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Energy variation in coal samples with different particle sizes in the process of adsorption and desorption
Journal of Petroleum Science and Engineering 188 (2020) 106932
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Energy variation in coal samples with different particle sizes in the process of adsorption and desorption Tao Gao a, b, c, Dong Zhao a, b, c, *, Chen Wang b, Zengchao Feng b a
College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, 030024, China Key Laboratory of In Situ Property-Improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, 030024, China c Graduate Student Education and Innovation Center in Coal Mine Safety of Shanxi Province, Taiyuan, 030024, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Adsorption Desorption Particle size Temperature Energy variation
The study of energy variation rules in the process of gas adsorption and desorption in coal is important to predict mine outburst disasters. In this study, an adsorption-desorption experimental device was developed and used to evaluate six coal samples of various particle sizes under different pressure conditions. Temperature was measured in real-time throughout the process. The rate of temperature change, maximum temperature variation, and adsorption quantity increased with particle size and an increase in equilibrium pressure. There is a positive linear relationship between adsorption quantity and temperature variation. This study focused on energy vari ation during adsorption and desorption. The results showed that the smaller the coal particle size, the larger the reduction in surface free energy and the change in heat. With an increase in equilibrium pressure, the rate of reduction in the surface free energy slows gradually, indicating that there is a non-uniform adsorption potential field on the surface of coal. By comparing the difference of temperature and heat data between a columnar coal sample and a granular coal sample in the process of adsorption and desorption, it can be concluded that the surface free energy is converted into heat and deformation energy in the adsorption process, and the deformation energy is converted into heat energy in the desorption process.
1. Introduction With the continuous deepening of mines, the potential for serious gas outburst disasters is increasing (Wang et al., 2017, 2019a, 2019b; Tang et al., 2015). Variations in gas occurrence state (adsorption and desorption) in coal strongly influences the potential for outbursts (Lu et al., 2019; Wang et al., 2019a, 2019b). Some gas outburst simulations in laboratory had been carried out (Li et al., 2017). Therefore, a good understanding of gas adsorption and desorption mechanisms is neces sary (Lu et al., 2017). Macroscopic adsorption and desorption experi ments and theoretical research have been broadly conducted (Tan and Gubbins, 1990; Sobczyk, 2011; Shemshad et al., 2012; Wang et al., 2012; Zhao et al., 2016). Generally, the smaller the particle size of coal, the faster its adsorption rate (Zhao et al., 2019; Busch et al., 2004; Gruszkiewicz et al., 2009). Gruszkiewicz et al. (2009) selected three size fractions of crushed coal samples (40–150 μm, 1–2 mm, and 5–10 mm) for adsorp tion experiments and found that the adsorption rate of the 40–150 μm coal samples was significantly faster than that of the other two samples.
Kim et al. (2018) analyzed adsorption properties of subbituminous coal with six different particle sizes and concluded that the smaller the par ticle size of the coal, the larger the specific surface area, and the larger the Langmuir volume. Many researchers have conducted isothermal adsorption-desorption experiments on coal samples with different par ticle sizes and concluded that as the particle size of a coal sample de creases, the adsorption equilibrium pressure increases, and the gas desorption rate becomes faster (Soares et al., 2007; Lutynski and Gon zale, 2016; Jin et al., 2016). In addition, temperature has an important effect on the adsorption and desorption of gas. Yu (1992) determined that a temperature rise of 1 � C when equilibrium pressure was 5 MPa, caused methane adsorption to decrease by 0.12 cm3/g. Wierzbicki (2013) studied the kinetic adsorp tion characteristics (effective diffusion coefficient) of methane on coal using gravimetric analysis. The results showed that a positive linear correlation existed between the diffusion coefficient and temperature. Using isothermal adsorption experiments, Pini et al. (2010) concluded that the higher the experimental temperature, the lower the gas adsorption in coal. He et al. (2010) studied the gas migration process in
* Corresponding author. College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, 030024, China. E-mail address: [email protected] (D. Zhao). https://doi.org/10.1016/j.petrol.2020.106932 Received 16 April 2019; Received in revised form 28 October 2019; Accepted 7 January 2020 Available online 8 January 2020 0920-4105/© 2020 Elsevier B.V. All rights reserved.
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coal samples using the independently developed deep coal-rock tem perature-pressure coupling gas experiment system and confirmed that an increase in temperature induced a large amount of desorption of gas in coal samples. Yan et al. (2019) conducted low-temperature nitrogen and isothermal adsorption experiments, discovering that the adsorption free-energy and enthalpy of the coal samples decreased with an increase in temperature and adsorption heat was affected by an increase in pore volume and specific surface area. Gas adsorption is an exothermic process, and adsorption and desorption are reversible (Chaback and Morgan, 1996). Coal particle size, temperature, pressure, and other factors can change the tempera ture and energy of coal by the dynamic process of adsorption and desorption of gas in coal. An (1983) found that gas desorption is the main factor that causes the temperature of the working face of the coal wall to change through field experiments. The temperature change can reflect the gas content in the coal and thus serve as a predictive indicator for gas outburst. Wu et al. (2017) carried out outburst tests by using briquettes made from coal samples of different particle sizes. The results showed that as particle size becomes smaller, the magnitude of tem perature change increases and the rate of change becomes faster during the process of gas adsorption and desorption, and the law of temperature evolution with time approximately conforms to a natural logarithm relationship. Many researchers have measured the magnitude of tem perature change of coal during isothermal gas adsorption, and concluded that as gas pressure increases, the quantity of gas adsorbed increases, and the heat released in the adsorption process increases; they propose that the magnitude of temperature change can be used to pre dict gas outbursts (Guo et al., 2000; Yang et al., 2019; Liu et al., 2013; Zhang, 1993). A few investigators numerically analyzed a comparison of the elastic energy necessary for coal seam outburst with the internal gas energy released during gas outburst, and found that the energy released by gas desorption is the main cause of gas outburst (An et al., 2019). Research has concluded that coal is a dual porosity media, transportsorption system in which gas pressure causes compression of micropo rous regions and expansion of macropores (Katarzyna et al., 2016). Adsorption of CO2 in coal samples led to its swelling (Kowalczyk et al., 2010; Kim et al., 2011). Studies indicate that there is a linear relation between coal expansion and the volume of gas adsorbed in its structure (Paweł et al., 2015). Bakhshian and Sahimi (2017) studied deformation and swelling of porous media induced by sorption of carbon dioxide, discovering that the strain was released as the adsorbed gas was desorbed. A phenomenon of temperature change and energy conversion does exist in the process of gas adsorption and desorption. Most studies are based on isothermal adsorption experiments or outburst simulation ex periments. However, the temperature measurements are considerably affected by the external environment temperature and the effect of gas compression and expansion. In addition, the effect of coal sample deformation caused by gas adsorption and desorption is not considered. In this study, an adsorption-desorption device was designed and modi fied to measure temperature variation more accurately during adsorption-desorption experiments using different particle sizes and pressures. In addition, by comparing columnar and granular coal sam ples, the deformation energy was analyzed. Consequently, the energy change rules were systematically studied. These results are of signifi cance to the improvement of adsorption and desorption mechanisms and the prevention and control of coal and gas outburst.
Shanxi. After being moved from the well and sealed tightly, it was sent to the laboratory for later use. Proximate and elemental analyses, vitrinite reflectance, and the definition of coal type were performed for the coal samples (Table 1). Granite samples were obtained directly from the engineering site. The sample sizes required in this experiment are columnar (raw coal, 50 mm in diameter and 100 mm in length), and 2–4, 4–8, 8–16, 16–30, and 30–60 mesh. To improve the quality of the experimental results, CO2 was selected as the adsorbate because of its relatively high adsorption heat and adsorption capacity. Moreover, due to the stronger adsorption capacity of CO2 than CH4 in coal seams, it adsorbs more gas, which may cause more serious outburst disasters. Methane and CO2 both are adsorbed on the surface of coal, and the adsorption and desorption characteristics in similar manners in real coal seams (Ma et al., 2014; Liu et al., 2015; Zhao et al., 2019). 2.2. Experimental system An existing experimental device for gas adsorption and desorption in the laboratory was redesigned and improved by adding an insulation layer and a temperature measuring device. The structure of the experimental device is shown in Fig. 1. (Labels in Fig. 1: ①Gas cylinder; ②Needle valve; ③Pressure gauge; ④Insulating layer; ⑤ Reference tank; ⑥Temperature sensor no.4; ⑦Ball valve; ⑧Tempera ture sensor no.1; ⑨Temperature sensor no.3; ⑩Temperature sensor no.2; ⑪Temperature recorder; ⑫Sample tank ; ⑬Three-way valve; ⑭ Vacuum gauge; ⑮Vacuum pump) 2.3. Sample tank To measure the magnitude of temperature variation in coal samples during the experiment, the existing sample tank was modified. A photograph and structural diagram of the tank are shown in Figs. 2 and 3, respectively. The main modification is that two 8 mm casings are welded at equal distances on both sides of the outlet. The temperature sensor probes are in direct contact with the coal at different positions through the casings, which enables the transmission of temperature signals inside and outside the tank. RTV silicone rubber and AB rubber were used for sealing. The aerogel felt with low thermal conductivity is close to the inner wall of the adsorption tank; aerogel felt and tin foil tape are wrapped around the outer wall of the adsorption tank to achieve the effect of double-layer thermal insulation. As shown in Fig. 3, two temperature sensors were placed at different positions inside the columnar coal sample by methods that do not damage the structure of the coal body. During the process of gas injection and exhaust, the gas will compress and expand, causing variations in the coal body temperature; therefore, the data measured by the temperature sensor are partially caused by gas adsorption and desorption. Granite, which does not adsorb much gas, was selected to calibrate the experimental tank. By measuring the maximum temperature variation of the granite sample in the process of gas injection and exhaust, the thermal effect of gas compression and expansion was offset. Due to the unevenness of coal structure, the adsorption character istics of coal at different positions are different. Therefore, two tem perature sensors were used in the experiment to test the temperature variation at the upper and lower positions of coal. By comparing the experimental data measured by the two temperature sensors, it was found that the rule of temperature variation with time was basically the same. In the adsorption process, the temperature first increases, then tends to stabilize and decreases finally. While in the desorption process, the temperature first decreases, then tends to stabilize and increases finally. However, the temperature stability time measured by no.2 sensor was greater than that of no.1 sensor, which indicates that the no.2 sensor was less affected by the temperature of the outside environment, so the temperature variation was measured more accurately. So, the temperature data measured by the no.2 sensor were selected as the
2. Experiments 2.1. Coal samples The coal sample used in this experiment is a lean coal, which is a fresh and gangue-free lump coal sample taken from the #3 coal seam working face in the San-Yuan Zhongneng Coal mine. The location of the Coal mine is Changzhi of Shanxi Province in China, which in southeast of 2
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Table 1 Results of the coal sample analyses. Origin of coal sample San-Yuan Zhongneng
Proximate analysis (%)
R0max (%), Coal type
Elemental analysis (%)
Mad
Ash
VM
FC
S
R0max (%)
Coal type
C
H
O
P
0.70
12.02
12.19
75.09
0.34
1.90
Lean coal
89.18
3.97
5.00
0.02
Note: Ash and total sulfur (S) were calculated on a dry basis. Volatile matter (VM) and fixed carbon (FC) were calculated on a dried, ash-free basis. R0max is maximum vitrinite reflectance.
