Characterisation of competitive adsorption of CH₄ and CO₂ mixed components in a coal body based on molecular simulation

Abstract

The competitive adsorption behaviour of CH₄ and CO₂ in coalbed methane mining is crucial for the optimisation of CO₂ enhanced coalbed methane recovery (CO₂-ECBM) technology. In this paper, the giant canonical monte carlo (GCMC) method is used to simulate and study the competitive adsorption behaviour of CH₄ and CO₂ mixed component gases at different temperatures, pressures, water contents and molar ratios. The results showed that: the competitive adsorption capacity of CO₂ was significantly stronger than that of CH₄, and the increase of its molar ratios could effectively replace CH₄; The increase of temperature or water contents decreased the heat of adsorption of equivalence of CH₄ and CO₂ as well as the Langmuir parameters a and b, which weakened the adsorption selectivity of CO₂, but the competitive ad-sorption advantage was still maintained; the competitive adsorption ad-vantages of CH₄ were also maintained. However, its competitive adsorption advantage was still maintained; under the conditions of low pressure and high CO₂ molar ratios, CO₂ had the strongest ability to drive CH₄; under the conditions of high pressure and low CO₂ molar ratios, the adsorption selec-tivity coefficient of CO₂ to CH₄ decreased the most; there was an upper limit of saturation in the driving of CH₂ by CO₂, and even if the molar ratios con-tinued to increase, the driving efficiency was still low. There is a sat-uration limit for the substitution of CH₄ by CO₂, and even if the molar ratios of CO₂ continues to increase, the substitution efficiency is not significantly improved. This study reveals the competitive adsorption mechanism between CH₄ and CO₂ at the mo-lecular level, and provides a theoretical basis for optimising CO₂-ECBM technology and improving CBM recovery.

Introduction

Against the background of global warming and the ‘dual-carbon’ goal, the devel-opment and utilisation of coalbed methane (CBM) has become a research hotspot for scholars at home and abroad¹. CBM, mainly composed of CH₄, is a chained aliphatic hydrocarbon gas stored in coal seams, which is a green and efficient clean energy source⁵. With the increasing concern for environmental protection and energy transi-tion, the efficient extraction and utilisation of CBM is becoming more and more crucial. However, temperature and pressure are important factors affecting the gas adsorption capacity of the coal body during CBM production⁶. In addition, the presence of mois-ture is also a challenge because moisture affects the CBM transport pro-cess⁸, which in turn affects the mining efficiency. Currently, CO₂-ECBM technology has be-come the focus of current research¹¹. By injecting car-bon dioxide into the coalbed, CO₂-ECBM technology can not only effectively enhance the production of coalbed me-thane, but also achieve the sequestra-tion of CO₂ to achieve the goal of emission reduc-tion¹³. Many scholars have carried out a large number of studies, Liang et al.¹⁷, Sun et al.¹⁸, Krooss et al.¹⁹, Day et al.²⁰, Li et al.²¹, Zhou et al.²² have all demon-strated that CO₂ injection is able to effectively replace methane in the coals through experimental studies of gas adsorption in the coal body. However, it was found that the increase in temperature, the change in pressure, and the presence of moisture can significantly affect the gas adsorption capacity and replacement efficiency. Specifically, an increase in pressure usually promotes gas adsorption, but the addition of moisture inhibits gas adsorption. In con-trast, an increase in temperature leads to a more signif-icant desorption effect of CO₂, reducing its adsorption capacity in the coal body. As a powerful tool, molecular simulation technology makes up for the limitations of tradi-tional macroscopic experimental methods at the microscopic level. Through molecular simulation, we are able to study in depth the adsorption behaviour and diffusion characteristics of gases in the coal body, and thus predict more ac-curately the extrac-tion efficiency of CBM and the optimization potential of CO₂-ECBM technology. The GCMC method simulation is widely used in the study of adsorption behaviour of coalbed methane, which can effectively simulate the competitive adsorption behaviour of gases in the coal body. In recent years, with the continuous advancement of simulation methods and computing power, the coupled use of GCMC and molecular dynamics (MD) simulations has become an important means of studying the transport behaviour of gases in porous media. This approach not only captures adsorption states but also further characterises diffusion paths and migration characteristics. At the same time, some scholars have introduced machine learning algorithms to perform data-driven modelling of coal structural parameters or adsorption performance, thereby improving the predictive efficiency and broad adaptability of simulations. Zhou Junping et al.²³, Wu et al.²⁴, Li et al.²⁵, Jia et al.²⁶, Zhao et al.²⁷, Sa et al.²⁸, Xu et al.²⁹, Xiang et al.³⁰, Dang et al.³¹, Long et al.³², Jin et al.³³, Zhang et al.³⁴, and Sun et al.³⁵ have all used the GCMC method simu-lation to to study the competitive adsorption behaviour of gases. It was shown that the adsorption capacity of the coal body for CO₂ was significantly stronger than that of CH₄, and the presence of moisture significantly affected the adsorption behaviour of the gas. Although a large number of studies have been conducted to reveal the effects of temperature, pressure, moisture and gas molar ratios on gas adsorption behaviours during CBM mining, macro-scopic experiments and so on cannot directly reflect the microscopic competi-tive adsorption behaviours of gases in the coal body. Therefore, Jundong Wu-caiwan coal was selected as the research object in this paper³⁶. The GCMC method was used to study the adsorption behaviours of CH₄ and CO₂ mixed components in the coal body under different conditions of temperature, pressure, wa-ter contents and molar ratios. Through the analysis of key pa-rameters such as adsorp-tion isotherms, Langmuir fitting parameters, and adsorption selectivity coefficients, the competitive adsorption characteristics of the two gases in the coal body were re-vealed, which provided a theoretical basis for optimising the CBM recovery and CO₂-ECBM technology.

