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

Abstract

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 CH 4 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.

Fractal characteristics of coal samples utilizing image analysis and gas adsorption

“…The calculation of fractal dimensions by the FHH model using LP-N 2 GA data has been discussed detailedly in the literature [75], and the surface fractal dimension calculation can be described as [34,40]:…”

“…During the process of coalification, the polycondensation of coal molecules occurs. As a consequence, the number of mesopores and the mesopore diameter decrease [34,80], making the pore distribution homogeneous and the coal surface more regular. This polycondensation effect of coal molecules results in the decline of D values with increasing metamorphism in lower rank coals.…”

“…As the coalification continuously rises (V daf . < 15%), the polycondensation effect intensifies, leading to coal bulk further compaction [24,34]. Because of the uneven bulk compaction, the internal stress of coal is larger than the strength of coal, then the micropores and cracks gradually develop in coals.…”

“…Pan et al [32] and Deng et al [33] also suggested that coal rank, mineral composition and deformation structures have a significant effect on microstructures of coals. Wang and co-workers [34] found that coalification makes pore structure more complex and pore surface rougher and that the fractal dimensions of seepage pores and adsorption pores gradually increase with increasing coal rank for middle-high rank coals.…”

“…Based on the physical description of the fractal surfaces, pore structure and fractal dimension were investigated by Correa et al [41] and Yang et al [42], indicating that micropores have a greater effect on fractal dimension than mesopores and macropores, and fractal dimension is negatively correlated with permeability. Previous research has demonstrated that the fractal dimension of coal can be acquired by various methods, including mercury intrusion [34,43,44], transmission electron microscopy (TEM) [45,46], scanning electron microscopy (SEM) [47,48], nuclear magnetic resonance (NMR) [49,50], field-mission SEM [40,51], small-angle X-ray scattering (SAXS) [52][53][54], and gas adsorption [39,42,55]. Among these fractal methods, low-pressure N 2 gas adsorption (LP-N 2 GA) analysis has been shown to be an effective method of characterizing fractal dimensions of coal [34,40,56].…”