Gas desorption characteristics of the high-rank intact coal and fractured coal

International Journal of Mining Science and Technology 25 (2015) 819–825

Contents lists available at ScienceDirect

International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst

Gas desorption characteristics of the high-rank intact coal and fractured coal Lu Shouqing a,b,⇑, Cheng Yuanping a,b,c, Qin Liming a,b, Li Wei a,b, Zhou Hongxing a,b, Guo Haijun a,b a

National Engineering Research Center of Coal Gas Control, China University of Mining & Technology, Xuzhou 221116, China School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China c Key Laboratory of Gas and Fire Control for Coal Mine, China University of Mining & Technology, Xuzhou 221116, China b

a r t i c l e

i n f o

Article history: Received 3 January 2015 Received in revised form 27 February 2015 Accepted 17 April 2015 Available online 5 August 2015 Keywords: Intact coal Fractured coal Isosteric adsorption heat Diffusion coefficient

a b s t r a c t The objective of this work is to study the gas desorption characteristics of the high-rank intact coal and fractured coal. The gas adsorption, mercury porosimetry and gas desorption experiments were carried out in this study. Then, the theories of thermodynamics, diffusion mechanism and desorption kinetics were used to estimate the gas desorption characteristics. The results of gas adsorption experiments show that the initial isosteric adsorption heat of the intact coal is greater than that of the fractured coal, indicating that the gas molecules desorb more easily from fractured coal than intact coal. Using the mercury porosimetry, we find that the diffusion channels of fractured coal are more developed than those of intact coal. The difficult diffusion form dominates in the intact coal during the gas diffusing, while the easy diffusion form dominates in the fractured coal. The results of gas desorption experiments show that the initial gas desorption volume and velocity of the fractured coal are both greater than those of the intact coal. Using the Fick diffusion law, the study calculates the gas diffusion coefficients of the intact coal and fractured coal. The diffusion coefficients of the fractured coal are 2 times and 10 times greater than those of the intact coal at the time of 0–120 and 0–10 min, respectively. Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

1. Introduction Under the comprehensive function of geostress, gas pressure and coal structure, coal and gas outburst is caused by mining [1–3]. Certain conditions must be met for the occurrence of coal and gas outburst. It is found that most of the coal and gas outburst accidents occurred in the thick fractured coal zone [4]. Coal is a special type of rock with a low strength and high Poisson’s ratio and is uniquely sensitive to stress and strain. Therefore, fractured coal can well record the effect of tectonic stress in stratum [5,6]. The thickening of the fractured coal zones increases the capacity for gas storage. It may act as tectonic screens, blocking gas migration, which can lead to high-pressure pockets of gas pressure. Therefore, a thick zone of the fractured coal provides favorable conditions for coal and gas outbursts [7]. Intact coal means that the coal is not or less affected by the tectonic stress, so its intact structure remains relatively complete. Fractured coal is the one that experiences plastic, ductile and flowing deformation due to repeated tectonic activities. Gas desorption

⇑ Corresponding author. Tel.: +86 13914895276. E-mail address: [email protected] (S. Lu).

characteristics of the fractured coal play a key role in the occurrence and development of coal and gas outburst [8]. Li found that the initial gas desorption velocity of the fractured coal was 1.36– 2.84 times greater than that of the intact coal [9]. Wen thought that the gas desorption velocity of the fractured coal was much higher than that of the intact coal when the CH4 desorption volumes were compared in the same place [10]. Zhang suggested that the gas desorption volume and diffusion coefficient of the fractured coal in the first 60 min were both higher than those of the intact coal [11]. Using an MD method for simulation, Jing found that the diffusion coefficient of the fractured coal was greater than that of the intact coal. The results also showed that the activation energy for diffusion in the fractured coal was less than that in the intact coal, so it is easier for gas molecules to diffuse in the fractured coal [12]. Despite many achievements of the gas desorption characteristics have been obtained, most of them focused on the desorption kinetics of the intact coal and fractured coal. Few of the results are made to comprehensively analyze the thermodynamics, diffusion mechanism and desorption kinetics. Considering those factors above, the study investigates the gas desorption characteristics of the high-rank intact coal and fractured coal. It not only can provide a new research method, but also will help us to find the reason for

http://dx.doi.org/10.1016/j.ijmst.2015.07.018 2095-2686/Ó 2015 Published by Elsevier B.V. on behalf of China University of Mining & Technology.

