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Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals
Journal of Petroleum Science and Engineering 146 (2016) 856–865
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Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals Zhaoping Meng a,b,n, Shanshan Liu a, Guoqing Li c,d a
College of Geosciences and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China Key Laboratory of Geological Hazards on Three Gorges Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002, China c Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China d Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 1 August 2015 Received in revised form 15 June 2016 Accepted 25 July 2016 Available online 26 July 2016
Methane adsorption capacity of coal is one of the major factors influencing total gas-in-place and it is linked to coal structure, while our knowledge of it is still limited. In this study, coal structures were classified into five categories, including integrated, blocky, cataclastic, granulated and mylonitized. The integrated and blocky coals are intact and the other three are deformed. To investigate the methane adsorption capacity, adsorption potential and surface free energy of different structure coals, four high rank coal samples were collected from No.3 coal seam of the Zhaozhuang coal mine in Southeastern Qinshui Basin China, and a series of methane isothermal adsorption experiments and pore distribution measurements were carried out in the laboratory, and then the adsorption potential and the surface free energy were analyzed. It turns out that, there are no obvious differences in the methane adsorption capacity for the various structure coals when the equilibrium pressure is below 2 MPa; while there are significant differences when the pressure is above 2 MPa. The higher the gas pressure is, the greater the difference is. Methane adsorption capacity of different structure coals can be arranged in a descending order: mylonitized 4granulated 4cataclastic 4 intact. That is, the methane adsorption capacity is higher for the deformed coal samples than for the intact ones. Both the specific surface area and the pore volume increase with the increase of deformation degree. Mesopores contribute most to the total pore volume; while the adsorption pores have the greatest contribution to the total specific surface area. The adsorption potential and surface free energy for different-structure coals under the same adsorption space volume increase with the increase of deformation degree. Adsorption potential decreases with the increase of adsorption space, and the adsorption potential of micropore is considerably higher than that of mesopore and macropore. As the temperature rises, the cumulative reduction of surface free energy decreases and the surface free energy decreases significantly at each equilibrium pressure. Methane adsorption in coal is dominated by adsorption potential and surface free energy. & 2016 Elsevier B.V. All rights reserved.
Keywords: Coalbed methane Coal structure Adsorption capacity Adsorption potential Surface free energy
1. Introduction Almost all of the late Paleozoic coal-bearing basins in China experienced one or a few tectonic movements of various scales in geological history and thereby the coal seams were tectonically deformed to various extents (Meng et al., 2010; Hou et al., 2012; Zhou et al., 2014; Jia et al., 2015). According to “Regulations on the Prevention and Cure of Coal and Gas Outburst”, an official standard document in China, coal structures are classified into five categories including type I- nondestructive coal, type II- destructive
n Corresponding author at: College of Geosciences and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China. E-mail address: [email protected] (Z. Meng).
coal, type III - strongly destructive coal, type IV - sheared coal and type V- pulverized coal. Type III, IV and V are collectively referred as deformed coals and Type I and II as intact ones. The intact coals have well-preserved sedimentary structures. Nearly all of the coal and gas outburst accidents occur in the deformed coal seams. Gas drainage neither from surface nor from underground has ever achieved a good result. Therefore, the research on adsorption mechanism in various-structure coals encompasses theoretical and practical significances for coalbed methane (CBM) development. Methane adsorption capacity in various-structure coals has been widely discussed by many researchers. Guo et al. (1996) summarized the characteristics of coal and gas outburst that occurred in the deformed coals in the Pingdingshan mining district, China. Zhang et al. (2007), Chikatamarla and Peter (1999), Yves
http://dx.doi.org/10.1016/j.petrol.2016.07.026 0920-4105/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Nomenclature
ε P0 Pi P R T Pc Tc w
is the adsorption potential, J/mol; is the saturation vapor pressure of the adsorbate at temperature T, MPa; is the equilibrium pressure of ideal gas under a constant temperature, MPa; is the equilibrium pressure, MPa; is the Universal Gas Constant, J/(mol K); is the absolute temperature, K; is the critical pressure of adsorbate, MPa; for methane, Pc is 4.62 MPa; is the critical temperature of adsorbate, K.; for methane, Tc is 190.6 K. is the adsorption volume, cm3/g;
et al. (2013), Skoczylas et al. (2014) and Li et al. (2015) concluded, from the analyses of coal petrography, coal quality, and isothermal adsorption/desorption tests under equilibrium moisture condition, that the methane adsorption capacity of coal is greatly related to its physical and chemical structures, porous structure, and particle size and so on; with the increasing degree of coal deformation, Langmuir volume increases gradually; the deformed coals usually have a higher methane adsorption capacity and a higher risk level of coal and gas outburst than the intact ones. Wang and Yang (1980) proposed that there are no significant differences in methane adsorption property between the intact and the deformed coal samples and the tectonic stress do not affect the micropore volume or the specific surface area of coals. Based on the isothermal adsorption experiments for three coal samples of different deformation degrees in Huaibei mining area, China, Pan et al. (2012) considered that the adsorption capacity of coal at low temperature (30 °C) increases with the increasing deformation degree, and there are no significant differences in the adsorption capacity among various structure coals at high temperatures (50 °C, 70 °C). CBM is mainly adsorbed on the inner surface of coal matrix. Knowledge of pore system and molecular structure of coal matrix is essential for the gas adsorption mechanism of coal. On the basis of detailed investigations on coal petrography, coal quality and isothermal adsorption lines, Faiz et al. (1992), Mastalerz et al. (2008) and Swanson et al. (2015) presented some conclusions: the pores in coal comprise micropores and mesopores; micropores provide the main adsorption space; pore volume and specific surface area are positively related to coal gas content and the adsorption capacity is positively related to coal rank, specific surface area, and fixed carbon content. Jiang and Ju (2004), Ju et al. (2009), and Hou et al. (2012), Pan et al. (2015) conducted experiments on the chemical structures of different types of deformed coals under the conditions of high temperature and high pressure and they considered that, with the rise of deformation degree, the basic structure unit of coal enlarges rapidly; aromatization and ring condensation enhance obviously; macromolecular structure deformation of coal would cause the change of nanoscale porous structure; in the same metamorphism environment, due to the tectonic stress, for the deformed coals, the amount of mesopores decreases significantly while the amount of micropores increases gradually. Thermodynamics is a branch of physics and focuses on the heat and temperature and their relation to energy and work. Thermodynamic variation is a macroscopical representation of methane adsorption capacity on coals. And adsorption potential, surface free energy can be used to quantitatively characterize
M Vad
ρad
s
Γ
V V0 S VL PL
Δγ
857
is the molecular mass of adsorbate, g/mol; is the volume of adsorbate in adsorbed state, mol/g; is the density of adsorbate in adsorbed state, g/cm3. is the surface tension, J/m2; is surface excess concentration, mol/m2; it is the methane concentration difference between the surface and inside of the coal; is methane adsorption volume, L; is the molar volume of gas under standard condition, 22.4 L/mol; is the pore surface area of coals, m2/g; is Langmuir volume, cm3/g; is Langmuir pressure, MPa; is the variation of surface free energy of coals after gas adsorption, J/m2.
the thermodynamics. Coal decreases spontaneously its surface free energy by decreasing the surface tension during adsorption. Therefore, the adsorption capacity of coals can be illustrated by the variation of surface free energy during adsorption. Jian et al. (2014) calculated the surface free energy variation of coals with intact and tectonic structure at different temperatures, and analyzed the cause of energy variation from dynamic metamorphism perspective. Cui et al. (2003) and Bai et al. (2014) investigated the relationship between adsorption capacity and adsorption heat of various structure coals and illustrated the microcosmic mechanism of heat generation during methane adsorption on coals by thermodynamic theory. G.Q. Li et al. (2014), X.J. Li et al. (2014) and Liu and Feng (2012) characterized the methane adsorption behavior on coal surface by estimating the adsorption heat and forces between the methane molecules and coal surfaces. Although a lot of efforts have been made on exploring the adsorption of different structure coals, our knowledge of it is still limited and there are still diverse viewpoints on it and the mechanism behind it is still obscure. This study was undertaken to investigate the methane adsorption behavior and thermodynamics in coals of different structures. The coal structures were first addressed, and then the measurement of pore distribution of coals of different structures and the methane isothermal adsorption experiments under different temperaturepressure conditions were carried out, and finally the thermodynamic characteristics were analyzed. The outcome of this research may deepen our understanding of the adsorption mechanism of coals and benefit the CBM exploration and development in the study area.
