CO₂ Sequestration Potential Competitive with H₂O and N₂ in Abandoned Coal Mines Based on Molecular Modeling

Abstract

To facilitate the local recycling of coal mine waste gas and investigate multi-component gas adsorption under high pressure conditions, this study develops a coal nanopore model using molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods and simulates the adsorption behavior of coal mine waste gas components (CO₂, H₂O, N₂) under varying pressure levels and gas molar ratios at 353.15 K. We evaluated the adsorption capacity and selectivity for both single-component and multi-component gases, quantifying adsorption interactions through adsorption heat, interaction energy, and energy distribution. The simulation results revealed that the contribution of the three gases to the total adsorption amount followed the order: H₂O > CO₂ > N₂. The selective adsorption coefficient of a gas exhibits an inverse correlation with its molar volume ratio. Isothermal heat adsorption of gases in coal was positive, decreasing sharply with increasing pressure before leveling off. Electrostatic interactions dominated CO₂ and H₂O adsorption, while van der Waals forces governed N₂ adsorption. As the gas mixture complexity increased, the overlap of energy distribution curves pronounced, highlighting competitive adsorption behavior. These findings offer a theoretical foundation for optimizing coal mine waste gas treatment and CO₂ sequestration technologies.

1. Introduction

Currently, anthropogenic carbon dioxide emissions from fossil fuel combustion exceed 40 billion tons per year, representing a significant contributor to global warming. CO₂ capture and storage (CCS) technology plays a pivotal role in addressing the Paris Agreement on Climate Change and achieving the goals of carbon peaking and carbon neutrality [1,2,3]. CCS involves separating CO₂ from industrial or related emission sources, transporting it to storage sites, and isolating it from the atmosphere over the long term. This process primarily consists of four components: capture, transportation, geological storage, and monitoring [4,5,6]. CO₂ geological storage, a key component of carbon capture, utilization, and storage (CCUS) technology, is a critical technological pathway toward achieving carbon neutrality. To ensure the stability and safety of CO₂ storage, CO₂ is primarily transported and stored underground in a supercritical state (pressure of 7.38 MPa and critical temperature of 31.1 °C) [6,7]. Coal mine waste gases include landfill gas (approximately 40–60% CH₄, 32–40% CO₂, 2–4% N₂, and 3–12% H₂O), coal mine drainage gas (H₂O content exceeding 20%), coal mine air exhaust gas (CH₄ content below 1%), and associated natural gas [8,9]. Water vapor, another greenhouse gas, also contributes to global warming [10]. With advancements in CO₂ capture and storage (CCS) and methane separation and utilization (MSU) technologies, capturing and storing coal mine exhaust gases during downstream mining processes can help mitigate the impact of mining on climate change [11].

The main factors influencing the adsorption capacity of coal mine waste gases include pore shape, pore size, and coal surface functional groups. Wang et al. [12,13] classified pore shapes into five types across three scales (nanoscale, microscale, and macroscale): spherical (0.2–0.8 μm), tubular (0.8–10 μm), slot (10–30 μm), slit (30–40 μm), and flat slit (50–240 μm). These pores exhibit a wide size range and a continuous distribution from microscale to mesoscale [14]. Among these, micropores with sizes below 10 nm contribute most significantly to adsorption capacity [15,16]. Pore size in coal also varies with the degree of coalification; as metamorphism increases, micropore content rises while macropore content decreases [16,17]. Dong et al. [18] found that pore size has a more substantial influence than functional groups in pure adsorption, with pore structure and original functional group contributions to CO₂/CH₄ adsorption quantified at 71% and 29%, and 83% and 17%, respectively. Liu et al. [19,20] discovered that oxygen-containing functional groups (e.g., -OH and -COOH) preferentially adsorb H₂O over CO₂. Yu et al. [21] determined the adsorption sequence of CO₂ and CH₄ on functional groups in vitrinite coal structures using simulations, which ranked as follows: -OH > -COOH > -C₂O > -CO > -C for CO₂, and -OH > -CO > -C₂O > -C > -COOH for CH₄.