Fig. 1. Structural diagram of the gas adsorption and desorption experi ment device.
Fig. 3. Structural drawing of the experimental tank.
were then added to the experimental tank after being weighed. The free space volume was calibrated by the helium expansion method. The experimental tank was then placed under vacuum for 8 h. Afterward, CO2 gas cylinders were connected to the experimental system for adsorption and desorption experiments. Pressure and temperature dur ing the experiment were recorded. Finally, the temperature and energy variation rules in the process of coal sample adsorption and desorption were calculated and analyzed with the collected data. 3. Results and discussion 3.1. Experimental results of temperature variation during the gas adsorption process 3.1.1. Effect of particle size on temperature during adsorption Fig. 4 shows the relationship between adsorption temperature of coal samples with different particle sizes over time when the maximum gas injection pressure is approximately 3 MPa. The maximum adsorption capacities and temperature changes of coal samples with different par ticle sizes are shown in Fig. 5. The experimental data show that under the same conditions, when particle size becomes smaller, the amount of CO2 gas adsorbed increases, the rate of temperature change coal sample becomes faster, and the magnitude of temperature change increase during the adsorption pro cess. The main reason for this is that the smaller the particle size of the coal, the larger its specific surface area, and the more space it has to absorb CO2 molecules. Therefore, under the same conditions, the coal can adsorb more CO2; and because adsorption is an exothermic physical process, the more gas adsorbed, the greater the amount of heat released, and the greater the temperature variation in the coal sample. Compared with the pulverized coal sample, the temperature of columnar coal sample stabilizes for a longer time after the temperature reaches the maximum value in Fig. 4. The reason for this is that the rate
Fig. 2. The experimental tank.
experimental results. 2.4. Experimental procedures The air-tightness of the device was checked before the experiment. The coal samples were dried at 60 � C in a drying oven for 6–8 h. They 3
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Fig. 6. Temperature variation during adsorption of 30–60 mesh coal samples under different equilibrium pressures.
Fig. 4. Adsorption temperature variation of coal samples with different particle sizes under 3 MPa.
Fig. 7. Maximum adsorption quantity and temperature variation of 30–60 mesh coal samples under different equilibrium pressures.
Fig. 5. Maximum adsorption quantity and temperature variation of coal sam ples with different particle sizes under 3 MPa.
Table 2 Results of fitting the graphs of adsorption quantity and temperature variation.
of gas adsorption in the interior space of columnar coal is smaller than that of pulverized coal sample. The heat generated by the adsorption inside the coal body conducts to the coal surface slowly and continu ously, which reduces the heat dissipation from the experimental tank to the outside environment and keeps the temperature of the coal body stable for a long time. 3.1.2. Effect of equilibrium pressure on temperature during adsorption Fig. 6 shows the adsorption temperature variation of 30–60 mesh coal samples with time. The maximum amount of gas adsorbed (adsorption quantity) and the temperature variation of the coal sample are shown in Fig. 7. The adsorption quantity (y) and temperature vari ation (x) of coal samples were fitted. The fitting results are shown in Table 2. The results show that there is a positive linear relationship. Coal is a porous medium with a complex pore structure. A higher gas injection pressure means that more CO2 gas is in the experimental tank and more gas can be adsorbed. With an increase in the equilibrium
Coal samples
Fitting equation
R2
2–4 mesh 4–8 mesh 8–16 mesh 16–30 mesh 30–60 mesh
y y y y y
0.9672 0.9509 0.9851 0.9484 0.9740
¼ 0.1904x-0.8909 ¼ 0.1236xþ1.6555 ¼ 0.1683xþ1.2958 ¼ 0.1413xþ4.3130 ¼ 0.2186xþ3.1795
pressure, gas molecules gain higher kinetic energy, so they are able enter smaller cracks and surface pores in the coal body, which increases the adsorption quantity. This adsorption process will release heat, so the coal sample temperature will increase. However, owing to the slow adsorption rate of gas into the surface pores and cracks in the coal body, the rate of heating gradually becomes slower. When the rate of heating is close to the coal sample’s rate of heat transfer to the adsorption tank and the external environment, the temperature reaches its peak. Therefore, 4
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as the equilibrium pressure increases, the gap between the temperature change curves decreases, and the rate at which ΔTmax increases gradually decreases. 3.2. Experimental results of temperature variation during the gas desorption process 3.2.1. Effect of particle size on temperature during desorption Fig. 8 shows the temperature variation rule of the columnar, and 2–4, 4–8, 8–16, 16–30, and 30–60 mesh coal samples during the desorption process under an equilibrium pressure of approximately 0.32 MPa. Under this equilibrium pressure, as the particle size of coal sample de creases, the faster the temperature decreases. Gas desorption can cause the temperature of coal to decrease. As shown in Fig. 8, the temperature decreases first and then rises slowly during desorption. Desorption is an endothermic physical pro cess, a large amount of gas desorption results in rapid decrease of tem perature at the preliminary stage. As the desorption decreases the temperature tends to be stable. Then the coal temperature starts to rise with the influence of the external environment temperature. The peak temperature of the columnar coal sample is 1.76 � C, which is between those of the 2–4 mesh and 4–8 mesh samples. When compared with granular coal samples, the desorption rate of adsorbed gas from the internal pores and fractures of columnar coal samples is slower. This slow and continuous desorption and heat adsorption of gas reduces the influence of the external environment temperature on the coal body temperature, so that the temperature reduction caused by gas desorption from columnar coal is larger than that of 2–4 mesh coal. However, the total adsorption quantity of gas in columnar coal is less than that of granular coal, so the temperature reduction value is less than that of 4–60 mesh coal. It can also be seen from the temperature variation curve that the rate of the initial temperature reduction follows the rule that the smaller the particle size, the higher the rate, which also indicates that the desorption rate of columnar coal is less than that of 2–60 mesh granular coal.
Fig. 9. Temperature variation in desorption process of 30–60 mesh coal sam ples under different equilibrium pressures.
reduction in Fig. 9. The maximum temperature reductions for 30–60 mesh coal caused only by desorption are 3.38, 4.63, 5.80, 7.02 and 7.98 � C when the corresponding adsorption equilibrium pressure values are 0.2873, 0.5176, 0.8033, 1.1234 and 1.4199 MPa, respectively. These data show that there is an obvious corresponding relationship between maximum temperature reduction caused by gas desorption and adsorption equilibrium pressure. As the equilibrium pressure increases, the maximum temperature reduction increases. There is also a positive correlation between equilibrium pressure and gas adsorption quantity. The larger the change of coal temperature, the higher the gas storage in the coal, the higher the probability and danger of outburst. 3.3. Law of energy variation during the adsorption process
3.2.2. Effect of equilibrium pressure on temperature during desorption Fig. 9 shows the temperature variation of 30–60 mesh coal samples with time during desorption. Basically, when equilibrium pressure in creases, both the rate of initial temperature reduction and the magnitude of maximum temperature reduction for coal samples of the same particle size increases during desorption. Granite samples were used to calibrate the maximum temperature
3.3.1. Energy variation of coal samples with different particle sizes during adsorption Based on the experimental data, Δσ (the reduction value of surface free energy) and ΔEa (the heat variation value of per unit mass coal sample, the unit is J/g) under different pressure conditions were calculated by using Equation (1) (Liu and Meng, 2015) and thermody namic Equation (2). Δσ ¼
RTVL P lnð1 þ Þ PL V0 S
E ¼ cmΔT
(1) (2)
where, Δσ is the reduction value of surface free energy in J/m2, R is the gas constant 8.314 J/(mol � K), T is the experimental temperature in K, P is equilibrium pressure in MPa, VL is the Langmuir volume in ml/g, PL is the Langmuir pressure in MPa, V0 is the molar volume of gas at standard temperature and pressure 22.4 � 103 cm3/mol, S is the surface area of unit mass adsorbent in cm2/g, E is the heat variation value in J, c is the specific heat in J/(K � g), and ΔT is the temperature variation value in K. The variation of surface free energy reduction and heat variation of coal samples with different particle sizes during the adsorption process, when the equilibrium pressure is approximately 0.32 MPa, are shown in Fig. 10. As shown in the figure, the surface free energy reduction caused by the adsorption of columnar, and 2–4 mesh, 4–8 mesh, 8–16 mesh, 16–30 mesh and 30–60 mesh coal samples are 0.026 J/m2, and 0.028, 0.029, 0.033, 0.035 and 0.038 J/m2, respectively, when the adsorption reaches
Fig. 8. Temperature variation during desorption of coal samples with different particle sizes under an equilibrium pressure of approximately 0.32 MPa. 5
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Fig. 10. Energy variation during adsorption of coal samples with different particle sizes under the equilibrium pressure of approximately 0.32 MPa.