2. Coal structure and its classification Coal structure refers to the geological structure features of a coal seam affected by tectonic movement. Moderate structural deformation can increase fractures in coal and lead to an improved permeability of the coal seam. However, excessively high level stress can break an intact coal seam into fine particles and destroy the coal cleat system and thereby result in a significant reduction in permeability (Meng and Li, 2013). According to the morphological features of coal under different tectonic stress regimes, coal structures can be classified into five types, including integrated, blocky, cataclastic, granulated and mylonitized structures (Table 1) (after the China national standard “Classification of coal-body structure” (GB/T 30050-2013), modified). The first two types are intact coals and the latter three are deformed coals. The deformed
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
42–10 1–2 Easy to peel into small pieces by Angular, no significant orienta- Broken into tablets and occasionally including intact coal shivers, orientation arhand, medium hardness tion arrangement and rangement and displacement displacement Cracks and permeability Completed fracture system, Completed fracture system and Well-developed cleats, high Less developed cleats and cracks, low No cleats, cracks filled by coal powder, very low high permeability relatively high permeability permeability permeability permeability High hardness coefficient, rela- Strong adsorption capability, with the increase of deformation, coal strength decreases and initial velocity of methane emission Adsorption and emission High f (hardness coefficient) value, low initial velocity of gas tively low initial velocity of gas increases emission emission
coal is subjected to obvious variation in its composition, texture and structure due to tectonic movements. Affected by multi-stage tectonic movements, the original pore-fracture system of the coal seam was destroyed and therefore CBM adsorption, permeability properties were affected. For example, the deformed coal in the Qinshui basin has a high porosity, a great adsorption capacity; however, it has a very low permeability and thereby the gas desorption from coal is very slow (Meng et al., 2011). Tectonic movements also affect coal rank. It is generally considered that the deformed coal has a slightly higher rank than the intact in the same coal seam and in the same mining district and the thermal effect caused by tetonic movements promotes dynamic metamorphism of coal seam.
3. Materials and method 3.1. Description of the coal samples In order to analyze the methane adsorption of different structure coals, four samples were selected from No.3 coal seam of Shanxi Formation in Zhaozhuang Coal Mine, Southeastern Qinshui Basin, China. The coal seam, bearing a strip of vitrinite, belongs to semi-clarain coal clarain coal type. Clarain is the major composition in coal seams and durain is the secondary. The coal structures of these coal samples include intact, cataclastic, granulated and mylonitized coal. We carried out proximate analysis for these samples in terms of China national standards “Proximate analysis of coal (GB/T 212-2008)”. 3.2. Isothermal adsorption experiment
Note: f ¼ P/10 (P is the uniaxial compressive strength of rock, MPa)
Integrated, great hardness, blocky
Basic granule size/ mm Broken state
Broken into pieces and no significant angular structure Traceable stripe structure and angular nubby structure Stripe texture and horizontal bedding structure Layered and blocky structures with distinct bands
Compressed into blocks or solid particles, oriented into foliation and mostly scaly, with crumpled slip surface and mirror surface o1 Powder, pulverized into solid block, powder
Not easy to identify Visible Clearly visible Clearly visible
Visibility of macroscopic lithotype Texture and Structure
Granulated coal Cataclastic coal Blocky coal Integrated coal
Deformed coals Intact coals Characteristics contrast
Table 1 The characteristics of different structure coals (after China national standard “Classification of coal-body structure” (GB/T 30050-2013), modified).
Unable to identify
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Mylonitized coal
858
3.2.1. Experimental instrument The experiment instrument was an ISO-300 isothermal adsorption and desorption instrument made by TerraTek (USA). The measuring range of the equipment was 0–70 MPa in pressure and the upper limit of temperature was 150 °C. The purity of adsorbent gas (methane) was up to 99.99%. The measuring unit was arranged in the oil bath thermostat to keep the temperature constant. This measuring unit consisted of a stainless steel reference cell and a sample cell with capacities of 80 cm3 and 160 cm3, respectively. The reference cell and the sample cell are arranged in the oil bath thermostat container, and the resolution of the temperature measurement was controlled within 70.02 °C. The pressures in reference cell and the sample cell were monitored only with high-precision pressure transducers with an accuracy of 70.10%. The temperatures of the reference cell and sample cell were kept constant during the experiment. The schematic diagram of isothermal adsorption experimental setup is shown in Fig. 1. 3.2.2. Experimental method In this study, the testing procedure followed the national standard of “Experimental Method of High-Pressure Adsorption Isothermal to Coal-Capacity Method” (GB/T19560-2008). For isothermal adsorption test, 100–120 g coal sample with the size of 0.2–0.25 mm (60–80 mesh) were selected. Before the isothermal adsorption experiment, proximate analysis was conducted to determine the moisture, ash, volatile matter, and fixed carbon content of the coal samples and to allow a conversion of adsorption volume of the coal samples under different conditions. According to the coal reservoir conditions, isothermal adsorption experiments were performed at a maximum pressure of 14 MPa and at temperatures of 25 °C, 35 °C, and 45 °C, and under equilibrium moisture condition.
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
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Pressure transducer
Water bath
Sample cell
Globe valve
Reference cell
Gas inlet
Vent
Vacuum pump Fig. 1. schematic diagram of isothermal adsorption experimental setup.
3.3. Measurement of pore size distribution using a low temperature nitrogen adsorption and desorption method
adsorbate can be described as follows (Ramirez-Pastor and Bulnes, 2000; Bai et al., 2014).
ASAP2020 specific surface area and porosity analyzer produced by America Micromeritics Instrument Corporation was used to measure the pore size distribution using a low temperature nitrogen adsorption and desorption method. The coal samples were crushed into particles of 0.2–0.25 mm in diameter (60–80 mesh). The mass of each sample for this experiment was 2–3 g. The adsorbate was nitrogen with purity of nearly 99.999%. The temperature was reflected with liquid nitrogen and the particle size was 0.35–500 nm in diameter. We placed the dried and degassed sample into dewars full of liquid nitrogen. The experiment system started the adsorption experiment under the preset pressure to obtain nitrogen adsorption under different pressures. General models to describe the specific surface area and volume of solid porous media are Brunauer–Emmett–Teller multimolecular (BET) layer adsorption formula and Barret–Joyner–Halenda (BJH) model, separately (Gregg et al., 1967).
ε=
3.4. Calculation of adsorption potential and surface free energy related to isothermal adsorption 3.4.1. Adsorption potential and adsorption space volume The concept of adsorption potential was first applied to physical adsorption by Polanyi in 1914. This concept reflects the variation of Gibbs free energy under the adsorption of 1 mol mass. Adsorption potential theory assumes that solids have surface gravity field just like the planets. Thus, the adsorbed molecules around solids can be adsorbed under the effect of surface force field. These adsorbed molecules can also form multilayer adsorption on the surface of the solids. Unlike Langmuir monolayer model, the adsorption potential theory needs no physical model. The gas adsorption in micropores is considered as a process of volume filling based on potential intensity rather than layer upon layer. Volume filling theoretical system, derived from adsorption potential theory, is the most well shaped and practical system to describe the adsorption behavior of microporous adsorbents. The characteristic curve of adsorption potential is not dependent on temperature because adsorption potential represents the work accomplished with the temperature-independent dispersion forces. The work accomplished with adsorption potential field during the transform from gaseous to adsorbed state for unit mass of
∫P
P0
i
P RT dP = RTln 0 P Pi
(1)
where ε is the adsorption potential, J/mol; P0 is the saturation vapor pressure of the adsorbate at temperature T, MPa; Pi is the equilibrium pressure of ideal gas under a constant temperature, MPa; P is the equilibrium pressure, MPa; R is the Universal Gas Constant, J/(mol K); and T is the absolute temperature, K. The temperatures at which the coal adsorbs methane are usually above the critical temperature of the methane. Therefore, Dubinin proposed an empirical formula to calculate the virtual saturation vapor pressure of the adsorbate under supercritical conditions:
P0 = PC (
T 2 ) , TC
(2)
where Pc is the critical pressure, MPa; and Tc is the critical temperature of adsorbate, K. For methane, Pc is 4.62 MPa, and Tc is 190.6 K. Adsorption space volume represents the volume occupied by the adsorbed gas in the adsorbents, and it can characterize the micropore structure. It is presented as follows.
w=Nad
M , ρad
(3) 3
where w is adsorption volume, cm /g; M is the molecular mass of adsorbate, g/mol; Nad is the volume of adsorbate in adsorbed state, mol/g; and ρad is the density of adsorbate in adsorbed state, g/cm3. The empirical formula to calculate ρad is presented as follows:
ρad =
8PC M. RTC
(4)
3.4.2. Surface free energy A coal macromolecule is composed of a core of two or three condensed aromatic rings (20–80% organic carbon) and alkyl groups and functional groups on the periphery. When the surface pores are well developed, the vacancy on one side of the carbon atoms on the surface leads to force unbalances. Thus, carbon atoms tend to move inward under the influence of the inward force. The energy obtained in such a tendency is defined as surface
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
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Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
free energy. According to surface chemistry theory (Jian et al., 2014), the Gibbs equation describing surface tension reduction during adsorption of methane in coal is as follows.
