Effects of added moisture on methane adsorption of coal with different degrees of metamorphism: An experimental and molecular dynamics simulation studies

Abstract

This article combines physical experiments and molecular simulations to explore the influence of added water on CH₄ adsorption in different metamorphism grade coal. Specifically, under different pressures, when the added water content increases from 0% to 6.26%, the CH₄ adsorption amount in high metamorphic coal decreases by approximately 76.72%, 63.82%, 59.53%, 56.10%, and 50.94%. However, the decrease in adsorption amount of gas fat coal and long flame coal is lower than that of anthracite, indicating that the effect of adding water on CH₄ adsorption is most significant in high metamorphic coal, while the effect on medium and low metamorphic coal is relatively small. The average adsorption heat of CH₄ molecules in anthracite is 1.66 times of that of gas fat coal and 2.40 times of that of long flame coal, and the higher the added moisture content, the lower the adsorption heat of CH₄, the more significant the inhibitory effect of added moisture on CH₄ adsorption by coal.

Keywords

adding water different metamorphic coals adsorption amount adsorption heat adsorption energy

Introduction

China is a country that relies mainly on coal as its primary energy source, and is the biggest producer and consumer of coal in the world (Liu, 2022), but more than 95% of China's high-gas mines belong to low-permeability coal bed, so in order to weaken the difficulty of gas extraction and reduce the occurrence of gas accidents, coal bed water injection has become an active method to improve coal bed permeability (Chen et al., 2017; National Bureau of Statistics, 2024). Coal seam infusion can use water as a power to improve coal seam permeability, thereby achieving rapid and efficient extraction of coal bed gas (Wang et al., 2022; Yuan et al., 2015). Therefore, studying the influence of adding moisture on methane sorption in coals of different rank has important practical meaning for optimizing the development strategy of coalbed methane resources.

Along with coalification, pore size plays different roles in different coal stages (Prinz et al., 2004). The related research shows that (Liu et al., 2018; Prinz et al., 2004) the disaggregation of various functional groups and the aggregation of molecular structure in coal macromolecules can also influence the difference of micropore structure. As a wetting fluid, H₂O molecules can replace CH₄ molecules adsorbed on the coal surface, leading to a decrease in the coal's ability to adsorb methane (Song et al., 2022; Xu et al., 2021). Joubert et al. (1974) and Krooss et al. (2002) demonstrated that the water content in coal increased, the methane adsorption amount decreased, and could inhibit the desorption of methane from coal. When the humidity attains a certain value, the adsorption amount of coal reduces as the raise of humidity, when achieving a certain value, the raise of humidity will not have an effect on coal adsorption (Joubert et al., 1973). Yue and Li et al. found in their research that water can serve as an effective substitute, occupying the methane adsorption sites in coal (Li et al., 2023b; Yue et al., 2019). Furthermore, researchers such as Zhang and Gao (Zhang et al., 2018, 2020) have discovered that the sorption behavior of H₂O molecules on coal surfaces is due to their complex interactions with the coal matrix, which includes intermolecular forces and the formation of hydrogen bonds. Xia et al. (2020) investigated the sorption of H₂O molecules on low-degree coals surface, indicating that H₂O molecules are more likely to adsorb at oxygen sites. You et al. (2018, 2019) studied the structure of H₂O molecules adsorbed on the bituminous coals surface. The results showed that the H₂O molecules tended to adsorb carboxyl groups, and Li et al. (2023a) confirmed that there is a negative relationship between the adsorption amount of methane and the adsorption amount of water content. Xia et al. (2019a) have shown that the oxygen functional groups of low-degree coals are primarily composed of carboxyl and hydroxyl groups. Zhu et al. (2021a, 2021b) created the isothermal curve of methane adsorption at different water content, and confirmed that the presence of water leads to a sudden decline in CH₄ sorption capacity. Arif (Arif et al., 2016) and Zheng (Zheng et al., 2020) analyzed the influence of the existence of H₂O on CH₄ sorption behavior, and came to conclusion that polar H₂O molecules would preferentially occupy adsorption positions in coal compared with CH₄ molecules. Clarkson et al. (Clarkson and Bustin, 2000) proved that humidity can lower the adsorption performance of coal. Gao (Gao et al., 2020) has proved that the coal matrix surface has a greater affinity for CH₄ molecules compared to H₂O molecules. Zhang-H et al. (Chen et al., 2018; Zhu et al., 2021a) demonstrated through simulations of methane adsorption amount at different water content conditions that a raise in water content results in a sudden decrease in methane sorption amount. Deng et al. (2019) proved that as the CH₄ adsorption amount and temperature gradually increase, the CH₄ adsorption heat decreases. Niu and Zhang (Niu et al., 2018; Zhang et al., 2015) showed that increasing the water content can lead to a decrease in the sorption capacity of low-rank coal molecules for CH₄ molecules.

