Coal Gas Adsorption Research Articles

Study on the Gas Desorption Regularity of Coal Lumps of Different Sizes under Different Water Inundation Conditions

Coal mining can create a large number of abandoned areas, which contain rich coalbed methane resources. The adsorption principle of the residual gas left in the abandoned coal seam is an important part of the coalbed methane resources. The adsorption principle is of great significance for the evaluation of coalbed methane resources in abandoned coal seams. However, due to the complex environment in abandoned coal seams, the factors affecting the adsorption of residual gas in abandoned coal seams are also complex. Among them, water is an important factor that affects the adsorption of residual gas in abandoned coal seams. The study on the adsorption of residual gas in abandoned coal seams with different water infiltration conditions is less. Therefore, a block coal water-flooding gas adsorption device was developed for the study of block coal gas adsorption, and the adsorption rate of block coal gas under different water infiltration conditions was determined experimentally.

Analysis of the influence of temperature on the adsorption characteristics of coal: Insights into low-temperature outburst prevention

To investigate the impact of temperature reduction on the adsorption characteristics of coal gas, isothermal adsorption experiments were conducted with environmental temperature and moisture content as variables. The changing patterns of coal gas adsorption characteristics, adsorption theories, and the temperature response of gas expansion were analyzed. Furthermore, engineering measures for preventing gas outbursts in low-temperature frozen coal seams were discussed. The research findings indicate that the gas adsorption capacity of coal samples increases with decreasing temperature. When the temperature decreases from 20 to −20 °C, the gas adsorption capacity decreases from 34.48 to 19.90 cm3/g. At the same pressure, the adsorption capacity of coal is negatively correlated with temperature. The Langmuir theoretical model better reflects the adsorption law of coal gas under different environmental temperature conditions, wherein the adsorption constant “a” gradually decreases with the increase in environmental temperature. In a closed space, the gas pressure decreases with the decrease in coal sample temperature. Under the new adsorption equilibrium, the gas pressure in the coal sample tank decreases linearly with temperature. Coal samples with different moisture contents exhibit varying degrees of decrease in gas pressure at the same temperature, with higher moisture content resulting in a smaller decrease in gas pressure. Gas expansion energy decreases linearly with the decrease in coal temperature, indicating a proportional relationship between gas expansion energy and coal gas release pressure. As the temperature decreases, the gas pressure decreases. Based on the experimental results, a synergistic approach for preventing gas outbursts through low-temperature freezing is proposed.

A fractal Langmuir adsorption equation on coal: Principle, methodology and implication

The conventional Langmuir equation based on the homogeneous surface assumption is failed to characterize the gas adsorption in coal due to the obvious heterogeneity of the coal pore structure. Therefore, a novel fractal Langmuir adsorption equation on coal is proposed based on the fractal characteristics of pore heterogeneity in this paper. The pore heterogeneity in coal displays obvious fractal characteristic, which can be characterized by the fractal dimension for the pore tortuosity (DT) and the fractal dimension for the pore size (Df). The principles and methodology of the proposed equation are deduced and summarized in detail. The results indicate that the proposed fractal Langmuir adsorption equation displays the higher accuracy than the conventional Langmuir equation. In addition, the proposed equation is the function of the fractal dimension for the pore tortuosity (DT), the fractal dimension for the pore size (Df), and the pore structural parameters (λmin, λmax, and L), which more comprehensively reflects the influence of pore structure on adsorption. Therefore, the proposed equation achieves a breakthrough in the calculation of the methane adsorption capacity in pores with different pore diameter ranges, which can more accurately reflect the methane adsorption in coal pores compared with the conventional Langmuir equation.