Fig. 11. Energy variation during adsorption of columnar coal samples under different equilibrium pressures.
equilibrium at the equilibrium pressure of approximately 0.32 MPa. As the particle size of the coal sample decreases, the surface free energy reduction during adsorption becomes larger. That is, as the power of coal to adsorb gas increases, the gas adsorption quantity also corre spondingly increases. A decrease in surface free energy will lead to an increase in coal temperature, and the adsorption heat value per unit mass of coal will also increase. In Fig. 10, the adsorption heat values of five granular coal samples, 2–4, 4–8, 8–16, 16–30 and 30–60 mesh, are 3.18, 5.68, 7.37, 10.73, and 12.28 J/g, respectively. The heat value increases gradually, which proves that the surface free energy is transformed into adsorption heat. The adsorption heat value of the columnar coal sample is 5.93 J/g, which is greater than that of the 2–4 and 4–8 mesh coal samples. Columnar coal is whole, and its internal deformation is restricted, so its deformation energy is less than that of granular coal in the adsorption process. According to the principle of energy transformation, more surface free energy in the columnar coal sample is converted into heat in the adsorption process. In other words, surface free energy is converted into heat and deformation energy in the adsorption process.
Fig. 12. Energy variation during adsorption of 30–60 mesh coal samples under different equilibrium pressures.
3.3.2. Energy variation in different equilibrium pressure during adsorption During the adsorption process, the variation of surface free energy and heat of columnar and 30–60 mesh coal samples under different equilibrium pressures are shown in Figs. 11 and 12, respectively. During the adsorption experiment, as the equilibrium pressure in creases, the surface free energy reduction in both columnar and granular coal samples decreases gradually, and the rate of decline decreases gradually. The reason is that as equilibrium pressure increases, the ability of the coal sample to adsorb CO2 increases, and there is a cor responding increase in the adsorption quantity. However, there is a nonuniform adsorption potential field on the surface of coal, and the adsorption performance of each site is different. Gas will preferentially occupy adsorption sites with a larger adsorption potential on the coal surface. As a result, as adsorption progresses, the adsorption sites with larger adsorption potentials decrease, so the adsorption becomes increasingly difficult. The law of adsorption heat value changing with equilibrium pressure is consistent with that of the reduction in surface free energy with equilibrium pressure. That is, it gradually increases and then flattens out. The reason is that the surface free energy is converted into the heat of adsorption. When the equilibrium pressure conditions are 0.305, 0.515, 0.811, 1.071, and 1.251 MPa, the corresponding adsorption heat values of the
columnar coal samples are 5.93, 6.64, 6.95, 6.83, and 6.50 J/g, respectively. When the equilibrium pressure is within a range of 0.305 MPa–0.811 MPa, as the equilibrium pressure increases, the heat value gradually increases and the rate of change slows down. When the equilibrium pressure becomes greater than 0.811 MPa, the heat essen tially remains unchanged. The reason is that as the pressure rises, the gas will enter deeper and into smaller pores in the coal body for adsorption and cause expansion deformation. The adsorption quantity will increase, and the surface free energy will decrease. However, the surface free energy of this part will be transformed into the deformation energy of the coal, resulting in adsorption heat value remaining constant. 3.4. Law of energy variation during the desorption process 3.4.1. Energy variation of coal samples with different particle sizes during desorption The desorption heat variation of coal samples with different particle sizes, when the equilibrium pressure is approximately 0.32 MPa, was taken as an example to analyze the influence of coal sample particle size on desorption. As shown in Fig. 13, the desorption heat of five granular coal samples, 2–4 mesh, 4–8 mesh, 8–16 mesh, 16–30 mesh, and 30–60 6
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mesh are 1.97, 3.12, 3.84, 4.31, and 4.93 J/g, respectively. As the particle size decreases, the desorption quantity under the same condi tion increases, the temperature change increases, and the desorption heat value per unit mass of coal sample increases. Under the same conditions, the desorption heat value of the column coal sample is 2.57 J/g, which is greater than that of 2–4 mesh. There is a nonlinear tendency for heat energy changing with particle sizes decreasing. The reason is that the deformation energy of the granular coal sample is larger than that of the columnar coal sample during the adsorption process. The deformation energy is converted into heat en ergy in the desorption process. So, the temperature and heat energy increments of column coal are smaller than that of 2–4 mesh, because the deformation energy was contributed to temperature increasing. And finally, the total temperature and heat energy declined value of column coal is bigger than that of 2–4 mesh. During coal and gas outburst process, much of the deformation energy was converted into heat en ergy, and the total temperature changes of coal were increased. But before its occurrence, the energy conversion was reverse, so some cooling phenomenon could be appeared on the surface of coal seam. From the viewpoints of energy conversion during gas adsorption and desorption, the above results could be used to prevent coal and gas outburst.
Fig. 14. Heat variation in desorption of 30–60 mesh coal samples under different equilibrium pressures.
3.4.2. Energy variation in different equilibrium pressure during desorption During the desorption process, the heat variation of 30–60 mesh coal sample under different equilibrium pressures is shown in Fig. 14. In general, the greater the equilibrium pressure, the more heat absorbed by the desorption of CO2, and the change rate decreases gradually. Fig. 15 shows the variation of heat in the desorption process of columnar coal samples under different equilibrium pressures. Compared with the heat variation law of granular coal samples of 30–60 mesh, the greater the equilibrium pressure, the more heat is absorbed by gas desorption, but the change rate shows an increasing trend. Because, with an increase in pressure, the shrinkage of the columnar coal sample is more significant, which lowers the temperature. These results also prove that the desorption heat calculated according to the temperature in cludes the heat absorbed by the adsorption and the heat absorbed by the coal shrinkage. 4. Conclusions In this paper, the adsorption and desorption experiments of coal
Fig. 15. Heat variation in desorption of columnar coal samples under different equilibrium pressures.
samples with different particle sizes were conducted and the tempera ture and energy variations during this process were analyzed. The cur rent study sheds light on preventing gas outburst and improving the safety of coal mining. (1) The smaller the particle size of coal sample, the more gas is adsorbed, the more the maximum temperature change rate and temperature change in the process of adsorption and desorption. For coal samples of the same particle size, the greater the equi librium pressure, the greater the adsorption quantity, the faster the rate of temperature variation, and the higher the maximum temperature variation are during the process of adsorption and desorption. (2) The reduction in surface free energy and the variation in heat in the process of adsorption and desorption increase with decreased particle size and increased equilibrium pressure. With an increase in equilibrium pressure, the reduction in surface free energy of coal samples with different particle sizes gradually slows, which
Fig. 13. Desorption heat variation of coal samples with different particle sizes under the equilibrium pressure of approximately 0.32 MPa. 7
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indicates that there is a non-uniform adsorption potential field on the surface of coal. (3) By comparing the differences in heat data between columnar and granular coal samples in the process of adsorption and desorp tion, it can be concluded that the reduction in surface free energy is converted into adsorption heat and deformation energy during the process of adsorption, and the deformation energy is con verted into heat energy during the desorption process.