Models and methods

Coal model construction and parameters

The molecular structure model was constructed in this paper using the coal of Wucaiwan, Zhundong as a research object, C₂₀₆H₁₂₈O₃₆N₂³⁶, which is dominated by aromatic carbon and has a lamellar structure. In the molecule, fatty carbon atoms primarily exist in a branched form or are attached to the side of the aromatic ring, with relatively long alkane side chains, exhibiting a certain degree of spatial conformation complexity. In terms of heteroatom distribution, oxygen primarily exists in the form of ether bonds (C-O), while nitrogen is mainly incorporated into the aromatic skeleton in pyrrole-type (N-5) and pyridine-type structures, with pyrrole-type nitrogen accounting for a higher proportion; sulphur atoms exist in the form of sulphone groups. The aromatic rings are primarily tri-substituted and tetra-substituted structures, indicating a high degree of substitution and aromaticity in the molecules. Compared to typical coal molecular models (such as the Wiser model), the coal molecular structure adopted in this study is primarily composed of aromatic carbon, with a more compact structure and shorter side chains, better aligning with the microscopic characteristics of medium-to-high-rank coal (such as coal rich in vitrinite). In contrast, the Wiser model has a higher proportion of aliphatic carbon and a more loosely structured arrangement, making it more suitable for studies of lower-rank coals.

First, the coal molecule was optimised by the Forcite module of Materials Studio 2020 using the Geometry Optimisation and Anneal methods to obtain the optimised structure as shown in Fig. 1a. Next, the Amorphous Cell module was used to construct the original cell structure of the coal molecule. By selecting six coal molecules with energy minimisation and geometry optimisation and annealing treatment, these molecules were placed in the simulation box using random distribution. In order to simulate the actual environment of the CBM, periodic boundary conditions were also set, resulting in the final coal structure model as shown in Fig. 1b. The density test was performed by Dynamic in the Forcite module and the density of the structural model was cal-culated to be 1.253 g/cm³. To further investigate the pore properties of the coal structure, based on the Connolly surface theory (Connolly, 1983) and its calculations, the AtomVolumes & Surfaces tool was used and a rigid probe molecule with a radius of 0.75 Å was used to determine the Connolly surface of the coal structure model and the internal pore size distribution of the coal body was obtained. The total volume can be divided into two parts: one part is the volume occupied by the coal skeleton, which is 19533.78 ų, and the other part is the pore volume, which is 5108.39 ų. This verifies the pore structure characteristics of the coal body and indicates that the constructed coal structure model has high rationality in terms of pore characteristics, making it suitable for simulation studies on gas adsorption and diffusion characteristics. Where the grey area represents the occupied volume and the blue area represents the void volume, and the pore distribution of the coal body is shown in Fig. 1c.

Fig. 1.

Molecular structure and modelling of coal.

