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Pulverization characteristics of coal from a strong outburst-prone coal seam and their impact on gas desorption and diffusion properties
Journal of Natural Gas Science and Engineering 33 (2016) 867e878
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Pulverization characteristics of coal from a strong outburst-prone coal seam and their impact on gas desorption and diffusion properties Haijun Guo a, b, Yuanping Cheng a, b, c, *, Ting Ren c, Liang Wang a, b, **, Liang Yuan a, b, Haina Jiang a, b, Hongyong Liu a, b a b c
Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China National Engineering Research Center of Coal Gas Control, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China School of Civil, Mining & Environmental Engineering, University of Wollongong, NSW 2522, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2016 Received in revised form 8 June 2016 Accepted 13 June 2016 Available online 16 June 2016
Coal seams that are prone to strong outbursts have low strength and cause heavy structural damage to the seam. Their outburst risk is highly related to the release of the adsorbed coalbed gas, which is controlled by the gas desorption and diffusion characteristics of coal. In the Haizi Coal Mine, China, an extremely high gas outburst risk was detected, and the coals from this area were found to have an unprecedented high degree of fragmentation and were present in the pulverized state. To explain the pulverization characteristics of the pulverized coal, the related physical parameters were investigated; the gas desorption and diffusion properties of the pulverized coal were analyzed and compared with those of the unpulverized coal. The results indicated that the pulverized coal could easily reach the required degree of fragmentation for a coal and gas outburst to occur. Furthermore, the pore volume and specific surface area of the pulverized coal differed according to the coal particle size. Compared with the unpulverized coal, the gas desorption and diffusion properties of the pulverized coal were largely varied, and the pore structure of the pulverized coal was much simpler. The formation of pulverized coal is believed to be closely related to complex geological conditions. © 2016 Elsevier B.V. All rights reserved.
Keywords: Pulverization characteristics Coal particle size Pore structure Gas desorption Gas diffusion
1. Introduction Coal mine gas accidents, which are usually caused by gas outbursts, constitute an increasing portion of coal-mine fatalities in China. The fundamental mechanisms causing coal and gas outbursts remain a mystery due to numerous contributing factors (Fisne and Esen, 2014; Shepherd et al., 1981; Singh, 1984; Skoczylas et al., 2014; Wang et al., 2014c; Wierzbicki and Skoczylas, 2014). Many attempts have been made to explain the outburst process, and various hypotheses and theoretical models have been proposed (Barron and Kullmann, 1990; Hodot, 1966; Lunarzewski, 1998; Skoczylas et al., 2014). However, limited progress has been made
* Corresponding author. Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. ** Corresponding author. Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. E-mail addresses: [email protected] (Y. Cheng), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.jngse.2016.06.033 1875-5100/© 2016 Elsevier B.V. All rights reserved.
toward understanding and predicting such catastrophes due to a lack of field data, which attributed to difficulties in observing outburst events. The original structure of coal can quite easily be destroyed by tectonic movement, which could result in the formation of tectonic coal (Jian et al., 2015). Many scholars have found that coal and gas outbursts are related to geologic tectonism during their investigation of sites where outbursts occurred (Airey, 1968; Cheng et al., 2013; Sachsenhofer et al., 2012; Zhang et al., 2016). Not all tectonic coal will result in coal and gas outbursts; however, tectonic coal was often found in these sites. Therefore, many authors have emphasized that the presence of tectonic coal ought to be treated as a factor that increases the outburst risk (Sachsenhofer et al., 2012; Skoczylas and Wierzbicki, 2014). Previous studies have mainly focused studying coal powder collected from accident sites where coal and gas outbursts occurred, whereas few studies have been conducted on the characterization of the structure of natural pulverized coal and its rules of gas occurrence and migration. The No. 7 coal seam of the Haizi Coal Mine in the Huaibei Coalfield, China, is a typical pulverized coal seam with an incomparably high degree of
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fragmentation, very small coal particle size, and quite low strength (the Protodyakonov coefficient is typically less than 0.15). In this paper, the drilling cuttings gas desorption index (Dh2) was introduced to describe the outburst risk of pulverized coal. The index represents the pressure created by the desorbed gas releasing from 50 g of coal cuttings with a 1e3 mm particle size during the 2 min between the third and fifth minute, which reflects the coal gas desorption ability at the initial stage of pressure relief. The larger the value of Dh2, the higher is the propensity toward outburst. According to experience at sites in China, the threshold values of Dh2 are 200 Pa and 160 Pa for dry and wet coal, respectively (SACMS, 2009). The field data indicated that, after a long period of gas drainage in the No. 7 coal seam of the Haizi Coal Mine, the gas pressure and content decreased to less than 0.15 MPa and 2 m3/t, respectively (the values of which are far less than the stated threshold values of 0.74 MPa and 8 m3/t in China); however, the value of Dh2 was still greater than 350 Pa, which is much higher than the threshold value. An examination of the pulverization characteristics of the outburst coal in the Haizi Coal Mine was conducted to reveal the mechanism of this unusual phenomenon. The Protodyakonov coefficient, microstructure and particle size distribution of the coal were measured, and the proximate analysis and pore structure were tested on pulverized coal with different particle sizes. Meanwhile, the gas desorption and diffusion properties of pulverized coal were analyzed and compared with those of unpulverized coal. Furthermore, the causes of formation of pulverized coal was discussed.
2. Pulverized coal and its pulverization characteristics 2.1. Coal samples Pulverized coal samples were collected from the No. 7 coal seam of the Haizi Coal Mine. As shown in Fig. 1, the degree of fragmentation of pulverized coal is extremely high, which is very similar to the coal powder produced after a coal and gas outburst. The partial enlargement of the image indicates that most of the coal particles present loose flakes and irregular lenticular fragments. The stratification and endogenic fracture of the coal are difficult to identify, and the extended fracture surfaces of the coal fragments are very unstable. Additionally, a pronounced pulverized appearance can be recognized in some cross-sections of the coal particles. The Protodyakonov coefficient, petrographic composition and adsorption constants were measured. The results are shown in Table 1.
To recognize the microscopic structure characteristics of pulverized coal, the microstructure of the coal samples was analyzed by scanning the selected smooth surfaces of the coal particles. The scanning electron microscope (SEM) images are shown in Fig. 2. As shown in Fig. 2(a) and (d), there are many fractures with irregular shapes on the surfaces of coal particles. Fig. 2(b) and (c) indicate that the fracture surface is very uneven, there are distinctive layered structures, and the surfaces are characterized by having many concaveeconvex structures. Fig. 2(d) also shows that the surfaces of the coal particles are very rough and there are many sub-micron particles, such as clay particles and tiny coal fragments, which are attached to the coal particle surfaces. Previous studies (Qu, 2010) have indicated that the microscopic layered structure of pulverized coal is very fragile, resulting in the low strength and hardness of the coal and in a the coal mass that is extremely easy to break.