Recent studies have utilized molecular mechanics, molecular dynamics, and Monte Carlo methods to calculate gas adsorption in coal [22,23,24,25], providing theoretical insights for managing gas hazards and improving energy utilization in air-mining regions [26]. Si et al. [27] examined the competitive adsorption behaviors of CO₂/N₂/O₂ at low pressures, while Li et al. [28] analyzed the competitive adsorption of CO₂, N₂, and CH₄ at different temperatures and studied changes in coal pore structure before and after gas injection. Liu et al. [29] simulated the adsorption of CH₄, CO₂, H₂O, and N₂ on a heterogeneous coal surface model using GCMC. Guo et al. [30] employed MC simulations to investigate competitive saturation adsorption of O₂ and H₂O within coal particles and applied the lattice Boltzmann method (LBM) to study intra- and inter-particle convection, diffusion, and adsorption. Despite these advancements, the microcosmic mechanism for coal mine waste gas adsorption remains underexplored.

The adsorption behavior of CH₄ in coal seam pores, including competitive adsorption in both single-component and multi-component systems, has been extensively studied in recent years [13,31]. Cai et al. [32] investigated the regulatory role of pore structure evolution in coal on the three-phase transformation mechanism of methane. Esen et al. [33] analyzed the effects of different coal lithotypes on methane adsorption and the efficiency of coalbed methane production. However, studies focusing on the subcomponents of coal mine waste gas (CO₂, H₂O, N₂) under high fugacity conditions remain relatively scarce.

Accordingly, a nanopore model was constructed, and grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations were conducted to examine coal mine waste gas adsorption. Three gases (CO₂, H₂O, N₂) with varying pressure and molar concentrations were targeted to explore the competitive adsorption mechanisms and their underlying causes. This study extends existing gas adsorption models by incorporating competitive adsorption mechanisms among CO₂, H₂O, and N₂, a critical but underexplored factor in coal mine gas management. By bridging molecular simulations with macroscopic carbon sequestration predictions, we provide a multiscale theoretical framework, offering practical insights for optimizing underground coal seam storage.

2. Models and Methodology

2.1. Construction and Optimization of Models

Proximate analysis serves as a fundamental methodology for characterizing coal composition, systematically dividing coal constituents into four principal components under standardized conditions: moisture (M), ash (A), volatile matter (V), and fixed carbon (FC). This analytical approach employs conditional testing procedures, where each component is quantified through specific thermal treatment protocols—moisture by oven drying, ash by high-temperature combustion, volatile matter through pyrolysis under inert atmosphere, and fixed carbon by mass balance calculation.

Air-dried basis (ad) reflects coal samples that have equilibrated with laboratory humidity conditions, where only surface moisture is removed while inherent moisture remains. Dry basis (d) eliminates all moisture content. These bases are measured under idealized conditions and can be converted to each other. Dry ash-free basis (daf) is the core benchmark system for industrial analysis of solid fuels and refers to the organic combustible content of the fuel after the hypothetical removal of all water and ash from the fuel [34]. These bases are measured under idealized conditions and can be converted to each other.

The coal samples were produced in the northern Ordos Basin. In this study, the coal samples were analyzed by proximate and maceral composition analysis. The results are shown in

Table 1. Analysis results of coal samples.

Proximate Analysis				Maceral Composition Analysis
Mad 1	Ad 2	Vdaf 3	FCdaf 4	Vitrinite	Inertinite	Liptinite	Mineral
0.73%	15.32%	14.88%	85.12%	90.63%	3.13%	3.51%	2.73%

¹ Moisture content on air-dried basis; ² ash yield on air-dried basis; ³ volatile matter on dry ash-free basis; ⁴ fixed carbon on dry ash-free basis.

.

