Characterization of microscopic adsorption of CH4/O2 on lignite based on molecular dynamics

Dameng Gao; Xinyue Xu; Lin Hong; Dan Zheng; Peng Lu

doi:10.25259/AJC_483_2025

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Original Article

2025

:18;

4832025

doi:

10.25259/AJC_483_2025

Characterization of microscopic adsorption of CH₄/O₂ on lignite based on molecular dynamics

Dameng Gao, Xinyue Xu^,*, Lin Hong, Dan Zheng, Peng Lu

Department of Key Laboratory of Mine Thermodynamic Disaster & Control of Ministry of Education, College of Safety Science & Engineering, Liaoning Technical University, No.188 Longwan South Street, Huludao, 125105, Liaoning, China

* Corresponding author: E-mail address: 15633141990@163.com (X. Xu)

Received: 2025-05-08, Accepted: 2025-07-11, Epub ahead of print: 2025-09-25, Published: 2025-10-30

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This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

Abstract

To reveal the factors that influence the competitive adsorption of CH₄/O₂ gases in lignite, the adsorption characteristics and thermodynamic properties of lignite on single components of CH₄ and O₂ and different proportions of the mixed components at different temperatures and pressures were simulated using the gas-phase molecular-based cluster (GCMC) model. The result shows that: a) In the context of single-component adsorption, the quantity of gas adsorbed increases in proportion to pressure and decreases in proportion to temperature. Concurrently, the isosteric heat of adsorption exhibits a tendency to first decrease and then increase, in conjunction with pressure. b) In the context of the adsorption of a binary mixture of CH₄/O₂ at a ratio of 5:5 by lignite, it was observed that the amount of CH₄ adsorbed exceeds the amount of O₂ adsorbed at low pressure, and conversely at high pressure. c) The adsorption selectivity coefficients of CH₄ on O₂ demonstrated a decreasing trend with increasing pressure; under all three ratios, CH₄ arrived first to reach the dominant adsorption site.

Keywords

Adsorption selectivity

Competitive adsorption

GCMC

Lignite

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1. Introduction

Spontaneous combustion of coal is a common yet dangerous phenomenon during coal mining and storage. It leads to the waste of coal resources and may cause gas explosions and other accidents that seriously threaten the safety of mines. Additionally, the gases produced by coal combustion, such as CO₂ and SO₂, pollute the environment and seriously affect both the safe production of coal and the ecological environment [1]. At present, the micro-mechanism of coal spontaneous combustion has not been completely clarified. The coal-oxygen composite theory is one of the most concerned theories, in which the CH₄ endogenous to the coal body will have an effect on the contact between O₂ and coal active adsorption sites, which in turn affects the process of the coal-oxygen composite reaction [2]. Therefore, to explore the mechanism of low-temperature oxidation of coal in the extraction zone, it is necessary to deeply study the adsorption characteristics of coal on CH₄ and O₂ gases under multi-field coupling.

The factors influencing CH₄ adsorption by coal extend beyond coal seam pressure and temperature, also encompassing the intrinsic nature of the coal, the water content within the seam, the degree of coal metamorphism, and the presence of other gases in the seam [3]. Bustin et al. [4] found that CH₄ adsorption did not vary uniformly with coal rank or coal type. Clarkson et al. [5] investigated the effects of coal composition and moisture content on the adsorption of single-component CH₄ and CO. Bronisław et al. [6] determined the diffusion of n-alkanes at the coal/water interface and the free energies of coal particles for their interaction in the aqueous phase. Li et al. [7] found that the higher microporous development of coal samples favors the displacement of N₂ and CH₄ by CO₂. Zhu et al. [8] experimentally verified that the maximum adsorption capacity of anthracite coal for CO₂, CH₄, and N₂ was larger than that of bituminous coal at different temperatures. Wang et al. [9] found that the O₂ concentration in the oxidizing process of coal was reduced by the effect of the gas content, which led to a weakening of the coal-oxygen complex reaction. Xiao et al. [10,11] experimentally revealed the relationship between CO₂, CH₄, and N₂ adsorption capacity with temperature. Mustafa et al. [12,13] found that bituminous coal has a high adsorption affinity for CO₂, CH₄, and N₂, and the CO₂ adsorption capacity is significantly higher than that of CH₄ and N₂.

