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Molecular simulation of the CH4/CO2/H2O adsorption onto the molecular structure of coal

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Abstract

Clarification of the molecular mechanism underlying the interaction of coal with CH4, CO2, and H2O molecules is the basis for an in-depth understanding of the states of fluid in coal and fluid-induced coal swelling/contraction. In terms of instrumental analysis, molecular simulation technology based on molecular mechanics/dynamics and quantum chemistry is a powerful tool for revealing the relationship between the structure and properties of a substance and understanding the interaction mechanisms of physical-chemical systems. In this study, the giant canonical ensemble Monte Carlo (GCMC) and molecular dynamics (MD) methods were applied to investigate the adsorption behavior of a Yanzhou coal model (C222H185N3O17S5). We explored the adsorption amounts of CH4, CO2, and H2O onto Yanzhou coal, the adsorption conformation, and the impact of oxygen-containing functional groups. Furthermore, we revealed the different adsorption mechanisms of the three substances using isosteric heat of adsorption and energy change data. (1) The adsorption isotherms of the mono-component CH4, CO2, and H2O were consistent with the Langmuir model, and their adsorption amounts showed an order of CH4<CO2<H2O. In addition, high temperatures were non-conducive to adsorption. When the three components of CH4/CO2/H2O were mixed (at a molar ratio of 1:1:1) for adsorption, only the adsorption curve of H2O was consistent with the Langmuir model. (2) The mean values of the isosteric heat of adsorption of CH4, CO2, and H2O were 22.54, 36.90, and 37.82 kJ/mol, respectively; that is, H2O>CO2>CH4. In addition, at higher temperatures, the isosteric heat of adsorption decreased; pressure had no significant effect on the heat of adsorption. (3) CH4 molecules displayed an aggregated distribution in the pores, whereas CO2 molecules were cross arranged in pairs. Regarding H2O molecules, under the influence of hydrogen bonds, the O atom pointed to surrounding H2O molecules or the H atoms of coal molecules in a regular pattern. The intermolecular distances of the three substances were 0.421, 0.553, and 0.290 nm, respectively. The radial distribution function (RDF) analysis showed that H2O molecules were arranged in the most compact fashion, forming a tight molecular layer. (4) H2O molecules showed a significantly stratified distribution around oxygen-containing functional groups on the coal surface, and the bonding strength showed a descending order of hydroxyl> carboxyl>carbonyl. In contrast, CO2 and CH4 showed only slightly stratified distributions. (5) After the adsorption of CH4, CO2, and H2O, the total energy, the energy of valence electrons, and the non-bonding interaction of the system in the Yanzhou coal model all decreased. The results regarding the decrease in the total energy of the system indicated an order of H2O>CO2>CH4 in terms of the adsorption priority of the Yanzhou coal model. The results regarding the decrease in the energy of valence electrons showed that under certain geological conditions, a pressure-induced “coal strain” could lead to a structural rearrangement during the interaction of coal with fluid to form a more stable conformation, which might be the molecular mechanism of coal swelling resulting from the interaction between fluid and coal. An analysis of the contribution of Van der Waals forces, electrostatic interactions and hydrogen bonds to the decrease in non-bonding interactions revealed the mechanism underlying the interactions between coal molecules and the three substances. The interaction between coal molecules and CH4 consisted of typical physical adsorption, whereas that between coal molecules and CO2 consisted mainly of physical adsorption combined with weak chemical adsorption. The interaction between coal molecules and H2O is physical and chemical.

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Authors and Affiliations

  1. Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province, Department of Earth Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024, China

    JianHua Xiang, FanGui Zeng, HuZhen Liang, Bin Li & XiaoXia Song

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  1. JianHua Xiang
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Xiang, J., Zeng, F., Liang, H. et al. Molecular simulation of the CH4/CO2/H2O adsorption onto the molecular structure of coal. Sci. China Earth Sci. 57, 1749–1759 (2014). https://doi.org/10.1007/s11430-014-4849-9

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