Li, H., Feng, Z.C., Zhao, D., Duan, D., 2017. Simulation experiment and acoustic emission study on coal and gas outburst. Rock Mech. Rock Eng. 50, 2193–2205. Liu, J.K., He, X.Q., Wang, C.X., 2013. Measurement of temperature variation in coal gas desorption based on infrared imaging technology. J. Liaoning Tech. Univ. 32 (9), 1161–1165 (in Chinese). Liu, S.S., Meng, Z.P., 2015. Study on energy variation of different coal-body structure coals in the process of isothermal adsorption. J. China Coal Soc. 40 (6), 1422–1427 (in Chinese). Liu, Z.X., Feng, Z.C., Zhang, Q.M., Zhao, D., Guo, H.Q., 2015. Heat and deformation effects of coal during adsorption and desorption of carbon dioxide. J. Nat. Gas Sci. Eng. 25, 242–252. https://doi.org/10.1016/j.jngse.2015.04.024. Lu, S.Q., Zhang, Y.L., Sa, Z.Y., Si, S.F., 2019. Evaluation of the effect of adsorbed gas and free gas on mechanical properties of coal. Environ. Earth Sci. 78 (6), 1–15. https:// doi.org/10.1007/s12665-019-8222-3. Lutynski, M., Gonzalez, M.A.G., 2016. Characteristics of carbon dioxide sorption in coal and gas shale-The effect of particle size. J. Nat. Gas Sci. Eng. 28, 558–565. https:// doi.org/10.1016/j.jngse.2015.12.037. Lu, W.D., Wang, J.R., Ju, Y.W., 2017. Microscopic mechanism of adsorption-desorption in coal and gas outburst process. J. Nanosci. Nanotechnol. 17 (9), 6894–6898. https://doi.org/10.1166/jnn.2017.14460. Ma, D.M., Li, L.X., Li, X.P., Bai, H.D., Wang, J., Liu, H.N., Li, F.Q., 2014. Contrastive experiment of adsorption-desorption between CH4 and CO2 in coal seam 4 of Dafosi coal mine. J. China Coal Soc. 39, 1938–1944 (in Chinese). Pini, R., Ottiger, S., Burlini, L., Storti, G., Mazzotti, M., 2010. Sorption of carbon dioxide, methane and nitrogen in dry coals at high pressure and moderate temperature. Int. J. Greenh. Gas Control 4 (1), 90–101. https://doi.org/10.1016/j.ijggc.2009.10.019. Paweł, B., Katerzyna, Z., Miroslawa, B., 2015. Expansion of hard coal accompanying the sorption of methane and carbon dioxide in isothermal and non-isothermal processes. Energy Fuels 29 (3), 1899–1904. https://doi.org/10.1021/ef502312p. Shemshad, J., Aminossadati, S.M., Bowen, W.P., Kizil, M.S., 2012. Effects of pressure and temperature fluctuations on near-infrared measurements of methane in underground coal mines. Appl. Phys. B 106 (4), 979–986. https://doi.org/10.1007/s00340-0114801-z. Sobczyk, J., 2011. The influence of sorption processes on gas stresses leading to the coal and gas outburst in the laboratory conditions. Fuel 90 (3), 1018–1023. https://doi. org/10.1016/j.fuel.2010.11.004, 2011. Soares, J.L., Oberziner, A.L.B., Jose, H.J., Rodrigues, A.E., Moreira, R.F.P.M., 2007. Carbon dioxide adsorption in Brazilian coals. Energy Fuels 21 (1), 209–215. https:// doi.org/10.1021/ef060149h. Tan, Z., Gubbins, K.E., 1990. Adsorption in carbon micropores at supercritical temperature. J. Phys. Chem. 94 (15), 6061–6069. https://doi.org/10.1021/ j100378a079. Tang, J., Jiang, C.L., Chen, Y.J., Li, X.W., Wang, G.D., Yang, D.D., 2015. Line prediction technology for forecasting coal and gas outbursts during coal roadway tunneling. J. Nat. Gas Sci. Eng. 34, 412–418. https://doi.org/10.1016/j.jngse.2016.07.010. Wang, C.J., Yang, S.Q., Li, X.W., Li, J.H., Jiang, C.L., 2019a. Comparison of the initial gas desorption and gas-release energy characteristics from tectonically-deformed and primary-undeformed coal. Fuel 238, 66–74. https://doi.org/10.1016/j. fuel.2018.10.047. Wang, C.J., Yang, S.Q., Jiang, C.L., Yang, D.D., Zhang, C.J., Li, X.W., Chen, Y.J., Tang, J., 2017. A method of rapid determination of gas pressure in a coal seam based on the advantages of gas spherical flow field. J. Nat. Gas Sci. Eng. 45, 502–510. https://doi. org/10.1016/j.jngse.2017.05.021. Wang, G., Cheng, W.M., Pan, G., 2012. Influence of temperature on coal’s adsorbing capability. J. Saf. Environ. 12 (5), 231–233. https://doi.org/10.1007/s11783-0110280-z. Wang, G., Guo, Y.Y., Du, C.A., Sun, L.L., Liu, Z.Y., Wang, Y., Cao, J.J., 2019b. Experimental study on damage and gas migration characteristics of gas-bearing coal with different pore structures under sorption-sudden unloading of methane. Geofluids 1–11. Wierzbicki, Miroslaw, 2013. Changes in the sorption/diffusion kinetics of a coal-methane system caused by different temperatures and pressures. Gospodarka Surowcmi Mineralnymi-Miner. Resour. Manag. 29 (4), 155–168. Wu, X., Li, J., Li, G.H., Nie, W., Tu, Z.H., 2017. Experimental study on thermodynamic process of coal and gas outburst. China Saf. Sci. J. 27 (5), 105–110 (in Chinese). Yan, M., Bai, Y., Li, S.G., Lin, H.F., Yan, D.J., Shu, C.M., 2019. Factors influencing the gas adsorption thermodynamic characteristics of low-rank coal. Fuel 248, 117–126. https://doi.org/10.1016/j.fuel.2019.03.064. Yang, T., Chen, P., Li, B., Nie, B.S., Zhu, C.J., Ye, Q.S., 2019. Potential safety evaluation method based on temperature variation during gas adsorption and desorption on coal surface. Saf. Sci. 113, 336–344. https://doi.org/10.1016/j.ssci.2018.11.027. Yu, Q.X., 1992. Mine Gas Prevention. China university of mining and technology press, Xuzhou (in Chinese). Zhao, W., Cheng, Y.P., Jiang, H.N., Jin, K., Wang, H.F., Wang, L., 2016. Role of the rapid gas desorption of coal powders in the development stage of outbursts. J. Nat. Gas Sci. Eng. 28, 491–501. https://doi.org/10.1016/j.jngse.2015.12.025. Zhao, D., Zhang, C., Chen, H., Feng, Z.C., 2019. Experimental study on gas desorption characteristics for different coal particle sizes and adsorption pressures under the action of pressured water and superheated steam. J. Pet. Sci. Eng. 179, 948–957. https://doi.org/10.1016/j.petrol.2019.05.027. Zhang, C.G., 1993. Survey research on the relationship between gas desorption and temperature of coal samples by using the infrared thermometer. Saf. Coal Mines (9), 17-18þ25-49. (in Chinese).
Since the gas adsorption quantity is found to be highly dependent on temperature, thus it is suggested that the temperature of coal body under mining can be used as a parameter to monitor the gas storage in coal, and potentially predict the gas outburst. Acknowledgements We sincerely thank the editors and anonymous reviewers for improving the quality of this manuscript. This study was supported by the National Natural Science Foundation of China (Grant No. 51304142 and No. 21373146), the Program for Outstanding Innovation Teams of Higher Learning Institutions (Grant Year 2014), the Project of Applied Foundation Research (Grant No. 201801D121283 and 201801D221366), and the Education Reform Project about Graduate Students of Shanxi Province in China (2018JG35). We thank Elsevier Language Service for English language editing, and we sincerely thank Sanyuan Zhongneng Coal Mine Co., Ltd in Shanxi Province of China for providing raw coal samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.petrol.2020.106932. References An, F.H., Yuan, Y., Chen, X.J., Li, Z.Q., Li, L.Y., 2019. Expansion energy of coal gas for the initiation of coal and gas outbursts. Fuel 235, 551–557. https://doi.org/ 10.1016/j.fuel.2018.07.132. An, Z.X., 1983. Determine the risk of coal seam outburst by using temperature of coal seam. Saf. Coal Mines (11), 43–46 (in Chinese). Busch, A., Gensterblum, Y., Krooss, B.M., Littke, R., 2004. Methane and carbon dioxide adsorption-diffusion experiments on coal: upscaling and modeling. Int. J. Coal Geol. 60 (2–4), 151–168. https://doi.org/10.1016/j.coal.2004.05.002. Bakhshian, S., Sahimi, M., 2017. Adsorption-induced swelling of porous media. Int. J. Greenh. Gas Control 57, 1–13. https://doi.org/10.1016/j.ijggc.2016.12.011. Chaback, J.J., Morgan, D., Yee, D., 1996. Sorption irreversibilities and mixture compositional behavior during enhanced coal bed methane recovery processes. SPE Gas Technol. Symp. 431–438. https://doi.org/10.1016/S0140-6701(99)96326-9. Gruszkiewicz, M., Naney, M., Blencoe, J., Cole, D.R., Pashin, J.C., Carroll, R.E., 2009. Adsorption kinetics of CO2, CH4, and their equimolar mixture on coal from the Black Warrior Basin, West-Central Alabama. Int. J. Coal Geol. 77 (1–2), 23–33. https://doi. org/10.1016/j.coal.2008.09.005. Guo, L.W., Yu, Q.X., Wang, K., 2000. Experimental study on change in coal temperature during adsorbing gas. J. China Inst. Min. Technol. 29 (3), 65–67 (in Chinese). He, M.C., Wang, C.G., Li, D.J., Liu, J., Zhang, X.H., 2010. Desorption characteristics of adsorbed gas in coal samples under coupling temperature and uniaxial compression. Chin. J. Rock Mech. Eng. 29 (5), 865–872 (in Chinese). Jin, K., Cheng, Y.P., Liu, Q.Q., Zhao, W., Wang, L., Wang, F., Wu, D.M., 2016. Experimental investigation of pore structure damage in pulverized coal: implications for methane adsorption and diffusion characteristics. Energy Fuels 30 (12), 10383–10395. https://doi.org/10.1021/acs.energyfuels.6b02530. Kowalczyk, P., Furmaniak, S., Gauden, P.A., Terzyk, A.P., 2010. Carbon dioxide adsorption-induced deformation of microporous carbons. J. Phys. Chem. C 114 (11), 5126–5133. https://doi.org/10.1021/jp911996h. Kim, H.J., Shi, Y., He, J., Lee, H.H., Lee, C.H., 2011. Adsorption characteristics of CO2 and CH4 on dry and wet coal from subcritical to supercritical conditions. Chem. Eng. J. 171 (1), 45–53. https://doi.org/10.1016/j.cej.2011.03.035. Katarzyna, C., Katarzyna, Z., Bronislaw, B., Pawel, B., 2016. Kinetic models assessment for swelling of coal induced by methane and carbon dioxide sorption. Adsorption 22 (4–6), 791–799. https://doi.org/10.1007/s10450-016-9775-z. Kim, D.H., Lee, C.C., Lee, Y.S., 2018. The CH4-CO2 adsorption characteristics of subbituminous coal with different particle sizes. J. Korean Soc. Miner. Energy Resour. Eng. 55 (1), 37–48 (in Korean).