2.2. Particle size distribution of pulverized coal The coal samples were separated into different size ranges, including <0.01 mm, 0.01e0.074 mm, 0.074e0.1 mm, 0.1e0.2 mm, 0.2e0.25 mm, 0.25e0.5 mm, 0.5e1 mm, 1e3 mm and >3 mm, and the mass of each size range was determined. To ensure replication, a sieve analysis test was conducted in triplicate. The results are shown in Table 2, which indicates that there are no obvious differences between each test; thus, the average value is taken for analysis. Table 2 indicates that most of the pulverized coal particles are less than 3 mm, and the coal with a particle size greater than 3 mm accounts for only 7.02%. Among the samples, the percentage of coal particles that lie in the hundred micron size range (0.1e1 mm) is the greatest (46.7%), those at the millimeter level (>1 mm) account for 25.7%, the ten micron-sized (0.01e0.1 mm) coal particles account for 18.46%, and the micron-sized (<0.01 mm) coal particles account for 9.14%. Because the samples were directly and quickly tested after collection from the sites in sealed canisters without crushing, this result is considered to be a good representation of the actual coal particle size distribution in the coal seam. Our research group studied the coal powder samples collected from the accident sites of the Machang, Yangquan and Bailongshan coal mines, China, where coal and gas outbursts occurred, and analyzed their particle size distributions. The results are shown in Table 3. A comparison of the data between Tables 2 and 3 indicates that pulverized coal and coal powder sampled from coal and gas outburst accident sites are very similar in terms of their particle size
Fig. 1. Image of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine.
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Table 1 Basic physical parameters of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine. Sampling location
f
The No. 7 coal seam in the Haizi Coal Mine
0.12
Petrographic composition (vol %)
R0 (%)
VC
IC
LC
MC
86.45
11.32
0
2.23
2.72
Adsorption constant VL (m3/t)
PL (MPa)
46.85
1.03
f ¼ Protodyakonov coefficient; VC ¼ Vitrinite content; IC ¼ nertinite content; LC ¼ Liptinite content; MC ¼ Mineral content; R0 ¼ Maximum vitrinite reflectance; VL ¼ Langmuir volume; PL ¼ Langmuir pressure.
Fig. 2. SEM images of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine.
Table 2 Mass distribution of pulverized coal with different particle sizes. Particle size range
>3 mm 1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.1e0.2 mm 0.074e0.1 mm 0.01e0.074 mm <0.01 mm
First
Second
Third
Percentage (%)
Mass (g)
Percentage (%)
Mass (g)
Percentage (%)
Mass (g)
Percentage (%)
78.38 206.90 154.93 142.36 89.82 128.43 94.63 107.12 100.82
7.10 18.75 14.04 12.90 8.14 11.64 8.58 9.71 9.14
60.50 165.00 123.65 111.88 75.83 99.93 78.85 86.86 80.80
6.85 18.68 14.00 12.67 8.58 11.31 8.93 9.83 9.15
67.23 176.10 133.19 118.43 80.37 111.01 82.43 91.13 86.37
7.10 18.61 14.08 12.52 8.49 11.73 8.71 9.63 9.13
distribution. Moreover, the percentage of pulverized coal where the particle size is less than 3 mm is even greater than that of coal powder. The results indicate that pulverized coal could easily reach the required degree of fragmentation for a coal and gas outburst. It is generally considered that the adsorbed gas in coal must experience diffusion and seepage after being desorbed (Barenblatt et al., 1960). The most common fluid model for describing the two gas flow types is the dual porosity media model, which was first proposed by Barenblatt (Barenblatt et al., 1960). In the field of CBM exploitation, the widely used dual porosity media model is a simplified model derived from Barenblatt’s model (Lim and Aziz,
7.02 18.68 14.04 12.69 8.41 11.56 8.74 9.72 9.14
1995; Perera et al., 2012; Vishal et al., 2013, 2015; Warren and Root, 1963). In the field of CBM exploitation, the widely used dual porosity media model is a simplified model derived from Barenblatt’s model. In the dual porosity media model, the coal mass is divided into the smallest units, namely, coal matrices, by fractures, and the naturally fractured reservoirs consist of pores in the coal matrices and fractures around the coal matrices (Busse et al., 2014; Connell et al., 2016; Gilman and Beckie, 2000; Thararoop et al., 2012). The adsorbed gas is primarily stored in the pores of the coal matrices. When coal is impacted by external factors, such as mining, the gas concentration difference between pores and the
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Table 3 Particle size distributions of coal powder from the sites where coal and gas outbursts occurred. Particle size range
Mass percentage (%)
>4 mm 3e4 mm 1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.08e0.2 mm 0.01e0.08 mm <0.01 mm
The machang coal mine
The yangquan coal mine
The bailongshan coal mine
19.02 4.48 9.20 12.04 15.17 20.67 14.01 3.95 1.46
23.02 6.22 12.28 13.19 11.35 7.54 17.75 7.10 1.55
29.32 7.87 14.63 15.13 11.69 3.81 9.89 6.99 0.66
environment causes the gas in the coal matrices to diffuse into fractures through which the gas then flows. It is observed that the smaller the size of the coal particle, the fewer the number of external fractures that gas molecules can pass through when they migrate in the coal mass. When gas molecules escape from pulverized coal, the path is much shorter and the resistance is also much smaller compared with unpulverized coal. The degree of fragmentation of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine is very similar to that of coal powder produced after coal and gas outbursts. The resistance of this type of coal to highpressure gas is extremely low; hence, the possibility of a coal and gas outburst occurring in a pulverized coal seam is much higher than in an unpulverized coal seam during excavating or mining. 2.3. Proximate analysis Proximate analysis of the pulverized coal with different particle sizes was conducted with a 5EeMAG6600 proximate analyzer. The result is shown in Table 4. Table 4 indicates that the moisture, ash, volatile and fixed carbon contents of pulverized coal vary with the particle size, and among them, the variation of the ash and fixed carbon contents are the most apparent. The moisture first increases then decreases with decreasing particle size, and the maximum value of 2.68% lies in the particle size range of 0.1e1 mm, but the overall variation with size range is very small. The ash content decreases first and then increases with decreasing particle size, and a minimum of 17.09% was found in the particle size range of 0.1e0.2 mm. The volatile content increases gradually as the particle size decreases. The fixed carbon content increases first and then decreases with decreasing particle size and reaches a maximum of 72.17% in the particle size range of 0.1e0.2 mm. The same results were obtained in previous studies (Cao et al., 2003; Küçük et al., 2003). We believe that these changes, which reveal the impact of crushing on the microscopic properties of coals, are inseparable from the intensification of the organic
matter activity in the coal mass when pulverized coal was formed. 3. Pore structure, gas desorption and diffusion properties of pulverized coal 3.1. N2 adsorption/desorption test and the pore size distribution The N2 adsorption/desorption test is typically used to measure the pore size distribution in porous materials, the principle of which is as follows (De Boer, 1958; Mastalerz et al., 2012; Wang et al., 2014a): with porous materials as the adsorbent and N2 as the adsorbate, the volume of gas adsorption is recorded while the gas pressure gradually increases to the saturated vapor pressure at a constant temperature of 77 K. The adsorption isotherm is obtained from the volume and specific surface area of the pores and pore sizes or the relative pressure. The N2 adsorption/desorption test can describe the pore structure and avoid damage to the pores of porous materials as a result of high pressure (Guo et al., 2015; Wang et al., 2014a). To quantitatively analyze the characteristics of coal pores, the IUPAC classification for coal/rock pore sizes was used as follows: micropores (<2 nm in diameter), mesopores (2e50 nm in diameter) and macropores (>50 nm in diameter) (IUPAC, 1994). The variation in the pore size distribution curves of pulverized coal with different particle sizes is shown in Fig. 3. The shape of the pore size distribution curves is determined by the pore structure. From Fig. 3, the variation of the pore size distribution curves of pulverized coal with different particle sizes is found to be similar. Moreover, the range of the pulverized coal pore size measured by the N2 adsorption/desorption test is
Table 4 Proximate analyses of coal samples with different particle sizes. Particle size range
1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.1e0.2 mm 0.074e0.1 mm 0.01e0.074 mm <0.01 mm
Proximate analyses (wt %) Mad
Ad
Vdaf
FC
2.58 2.68 2.65 2.61 2.49 2.42 2.31 2.23
32.73 26.71 24.00 21.15 17.09 18.53 19.61 21.26
6.91 7.17 7.47 7.92 8.26 8.87 9.36 10.24
57.77 66.35 67.89 69.93 72.17 70.15 68.34 66.16
Mad ¼ Moisture content; Ad ¼ Ash content; Vdaf ¼ Volatile content; FC ¼ Fixed carbon basis.