820

S. Lu et al. / International Journal of Mining Science and Technology 25 (2015) 819–825

the greater gas desorption capacity of the fractured coal. Finally, it is greatly important for the reasonable prediction and prevention of the fractured coal and gas outburst. 2. Method and theory

ffiffi

DHad p = T RT

bðTÞ ¼ b0 e

ð2Þ

where b0 is a constant associated with the molecular weight of adsorbed gas; DHad the adsorption enthalpy, J/mol; R the ideal gas constant of 8.414 J/(mol K); and T the equilibrium temperature, K. Eq. (2) can be rewritten as follows:

2.1. Experimental coal samples and method

pffiffiffi DHad lnðb T Þ ¼ lnðb0 Þ  RT

The coal samples were selected from the No. 3 coal seam of Daning Coal Mine. With the hazards of coal and gas outbursts, the No. 3 coal seam is a high-rank anthracite. The No. 3 coal seam often contains several intact coal and fractured coal sub-layers, and the middle-lower part of the No. 3 coal seam is a deformed sub-layer with a thickness of 0.2–0.5 m. Fig. 1 shows the scene of the intact coal and fractured coal sub-layers distribution. The intact coal and fractured coal for the gas desorption experiments were selected from different sub-layers at the same location. The coal samples were crushed into the particle with the size 1–3 mm. 99.9% CH4 gas and the gas desorption instrument of geological exploration (FHJ-5) were used in the gas desorption experiments. First, the 50 g coal samples were placed into the sample cell. The sample cell was connected to a vacuum pump at a pressure of 13 kPa for 24 h. Second, CH4 was injected into the sample cell. The sample cell was placed in a water bath at a temperature of 303 K. Finally, the samples were held at a desired high pressure (0.8, 1.3, 2.4, 3.3 or 4.0 MPa) for a few days to reach the sorption equilibrium. Then, the valve of the cell was opened. Until the pressure gauge of the cell was 0 MPa, the sample cell was connected to the gas desorption instrument of geological exploration (FHJ-5) for 120 min. Fig. 2 presents the main equipments for the gas desorption experiments. Table 1 displays the key parameters, including the proximate analysis, macerals, the maximum vitrinite reflectance and firmness coefficient of the intact coal and fractured coal.

pffiffiffi According to Eq. (3), there is a linear relation between lnðb T Þ and 1/T, and the adsorption enthalpy (DHad) and constant (b0) can be calculated by the formula fitting method. The isosteric adsorption heat (Qst) can be defined as the enthalpy change (DHst) when n mol gas is adsorbed by adsorbent with a constant surface area (S) at a constant temperature and pressure. The isosteric adsorption heat can indirectly reflect the intermolecular forces between adsorbent and adsorbate molecules. According to the Clausius–Clapeyron equation, the isosteric adsorption heat can be expressed as follows [16]:

2.2. Thermodynamic model for the isosteric adsorption heat There are numerous theoretical isothermal sorption models, including Langmuir monolayer sorption model, BET multilayer sorption model and Polyanyi sorption model. The adsorption behavior and mechanism of different gases on coal were discussed by various international and domestic academics [13,14]. The Langmuir model is commonly used to research the adsorption characteristics of coal in its concise form. Therefore, Langmuir model and its equation used in this paper are as follows:

V¼

abP 1 þ bP

ð1Þ

where a is the limiting adsorption volume of dry ash free, mL/g daf; b an adsorption constant, MPa1; and V the gas adsorbed volume of coal per unit mass at a constant temperature and pressure, mL/g daf. According to the thermodynamics, the relation between b in the Langmuir adsorption model and the temperature T can be expressed as follows [15]: Intact coal

Q st ¼ DHst ¼

    @ DHst @ ln P ¼ RT 2 @T V @n T;P;S

ð3Þ

ð4Þ

Chen believes that there is a linear relation between Langmuir adsorption constant a and temperature T [17]. The Eq. (2) is connected to the Eq. (4), then

Q st ¼ DHst ¼ DHad þ

RT RT 2 @a þ 2 V  a @T

ð5Þ

where R is a known quantity. And the relation of a and temperature T, @a=@T, DHad and b0 can be calculated. Therefore, the isosteric adsorption heat Qst is a function of gas adsorbed volume V and temperature T. 2.3. Dynamics model for the diffusion coefficient Gas diffusion in coal is monitored by Fick diffusion law. The diffusion coefficient D is one of the most important kinetic parameters for gas diffusion in coal. It can intuitively reflect the ability of gas diffusion in coal. The method that fitting the data of the gas desorption experiments is the simplest one for getting the diffusion coefficient. In this case, the coal particles are supposed into spherical particles of the same size. Based on the Fick diffusion law, the differential equation can be solved under the given initial conditions and boundary conditions. The analytical solution can be obtained as follows [18]:

1

1 Qt 6 X 1 n2 Bt ¼ e Q 1 p2 n¼1 n2

ð6Þ

where Qt is the gas accumulated desorption volume of the coal particles at time t, mL/g; and Q1 the gas limiting desorption volume of the coal particles, mL/g; B = pD/r2; D the diffusion coefficient, and r the radius of spheres, m2/s. The analytical solution is numerically simulated by the computer, then the equation can be obtained as follows [19]:

Q t =Q 1 ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1  eKBt

ð7Þ

Fractured coal

where K, a constant, is equal to 1. Eq. (7) can be rewritten as follows:

lnð1  ðQ t =Q 1 Þ2 Þ ¼ KBt

Fig. 1. Scene of the intact coal and fractured coal sub-layers distribution.

ð8Þ

According to Eq. (8), there is a linear relation between ln(1  (Qt/Q1)2) and time t. KB is the slope of the line. In the laboratory, t and Qt are both known quantities. The gas limiting desorption volume (Q1) can be calculated by Eq. (9) [20]. Then, KB can be gained by linear regression. The diffusion coefficient (D) is got.

821

S. Lu et al. / International Journal of Mining Science and Technology 25 (2015) 819–825 Pressure gauges

Pressure gauges

Decompression cell

Four-way valve Water bath Sample cell

Vacuum pump

CH 4

Measuring cylinder

Fig. 2. Main apparatuses for the gas desorption experiments.

Table 1 Key parameters of the intact coal and fractured coal. Sample no.

Proximate analysis (%, by weight)

Macerals (%, by volume)

Mad

Aad

Vdaf

Vitrinite

Inertinite

Mineral

Intact coal Fractured coal

1.46 1.95

18.14 22.15

12.29 11.01

75.79 84.35

21.87 6.63

2.44 6.02

Ro,max (%)

Firmness coefficient

3.16 3.29

2.00 0.33

Table 2 Langmuir adsorption constants of the intact coal and fractured coal. Sample no.

Langmuir constant

Intact coal

Constant Constant Constant Constant

Fractured coal

a b a b

(mL/g daf) (MPa1) (mL/g daf) (MPa1)

303 (K)

313 (K)

323 (K)

333 (K)

51.8892 1.3778 50.7703 1.1005

49.1426 1.2013 48.5056 0.9786

46.1807 1.0902 45.5454 0.9291

44.5580 0.9247 43.0619 0.7862

 Q1 ¼

 abP 1 abP 0  ð1  wf  Af Þ  1 þ bP1 1 þ bP 0

Temperature

ð9Þ

where P1 is the definite equilibrium pressure, MPa; P0 the atmospheric pressure in the laboratory, MPa; wf the moisture of coal, %; and Af the ash of coal, %.