−dσ = RT Γd(lnP),
(5)
where s is the surface tension, J/m2; and Γ is the surface excess concentration, mol/m2, which is the methane concentration difference between the surface and inside of the coal:
Γ=
V , SV0
(6)
where V is the methane adsorption volume, L; V0 is the molar volume of gas under standard condition, 22.4 L/mol; and S is the specific surface area of coals, m2/g. V can be determined with the Langmuir equation:
V=VL·P/(PL +P),
(7) 3
where VL is the Langmuir volume, cm /g; and PL is the Langmuir pressure, MPa. VL represents the greatest adsorption capacity of coals and PL is the equilibrium pressure at which the corresponding adsorption volume reaches one-half of the Langmuir volume. Eq. (6) is solved for Γ, and Eq. (7) is solved for V. Then, Γ and V are substituted into Eq. (5). Eq. (5) can be integrated and rewritten as follows.
∆γ=
VLRT P ln (1+ ), V0S PL
(8)
where Δγ is the variation of surface free energy of coals due to gas adsorption, J/m2. The change of surface free energy at a given pressure can be obtained with the differentiation of Eq. (8), and the formula is as follows:
∆γP =
VLRTPL . V0S(PL + P )
(9)
4. Results and discussions 4.1. Adsorption properties of different structure coals Proximate analysis results of the coal samples were listed in Table 2. The isothermal adsorption experimental results are shown in Fig. 2. Note that, in this paper we just showed the error bars of experimental data with an assumed relative error of 7 5% and we did not aware of the real experimental errors. From Fig. 2 and Table 3, we can see that the isothermal adsorption curves of four coal samples show slight differences at 0– 2 MPa pressure. As the pressure rises, the methane adsorption capacities of different coals are shown in a decreasing order: mylonitized coal 4granulated coal 4 cataclastic coal 4 intact coal. The higher the pressure is, the greater the differences of methane adsorption volume for different coal type are. Consequently, under the same temperature, the Langmuir volume (VL) is in sequence of
Fig. 2. Isotherm adsorption of methane on various coals under different temperatures.
mylonitized4granulated 4 cataclastic 4intact. Coal methane adsorption capacities are significantly different under high temperature (45 °C) in this study. However, Pan et al. (2012) proposed that the various deformed medium rank coal samples (with a vitrinite reflectance of about 0.9%) do not show significant differences in coal methane adsorption capacity at high temperature (50 °C), and their adsorption isotherms are similar. The samples in
Table 2 Proximate analysis results of coal samples. Sample ID.
Coal structure
Temperature (°C)
Humidity (%)
Moisture content (%)adM
Ash (%)
Volatile (%)
ZZ-1 ZZ-2 ZZ-3 ZZ-4
Intact coal Cataclastic coal Granulated coal Mylonitic coal
15 15 15 15
17 17 17 17
1.3 0.77 0.83 1.02
11.04 11.91 12.7 9.83
9.76 8.65 10.23 5.42
Note: adM, air-dried moisture content
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Table 3 Adsorption capacities of different structural coals. Sample ID Coal structure Temperature °C Langmuir Volume, cm3/g-
Langmuir Pressure, MPa
ZZ-1
Intact
25 35 45
32.26 28.41 27.25
2.40 2.42 2.74
ZZ-2
Cataclastic
25 35 45
37.04 34.60 32.57
2.44 2.72 3.04
ZZ-3
Granulated
25 35 45
36.76 34.84 33.56
2.23 3.12 3.07
ZZ-4
Mylonitized
25 35 45
40.32 36.50 36.36
2.52 2.66 3.61
this study are high-rank coals. The pore size distribution of coal varies significantly with coal rank and the methane adsorption capacity is much higher for some high rank coals than for low and medium coals and thereby the variation law of methane adsorption capacity under different temperatures are quite different. Coal is a porous medium, which contains large internal surface area, and adsorption is one of its natural attributes. Given the Van der Waals’ force between methane molecules and inner surface of coal matrix, most methane molecules in the coal exist as an adsorbed state mainly on the surface of coal matrix, which is also known as physical adsorption. The structures of coal and graphite are similar. Kaplan (1986) maintained that the binding energy between two aromatic microchip layers in the graphite is approximately 5.4 kJ mol 1, caused by the Van der Waals’ force. The liquefaction heat of methane is merely 9.4 kJ mol 1, implying that
861
its saturated bonding structure exhibits chemical inertness state. The nature of methane adsorption is a mutual attraction between methane molecule and coal macromolecule when the methane molecules stay on the pore surface of coal for a short period. The adsorption capacity of coal is a function of temperature, adsorbate, pressure, and coal properties. 4.2. Effect of temperatures and pressures on coal methane adsorption Fig. 3 shows that the adsorption capacity of coal is closely related to the temperature. At a certain pressure, the methane adsorption volume decreases as the temperature increases. Conspicuous differences have been shown on adsorption capacity of coal to methane under various pressures. As a whole, the methane adsorption volume increases with the increment of pressure for all coal samples but in different scales. The adsorption curves of different structural coals rise linearly under low pressures; as the pressure increases, the curves rise gently and eventually become flat, thereby reaching a saturated adsorption volume. The shape of each curve subdues to Langmuir equation. Coupling effects have been achieved through temperature and pressure on adsorption of coal to methane. Figs. 2 and 3 illustrate that the adsorption curves at low pressure rise dramatically with the increase of pressure, and these curves are similar under different temperatures. This observation demonstrates that pressure is the leading factor controlling methane adsorption, and adsorption capacity is more sensitive to pressure variation than temperature. At high pressure, the growth rate of adsorption volume decreases rapidly, and the discrepancy of adsorption curves among different temperatures increases, which is mainly triggered by temperature. Accordingly, we can conclude that pressure has a deeper influence on the methane adsorption capacity than the temperature does at low
Fig. 3. Isothermal adsorption curves of various coals (a—intact coal; b—cataclastic coal; c—granulated coal; d—mylonitized coal).
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
55.47 80.06 80.30 87.68 44.53 19.94 19.70 12.32 0.008876 0.036830 0.057817 0.232361 0.007124 0.009170 0.014183 0.032639
0.016 0.046 0.072 0.232
Adsorption pore Mesopore
26.52 42.08 41.51 50.07 73.48 57.92 58.49 49.93 Undeformed structure Fragmented structure Granulated structure Mylonitized structure ZZ-1 ZZ-2 ZZ-3 ZZ-4
Note: adsorption pore includes micropore and transitional pore
0.000381 0.000614 0.000962 0.002475 0.000101 0.000258 0.000399 0.001239
Coal structure
Adsorption pore
pressures; while the temperature gradually becomes the leading factor affecting the methane adsorption at high pressures. 4.3. Effect of porous characteristics on methane adsorption in coal
0.000280 0.000356 0.000563 0.001236
Adsorption pore Mesopore Mesopore
Fig. 4. Characteristics of pore distribution of various coals (a—pore volume; b— pore surface area).
Sample ID
Table 4 Pore distribution of the coal samples.
Total volume
Pore volume proportion (%) Pore volume (ml/g)
Pore surface area (m2/g)
Total pore surface area
Mesopore
Adsorption pore
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Pore surface area proportion (%)
862
Coal is a dual-porosity media, hereby, we use the results of lowtemp liquid nitrogen adsorption to characterize the pore size distribution. Coal matrix pores are always complex and heterogeneous and can be classified into four types in terms of their genesis: primary pore, metamorphic pore, epigenetic pore and mineral pore. Adsorption capacity of coal is mainly related to nano-pores of organic matter, so we focus on the pore size distribution. Coal porous structure refers to different shapes, size distribution, and interconnected relationships of pores and pore throats in coal. It is also generally represented by characterization of pore specific surface area, pore volume, pore model, and pore size distribution, which can be acquired by low-temperature nitrogen adsorption and desorption experiment. Pore size classification is based on the standard proposed by Hotot (1961), which divided pores into micropores ( o10 nm), transitional pores (10–100 nm), mesopores (100–1000 nm), and macropores (4 1000 nm) via a spatial scale. In his work, both micropores and transitional pores are called adsorption pores. The experimental results of lowtemperature nitrogen adsorption and desorption test about pore volume and specific surface area of different coals are shown in Table 4 and Fig. 4. The pore size and pore volume distribution of coal samples were further calculated with BJH model. This model is the recommended one in China national standards “Analysis of mesopores and macropores by gas adsorption (GB/T 21650.2— 2008/IS0 15901-2: 2006)”. And it is widely used by many researches (Tian et al., 2015; Ross and Bustin, 2009; Wang et al., 2014).