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Energy variation in coal samples with different particle sizes in the process of adsorption and desorption Tao Gao a, b, c, Dong Zhao a, b, c, *, Chen Wang b, Zengchao Feng b a
College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, 030024, China Key Laboratory of In Situ Property-Improving Mining of Ministry of Education, Taiyuan University of Technology, Taiyuan, 030024, China c Graduate Student Education and Innovation Center in Coal Mine Safety of Shanxi Province, Taiyuan, 030024, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Adsorption Desorption Particle size Temperature Energy variation
The study of energy variation rules in the process of gas adsorption and desorption in coal is important to predict mine outburst disasters. In this study, an adsorption-desorption experimental device was developed and used to evaluate six coal samples of various particle sizes under different pressure conditions. Temperature was measured in real-time throughout the process. The rate of temperature change, maximum temperature variation, and adsorption quantity increased with particle size and an increase in equilibrium pressure. There is a positive linear relationship between adsorption quantity and temperature variation. This study focused on energy vari ation during adsorption and desorption. The results showed that the smaller the coal particle size, the larger the reduction in surface free energy and the change in heat. With an increase in equilibrium pressure, the rate of reduction in the surface free energy slows gradually, indicating that there is a non-uniform adsorption potential field on the surface of coal. By comparing the difference of temperature and heat data between a columnar coal sample and a granular coal sample in the process of adsorption and desorption, it can be concluded that the surface free energy is converted into heat and deformation energy in the adsorption process, and the deformation energy is converted into heat energy in the desorption process.
1. Introduction With the continuous deepening of mines, the potential for serious gas outburst disasters is increasing (Wang et al., 2017, 2019a, 2019b; Tang et al., 2015). Variations in gas occurrence state (adsorption and desorption) in coal strongly influences the potential for outbursts (Lu et al., 2019; Wang et al., 2019a, 2019b). Some gas outburst simulations in laboratory had been carried out (Li et al., 2017). Therefore, a good understanding of gas adsorption and desorption mechanisms is neces sary (Lu et al., 2017). Macroscopic adsorption and desorption experi ments and theoretical research have been broadly conducted (Tan and Gubbins, 1990; Sobczyk, 2011; Shemshad et al., 2012; Wang et al., 2012; Zhao et al., 2016). Generally, the smaller the particle size of coal, the faster its adsorption rate (Zhao et al., 2019; Busch et al., 2004; Gruszkiewicz et al., 2009). Gruszkiewicz et al. (2009) selected three size fractions of crushed coal samples (40–150 μm, 1–2 mm, and 5–10 mm) for adsorp tion experiments and found that the adsorption rate of the 40–150 μm coal samples was significantly faster than that of the other two samples.
Kim et al. (2018) analyzed adsorption properties of subbituminous coal with six different particle sizes and concluded that the smaller the par ticle size of the coal, the larger the specific surface area, and the larger the Langmuir volume. Many researchers have conducted isothermal adsorption-desorption experiments on coal samples with different par ticle sizes and concluded that as the particle size of a coal sample de creases, the adsorption equilibrium pressure increases, and the gas desorption rate becomes faster (Soares et al., 2007; Lutynski and Gon zale, 2016; Jin et al., 2016). In addition, temperature has an important effect on the adsorption and desorption of gas. Yu (1992) determined that a temperature rise of 1 � C when equilibrium pressure was 5 MPa, caused methane adsorption to decrease by 0.12 cm3/g. Wierzbicki (2013) studied the kinetic adsorp tion characteristics (effective diffusion coefficient) of methane on coal using gravimetric analysis. The results showed that a positive linear correlation existed between the diffusion coefficient and temperature. Using isothermal adsorption experiments, Pini et al. (2010) concluded that the higher the experimental temperature, the lower the gas adsorption in coal. He et al. (2010) studied the gas migration process in
* Corresponding author. College of Safety and Emergency Management Engineering, Taiyuan University of Technology, Taiyuan, 030024, China. E-mail address: [email protected] (D. Zhao). https://doi.org/10.1016/j.petrol.2020.106932 Received 16 April 2019; Received in revised form 28 October 2019; Accepted 7 January 2020 Available online 8 January 2020 0920-4105/© 2020 Elsevier B.V. All rights reserved.
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coal samples using the independently developed deep coal-rock tem perature-pressure coupling gas experiment system and confirmed that an increase in temperature induced a large amount of desorption of gas in coal samples. Yan et al. (2019) conducted low-temperature nitrogen and isothermal adsorption experiments, discovering that the adsorption free-energy and enthalpy of the coal samples decreased with an increase in temperature and adsorption heat was affected by an increase in pore volume and specific surface area. Gas adsorption is an exothermic process, and adsorption and desorption are reversible (Chaback and Morgan, 1996). Coal particle size, temperature, pressure, and other factors can change the tempera ture and energy of coal by the dynamic process of adsorption and desorption of gas in coal. An (1983) found that gas desorption is the main factor that causes the temperature of the working face of the coal wall to change through field experiments. The temperature change can reflect the gas content in the coal and thus serve as a predictive indicator for gas outburst. Wu et al. (2017) carried out outburst tests by using briquettes made from coal samples of different particle sizes. The results showed that as particle size becomes smaller, the magnitude of tem perature change increases and the rate of change becomes faster during the process of gas adsorption and desorption, and the law of temperature evolution with time approximately conforms to a natural logarithm relationship. Many researchers have measured the magnitude of tem perature change of coal during isothermal gas adsorption, and concluded that as gas pressure increases, the quantity of gas adsorbed increases, and the heat released in the adsorption process increases; they propose that the magnitude of temperature change can be used to pre dict gas outbursts (Guo et al., 2000; Yang et al., 2019; Liu et al., 2013; Zhang, 1993). A few investigators numerically analyzed a comparison of the elastic energy necessary for coal seam outburst with the internal gas energy released during gas outburst, and found that the energy released by gas desorption is the main cause of gas outburst (An et al., 2019). Research has concluded that coal is a dual porosity media, transportsorption system in which gas pressure causes compression of micropo rous regions and expansion of macropores (Katarzyna et al., 2016). Adsorption of CO2 in coal samples led to its swelling (Kowalczyk et al., 2010; Kim et al., 2011). Studies indicate that there is a linear relation between coal expansion and the volume of gas adsorbed in its structure (Paweł et al., 2015). Bakhshian and Sahimi (2017) studied deformation and swelling of porous media induced by sorption of carbon dioxide, discovering that the strain was released as the adsorbed gas was desorbed. A phenomenon of temperature change and energy conversion does exist in the process of gas adsorption and desorption. Most studies are based on isothermal adsorption experiments or outburst simulation ex periments. However, the temperature measurements are considerably affected by the external environment temperature and the effect of gas compression and expansion. In addition, the effect of coal sample deformation caused by gas adsorption and desorption is not considered. In this study, an adsorption-desorption device was designed and modi fied to measure temperature variation more accurately during adsorption-desorption experiments using different particle sizes and pressures. In addition, by comparing columnar and granular coal sam ples, the deformation energy was analyzed. Consequently, the energy change rules were systematically studied. These results are of signifi cance to the improvement of adsorption and desorption mechanisms and the prevention and control of coal and gas outburst.
Shanxi. After being moved from the well and sealed tightly, it was sent to the laboratory for later use. Proximate and elemental analyses, vitrinite reflectance, and the definition of coal type were performed for the coal samples (Table 1). Granite samples were obtained directly from the engineering site. The sample sizes required in this experiment are columnar (raw coal, 50 mm in diameter and 100 mm in length), and 2–4, 4–8, 8–16, 16–30, and 30–60 mesh. To improve the quality of the experimental results, CO2 was selected as the adsorbate because of its relatively high adsorption heat and adsorption capacity. Moreover, due to the stronger adsorption capacity of CO2 than CH4 in coal seams, it adsorbs more gas, which may cause more serious outburst disasters. Methane and CO2 both are adsorbed on the surface of coal, and the adsorption and desorption characteristics in similar manners in real coal seams (Ma et al., 2014; Liu et al., 2015; Zhao et al., 2019). 2.2. Experimental system An existing experimental device for gas adsorption and desorption in the laboratory was redesigned and improved by adding an insulation layer and a temperature measuring device. The structure of the experimental device is shown in Fig. 1. (Labels in Fig. 1: ①Gas cylinder; ②Needle valve; ③Pressure gauge; ④Insulating layer; ⑤ Reference tank; ⑥Temperature sensor no.4; ⑦Ball valve; ⑧Tempera ture sensor no.1; ⑨Temperature sensor no.3; ⑩Temperature sensor no.2; ⑪Temperature recorder; ⑫Sample tank ; ⑬Three-way valve; ⑭ Vacuum gauge; ⑮Vacuum pump) 2.3. Sample tank To measure the magnitude of temperature variation in coal samples during the experiment, the existing sample tank was modified. A photograph and structural diagram of the tank are shown in Figs. 2 and 3, respectively. The main modification is that two 8 mm casings are welded at equal distances on both sides of the outlet. The temperature sensor probes are in direct contact with the coal at different positions through the casings, which enables the transmission of temperature signals inside and outside the tank. RTV silicone rubber and AB rubber were used for sealing. The aerogel felt with low thermal conductivity is close to the inner wall of the adsorption tank; aerogel felt and tin foil tape are wrapped around the outer wall of the adsorption tank to achieve the effect of double-layer thermal insulation. As shown in Fig. 3, two temperature sensors were placed at different positions inside the columnar coal sample by methods that do not damage the structure of the coal body. During the process of gas injection and exhaust, the gas will compress and expand, causing variations in the coal body temperature; therefore, the data measured by the temperature sensor are partially caused by gas adsorption and desorption. Granite, which does not adsorb much gas, was selected to calibrate the experimental tank. By measuring the maximum temperature variation of the granite sample in the process of gas injection and exhaust, the thermal effect of gas compression and expansion was offset. Due to the unevenness of coal structure, the adsorption character istics of coal at different positions are different. Therefore, two tem perature sensors were used in the experiment to test the temperature variation at the upper and lower positions of coal. By comparing the experimental data measured by the two temperature sensors, it was found that the rule of temperature variation with time was basically the same. In the adsorption process, the temperature first increases, then tends to stabilize and decreases finally. While in the desorption process, the temperature first decreases, then tends to stabilize and increases finally. However, the temperature stability time measured by no.2 sensor was greater than that of no.1 sensor, which indicates that the no.2 sensor was less affected by the temperature of the outside environment, so the temperature variation was measured more accurately. So, the temperature data measured by the no.2 sensor were selected as the
2. Experiments 2.1. Coal samples The coal sample used in this experiment is a lean coal, which is a fresh and gangue-free lump coal sample taken from the #3 coal seam working face in the San-Yuan Zhongneng Coal mine. The location of the Coal mine is Changzhi of Shanxi Province in China, which in southeast of 2
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Table 1 Results of the coal sample analyses. Origin of coal sample San-Yuan Zhongneng
Proximate analysis (%)
R0max (%), Coal type
Elemental analysis (%)
Mad
Ash
VM
FC
S
R0max (%)
Coal type
C
H
O
P
0.70
12.02
12.19
75.09
0.34
1.90
Lean coal
89.18
3.97
5.00
0.02
Note: Ash and total sulfur (S) were calculated on a dry basis. Volatile matter (VM) and fixed carbon (FC) were calculated on a dried, ash-free basis. R0max is maximum vitrinite reflectance.