3. Results and discussion 3.1. Thermodynamics characteristics of the coal samples To obtain the isosteric adsorption heats, the isothermal CH4 adsorption experiments of the coal samples are carried out at different temperatures (303, 313, 323 or 333 K). The experimental data are corrected by the moisture and ash. Then, the Langmuir adsorption constants of dry ash-free are obtained as shown in Table 2. pffiffiffi According to Eq. (9), the linear relations between lnðb T Þ and 1/T are shown in Fig. 3(a). The calculated DHad and b0 are shown in Table 3. The linear relations between a and T are shown in Fig. 3(b), and the fitting relationships are listed in Table 3. The parameters in Table 3 are taken into the Eq. (9). Then, the relations for the isosteric adsorption heats of the intact coal and fractured coal are obtained as shown in Fig. 4. There are two reasons for the isosteric adsorption heat changing with the adsorption volume [21]. First, the intermolecular forces between adsorbent and adsorbate molecules will increase with

an increase of adsorption. It will lead to the increase of isosteric adsorption heat with the increase of adsorption. Second, the surface of coal is uneven and anisotropic, which leads to the unequal adsorption energy on the coal surface [22]. The gas molecules are initially adsorbed on the coal surface with higher adsorption energy. It will intensify the non-uniformity phenomenon on the coal surface. Then, the isosteric adsorption heat decreases with the increase of adsorption. At the same temperature, the isosteric adsorption heats of the coal samples increase slowly with the increase of adsorption. Therefore, the intermolecular forces come first, then followed by the uneven and anisotropic of the coal surface. At the same adsorption volume, the isosteric adsorption heats will increase with the increase of temperature. Although this will increase the chances of the gas molecules colliding with the coal surface, the coal surface needs more adsorption potential energy to capture the gas molecules. In this case, part of the coal surface cannot capture the gas molecules, while the gas molecules can be captured at a lower temperature. At the same conditions, the isosteric adsorption heat of the intact coal is greater than that of the fractured coal. The initial isosteric adsorption heats of the intact coal and fractured coal are 14.46 and 12.73 kJ/mol, respectively. The initial isosteric adsorption heat directly reflects the relation between coal surface and gas molecules [23]. The results demonstrate that the intermolecular forces of the intact coal are stronger than those of the fractured coal. Therefore, gas molecules desorb more easily from the fractured coal than the intact coal.

822

S. Lu et al. / International Journal of Mining Science and Technology 25 (2015) 819–825 60

3.3

Intact coal Fractured coal

Adsorption constant a (mL/g·daf)

3.2

ln(b*T 0.5)

3.1 3.0 2.9 2.8 2.7 0.0031

0.0032

50 45 40 35 300

2.6 0.0030

55

0.0033

305

310

320

315

325

330

335

Temperature T (K)

1/T (K-1)

(b) Adsorption constant b changes with temperature T

(a) Adsorption constant a changes with temperature T

Fig. 3. Adsorption constants a, b change with temperature T.