The micro-organic structure of coal varies with different degree metamorphic, and the influence of added water on methane sorption of different rank metamorphic coals is also different. Therefore, this article selects three kinds of coal with different grade metamorphism, which stand for coal with low rank of metamorphism (long flame coal), medium grade of metamorphism (gas fat coal), and high grade of metamorphism (anthracite). Through experimental research on methane adsorption amount in coal under different water contents (1.32%, 3.85%, 4.46%, and 6.26%), different pressures and different grades metamorphism, the complexity and differences in the mechanism of moisture on methane adsorption are revealed, and how these differences change in pace with the increase of coal sample metamorphism. Combined with molecular simulation methods, this study aims to reveal at the microscopic level how the presence of H₂O molecules affects the intermolecular interactions and adsorption energy of CH₄ molecules. To explore the consequence of additional moisture on methane sorption in coal with different metamorphic degrees, it helps to optimize the development strategy in coalbed methane resources, and offer scientific basis and theoretical understanding of improving coal mine safety production.

Experiments

Materials

The used anthracite is the coal sample from Duanshi coal mine. The used gas fat comes from Liujialiang coal mine. The long flame comes from Banshi coal mine, Yanbian Korean. The collected coal samples are crushed, the adsorption constants are determined, and the results of the measured adsorption constant are shown in Table 1.

Table 1.

Adsorption constant.

Adsorption constant	a (m³/t)	b (MPa⁻¹)
Anthracite	38.43	1.07
Gas fat coal	13.831	0.848
Long flame coal	13.477	2.585

The collected lump coals are broken and 1 to 3 mm granular coals are selected. After drying in the vacuum drying box, it is sealed. Set the operating temperature of the vacuum drying box at 80°C to 85°C for 6 hours. The industrial analysis results are shown in Table 2.

Table 2.

Industrial analysis results.

Industrial analysis	M_ad (%)	A_ad (%)	V_ad (%)
Anthracite	1.68	17.71	7.83
Gas fat coal	1.45	31.88	30.82
Long flame coal	8.54	13.12	46.90

The adsorption constants a of anthracite, gas fat coal, and long flame coal are, respectively, 38.43, 13.831, and 13.477 m³/t, and the adsorption constants b are 1.07, 0.848, and 2.585 MPa. Among them, anthracite has the strongest adsorption capacity, while long flame coal and gas fat coal have weaker adsorption capacity. Anthracite has the highest porosity, the porosity of long flame coal is the lowest.

Experimental procedure

After weighing an appropriate amount of coal sample, it is placed in the vacuum drying box and heated for 6 hours for dehydrating treatment. Then, weigh 60 g of dry coal sample and place it in a coal sample tank. Finally, place it into the constant temperature water bath device, fixed temperature is set at 303.15 K, and the adsorption pressures are 0.5, 1, 1.5, 2.0, and 3.0 MPa, respectively. After injecting methane gas, close the charging valve and leave it for more than 12 hours to make methane adsorption equilibrium in coal sample. According to the weight of coal particles, measure the required water content when the water content is 1.32%, 3.85%, 4.46%, and 6.26%, respectively. The high-pressure water injection device is used to complete the water injection and conduct adsorption experiments. The pressure display screen is used to record the sorption pressure of methane on coal of different grades metamorphism at different moisture contents (Figure 1).

