Development characteristics of coal microfracture and coal petrology control under cyclic high voltage electrical pulse

Based on the open experimental platform of cycle high voltage electrical pulse generator, the impact cracking experiments of three kinds of coal samples with different metamorphic degrees, namely, fat coal, lean coal and anthracite were carried out under two kinds of energy loading methods: metal wire and energetic material. Then, the characteristics of coal rank and load response of microfracture were analyzed by the polarizing microscope, and the relationship between microfracture evolution and macerals was studied. The results show that the increasing trend between the density of microcracks and the impact times is nonlinear, which can be roughly divided into three stages: slow increase in the initial stage, rapid increase in the middle stage, and stabilization in the later stage in the first place. This means that the cracking effect is not better with more cyclic impacts, and there is an optimal impact number. Second, the microfracture density in coal under wire loading is more developed than that under energetic material loading. In general, the development degree of micro fissures in fat coal is higher than that in lean coal, and that in lean coal is higher than that in anthracite. It shows that the cyclic high-voltage electrical pulse has "double bottom effect" in fracturing effect. Namely, when both low energy loading condition and low rank coal are available, the cracking effect is better. Third, there are some differences and emphases in the development of microfractures among the macerals. Vitrinite is the most developed, followed by inertinite, and exinite is the least developed. This is mainly due to the advantages of vitrinite, such as volume content(space dominance), original fissure density(location advantage) and brittleness(mechanical superiority). Fourth, the microcracks in the macerals can be classified into four main types: through the macerals, confined within the macerals, developing along the boundary of the macerals, showing oblique morphology and progressive tensile shear extension. Finally, these conclusions are of important theoretical significance for further revealing the microscopic mechanism of coal seam fracture induced by cyclic high voltage electric pulse, clarifying the target coal seam in the engineering practice, optimizing the working process parameters and improving the operation results.