The slit nanopore model was constructed using the coal molecular fragment shown in Figure 1a. The amorphous cell module and geometry optimization via the molecular dynamics were applied to achieve the model with the lowest energy, as depicted in Figure 1b. The chemical formula for the model is C₂₈₈₄H₂₃₅₂N₂₈O₂₁₀ (C: 85.00%, H: 5.77%, N: 0.96%, O: 8.25%). The pore width was set to 40 Å, with periodic boundaries and lattice parameters defined as α = β = γ = 90°, and edge lengths of A = 38.3150 Å, B = 38.3150 Å, and C = 86.6008 Å. The total density of the gap-containing model was 0.531417 g/cm³, with additional parameters listed in

Table 2. Analog parameters settings.

Setting	Amorphous Cell Parameters	Setting	Sorption Parameters
Task	Packing	Task	Adsorption isotherm
Quality	Fine	Method	Metropols
Temperature	353.15 K	Temperature	353.15 K
Force field	COMPASS	Fugacity steps	10
Charges	Forcefield-assigned	Charges	Forcefield-assigned
Electrostatic	Ewald	Quality	Customized
van der Waals	Atom-based	Equilibration steps	10,000
		Production steps	100,000
		Force field	COMPASS
		Electrostatic	Atom-based
		van der Waals	Atom-based

.

Molecular models of CO₂, H₂O, and N₂ were constructed using Material Studio 2023 (Dassault Systèmes BIOVIA, San Diego, USA), and optimized with the Compass force field to achieve the lowest molecular energy.

2.2. Simulation and Analytical Methods

In this study, the adsorption properties of CO₂, N₂, and H₂O at 353.15 K were simulated using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) methods. GCMC is suitable for simulating adsorption processes in open systems. Under constant temperature, volume, and chemical potential conditions, it simulates the interactions between gas molecules and adsorbent surfaces through stochastic operations such as random displacement, insertion, and deletion of molecules, thereby obtaining key data such as adsorption isotherms. MD, based on Newton’s laws of motion, tracks molecular trajectories in phase space by solving equations of motion, enabling the study of molecular dynamic behaviors and interaction energies [35].

Simulations were conducted at pressure ranging from 0 to 30 MPa. As shown in

Table 3. Gas pressure settings.

NO.	Adsorbate			NO.	Adsorbate
	Set Pressure (MPa)				Set Pressure (MPa)
	CO2	N2	H2O		CO2	N2	H2O
1	30	/	/	14	3	/	27
2	/	30	/	15	9	/	21
3	/	/	30	16	15	/	15
4	3	27	/	17	21	/	9
5	9	21	/	18	27	/	3
6	15	15	/	19	10	10	10
7	21	9	/	20	15	7.5	7.5
8	27	3	/	21	7.5	15	7.5
9	/	3	27	22	7.5	7.5	15
10	/	9	21
11	/	15	15
12	/	21	9
13	/	27	3

, the gases studied included single-component CO₂, H₂O, and N₂, as well as two-component gas mixtures with molar volume ratios of 1:9, 3:7, 5:5, 7:3, and 9:1 (CO₂/H₂O, CO₂/N₂, and H₂O/N₂), and three-component gas mixtures with molar volume ratios of 1:1:1, 1:1:2, 1:2:1, and 2:1:1 (CO₂/H₂O/N₂).

The small molecule models of CO₂, H₂O, and N₂ gases with the lowest energy after geometry optimization in Materials Studio were simulated in the Sorption module using the parameters shown in Table 2.

To quantify the gas adsorption capacity, adsorption selectivity $S_{i/j}$ was used to describe the degree of relative enrichment of the two-component gas in the adsorbed phase. The selective adsorption $S_{i/j}$ of gas i on gas j is defined, as shown in (1) [36]

$$S_{i/j}={\displaystyle \frac{x_i/x_j}{y_i/y_j}}$$

(1)

where x_i (x_j) and y_i (y_j) denote the substance fraction of gas i (j) in the adsorption phase and gas phase, respectively. If $S_{i/j}$ > 1, it indicates that the adsorbent has a stronger affinity for gas i and is more easily enriched in the adsorption phase. The greater the adsorption selectivity, the better the separation performance of the gas mixture.