Figure 1.

Experimental design.

Measurement of pipeline volume

Calculate the volume filled into the coal sample tank and pipework according to formula 1.

V = Q \cdot \frac{P_{1} Z_{2} (273.2 + t_{2})}{P_{2} Z_{1} (273.2 + t_{2})}

(1)

In the formula, Q is the volume of adsorbed methane, mL; $P_{1}$ the gas pressure of coal sample tank after venting, MPa; $P_{2}$ the gas pressure of coal sample tank after inflation, MPa; $Z_{1}$ and $Z_{2}$ is the compressibility coefficients of methane at temperatures $t_{1}$ , $P_{1}$ and $t_{2}$ , $P_{2}$ , respectively, dimensionless; $t_{1}$ the ambient temperature of adsorbed methane gas, °C; $t_{2}$ the environmental temperature of coal sample tank and pipeline, °C; V the total volume of the coal sample tank and pipe, mL.

Fitting

At the same ratio of coverage, even surface and fixed temperature, the Langmuir adsorption isotherm is a perfect single-layer adsorption equation that can better illustrate chemical adsorption or physical adsorption with particularly strong gas–solid adsorption force. Its assumption is that the surface of the adsorbate and adsorbent is even and the sorption between gas and solid is a single-layer adsorption. The formula is as follows:

V = \frac{a b p}{1 + b p}

(2)

In the formula, V is the amount of adsorption at equilibrium pressure P under constant conditions, cm³/g; P the equilibrium pressure of gas adsorption, MPa; a reflects the maximum adsorption capacity of the adsorbent surface, the limit value of the adsorption volume V when the pressure P approaches infinity, cm³/g; and b the sorption constant of gas, representing the capacity of the solid surface's ability to adsorb gas.

Simulation

Simulation model

The coal molecular structure model (Ge et al., 2020; Yan, 2019; You, 2018) and coal molecular parameters of three different rank coals were used in the simulation respectively as shown in Figure 2.

Figure 2.

Detailed parameters of coal molecule.

Periodic boundary conditions were constructed using three coal molecular structure models (anthracite, gas fat coal, and long flame coal) after annealing optimization. After simulating the density of the model, potential energy density plots were drawn for three different degrees metamorphic coal (see Figure 3 for details). In general, the optimal density of coal molecules is considered to be the density value of the second local potential energy smallest point after crossing the first potential energy lowest point. Therefore, it can be determined that the molecular model density of anthracite is 1.3 g/cm³, that of gas fat coal is 1.25 g/cm³, and that of long flame coal is 1.25 g/cm³.

Figure 3.

Potential energy–density relationship of coal molecules structure model.

Saturated adsorption methane molecular model

The MS is used to import the lowest energy coal molecules with different grades of metamorphism under periodic boundary conditions into the Sorption module. Select the Locate task to construct the adsorption models. The simulation parameters for the maximum loading step and maximum production step are set at 100,000 steps, the structural model of saturated sorption of CH₄ molecules and H₂O molecules by coal molecules, respectively. The adsorption simulation sequence is shown in Figure 4.

Figure 4.

The adsorption simulation sequence.

During the adsorption process, record and calculate the energy difference of each component before and after CH₄ molecules adsorption, as shown in Tables 3 to 5. Positive values show rise in energy after adsorption, while negative values show a decline in energy after adsorption.

Table 3.

Energy difference of anthracite molecules before and after adsorption of different amounts of CH₄ molecules.