Model construction for different moisture contents

On the basis of the above model, in order to investigate the effect of dif-ferent water contents on the gas adsorption behaviour of the coal body, in this paper, different numbers of water molecules were pre-adsorbed in the coal structure model using the Sorption module in Materials Studio 2020, in particular, the Locate task item. Specifically, 0, 15, 35, and 70 H₂O molecules were adsorbed in the coal body in order to construct the coal body model with water contents of 0%, 1.40%, 3.28%, and 6.55%, respectively, which are calculated as follows in Eq. (1). The changes of coal body structure under different water contents conditions are shown in Fig. 2.

Fig. 2.

Structural modelling of coal with different water contents.

As seen in Fig. 2, with the increase of water contents, water molecules gradually fill the pores of the coal, resulting in a decrease of the effective volume, which limits the gas adsorption. At low water contents, water molecules are mainly combined with polar functional groups of coal through hy-drogen bonding. Whereas, under high water contents conditions, water molecules fill the pores and may form water bridges or a continuous water film, which further hinders gas adsorption³⁷. Water molecules preferentially occupy the polar adsorption sites of coal, leading to a decrease in gas adsorption, which in turn reduces the adsorption capacity of the coal body. Therefore, the moisture content has a significant effect on the pore structure and adsorption characteristics of coal bodies. In the study of gas adsorption in coal bodies, the change of water contents must be fully considered in order to more accurately reflect the behaviour of gases in coal bodies.

Adsorption simulation programme

The adsorption behaviour of CH₄ and CO₂ in the coal body under different conditions was simulated using the Fix pressure task item of the Sorption module in Materials Studio 2020. By adjusting the simulation pressure range (0–10 MPa), the cellular configuration, adsorption isothermal curves, Langmuir fitting parameters, and adsorption selectivity coefficient curves of the coal body under different conditions were obtained, which provided data support for the indepth understanding of the adsorption characteristics of gases in the coal body. The simulation scheme was set up as follows: (1) In order to investi-gate the effect of molar ratios on the adsorption of two-component CH₄ and CO₂, the molar ratios of the two gases, CH₄ and CO₂, were set as 7:3, 1:1, 3:7, and 1:9, and the moisture content of the coal body was 1.41%, and the tem-perature was 293.15 K. (2) In order to examine the temperature effect on the adsorption behaviours of two-component effect, the CH₄ to CO₂ molar ratios was kept at 7:3, and the temperatures were set to 293.15 K, 303.15 K and 313.15 K. The water contents of the coal body was 1.41%; (3) in order to ana-lyse the effect of the water contents, the molar ratios of CH₄ to CO₂ was set to 3:7, and the temperatures were 293.15 K. The The water contents of the coal body was set to 0%, 1.41%, 3.28% and 6.55%.

Adsorption simulation parameters

Gas adsorption simulation is based on GCMC method, and the calcula-tion method selects Fixed pressure or Adsorption isotherm task item in Sorption module³⁹. Some scholars simulate the construction of the crystal cell with detailed expressions⁴⁰, in order to avoid the influence of unstable pressure on the simulation results, so the Fixed pressure task item was selected for the molecular simulation calculation. Simulation parameter set-tings: the calculation method is Metropolis, the accuracy is Fine, the simula-tion loading equilibrium step is 1 × 10⁶, the total number of process steps is 1 × 10⁷, the simulation force field is Compass, and the charge equilibrium method assigned by Forcefield is used to carry out the simulation. Based on the set charge of each atom, the convergence accuracy of the initial charge was set to 5.0 × 10⁻⁴, the Coulomb force was calculated by Ewald, and the van der Waals force and hydrogen bonding interaction was calculated by Atom based with a truncation radius of 18.5 Å. The Compass, force field and a cutoff radius of 18.5 Å were selected, with parameter settings based on simulation experience from existing coal-related systems. Adsorption and diffusion trends remained consistent across different cutoff radii, indicating that the results are insensitive to parameter changes, and the selected settings exhibit good stability and applicability. Furthermore, the COMPASS force field has been thoroughly validated in studies of organic solid and gas adsorption, effectively describing the non-bonding interactions between coal structures and gas molecules^30,39.

Adsorption calculation model

In order to investigate the effect of different water contents on the gas adsorption behaviour in the coal body, Eq. (1) is calculated as follows⁴³:

1

where is the water contents, %; is the molar mass of water, g/mol; and is the total molar mass of the coal structure model, g/mol.