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
863
Fig. 5. Characteristic curve of adsorption potential on various coals at 35 °C.
However, this model has its own shortcomings, because it assumes that (1) the pores are cylindrical (more precisely that pore volume and capillary volume are related to each other as the square of some adequate measure of their cross sections) and (2) the amount of adsorbate in equilibrium with the gas phase is retained by the adsorbent by two mechanisms: (a) physical adsorption on the pore walls and (b) capillary condensation in the inner capillary volume (Barrett et al., 1951). While the pores of coal matrix do not strictly conforms to these two assumptions. Table 4 and Fig. 4 show the measurement range covers from 1.7 to 435.9 nm in diameter and the pores less than 1.7 nm will contribute to the surface area and adsorption capacity but are not within the measurement range of low-temperature nitrogen adsorption method. Fig. 4 illustrates that the pore volume and specific surface area increase as the deformation degree of coal increases from the intact coal sample to the mylonitized coal. The pore volume and specific surface area for the intact, cataclastic, and granulated coal samples are similar, whereas the pore volume and specific surface area of the mylonitized coal sample are considerably higher than those of the other three. In general, the pore volume and the specific surface area of pores with different sizes are in the decreasing order: mylonitized coal 4 granulated coal 4cataclastic coal 4intact coal. This observation indicates that the mesopores contribute most to the total pore volume, whereas the adsorption pores have the greatest contribution (more than 50%) to the total specific surface area in all coal types. However, the mesopores provide a relatively small contribution to the specific surface area. Moreover, the proportions of both adsorption pore and mesopore in the pore volume and specific surface area increase from the intact coal to the mylonitized coal. 4.4. Analysis on adsorption potential and surface free energy calculation results 4.4.1. Adsorption potential and adsorption space volume The characteristic curve of adsorption potential is not dependent on the temperature because the adsorption potential represents the work accomplished with the temperature-independent dispersion forces. According to the adsorption potential theory, the characteristic curve of adsorption potential at a temperature of 35 °C is drawn here as a representative. Fig. 5 illustrates that the adsorption potential of mylonitized coal is the largest, followed by granulated coal, cataclastic coal, and intact coal under the same adsorption space volume. The greater the adsorption potential is, the less the adsorption pressure needed for adsorbing the same methane volume is. Hence, the coals with a high adsorption potential have a strong adsorption capacity, which is consistent with the results of the isothermal adsorption experiment. As the adsorption space volume is approximate to 0,
Fig. 6. Cumulative reduction of surface free energy on various coals (a—25 °C, b— 35 °C, c—45 °C).
characteristic curves approach Y axis. The adsorption potential is strong in micropores. Moreover, methane molecules occupy pore volume sequentially through volume filling mechanism in terms of the adsorption potential strength. Thus, the micropores are first occupied by the methane molecules. Adsorption potential decreases as adsorption space volume increases. Thus, adsorption occurs later in mesopore and macropore than in micropore. The strong adsorption potential in micropores is the result of the overlapped Van der Waals’ force from the pore walls, given that the micropore diameter is considerably small. 4.4.2. Surface free energy The results of surface free energy calculation are shown in Figs. 6–8. Fig. 6 illustrates that the mylonitized coal sample has the maximum surface free energy variation under various temperatures, followed by granulated coal, cataclastic coal, and intact coal. As we know, any object tends to decrease its surface free energy because an object stabilizes with minimal surface free energy.
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
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Fig. 8. Surface free energy change on granulated coals under various temperatures (a—cumulative reduction, b—reduction rate).
Fig. 7. Reduction rate of surface free energy on various coals (a—25 °C, b—35 °C, c —45 °C).
Coals decrease their surface free energy through methane adsorption; the larger the methane adsorption volume is, the greater the reduction of surface free energy is. Therefore, surface free energy can reflect the methane adsorption capacity of various coals. Fig. 7 shows the reduction of surface free energy under given pressures. Surface free energy variation decreases at high pressures, indicating that adsorption behavior becomes increasingly difficult as the adsorption continues. Adsorption potential theory indicates that methane occupies the spots sequentially in terms of potential strength. Compared with the cumulative reduction of surface free energy in Fig. 6, the rate of surface free energy reduction exhibits an inverse relationship, that is, the rate of surface free energy is the highest for the intact coal sample and is the lowest for the mylonitized coal sampleunder each pressure point. Temperature is a macroscopical representation of microcosmic molecules’ thermal motion. It has a significant effect on the kinetic energy of molecules. For the granulated coal, the cumulative
reduction of surface free energy decreases with the increase of temperature (Fig. 8a). On the contrary, the reduction rate of surface free energy increases with the increase of temperature (Fig. 8b). Kinetic energy is known to be directly related to temperature. Highly reactive methane molecules cannot be adsorbed easily. Thus, the methane adsorption capacity on coals is weak at a high temperature. The reduction rate of surface free energy decreases rapidly under each equilibrium pressure at high temperatures, because the kinetic energy of methane molecules increases with the increase of temperature. The nature of gas adsorption onto solid is characterized by the attraction forces between gas molecule and solid molecule (Li et al., 2014). Coal adsorption to methane is physical adsorption at normal temperatures. In fact, physical adsorption is caused by Van der Waals’ forces, which mainly consist of electric forces, introduction forces, and diffusion forces. Some researchers proved that among the three forces, the diffusion force is the major force related to the methane adsorption on coals. For example, Chen et al. (2000) calculated the adsorption potential of methane adsorption on coals by adopting a quantum chemistry approach. And they concluded that the adsorption potential is mainly caused by the diffusion force based on the potential range calculated. Kaplan (1986) held the view that the structure of the coal is similar to that of graphite. Thus, the binding energy of graphite among aromatic carbon layers of approximately 5.4 kJ/mol can demonstrate that the leading diffusion force existed in graphite is also predominant in coals. In methane adsorption process in coal, methane molecules lose a certain amount of potential energy under the attraction forces. Then, the molecules enter the adsorption potential well, which are consequently adsorbed. The decrease of energy accompanies the release of heat. The more frequently potential energy changes, the more the energy will be released in the
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
system and the easier the adsorption behavior will perform, thereby the coal has a great adsorption capacity. Thus, the adsorption capacity of coals can be represented by surface free energy variation.
5. Conclusions
(1) The methane adsorption capacity of various coals in the study area could be arranged in a decreasing order: mylonitized coal 4granulated coal 4cataclastic coal 4intact coal. The methane adsorption capacity of the deformed coal is considerably higher than that of the intact ones in this study area. Methane adsorption capacity decreases with the increase of temperature. Pressure has a more significant effect on the methane adsorption at low pressures than temperature, whereas with the rise of pressure, temperature gradually becomes the leading factor influencing adsorption capacity. (2) In various ranges of pore size, the pore specific surface area and pore volume for the coal samples in this study area could be listed in a decreasing order: mylonitized4 granulated4cataclastic 4intact. This result is consistent with their isothermal adsorption capacities. The pore volume is mainly dominated by the mesopores and specific surface area is largely controlled by the adsorption pores. (3) Under the same adsorption space volume, the adsorption potential and surface free energy of different-structure coals could be presented in a decreasing order: mylonitized4 granulated4cataclastic 4intact. The adsorption potential decreases with the increase of adsorption space and the adsorption potential of micropore is much higher than that of mesopore and macropore. (4) The rise of temperature results in a decrease of the cumulative reduction of surface free energy and a significant decrease of the reduction rate of surface free energy under each equilibrium pressure. The adsorption of methane in coal is dominated by the adsorption potential and the surface free energy.
Acknowledgements This work is financially supported by the National Basic Research Program of China (973 Program under Project no. 2012CB214705), National Natural Science Foundation of China (Grant nos. 41372163 and 41172145), Key Scientific and Technology Project of CBM Foundation in Shanxi (Grant nos. MQ2014-01 and MQ2014-12), Shanxi Provincial Basic Research Program—Coal Bed Methane Joint Research Foundation (Grant nos. 2015012014 and 2014012001), the National Science and Technology Major Project of the Ministry of Science and Technology of China During “13th Five-Year Plan” (Grant nos. 2016ZX05067001-006 and 2016ZX05067001-007) and the Natural Science Foundation of Hubei Province (Grant no. 2014CFB169). The authors are grateful to Key Laboratory of Integrated Extraction of Coal and Coal Bed Methane China National Energy Administration, Shanxi Jincheng Anthracite Mining Group Ltd. for the experimental support. We also thank Michael Dawson from Telamonn Energy Services Inc. in Canada for language revision.