Table 3 Relation between Langmuir constant and temperature. pffiffiffi Relations between lnðb T Þ and 1/T pffiffiffi lnðb T Þ = 1143.8/T  0.5926 pffiffiffi lnðb T Þ = 907.77/T  0.0374

Sample no. Intact coal Fractured coal

DHad (J/mol)

b0

Relation between a and T

9509.55

0.5529

a = 0.2496T + 127.34

7547.20

0.9633

a = 0.2609T + 129.96

40

adsorption Isosteric heat of (kJ/mol)

adsorption Isosteric heat of (kJ/mol)

40

22.87 kJ/mol

30 16.11 kJ/mol

20

10 0

14.46 kJ/mol

330 5 Ad

10

15 sor pti 20 (mL on co / g nstan ·da f) t

310 25

e( tur era mp e T

22.24 kJ/mol

14.52 kJ/mol

20

12.73 kJ/mol

10 0

330 5

320

17.91 kJ/mol

30

K)

(a) Isosteric adsorption heat of the intact coal

10 16.50 kJ/mol sor 15 pti 20 (mL on co /g·d nstan 25 af ) t

Ad

320 ) (K re atu r e mp Te

310

(b) Isosteric adsorption heat of the fractured coal

Fig. 4. Isosteric adsorption heats of the intact coal and fractured coal.

3.2. Gas diffusion mechanism of the coal samples The mercury porosimetry experiments were carried out to compare the diffusion channels of the two coals. Fig. 5 shows the pore size distribution of the intact coal and fractured coal. According to Fig. 5, the fractured coal pores with a diameter of 10–1000 nm and >1000 nm are 4.8 and 12.3 times greater than those of the intact coal, respectively. However, the pores with a diameter less than 10 nm of the intact coal are greater than those of the fractured coal. The pores of the intact coal are primarily made up of the pores with a diameter less than 10 nm, accounting for 61.34%. The pores of the fractured coal are primarily composed of the pores with a diameter more than 10 nm, accounting for 84.72%. It is possible to conclude that the diffusion channels of the fractured coal are more developed than those of the intact coal. According to the molecular motion theory, gas diffusion in coal is caused by the irregular thermal motion of gas molecules. The gas diffusion model of coal can be classified according to the relations between the pore diameter and the mean free path of gas molecules [24]. When d P 10k, the collisions mainly occur between gas molecules, called as Fick diffusion. When 0.1k < d < 10k, the

collisions of gas molecules are equivalently important to the collisions of gas molecules and pore wall, named as transition diffusion. When d 6 0.1k, the collisions mainly occur between gas molecules and pore wall, termed as Knudsen diffusion. On the conventional condition, the mean free path of gas is 100 nm. The pore diameters for Fick diffusion, transition diffusion and Knudsen diffusion are d P 1000 nm, 10 nm < d < 1000 nm and d 6 10 nm, respectively [25]. Combining the pore size distribution in Fig. 5(b), the gas diffusion model of the intact coal comes Knudsen diffusion first, transition diffusion second and Fick diffusion last. However, the gas diffusion model of the fractured coal comes transition diffusion first, Fick diffusion second and Knudsen diffusion last. Therefore, the difficult diffusion form dominates in the intact coal during the gas diffusing, while the easy diffusion form dominates in the fractured coal. Because the gas in the macropores can easily diffusing outside at the beginning, Fick diffusion is the main form of gas diffusion [26]. According to Fig. 5, the pores with a diameter more than 1000 nm of the fractured coal are much greater than those of the intact coal. At the beginning of the gas desorption, the diffusion capacity of the fractured coal is much higher than that of the intact coal.

823

S. Lu et al. / International Journal of Mining Science and Technology 25 (2015) 819–825 0.06

0.0040

0.04 0.0219

0.0165

0.02

0.0104

0.0034

0.0025 0

10 nm

0.0020

1000 nm

10-1000 nm

Pore size (nm) 80

0.0015

Pore volume percent (%)

Pore volume (mL/g)

0.0030

0.0497 0.0418

Pore volume (mL/g)

Intact coal Fractured coal

0.0035

0.0010 0.0005

100

1000

10000

46.02

40 15.28

0

100000

38.7

29.14

20

0 10

61.34

60

9.52

10 nm

1000 nm

10-1000 nm

Pore size (nm)

Pore size distribution (nm) (a) Pore volume changing with pore size distribution

(b) Pore volume and pore volume percent distribution

Fig. 5. Pore size distribution of the intact coal and fractured coal.