Number of CH₄ (n)	Total energy difference (kcal/mol)	Valence electron energy difference (kcal/mol)				Nonbonding energy different (kcal/mol)
Number of CH₄ (n)	Total energy difference (kcal/mol)	Bond	Angle	Torsion	Inversion	H-bond	Van der Waals	Electrostatic
1	−0.015	0	0	0	0	0	0.06	−0.076
2	−0.238	0	0	0	0	0	−0.146	−0.092
3	−0.157	0	0	0	0	0	0.062	−0.219
4	−0.762	0	0	0	0	0	0.045	−0.807
5	−0.840	0	0	0	0	0	0.124	−0.963
6	−2.936	0	0	0	0	0	−1.688	−1.248
7	−3.360	0	0	0	0	0	−2.60	−0.76
8	−4.728	0	0	0	0	0	−0.931	−3.797
9	−10.116	0	0	0	0	0	−5.107	−5.009
10	−17.961	0	0	0	0	0	−14.235	−3.726
11	−10.767	0	0	0	0	0	−6.202	−4.565
12	−7.737	0	0	0	0	0	−6.092	−1.645
13	−7.261	0	0	0	0	0	−3.876	−3.385

Table 4.

Energy difference of gas-rich coal molecules before and after adsorption of different amounts of CH₄ molecules.

Number of CH₄ (n)	Total energy difference (kcal/mol)	Valence electron energy difference (kcal/mol)				Nonbonding energy different (kcal/mol)
Number of CH₄ (n)	Total energy difference (kcal/mol)	Bond	Angle	Torsion	Inversion	H-bond	Van der Waals	Electrostatic
1	−0.013	0	0	0	0	0	0.049	−0.062
2	−0.030	0	0	0	0	0	−0.028	−0.002
3	−0.147	0	0	0	0	0	−0.001	−0.146
4	−0.417	0	0	0	0	0	−0.584	0.167
5	−1.026	0	0	0	0	0	−1.01	−0.016
6	−4.353	0	0	0	0	0	−3.752	−0.601
7	−9.875	0	0	0	0	0	−9.468	−0.407
8	−12.265	0	0	0	0	0	−10.931	−1.334
9	−4.681	0	0	0	0	0	−4.334	−0.347

Table 5.

Energy difference of long flame coal molecules before and after adsorption of different amounts of CH₄ molecules.

Number of CH₄ (n)	Total energy difference (kcal/mol)	Valence electron energy difference (kcal/mol)				Nonbonding energy different (kcal/mol)
Number of CH₄ (n)	Total energy difference (kcal/mol)	Bond	Angle	Torsion	Inversion	H-bond	Van der Waals	Electrostatic
1	−0.403	0	0	0	0	0	−0.312	−0.091
2	−1.510	0	0	0	0	0	−0.851	−0.659
3	−2.596	0	0	0	0	0	−1.517	−1.079
4	−4.200	0	0	0	0	0	−4.299	0.099
5	−5.533	0	0	0	0	0	−6.242	0.709
6	−5.335	0	0	0	0	0	−2.667	−2.668

According to Tables 3 to 5, it can be concluded that during coal molecules adsorbing CH₄ molecules, only H-bond energy is involved, indicating that the sorption of CH₄ by coal molecules belongs to physisorptio (Chen et al., 2023; Pan et al., 2018; Uanl et al., 1991). The van der Waals energy of anthracite, gas fat coal, and long flame coal molecules accounts for 60.69%, 91.62%, and 76.23% of the total energy difference, respectively.

In the light of the simulation results, the system energy diagrams for the adsorption of CH₄ by anthracite, gas fertilizer coal, and long flame coal are drawn, as shown in Figure 5.

Figure 5.

Total energy difference of coal molecules adsorbing methane molecules.

