Micro morphology and pore structure of macerals in coal observed by atomic force microscopy (AFM)

Coal is not only important energy resource, but also potential source of materials. As energy resource, coal has these physical properties which are important for comprehensive applications and transformations. For example, the pore structure of coal is crucial for the reservoir, adsorption, and migration and desorption of coal bed methane. As feedstock of materials, coal can provide good raw material for special technological fields, such as graphite and graphene from coal.  Macerals are the main components of coal, which are significant for properties and applications of coal. Taking coal sample (XJH) from No. 2 coal seam of Xiaojihan coal mine, Yuheng mining area as the object of this study, macerals were observed by atomic force microscope (AFM). Micro morphology and pore structure of vitrinite, semifusinite and fusinite are analyzed to provide fundamentals for clean and comprehensive application of coal in this area. Results show that the micro morphology is featured by granular structure with randomly distributed granules. Vitrinite has the largest Ds value, indicating a complex micro morphological structure. Granules accumulate densely and fluctuate in large extent. Semifusinite has a medium Ds value. Fusinite has the smallest Ds value, and granules accumulate dispersedly and fluctuate in small extent. As for pore structure, vitrinite has the lowest value of mean pore size and volume, but has the largest pore number. It results from the micro pores sized below 2 nm. The pore structure of vitrinite is beneficial to reservoir and adsorption of coal bed methane. Inertinite has larger pore size and volume than vitrinite, but the pore number is less. This is mainly contributed by pores sized in 2～50 nm. The pore structure of inertinite can provide beneficial space for the diffusion of coal bed methane. Inertinite is dominated in pores sized in 2～50 nm, but fractal dimension of pores (D) of semifusinite is larger than fusinite, indicating pores in semifusinite has larger irregular degree than fusinite.