The number of molecules adsorbed per unit cell (Average molecules/cell), i.e., the number of molecules of adsorbent per cell, was obtained by simulation with the GCMC method, while the unit commonly used in the experiments is mmol/g, and the Eq. (2) for the unit conversion is as follows⁴⁴:

2

where is the adsorption volume, mmol/g; is the number of molecules of the adsorbent within a single crystal cell; _a is Avogadro’s constant, 6.02 × 10²³; is the mass of the crystal cell unit, M = 3.099 × 10⁻²⁰ g.

In order to analyse the gas adsorption behaviour,, which is widely used to describe the adsorption behaviour of gases on solid surfaces, and is suitable for analysing the adsorption characteristics of CH₄ and CO₂ in coal bodies.Equation (3) of the Langmuir model is as follows⁴⁵:

3

where is the gas adsorption volume, mmol/g; is the saturation adsorption volume, mmol/g; is the adsorption equilibrium parameter, MPa⁻¹; and is the equilibrium pressure, MPa.

In order to further analyse the competitive adsorption characteristics, the competitive adsorption relationship between CH₄ and CO₂ can be intuitively determined by calculating the adsorption selectivity coefficient. When the selectivity coefficient of CO₂ to CH₄ is greater than 1, it indicates that CO₂ has a stronger competitive adsorption capacity. The formula (4) for the selectivity coefficient is as follows⁴⁷:

4

where is the CO₂ to CH₄ adsorption selectivity coefficient; is the molar ratios of CO₂; is the molar ratios of CH₄; and / refer to the molar ratios of the CO₂ and CH₄ gases in the free state.

Results and discussion

The influence of molar ratios and pressures

Adsorption isothermal curve analysis

The Langmuir model was used to fit the simulation results. Equations (2) and (3) were used to obtain the isothermal curves of CH₄ and CO₂ adsorption under different molar ratios conditions, and the results are shown in Fig. 3. The corresponding fitting parameters were obtained by Langmuir fitting as shown in Table 1.

Fig. 3.

Adsorption isotherms of CH₄ and CO₂ at different molar ratios.

Table 1.

Langmuir fitting parameters for CH₄ and CO₂ at different molar ratios.

Molar ratios CH4:CO2	a/(mmol·g−1)		b/MPa−1		R 2	R 2
	CH4	CO2	CH4	CO2	CH4	CO2
7:3	0.276	1.039	14.289	17.783	0.99	0.98
1:1	0.233	1.162	13.936	17.792	0.98	0.99
3:7	0.159	1.301	13.709	18.594	0.99	0.99
1:9	0.121	1.353	10.078	18.891	0.98	0.98

From Fig. 3; Table 1, it can be seen that the adsorption amount of CO₂ was significantly higher than that of CH₄ under the four molar ratios, and the fitting accuracies R² were all greater than 0.98. The increase in the adsorption amount of CO₂ decreased gradually when the CH₄:CO₂ molar ratios was changed from 7:3 to 1:9, which indicated that there was an upper limit to the CH₄ repulsion by CO₂. drive existed an upper limit.The adsorption of CO₂ increased nonlinearly with the increase of molar ratios and reached 91.8% at 1:9, whereas the adsorption of CH₄ decreased nonlinearly. The adsorption of both gases increased with increasing pressure, but the increase in adsorption slowed down at high pressures, and the adsorption of CO₂ increased faster than that of CH₄. According to the Langmuir fitting parameters, the Langmuir constant a for CH₄ decreased with increasing CO₂ molar ratios, whereas the value of a for CO₂ increased with increasing molar ratios, suggesting that CO₂ dominated the competing adsorption and inhibited the adsorption of CH₄. This is consistent with the conclusions of Liang¹⁷, Sun¹⁸, and other studies. Similarly, the b value of CH₄ decreased with increasing CO₂ molar ratios while the b value of CO₂ showed an increasing trend, which further confirmed the inhibition of CH₄ adsorption by CO₂²¹. It was shown that increasing the molar ratios of CO₂ significantly increased its adsorption while decreasing the adsorption of CH₄, suggesting the dominance of CO₂ in competitive adsorption. Although high pressure can promote the adsorption amount of the gas, its growth tends to level off at high pressure. It should be noted that when the CO₂ molar ratios is too high, the enhancement of CH₄ displacement efficiency is limited despite the increase of CO₂ adsorption.