3. Results

3.1. Adsorption Simulation Results

The adsorption results of the three-component gases at pressure loading to 30 MPa equilibrium are shown in Figure 2. These include the blue-dashed CO₂ density distribution, the red-dashed H₂O density distribution, and the green-dashed N₂ density distribution.

3.2. Isothermal Adsorption Capacity

As shown in Figure 3a, the adsorption amounts of the three gases (H₂O, CO₂, and N₂) exhibit a monotonic increase with increasing pressure. The adsorption isotherm of CO₂ rises rapidly at low pressure before gradually reaching saturation, while the adsorption rate of H₂O is faster at low pressure but slows as pressure increases. In contrast, the adsorption of N₂ follows a nearly linear trend. At 30 MPa, the adsorption amounts of H₂O, CO₂, and N₂ are 1428.56 mol/mol, 1018.66 mol/mol, and 433.40 mol/mol, respectively. When pressure exceeds 12 MPa, the adsorption capacity of H₂O exceeds that of CO₂, with the overall adsorption order being H₂O > CO₂ > N₂. This suggests that H₂O adsorbs more strongly than CO₂ at nanopore sites, primarily due to the abundance of polar functional groups (e.g., hydroxyl, carboxyl, carbonyl, and ether bonds) on the nanopore surfaces of coal beds [19,37]. H₂O, with its strong polarity and permanent electric dipole moment, forms hydrogen bonds and electrostatic interactions with these surface groups. CO₂, although less polar, can still undergo chemical adsorption with acidic surface groups such as carbonyl. N₂, being non-polar, is adsorbed only through weaker intermolecular interactions such as van der Waals forces, leading to its lower adsorption [37].

Figure 3b–d present the adsorption of two-component gas. At any molar volume ratio, the contribution to total adsorption shows a similarity to that of the single component. At 30 MPa and a molar component ratio of 1, the gas adsorption for the CO₂/H₂O binary system is 373.37 mol/mol for CO₂ and 698.43 mol/mol for H₂O, with H₂O adsorption being 1.87 times that of CO₂. This is 1.40 times greater than the adsorption of single-component gases under the same conditions, indicating that H₂O dominates competitive adsorption in the CO₂/H₂O system. In the CO₂/N₂ system, the adsorption amounts are 428.82 mol/mol for CO₂ and 143.69 mol/mol for N₂, with CO₂ adsorption being 2.98 times that of N₂, which is 2.35 times greater than the adsorption in a single-component gas system. This demonstrates CO₂’s competitiveness over N₂. In the H₂O/N₂ binary system, the adsorption of H₂O consistently exceeds that of N₂ across various molar component ratios.

Figure 3e illustrates the adsorption behavior of CO₂/H₂O/N₂ mixtures. In both binary and ternary competitive adsorption systems, the saturated adsorption amounts are lower than those of the corresponding single-component systems, with adsorption amounts increasing with the molar volume fractions of CO₂, H₂O, and N₂.

3.3. Competitive Adsorption

In Figure 4a, the adsorption selectivity $S_{H_{2}O/{CO}_2}$ is highest when the initial molar volume of CO₂ in the gas phase is high. The selectivity initially decreases rapidly with increasing pressure before exhibiting an upward trend. At low pressures, the adsorption sites are primarily occupied by H₂O, which, due to its significant polarity, is preferentially enriched, leading to a higher H₂O/CO₂ adsorption selectivity. As the pressure surpasses 6 MPa, CO₂ gradually occupies the adsorption sites, diminishing the selective adsorption advantage of H₂O. In Figure 4b, the adsorption selectivity decreases sharply and then stabilizes with increasing pressure at any molar fraction. The highest $S_{{CO}_2/N_2}$ values are observed at lower CO₂ molar volume (with $y_{{CO}_2}:y_{N_2}$ = 1:9), with high selectivity peaks, particularly at low pressures. The adsorption selectivity pattern in Figure 4c is similar to that in Figure 4b, but $S_{H_{2}O/N_2}$ shows an increasing trend from 6 to 30 MPa, in contrast with $S_{{CO}_2/N_2}$. The highest $S_{H_{2}O/N_2}$ values are found at lower molar volumes of H₂O (with $y_{H_{2}O}:y_{N_2}$ = 1:9). In summary, the smaller the molar volume ratio of the gas, the greater difference in the adsorption capacities of the two-component gas system, and the higher the selective adsorption coefficient for the corresponding gas.