Visual characterization of coal gas desorption using infrared radiation

Gas monitoring is an important means of preventing gas disasters in coal mines. Correct gas monitoring methods can be used to locate abnormal areas of gas emission on the surface of coal bodies. Therefore, an infrared radiation (IR) testing system of coal gas adsorption and desorption is established in the present work, and the IR-based measurement of coal gas desorption is conducted. The corresponding relationship between amount of gas desorption and the infrared radiation temperature (IRT) difference in coal gas desorption process is studied. The general formula of amount of gas desorption of coal based on the average infrared radiation temperature (AIRT) difference is established, and the visual characterization of amount of gas desorption of coal based on IRT difference is realized. The results show that the process of coal gas desorption is a cooling process, and AIRT difference, in the process of coal gas desorption shows a downward trend. There is a linear relationship between the amount of gas desorption of coal and the AIRT difference. The gas desorption curve of coal based on the AIRT difference is consistent with the measured curve. The cloud diagram of coal gas desorption under different adsorption equilibrium pressures can better reflect the variation characteristics of gas desorption in the process of coal gas desorption, and quantitatively characterize the distribution of gas desorption in different regions of the coal. The research results will help to accurately grasp the regional distribution characteristics of coal gas emissions and provide theoretical support for accurate monitoring and control of gas disaster risk at the mining face.

Insight into difference in high-pressure adsorption-desorption of CO2 and CH4 from low permeability coal seam of Huainan-Huaibei coalfield, China

This study aimed to investigate the differential characteristics of high-pressure adsorption-desorption of supercritical CO2 and CH4 at main temperatures of 36 ℃, 42 ℃, and 48 ℃ and the pressure of 0–20 MPa in low permeability coal, a case from the tectonic coal area of Huainan-Huaibei coalfield, eastern China. Combining the variation characteristics of adsorption capacity with pressure, gas free phase density, the prediction of CO2 adsorbed phase density, the correction method of gas adsorption capacity, the sequestration capacity of CO2 in low permeability coal, and the adsorption hysteresis index of adsorbed gas were assessed. The controlling factors of gas adsorption capacity were revealed based on the general discussion of the relation between the gas adsorption capacity and the conditions such as temperature, pressure, coal maceral, coal quality, and pore structure. The results show that the lowest equilibrium pressure associated with the CH4 excess adsorption peak is 9 MPa, which is larger than most of the experimental pressures for the CH4 isothermal adsorption test. CO2 excess adsorption capacity of the different coal samples has characteristic of an obvious peak at the experimental pressure near 6 MPa under different experimental temperatures, all CO2 excess adsorption capacity changes with CO2 free phase density in a two-step pattern, CH4 or CO2 adsorbed phase density can be predicted by the truncation method. Desorption of CH4 or CO2 in coal has an obvious "hysteresis loop", the cumulative adsorption hysteresis index (CAHI) and the incremental adsorption hysteresis index (IAHI) of CH4 based on the calculation of CH4 excess adsorption capacity are controlled by the temperature and pressure, it has the obvious rise during the stage of low pressure (less than 4 MPa), whether IAHI or CAHI of CO2, it shows characteristics of complex changes. Variations in CAHI and IAHI for CH4 or CO2 are governed by the development of pore structure in coal, and the temperature. Coal maceral, coal quality, and coal grade have an effect on gas adsorption in coal by affecting the growth, filling, and deformation of pores in coal; the adsorption capacity of the gas depends both on the surface area of the pores and the pore volume of the pores in the coal. The differential mechanism for the gas adsorption capacity from the low permeability coal requires further understanding.

Experimental Study of the Pore Structure and Gas Desorption Characteristics of a Low-Rank Coal: Impact of Moisture.