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Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
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Journal of Petroleum Science and Engineering journal homepage: www.elsevier.com/locate/petrol
Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals Zhaoping Meng a,b,n, Shanshan Liu a, Guoqing Li c,d a
College of Geosciences and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China Key Laboratory of Geological Hazards on Three Gorges Reservoir Area, Ministry of Education, China Three Gorges University, Yichang 443002, China c Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China d Faculty of Earth Resources, China University of Geosciences, Wuhan 430074, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 1 August 2015 Received in revised form 15 June 2016 Accepted 25 July 2016 Available online 26 July 2016
Methane adsorption capacity of coal is one of the major factors influencing total gas-in-place and it is linked to coal structure, while our knowledge of it is still limited. In this study, coal structures were classified into five categories, including integrated, blocky, cataclastic, granulated and mylonitized. The integrated and blocky coals are intact and the other three are deformed. To investigate the methane adsorption capacity, adsorption potential and surface free energy of different structure coals, four high rank coal samples were collected from No.3 coal seam of the Zhaozhuang coal mine in Southeastern Qinshui Basin China, and a series of methane isothermal adsorption experiments and pore distribution measurements were carried out in the laboratory, and then the adsorption potential and the surface free energy were analyzed. It turns out that, there are no obvious differences in the methane adsorption capacity for the various structure coals when the equilibrium pressure is below 2 MPa; while there are significant differences when the pressure is above 2 MPa. The higher the gas pressure is, the greater the difference is. Methane adsorption capacity of different structure coals can be arranged in a descending order: mylonitized 4granulated 4cataclastic 4 intact. That is, the methane adsorption capacity is higher for the deformed coal samples than for the intact ones. Both the specific surface area and the pore volume increase with the increase of deformation degree. Mesopores contribute most to the total pore volume; while the adsorption pores have the greatest contribution to the total specific surface area. The adsorption potential and surface free energy for different-structure coals under the same adsorption space volume increase with the increase of deformation degree. Adsorption potential decreases with the increase of adsorption space, and the adsorption potential of micropore is considerably higher than that of mesopore and macropore. As the temperature rises, the cumulative reduction of surface free energy decreases and the surface free energy decreases significantly at each equilibrium pressure. Methane adsorption in coal is dominated by adsorption potential and surface free energy. & 2016 Elsevier B.V. All rights reserved.
Keywords: Coalbed methane Coal structure Adsorption capacity Adsorption potential Surface free energy
1. Introduction Almost all of the late Paleozoic coal-bearing basins in China experienced one or a few tectonic movements of various scales in geological history and thereby the coal seams were tectonically deformed to various extents (Meng et al., 2010; Hou et al., 2012; Zhou et al., 2014; Jia et al., 2015). According to “Regulations on the Prevention and Cure of Coal and Gas Outburst”, an official standard document in China, coal structures are classified into five categories including type I- nondestructive coal, type II- destructive
n Corresponding author at: College of Geosciences and Surveying Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China. E-mail address: [email protected] (Z. Meng).
coal, type III - strongly destructive coal, type IV - sheared coal and type V- pulverized coal. Type III, IV and V are collectively referred as deformed coals and Type I and II as intact ones. The intact coals have well-preserved sedimentary structures. Nearly all of the coal and gas outburst accidents occur in the deformed coal seams. Gas drainage neither from surface nor from underground has ever achieved a good result. Therefore, the research on adsorption mechanism in various-structure coals encompasses theoretical and practical significances for coalbed methane (CBM) development. Methane adsorption capacity in various-structure coals has been widely discussed by many researchers. Guo et al. (1996) summarized the characteristics of coal and gas outburst that occurred in the deformed coals in the Pingdingshan mining district, China. Zhang et al. (2007), Chikatamarla and Peter (1999), Yves
http://dx.doi.org/10.1016/j.petrol.2016.07.026 0920-4105/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Nomenclature
ε P0 Pi P R T Pc Tc w
is the adsorption potential, J/mol; is the saturation vapor pressure of the adsorbate at temperature T, MPa; is the equilibrium pressure of ideal gas under a constant temperature, MPa; is the equilibrium pressure, MPa; is the Universal Gas Constant, J/(mol K); is the absolute temperature, K; is the critical pressure of adsorbate, MPa; for methane, Pc is 4.62 MPa; is the critical temperature of adsorbate, K.; for methane, Tc is 190.6 K. is the adsorption volume, cm3/g;
et al. (2013), Skoczylas et al. (2014) and Li et al. (2015) concluded, from the analyses of coal petrography, coal quality, and isothermal adsorption/desorption tests under equilibrium moisture condition, that the methane adsorption capacity of coal is greatly related to its physical and chemical structures, porous structure, and particle size and so on; with the increasing degree of coal deformation, Langmuir volume increases gradually; the deformed coals usually have a higher methane adsorption capacity and a higher risk level of coal and gas outburst than the intact ones. Wang and Yang (1980) proposed that there are no significant differences in methane adsorption property between the intact and the deformed coal samples and the tectonic stress do not affect the micropore volume or the specific surface area of coals. Based on the isothermal adsorption experiments for three coal samples of different deformation degrees in Huaibei mining area, China, Pan et al. (2012) considered that the adsorption capacity of coal at low temperature (30 °C) increases with the increasing deformation degree, and there are no significant differences in the adsorption capacity among various structure coals at high temperatures (50 °C, 70 °C). CBM is mainly adsorbed on the inner surface of coal matrix. Knowledge of pore system and molecular structure of coal matrix is essential for the gas adsorption mechanism of coal. On the basis of detailed investigations on coal petrography, coal quality and isothermal adsorption lines, Faiz et al. (1992), Mastalerz et al. (2008) and Swanson et al. (2015) presented some conclusions: the pores in coal comprise micropores and mesopores; micropores provide the main adsorption space; pore volume and specific surface area are positively related to coal gas content and the adsorption capacity is positively related to coal rank, specific surface area, and fixed carbon content. Jiang and Ju (2004), Ju et al. (2009), and Hou et al. (2012), Pan et al. (2015) conducted experiments on the chemical structures of different types of deformed coals under the conditions of high temperature and high pressure and they considered that, with the rise of deformation degree, the basic structure unit of coal enlarges rapidly; aromatization and ring condensation enhance obviously; macromolecular structure deformation of coal would cause the change of nanoscale porous structure; in the same metamorphism environment, due to the tectonic stress, for the deformed coals, the amount of mesopores decreases significantly while the amount of micropores increases gradually. Thermodynamics is a branch of physics and focuses on the heat and temperature and their relation to energy and work. Thermodynamic variation is a macroscopical representation of methane adsorption capacity on coals. And adsorption potential, surface free energy can be used to quantitatively characterize
M Vad
ρad
s
Γ
V V0 S VL PL
Δγ
857
is the molecular mass of adsorbate, g/mol; is the volume of adsorbate in adsorbed state, mol/g; is the density of adsorbate in adsorbed state, g/cm3. is the surface tension, J/m2; is surface excess concentration, mol/m2; it is the methane concentration difference between the surface and inside of the coal; is methane adsorption volume, L; is the molar volume of gas under standard condition, 22.4 L/mol; is the pore surface area of coals, m2/g; is Langmuir volume, cm3/g; is Langmuir pressure, MPa; is the variation of surface free energy of coals after gas adsorption, J/m2.
the thermodynamics. Coal decreases spontaneously its surface free energy by decreasing the surface tension during adsorption. Therefore, the adsorption capacity of coals can be illustrated by the variation of surface free energy during adsorption. Jian et al. (2014) calculated the surface free energy variation of coals with intact and tectonic structure at different temperatures, and analyzed the cause of energy variation from dynamic metamorphism perspective. Cui et al. (2003) and Bai et al. (2014) investigated the relationship between adsorption capacity and adsorption heat of various structure coals and illustrated the microcosmic mechanism of heat generation during methane adsorption on coals by thermodynamic theory. G.Q. Li et al. (2014), X.J. Li et al. (2014) and Liu and Feng (2012) characterized the methane adsorption behavior on coal surface by estimating the adsorption heat and forces between the methane molecules and coal surfaces. Although a lot of efforts have been made on exploring the adsorption of different structure coals, our knowledge of it is still limited and there are still diverse viewpoints on it and the mechanism behind it is still obscure. This study was undertaken to investigate the methane adsorption behavior and thermodynamics in coals of different structures. The coal structures were first addressed, and then the measurement of pore distribution of coals of different structures and the methane isothermal adsorption experiments under different temperaturepressure conditions were carried out, and finally the thermodynamic characteristics were analyzed. The outcome of this research may deepen our understanding of the adsorption mechanism of coals and benefit the CBM exploration and development in the study area.
