Micro-pores and fractures of coals analysed by field emission scanning electron microscopy and fractal theory

•Nanoscale pores were analysed by FESEM and fractal theory.•Nanopores in coal matrix can be categorised as micro-pores or micro-fractures.•Micro-pores are round or elliptical and micro-fractures are irregular bending.•As coalification, the average width of micro-pores changes very small.•With increasing coal rank, the average width of micro-fractures changes regularly.

In this study, the micro-pores and fractures of six coal samples with different metamorphic degrees (Ro,m = 0.75–2.80%) were analysed using field emission scanning electron microscopy (FESEM). The micro-fractures and pores were quantitatively classified based on fractal theory. Our results show that pores in coal matrix can be categorised as either micro-pores or micro-fractures according to fractal theory. Micro-pores exhibit round or elliptical shapes, whereas micro-fractures are bent and exhibit irregular shapes, which are connected with other micro-fractures via round or elliptical channels. With increasing metamorphic degrees, the distribution of the total surface porosity of micro-pores and micro-fractures is characterised by an inverted “U” shape. The total surface porosity is at its greatest during the high-rank bituminous coal stage. However, the correlation between coal metamorphic degree and surface densities of micro-pores and micro-fractures varies. During the low- and medium-rank stages, the surface frequency and the average width of micro-pores and fractures increase with increasing coal rank. During the high-rank coal stage, with increasing coal rank, the surface frequency of micro-pores and fractures rapidly increases but as a whole is lower than those during the low- and medium-rank stages. The average width of micro-fractures decreases with increasing average surface frequency in a logarithmic relationship; in contrast, the average width of micro-pores increases logarithmically with increasing average surface frequency.