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.