Coal–water interactions have a prominent impact on the prediction of coal mine gas disasters and coalbed methane extraction. The change of characteristics in the microscopic pores of coal caused by the existence of water is an important factor affecting the diffusion and migration of gas in coal. The low-pressure nitrogen adsorption experiments and gas desorption experiments of a low-rank coal with different equilibrium moisture contents were conducted. The results show that both the specific surface area and pore volume decrease significantly as the moisture content increases, and the micropores (pore diameter <10 nm) are most affected by the water adsorbed by coal. In particular, for a water-equilibrated coal sample at 98% relative humidity, micropores with pore sizes smaller than 4 nm as determined by the density functional theory model almost disappear, probably due to the blocking effects of water clusters and capillary water. In this case, micropores with a diameter less than 10 nm still contribute most of the specific surface area for gas adsorption in coal. Furthermore, the fractal dimensions at relative pressures of 0–0.5 (D1) and 0.5–1 (D2) calculated by the Frenkel–Halsey–Hill model indicate that when the moisture content is less than 4.74%, D1 decreases rapidly, whereas D2 shows a slight reduction as the moisture content increased. In contrast, when the moisture content exceeds 4.74%, further increases in the moisture content cause D2 to decrease significantly, while there is nearly no change for D1. The correlation analyses show that the ultimate desorption volume and initial desorption rate are closely related to the fractal dimension D1, while the desorption constant (Kt) mainly depends on the fractal dimension D2. Therefore, the gas desorption performances of coal have a close association with the pore properties of coal under water-containing conditions, which indicate that the fluctuation in moisture content should be carefully considered in the evaluation of gas diffusion and migration performances of in situ coal seams.

Low Nuclear Magnetic Resonance Experimental Study on Gas Adsorption of High-Rank Coals with Different Beddings.

In order to deeply study the influence of the coal bedding structure on coal gas adsorption, low nuclear magnetic resonance (LNMR) and a confining pressure loading system were used to carry out the LNMR experiment of gas adsorption of high-rank coals with different beddings under different confining pressures. The results showed that the amount of gas adsorption of high-rank coals with different beddings increases with time and decreases with the increase of confining pressure. In the process from low confining pressure to high confining pressure, the coal sample with oblique bedding (bedding angles 30°, 45°, and 60°) has the largest average increment of gas adsorption, followed by the coal sample with vertical bedding (bedding angle 90°), and the coal sample with parallel bedding has the smallest increment of gas adsorption (bedding angle 0°). The linear function relation between the different-bedding high-rank coal gas adsorption state and the confining pressure is y = a – bx. The relation between the free peak area and the confining pressure conforms to the exponential function y = a + bexp(cx). Different-bedding high-rank coal adsorption peaks and the peak area decrease with the increase of confining pressure, and the free peak continues to move to the left; that is, the large pores gradually shrink. With the increase of angle and bedding, the area of the adsorption peak increases first and then decreases, presenting an “inverted V” shape on the whole. The area of the free peak decreases first and then increases, presenting a “V” shape on the whole.

Adsorption and deformation characteristics of coal under liquid nitrogen cold soaking

The research on gas adsorption deformation characteristics of coal under liquid nitrogen cold soaking deserves investigation, as most of the investigation efforts in this field are in a preliminary stage. In this article, the experiments of coal adsorption deformation under different times of liquid nitrogen cold soaking (0, 1, 3, 5, 7 and 9) were carried out. The variation of coal gas adsorption capacity and volumetric strain under different times of liquid nitrogen cold soaking were investigated. Based on thermodynamics, the adsorption strain model of coal was developed, and the relationship between coal adsorption capacity and volumetric strain was discussed. The research results show that: (i) the coal adsorption capacity and volumetric strain both gradually decrease with the increase of the number of liquid nitrogen cold soaking; (ii) the saturated gas adsorption capacity and limit volume strain of coal both show a decreasing trend with the increase of liquid nitrogen cold soaking times; (iii) the volumetric strain of coal under the different times of liquid nitrogen cold soaking first accelerates and then slowly rises with the increase of adsorption capacity. After liquid nitrogen cold soaking, the volumetric strain growth rate of coal gradually decreases; (iv) the coal under different cold soaking times reaches the limit volumetric strain at a pressure (P) that is higher than the pressure (p) required to reach the saturation adsorption capacity, and the values of p and P both show a decreasing trend with the increase of liquid nitrogen cold soaking times; (v) the model results have good predictive effect.