As can be seen from Figure 4, when a quantity of CH₄ adsorbed by anthracite, gas fat coal and long flame coal is less than 10, 8, and 3, respectively, the total energy difference of the entire system still shows a downward trend. When the number of CH₄ adsorbed by anthracite, gas fat coal and long flame coal exceeds 10, 8, and 3, the energy difference of the system rapidly raises. This is because the adsorption of CH₄ on coal is an exothermic process, and the larger the energy difference, the stronger the interaction between coal molecules and CH₄ molecules. Therefore, the saturated adsorption amount of coal molecules is a quantity of CH₄ molecules when the total energy difference of whole system achieves its lowest point. At this point, coal begins to repel CH₄ molecules, resulting in a decrease adsorption capacity of coal for CH₄ molecules and a raise in the total energy difference of the whole system. Therefore, the saturated adsorption amount of anthracite, gas fat coal, and long flame coal are 10 molecules/(u. c), 8 molecules/(u. c), and 3 molecules/(u. c), respectively.

Construct additional moisture model

The coal-methane-water ternary molecular structure model with water content of 1.32%, 3.85%, 4.46%, and 6.26% is constructed by using the Locate task. The molecular structure of anthracite was loaded with 2, 7, 8, and 11 H₂O molecules, gas fat coal is loaded with 2, 5, 6, and 9 H₂O molecules, and long flame coal is loaded with 1, 3, 4, and 5 H₂O molecules, respectively. Then, using the adsorption isotherm task, the accuracy is set to Customized, the temperature is 303.15 K, the Equilibration and Production steps are 1,000,000, the Fugacity steps are 20, and the adsorption pressures are 0.5, 1, 2, and 3 MPa, the force field is Dreiding, and Use current charges are selected. The models of added water coal are shown in Figure 6.

Figure 6.

The model of added water coal. (a) Anthracite, (b) gas fat coal, and (c) long flame coal.

Simulation calculation method

The boundary condition of molecular structure model is 12.5 × 1 × 0.5 Å³, α = β = γ = 90°. The Forcite module in MS is used for energy optimization, geometric optimization, and annealing treatment. The Smart algorithm is selected to completely remove the disadvantageous interactions between atoms, and the influence of coal with different ranks of metamorphism on the sorption of H₂O is studied. Due to the minimal impact of environmental pressure on the whole process, NVT is chosen (Liang et al., 2016; Zhang et al., 2016), a Nosé thermostat is used, and the temperature is 303.15 K (Nose, 1991). The whole simulation time is 600 ps, which is enough for energy balance, the final 200 ps are used to get trajectory data for further study (Xia et al., 2019b).

Results and discussion

CH₄ adsorption amount

The molecular models of coal are first saturated to adsorb CH₄ molecules, and then different amounts of H₂O molecules are adsorbed to form coal with moisture contents of dry coal, 1.32%, 3.85%, 4.46%, and 6.26%, respectively.

The adsorption pressures at different water contents are recorded, and the CH₄ adsorption amounts of coal are calculated. The experimental outcomes are shown in Table 6, and are plotted in Figure 7.

Figure 7.

Adsorption amounts of CH₄ in coal.

Table 6.

CH₄ adsorption amounts in coal.

Pressure (MPa)	Anthracite					Gas fat coal					Long flame coal
Pressure (MPa)	Dry coal	1.32%	3.85%	4.46%	6.26%	Dry coal	1.32%	3.85%	4.46%	6.26%	Dry coal	1.32%	3.85%	4.46%	6.26%
0.5	11.839	5.92	4.279	3.525	2.756	4.102	3.113	2.503	1.978	1.427	3.662	2.929	1.814	1.099	0.897
1	17.127	11.369	8.937	7.603	6.196	6.411	5.124	4.180	3.511	2.971	4.701	3.911	2.776	2.003	1.666
1.5	20.406	14.453	11.638	10	8.259	7.824	6.535	5.669	4.844	4.063	5.703	4.817	3.628	2.661	2.182
2	22.567	16.721	13.71	11.871	9.908	8.541	7.438	6.441	5.662	4.878	6.314	5.349	4.285	3.184	2.814
3	25.24	19.833	16.679	14.603	12.381	9.526	8.530	7.738	6.989	6.276	7.028	6.071	5.234	3.764	3.174