Fig. 3. Pore size distribution curves of pulverized coal with different particle sizes.
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predominantly <2 nm (micropores). For coal with a particle size of less than 0.5 mm, the pore volume with a pore width of 2e5 nm also accounts for a significant proportion. The pore volume and specific surface area are essential indices for the evaluation of the gas adsorption/desorption capacity of coal. The total pore volume and specific surface area of pulverized coal with different particle sizes were calculated, and the results are shown in Table 5. According to Table 5, the specific surface area of the pores is found to be well correlated with the pore volume, and the total pore volume and specific surface area of pulverized coal continually increase as the particle size of pulverized coal decreases. This observation is thought to be closely related to the damage to the coal structure and the reopening of closed coal pores that occurred when pulverized coal was formed. 3.2. Desorption experiments and diffusion coefficients of pulverized coal
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(mm/s), C is the volume fraction of the adsorbed gas in coal matrices, vC=vx is the concentration gradient in the direction of gas diffusion, and D is the diffusion coefficient (mm2/s). To solve Eq. (1), it is generally assumed that coal particles are isotropic and homogeneous spherical particles. Some scholars (Jian et al., 2012; Pillalamarry et al., 2011; Walker and Mahajan, 1978; Zhang, 2008) studied Fick’s diffusion law and obtained its analytical solution (if D is constant), which is as follows:
pffiffiffiffi D pffiffi Qt 6 D t3 2t F¼ ¼ pffiffiffi Q∞ r p r
(2)
where F is the gas desorption ratio per unit mass of coal, Qt is the desorption capacity per unit mass of coal at time t (m3/t), Q∞ is the limit of the desorption capacity per unit mass of coal as time t tends to infinity (m3/t), and r is the radius of the coal particle (mm). If D depends on time, then (Jian et al., 2012)
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Z
Z
t
Ddt
t
Ddt
3.2.1. Desorption experiments Studies on the processes associated with the occurrence and development of coal gas disasters indicate that coal and gas outbursts are accompanied by the rapid desorption of coal gas, where large amounts of gas desorbs from the coal mass in an extremely short time (Skoczylas et al., 2014; Wierzbicki and Skoczylas, 2014; Xue et al., 2014). Thus, gas desorption experiments were performed according to the following steps: First, the coal samples were manipulated to achieve adsorption equilibrium at 1 MPa; second, the free gas in the coal sample tank was rapidly released and the desorption data after relief of the gas pressure were recorded. The gas desorption volume and initial gas desorption rate are shown in Fig. 4. From Fig. 4, the gas desorption capacity of pulverized coal is found to increase with the decreasing particle size, which shows exact agreement with the coal pore characteristics. For each coal particle size, the gas desorption rate is relatively high at the initial stage and then decreases over time and eventually stabilizes. The initial gas desorption rate increases as the coal particle size decreases. This indicates that the higher the degree of fragmentation of coal, the stronger the desorption ability, especially the initial desorption ability.
6 F ¼ pffiffiffi
3.2.2. Diffusion coefficients Gas diffusion in a coal mass is a very complex process, and it is thought that the process can be described by Fick’s diffusion law re et al., 2010; Clarkson and Bustin, 1999; Lito et al., 2015; (Charrie Pillalamarry et al., 2011):
Therefore, y as a function of t can be calculated based on the gas desorption data. Then, the diffusion coefficients D at a given time can be obtained from:
vC q ¼ D vx
(1)
where q is the diffusion rate of gas flowing through a unit area Table 5 Total pore volume and specific surface area of pulverized coal with different particle sizes. Particle size range
Pore volume (102 mL/g)
Specific surface area (m2/g)
1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.1e0.2 mm 0.074e0.1 mm 0.01e0.074 mm <0.01 mm
0.82 1.14 1.38 1.63 1.85 2.40 2.55 3.09
6.00 10.03 11.49 14.52 17.03 23.80 27.36 30.49
0
p
3
r
0
r2
(3)
In Eq. (3), we assume that
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Z t
Ddt 0
x¼
(4)
r
Thus, Eq. (3) is a quadratic equation for x:
6 3x2 pffiffiffi x þ F ¼ 0
p
(5)
The value of x is derived as:
1 x ¼ pffiffiffi
p
rffiffiffiffiffiffiffiffiffiffiffiffi 1 F p 3
Then, let y ¼
y¼r
D¼
2
dy dt
1 pffiffiffi
p
Rt 0
(6) Ddt ¼ r 2 x2 . Namely,
rffiffiffiffiffiffiffiffiffiffiffiffi!2 1 F p 3
(7)
(8)
To smooth the data in the above differential calculation, F calculated from the sixth order polynomial is used to calculate y, and central differentials are used to obtain D. The diffusion coefficients of pulverized coal with different particle sizes are shown in Fig. 5. From Fig. 5, it can be seen that the diffusion coefficient of pulverized coal is not constant. To compare diffusion coefficients of coal with different particle sizes, the data from the first 20 min were selected and are shown in Table 6. Fig. 5 and Table 6 reveal that for pulverized coal with the same particle size, the diffusion coefficient decreases with increasing desorption time. Specifically, the diffusion coefficient decreases rapidly at the initial stage and then tends to stabilize after decreasing to a certain extent over time. It is considered that the pores near the external surface of coal matrices, especially macropores, desorbed first during the initial desorption stage, and then,
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Fig. 4. Gas desorption volume and initial gas desorption rate of pulverized coal with different particle sizes.
Fig. 5. Diffusion coefficients of pulverized coal with different particle sizes.
the gas from the inner pores of the coal matrices has to pass through a longer path and overcome a larger resistance. When some gas molecules diffuse from the coal matrices, the pore gas pressure in the coal matrices decreases. This leads to the shrinkage of the coal matrices and a decrease in the pore dimension (Harpalani and Chen, 1995, 1997; Lu et al., 2015b). As a result, the resistance to the diffusion of the subsequent gas molecules from the coal matrices increases greatly. Therefore, the diffusion coefficient decreases over time. From Fig. 5 and Table 6, the diffusion coefficient is also found to decrease with decreasing particle size. According to diffusion theory (Nie et al., 2013), the effective diffusion area is known to have a significant effect on the diffusion coefficient. As the particle size decreases, the pore surface area increases and the average pore
width decreases. As a result, the diffusion length is limited by the spatial scale of the pores, which leads to a decrease in the effective diffusion area. 3.3. Experimental comparative analysis To quantitatively analyze the difference of gas desorption between pulverized coal and unpulverized coal, anthracite coal (R0 ¼ 3.27%) from the Wolonghu Coal Mine and bituminous coal (R0 ¼ 1.17%) from the Linhuan Coal Mine were tested. The hardness of these two unpulverized coal samples is high; therefore, the samples were crushed by a pulverizer first; then, the coal samples were screened using sifters to obtain the desired particle size ranges of 0.01e0.074 mm, 0.1e0.2 mm, 0.25e0.5 mm and 1e3 mm.