Temperature-pressure coupling effect on gas desorption characteristics in coal during low-variable temperature process

Accurate determination of coalbed methane (CBM) content can not only prevent and control coal and gas outburst disasters in mines, but also facilitate the prediction of coal seam methane resource reserves, which is significant for injecting carbon dioxide to enhance CBM recovery. The freezing core sampling technique proposed in recent years for accurate determination of the CBM content has faced temperature-pressure coupling effects on gas desorption in coalin a low-variable temperature environment. In this paper, we utilized a self-developed high-low temperature alternating adsorption-desorption experimental device to study the gas desorption process in coal under temperature-pressure coupling conditions in a low-variable temperature environment. The experimental results show that the main promotion and inhibition effects of temperature on the gas adsorption and desorption in coal in a low-temperature environment were consistent with those in an ambient temperature environment, i.e., the decrease of temperature promoted coal gas adsorption, while the increase of the temperature promoted coal gas desorption. It is found that throughout the temperature change process (TCP), there existed a ‘pseudo-desorption’ phenomenon, and the cumulative desorption amount showed a consecutive rising-declining-rising trend and finally stabilized. The whole process can be regarded as consisting of a main desorption process and a retrograde desorption process. In the main desorption process with the same TCP, the cumulative desorption amount and initial desorption rate increased with the increase of initial equilibrium pressure. In the retrograde desorption process, there existed a peak retrograde desorption rate, and the staged retrograde desorption amount conformed to the linear decrease law. A comprehensive analysis of the temperature-pressure coupling effect on gas desorption throughout the TCP was conducted. Thus, we proposed some considerations and precautions for the configuration and supply monitoring of the cold source in the process of the measurement of CBM content using the freeze-coring method.

Experimental study on the influence of aerated gas pressure and confining pressure on low-rank coal gas adsorption process

To deeply study the variation characteristics of the gas content in the process of gas adsorption for coal samples under different gas pressures and confining pressures, low-field nuclear magnetic resonance technology was used to carry out experimental research on the gas adsorption of coal. The relationship between the T2 spectrum amplitude integral and the gas quantity was analyzed. The results show the following: (1) When the samples were inflated for 11 h at each gas pressure point (0.31, 0.74, 1.11, and 1.46 MPa), after ∼5 h of adsorption, the amount of adsorbed gas exceeded 85.0% of the total adsorption capacity; additionally, as the adsorption time increased, the amount of adsorbed gas gradually tended to stabilize. When the gas pressure was &gt;1 MPa, the amount of adsorbed gas exceeded 90.0% of the total adsorption capacity; Higher the pressure of aerated gas, greater the gas pressure gradient or concentration gradient on the surface of the coal sample and the greater the driving force for gas molecules to seep or diffuse into the coal sample. (2) When the samples were inflated for 11 h at each confining pressure point (3, 4, 5, and 7 MPa), the adsorbed gas increased by ∼85.0% of the total adsorbed gas in the first 5 h. When the pressure was &lt;5 MPa, the amount of adsorbed gas exceeded 85.0% of the total amount of adsorption; that is, the increase in adsorbed gas was the largest at ∼5 h in the adsorption process for the columnar coal sample under different confining pressures, and the increase was ∼5.0% from 7–11 h. When the large pores in the coal sample closed, the amount of gas that seeped into the deep part of the coal sample within the same aeration time was reduced.

