Automatic identification and quantification of surface nanoscale pore morphology in coals of different ranks based on AFM, SEM and LP-NGA.

The pore structure characteristics of coal are crucial for coalbed methane adsorption and migration, carbon storage, and safety in deep coal mining. Although traditional methods can detect pore volume and distribution, they are limited in analysing pore morphology and surface properties. This study employs multiscale techniques including AFM (Atomic force microscopy), SEM (Scanning electron microscopy), and LP-NGA (Low-Pressure nitrogen gas adsorption) to systematically analyse the impact of coal rank changes on pore structure and its evolutionary process, covering coals from medium-volatile to low-volatile bituminous and anthracite coals. AFM reveals the three-dimensional morphology and quantitative parameters of nanopores, SEM observes meso- and micropore structures, and LP-NGA verifies pore size distribution. As coal rank increases, surface roughness decreases significantly, the number of pores increases, the average pore diameter decreases, pore morphology transforms from irregular to circular, and porosity increases. Specifically, as the rank of coal increases, the number of nanoring structures rises, while their diameters decrease. Changes in coal rank profoundly affect the nanoring structure, consistent with the evolutionary trend of surface morphology. The combination of AFM and LP-NGA reveals the role of micropores in gas adsorption. This research not only provides a new perspective for understanding the influence of coal rank changes on pore structure characteristics but also offers a theoretical foundation for coalbed methane development, geological sequestration of carbon dioxide, design of coal-based functional materials, and coal mine safety prevention and control.