Analysis of adsorption selectivity coefficient

As can be seen from Fig. 4, the adsorption selectivity of CO₂ on CH₄ increased significantly with increasing CO₂ molar ratios. This was attributed to the fact that high CO₂ molar ratios enhanced its competitive adsorption capacity in the coal body²³. At low pressure (0.1 MPa), the selectivity coefficient increased from 6.47 to 23.02, and the results were consistent with the study of Dang et al.³¹. When the CO₂ molar ratios is low, the selectivity coefficient tends to decrease with increasing pressure. Whereas, at high CO₂ molar ratios, the selectivity coefficient increased at the initial stage and then slowly decreased. It was shown that elevating the CO₂ content could enhance the competitive adsorption capacity at the initial stage of adsorption, but the advantage gradually weakened as CO₂ reached a certain saturation level. Therefore, the molar ratios and pressure of CO₂ are the key factors affecting the adsorption selectivity coefficient. Therefore, under the conditions of low pressure and high CO₂ molar ratios (0.1 MPa, CH₄:CO₂=1:9), CO₂ has the strongest ability to displace CH₄, and the adsorption selectivity coefficient reaches a maximum of 23.02, which is suitable for realising the high efficiency displacements of CO₂ on CH₄. While under the conditions of high pressure and low CO₂ molar ratios (10 MPa, CH₄:CO₂=7:3), the adsorption selectivity coefficient was the smallest, which was only 4.39, indicating that this condition was not conducive to the efficient recovery of CH₄.

Fig. 4.

The adsorption selectivity coefficient curves of CH₄ to CO₂ at different molar ratios.

The influence of temperature and pressures

Adsorption isothermal curve analysis

The adsorption isothermal curves of CH₄ and CO₂ at different temperatures were obtained by using the Langmuir model of Eqs. (2) and (3) and fitting it with the above simulated data as shown in Fig. 5. The corresponding Langmuir fitting parameters are shown in Table 2.

Fig. 5.

Adsorption isotherms of CH₄ and CO₂ at different temperatures.

Table 2.

Langmuir fitting parameters of CH₄ and CO₂at different temperatures.

temperature/K	a/(mmol·g−1)		b/MPa−1		R 2	R 2
	CH4	CO2	CH4	CO2	CH4	CO2
293.15	0.286	1.039	14.289	17.783	0.99	0.98
303.15	0.248	0.917	8.172	10.381	0.98	0.99
313.15	0.213	0.729	5.246	6.657	0.99	0.99

According to the data in Fig. 5; Table 2, the fitting accuracies R² were all greater than 0.98. The adsorption amounts of both CH₄ and CO₂ decreased with increasing temperature, which is in accordance with the thermodynamic characteristics of physical adsorption. Meanwhile, the pressure increase significantly increased the adsorption amount of both gases. In the interval of 0.1–2 MPa, the adsorption amount showed a rapid increase, while when the pressure exceeded 6 MPa, the growth rate of adsorption amount slowed down and approached saturation²³. This indicates that the adsorption sites were gradually filled. The adsorption amount of CO₂ was always significantly higher than that of CH₄, especially at 293.15 K and 10 MPa, the adsorption amount of CO₂ was 1.035mmol/g, while that of CH₄ was 0.273mmol/g, which suggests that CO₂ has stronger adsorption capacity and is more likely to occupy the adsorption sites in the pores of the coal seam. This is consistent with the conclusions of studies by He⁴⁸ and others. In addition, with increasing temperature, the adsorption of CO₂ decreases more significantly than that of CH₄, and this phenomenon is also reflected in the changes of Langmuir-fitted parameters a and b. The increase in temperature caused the parameters a and b to decrease, indicating that the high temperature inhibited the adsorption process, leading to a decrease in the residence time of the gas in the coal pores, while the enhanced thermal movement of the gas molecules further reduced the adsorption equilibrium concentration.

Analysis of adsorption selectivity coefficient

The adsorption selectivity coefficient of CO₂ relative to CH₄ is calculated by simulation data and Formula (4), and the specific change trend is shown in Fig. 6.

Fig. 6.

Adsorption selectivity coefficient curves of CO₂ to CH₄ at different temperatures.