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Table 6 Diffusion coefficients of pulverized coal with different particle sizes in the first 20 min. Time (s)
Diffusion coefficient (mm2/s) 1e3 mm
30 60 90 120 150 180 210 240 300 360 420 480 540 600 900 1200
5.445 3.608 2.818 2.350 2.039 1.788 1.611 1.469 1.286 1.096 9.762 8.810 7.993 7.340 5.227 4.075
0.5e1 mm 5
10 105 105 105 105 105 105 105 105 105 106 106 106 106 106 106
1.168 7.655 6.228 4.655 3.847 3.602 3.253 2.982 2.582 2.317 2.132 1.965 1.814 1.674 1.244 9.776
5
10 106 106 106 106 106 106 106 106 106 106 106 106 106 106 107
0.25e0.5 mm
0.2e0.25 mm
6
6
3.579 2.694 2.171 1.835 1.584 1.409 1.267 1.153 9.921 8.612 7.713 6.946 6.353 5.839 4.143 3.203
10 106 106 106 106 106 106 106 107 107 107 107 107 107 107 107
2.186 1.628 1.374 1.127 9.588 9.016 8.029 7.264 6.198 5.228 4.586 4.152 3.679 3.329 2.299 1.739
10 106 106 106 107 107 107 107 107 107 107 107 107 107 107 107
The gas desorption characteristics of the two unpulverized coal samples with different particle sizes were tested and compared with those of pulverized coal under the same conditions. The results are shown in Figs. 6 and 7. Figs. 6 and 7 indicate that the desorption capacity and the initial average gas desorption rate of pulverized coal are higher than those of the two unpulverized coal samples. The degree of metamorphism is known to have a significant effect on the gas desorption capacity of coal. Generally, the higher the degree of metamorphism, the stronger the gas desorption capacity of coal, which is consistent with the gas desorption results of anthracite coal from the Wolonghu Coal Mine and bituminous coal from the
0.1e0.2 mm 1.174 8.535 7.056 5.814 5.157 4.518 4.068 3.735 3.155 2.772 2.500 2.245 2.038 1.867 1.321 1.018
6
10 107 107 107 107 107 107 107 107 107 107 107 107 107 107 107
0.074e0.1 mm 5.254 4.171 3.377 2.738 2.266 1.975 1.795 1.591 1.363 1.189 1.054 9.539 8.551 8.021 5.514 4.251
7
10 107 107 107 107 107 107 107 107 107 107 108 108 108 108 108
0.01e0.074 mm 1.646 1.212 9.754 8.037 6.918 6.029 5.354 4.852 4.120 3.492 3.047 2.712 2.451 2.243 1.546 1.179
7
10 107 108 108 108 108 108 108 108 108 108 108 108 108 108 108
<0.01 mm 2.664 1.911 1.531 1.248 1.069 9.447 8.459 7.673 6.451 5.686 5.080 4.568 4.173 3.833 2.714 2.097
109 109 109 109 109 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010
Linhuan Coal Mine. However, Figs. 6 and 7 show that the gas desorption law of pulverized coal is irregular, which indicates that the degree of metamorphism is not the primary factor affecting the gas desorption ability of pulverized coal. To a large extent, it is the decrease of the coal matrix size and damage to the coal pore structure during the formation of pulverized coal that determine the coal gas desorption properties. As a result, pulverized coal has both a higher gas desorption capacity and initial gas desorption rate; this is the reason that the value of Dh2 is still greater than 350 Pa, which is much higher than the threshold value when the gas pressure and content decreased to less than 0.15 MPa and 2 m3/ t, respectively, after a long period of gas drainage in the No. 7 coal
Fig. 6. Gas desorption curves of pulverized coal and unpulverized coal.
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Fig. 7. Initial average gas desorption rate of pulverized coal and unpulverized coal.
seam of the Haizi Coal Mine. The diffusion coefficient is recognized as a critical parameter for evaluating the methane diffusion properties in coal matrices re et al., 2010; Jin et al., 2015; Lito et al., 2015; Lu et al., (Charrie 2015a; Walker and Mahajan, 1978; Xu et al., 2015). In our research, the diffusion coefficient of pulverized coal was also compared with that of unpulverized coal. The results are shown in Fig. 8. Fig. 8 shows that the diffusion coefficient of pulverized coal is greater than that of unpulverized coal in the initial stage. Over time, the difference in the diffusion coefficient between unpulverized coal and pulverized coal becomes increasingly smaller, and the diffusion coefficient of unpulverized coal eventually becomes equal to or greater than that of pulverized coal. As mentioned above, the coal matrices shrink when the pore gas pressure decreases due to gas diffusion. The strength and hardness of the pulverized coal mass are lower than those of unpulverized coal; consequently, the pulverized coal skeleton is more easily affected. Thus, for pulverized coal, over time, the decreasing magnitude of the diffusion coefficient is more obvious than for unpulverized coal.
pores have a significant effect on gas storage, desorption and transport in coal. When pulverized coal was formed, many closed pores were transformed into open pores, and some semi-enclosed pores, such as the spherical and ink bottle types, might have deformed into cylindrical, conical or other types of pores. In addition, many interconnected pores were likely divided into many simple pores, and some long pores were possibly divided into several parts as the coal matrices were severely damaged. In other words, the pores in coal matrices become increasingly simpler during the generation of the pulverized coal. These changes caused the paths for gas entering or leaving pores to become short and caused the resistance to decrease significantly. Thus, for pulverized coal, the gas desorption capacity and initial gas desorption rate are extremely high, and the desorption and diffusion properties are quite different from those of unpulverized coal. If mining activities break the gas adsorption equilibrium state, compared with unpulverized coal, the gas in pulverized coal could much more rapidly be desorbed and diffuse from coal pores to fractures and then flow out of the coal. There is no doubt that the uncontrollable emission of such a large quantity of gas would quite easily initiate the coal and gas outburst.
3.4. Evolution mechanism of the pulverized coal pore structure Generally, the shape of pores in coal matrices is irregular. Based on the pore shapes, the types of pores in coal matrices can be classified into cylindrical, conical, spherical or ink bottle, slits and interstices, which are illustrated in Fig. 9(a) (Rouquerol et al., 1999; Wang et al., 2014a). According to the differences in pore connectivity, the types of pores in coal matrices can also be classified as passing, interconnected, dead end and closed pores, which are illustrated in Fig. 9 (b). The former three types (passing, interconnected and dead end) are also referred to as open pores. Open
4. Formation causes of pulverized coal A coal and gas outburst is a process in which the energy stored in the coal seam is released unsteadily, resulting in large amounts of coal (rock) and gas being expelled in an extremely short period of time (Beamish and Crosdale, 1998; Briggs, 1921; Hargraves, 1983; Lama and Bodziony, 1996; Shepherd et al., 1981). The energy is primarily provided by the gas stored in the coal, and the premise is that there is a large amount of gas rapidly desorbing from the coal
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875
Fig. 8. Diffusion coefficients of pulverized coal and unpulverized coal.
Fig. 9. Types of pores in coal matrices.
mass that could provide enough energy in an extremely short time. According to a laboratory experiment and site investigation, pulverized coal from the No. 7 coal seam in the Haizi Coal Mine possesses all of the conditions. Nowadays, many scholars are studying the reasons for the formation of tectonic coal or pulverized coal, among which the most important reason is the complex set of geological conditions.