4. Discussion

To clarify the adsorption mechanism, the controlling factors for competitive adsorption were identified through a quantitative analysis of the total interaction energy, the heat of adsorption for different gases, and the energy distribution.

4.1. Adsorption Heat Influences

The magnitude of the heat of adsorption can measure the degree of adsorption strength. The larger the heat of adsorption, the stronger the interaction between the adsorbed molecules and the solid surface, the easier the molecules are captured on the adsorption sites, and the stronger the adsorption [38]. The heat of adsorption changes during the loading process were observed by the visual molecular simulation tool, and the heat of adsorption of the gas after reaching equilibrium was statistically calculated.

The change rule of heat of adsorption of CO₂, N₂, and H₂O with pressure is shown in Figure 5a, the size of heat of adsorption of CO₂ and H₂O decreases and then increases with the increase in pressure, and the range of change of heat of adsorption of N₂ is relatively small. At 4.925–9.011 kcal/mol, the range of change of heat of adsorption of H₂O is larger at 3.892–10.016 kcal/mol, which indicates that the pressure of CO₂ is less than 20 MPa. The heat of adsorption of N₂ decreases with the increase in pressure and then stabilizes at about 2.000 kcal/mol. In general, the heat of adsorption of CO₂ and H₂O is higher than that of N₂. In the early stage of adsorption, the high-energy sites on the surface are preferentially occupied and the heat of adsorption is higher. As the adsorption sites are gradually occupied, the remaining sites have lower energy and the heat of adsorption decreases.

The change of heat of adsorption of the two-component system with the pressure and the composition of the system gas is shown in Figure 5b–d. The magnitude of the heat of adsorption of CO₂ and H₂O decreases and then increases with the increase in pressure, and the change in the heat of adsorption of N₂ is not obvious under high pressure. The heat of adsorption of CO₂ in the CO₂/H₂O system is 7.336 kcal/mol at 90% CO₂ content and 30 MPa, which is larger compared with 5.128 kcal/mol in the CO₂/N₂ system, indicating that H₂O has less inhibitory effect on CO₂ adsorption in the nanopore space of coal beds compared to N₂. The combinations of higher molar concentration of N₂ present a greater heat of adsorption, and the increase in molar concentration increased the competitive adsorption capacity of N₂ in the two-component gas.

In Figure 5e, the heat of adsorption of CO₂ is higher in the combination CO₂/H₂O/N₂ = 1/1/2 than in CO₂/H₂O/N₂ = 1/2/1 at pressure lower than 6 MPa, and the opposite trend is observed at pressure above 6 MPa, indicating that the adsorption capacity of CO₂ molecules increases with higher pressure and higher gas water content. The trend in heat of adsorption for the two-component and three-component systems of CO₂, N₂, and H₂O are similar to those of the single-component system. The promotion of CO₂ adsorption by H₂O and the difference in adsorption amounts of the three gases by the average gas adsorption heat clear response, as shown in Figure 5f, corroborate the aforementioned arguments.

4.2. Interaction Energy of the System Influences

Interaction energy represents the change in the total energy of the system before and after adsorption. The greater the absolute value of the interaction energy, the more stable the adsorption system becomes [39]. It mainly includes electrostatic energy and van der Waals energy; electrostatic energy is mainly caused by hydrogen bonding and electrostatic force, and van der Waals energy is mainly caused by van der Waals force.