As seen in Fig. 6, the increase in temperature led to a decrease in the adsorption selectivity coefficient of CO₂ over CH₄, indicating that the increase in temperature led to a weakening of the adsorption capacity of CO₂, whereas the change in the adsorption capacity of CH₄ was small. Since adsorption is an exothermic process, the increase in temperature weakened the intermolecular interaction force, which led to a decrease in CO₂ adsorption, whereas CH₄ was less affected by the change in temperature due to its weaker polarity and smaller adsorption potential energy⁴⁹. The selectivity of CO₂ adsorption was higher at pressures of 0–1 MPa, reaching a maximum value of 6.69 at 293.15 K. CO₂ was more likely to be preferentially adsorbed at low pressures, which was mainly attributed to its stronger adsorption capacity and higher heat of adsorption of the equivalent amount⁵⁰. The adsorption selectivity of CO₂ to CH₄ decreased significantly with the pressure of 5 ~ 10 MPa. This is due to the gradual saturation of coal body adsorption sites at high pressure and the enhanced competitive adsorption of CO₂ and CH₄, resulting in a gradual decrease in the adsorption selectivity of CO₂ on CH₄. This is consistent with the findings of Wu⁴⁷ et al. It was shown that the increase in temperature decreased the adsorption selectivity of CO₂ on CH₄. At low pressure, the adsorption selectivity of CO₂ is higher and its preferential adsorption capacity is higher. However, with the increase of pressure, the competitive adsorption effect of CH₄ increased, so that the adsorption selectivity of CO₂ to CH₄ gradually decreased.

The influence of moisture contents and pressures

Adsorption isothermal curve analysis

The adsorption isothermal curves of CH₄ and CO₂ at different water contents were obtained by using the Langmuir model of Eqs. (2) and (3) and fitted with the above simulated data, as shown in Fig. 7. The corresponding Langmuir fitting parameters are shown in Table 3.

Fig. 7.

Adsorption isotherms of CH₄ and CO₂ at different moisture contents.

Table 3.

Langmuir fitting parameters of CH₄ and CO₂ under different water contents.

water contents/%	a/(mmol·g−1)		b/MPa−1		R 2	R 2
	CH4	CO2	CH4	CO2	CH4	CO2
0	0.389	1.227	15.182	19.128	0.99	0.99
1.41	0.276	1.039	14.289	17.783	0.99	0.99
3.28	0.219	0.821	7.353	8.357	0.99	0.99
6.55	0.158	0.574	4.481	5.284	0.99	0.99

As can be seen from Fig. 7; Table 3, the fitting accuracies R² were all greater than 0.99. The increase in water contents significantly inhibited the adsorption of CH₄ and CO₂. The decrease of CO₂ adsorption was larger than that of CH₄, which indicated that the increase of water contents had a more significant effect on the adsorption of CO₂. This is because water molecules preferentially occupy the active sites on the pore surface of the coal body and form competitive adsorption with gas molecules, hindering the displacement of CH₄ by CO₂. This is consistent with the conclusions of Gao⁵¹ et al. With increasing water contents, the Langmuir parameter a values of both CH₄ and CO₂ showed a decreasing trend, indicating a decrease in the maximum adsorption capacity. Meanwhile, the b values of both also decreased with increasing water contents, indicating that the influence of water molecules weakened the adsorption binding energy of the gas to the surface of the coal body²⁵. Under the same conditions, the adsorption of CO₂ was always higher than that of CH₄, and its Langmuir parameter a and b values were greater than those of CH₄, indicating that the adsorption capacity and bonding strength of CO₂ were stronger in the coal body. In addition, the adsorption amounts of both CH₄ and CO₂ tended to increase with increasing pressure, but the increase decreased with increasing water contents. High pressure helps gas molecules to enter the pores, but high water contents weakens this effect, which is especially significant for CO₂.

Analysis of adsorption selectivity coefficient

In order to further analyze the effect of different water contents on the competitive adsorption behavior of CH₄ and CO₂, the adsorption selectivity coefficient curves of two-component CH₄ and CO₂ with water contents of 0%, 1.41%, 3.28% and 6.55% were obtained by referring to the simulation data and Formula (4), as shown in Fig. 8.

Fig. 8.

Adsorption isotherms of CH₄ and CO₂ at different moisture contents.