4.1. Influence of regional tectonic movement The Haizi Coal Mine is located in the Huaibei Coalfield. The complex geological structure in the Huaibei Coalfield is primarily affected and controlled by the plate movement. The continuous sedimentation of strata began in the middle Carboniferous before successively experiencing three tectonic epochs, i.e., the Indosinian,
Yanshan and Himalayan movements (Wang et al., 2014c). The overlap of multiple periods of tectonic movements resulted in the reticular fault tectonic framework of the Huaibei Coalfield, which is shown in Fig. 10. Different grades of the tectonic movements and stress fields control not only the range and intensity of the tectonism but also the occurrence and distribution of CBM. Furthermore, they also determine the migration conditions of the coal seam and the degree of fragmentation of the coal. Fig. 10 indicates that the Haizi Coal Mine is located in the fault block enclosed by the Subei, GuangwueGuzhen, XiayieGushi and Fengwo faults and lies in the northwest portion of the Tongting anticline. This region is a general monoclinal structure with approximately an EeW strike and tilting northwards. According to the statistical results, there are 54 faults per square kilometer in the No. 7 coal seam of the Haizi Coal Mine, which are mainly normal
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Fig. 10. Geological structure distribution of the Huaibei Coalfield.
faults that suffered from extension tectonism. The Subei fault strikes EeW and cuts the Fengwo and XiayieGushi faults, which strike approximately NeS and are located in the eastern and western portion of the Haizi Coal Mine, respectively. The direction of the horizontal stress is approximately eastewest. Therefore, the relative movement between the roof and floor of the coal seam can easily occur when there is unbalanced tectonic movement in the strata (Li et al., 2011). In the long geological period since the deposition of coal, the large horizontal stress resulted in a substantial layereslide movement between the upper strata and underlying strata of the No. 7 coal seam in the Haizi Coal Mine. As a result, the raw coal was severely damaged and numerous damaged faces, e.g., the layered structure, were formed. For all of these reasons, the strength of the coal mass was substantially reduced and is easily further broken with additional stress. 4.2. Damage caused by the magmatic intrusion In the long geological history, the Huaibei Coalfield not only
experienced multiple periods of tectonic movement but was also greatly influenced by magmatic intrusion accompanying the tectonic movement. The magmatic intrusion that occurred in the Yanshanian of the Mesozoic was recognized to be the strongest, which plays an important role in damaging the coal mass. The magmatic rock in the Haizi Coal Mine was primarily squeezed into the strata, and the profile of the magmatic intrusion direction is shown in Fig. 11. Along with the magma squeezed into the strata with an average thickness of 120 m, the coal mass of the No. 7 coal seam under extremely thick magmatic rock was affected by both the gravity stress of the overlying rocks and the large extrusion stress caused by the magmatic intrusion (Stewart et al., 2005; Wang et al., 2014b), and the extrusion stress proportionally increased as the thickness of the overlying magmatic rocks increased. Geological exploration indicates that the thickest of the overlying magmatic rocks in the No. 7 coal seam of the Haizi Coal Mine approaches 170 m. The large extrusion stress intensifies the layereslide movement between the upper strata and underlying strata of the No. 7 coal seam. Under the combined effect of multiple
Fig. 11. Profile of the magma intrusion direction in the Haizi Coal Mine.
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stresses, the coal experienced additional serious destruction. The result is that the activity of organic components in the coal mass was intensified. Then, tectonic coalification occurred in the coal mass and pulverized coal was eventually formed. For pulverized coal, the extreme decrease in the coal matrix size and the heavy damage to the coal pore structure caused by fragmentation determine its increased gas desorption and diffusion properties, which results in an unprecedented high potential for coal and gas outbursts in the pulverized coal seam. This research can provide a reference for the prediction of coal and gas outbursts in similar coal seams and lead to the improvement of safety conditions in the coal mining industry in the future. 5. Conclusions 1) The degree of fragmentation of pulverized coal is extremely high and very similar to coal powder produced after a coal and gas outburst. For pulverized coal, proximate analyses vary with the decreasing particle size, the pore size distribution with the different particle size is similar, and the total pore volume and specific surface area increase continually as the particle size decreases. According to the analysis for pore types, pores in the coal matrices became increasingly simple as pulverized coal was generated. 2) The gas desorption ability decreases with the increasing particle size of pulverized coal, and the diffusion coefficient decreases as the particle size decreases. For pulverized coal with the same particle size, the diffusion coefficient decreases rapidly at the initial stage and then tends to stabilize after decreasing to some extent, and the decreasing amplitude is more significant over time compared with unpulverized coal. 3) Under the combined effect of the large horizontal stress and magmatic intrusion, raw coal could easily turn into pulverized coal. Pulverized coal not only has an extremely decreased matrix size but also severely damaged pore structures, which leads to an unprecedented high potential for coal and gas outbursts. Acknowledgments The authors are grateful to the financial support from the sponsorship of Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents, the Qing Lan Project, the National Natural Science Foundation of China (No. 51574229, No. 51404260, No. 51304204), the Fundamental Research Funds for the Central Universities (2014XT02) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References Airey, E.M., 1968. Gas emission from broken coal. An experimental and theoretical investigation. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 5, 475e494. Barenblatt, G.I., Zheltov, I.P., Kochina, I.N., 1960. Basic concepts in the theory of seepage of homogeneous liquids in fissured rocks [strata]. J. Appl. Math. Mech. 24, 1286e1303. Barron, K., Kullmann, D., 1990. Modelling of outbursts at #26 Colliery, Glace Bay, Nova Scotia. Part 2: proposed outburst mechanism and model. Min. Sci. Technol. 11, 261e268. Beamish, B.B., Crosdale, P.J., 1998. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int. J. Coal Geol. 35, 27e55. Briggs, H., 1921. Characteristics of outbursts of gas in mines. Trans. Inst. Min. Eng. 61, 119e146. Busse, J., Scheuermann, A., Galindo-Torres, S.A., Bringemeier, D., Li, L., 2014. In-situ and Laboratory Measurements of Coal Matrix and Cleat Permeability. Crc PressTaylor & Francis Group, Boca Raton. Cao, Y., Davis, A., Liu, R., Liu, X., Zhang, Y., 2003. The influence of tectonic deformation on some geochemical properties of coalsda possible indicator of outburst potential. Int. J. Coal Geol. 53, 69e79. re, D., Pokryszka, Z., Behra, P., 2010. Effect of pressure and temperature on Charrie
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Pulverization characteristics of coal from a strong outburst-prone coal seam and their impact on gas desorption and diffusion properties Haijun Guo a, b, Yuanping Cheng a, b, c, *, Ting Ren c, Liang Wang a, b, **, Liang Yuan a, b, Haina Jiang a, b, Hongyong Liu a, b a b c
Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China National Engineering Research Center of Coal Gas Control, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China School of Civil, Mining & Environmental Engineering, University of Wollongong, NSW 2522, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 March 2016 Received in revised form 8 June 2016 Accepted 13 June 2016 Available online 16 June 2016
Coal seams that are prone to strong outbursts have low strength and cause heavy structural damage to the seam. Their outburst risk is highly related to the release of the adsorbed coalbed gas, which is controlled by the gas desorption and diffusion characteristics of coal. In the Haizi Coal Mine, China, an extremely high gas outburst risk was detected, and the coals from this area were found to have an unprecedented high degree of fragmentation and were present in the pulverized state. To explain the pulverization characteristics of the pulverized coal, the related physical parameters were investigated; the gas desorption and diffusion properties of the pulverized coal were analyzed and compared with those of the unpulverized coal. The results indicated that the pulverized coal could easily reach the required degree of fragmentation for a coal and gas outburst to occur. Furthermore, the pore volume and specific surface area of the pulverized coal differed according to the coal particle size. Compared with the unpulverized coal, the gas desorption and diffusion properties of the pulverized coal were largely varied, and the pore structure of the pulverized coal was much simpler. The formation of pulverized coal is believed to be closely related to complex geological conditions. © 2016 Elsevier B.V. All rights reserved.
