Experimental Study on the Pore Structure of Middle- and Low-Rank Coal and Its Influence on Methane Adsorption

Abstract

The pore characteristics of coal have an important influence on the coal adsorption of methane. The liquid nitrogen adsorption method was used to study the pore structure of low- and middle-rank coal samples from two aspects: pore specific surface area and pore shape. Low-field nuclear magnetic resonance (NMR) technology was used to study the difference in methane adsorption of coal samples under the same adsorption conditions. The influence and mechanism of the pore structure of middle- and low-rank coal samples on gas adsorption are discussed and analyzed. The results show the following: (1) There are differences in the adsorption capacity of medium- and low-rank coal samples. Within 1 h of methane inflation, the methane adsorption capacity of middle-rank coals such as Henan Pingdingshan (PDS) and Shanxi Wangzhuang (WZ) and Xinjiang low-rank coals (XJ) increases by 36.4%, 31.7%, and 35.8%, respectively, and as the inflation time is extended to the cumulative inflation of 10 h, the order of the increase in methane adsorption capacity is still PDS>XJ>WZ. (2) The pore types of the experimental coal samples are mostly wedge-shaped pores and ink bottle-shaped pores. The order of the BET specific surface area of the coal samples is in the order of WZ>PDS> XJ, which means that the WZ samples can provide more adsorption sites for gas molecule adsorption, but under the same adsorption conditions, the WZ sample has the least amount of gas adsorbed. Further analysis shows that the order of the tortuosity of the coal sample is WZ>XJ>PDS, which means that the WZ sample has a large pore tortuosity, a long channel for methane molecules to diffuse into the internal pores of the coal sample, and large diffusion resistance. It is the main reason why the methane adsorption capacity of WZ coal samples is less than that of PDS and XJ coal samples in the same adsorption conditions.

Experimental Study on the Effect of Gas Pressure on the Pore Structure and Dynamic Mechanical Properties of Coal

“…To study the effect of gas pressure on the structural change characteristics of the internal pores of coal, many scholars have done much experimental research on this. In recent years, not much research has been done on the use of NMR technology in the field of coal, and some scholars have utilized low-field nuclear magnetic resonance (NMR) technology to study the structural change characteristics of coal bodies affected by different pressures. , To study the effect of gas on the microstructure of coal/rock bodies under a stress state, Liu et al and others conducted gas adsorption tests on raw coal samples under different gas pressure conditions using nuclear magnetic resonance (NMR) technology. The results showed that with the increase of gas pressure, the coal samples reached adsorption equilibrium for a longer period of time and had a significant expansion of the medium and large pores.…”

Influence of Tectonically Deformed Coal-Based Activated Carbon and Its Surface Modification on Methane Adsorption

“…Li 14 revealed distinctive fractal characteristics in the pore structures of differently ranked coal samples, displaying an asymmetric U-shaped pattern and reflecting the influence of coalification on pore fractal dimensions. Jia et al 15 revealed that the greater tortuosity of the middle-and low-ranked coal was responsible for a low methane uptake. Oxygen-containing functional groups (OFGs) on the coal surface influenced methane adsorption behavior by interacting with methane molecules.…”

“…Li revealed distinctive fractal characteristics in the pore structures of differently ranked coal samples, displaying an asymmetric U-shaped pattern and reflecting the influence of coalification on pore fractal dimensions. Jia et al revealed that the greater tortuosity of the middle- and low-ranked coal was responsible for a low methane uptake. Oxygen-containing functional groups (OFGs) on the coal surface influenced methane adsorption behavior by interacting with methane molecules. , Sun et al revealed that the gas adsorption capacity was negatively correlated with the ash content and OFGs, while positively correlated with the vitrinite content in a study via synchrotron radiation nanocomputed tomography and small-angle X-ray scattering.…”