Experimental analysis of pore structure and fractal characteristics of soft and hard coals with same coalification

Abstract

Accurate and quantitative investigation of the physical structure and fractal geometry of coal has important theoretical and practical significance for coal bed methane (CBM) development and the prevention of dynamic disasters such as coal and gas outbursts. This study investigates the pore structure and fractal characteristics of soft and hard coals using nitrogen and carbon dioxide (N2/CO2) adsorption. Coal samples from Pingdingshan Mine in Henan province of China were collected and pulverized to the required size (0.20–0.25 mm). N2/CO2 adsorption tests were performed to evaluate the specific surface area (SSA), pore size distribution (PSD), and pore volume (PV) using Braunuer-Emmett-Teller (BET), Barrett-Joyner-Halenda (BJH), and Density Functional Theory (DFT). The pore structure was characterized based on the theory of fractal dimensions. The results unveiled that the strength of coal has a significant influence on pore structure and fractal dimensions. There are significant differences in SSA and PV between both coals. The BJH-PV and BET-SSA obtained by N2-adsorption for soft coal are 0.029–0.032 cm3/g and 3.523–4.783 m2/g. While the values of PV and SSA obtained by CO2-adsorption are 0.037–0.039 cm3/g and 106.016–111.870 m2/g. Soft coal shows greater SSA and PV than hard coal, which is consistent with the adsorption capacity ($${V}_{\mathrm{L}}$$ V L ). The fractal dimensions of soft and hard coal are respectively different. The Ding coal exhibits larger D1 and smaller D2, and the reverse for the Wu coal seam is observed. The greater the value of D1 (complexity of pore surface) of soft coal is, the larger the pore surface roughness and gas adsorption capacity is. The results enable us to conclude that the characterization of pores and fractal dimensions of soft and hard coals is different, tending to different adsorption/desorption characteristics. In this regard, the results provide a reference for formulating corresponding coal and gas outburst prevention and control measures.

Quantitative Analysis of the Effect of NaClO Stimulation in Anthracite and Bituminous Coal with FTIR and LF-NMR

“…Fractal dimension is usually used to characterize the heterogeneity of pore structure in coal . As the single T 2 cutoff model cannot accurately identify the state and migration behavior of fluids in rock, a dual T 2 cutoff model has been introduced corresponding to the three-peak characteristics of the T 2 spectra of coal samples before and after oxidation.…”

“…Fractal dimension is usually used to characterize the heterogeneity of pore structure in coal. 52 As the single T 2 cutoff model cannot accurately identify the state and migration behavior of fluids in rock, 53 a dual T 2 cutoff model has been introduced corresponding to the threepeak characteristics 54 of the T 2 spectra of coal samples before and after oxidation. Based on the fractal theory, 55 the cumulative The LF-NMR T 2 spectra consist of volume relaxation (T 2B ), diffusion relaxation (T 2D ) and surface relaxation (T 2S ), which can be expressed as…”

Occurrence of moisture in deep gas-bearing shale matrix and its impacts on methane adsorption/desorption capability under favorable reservoir conditions

“… 53 Additionally, the differential mesopore volume distribution profile was generated by applying the DFT combined with adsorption amounts. 54 …”

“…53 Additionally, the differential mesopore volume distribution prole was generated by applying the DFT combined with adsorption amounts. 54 The aforementioned characterizations were conducted to the dry and moist samples. It is noteworthy that the degassing operated at the vacuum degree of 0.1325 Pa and the temperature of 333.15 K for 24.00 h was only performed to the dry samples to completely remove residual moisture and gas.…”

The Physical Behavior of Protected Coal Seams Based on Triaxial Unloading Conditions

“…Protective seam mining can realize both the stress unloading of the coal body of the protected seam, which reduces the possibility of high-energy disasters, and the resolution of the original adsorbed state gas in the coal body of the protected seam [12], which provides conditions for the extraction of gas from the protected seam. Gas threatens the safe production of coal mines [13][14][15], but it is also a clean energy source [16]. It is economical (and indeed sensible) to conduct coal seam mining while also achieving the extraction and utilization of unburdened gas [17].…”