Experimental research on the effect of H2S solution on pore structure of low-rank coal

Based on the special geological phenomena that the high concentration of hydrogen sulfide (H2S) exists in coal seam and coal measure water in many low-rank coal mining areas in Xinjiang, the low-rank coal of Shaerhu coal mine was taken as the basic study object. The experimental procedure consists of coal samples preparation, high pressure mercury injection porosimetry (HPMIP) and field emission scanning electron microscope (FESEM) before acidization, H2S solution preparation, sample solution immersion and HPMIP and FESEM after acidization. The acidization of the H2S solution and coal was physically simulated, and the dynamic change of pore structure before and after H2S solution acidizing was studied in detail. The results show that after the H2S solution acidizing, the total pore volume of coal increased by 23.09%, the macropore volume increased substantially by 66. 10% and the mesopore volume increased by 4.86%, which indicated that the pore connectivity was significantly improved. The pore types before and after acidization were dominated by open pores, and the open pores volume increased significantly by 21.18% after acidization. The change of total pore specific surface area was not obvious before and after acidization, the increase of pore specific surface area of micro- and transition pores were 0.42% and 1.37% respectively, and the increase of pore specific surface area of macropores was 60.00%. After the H2S solution acidizing, the fractal dimensions of thermal model decreased which demonstrated that the pore surface became smoother, while fractal dimensions of the Menger sponge model increased which demonstrated that the pore space morphology and structure became more complex. The improvement mechanism of the H2S solution on coal pore structure is mainly derived from the following two aspects: the positive effect caused by the dissolution of carbonate minerals in the coal, which increase the pore volume; the negative effect caused by clay minerals which can disperse, expand, and precipitate into the pores, that reduce the connectivity of the pores. However, the effect of positive effect was larger than the negative effect, indicating the total pore volume of coal increased and the pore connectivity improved. after acidization was compared and analyzed, and the improvement mechanism of coal pore structure was also revealed. The research results are helpful to reveal the correlation between the high concentration of H2S and high CBM production. © 2020, Editorial Office of Journal of China Coal Society. All right reserved.