Keywords: Pulverization characteristics Coal particle size Pore structure Gas desorption Gas diffusion
1. Introduction Coal mine gas accidents, which are usually caused by gas outbursts, constitute an increasing portion of coal-mine fatalities in China. The fundamental mechanisms causing coal and gas outbursts remain a mystery due to numerous contributing factors (Fisne and Esen, 2014; Shepherd et al., 1981; Singh, 1984; Skoczylas et al., 2014; Wang et al., 2014c; Wierzbicki and Skoczylas, 2014). Many attempts have been made to explain the outburst process, and various hypotheses and theoretical models have been proposed (Barron and Kullmann, 1990; Hodot, 1966; Lunarzewski, 1998; Skoczylas et al., 2014). However, limited progress has been made
* Corresponding author. Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. ** Corresponding author. Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. E-mail addresses: [email protected] (Y. Cheng), [email protected] (L. Wang). http://dx.doi.org/10.1016/j.jngse.2016.06.033 1875-5100/© 2016 Elsevier B.V. All rights reserved.
toward understanding and predicting such catastrophes due to a lack of field data, which attributed to difficulties in observing outburst events. The original structure of coal can quite easily be destroyed by tectonic movement, which could result in the formation of tectonic coal (Jian et al., 2015). Many scholars have found that coal and gas outbursts are related to geologic tectonism during their investigation of sites where outbursts occurred (Airey, 1968; Cheng et al., 2013; Sachsenhofer et al., 2012; Zhang et al., 2016). Not all tectonic coal will result in coal and gas outbursts; however, tectonic coal was often found in these sites. Therefore, many authors have emphasized that the presence of tectonic coal ought to be treated as a factor that increases the outburst risk (Sachsenhofer et al., 2012; Skoczylas and Wierzbicki, 2014). Previous studies have mainly focused studying coal powder collected from accident sites where coal and gas outbursts occurred, whereas few studies have been conducted on the characterization of the structure of natural pulverized coal and its rules of gas occurrence and migration. The No. 7 coal seam of the Haizi Coal Mine in the Huaibei Coalfield, China, is a typical pulverized coal seam with an incomparably high degree of
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fragmentation, very small coal particle size, and quite low strength (the Protodyakonov coefficient is typically less than 0.15). In this paper, the drilling cuttings gas desorption index (Dh2) was introduced to describe the outburst risk of pulverized coal. The index represents the pressure created by the desorbed gas releasing from 50 g of coal cuttings with a 1e3 mm particle size during the 2 min between the third and fifth minute, which reflects the coal gas desorption ability at the initial stage of pressure relief. The larger the value of Dh2, the higher is the propensity toward outburst. According to experience at sites in China, the threshold values of Dh2 are 200 Pa and 160 Pa for dry and wet coal, respectively (SACMS, 2009). The field data indicated that, after a long period of gas drainage in the No. 7 coal seam of the Haizi Coal Mine, the gas pressure and content decreased to less than 0.15 MPa and 2 m3/t, respectively (the values of which are far less than the stated threshold values of 0.74 MPa and 8 m3/t in China); however, the value of Dh2 was still greater than 350 Pa, which is much higher than the threshold value. An examination of the pulverization characteristics of the outburst coal in the Haizi Coal Mine was conducted to reveal the mechanism of this unusual phenomenon. The Protodyakonov coefficient, microstructure and particle size distribution of the coal were measured, and the proximate analysis and pore structure were tested on pulverized coal with different particle sizes. Meanwhile, the gas desorption and diffusion properties of pulverized coal were analyzed and compared with those of unpulverized coal. Furthermore, the causes of formation of pulverized coal was discussed.
2. Pulverized coal and its pulverization characteristics 2.1. Coal samples Pulverized coal samples were collected from the No. 7 coal seam of the Haizi Coal Mine. As shown in Fig. 1, the degree of fragmentation of pulverized coal is extremely high, which is very similar to the coal powder produced after a coal and gas outburst. The partial enlargement of the image indicates that most of the coal particles present loose flakes and irregular lenticular fragments. The stratification and endogenic fracture of the coal are difficult to identify, and the extended fracture surfaces of the coal fragments are very unstable. Additionally, a pronounced pulverized appearance can be recognized in some cross-sections of the coal particles. The Protodyakonov coefficient, petrographic composition and adsorption constants were measured. The results are shown in Table 1.
To recognize the microscopic structure characteristics of pulverized coal, the microstructure of the coal samples was analyzed by scanning the selected smooth surfaces of the coal particles. The scanning electron microscope (SEM) images are shown in Fig. 2. As shown in Fig. 2(a) and (d), there are many fractures with irregular shapes on the surfaces of coal particles. Fig. 2(b) and (c) indicate that the fracture surface is very uneven, there are distinctive layered structures, and the surfaces are characterized by having many concaveeconvex structures. Fig. 2(d) also shows that the surfaces of the coal particles are very rough and there are many sub-micron particles, such as clay particles and tiny coal fragments, which are attached to the coal particle surfaces. Previous studies (Qu, 2010) have indicated that the microscopic layered structure of pulverized coal is very fragile, resulting in the low strength and hardness of the coal and in a the coal mass that is extremely easy to break.
2.2. Particle size distribution of pulverized coal The coal samples were separated into different size ranges, including <0.01 mm, 0.01e0.074 mm, 0.074e0.1 mm, 0.1e0.2 mm, 0.2e0.25 mm, 0.25e0.5 mm, 0.5e1 mm, 1e3 mm and >3 mm, and the mass of each size range was determined. To ensure replication, a sieve analysis test was conducted in triplicate. The results are shown in Table 2, which indicates that there are no obvious differences between each test; thus, the average value is taken for analysis. Table 2 indicates that most of the pulverized coal particles are less than 3 mm, and the coal with a particle size greater than 3 mm accounts for only 7.02%. Among the samples, the percentage of coal particles that lie in the hundred micron size range (0.1e1 mm) is the greatest (46.7%), those at the millimeter level (>1 mm) account for 25.7%, the ten micron-sized (0.01e0.1 mm) coal particles account for 18.46%, and the micron-sized (<0.01 mm) coal particles account for 9.14%. Because the samples were directly and quickly tested after collection from the sites in sealed canisters without crushing, this result is considered to be a good representation of the actual coal particle size distribution in the coal seam. Our research group studied the coal powder samples collected from the accident sites of the Machang, Yangquan and Bailongshan coal mines, China, where coal and gas outbursts occurred, and analyzed their particle size distributions. The results are shown in Table 3. A comparison of the data between Tables 2 and 3 indicates that pulverized coal and coal powder sampled from coal and gas outburst accident sites are very similar in terms of their particle size
Fig. 1. Image of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine.