2. Coal structure and its classification Coal structure refers to the geological structure features of a coal seam affected by tectonic movement. Moderate structural deformation can increase fractures in coal and lead to an improved permeability of the coal seam. However, excessively high level stress can break an intact coal seam into fine particles and destroy the coal cleat system and thereby result in a significant reduction in permeability (Meng and Li, 2013). According to the morphological features of coal under different tectonic stress regimes, coal structures can be classified into five types, including integrated, blocky, cataclastic, granulated and mylonitized structures (Table 1) (after the China national standard “Classification of coal-body structure” (GB/T 30050-2013), modified). The first two types are intact coals and the latter three are deformed coals. The deformed
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
42–10 1–2 Easy to peel into small pieces by Angular, no significant orienta- Broken into tablets and occasionally including intact coal shivers, orientation arhand, medium hardness tion arrangement and rangement and displacement displacement Cracks and permeability Completed fracture system, Completed fracture system and Well-developed cleats, high Less developed cleats and cracks, low No cleats, cracks filled by coal powder, very low high permeability relatively high permeability permeability permeability permeability High hardness coefficient, rela- Strong adsorption capability, with the increase of deformation, coal strength decreases and initial velocity of methane emission Adsorption and emission High f (hardness coefficient) value, low initial velocity of gas tively low initial velocity of gas increases emission emission
coal is subjected to obvious variation in its composition, texture and structure due to tectonic movements. Affected by multi-stage tectonic movements, the original pore-fracture system of the coal seam was destroyed and therefore CBM adsorption, permeability properties were affected. For example, the deformed coal in the Qinshui basin has a high porosity, a great adsorption capacity; however, it has a very low permeability and thereby the gas desorption from coal is very slow (Meng et al., 2011). Tectonic movements also affect coal rank. It is generally considered that the deformed coal has a slightly higher rank than the intact in the same coal seam and in the same mining district and the thermal effect caused by tetonic movements promotes dynamic metamorphism of coal seam.
3. Materials and method 3.1. Description of the coal samples In order to analyze the methane adsorption of different structure coals, four samples were selected from No.3 coal seam of Shanxi Formation in Zhaozhuang Coal Mine, Southeastern Qinshui Basin, China. The coal seam, bearing a strip of vitrinite, belongs to semi-clarain coal clarain coal type. Clarain is the major composition in coal seams and durain is the secondary. The coal structures of these coal samples include intact, cataclastic, granulated and mylonitized coal. We carried out proximate analysis for these samples in terms of China national standards “Proximate analysis of coal (GB/T 212-2008)”. 3.2. Isothermal adsorption experiment
Note: f ¼ P/10 (P is the uniaxial compressive strength of rock, MPa)
Integrated, great hardness, blocky
Basic granule size/ mm Broken state
Broken into pieces and no significant angular structure Traceable stripe structure and angular nubby structure Stripe texture and horizontal bedding structure Layered and blocky structures with distinct bands
Compressed into blocks or solid particles, oriented into foliation and mostly scaly, with crumpled slip surface and mirror surface o1 Powder, pulverized into solid block, powder
Not easy to identify Visible Clearly visible Clearly visible
Visibility of macroscopic lithotype Texture and Structure
Granulated coal Cataclastic coal Blocky coal Integrated coal
Deformed coals Intact coals Characteristics contrast
Table 1 The characteristics of different structure coals (after China national standard “Classification of coal-body structure” (GB/T 30050-2013), modified).
Unable to identify
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Mylonitized coal
858
3.2.1. Experimental instrument The experiment instrument was an ISO-300 isothermal adsorption and desorption instrument made by TerraTek (USA). The measuring range of the equipment was 0–70 MPa in pressure and the upper limit of temperature was 150 °C. The purity of adsorbent gas (methane) was up to 99.99%. The measuring unit was arranged in the oil bath thermostat to keep the temperature constant. This measuring unit consisted of a stainless steel reference cell and a sample cell with capacities of 80 cm3 and 160 cm3, respectively. The reference cell and the sample cell are arranged in the oil bath thermostat container, and the resolution of the temperature measurement was controlled within 70.02 °C. The pressures in reference cell and the sample cell were monitored only with high-precision pressure transducers with an accuracy of 70.10%. The temperatures of the reference cell and sample cell were kept constant during the experiment. The schematic diagram of isothermal adsorption experimental setup is shown in Fig. 1. 3.2.2. Experimental method In this study, the testing procedure followed the national standard of “Experimental Method of High-Pressure Adsorption Isothermal to Coal-Capacity Method” (GB/T19560-2008). For isothermal adsorption test, 100–120 g coal sample with the size of 0.2–0.25 mm (60–80 mesh) were selected. Before the isothermal adsorption experiment, proximate analysis was conducted to determine the moisture, ash, volatile matter, and fixed carbon content of the coal samples and to allow a conversion of adsorption volume of the coal samples under different conditions. According to the coal reservoir conditions, isothermal adsorption experiments were performed at a maximum pressure of 14 MPa and at temperatures of 25 °C, 35 °C, and 45 °C, and under equilibrium moisture condition.
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
859
Pressure transducer
Water bath
Sample cell
Globe valve
Reference cell
Gas inlet
Vent
Vacuum pump Fig. 1. schematic diagram of isothermal adsorption experimental setup.
3.3. Measurement of pore size distribution using a low temperature nitrogen adsorption and desorption method
adsorbate can be described as follows (Ramirez-Pastor and Bulnes, 2000; Bai et al., 2014).
ASAP2020 specific surface area and porosity analyzer produced by America Micromeritics Instrument Corporation was used to measure the pore size distribution using a low temperature nitrogen adsorption and desorption method. The coal samples were crushed into particles of 0.2–0.25 mm in diameter (60–80 mesh). The mass of each sample for this experiment was 2–3 g. The adsorbate was nitrogen with purity of nearly 99.999%. The temperature was reflected with liquid nitrogen and the particle size was 0.35–500 nm in diameter. We placed the dried and degassed sample into dewars full of liquid nitrogen. The experiment system started the adsorption experiment under the preset pressure to obtain nitrogen adsorption under different pressures. General models to describe the specific surface area and volume of solid porous media are Brunauer–Emmett–Teller multimolecular (BET) layer adsorption formula and Barret–Joyner–Halenda (BJH) model, separately (Gregg et al., 1967).
ε=
3.4. Calculation of adsorption potential and surface free energy related to isothermal adsorption 3.4.1. Adsorption potential and adsorption space volume The concept of adsorption potential was first applied to physical adsorption by Polanyi in 1914. This concept reflects the variation of Gibbs free energy under the adsorption of 1 mol mass. Adsorption potential theory assumes that solids have surface gravity field just like the planets. Thus, the adsorbed molecules around solids can be adsorbed under the effect of surface force field. These adsorbed molecules can also form multilayer adsorption on the surface of the solids. Unlike Langmuir monolayer model, the adsorption potential theory needs no physical model. The gas adsorption in micropores is considered as a process of volume filling based on potential intensity rather than layer upon layer. Volume filling theoretical system, derived from adsorption potential theory, is the most well shaped and practical system to describe the adsorption behavior of microporous adsorbents. The characteristic curve of adsorption potential is not dependent on temperature because adsorption potential represents the work accomplished with the temperature-independent dispersion forces. The work accomplished with adsorption potential field during the transform from gaseous to adsorbed state for unit mass of
∫P
P0
i
P RT dP = RTln 0 P Pi
(1)
where ε is the adsorption potential, J/mol; P0 is the saturation vapor pressure of the adsorbate at temperature T, MPa; Pi is the equilibrium pressure of ideal gas under a constant temperature, MPa; P is the equilibrium pressure, MPa; R is the Universal Gas Constant, J/(mol K); and T is the absolute temperature, K. The temperatures at which the coal adsorbs methane are usually above the critical temperature of the methane. Therefore, Dubinin proposed an empirical formula to calculate the virtual saturation vapor pressure of the adsorbate under supercritical conditions:
P0 = PC (
T 2 ) , TC
(2)
where Pc is the critical pressure, MPa; and Tc is the critical temperature of adsorbate, K. For methane, Pc is 4.62 MPa, and Tc is 190.6 K. Adsorption space volume represents the volume occupied by the adsorbed gas in the adsorbents, and it can characterize the micropore structure. It is presented as follows.
w=Nad
M , ρad
(3) 3
where w is adsorption volume, cm /g; M is the molecular mass of adsorbate, g/mol; Nad is the volume of adsorbate in adsorbed state, mol/g; and ρad is the density of adsorbate in adsorbed state, g/cm3. The empirical formula to calculate ρad is presented as follows:
ρad =
8PC M. RTC
(4)
3.4.2. Surface free energy A coal macromolecule is composed of a core of two or three condensed aromatic rings (20–80% organic carbon) and alkyl groups and functional groups on the periphery. When the surface pores are well developed, the vacancy on one side of the carbon atoms on the surface leads to force unbalances. Thus, carbon atoms tend to move inward under the influence of the inward force. The energy obtained in such a tendency is defined as surface
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
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free energy. According to surface chemistry theory (Jian et al., 2014), the Gibbs equation describing surface tension reduction during adsorption of methane in coal is as follows.
