Micrometer-scale fractures in coal related to coal rank based on micro-CT scanning and fractal theory

•Microfractures were analyzed using micro-CT scanning technique and fractal theory.•Type B microfractures are mainly developed in the experimental coal samples.•Microfracture length, width, volume, and aperture reduces as coal rank increases.•Quantity and fractal dimensions of microfractures increase as coal rank increases.•CT porosity changes with fractal dimensions, displaying a ‘U-shaped’ trend.