According to Figure 7, in a dry state, the adsorption amounts of CH₄ increase with the raise of adsorption pressure. Therefore, increasing the adsorption pressure under constant temperature conditions is beneficial for coal to adsorb CH₄ and H₂O, and the greater the CH₄ adsorption amount of anthracite. This is because anthracite has a complex structure and high porosity, mainly consisting of micropores and small holes. Gas fat coal has more developed micropores, while long flame coal has uneven pore size distribution, the degree of micropore development is relatively low. Due to micropores being the primary site for gas sorption, anthracite has the strongest adsorption capacity for methane, long flame coal has the least adsorption capacity. Dry coal has the highest CH₄ adsorption amounts, and the bigger the water content, the less the CH₄ adsorption amount in coal. This indicates that coals have a greater affinity for H₂O than CH₄. Adding H₂O will occupy the dominant adsorption sites in coal, replacing some already adsorbed CH₄, resulted in a decline in CH₄ adsorption amount. The taller the H₂O content, the smaller the CH₄ sorption amount.

Variation of methane adsorption rate

The change value of CH₄ adsorption amount is calculated (the change value is the difference in CH₄ sorption amount between dry coal and the coal sample after adding water). The change value is plotted in Figure 8.

Figure 8.

Changes of CH₄ adsorption amount in coal.

According to Figure 8, at the equal sorption equilibrium pressure and added water content, the change in CH₄ adsorption amount of anthracite is the most, while the change in long flame coal is the least. This is because with the raise of coal rank increases, the interaction between coal pore surfaces and H₂O molecules become greater, and H₂O can replace more CH₄, leading to a reduce CH₄ adsorption amount. Therefore, it can be concluded that the impact of added moisture on CH₄ sorption for high metamorphic coal is the greatest, while the effect on CH₄ adsorption in medium metamorphic coal and low metamorphic coal is relatively small.

Isothermal adsorption heat

At different sorption equilibrium pressure and moisture content, the CH₄ isothermal adsorption heat data of anthracite, gas fat coal, and long flame coal are obtained by isothermal adsorption simulation, which are shown in Table 7 and plotted in Figure 9.

Figure 9.

Isothermal adsorption heat curved line of CH₄ molecules in coal structure under different pressures.

Table 7.

Isothermal adsorption heat of CH₄ molecule in coal.

Pressure (MPa)	Anthracite					Gas fat coal					Long flame coal
Pressure (MPa)	Dry coal	1.32%	3.85%	4.46%	6.26%	Dry coal	1.32%	3.85%	4.46%	6.26%	Dry coal	1.32%	3.85%	4.46%	6.26%
0.5	7.221	6.024	5.699	5.344	4.922	4.424	4.016	3.798	3.44	3.264	3.021	2.469	1.839	1.429	0.853
1	8.284	6.856	6.399	5.894	5.434	5.01	4.586	4.183	3.77	3.489	3.438	2.79	2.282	1.902	1.319
1.5	8.816	7.348	6.6081	6.094	5.572	5.348	4.894	4.489	3.981	3.571	3.704	2.984	2.574	2.14	1.507
2	9.229	7.458	6.759	6.186	5.672	5.516	5.098	4.642	4.142	3.755	3.857	3.15	2.735	2.333	1.658
3	9.534	7.824	6.927	6.261	5.730	5.735	5.364	4.864	4.28	3.839	5.364	4.864	4.28	3.839	5.364

According to Figure 9, at the same coal sample, the adsorption heat of CH₄ decreases with the increase of additional water content, which indicates that increasing moisture reduces CH₄ adsorption capacity. Because H₂O is easier to combine with the chemical bonds on the surface of coal molecules and adsorb coal molecules with hydrophilic functional groups inside. When injecting added water into coal that adsorbs gas, H₂O molecules will compete with CH₄ molecules in the saturated state of adsorption. Due to the absolute dominance of H₂O molecules, the incoming H₂O molecules will replace the adsorbed CH₄ molecules, leading to the decline in CH₄ adsorption in coal. With add of the metamorphism grade of coal, the impact of moisture on coal adsorption of CH₄ molecules becomes more significant. At the same conditions, the heat of CH₄ adsorption in anthracite is always greater than that of gas fat coal and long flame coal, indicating that anthracite has a greater ability to adsorb CH₄ than gas fat coal and long flame coal.