Experimental Research on Adsorption Kinetic Characteristics of CH4, CO2, and N2 in Coal from Junggar Basin, China, at Different Temperatures

The adsorption kinetic characteristics of CH4, CO2, and N2 in coal provide crucial information for increasing coal seam gas recovery. In this study, isothermal adsorption kinetics experiments were conducted using automatic high-pressure gas adsorption analyzer to reveal differences in adsorption kinetic characteristics of CH4, CO2, and N2 in coal. The research shows that, before reaching adsorption equilibrium, the adsorption ratio of CO2 was higher than that of N2 while that of CH4 was the smallest. The initial adsorption rates of CO2, CH4, and N2 were 1998, 205, and 76 cm3 g−1 h−1, respectively. The order of decreasing gas adsorption rate with respect to the fluctuation zone was CO2 → N2 → CH4. Before reaching adsorption equilibrium, the gas adsorption ratio at high temperature was higher than that at low temperature. The adsorption rates (Rt) of gas were divided into a rapid decay zone (Rt > 0.1 Qe/h), slow decay zone (0.01 Qe/h < Rt < 0.1 Qe/h), and an adsorption equilibrium zone (|Rt| < 0.01 Qe/h). Among the pseudo-first-order kinetic equation, the Fickian diffusion model and the dynamic diffusion model, the latter can best describe the adsorption kinetic process of coal. This work is expected to improve the theoretical basis of enhancing coal seam gas recovery and enrich the theory of gas adsorption in coal.

Exploring the adsorption and diffusion characteristics of tectonic coal at different mass ratios based on the specific surface Gibbs function

Tectonic coal is a soft coal, and the integrity of a tectonic coal seam can become compromised by the formation of cracks under the action of tectonic stresses. A series of tests were conducted to investigate the effect of the mass ratios of tectonic coal on its rock characteristics, including coal gas adsorption tests, specific surface measurements, and initial gas velocity determinations. The study showed that the adsorption value of coal sample gradually increased with increases in the mass ratio of tectonic coal. When the tectonic coal mass ratio reached 50%, the adsorption of the coal sample and initial gas velocity Δp reached highest values. In addition, a mathematical model using the Gibbs function was developed to determine the relationship between the initial gas velocity and the different mass ratios of tectonic coal. This research provides a theoretical foundation for further research on the control of coal and gas outbursts.

Energy Calculation and Simulation of Methane Adsorbed by Coal with Different Metamorphic Grades.

The law of specific surface energy variation during the adsorption process is an important basis for studying the mechanisms of coal gas adsorption. Based on the theory of adsorption energy, eight coal samples with different ranks were analyzed using an isothermal adsorption experiment at three different temperatures (30, 40, and 50 °C) and six different pressures (0.1, 0.2, 0.6, 1.0, 1.4, and 1.8 MPa). Then, the single-layer adsorption model and multilayer adsorption model were used to calculate the energy variation during the adsorption process. Just like the adsorption capacity, it is clear that the specific surface energy is inversely proportional to temperature and proportional to gas pressure. The energy difference between the single-layer adsorption and the multilayer adsorption calculation is large. Therefore, the adsorption energy was calculated based on the calorific value, and the comparative analysis shows that the specific surface energy based on the multilayer adsorption model can better reflect the gas adsorption capacity than the single-layer adsorption model. The mechanisms of gas adsorption were explored, such as intermolecular force, energy variation, and specific surface area. The adsorption energy was simulated, which indicated that the energy variation is affected by both coal physical properties and internal chemical structure.

Influence of gas migration on permeability of soft coalbed methane reservoirs under true triaxial stress conditions.

