Influence of Coalification on the Pore Characteristics of Middle High Rank Coal

The pore size distribution, pore shape and connectivity, and fractal characteristics are investigated to determine the pore characteristics of three different samples of middle high rank coal. Pores of more than and less than 10 nm were measured using mercury intrusion porosimetry (MIP) and gas adsorption, respectively. The pore size distribution was verified with the initial methane diffusion rate and CH4 desorption. Fractal dimensions of seepage pores and adsorption pores were counted using the results from MIP and gas adsorption, respectively. First, the results show that micropores and transition pores occupy the most volume and specific surface area. Micropores and transition pores, as well as porosity, gradually increase as coal rank increases. Second, the fractal dimensions of seepage pores and adsorption pores gradually increase with increasing coal rank, which shows that coalification makes pore structure more complex and pore surface rougher. Additionally, the fractal dimensions of bigger pores are greater than those of smaller pores, implying that the surface and structure of bigger pores is rougher and more complex than those of smaller pores, respectively. Finally, the connectivity of coal has a close relationship with macropores rather than coal rank.