−dσ = RT Γd(lnP),
(5)
where s is the surface tension, J/m2; and Γ is the surface excess concentration, mol/m2, which is the methane concentration difference between the surface and inside of the coal:
Γ=
V , SV0
(6)
where V is the methane adsorption volume, L; V0 is the molar volume of gas under standard condition, 22.4 L/mol; and S is the specific surface area of coals, m2/g. V can be determined with the Langmuir equation:
V=VL·P/(PL +P),
(7) 3
where VL is the Langmuir volume, cm /g; and PL is the Langmuir pressure, MPa. VL represents the greatest adsorption capacity of coals and PL is the equilibrium pressure at which the corresponding adsorption volume reaches one-half of the Langmuir volume. Eq. (6) is solved for Γ, and Eq. (7) is solved for V. Then, Γ and V are substituted into Eq. (5). Eq. (5) can be integrated and rewritten as follows.
∆γ=
VLRT P ln (1+ ), V0S PL
(8)
where Δγ is the variation of surface free energy of coals due to gas adsorption, J/m2. The change of surface free energy at a given pressure can be obtained with the differentiation of Eq. (8), and the formula is as follows:
∆γP =
VLRTPL . V0S(PL + P )
(9)
4. Results and discussions 4.1. Adsorption properties of different structure coals Proximate analysis results of the coal samples were listed in Table 2. The isothermal adsorption experimental results are shown in Fig. 2. Note that, in this paper we just showed the error bars of experimental data with an assumed relative error of 7 5% and we did not aware of the real experimental errors. From Fig. 2 and Table 3, we can see that the isothermal adsorption curves of four coal samples show slight differences at 0– 2 MPa pressure. As the pressure rises, the methane adsorption capacities of different coals are shown in a decreasing order: mylonitized coal 4granulated coal 4 cataclastic coal 4 intact coal. The higher the pressure is, the greater the differences of methane adsorption volume for different coal type are. Consequently, under the same temperature, the Langmuir volume (VL) is in sequence of
Fig. 2. Isotherm adsorption of methane on various coals under different temperatures.
mylonitized4granulated 4 cataclastic 4intact. Coal methane adsorption capacities are significantly different under high temperature (45 °C) in this study. However, Pan et al. (2012) proposed that the various deformed medium rank coal samples (with a vitrinite reflectance of about 0.9%) do not show significant differences in coal methane adsorption capacity at high temperature (50 °C), and their adsorption isotherms are similar. The samples in
Table 2 Proximate analysis results of coal samples. Sample ID.
Coal structure
Temperature (°C)
Humidity (%)
Moisture content (%)adM
Ash (%)
Volatile (%)
ZZ-1 ZZ-2 ZZ-3 ZZ-4
Intact coal Cataclastic coal Granulated coal Mylonitic coal
15 15 15 15
17 17 17 17
1.3 0.77 0.83 1.02
11.04 11.91 12.7 9.83
9.76 8.65 10.23 5.42
Note: adM, air-dried moisture content
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Table 3 Adsorption capacities of different structural coals. Sample ID Coal structure Temperature °C Langmuir Volume, cm3/g-
Langmuir Pressure, MPa
ZZ-1
Intact
25 35 45
32.26 28.41 27.25
2.40 2.42 2.74
ZZ-2
Cataclastic
25 35 45
37.04 34.60 32.57
2.44 2.72 3.04
ZZ-3
Granulated
25 35 45
36.76 34.84 33.56
2.23 3.12 3.07
ZZ-4
Mylonitized
25 35 45
40.32 36.50 36.36
2.52 2.66 3.61
this study are high-rank coals. The pore size distribution of coal varies significantly with coal rank and the methane adsorption capacity is much higher for some high rank coals than for low and medium coals and thereby the variation law of methane adsorption capacity under different temperatures are quite different. Coal is a porous medium, which contains large internal surface area, and adsorption is one of its natural attributes. Given the Van der Waals’ force between methane molecules and inner surface of coal matrix, most methane molecules in the coal exist as an adsorbed state mainly on the surface of coal matrix, which is also known as physical adsorption. The structures of coal and graphite are similar. Kaplan (1986) maintained that the binding energy between two aromatic microchip layers in the graphite is approximately 5.4 kJ mol 1, caused by the Van der Waals’ force. The liquefaction heat of methane is merely 9.4 kJ mol 1, implying that
861
its saturated bonding structure exhibits chemical inertness state. The nature of methane adsorption is a mutual attraction between methane molecule and coal macromolecule when the methane molecules stay on the pore surface of coal for a short period. The adsorption capacity of coal is a function of temperature, adsorbate, pressure, and coal properties. 4.2. Effect of temperatures and pressures on coal methane adsorption Fig. 3 shows that the adsorption capacity of coal is closely related to the temperature. At a certain pressure, the methane adsorption volume decreases as the temperature increases. Conspicuous differences have been shown on adsorption capacity of coal to methane under various pressures. As a whole, the methane adsorption volume increases with the increment of pressure for all coal samples but in different scales. The adsorption curves of different structural coals rise linearly under low pressures; as the pressure increases, the curves rise gently and eventually become flat, thereby reaching a saturated adsorption volume. The shape of each curve subdues to Langmuir equation. Coupling effects have been achieved through temperature and pressure on adsorption of coal to methane. Figs. 2 and 3 illustrate that the adsorption curves at low pressure rise dramatically with the increase of pressure, and these curves are similar under different temperatures. This observation demonstrates that pressure is the leading factor controlling methane adsorption, and adsorption capacity is more sensitive to pressure variation than temperature. At high pressure, the growth rate of adsorption volume decreases rapidly, and the discrepancy of adsorption curves among different temperatures increases, which is mainly triggered by temperature. Accordingly, we can conclude that pressure has a deeper influence on the methane adsorption capacity than the temperature does at low
Fig. 3. Isothermal adsorption curves of various coals (a—intact coal; b—cataclastic coal; c—granulated coal; d—mylonitized coal).
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
55.47 80.06 80.30 87.68 44.53 19.94 19.70 12.32 0.008876 0.036830 0.057817 0.232361 0.007124 0.009170 0.014183 0.032639
0.016 0.046 0.072 0.232
Adsorption pore Mesopore
26.52 42.08 41.51 50.07 73.48 57.92 58.49 49.93 Undeformed structure Fragmented structure Granulated structure Mylonitized structure ZZ-1 ZZ-2 ZZ-3 ZZ-4
Note: adsorption pore includes micropore and transitional pore
0.000381 0.000614 0.000962 0.002475 0.000101 0.000258 0.000399 0.001239
Coal structure
Adsorption pore
pressures; while the temperature gradually becomes the leading factor affecting the methane adsorption at high pressures. 4.3. Effect of porous characteristics on methane adsorption in coal
0.000280 0.000356 0.000563 0.001236
Adsorption pore Mesopore Mesopore
Fig. 4. Characteristics of pore distribution of various coals (a—pore volume; b— pore surface area).
Sample ID
Table 4 Pore distribution of the coal samples.
Total volume
Pore volume proportion (%) Pore volume (ml/g)
Pore surface area (m2/g)
Total pore surface area
Mesopore
Adsorption pore
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Pore surface area proportion (%)
862
Coal is a dual-porosity media, hereby, we use the results of lowtemp liquid nitrogen adsorption to characterize the pore size distribution. Coal matrix pores are always complex and heterogeneous and can be classified into four types in terms of their genesis: primary pore, metamorphic pore, epigenetic pore and mineral pore. Adsorption capacity of coal is mainly related to nano-pores of organic matter, so we focus on the pore size distribution. Coal porous structure refers to different shapes, size distribution, and interconnected relationships of pores and pore throats in coal. It is also generally represented by characterization of pore specific surface area, pore volume, pore model, and pore size distribution, which can be acquired by low-temperature nitrogen adsorption and desorption experiment. Pore size classification is based on the standard proposed by Hotot (1961), which divided pores into micropores ( o10 nm), transitional pores (10–100 nm), mesopores (100–1000 nm), and macropores (4 1000 nm) via a spatial scale. In his work, both micropores and transitional pores are called adsorption pores. The experimental results of lowtemperature nitrogen adsorption and desorption test about pore volume and specific surface area of different coals are shown in Table 4 and Fig. 4. The pore size and pore volume distribution of coal samples were further calculated with BJH model. This model is the recommended one in China national standards “Analysis of mesopores and macropores by gas adsorption (GB/T 21650.2— 2008/IS0 15901-2: 2006)”. And it is widely used by many researches (Tian et al., 2015; Ross and Bustin, 2009; Wang et al., 2014).
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
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863
Fig. 5. Characteristic curve of adsorption potential on various coals at 35 °C.