Fig. 1. Structural diagram of the gas adsorption and desorption experi ment device.
Fig. 3. Structural drawing of the experimental tank.
were then added to the experimental tank after being weighed. The free space volume was calibrated by the helium expansion method. The experimental tank was then placed under vacuum for 8 h. Afterward, CO2 gas cylinders were connected to the experimental system for adsorption and desorption experiments. Pressure and temperature dur ing the experiment were recorded. Finally, the temperature and energy variation rules in the process of coal sample adsorption and desorption were calculated and analyzed with the collected data. 3. Results and discussion 3.1. Experimental results of temperature variation during the gas adsorption process 3.1.1. Effect of particle size on temperature during adsorption Fig. 4 shows the relationship between adsorption temperature of coal samples with different particle sizes over time when the maximum gas injection pressure is approximately 3 MPa. The maximum adsorption capacities and temperature changes of coal samples with different par ticle sizes are shown in Fig. 5. The experimental data show that under the same conditions, when particle size becomes smaller, the amount of CO2 gas adsorbed increases, the rate of temperature change coal sample becomes faster, and the magnitude of temperature change increase during the adsorption pro cess. The main reason for this is that the smaller the particle size of the coal, the larger its specific surface area, and the more space it has to absorb CO2 molecules. Therefore, under the same conditions, the coal can adsorb more CO2; and because adsorption is an exothermic physical process, the more gas adsorbed, the greater the amount of heat released, and the greater the temperature variation in the coal sample. Compared with the pulverized coal sample, the temperature of columnar coal sample stabilizes for a longer time after the temperature reaches the maximum value in Fig. 4. The reason for this is that the rate
Fig. 2. The experimental tank.
experimental results. 2.4. Experimental procedures The air-tightness of the device was checked before the experiment. The coal samples were dried at 60 � C in a drying oven for 6–8 h. They 3
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Fig. 6. Temperature variation during adsorption of 30–60 mesh coal samples under different equilibrium pressures.
Fig. 4. Adsorption temperature variation of coal samples with different particle sizes under 3 MPa.
Fig. 7. Maximum adsorption quantity and temperature variation of 30–60 mesh coal samples under different equilibrium pressures.
Fig. 5. Maximum adsorption quantity and temperature variation of coal sam ples with different particle sizes under 3 MPa.
Table 2 Results of fitting the graphs of adsorption quantity and temperature variation.
of gas adsorption in the interior space of columnar coal is smaller than that of pulverized coal sample. The heat generated by the adsorption inside the coal body conducts to the coal surface slowly and continu ously, which reduces the heat dissipation from the experimental tank to the outside environment and keeps the temperature of the coal body stable for a long time. 3.1.2. Effect of equilibrium pressure on temperature during adsorption Fig. 6 shows the adsorption temperature variation of 30–60 mesh coal samples with time. The maximum amount of gas adsorbed (adsorption quantity) and the temperature variation of the coal sample are shown in Fig. 7. The adsorption quantity (y) and temperature vari ation (x) of coal samples were fitted. The fitting results are shown in Table 2. The results show that there is a positive linear relationship. Coal is a porous medium with a complex pore structure. A higher gas injection pressure means that more CO2 gas is in the experimental tank and more gas can be adsorbed. With an increase in the equilibrium
Coal samples
Fitting equation
R2
2–4 mesh 4–8 mesh 8–16 mesh 16–30 mesh 30–60 mesh
y y y y y
0.9672 0.9509 0.9851 0.9484 0.9740
¼ 0.1904x-0.8909 ¼ 0.1236xþ1.6555 ¼ 0.1683xþ1.2958 ¼ 0.1413xþ4.3130 ¼ 0.2186xþ3.1795
pressure, gas molecules gain higher kinetic energy, so they are able enter smaller cracks and surface pores in the coal body, which increases the adsorption quantity. This adsorption process will release heat, so the coal sample temperature will increase. However, owing to the slow adsorption rate of gas into the surface pores and cracks in the coal body, the rate of heating gradually becomes slower. When the rate of heating is close to the coal sample’s rate of heat transfer to the adsorption tank and the external environment, the temperature reaches its peak. Therefore, 4
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as the equilibrium pressure increases, the gap between the temperature change curves decreases, and the rate at which ΔTmax increases gradually decreases. 3.2. Experimental results of temperature variation during the gas desorption process 3.2.1. Effect of particle size on temperature during desorption Fig. 8 shows the temperature variation rule of the columnar, and 2–4, 4–8, 8–16, 16–30, and 30–60 mesh coal samples during the desorption process under an equilibrium pressure of approximately 0.32 MPa. Under this equilibrium pressure, as the particle size of coal sample de creases, the faster the temperature decreases. Gas desorption can cause the temperature of coal to decrease. As shown in Fig. 8, the temperature decreases first and then rises slowly during desorption. Desorption is an endothermic physical pro cess, a large amount of gas desorption results in rapid decrease of tem perature at the preliminary stage. As the desorption decreases the temperature tends to be stable. Then the coal temperature starts to rise with the influence of the external environment temperature. The peak temperature of the columnar coal sample is 1.76 � C, which is between those of the 2–4 mesh and 4–8 mesh samples. When compared with granular coal samples, the desorption rate of adsorbed gas from the internal pores and fractures of columnar coal samples is slower. This slow and continuous desorption and heat adsorption of gas reduces the influence of the external environment temperature on the coal body temperature, so that the temperature reduction caused by gas desorption from columnar coal is larger than that of 2–4 mesh coal. However, the total adsorption quantity of gas in columnar coal is less than that of granular coal, so the temperature reduction value is less than that of 4–60 mesh coal. It can also be seen from the temperature variation curve that the rate of the initial temperature reduction follows the rule that the smaller the particle size, the higher the rate, which also indicates that the desorption rate of columnar coal is less than that of 2–60 mesh granular coal.
Fig. 9. Temperature variation in desorption process of 30–60 mesh coal sam ples under different equilibrium pressures.
reduction in Fig. 9. The maximum temperature reductions for 30–60 mesh coal caused only by desorption are 3.38, 4.63, 5.80, 7.02 and 7.98 � C when the corresponding adsorption equilibrium pressure values are 0.2873, 0.5176, 0.8033, 1.1234 and 1.4199 MPa, respectively. These data show that there is an obvious corresponding relationship between maximum temperature reduction caused by gas desorption and adsorption equilibrium pressure. As the equilibrium pressure increases, the maximum temperature reduction increases. There is also a positive correlation between equilibrium pressure and gas adsorption quantity. The larger the change of coal temperature, the higher the gas storage in the coal, the higher the probability and danger of outburst. 3.3. Law of energy variation during the adsorption process
3.2.2. Effect of equilibrium pressure on temperature during desorption Fig. 9 shows the temperature variation of 30–60 mesh coal samples with time during desorption. Basically, when equilibrium pressure in creases, both the rate of initial temperature reduction and the magnitude of maximum temperature reduction for coal samples of the same particle size increases during desorption. Granite samples were used to calibrate the maximum temperature
3.3.1. Energy variation of coal samples with different particle sizes during adsorption Based on the experimental data, Δσ (the reduction value of surface free energy) and ΔEa (the heat variation value of per unit mass coal sample, the unit is J/g) under different pressure conditions were calculated by using Equation (1) (Liu and Meng, 2015) and thermody namic Equation (2). Δσ ¼
RTVL P lnð1 þ Þ PL V0 S
E ¼ cmΔT
(1) (2)
where, Δσ is the reduction value of surface free energy in J/m2, R is the gas constant 8.314 J/(mol � K), T is the experimental temperature in K, P is equilibrium pressure in MPa, VL is the Langmuir volume in ml/g, PL is the Langmuir pressure in MPa, V0 is the molar volume of gas at standard temperature and pressure 22.4 � 103 cm3/mol, S is the surface area of unit mass adsorbent in cm2/g, E is the heat variation value in J, c is the specific heat in J/(K � g), and ΔT is the temperature variation value in K. The variation of surface free energy reduction and heat variation of coal samples with different particle sizes during the adsorption process, when the equilibrium pressure is approximately 0.32 MPa, are shown in Fig. 10. As shown in the figure, the surface free energy reduction caused by the adsorption of columnar, and 2–4 mesh, 4–8 mesh, 8–16 mesh, 16–30 mesh and 30–60 mesh coal samples are 0.026 J/m2, and 0.028, 0.029, 0.033, 0.035 and 0.038 J/m2, respectively, when the adsorption reaches
Fig. 8. Temperature variation during desorption of coal samples with different particle sizes under an equilibrium pressure of approximately 0.32 MPa. 5
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Fig. 10. Energy variation during adsorption of coal samples with different particle sizes under the equilibrium pressure of approximately 0.32 MPa.