As shown in Figure 6a, the interaction energy follows the trend H₂O > CO₂ > N₂ in single-component gases, in which the interaction energy between H₂O and CO₂ is dominated by the electrostatic energy (which is about 76% of the interaction energy of CO₂), the interaction energy of H₂O is dominated by the electrostatic energy, and the interaction energy of N₂ is completely dominated by the van der Waals energy.

As shown in Figure 6b–d, at 353.15 K and 30 MPa, with the same gas molar occupancy, the interaction energy in the CO₂/H₂O system is −4666.56334 kcal/mol, in the CO₂/N₂ system is −1337.28507 kcal/mol, and in the H₂O/N₂ system is −4009.13858 kcal/mol. This indicates that the stability of the binary mixed systems follows the order: CO₂/H₂O > H₂O/N₂ > CO₂/N₂, with the strongest interaction observed between CO₂ and H₂O. As the pressure increases, the interaction energies of all the binary mixed systems show a slow upward trend. In the CO₂/H₂O and H₂O/N₂ systems, electrostatic energy predominantly governs the interaction energy, while in the CO₂/N₂ system, van der Waals forces become more influential, and the role of electrostatic energy diminishes. In the CO₂/H₂O system, the total interaction energy decreases with an increase in the CO₂ molar fraction, leading to reduced adsorption stability of the gas mixture. Conversely, in the CO₂/N₂ system, the total interaction energy increases as the CO₂ molar fraction increases, with N₂ playing a lesser role in the interaction energy.

As shown in Figure 6e, in the three-component gas mixture, the total gas interaction energy exhibits a gradual and steady increase with rising pressure. The CO₂/H₂O/N₂ = 1/2/1 mixture has the highest total interaction energy among the four different gas combinations, indicating that increasing the H₂O content in the gas mixture contributes to greater adsorption system stability. The interaction energies of the gas mixtures, averaged over molar volume ratios between 0 and 30 MPa, are displayed in Figure 6f. The results indicate that the average gas adsorption energy of the CO₂/H₂O two-component system is 3.51 times higher than that of the CO₂/N₂ system, while the average gas adsorption energy of the H₂O/N₂ system is more than 2.61 times higher than that of the CO₂/N₂ system.

4.3. Energy Distribution of Different Gases Influences

The peak position and width of the energy distribution reflect the adsorption stability of the gas molecules and the energy distribution of the adsorption sites, the higher the absolute value of the interaction energy, the more favorable the adsorption sites occupied by the gas, and the more stable the combination with the coal molecules [40].

The adsorption energies of the single-component gases are shown in Figure 7a. The adsorption energy distribution of H₂O is the widest, with the smoothest energy distribution curve; the adsorption energy distribution of N₂ is the narrowest, with the highest peak value; and the energy distribution of CO₂ lies between them. The most significant adsorption energy of H₂O is −16.35 kcal/mol, that of CO₂ is −8.15 kcal/mol, and that of N₂ is −0.55 kcal/mol. This indicates that H₂O is the dominant gas (significantly enhancing the stability of the system), CO₂ exhibits a strong competitiveness, while N₂ has a relatively weak competitive effect.

The adsorption energy distributions of the two-component gas mixtures (CO₂/H₂O, H₂O/N₂, and CO₂/N₂) are shown in Figure 7b–d. The larger the overlap of the interaction energy distribution curves, the more intense the competitive adsorption between the gases. In other words, the competitive intensity in the binary mixed system follows the order: CO₂/H₂O > H₂O/N₂ > CO₂/N₂. In the CO₂/N₂ mixed system, the absolute value of the most significant adsorption energy of CO₂ increases from 3.85 kcal/mol to 4.45 kcal/mol as the molar concentration of CO₂ increases, and the interaction energy distribution curve shifts slightly to the left. In the H₂O/N₂ hybrid system, as the H₂O concentration increases, the absolute value of the most significant adsorption energy of H₂O increases from 5.55 kcal/mol to 6.25 kcal/mol, with the distribution curve shifting leftward. In the CO₂/N₂ mixture system, as the CO₂ molar concentration increases, N₂ gradually exhibits two gas absorption peaks, with the absolute value of the adsorption energy corresponding to the first gas absorption peak being lower than that of the second peak. Comparing the interaction energy distribution curves of single-component and two-component adsorption, it is evident that the shifts of the CO₂ and H₂O curves in the competitive adsorption system are larger, while the shifts of the N₂ curve are smaller and concentrated mainly in the high-energy region. This further confirms that the role of N₂ in the adsorption system is weaker.