The molecular simulation method can provide detailed information such as gas adsorption amounts, avoid the influence of external environment and other factors on the experimental results, and provide a complementary reference for experimental studies. Zhang et al. [14,15] employed the Grand Canonical Monte Carlo (GCMC) method to investigate the adsorption of single-component gases and binary gas mixtures with varying proportions in coal, highlighting that the adsorption capacity followed the order CO₂>CH₄. Hong et al. [16] examined the adsorption behavior of coal molecules towards CO₂, CH₄, and N₂ under different water content conditions. Jiang et al. [17] found that the adsorption potential wells for CO₂ molecules on the coal surface were much larger than those for CH₄ molecules. Cheng et al. [18,19] investigated the adsorption characteristics of CO₂ and N₂ in coal using the GCMC method. Qu et al. [20] revealed the adsorption capacity of coal molecules at different temperatures, pressures, and molar fractions under CO₂ and N₂, and the pressure and molar fraction conditions of CO₂, O₂, and CH₄ adsorption and diffusion microscopic mechanism. Zhou et al [21] explored the microscale interactions underlying the competitive adsorption of coal-fired flue gases on coal surfaces, demonstrating that competitive adsorption phenomena occur as these gases are adsorbed. Zhang et al [22,23] found that elevated pressure inhibited the adsorption of three gases, namely, CH₄, O₂, and N₂, and the selectivity coefficient associated with coal’s adsorption capacity for the CH₄/O₂ mixture exhibited a decline as the temperature increased. Dong et al [24] discovered that in low-pressure or shallowly buried deep coal seams, the adsorption selectivity of CO₂ over CO is significantly advantageous, and the adsorption selectivity of O₂/CO does not change significantly with pressure.

Regarding the thermodynamic phenomena of coal adsorbed gases, Zhang et al.[25] established a model for the surface diffusion coefficient of coal adsorbed gases by applying the isosteric heat of adsorption, coverage, and blocking coefficients to calculate the surface diffusion blocking coefficient, and Ma et al.[26,27] found that the heat of adsorption of CO₂ was much larger than that of CH₄ in both pressurization and depressurization processes, and that CO₂ dominated the competition between the two gases for adsorption on the surface of the coal pores. Jia et al. [28] concluded that when the gas itself has a larger heat of adsorption, it is more affected by the temperature. Zhang et al. [29] pointed out that the addition of CH₄ and N₂ affects the adsorption capacity of CO₂, and when the adsorption amount tends to be stabilized, the heat of adsorption curve floats in a small range.

In summary, there are relatively few simulation studies on the adsorption characteristics of coal on CH₄ and O₂, especially the lack of in-depth studies on the adsorption behaviors of CH₄ and O₂ under multi-field coupling conditions such as gas flow, temperature, and pressure fields. In view of this, this paper adopts the GCMC method to carry out the research on the adsorption characteristics of lignite to CH₄ and O₂ single-component gases as well as different ratios of binary gas mixtures of CH₄ and O₂ at different temperatures and pressures, and the microcosmic mechanism of lignite oxidative spontaneous combustion was investigated in detail, which lays a theoretical foundation for the technological measures of preventing and controlling the spontaneous combustion of coals.

2. Materials and Methods

The coal sample model was chosen from Jungar lignite [30], which has a molecular formula of C₁₈₄H₁₈₂O₃₈N₂. The Materials Studio software was used to construct the structural models of CH₄, O₂, and coal macromolecules, respectively.

The Forcite module was used for molecular mechanics optimization, and the parameter settings have been shown in Table 1. The annealed molecular structure model and the optimized coal macromolecular structure model are shown in Figure 1.

(a) Molecular structure model after annealing, (b) Optimized coal macromolecular structure model.

Table 1. Parameter settings for structural optimization.