The permeability of the coal body is the key parameter restricting the efficient extraction of coalbed methane, and scholars have analysed it from two angles of the change of stress state and porosity of the coal body. However, there is still a lack of study on the mechanism of gas migration and movement in soft coalbed methane reservoir under the coupling between the true triaxial stress field (maximum principal stress σ1 > intermediate principal stress σ2 > minimum principal stress σ3) and the gas pressure field. In this paper, the coal gas adsorption and seepage experiments are conducted through the self-developed true triaxial ‘gas–solid’ coupled coal mass seepage system with gas as the adsorption and seepage medium and coal briquette taking the place of soft coalbed methane reservoirs. Furthermore, the coal gas adsorption deformation model and the permeability evolution model taking gas adsorption into account are developed. Through analysis of both experimental and theoretic results, the main conclusions are drawn as follows: (i) With the increase in gas pressure, the adsorption deformation variation of coal mass is divided into a slow growth zone, a stable growth zone and a rapid growth zone. (ii) The gas adsorption deformation model developed can predict the variation trend of coal mass adsorption volumetric strains for different types of soft coalbeds, and the fitting variance of experimental and theoretical volumetric strains is above 98%. (iii) With the increase in maximum principal stress difference, the coal permeability variation curve shows two obvious turning points, which can be divided into a slow reduction zone, a rapid reduction zone and a steady reduction zone. (iv) The permeability model of coal mass considering the gas adsorption effect can reflect the variation characteristics of permeability in the rapid reduction zone, and the overall fitting variance of experimental and theoretical permeabilities is above 91%. The above results could provide a reliable experimental and theoretical basis for improving coalbed methane extraction rates.

The binary gas sorption in the bituminous coal of the Huaibei Coalfield in China

Knowledge of the gas sorption characteristics of a coal not only helps to explain the mechanism of enhanced coalbed methane recovery but also provides an important basis for simultaneous coal and gas extraction. In consequence, the pure and binary gas excess sorption capacity of methane, carbon dioxide, and nitrogen of bituminous coal samples derived from the Xutuan Coal Mine in Huaibei coalfield, in Anhui Province in China, was measured using the volumetric method. The fitting analysis of the pure gas Langmuir adsorption model was carried out. The binary gas excess sorption measurement showed that the final sorption capacity of bituminous samples was the same no matter what the gas adsorption order of competitive adsorption and displacement adsorption. Hence, coal gas adsorption is physical adsorption, i.e. the different adsorption and desorption process of gas molecules does not affect the final adsorption amount of coal to each component of gas. Using the fitting parameters obtained by the Langmuir equation, the extended Langmuir equation was used to predict the adsorption capacity for each component of the binary gas. The comparison between predicted adsorption capacity and measured adsorption capacity showed that the extended Langmuir equation can better describe the trend of the adsorption isotherm curves of a binary gas under different pressures. The separation coefficient and displacement coefficient were defined from Langmuir adsorption theory. The separation coefficient involves the proportion of each component in the free phase and the proportion of each component in the adsorption phase. The displacement coefficient involves the displacement ability of gas molecules at adsorption sites by free gas molecules.

Experimental comparisons of gas adsorption, sorption induced strain, diffusivity and permeability for low and high rank coals

Because of coal pore structure, the adsorption and diffusion behaviors are quite different for low and high rank coals. Differences of gas adsorption-diffusion and adsorption deformation of low and high rank coal and its permeability evolution were carried out in isothermal adsorption experiment and desorption-seepage testing system. The law of adsorption and adsorption induced strain, diffusion of low and high rank coal and its influence on the coal permeability were revealed. It turns out that, the methane adsorption-diffusion and adsorption induced-strain of low and high rank coal increase with the increase of the adsorption equilibrium pressure. Because of the control of pore structure, the diffusion property of low rank coal sample is higher than that of high rank coal sample. And the strain perpendicular to the bedding plane is higher than that parallel to the bedding plane. Sorption-induced strain of high rank coal is higher than that of low rank coal, which is related to the amount of gas adsorption in coal. The Langmuir volumetric strain is about twice as high as that of the low rank coal. The law of gas adsorption deformation of coal can be described by a Langmuir isotherm adsorption equation. The influence of methane adsorption-induced swelling strain on the permeability of high rank coal is higher than that of low rank coal. At a constant effective stress, the permeability of low rank coal is higher than that of high rank coal, and with the increases of adsorption equilibrium pressure, the permeability decline by a negative exponential function of different rank coals; the rate of permeability reduction of high rank coal is higher than that of low rank coal.