As seen in Fig. 8, the adsorption selectivity coefficient of CO₂ on CH₄ decreased significantly with the increase of water contents. This is mainly due to the fact that water molecules reduce the adsorption energy of CO₂²⁵. This weakened the preferential adsorption of CO₂ in the pores of the coal body, i.e., the presence of water molecules affected CO₂ to a greater extent than CH₄, altering the competitive adsorption effect between CO₂ and CH₄. The selectivity coefficient was up to 7.88 under the condition of 0% water contents, while the value decreased to 4.06 when the water contents increased to 6.55%.The highest adsorption selectivity of CO₂ to CH₄ was observed in the pressure interval of 0.1-1 MPa, while the selectivity coefficient gradually decreased in the pressure interval of 5–10 MPa. This may be attributed to the increased competition between gas molecules at higher pressures as well as the increased adsorption saturation of the pores of the coal seam, which reduces the adsorbable space. At high water contents of 6.55%, water molecules are prone to form a water film inside the micropores and mesopores of the coal seam⁵¹, and water molecules have a significant inhibitory effect on CO₂. Nevertheless, CO₂ still maintains the adsorption advantage, indicating that it still has some adsorption capacity in the competitive adsorption mechanism. Under high-pressure and high-moisture conditions, the selective adsorption coefficient of CO₂ shows a significant downward trend, indicating that its adsorption advantage is jointly constrained by thermodynamic adsorption saturation effects and kinetic diffusion resistance. Therefore, the adsorption advantage of CO₂ does not dominate under all conditions, and its thermodynamic and kinetic constraints must be considered comprehensively.

Conclusions

This paper investigates the adsorption behaviour of a CH₄/CO₂ mixture in a coal body using the GCMC simulation method. Analysing the adsorption isothermal curves, Langmuir fitting parameters and adsorption selectivity coefficients of the gases under different conditions revealed the law of competitive adsorption and confirmed the dominant position of CO₂. The results of the study provide theoretical support for enhancing CBM extraction rates, optimising CO₂ sequestration, and advancing CO₂-ECBM technology. The main conclusions are as follows:

1. The adsorption amounts of both CH₄ and CO₂ decreased with increasing temperature or water contents; however, the adsorption stability of CO₂ was stronger.

2. The adsorption of both gases increased with increasing pressure, but excessive pressure limited the increase in adsorption. Increasing water contents decreased the overall adsorption level of the isothermal curve but did not alter the adsorption trend with pressure.The Langmuir parameters a and b for both gases decreased with increasing temperature or water contents. The values of a and b for CO₂ were consistently higher than those for CH₄ and CO₂ exhibited greater sensitivity to changes in moisture and temperature.

3. As the CO₂ molar ratios increases, CO₂ exhibits a significant competitive adsorption advantage. However, even if the CO₂ molar ratios continues to increase, there is still an upper limit to CH₄ displacement. Displacement of CH₄ by CO₂ is strongest at low pressures and high CO₂ ratios, and the adsorption selectivity coefficient of CO₂ decreases most at high pressures and low CO₂ ratios.

4. An elevated temperature or increased water contents has a more significant effect on CO₂, particularly on the desorption effect. Meanwhile, the trend of CH₄ reduction is relatively slow. In the low-pressure range, gas adsorption increases rapidly with pressure, tending to level off near the saturation point. High temperatures or the presence of water molecules inhibit the adsorption process and reduce the adsorption selectivity coefficient of CO₂ for CH₄.

Author contributions

“Conceptualization, Qingshui Ma and Jiayi Zong.; methodology, Xingyu Wu and Qingshui Ma.; software, Qingshui Ma and Xingyu Wu.; validation, Jiayi Zong and Chao Zhang; formal analysis, Tinggui Jia; investigation, Jiayi Zong; resources, Qingshui Ma.; data curation, Jiayi Zong.; writ-ing—original draft preparation, Qingshui Ma; writing—review and editing, Xingyu Wu.; visual-ization, Chao Zhang; supervision, Tinggui Jia; project administration, Xingyu Wu.; funding ac-quisition, Tinggui Jia. All authors have read and agreed to the published version of the manuscript.”

Funding

Natural Science Foundation of Inner Mongolia Autonomous Region (2022LHMS05019,2022LHMS05020)

Data availability

The articles published in this paper contain all data generated or analyzed during the research period. The authors will provide relevant data upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Institutional review board statement

This research does not involve ethics.

Conflict of interest

The authors declare no competing financial interest.

The articles published in this paper contain all data generated or analyzed during the research period. The authors will provide relevant data upon reasonable request.

Software Information :

Version No. : Materials Studio 2020.

Official URL : https://www.3ds.com/products/biovia/materials-studio.