Superfine comminution characteristics of low-rank coal pyrolysis semicokes and evolution of fragmentation fractal dimension

•The millimeter size comminution process conforms to the Steacy model.•The scale-depended fractal model could describe the superfine comminution process.•Pore structure and micro-defects have a significant impact on the comminution process.•With size decrease, the comminution mechanism shifts from shattering to abrasion.