Dynamic Evolution of Nanoscale Pores of Different Rank Coals Under Solvent Extraction

During the coalification process, coalbed methane (CBM) is formed and mainly adsorbed in the pores of coal. Pore structure evolution is critical to CBM adsorption/desorption and extraction. This paper puts forward two parameters, namely the variety degree x and variety gene σ, for characterizing pore structure through mercury injection tests. Then, under extraction with different solvents, the dynamic evolution characteristics of nanoscale pores are addressed and quantified by taking four different rank coals (lignite, medium-volatile bituminous coal, low-rank anthracite and mediumrank anthracite) from different coal mines of China as the study object. The results indicate that the content of meso- and macropores after solvent extraction is much larger, but that there is no obvious law with the content of transition pores and micropores in the size range of 50-7.2 nm, according to the basic data sets of specific surface area (SSA) and pore volume (PV) of all coal samples. This phenomenon can be explained by the pore increase and expansion effects in nanoscale pores during solvent extraction. Generally, with the increasing of the solvent extraction degree, the difference in variety degree x with respect to the total PV and total SSA of different coals shows a significant decreasing trend, which expresses a homogeneous development in the change in pore structure. In regard to different solvents, benzene mainly causes pore expansion in meso- and macropores, and CS₂ has a great effect on micropores. Whereas acetone plays an important role in mesopores and transition pores with pore expansion, THF has various effects on different size pores. Further study with higher variety gene σ values shows that the total PV mainly depends on the change in the absolute content of meso- and macropores. While the change in the absolute content of transition pores and micropores (less than 50 nm) has a great influence on the total SSA. As the extraction degree increases, the influence of the transition pores and micropores on the total PV is increased, and then, the content of meso- and macropores also plays an important role on the total SSA. However, this effect is highly different for raw coals of different ranks.