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Table 1 Basic physical parameters of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine. Sampling location
f
The No. 7 coal seam in the Haizi Coal Mine
0.12
Petrographic composition (vol %)
R0 (%)
VC
IC
LC
MC
86.45
11.32
0
2.23
2.72
Adsorption constant VL (m3/t)
PL (MPa)
46.85
1.03
f ¼ Protodyakonov coefficient; VC ¼ Vitrinite content; IC ¼ nertinite content; LC ¼ Liptinite content; MC ¼ Mineral content; R0 ¼ Maximum vitrinite reflectance; VL ¼ Langmuir volume; PL ¼ Langmuir pressure.
Fig. 2. SEM images of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine.
Table 2 Mass distribution of pulverized coal with different particle sizes. Particle size range
>3 mm 1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.1e0.2 mm 0.074e0.1 mm 0.01e0.074 mm <0.01 mm
First
Second
Third
Percentage (%)
Mass (g)
Percentage (%)
Mass (g)
Percentage (%)
Mass (g)
Percentage (%)
78.38 206.90 154.93 142.36 89.82 128.43 94.63 107.12 100.82
7.10 18.75 14.04 12.90 8.14 11.64 8.58 9.71 9.14
60.50 165.00 123.65 111.88 75.83 99.93 78.85 86.86 80.80
6.85 18.68 14.00 12.67 8.58 11.31 8.93 9.83 9.15
67.23 176.10 133.19 118.43 80.37 111.01 82.43 91.13 86.37
7.10 18.61 14.08 12.52 8.49 11.73 8.71 9.63 9.13
distribution. Moreover, the percentage of pulverized coal where the particle size is less than 3 mm is even greater than that of coal powder. The results indicate that pulverized coal could easily reach the required degree of fragmentation for a coal and gas outburst. It is generally considered that the adsorbed gas in coal must experience diffusion and seepage after being desorbed (Barenblatt et al., 1960). The most common fluid model for describing the two gas flow types is the dual porosity media model, which was first proposed by Barenblatt (Barenblatt et al., 1960). In the field of CBM exploitation, the widely used dual porosity media model is a simplified model derived from Barenblatt’s model (Lim and Aziz,
7.02 18.68 14.04 12.69 8.41 11.56 8.74 9.72 9.14
1995; Perera et al., 2012; Vishal et al., 2013, 2015; Warren and Root, 1963). In the field of CBM exploitation, the widely used dual porosity media model is a simplified model derived from Barenblatt’s model. In the dual porosity media model, the coal mass is divided into the smallest units, namely, coal matrices, by fractures, and the naturally fractured reservoirs consist of pores in the coal matrices and fractures around the coal matrices (Busse et al., 2014; Connell et al., 2016; Gilman and Beckie, 2000; Thararoop et al., 2012). The adsorbed gas is primarily stored in the pores of the coal matrices. When coal is impacted by external factors, such as mining, the gas concentration difference between pores and the
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Table 3 Particle size distributions of coal powder from the sites where coal and gas outbursts occurred. Particle size range
Mass percentage (%)
>4 mm 3e4 mm 1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.08e0.2 mm 0.01e0.08 mm <0.01 mm
The machang coal mine
The yangquan coal mine
The bailongshan coal mine
19.02 4.48 9.20 12.04 15.17 20.67 14.01 3.95 1.46
23.02 6.22 12.28 13.19 11.35 7.54 17.75 7.10 1.55
29.32 7.87 14.63 15.13 11.69 3.81 9.89 6.99 0.66
environment causes the gas in the coal matrices to diffuse into fractures through which the gas then flows. It is observed that the smaller the size of the coal particle, the fewer the number of external fractures that gas molecules can pass through when they migrate in the coal mass. When gas molecules escape from pulverized coal, the path is much shorter and the resistance is also much smaller compared with unpulverized coal. The degree of fragmentation of pulverized coal from the No. 7 coal seam in the Haizi Coal Mine is very similar to that of coal powder produced after coal and gas outbursts. The resistance of this type of coal to highpressure gas is extremely low; hence, the possibility of a coal and gas outburst occurring in a pulverized coal seam is much higher than in an unpulverized coal seam during excavating or mining. 2.3. Proximate analysis Proximate analysis of the pulverized coal with different particle sizes was conducted with a 5EeMAG6600 proximate analyzer. The result is shown in Table 4. Table 4 indicates that the moisture, ash, volatile and fixed carbon contents of pulverized coal vary with the particle size, and among them, the variation of the ash and fixed carbon contents are the most apparent. The moisture first increases then decreases with decreasing particle size, and the maximum value of 2.68% lies in the particle size range of 0.1e1 mm, but the overall variation with size range is very small. The ash content decreases first and then increases with decreasing particle size, and a minimum of 17.09% was found in the particle size range of 0.1e0.2 mm. The volatile content increases gradually as the particle size decreases. The fixed carbon content increases first and then decreases with decreasing particle size and reaches a maximum of 72.17% in the particle size range of 0.1e0.2 mm. The same results were obtained in previous studies (Cao et al., 2003; Küçük et al., 2003). We believe that these changes, which reveal the impact of crushing on the microscopic properties of coals, are inseparable from the intensification of the organic
matter activity in the coal mass when pulverized coal was formed. 3. Pore structure, gas desorption and diffusion properties of pulverized coal 3.1. N2 adsorption/desorption test and the pore size distribution The N2 adsorption/desorption test is typically used to measure the pore size distribution in porous materials, the principle of which is as follows (De Boer, 1958; Mastalerz et al., 2012; Wang et al., 2014a): with porous materials as the adsorbent and N2 as the adsorbate, the volume of gas adsorption is recorded while the gas pressure gradually increases to the saturated vapor pressure at a constant temperature of 77 K. The adsorption isotherm is obtained from the volume and specific surface area of the pores and pore sizes or the relative pressure. The N2 adsorption/desorption test can describe the pore structure and avoid damage to the pores of porous materials as a result of high pressure (Guo et al., 2015; Wang et al., 2014a). To quantitatively analyze the characteristics of coal pores, the IUPAC classification for coal/rock pore sizes was used as follows: micropores (<2 nm in diameter), mesopores (2e50 nm in diameter) and macropores (>50 nm in diameter) (IUPAC, 1994). The variation in the pore size distribution curves of pulverized coal with different particle sizes is shown in Fig. 3. The shape of the pore size distribution curves is determined by the pore structure. From Fig. 3, the variation of the pore size distribution curves of pulverized coal with different particle sizes is found to be similar. Moreover, the range of the pulverized coal pore size measured by the N2 adsorption/desorption test is
Table 4 Proximate analyses of coal samples with different particle sizes. Particle size range
1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.1e0.2 mm 0.074e0.1 mm 0.01e0.074 mm <0.01 mm
Proximate analyses (wt %) Mad
Ad
Vdaf
FC
2.58 2.68 2.65 2.61 2.49 2.42 2.31 2.23
32.73 26.71 24.00 21.15 17.09 18.53 19.61 21.26
6.91 7.17 7.47 7.92 8.26 8.87 9.36 10.24
57.77 66.35 67.89 69.93 72.17 70.15 68.34 66.16
Mad ¼ Moisture content; Ad ¼ Ash content; Vdaf ¼ Volatile content; FC ¼ Fixed carbon basis.
Fig. 3. Pore size distribution curves of pulverized coal with different particle sizes.