Sorption energy

The sorption energy of CH₄ or H₂O molecules to some extent determines their gas content in coal seams. The bigger the absolute value of the sorption energy, the bigger the interactive force between coal molecules and CH₄ molecules or H₂O molecules.

The sorption energy of CH₄ or H₂O on coal with different degree metamorphic is simulated, as shown in Table 8 and Figure 10.

Figure 10.

The sorption energy of CH₄ or H₂O in coal.

Table 8.

The sorption energy of coal molecules to CH₄ molecules or H₂O molecules.

Coal molecules	CH₄(kcal/mol)	H₂O(kcal/mol)
Anthracite	−57.127	−192.078
Gas fat coal	−32.054	−140.725
Long flame coal	−18.421	−110.216

According to Table 8, the adsorption energy of anthracite for CH₄ molecules is 1.78 times that of gas fat coal and 3.10 times that of long flame coal, indicating that the greater the metamorphism grade of coal, the stronger the sorption ability of coal for CH₄ molecules. The sorption energy of H₂O molecules in anthracite, gas fat coal and long flame coal is higher than that of CH₄ molecules, because the molecular architecture of anthracite is more complex than that of gas fat coal molecules and long flame coal molecules, which is more conducive to adsorbing H₂O molecules.

Conclusion

The addition of moisture affects the adsorption between methane and coal, and the Valence electron energy and nonbonding energy between coal molecules and H₂O molecules are always higher than those between coal and CH₄ molecules, whether it is high metamorphic, medium metamorphic, or low metamorphic coal.

In isothermal adsorption heat simulation, the greater coal metamorphism degree in a dry state, the greater the ability to adsorb CH₄ molecules. For the same coal specimen, the bigger the added water content, the smaller the adsorption heat of CH₄ molecules, and the lower the CH₄ molecules adsorption capacity.

The adsorption energies of CH₄ and H₂O in anthracite are both greater than those of gas fat coal and long flame coal. For the identical coal specimen, the adsorption energy of H₂O has always been higher than that of CH₄ molecules, which means that coal molecules are stronger sorption capacity for H₂O than CH₄, and compared with CH₄, coal molecules are more inclined to adsorb H₂O.

Study limitation and future recommendation

This article investigates the adsorption capacity in coal of different degrees metamorphism for methane at different sorption pressures and water addition conditions, analyzes the influence and mechanization of water on coal methane adsorption, and does not consider the influence of temperature.

During the simulation in this article, it was not analyzed how H₂O molecules and CH₄ molecules act on coal molecules, as well as the dynamic displacement of specific position on the coal molecules are not analyzed.

The limited types of coal samples in this article may result in insufficient representativeness of the results, which cannot fully reflect the characteristics of all coal samples.

Footnotes

Author's contributions statement

SN is responsible for the results and analysis section of the article, processing of origin data and image editing in the later stage; XC for finalizing and revising the article; and TW and SN for all simulations of the article.

Data availability

The figures and tables data used to support the findings of this study are included within the article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Siyu Nie

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Effects of added moisture on methane adsorption of coal with different degrees of metamorphism: An experimental and molecular dynamics simulation studies

Abstract

Keywords

Introduction

Experiments

Materials

Experimental procedure

Measurement of pipeline volume

Fitting

Simulation

Simulation model

Saturated adsorption methane molecular model

Construct additional moisture model

Simulation calculation method

Results and discussion

CH4 adsorption amount

Variation of methane adsorption rate

Isothermal adsorption heat

Sorption energy

Conclusion

Study limitation and future recommendation

Footnotes

Author's contributions statement

Data availability

Declaration of conflicting interests

Funding

ORCID iD

References

CH₄ adsorption amount