution in Coal and the Associated Evolution of Fracture–Macropore Structure under Hot Flue Gas Treatment

Abstract Hot flue gas injection into coal seams shows considerable potential for enhancing gas drainage from low-permeability coal seams; however, how the carbonate mineralogical composition of coal controls its fracture–macropore response to CO2–H2O acidification remains poorly constrained. In this study, WL, DX, and DLT coal samples with markedly different carbonate mineral contents were selected for acidification experiments using a CO2–H2O system as the reactive analogue of hot flue gas. X-ray diffraction, stereomicroscopic observation, scanning electron microscopy, mercury intrusion porosimetry, and three-dimensional X-ray microscopy (3D-XRM) were jointly employed to elucidate the coupling between mineral dissolution and the evolution of fracture–macropore structures. The results show that pore-structure responses are strongly governed by the carbonate mineralogical basis of the coal. In WL and DX samples, calcite was preferentially dissolved, accompanied by the weakening of the mineral–matrix interface and local space release, which led to three characteristic evolution modes: expansion mode of pre-existing fractures, unblocking mode of closed pores, and generation mode of newly formed fractures. After three acidification cycles, the average surface fracture length of WL increased to 1.20 mm, while the fracture area ratio of DX rose to 9.82%. Correspondingly, the macropore volumes of WL and DX increased by 34.8 and 22.2%, respectively, representing the primary contribution to pore-volume enhancement. By contrast, the low-carbonate DLT sample showed a net reduction in total pore volume (−11.8%), driven mainly by mesopore blockage. These findings demonstrate that hot flue-gas acidification does not universally activate the fracture–macropore structure of coal. Its effectiveness depends on whether carbonate dissolution can be converted into an effective fracture–macropore connected space. This study provides a mineralogical basis for identifying coal seams suitable for hot flue gas stimulation and for optimizing related injection strategies.