Impacts of long-term water inrush on characteristics of coal gas adsorption and seepage

Frequent coal mine flooding is bound to impact not only the stope stress effects of working face, but also the absorption and seepage characteristics of coal mass. In this paper, we examined the effects of long-term water inrush on coal pore structure as well as gas adsorption and seepage characteristics using water-flooded Taoyuan Coal Mine, Anhui Province, China, as an example. Results showed that: 1) after water inrush, the volume, area, and porosity of coal samples, as well as the proportion of micro-pores decreased significantly with the distance from the water inrush site increasing; 2) the amount of gas adsorption of coal samples near the water inrush site was higher than that away from the site; 3) within 13 m from the site, the seepage capacity of coal gas increased; 4) the water seeped into the coal dissolves a mass of inorganic minerals and organic matter and carries them away. [Received: December 26, 2014; Accepted: June 13, 2015]

Impacts of long-term water inrush on characteristics of coal gas adsorption and seepage

Frequent coal mine flooding is bound to impact not only the stope stress effects of working face, but also the absorption and seepage characteristics of coal mass. In this paper, we examined the effects of long-term water inrush on coal pore structure as well as gas adsorption and seepage characteristics using water-flooded Taoyuan Coal Mine, Anhui Province, China, as an example. Results showed that: 1) after water inrush, the volume, area, and porosity of coal samples, as well as the proportion of micro-pores decreased significantly with the distance from the water inrush site increasing; 2) the amount of gas adsorption of coal samples near the water inrush site was higher than that away from the site; 3) within 13 m from the site, the seepage capacity of coal gas increased; 4) the water seeped into the coal dissolves a mass of inorganic minerals and organic matter and carries them away. [Received: December 26, 2014; Accepted: June 13, 2015]

Effect of an electrostatic field on gas adsorption and diffusion in tectonic coal

The characteristics of adsorption, desorption, and diffusion of gas in tectonic coal are important for the prediction of coal and gas outbursts. Three types of coal samples, of which both metamorphic grade and degree of damage is different, were selected from Tongchun, Qilin, and Pingdingshan mines. Using a series of experiments in an electrostatic field, we analyzed the characteristics of gas adsorption and diffusion in tectonic coal. We found that gas adsorption in coal conforms to the Langmuir equation in an electrostatic field. Both the depth of the adsorption potential well and the coal molecular electronegativity increases under the action of an electrostatic field. A Joule heating effect was caused by changing the coal–gas system conductivity in an electrostatic field. The quantity of gas adsorbed and ΔP result from competition between the depth of the adsorption potential well, the coal molecular electronegativity, and the Joule heating effect. ΔP peaks when the three factors control behavior equally. Compared with anthracite, the impact of the electrostatic field on the gas diffusion capacity of middle and high rank coals is greater. Compared with the original coal, the gas adsorption quantity, ΔP, and the gas diffusion capacity of tectonic coal are greater in an electrostatic field. In addition, the smaller the particle size of tectonic coal, the larger the ΔP.

Simulation of the interaction of methane, carbon dioxide and coal

Gas adsorption has an important influence on gas flow in a coal body. Research on the characteristics of coal and gas adsorption is the theoretical basis for studying gas flow in coal. In this paper, the interaction between methane, carbon dioxide and surface molecules of anthracite was simulated using the quantum chemistry method. Adsorption energy and adsorption configurations of different quantities of gas molecules absorbed on the coal surface were calculated. The results show that adsorption between coal and the two kinds of gas molecules is a physical adsorption process and there is an optimal configuration. Gas molecules are more easily adsorbed in the hydroxyl-containing side chain, while it is difficult for them to be adsorbed at the position of the benzene ring. Besides, carbon dioxide molecules are more readily adsorbed on the coal surface than methane molecules. The findings have an important significance in revealing the nature of gas adsorption in coal.