Task	Geometry optimization
Quality	Fine
Algorithm	Smart
Forcefield	Dreiding
Charges	Charge using QEq
Summation method	Atom based

To prevent the molecular mechanics optimization from obtaining only local energy minima, the optimized structure was again subjected to annealing kinetics optimization, and the parameter settings have been shown in Table 2.

Table 2. Parameter settings for annealing kinetics optimization.

Task	Anneal
Quality	Fine
Number of annealing cycles	5
Ensemble	NVT
Heating ramps per cycle	50
Dynamics steps per cycle	100

Coal is an irregular macromolecular structure dominated by aromatic structure without a fixed lattice structure. To construct a reasonable initial adsorption model of lignite, the Amorphous Cell module was used to load the basic structural units of lignite into the amorphous cell, and two basic structural units were chosen to be loaded to reflect the interactions between each coal molecule. This configuration was used in all subsequent adsorption simulations.

3. Results and Discussion

The adsorption simulation of single-component CH₄ and O₂ gases, as well as binary mixed gases with different molar ratios at varying temperatures, was carried out by the gas-phase molecular-based cluster model (GCMC) method. This study aimed to elucidate the adsorption mechanism of CH₄ and O₂ gases on coal at different temperatures and concentration ratios, with the binary component ratios set at 9:1, 7:3, and 5:5.

3.1. Adsorption of single-component CH₄, O₂ in lignite coal

3.1.1. Adsorption isotherms

The adsorption isotherm reflects the variation of adsorption with pressure or concentration at a constant temperature, and the adsorption results were fitted using the Langmuir model at temperatures of 263.15∼313.15 K and pressures of 0.1 MPa∼10 MPa, as shown in Figure 2. The Langmuir model has become an indispensable and important tool in the field of adsorption research due to its excellent theoretical foundation and wide applicability [31,32].

(a) CH4 and (b) O2, Adsorption isotherms of single-component gases in lignite at different temperatures.

In the simulated temperature and pressure ranges, the adsorption amounts of CH₄ and O₂ were 0.282∼6.507 mmol/g and 0.177∼8.725 mmol/g, respectively. The adsorption amounts of the two gases exhibited a gradual increase in conjunction with an increase in pressure. Furthermore, the adsorption isotherms demonstrated an overall parabolic upward trend. As the adsorption pressure increased, the adsorption amounts of CH₄ and O₂ exhibited a gradual rise, with a more pronounced increase observed during the low-pressure stage. Conversely, the increase in adsorption amount during the high-pressure stage became more gradual. In the low-pressure stage, by the lignite micropore adsorption potential, the gas can fully enter the lignite molecular model of the micropore, micropore filling occurs, occupying a larger adsorption space; As the pressure increases progressively, the number of effective adsorption sites on the coal surface gradually decreases, and the gas adsorption process reaches a plateau stage during which the rate of coal adsorption on the gas decelerates, and ultimately tends to be stabilized. At the same temperature, the adsorption quantity of CH₄ exceeds that of O₂ at low pressure, while the adsorption quantity of O₂ surpasses that of CH₄ at high pressure.

As shown in Figure 3, the adsorption amounts of both CH₄ and O₂ showed a decreasing trend with increasing temperature, indicating that increasing temperature is unfavorable for gas adsorption. At constant pressure, an increase in temperature results in a decrease in the amount of gas adsorbed. This observation indicates that the adsorption process is exothermic and classified as physical adsorption. As temperature increases, the activation energy of gas molecules rises, enabling them to acquire sufficient energy to overcome van der Waals forces. This diminishes the binding effect of the coal matrix on gas molecules and suppresses the exothermic adsorption process, thereby leading to a reduction in the adsorption capacity. The absolute value of the slope of O₂ was smaller than that of CH₄ at 1MPa, and the absolute value of the slope of O₂ was larger than that of CH₄ at 3∼9MPa, which indicated that the temperature influence of CH₄ was larger compared with that of O₂ at low pressure, and the absolute value of O₂ was larger than that of CH₄ at 3∼9MPa. under low-pressure conditions, CH₄ is more affected by temperature compared with O₂, and the opposite is true under high-pressure conditions.