However, this model has its own shortcomings, because it assumes that (1) the pores are cylindrical (more precisely that pore volume and capillary volume are related to each other as the square of some adequate measure of their cross sections) and (2) the amount of adsorbate in equilibrium with the gas phase is retained by the adsorbent by two mechanisms: (a) physical adsorption on the pore walls and (b) capillary condensation in the inner capillary volume (Barrett et al., 1951). While the pores of coal matrix do not strictly conforms to these two assumptions. Table 4 and Fig. 4 show the measurement range covers from 1.7 to 435.9 nm in diameter and the pores less than 1.7 nm will contribute to the surface area and adsorption capacity but are not within the measurement range of low-temperature nitrogen adsorption method. Fig. 4 illustrates that the pore volume and specific surface area increase as the deformation degree of coal increases from the intact coal sample to the mylonitized coal. The pore volume and specific surface area for the intact, cataclastic, and granulated coal samples are similar, whereas the pore volume and specific surface area of the mylonitized coal sample are considerably higher than those of the other three. In general, the pore volume and the specific surface area of pores with different sizes are in the decreasing order: mylonitized coal 4 granulated coal 4cataclastic coal 4intact coal. This observation indicates that the mesopores contribute most to the total pore volume, whereas the adsorption pores have the greatest contribution (more than 50%) to the total specific surface area in all coal types. However, the mesopores provide a relatively small contribution to the specific surface area. Moreover, the proportions of both adsorption pore and mesopore in the pore volume and specific surface area increase from the intact coal to the mylonitized coal. 4.4. Analysis on adsorption potential and surface free energy calculation results 4.4.1. Adsorption potential and adsorption space volume The characteristic curve of adsorption potential is not dependent on the temperature because the adsorption potential represents the work accomplished with the temperature-independent dispersion forces. According to the adsorption potential theory, the characteristic curve of adsorption potential at a temperature of 35 °C is drawn here as a representative. Fig. 5 illustrates that the adsorption potential of mylonitized coal is the largest, followed by granulated coal, cataclastic coal, and intact coal under the same adsorption space volume. The greater the adsorption potential is, the less the adsorption pressure needed for adsorbing the same methane volume is. Hence, the coals with a high adsorption potential have a strong adsorption capacity, which is consistent with the results of the isothermal adsorption experiment. As the adsorption space volume is approximate to 0,
Fig. 6. Cumulative reduction of surface free energy on various coals (a—25 °C, b— 35 °C, c—45 °C).
characteristic curves approach Y axis. The adsorption potential is strong in micropores. Moreover, methane molecules occupy pore volume sequentially through volume filling mechanism in terms of the adsorption potential strength. Thus, the micropores are first occupied by the methane molecules. Adsorption potential decreases as adsorption space volume increases. Thus, adsorption occurs later in mesopore and macropore than in micropore. The strong adsorption potential in micropores is the result of the overlapped Van der Waals’ force from the pore walls, given that the micropore diameter is considerably small. 4.4.2. Surface free energy The results of surface free energy calculation are shown in Figs. 6–8. Fig. 6 illustrates that the mylonitized coal sample has the maximum surface free energy variation under various temperatures, followed by granulated coal, cataclastic coal, and intact coal. As we know, any object tends to decrease its surface free energy because an object stabilizes with minimal surface free energy.
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
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Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
Fig. 8. Surface free energy change on granulated coals under various temperatures (a—cumulative reduction, b—reduction rate).
Fig. 7. Reduction rate of surface free energy on various coals (a—25 °C, b—35 °C, c —45 °C).
Coals decrease their surface free energy through methane adsorption; the larger the methane adsorption volume is, the greater the reduction of surface free energy is. Therefore, surface free energy can reflect the methane adsorption capacity of various coals. Fig. 7 shows the reduction of surface free energy under given pressures. Surface free energy variation decreases at high pressures, indicating that adsorption behavior becomes increasingly difficult as the adsorption continues. Adsorption potential theory indicates that methane occupies the spots sequentially in terms of potential strength. Compared with the cumulative reduction of surface free energy in Fig. 6, the rate of surface free energy reduction exhibits an inverse relationship, that is, the rate of surface free energy is the highest for the intact coal sample and is the lowest for the mylonitized coal sampleunder each pressure point. Temperature is a macroscopical representation of microcosmic molecules’ thermal motion. It has a significant effect on the kinetic energy of molecules. For the granulated coal, the cumulative
reduction of surface free energy decreases with the increase of temperature (Fig. 8a). On the contrary, the reduction rate of surface free energy increases with the increase of temperature (Fig. 8b). Kinetic energy is known to be directly related to temperature. Highly reactive methane molecules cannot be adsorbed easily. Thus, the methane adsorption capacity on coals is weak at a high temperature. The reduction rate of surface free energy decreases rapidly under each equilibrium pressure at high temperatures, because the kinetic energy of methane molecules increases with the increase of temperature. The nature of gas adsorption onto solid is characterized by the attraction forces between gas molecule and solid molecule (Li et al., 2014). Coal adsorption to methane is physical adsorption at normal temperatures. In fact, physical adsorption is caused by Van der Waals’ forces, which mainly consist of electric forces, introduction forces, and diffusion forces. Some researchers proved that among the three forces, the diffusion force is the major force related to the methane adsorption on coals. For example, Chen et al. (2000) calculated the adsorption potential of methane adsorption on coals by adopting a quantum chemistry approach. And they concluded that the adsorption potential is mainly caused by the diffusion force based on the potential range calculated. Kaplan (1986) held the view that the structure of the coal is similar to that of graphite. Thus, the binding energy of graphite among aromatic carbon layers of approximately 5.4 kJ/mol can demonstrate that the leading diffusion force existed in graphite is also predominant in coals. In methane adsorption process in coal, methane molecules lose a certain amount of potential energy under the attraction forces. Then, the molecules enter the adsorption potential well, which are consequently adsorbed. The decrease of energy accompanies the release of heat. The more frequently potential energy changes, the more the energy will be released in the
Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i
Z. Meng et al. / Journal of Petroleum Science and Engineering 146 (2016) 856–865
system and the easier the adsorption behavior will perform, thereby the coal has a great adsorption capacity. Thus, the adsorption capacity of coals can be represented by surface free energy variation.
5. Conclusions
(1) The methane adsorption capacity of various coals in the study area could be arranged in a decreasing order: mylonitized coal 4granulated coal 4cataclastic coal 4intact coal. The methane adsorption capacity of the deformed coal is considerably higher than that of the intact ones in this study area. Methane adsorption capacity decreases with the increase of temperature. Pressure has a more significant effect on the methane adsorption at low pressures than temperature, whereas with the rise of pressure, temperature gradually becomes the leading factor influencing adsorption capacity. (2) In various ranges of pore size, the pore specific surface area and pore volume for the coal samples in this study area could be listed in a decreasing order: mylonitized4 granulated4cataclastic 4intact. This result is consistent with their isothermal adsorption capacities. The pore volume is mainly dominated by the mesopores and specific surface area is largely controlled by the adsorption pores. (3) Under the same adsorption space volume, the adsorption potential and surface free energy of different-structure coals could be presented in a decreasing order: mylonitized4 granulated4cataclastic 4intact. The adsorption potential decreases with the increase of adsorption space and the adsorption potential of micropore is much higher than that of mesopore and macropore. (4) The rise of temperature results in a decrease of the cumulative reduction of surface free energy and a significant decrease of the reduction rate of surface free energy under each equilibrium pressure. The adsorption of methane in coal is dominated by the adsorption potential and the surface free energy.
Acknowledgements This work is financially supported by the National Basic Research Program of China (973 Program under Project no. 2012CB214705), National Natural Science Foundation of China (Grant nos. 41372163 and 41172145), Key Scientific and Technology Project of CBM Foundation in Shanxi (Grant nos. MQ2014-01 and MQ2014-12), Shanxi Provincial Basic Research Program—Coal Bed Methane Joint Research Foundation (Grant nos. 2015012014 and 2014012001), the National Science and Technology Major Project of the Ministry of Science and Technology of China During “13th Five-Year Plan” (Grant nos. 2016ZX05067001-006 and 2016ZX05067001-007) and the Natural Science Foundation of Hubei Province (Grant no. 2014CFB169). The authors are grateful to Key Laboratory of Integrated Extraction of Coal and Coal Bed Methane China National Energy Administration, Shanxi Jincheng Anthracite Mining Group Ltd. for the experimental support. We also thank Michael Dawson from Telamonn Energy Services Inc. in Canada for language revision.
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Please cite this article as: Meng, Z., et al., Adsorption capacity, adsorption potential and surface free energy of different structure high rank coals. J. Petrol. Sci. Eng. (2016), http://dx.doi.org/10.1016/j.petrol.2016.07.026i