20 0.8 MPa 1.3 MPa 2.4 MPa 3.3 MPa 4.0 MPa

60

Desorption volume (mL/g)

Desorption volume (mL/g)

80

40

20

0

20

40

60

80

100

16 12

8 4

0

120

20

40

60

80

100

Time (min)

Time (min)

(a) Gas desorption kinetics curves of the intact coal

(b) Gas desorption kinetics curves of the fractured coal

120

Fig. 6. Relations for gas accumulated desorption volume and time.

0

0

-0.01

-0.1

-0.02

0.10 0.05

-0.03 0

2

4

6

8

0.8 MPa 1.3 MPa 2.4 MPa 3.3 MPa 4.0 MPa

-0.15 -0.20

0

-0.3 0

-0.05 -0.10

-0.2

10 ln(1-(Qt/Q∞)2)

ln(1-(Qt/Q∞)2)

0

0.2

2

4

6

8

10

-0.2 -0.4 -0.6

-0.25

-0.8 0

20

40

60

80

100

0

120

20

40

60

80

100

120

Time (min)

Time (min) (a) Relations between ln(1-(Qt/Q∞)2)

(b) Relations between ln(1-(Qt/Q∞)2)

and time of the intact coal

and time of the fractured coal 2

Fig. 7. Relations between ln(1  (Qt/Q1) ) and time of the intact coal and fractured coal.

3.3. Characteristics of gas desorption kinetics of the coal samples 3.3.1. Characteristics of gas desorption kinetics curves of the coal samples The gas desorption experiments for gas desorption was carried out. Fig. 6 presents the relations for the gas accumulated desorption volume and time at different pressures.

According to Fig. 6, the gas accumulated desorption volume increases with time under a definite pressure. For any gas desorption curve, the initial slope of the curve is the largest, and the latter slope of the curve is less than the former one. Therefore, the initial gas desorption velocity is the largest. Then, the gas desorption velocity gradually decreases. For one coal sample, the gas desorption volume and desorption velocity at the same time are greater.

824

S. Lu et al. / International Journal of Mining Science and Technology 25 (2015) 819–825

Table 4 Gas diffusion coefficients of the coal samples. Sample no.

P (MPa)

0–120 (min) KB

0–10 (min) 2

D (m /s)

R 12

2

KB

D (m2/s)