Fig. 11. Energy variation during adsorption of columnar coal samples under different equilibrium pressures.
equilibrium at the equilibrium pressure of approximately 0.32 MPa. As the particle size of the coal sample decreases, the surface free energy reduction during adsorption becomes larger. That is, as the power of coal to adsorb gas increases, the gas adsorption quantity also corre spondingly increases. A decrease in surface free energy will lead to an increase in coal temperature, and the adsorption heat value per unit mass of coal will also increase. In Fig. 10, the adsorption heat values of five granular coal samples, 2–4, 4–8, 8–16, 16–30 and 30–60 mesh, are 3.18, 5.68, 7.37, 10.73, and 12.28 J/g, respectively. The heat value increases gradually, which proves that the surface free energy is transformed into adsorption heat. The adsorption heat value of the columnar coal sample is 5.93 J/g, which is greater than that of the 2–4 and 4–8 mesh coal samples. Columnar coal is whole, and its internal deformation is restricted, so its deformation energy is less than that of granular coal in the adsorption process. According to the principle of energy transformation, more surface free energy in the columnar coal sample is converted into heat in the adsorption process. In other words, surface free energy is converted into heat and deformation energy in the adsorption process.
Fig. 12. Energy variation during adsorption of 30–60 mesh coal samples under different equilibrium pressures.
3.3.2. Energy variation in different equilibrium pressure during adsorption During the adsorption process, the variation of surface free energy and heat of columnar and 30–60 mesh coal samples under different equilibrium pressures are shown in Figs. 11 and 12, respectively. During the adsorption experiment, as the equilibrium pressure in creases, the surface free energy reduction in both columnar and granular coal samples decreases gradually, and the rate of decline decreases gradually. The reason is that as equilibrium pressure increases, the ability of the coal sample to adsorb CO2 increases, and there is a cor responding increase in the adsorption quantity. However, there is a nonuniform adsorption potential field on the surface of coal, and the adsorption performance of each site is different. Gas will preferentially occupy adsorption sites with a larger adsorption potential on the coal surface. As a result, as adsorption progresses, the adsorption sites with larger adsorption potentials decrease, so the adsorption becomes increasingly difficult. The law of adsorption heat value changing with equilibrium pressure is consistent with that of the reduction in surface free energy with equilibrium pressure. That is, it gradually increases and then flattens out. The reason is that the surface free energy is converted into the heat of adsorption. When the equilibrium pressure conditions are 0.305, 0.515, 0.811, 1.071, and 1.251 MPa, the corresponding adsorption heat values of the
columnar coal samples are 5.93, 6.64, 6.95, 6.83, and 6.50 J/g, respectively. When the equilibrium pressure is within a range of 0.305 MPa–0.811 MPa, as the equilibrium pressure increases, the heat value gradually increases and the rate of change slows down. When the equilibrium pressure becomes greater than 0.811 MPa, the heat essen tially remains unchanged. The reason is that as the pressure rises, the gas will enter deeper and into smaller pores in the coal body for adsorption and cause expansion deformation. The adsorption quantity will increase, and the surface free energy will decrease. However, the surface free energy of this part will be transformed into the deformation energy of the coal, resulting in adsorption heat value remaining constant. 3.4. Law of energy variation during the desorption process 3.4.1. Energy variation of coal samples with different particle sizes during desorption The desorption heat variation of coal samples with different particle sizes, when the equilibrium pressure is approximately 0.32 MPa, was taken as an example to analyze the influence of coal sample particle size on desorption. As shown in Fig. 13, the desorption heat of five granular coal samples, 2–4 mesh, 4–8 mesh, 8–16 mesh, 16–30 mesh, and 30–60 6
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mesh are 1.97, 3.12, 3.84, 4.31, and 4.93 J/g, respectively. As the particle size decreases, the desorption quantity under the same condi tion increases, the temperature change increases, and the desorption heat value per unit mass of coal sample increases. Under the same conditions, the desorption heat value of the column coal sample is 2.57 J/g, which is greater than that of 2–4 mesh. There is a nonlinear tendency for heat energy changing with particle sizes decreasing. The reason is that the deformation energy of the granular coal sample is larger than that of the columnar coal sample during the adsorption process. The deformation energy is converted into heat en ergy in the desorption process. So, the temperature and heat energy increments of column coal are smaller than that of 2–4 mesh, because the deformation energy was contributed to temperature increasing. And finally, the total temperature and heat energy declined value of column coal is bigger than that of 2–4 mesh. During coal and gas outburst process, much of the deformation energy was converted into heat en ergy, and the total temperature changes of coal were increased. But before its occurrence, the energy conversion was reverse, so some cooling phenomenon could be appeared on the surface of coal seam. From the viewpoints of energy conversion during gas adsorption and desorption, the above results could be used to prevent coal and gas outburst.
Fig. 14. Heat variation in desorption of 30–60 mesh coal samples under different equilibrium pressures.
3.4.2. Energy variation in different equilibrium pressure during desorption During the desorption process, the heat variation of 30–60 mesh coal sample under different equilibrium pressures is shown in Fig. 14. In general, the greater the equilibrium pressure, the more heat absorbed by the desorption of CO2, and the change rate decreases gradually. Fig. 15 shows the variation of heat in the desorption process of columnar coal samples under different equilibrium pressures. Compared with the heat variation law of granular coal samples of 30–60 mesh, the greater the equilibrium pressure, the more heat is absorbed by gas desorption, but the change rate shows an increasing trend. Because, with an increase in pressure, the shrinkage of the columnar coal sample is more significant, which lowers the temperature. These results also prove that the desorption heat calculated according to the temperature in cludes the heat absorbed by the adsorption and the heat absorbed by the coal shrinkage. 4. Conclusions In this paper, the adsorption and desorption experiments of coal
Fig. 15. Heat variation in desorption of columnar coal samples under different equilibrium pressures.
samples with different particle sizes were conducted and the tempera ture and energy variations during this process were analyzed. The cur rent study sheds light on preventing gas outburst and improving the safety of coal mining. (1) The smaller the particle size of coal sample, the more gas is adsorbed, the more the maximum temperature change rate and temperature change in the process of adsorption and desorption. For coal samples of the same particle size, the greater the equi librium pressure, the greater the adsorption quantity, the faster the rate of temperature variation, and the higher the maximum temperature variation are during the process of adsorption and desorption. (2) The reduction in surface free energy and the variation in heat in the process of adsorption and desorption increase with decreased particle size and increased equilibrium pressure. With an increase in equilibrium pressure, the reduction in surface free energy of coal samples with different particle sizes gradually slows, which
Fig. 13. Desorption heat variation of coal samples with different particle sizes under the equilibrium pressure of approximately 0.32 MPa. 7
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indicates that there is a non-uniform adsorption potential field on the surface of coal. (3) By comparing the differences in heat data between columnar and granular coal samples in the process of adsorption and desorp tion, it can be concluded that the reduction in surface free energy is converted into adsorption heat and deformation energy during the process of adsorption, and the deformation energy is con verted into heat energy during the desorption process.