(a) CH4 and (b) O2, Variation of single-component CH4 and O2 adsorption with temperature at different pressures.

The adsorption isotherm fitting parameters have been shown in Table 3. The isothermal adsorption constants of the two gases were obtained after Langmuir fitting, and the R² was greater than 0.999, indicating that the curves were highly coincident. The adsorption constant a indicates the limiting adsorption capacity of coal on the gas, and the adsorption constant b indicates the degree of fast and slow adsorption of coal on the gas.

Table 3. Langmuir fitting parameters of adsorption isotherms of single-component CH₄, O_2.

Temperature/K	CH₄			O₂
Temperature/K	a/mmol/g	b/MPa	R²	a/mmol/g	b/MPa	R²
263.15	8.028	0.702	0.9997	12.763	0.297	0.9999
273.15	7.763	0.608	0.9997	12.434	0.253	0.9998
283.15	7.535	0.525	0.9999	12.238	0.218	0.9999
293.15	7.277	0.469	0.9998	12.091	0.185	0.9998
303.15	7.152	0.379	0.9998	11.885	0.162	0.9998
313.15	7.109	0.331	0.9996	11.332	0.148	0.9999

The kinetic diameters of the gas molecules were 3.8 × 10^-10m for CH₄ and 3.46 × 10^-10m for O₂, and the relationship between the adsorption constants of the two gases was a (O₂) > a (CH₄), which indicates that O₂ occupies more adsorption sites when adsorption occurs, resulting in an increase in adsorption. This is due to the small molecular kinetic diameter of O₂ [33], even if the pressure is small can quickly enter the pore to occur adsorption behavior, the more the pore space that can effectively adsorb O₂, the more the number of adsorption sites will be, and the higher the probability that O₂ will be adsorbed into the pore space. Consequently, the adsorption capacity exhibits an inverse relationship with the gas kinetic diameter, the larger the value of b, the lower the pressure needed to saturate the adsorption amount of coal samples, b (O₂) > b (CH₄), so the pressure needed to saturate the adsorption amount of CH₄ is lower than that of O₂, and the adsorption curve of a single group of CH₄ tends to be flat first.

3.1.2. Isosteric heat of adsorption

The isosteric heat of adsorption is influenced by the interaction between gas molecules and coal, as well as the interaction between gas molecules in general. The isosteric heat of adsorption for CH₄ and O₂ both exhibit a slight decrease with an increase in temperature. The heat of adsorption exhibits a decrease initially, followed by an increase as pressure increases. This phenomenon can be attributed to the predominance of gas-gas molecule interactions under low-pressure conditions, which results in a higher energy state, whereas under high-pressure conditions, the energy of coal-gas interactions contributes more.

As shown in Figure 4, at a pressure of 0.1 MPa, CH₄ and O₂ molecules preferentially occupy the active adsorption sites on the lignite surface. These active sites have stronger interactions with the gas molecules, so the adsorption heat released is larger; as the pressure increases, at a pressure of 1 MPa, the high-energy adsorption sites on the coal surface are gradually occupied, and the gas molecules can only be adsorbed on the sites with lower energies. This phenomenon leads to a decrease in the heat of adsorption. The heat of adsorption at 1 MPa to 10 MPa shows a slow increasing trend with the increase of adsorption amount. This is because an increase in pressure will lead to an increase in contact area and interaction force between CH₄, O₂, and the coal surface.

(a) CH4 and (b) O2, Isosteric heat of adsorption of single-component gases at different temperatures.

3.2. Adsorption of binary gas mixtures in coal

3.2.1. Adsorption isotherms

The competitive adsorption of a binary gas mixture of CH₄ and O₂ at temperatures ranging from 263.15 to 313.15 K and pressures ranging from 0.1 to 10 MPa was simulated, and the fitted adsorption isotherms have been shown in Figure 5.

(a) 263.15 K, (b) 273.15 K (c) 283.15 K, (d) 293.15 K, (e) 303.15 K and (f) 313.15 K, Adsorption isotherms for different ratios of binary gas mixtures.