R2 12

Intact coal

0.8 1.3 2.4 3.3 4.0

0.00152 0.00148 0.00167 0.00217 0.00220

2.5668  10 2.4993  1012 2.8201  1012 3.6645  1012 3.7151  1012

0.9994 0.9981 0.9988 0.9973 0.9960

0.00134 0.00141 0.00170 0.00250 0.00270

2.2628  10 2.3811  1012 2.8708  1012 4.2217  1012 4.5595  1012

0.9949 0.9973 0.9970 0.9989 0.9989

Fractured coal

0.8 1.3 2.4 3.3 4.0

0.00474 0.00388 0.00295 0.00303 0.00285

8.0044  1012 6.5521  1012 4.9816  1012 5.1167  1012 4.8128  1012

0.8227 0.7795 0.7855 0.7784 0.7580

0.02243 0.01818 0.01533 0.01556 0.01650

3.7877  1011 3.0700  1011 2.5888  1011 2.6276  1011 2.7863  1011

0.9593 0.9521 0.9439 0.9608 0.9262

At the same pressure, the initial gas desorption volume and velocity of the fractured coal are greater than those of the intact coal. After a certain period, the gas desorption curve of the fractured coal will be more flat than that of the intact coal, which suggests that the gas desorption velocity of the fractured coal will decrease more quickly than that of the intact coal. 3.3.2. Diffusion coefficient of gas desorption kinetics of the coal samples To compare the comprehensive and the initial diffusion coefficients, the experimental data of 0–120 and 0–10 min are fitted, respectively. Fig. 7 shows the relations between ln(1  (Qt/Q1)2) and time of the coal samples. Table 4 lists the gas diffusion coefficients. According to Fig. 7 and Table 4, we find that the Yang’s theory formula could well describe the gas desorption kinetics behavior of the intact coal at any time, with a high fitting precision (R2 > 0.99). It can also well describe that of the high-rank fractured coal at the time of 0–10 min, with a fitting precision (R2 > 0.90). However, it is fail to describe the gas desorption kinetics behavior of the fractured coal at the time of 0–120 min, with a low fitting precision (R2 < 0.90). At the time of 0–120 min, the gas diffusion coefficients of the intact coal and fractured coal are 2.4993  1012 to 3.7151  1012 m2/s and 4.8128  1012 to 8.0044  1012 m2/s, respectively. At the time of 0–10 min, the gas diffusion coefficients of the intact coal and fractured coal are 2.2628  1012 to 4.5595  1012 m2/s and 2.5888  1011 to 3.7877  1011 m2/s, respectively. The diffusion coefficients of the fractured coal are 2 times and 10 times greater than those of the intact coal at the time of 0–120 min and 0–10 min, respectively. The larger diffusion coefficient of the fractured coal is related to the weaker intermolecular forces and more macropores in it than that in the intact coal. In this paper, the range of the diffusion coefficient is between 1012 and 1011 m2/s, and it has the same order of magnitudes with the results obtained by Chen [27]. 4. Conclusions The theoretical analysis and experiments of the gas desorption characteristics are introduced, and a comprehensive analysis is made on the thermodynamics, diffusion mechanism and desorption kinetics. The conclusions are as follows: (1) From the thermodynamics aspect, the initial isosteric adsorption heat of 14.46 kJ/mol for the intact coal is greater than that of 12.73 kJ/mol for the fractured coal, indicating that the gas molecules desorb more easily from the fractured coal than the intact coal. (2) From the diffusion mechanism aspect, the pores with a diameter less than 10 nm account for 61.34% in the intact coal, and the pores with a diameter more than 10 nm

account for 84.72% in the fractured coal. The diffusion channels of the fractured coal are more developed than those of the intact coal. The difficult diffusion form dominates in the intact coal during the gas diffusing, and the easy diffusion form dominates in the fractured coal. (3) From the desorption kinetics aspect, the results show that the initial gas desorption volume and velocity of the fractured coal are both greater than those of the intact coal. The gas desorption velocity of the fractured coal will decrease more quickly than that of the intact coal. The diffusion coefficients of the fractured coal are 2 times and 10 times greater than those of the intact coal at the time of 0–120 min and 0–10 min, respectively. Therefore, the larger diffusion coefficient of the fractured coal is linked to the weaker intermolecular forces and more macropores in it than that in the intact coal.

