Effect of internal moisture on CH4 adsorption and diffusion of coal: A molecular simulation study

doi:10.1016/j.cplett.2021.139086

Chemical Physics Letters

Volume 783, 16 November 2021, 139086

https://doi.org/10.1016/j.cplett.2021.139086 Get rights and content

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The internal moisture can evidently change the structural properties of anthracite.
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A relatively increased internal moisture can reduce the adsorption property of CH₄.
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The occurrence of moisture inside anthracite can hinder the diffusion of CH₄.

Abstract

To reveal the influence of internal moisture on CH₄ adsorption and diffusion is of great significance to the development of coalbed CH₄. Here, we use molecular simulation methods to explore the influence of internal moisture on the microporous morphology, adsorption property, and diffusion behavior. The results show that internal moisture can change the structural properties of coal, and the pore structure parameters reduce with increase of the internal moisture. Adsorption property indicators indicate an increased internal moisture can reduce the adsorption property. In addition, a negative correlation between the internal moisture and self- and transport diffusivities coefficient is revealed.

Graphical abstract

Introduction

As a non-renewable energy source, coal occurs deep in the stratum, and it is also accompanied by clean and high-calorific hydrocarbons, the main component of which is CH₄ [1], [2], [3]. The coordinated and co-mining of coal and CH₄ can realize the safe and efficient development of dual energy sources [4], [5]. In general, the CH₄ stored in the coal seam is drained firstly, followed by coal seam mining [6]. The extraction of coalbed CH₄ not only ensures the safety of coal mining, but also realizes energy utilization and environmental protection [7]. To understand the principles behind CH₄ adsorption and diffusion is a key research problem. However, in-situ reservoirs contain inherent moisture, which must affect the adsorption and diffusion of CH₄ (see Fig. 1). Therefore, the effect of internal moisture on CH₄ adsorption and diffusion needs to be further explored.

Many reports have been published on the topics of gas adsorption and diffusion in coal, including the influence of internal moisture on it. For instance, some scholars have studied the gas adsorption and diffusion characteristics with different moisture content in coal based on various experimental methods [8], [9], [10], [11], [12], [13], [14], [15]. However, due to the limitations of the experimental method, it is difficult to evenly add and accurately measure the internal moisture, resulting in large differences in the experimental results obtained. To further obtain consistent results, we use molecular simulation methods to explore the influence of internal moisture on the microporous morphology, adsorption property and diffusion behavior. This research can provide molecular insights into the gas adsorption and diffusion in coal with different internal moisture.

Section snippets

Molecular model

Coal has a complex and diverse physicochemical structure, including multi-scale pores and multi-category functional groups [16], [17]. For a long time, many scholars have been trying to build a typical coal structure, such as Given in 1960, Wiser in 1975, Spiro in 1981, Shinn in 1984 and so on [18], [19]. The accuracy and rationality of the molecular model are critical to the simulation results. In this study, the coal molecular constructed by Meng et al., as shown in Fig. 2(a), is selected as

Microporous morphology

As a porous material, coal is composed of an aromatic skeleton and free pores, which can also be described as solid phase and void phase respectively [22]. The microporous morphology of coal greatly affects its adsorption and diffusion process; therefore, it is necessary to analyze and compare the pore structure of coal with different internal moisture.

In the simulation, the nitrogen probe with radius of 1.84 Å is used to predict the pore structure of coal. The relationships between pore

Conclusions

In this work, the effect of internal moisture on CH₄ adsorption and diffusion of coal is investigated through molecular simulation. The main results of the present research work can be summarized as follows:

(1) Internal moisture can evidently change the structural properties of coal, and the pore structure parameters gradually reduce with increase of the internal moisture. Changes in structural properties provide an evidence for the analysis of adsorption and diffusion characteristics.

(2)

CRediT authorship contribution statement

Min Hao: Conceptualization, Methodology, Software, Investigation, Writing–original draft. Chengmin Wei: Formal analysis, Resources, Writing–review & editing. : . Zhen Qiao: Formal analysis, Resources, Writing–review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are grateful to the financial support from the Fundamental Research Funds for the Central Universities (No. 2020YJSAQ05). Thanks to all the reviewers and editors for their work.

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Research paperEffect of internal moisture on CH4 adsorption and diffusion of coal: A molecular simulation study

Highlight

Abstract

Graphical abstract

Introduction

Section snippets

Molecular model

Microporous morphology

Conclusions

CRediT authorship contribution statement

Declaration of Competing Interest

Acknowledgment

Fuel

Energy

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

Int. J. Greenhouse Gas Control

Fuel

Fuel

J. Nat. Gas Sci. Eng.

J. Nat. Gas Sci. Eng.

Fuel

Particuology

Fuel

Fuel

Powder Technol.

Fuel

Fuel

Fuel

Fuel

J. Loss Prev. Process Ind.

J. Nat. Gas Sci. Eng.

Research paper
Effect of internal moisture on CH₄ adsorption and diffusion of coal: A molecular simulation study