The variation rule of the adsorption amount of the binary mixture of CH₄ and O₂ by lignite is analogous to that of single-component gas adsorption, and the pressure and adsorption amount demonstrate a positive correlation. The adsorption of CH₄ and O₂ by lignite exhibited an increase in response to an increase in pressure, and the gas molecules were easier to enter the pores of the coal under high pressure. The adsorption amounts of mixed gases all decreased with increasing temperature, indicating that the increase of temperature is unfavorable to the adsorption of gases by lignite, which is due to the fact that high temperature promotes the thermal movement of CH₄ and O₂, the increase of activation energy, the inhibition of molecules from the free state to the adsorption state, and the individual regionally stable gases will be transformed to exist in the free state due to the high temperature [28].

In the CH₄/O₂ binary gas system, when the CH₄ concentration is 90% or 70%, the adsorption capacity of CH₄ is greater than that of O₂. When the molar ratio of CH₄ to O₂ is 5:5, CH₄ exhibits a higher adsorption capacity than O₂ at low pressures. This phenomenon can be attributed to the fact that CH₄ has a higher boiling point compared to O₂. The higher the boiling point of the gas, which means that the adsorption trap is deeper, the smaller the rate of diffusion of the gas. The molecular interaction force of the gas is greater, and it is easier to adsorb the gas. The intermolecular force of CH₄ gas is larger, and it is easier to be adsorbed onto the surface of the porous medium.

When the pressure is higher than 8 or 9 MPa, the adsorption amount of O₂ is larger than that of CH₄, which indicates that the pressure increase is more favorable to the adsorption of O₂ by lignite. The gas molecules are compressed under high pressure, and the O₂ molecules are smaller in size, which can fill the micropores more efficiently, which facilitates the O₂ molecules occupying more adsorption sites on the surface of lignite, and sufficiently competing with CH₄ molecules for adsorption; and the O₂ has a high polarizability and quadrupole moment, so that the adsorption amount of O₂ is bigger under the combined effect of high pressure and polarity.

3.2.2. Adsorption selectivity

Adsorption selectivity is the ability exhibited by an adsorbent to preferentially adsorb certain substances, and in the study of competitive adsorption of multicomponent gases, adsorption selectivity calculations are used to quantify the competing adsorption capacity of two gases in a mixed system. The binary adsorption selectivity of gas A with O₂ is defined as shown in Eq (1):

(1)

S_{A ∕ O_{2}} = \frac{x_{A} ∕ x_{O_{2}}}{y_{A} ∕ y_{O_{2}}}

where x_A is the molar proportion of gas A in the adsorption component; x_O2 is the molar proportion of O₂ in the adsorption component; y_A and y_O2 are the molar proportions of gas A and O₂ in the free state, respectively. When S_A/O2 is greater than 1, it indicates that the adsorbent has a stronger adsorption advantage for gas A in the binary gas mixture [24].

To further quantitatively characterize the competitive adsorption of binary gases, the adsorption selectivity of binary gas mixtures in coal at different ratios was calculated, as shown in Figure 6.

(a) 9:1, (b) 7:3 and (c) 5:5, Desorption selectivity of binary gas mixture with different ratios.

The adsorption selectivity coefficients under the three ratios were 0.891∼1.998, 0.962∼1.987, and 0.97∼1.938, respectively, indicating that the competitive adsorption ability of lignite for the CH₄ gas was slightly stronger than that of O₂ in the adsorption process of binary mixed gases, and it had a certain adsorption advantage.

Under certain conditions of temperature and pressure, the variation interval of adsorption selectivity values becomes smaller for different gas components, indicating that the adsorption capacity of the adsorbent on each component tends to be balanced and the difference in selectivity decreases under the condition of low CH₄ concentration, which results in a reduction of the variation range of the selectivity values. The adsorption selectivity coefficients were examined at three distinct ratios. The results demonstrated a decreasing trend with increasing pressure, suggesting that elevated pressure negatively impacts the competitive adsorption capacity of CH₄ in binary gas mixtures. This observation aligns with the principles outlined by the adsorption isotherm.