Acknowledgments Financial support provided by the National Basic Research Program of China (No. 2011CB201204), the Natural Science Foundation for the Youth of China (Nos. 41202118 and 51204173). References [1] Cheng YP. Theories and engineering applications on coal mine gas control. Xuzhou: China University of Mining & Technology Press; 2010. [2] Wang HF, Cheng YP, Wang L. Regional gas drainage techniques in Chinese coal mines. Int J Min Sci Technol 2013;22(6):873–8. [3] Hungerford F, Ren T, Aziz N. Evolution and application of in-seam drilling for gas drainage. Int J Min Sci Technol 2013;23(4):543–53. [4] Zhang ZM, Zhang YG. Gas geology rule and gas prediction. Beijing: Coal industry Press; 2005. [5] Li M, Jiang B, Lin SF, Wang JL, Ji MJ, Qu ZH. Tectonically fractured coal types and pore structures in Puhe and Shanchahe coal mines in western Guizhou. Min Sci Technol 2011;21(3):353–7. [6] Jiang B, Qu ZH, Wang GGX, Li M. Effects of structural deformation on formation of coalbed methane reservoirs in Huaibei coalfield, China. Int J Coal Geol 2010;82(3–4):175–83. [7] Li HY. Major and minor structural features of a bedding shear zone along a coal seam and related gas outburst, Pingdingshan coalfield, northern China. Int J Coal Geol 2001;47(2):101–13. [8] Hao JS, Yuan CF, Zhang ZX. The tectonic coal and its effects on coal and gas outburst. J Jiaozuo Inst Technol (Nat Sci) 2000;19(6):403–6. [9] Li YB, Zhang YG, Zhang ZM, Jiang B. Experimental study on gas desorption of tectonic coal at initial stage. J China Coal Soc 2013;38(1):15–20. [10] Wen ZH, Wei JP, Wang DK, Wang C. Experimental study of gas desorption law of fractured coal. In: Proceedings of first international symposium on mine safety science and engineering, vol. 26; 2011. p. 1083–8. [11] Zhang ZG. Research on gas irradiation feature of tectonic coal. First international symposium on mine safety science and engineering, vol. 26; 2011. p. 154–9. [12] Jing W. Molecular simulation of adsorption and diffusion of methane in fractured coal. Taiyuan: Taiyuan University of Technology; 2010. [13] Clarkson CR, Bustin RM, Levy JH. Application of the mono/multilayer and adsorption potential theories to coal methane adsorption isotherms at elevated temperature and pressure. Carbon 1997;35(12):1689–705.

S. Lu et al. / International Journal of Mining Science and Technology 25 (2015) 819–825 [14] Dai SF, Zhang BB, Peng SP, Zhang XD, Chou CL. Models of pure CO2 and pure CH4 adsorption on the late paleozoic coals from the Kailuan coalfield, Hebei, China. Acta Geol Sin 2009;83(5):731–7. [15] Gasser RPH. An introduction to chemisorption and catalysis by metals. Oxford: Oxford Science Publishers, Clarendon Press; 1987. [16] Myers AL. Thermodynamics of adsorption in porous materials. Am Inst Chem Eng J 2002;48(1):145–9. [17] Chen CG, Xian XH, Zhang DJ, Xian XF. The dependence of temperature to the adsorption of methane on anthracite coal and its char. Coal Convers 1995;18(3):88–92. [18] Zhang YX. Geochemical kinetics. Boston: Princeton University Press; 2008. [19] Yang QL, Wang YA. Theory of methane diffusion from coal cuttings and its application. J China Coal Soc 1986;3:87–93. [20] Nie BS, Wang EY, Guo YY, Wu SY, Wei JF. Mathematical and physical model of gas diffusion through coal particles. J Liaoning Techn Univ (Nat Sci) 1999;18(6):582–5.

825

[21] Ruthven DM. Principle of adsorption and adsorption process. New York: John Wiley & Sons Inc.; 1984. [22] Yang RT, Saunders JT. Adsorption of gases on coals and heated-treated coals at elevated temperature and pressure. Fuel 1985;64:616–20. [23] Nodzen´ski A. Sorption and desorption of gases (CH4, CO2) on hard coal and active carbon at elevated pressures. Fuel 1998;77(11):1243–6. [24] He XQ, Nie BS. Diffusion mechanism of porous gases in coa1 seams. J China Univ Min Technol 2001;30(1):1–4. [25] Nie BS, He XQ, Wang EY. Mechanism and modes of gas diffusion in coal seams. China Saf Sci J 2000;10(6):24–8. [26] Jian X, Guan P, Zhang W. Carbon dioxide sorption and diffusion in coals: experimental investigation and modeling. Sci China: Earth Sci 2012;55(4):633–43. [27] Chen XJ, Cheng YP, He T, Li X. Water injection impact on gas diffusion characteristic of coal. J Min Saf Eng 2013;30(3):443–8.