Li, H., Feng, Z.C., Zhao, D., Duan, D., 2017. Simulation experiment and acoustic emission study on coal and gas outburst. Rock Mech. Rock Eng. 50, 2193–2205. Liu, J.K., He, X.Q., Wang, C.X., 2013. Measurement of temperature variation in coal gas desorption based on infrared imaging technology. J. Liaoning Tech. Univ. 32 (9), 1161–1165 (in Chinese). Liu, S.S., Meng, Z.P., 2015. Study on energy variation of different coal-body structure coals in the process of isothermal adsorption. J. China Coal Soc. 40 (6), 1422–1427 (in Chinese). Liu, Z.X., Feng, Z.C., Zhang, Q.M., Zhao, D., Guo, H.Q., 2015. Heat and deformation effects of coal during adsorption and desorption of carbon dioxide. J. Nat. Gas Sci. Eng. 25, 242–252. https://doi.org/10.1016/j.jngse.2015.04.024. Lu, S.Q., Zhang, Y.L., Sa, Z.Y., Si, S.F., 2019. Evaluation of the effect of adsorbed gas and free gas on mechanical properties of coal. Environ. Earth Sci. 78 (6), 1–15. https:// doi.org/10.1007/s12665-019-8222-3. Lutynski, M., Gonzalez, M.A.G., 2016. Characteristics of carbon dioxide sorption in coal and gas shale-The effect of particle size. J. Nat. Gas Sci. Eng. 28, 558–565. https:// doi.org/10.1016/j.jngse.2015.12.037. Lu, W.D., Wang, J.R., Ju, Y.W., 2017. Microscopic mechanism of adsorption-desorption in coal and gas outburst process. J. Nanosci. Nanotechnol. 17 (9), 6894–6898. https://doi.org/10.1166/jnn.2017.14460. Ma, D.M., Li, L.X., Li, X.P., Bai, H.D., Wang, J., Liu, H.N., Li, F.Q., 2014. Contrastive experiment of adsorption-desorption between CH4 and CO2 in coal seam 4 of Dafosi coal mine. J. China Coal Soc. 39, 1938–1944 (in Chinese). Pini, R., Ottiger, S., Burlini, L., Storti, G., Mazzotti, M., 2010. Sorption of carbon dioxide, methane and nitrogen in dry coals at high pressure and moderate temperature. Int. J. Greenh. Gas Control 4 (1), 90–101. https://doi.org/10.1016/j.ijggc.2009.10.019. Paweł, B., Katerzyna, Z., Miroslawa, B., 2015. Expansion of hard coal accompanying the sorption of methane and carbon dioxide in isothermal and non-isothermal processes. Energy Fuels 29 (3), 1899–1904. https://doi.org/10.1021/ef502312p. Shemshad, J., Aminossadati, S.M., Bowen, W.P., Kizil, M.S., 2012. Effects of pressure and temperature fluctuations on near-infrared measurements of methane in underground coal mines. Appl. Phys. B 106 (4), 979–986. https://doi.org/10.1007/s00340-0114801-z. Sobczyk, J., 2011. The influence of sorption processes on gas stresses leading to the coal and gas outburst in the laboratory conditions. Fuel 90 (3), 1018–1023. https://doi. org/10.1016/j.fuel.2010.11.004, 2011. Soares, J.L., Oberziner, A.L.B., Jose, H.J., Rodrigues, A.E., Moreira, R.F.P.M., 2007. Carbon dioxide adsorption in Brazilian coals. Energy Fuels 21 (1), 209–215. https:// doi.org/10.1021/ef060149h. Tan, Z., Gubbins, K.E., 1990. Adsorption in carbon micropores at supercritical temperature. J. Phys. Chem. 94 (15), 6061–6069. https://doi.org/10.1021/ j100378a079. Tang, J., Jiang, C.L., Chen, Y.J., Li, X.W., Wang, G.D., Yang, D.D., 2015. Line prediction technology for forecasting coal and gas outbursts during coal roadway tunneling. J. Nat. Gas Sci. Eng. 34, 412–418. https://doi.org/10.1016/j.jngse.2016.07.010. Wang, C.J., Yang, S.Q., Li, X.W., Li, J.H., Jiang, C.L., 2019a. Comparison of the initial gas desorption and gas-release energy characteristics from tectonically-deformed and primary-undeformed coal. Fuel 238, 66–74. https://doi.org/10.1016/j. fuel.2018.10.047. Wang, C.J., Yang, S.Q., Jiang, C.L., Yang, D.D., Zhang, C.J., Li, X.W., Chen, Y.J., Tang, J., 2017. A method of rapid determination of gas pressure in a coal seam based on the advantages of gas spherical flow field. J. Nat. Gas Sci. Eng. 45, 502–510. https://doi. org/10.1016/j.jngse.2017.05.021. Wang, G., Cheng, W.M., Pan, G., 2012. Influence of temperature on coal’s adsorbing capability. J. Saf. Environ. 12 (5), 231–233. https://doi.org/10.1007/s11783-0110280-z. Wang, G., Guo, Y.Y., Du, C.A., Sun, L.L., Liu, Z.Y., Wang, Y., Cao, J.J., 2019b. Experimental study on damage and gas migration characteristics of gas-bearing coal with different pore structures under sorption-sudden unloading of methane. Geofluids 1–11. Wierzbicki, Miroslaw, 2013. Changes in the sorption/diffusion kinetics of a coal-methane system caused by different temperatures and pressures. Gospodarka Surowcmi Mineralnymi-Miner. Resour. Manag. 29 (4), 155–168. Wu, X., Li, J., Li, G.H., Nie, W., Tu, Z.H., 2017. Experimental study on thermodynamic process of coal and gas outburst. China Saf. Sci. J. 27 (5), 105–110 (in Chinese). Yan, M., Bai, Y., Li, S.G., Lin, H.F., Yan, D.J., Shu, C.M., 2019. Factors influencing the gas adsorption thermodynamic characteristics of low-rank coal. Fuel 248, 117–126. https://doi.org/10.1016/j.fuel.2019.03.064. Yang, T., Chen, P., Li, B., Nie, B.S., Zhu, C.J., Ye, Q.S., 2019. Potential safety evaluation method based on temperature variation during gas adsorption and desorption on coal surface. Saf. Sci. 113, 336–344. https://doi.org/10.1016/j.ssci.2018.11.027. Yu, Q.X., 1992. Mine Gas Prevention. China university of mining and technology press, Xuzhou (in Chinese). Zhao, W., Cheng, Y.P., Jiang, H.N., Jin, K., Wang, H.F., Wang, L., 2016. Role of the rapid gas desorption of coal powders in the development stage of outbursts. J. Nat. Gas Sci. Eng. 28, 491–501. https://doi.org/10.1016/j.jngse.2015.12.025. Zhao, D., Zhang, C., Chen, H., Feng, Z.C., 2019. Experimental study on gas desorption characteristics for different coal particle sizes and adsorption pressures under the action of pressured water and superheated steam. J. Pet. Sci. Eng. 179, 948–957. https://doi.org/10.1016/j.petrol.2019.05.027. Zhang, C.G., 1993. Survey research on the relationship between gas desorption and temperature of coal samples by using the infrared thermometer. Saf. Coal Mines (9), 17-18þ25-49. (in Chinese).
Since the gas adsorption quantity is found to be highly dependent on temperature, thus it is suggested that the temperature of coal body under mining can be used as a parameter to monitor the gas storage in coal, and potentially predict the gas outburst. Acknowledgements We sincerely thank the editors and anonymous reviewers for improving the quality of this manuscript. This study was supported by the National Natural Science Foundation of China (Grant No. 51304142 and No. 21373146), the Program for Outstanding Innovation Teams of Higher Learning Institutions (Grant Year 2014), the Project of Applied Foundation Research (Grant No. 201801D121283 and 201801D221366), and the Education Reform Project about Graduate Students of Shanxi Province in China (2018JG35). We thank Elsevier Language Service for English language editing, and we sincerely thank Sanyuan Zhongneng Coal Mine Co., Ltd in Shanxi Province of China for providing raw coal samples. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.petrol.2020.106932. References An, F.H., Yuan, Y., Chen, X.J., Li, Z.Q., Li, L.Y., 2019. Expansion energy of coal gas for the initiation of coal and gas outbursts. Fuel 235, 551–557. https://doi.org/ 10.1016/j.fuel.2018.07.132. An, Z.X., 1983. Determine the risk of coal seam outburst by using temperature of coal seam. Saf. Coal Mines (11), 43–46 (in Chinese). Busch, A., Gensterblum, Y., Krooss, B.M., Littke, R., 2004. Methane and carbon dioxide adsorption-diffusion experiments on coal: upscaling and modeling. Int. J. Coal Geol. 60 (2–4), 151–168. https://doi.org/10.1016/j.coal.2004.05.002. Bakhshian, S., Sahimi, M., 2017. Adsorption-induced swelling of porous media. Int. J. Greenh. Gas Control 57, 1–13. https://doi.org/10.1016/j.ijggc.2016.12.011. Chaback, J.J., Morgan, D., Yee, D., 1996. Sorption irreversibilities and mixture compositional behavior during enhanced coal bed methane recovery processes. SPE Gas Technol. Symp. 431–438. https://doi.org/10.1016/S0140-6701(99)96326-9. Gruszkiewicz, M., Naney, M., Blencoe, J., Cole, D.R., Pashin, J.C., Carroll, R.E., 2009. Adsorption kinetics of CO2, CH4, and their equimolar mixture on coal from the Black Warrior Basin, West-Central Alabama. Int. J. Coal Geol. 77 (1–2), 23–33. https://doi. org/10.1016/j.coal.2008.09.005. Guo, L.W., Yu, Q.X., Wang, K., 2000. Experimental study on change in coal temperature during adsorbing gas. J. China Inst. Min. Technol. 29 (3), 65–67 (in Chinese). He, M.C., Wang, C.G., Li, D.J., Liu, J., Zhang, X.H., 2010. Desorption characteristics of adsorbed gas in coal samples under coupling temperature and uniaxial compression. Chin. J. Rock Mech. Eng. 29 (5), 865–872 (in Chinese). Jin, K., Cheng, Y.P., Liu, Q.Q., Zhao, W., Wang, L., Wang, F., Wu, D.M., 2016. Experimental investigation of pore structure damage in pulverized coal: implications for methane adsorption and diffusion characteristics. Energy Fuels 30 (12), 10383–10395. https://doi.org/10.1021/acs.energyfuels.6b02530. Kowalczyk, P., Furmaniak, S., Gauden, P.A., Terzyk, A.P., 2010. Carbon dioxide adsorption-induced deformation of microporous carbons. J. Phys. Chem. C 114 (11), 5126–5133. https://doi.org/10.1021/jp911996h. Kim, H.J., Shi, Y., He, J., Lee, H.H., Lee, C.H., 2011. Adsorption characteristics of CO2 and CH4 on dry and wet coal from subcritical to supercritical conditions. Chem. Eng. J. 171 (1), 45–53. https://doi.org/10.1016/j.cej.2011.03.035. Katarzyna, C., Katarzyna, Z., Bronislaw, B., Pawel, B., 2016. Kinetic models assessment for swelling of coal induced by methane and carbon dioxide sorption. Adsorption 22 (4–6), 791–799. https://doi.org/10.1007/s10450-016-9775-z. Kim, D.H., Lee, C.C., Lee, Y.S., 2018. The CH4-CO2 adsorption characteristics of subbituminous coal with different particle sizes. J. Korean Soc. Miner. Energy Resour. Eng. 55 (1), 37–48 (in Korean).
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