In Figure 7e, the overlapping portion of the interaction energy distribution curves in the three-component gas mixture is larger compared with that in the two-component mixtures. The energy distribution curves of CO₂ and H₂O approximately overlap, indicating enhanced competitive and synergistic interactions within the adsorption system. N₂ shows a more obvious two-gas absorption peak. The average most significant adsorption energies of the single-component, two-component, and three-component gases are shown in Figure 7f. The absolute values of the average most significant adsorption energies of N₂ in the two-component and three-component gas mixtures are higher compared with those in the single-component system. This suggests that N₂ achieves a more favorable competition for the adsorption sites, which contrasts with CO₂ and H₂O.

4.4. Coal Mine Waste Gas Geologic Sequestration Practice

The simulation results demonstrate that, when the H₂O content exceeds 10%, the CO₂ adsorption capacity decreases by approximately 20%. It is recommended that in actual production, the coal mine gas should undergo dehydration treatment prior to injection to maintain the H₂O molar fraction at ≤5%, thereby reducing its competitive adsorption with CO₂ for active sites. If the coal mine gas contains a high concentration of N₂, CO₂ purification technologies such as membrane separation should be employed to prevent the reduction in CO₂ adsorption capacity caused by N₂ dilution.

Under geological conditions similar to those in this study, a stepwise pressurization strategy is advised. The initial pressure should be controlled at approximately 10 MPa, where the CO₂ adsorption rate reaches its maximum. After stabilization, the pressure can be gradually increased to enhance the long-term stability of CO₂ sequestration. A framework for the implementation of underground storage of coal mine waste gas was designed as shown in Figure 8.

Currently, conventional adsorption analyzers, such as BET apparatuses, are primarily suitable for single-component gas testing or measurements under pressures < 10 MPa. Moreover, the simultaneous presence of H₂O and CO₂ in multi-component gas mixtures can lead to the corrosion of experimental setups. In future research, combining scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to examine nanopore structures, along with the development of a high-pressure multi-component gas adsorption system, represents a critical pathway from theoretical modeling to experimental validation.

5. Conclusions

This study accounts the results of competitive adsorption of coal mine waste gas in the nanopores by quantifying the parameters of heat of adsorption, interaction energy, and energy distribution. This study extends existing models of gas adsorption and provides practical insights for optimizing underground coal seam sequestration. The contribution to the total adsorption amount followed the sequence: H₂O > CO₂ > N₂. The smaller the corresponding molar volume ratio of the gases, the higher the selective adsorption coefficient of that gas. Adsorption results are influenced by three main factors.

(1) As pressure rises, the isothermal heat of adsorption curve first decreases sharply, followed by a gradual increase. Between 6 and 30 MPa, the slope of the isothermal heat of adsorption curve for H₂O and CO₂ is the steepest, while the curve for N₂ remains relatively flat. (2) At pressure between 3 and 30 MPa, the interaction energy of the gas mixtures increases gradually with increasing pressure. Electrostatic energy dominates the interaction energy of CO₂ and H₂O, while van der Waals energy governs the interaction energy of N₂. The average adsorption energy of the CO₂/H₂O and H₂O/N₂ combination is approximately three times that of the CO₂/N₂ combination. (3) As the degree of gas mixing increases, the overlap of the energy distribution curves intensifies. The energy distribution curves for CO₂ and H₂O shift significantly to the right, while N₂ shows more prominent dual adsorption peaks.