3.2.3. Potential energy distribution

The energy distribution diagram illustrates that the potential energy distribution of gas molecules in the coal, indicating the proportion of the number of molecules distributed in different energy regions of the gas molecules, with the lowest interaction energy region representing the preferred adsorption site [34]. As can be seen from Figure 7, the distribution form of potential energy roughly exhibits a normal distribution.

Adsorption potential energy curves of binary gas mixtures with different ratios at 0.1 MPa, 263.15 K.

The potential energy peaks of the best adsorption sites of CH₄ appeared in the range of -3.95∼-3.75 kcal/mol, which belonged to the low-energy region with strong interaction energy. CH₄ reached the advantageous adsorption sites first, indicating that CH₄ had more adsorption advantages compared with O₂. The peaks of the best adsorption sites of O₂ appeared in the region of -3.05 kcal/mol when the concentration of O₂ was at 50% and 100%, and the peaks of the best adsorption sites of O₂ appeared in the region of -2.95 kcal/mol at low concentration, which belonged to the region of -3.75 kcal/mol. The potential energy peak of the optimum adsorption site of O₂ at low concentrations appeared in the region of -2.95 kcal/mol, and the peak of the adsorption site was shifted to the right in the bicomponent compared to the single-component gases, which indicated that CH₄ inhibited the adsorption of O₂. The absolute values of the potential energy peaks do not change much when coal adsorbs different ratios of binary gas mixtures. For the CH₄/O₂ binary gas mixture system, the gas potential energy distribution is less affected by the gas components.

4. Conclusions

In the range of temperature 263.15∼313.15K and pressure 0∼10MPa, the increase of pressure promotes the adsorption of lignite to single-component CH₄ and O₂ gases, and the increase of temperature inhibits the adsorption behavior of coal to gases, and the adsorption constants a and b are negatively correlated with the temperature, which also indicates that high temperature is not conducive to the adsorption of gases. The isosteric heat of adsorption decreases and then increases, and the value changes with temperature and pressure are small.

In the context of a CH₄/O₂ binary gas system, it has been observed that when the content of CH₄ in the system reaches 90% or 70%, the adsorption amount of CH₄ exceeds that of O₂. Furthermore, under conditions where the molar ratio is 5:5, the adsorption amount of CH₄ surpasses that of O₂ at low pressure, while the adsorption amount of O₂ exceeds that of CH₄ at high pressure. It is noteworthy that an increase in pressure is more conducive to lignite adsorbing O₂.

The adsorption selectivity coefficients S_CH4/O2 are all decreasing with the increase of pressure. Elevated pressure has an adverse effect on the competitive adsorption capacity of CH₄ within the binary gas mixture. and the competitive adsorption ability of CH₄ gas is slightly marginally superior than that of O₂. In the three ratios, the potential distribution of the binary gas mixture remains relatively stable, and CH₄ reaches the advantageous adsorption sites first, indicating that CH₄ has more adsorption advantages compared with O₂. The potential distribution of the binary gas mixtures in the three ratios did not change much.

Acknowledgment

Liaoning Natural Science Foundation of China (Grant No. LJKQZ20222334), Liaoning Provincial Natural Science Foundation Outstanding Youth Fund Project (2024JH3/10200042), Liaoning Province “Xingliao Talent Program” Project (XLYC2403031)

CRediT authorship contribution statement

Dameng Gao: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing–original draft. Xinyue Xu: Conceptualization, Data curation, Writing–original draft, Methodology. Lin Hong: Conceptualization, Investigation, Methodology, Visualization. Dan Zheng: Conceptualization, Data curation, Formal analysis, Software, Validation. Jing Gong: Conceptualization, Data curation, Methodology. Peng Lu: Writing–original draft, Data curation.

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.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

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1. Introduction

2. Materials and Methods

3. Results and Discussion

3.1. Adsorption of single-component CH4, O2 in lignite coal
3.2. Adsorption of binary gas mixtures in coal

4. Conclusions

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