Study on structure evolution features of different rank coals under shock load

The efficient extraction and rational utilization of gas are of great significance for preventing and controlling the gas disaster，improving the air quality and increasing the supply of clean energy. In order to explore the influence of shock load on the microscopic pore structure and macromolecular morphological structure of coal，the lignite，bitumite and anthracite were taken as the research objects，and the Separated Hopkinson Pressure Bar (SHPB) impact test system was used to simulate the shock wave and stress wave of impact stress in the different attenuation processes. Combined with the test data of low temperature liquid nitrogen and Raman spectra，the characteristics and laws of structural evolution of different rank coals under the shock load were studied. The results showed that the total adsorption capacities，total specific surface areas and total pore volumes of lignite，bitumite and anthracite decreased under the effect of shock load. For example，when the shock load were 0，0.5，0.75 and 1.0 MPa，the total BJH specific surface areas of HM-0，HM-1，HM-2 and HM-3 were 7.066，6.611，4.468 and 3.548 m2/g，respectively. The micropores were gradually transformed into transition pores or mesopores，and the proportion of micropores decreased，while the sum of the proportion of transitional pores and mesopores increased. The shock load could promote the effective desorption of gas adsorbed in micropores and improve the diffusion and seepage velocity of gas in transitional pores and mesopores，which could well explain the macroscopic phenomena of the produce of excess coalbed methane in the process of disturbance in coal reservoir. With the increase of the shock load，the D1 peak positions of Raman spectrum for different rank coals gradually moved to the high wave number，the G peak positions gradually moved to the low wave number，and the ID1/IG gradually increased. After the shock load，the ID3/Ito tal of different rank coals decreased compared with the original coal sample. This suggestted that the shock load leaded to the destruction of the microcrystalline structure units in the macromolecular morphological structure of different rank coals，and the degree of microcrystalline growth increased towards the direction of disorder. The degree of intramolecular defects gradually increased，and the relative amount of amorphous carbon also decreased.