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predominantly <2 nm (micropores). For coal with a particle size of less than 0.5 mm, the pore volume with a pore width of 2e5 nm also accounts for a significant proportion. The pore volume and specific surface area are essential indices for the evaluation of the gas adsorption/desorption capacity of coal. The total pore volume and specific surface area of pulverized coal with different particle sizes were calculated, and the results are shown in Table 5. According to Table 5, the specific surface area of the pores is found to be well correlated with the pore volume, and the total pore volume and specific surface area of pulverized coal continually increase as the particle size of pulverized coal decreases. This observation is thought to be closely related to the damage to the coal structure and the reopening of closed coal pores that occurred when pulverized coal was formed. 3.2. Desorption experiments and diffusion coefficients of pulverized coal
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(mm/s), C is the volume fraction of the adsorbed gas in coal matrices, vC=vx is the concentration gradient in the direction of gas diffusion, and D is the diffusion coefficient (mm2/s). To solve Eq. (1), it is generally assumed that coal particles are isotropic and homogeneous spherical particles. Some scholars (Jian et al., 2012; Pillalamarry et al., 2011; Walker and Mahajan, 1978; Zhang, 2008) studied Fick’s diffusion law and obtained its analytical solution (if D is constant), which is as follows:
pffiffiffiffi D pffiffi Qt 6 D t3 2t F¼ ¼ pffiffiffi Q∞ r p r
(2)
where F is the gas desorption ratio per unit mass of coal, Qt is the desorption capacity per unit mass of coal at time t (m3/t), Q∞ is the limit of the desorption capacity per unit mass of coal as time t tends to infinity (m3/t), and r is the radius of the coal particle (mm). If D depends on time, then (Jian et al., 2012)
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Z
Z
t
Ddt
t
Ddt
3.2.1. Desorption experiments Studies on the processes associated with the occurrence and development of coal gas disasters indicate that coal and gas outbursts are accompanied by the rapid desorption of coal gas, where large amounts of gas desorbs from the coal mass in an extremely short time (Skoczylas et al., 2014; Wierzbicki and Skoczylas, 2014; Xue et al., 2014). Thus, gas desorption experiments were performed according to the following steps: First, the coal samples were manipulated to achieve adsorption equilibrium at 1 MPa; second, the free gas in the coal sample tank was rapidly released and the desorption data after relief of the gas pressure were recorded. The gas desorption volume and initial gas desorption rate are shown in Fig. 4. From Fig. 4, the gas desorption capacity of pulverized coal is found to increase with the decreasing particle size, which shows exact agreement with the coal pore characteristics. For each coal particle size, the gas desorption rate is relatively high at the initial stage and then decreases over time and eventually stabilizes. The initial gas desorption rate increases as the coal particle size decreases. This indicates that the higher the degree of fragmentation of coal, the stronger the desorption ability, especially the initial desorption ability.
6 F ¼ pffiffiffi
3.2.2. Diffusion coefficients Gas diffusion in a coal mass is a very complex process, and it is thought that the process can be described by Fick’s diffusion law re et al., 2010; Clarkson and Bustin, 1999; Lito et al., 2015; (Charrie Pillalamarry et al., 2011):
Therefore, y as a function of t can be calculated based on the gas desorption data. Then, the diffusion coefficients D at a given time can be obtained from:
vC q ¼ D vx
(1)
where q is the diffusion rate of gas flowing through a unit area Table 5 Total pore volume and specific surface area of pulverized coal with different particle sizes. Particle size range
Pore volume (102 mL/g)
Specific surface area (m2/g)
1e3 mm 0.5e1 mm 0.25e0.5 mm 0.2e0.25 mm 0.1e0.2 mm 0.074e0.1 mm 0.01e0.074 mm <0.01 mm
0.82 1.14 1.38 1.63 1.85 2.40 2.55 3.09
6.00 10.03 11.49 14.52 17.03 23.80 27.36 30.49
0
p
3
r
0
r2
(3)
In Eq. (3), we assume that
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi Z t
Ddt 0
x¼
(4)
r
Thus, Eq. (3) is a quadratic equation for x:
6 3x2 pffiffiffi x þ F ¼ 0
p
(5)
The value of x is derived as:
1 x ¼ pffiffiffi
p
rffiffiffiffiffiffiffiffiffiffiffiffi 1 F p 3
Then, let y ¼
y¼r
D¼
2
dy dt
1 pffiffiffi
p
Rt 0
(6) Ddt ¼ r 2 x2 . Namely,
rffiffiffiffiffiffiffiffiffiffiffiffi!2 1 F p 3
(7)
(8)
To smooth the data in the above differential calculation, F calculated from the sixth order polynomial is used to calculate y, and central differentials are used to obtain D. The diffusion coefficients of pulverized coal with different particle sizes are shown in Fig. 5. From Fig. 5, it can be seen that the diffusion coefficient of pulverized coal is not constant. To compare diffusion coefficients of coal with different particle sizes, the data from the first 20 min were selected and are shown in Table 6. Fig. 5 and Table 6 reveal that for pulverized coal with the same particle size, the diffusion coefficient decreases with increasing desorption time. Specifically, the diffusion coefficient decreases rapidly at the initial stage and then tends to stabilize after decreasing to a certain extent over time. It is considered that the pores near the external surface of coal matrices, especially macropores, desorbed first during the initial desorption stage, and then,
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Fig. 4. Gas desorption volume and initial gas desorption rate of pulverized coal with different particle sizes.
Fig. 5. Diffusion coefficients of pulverized coal with different particle sizes.
the gas from the inner pores of the coal matrices has to pass through a longer path and overcome a larger resistance. When some gas molecules diffuse from the coal matrices, the pore gas pressure in the coal matrices decreases. This leads to the shrinkage of the coal matrices and a decrease in the pore dimension (Harpalani and Chen, 1995, 1997; Lu et al., 2015b). As a result, the resistance to the diffusion of the subsequent gas molecules from the coal matrices increases greatly. Therefore, the diffusion coefficient decreases over time. From Fig. 5 and Table 6, the diffusion coefficient is also found to decrease with decreasing particle size. According to diffusion theory (Nie et al., 2013), the effective diffusion area is known to have a significant effect on the diffusion coefficient. As the particle size decreases, the pore surface area increases and the average pore
width decreases. As a result, the diffusion length is limited by the spatial scale of the pores, which leads to a decrease in the effective diffusion area. 3.3. Experimental comparative analysis To quantitatively analyze the difference of gas desorption between pulverized coal and unpulverized coal, anthracite coal (R0 ¼ 3.27%) from the Wolonghu Coal Mine and bituminous coal (R0 ¼ 1.17%) from the Linhuan Coal Mine were tested. The hardness of these two unpulverized coal samples is high; therefore, the samples were crushed by a pulverizer first; then, the coal samples were screened using sifters to obtain the desired particle size ranges of 0.01e0.074 mm, 0.1e0.2 mm, 0.25e0.5 mm and 1e3 mm.
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Table 6 Diffusion coefficients of pulverized coal with different particle sizes in the first 20 min. Time (s)
Diffusion coefficient (mm2/s) 1e3 mm
30 60 90 120 150 180 210 240 300 360 420 480 540 600 900 1200
5.445 3.608 2.818 2.350 2.039 1.788 1.611 1.469 1.286 1.096 9.762 8.810 7.993 7.340 5.227 4.075
0.5e1 mm 5
10 105 105 105 105 105 105 105 105 105 106 106 106 106 106 106
1.168 7.655 6.228 4.655 3.847 3.602 3.253 2.982 2.582 2.317 2.132 1.965 1.814 1.674 1.244 9.776
5
10 106 106 106 106 106 106 106 106 106 106 106 106 106 106 107
0.25e0.5 mm
0.2e0.25 mm
6
6
3.579 2.694 2.171 1.835 1.584 1.409 1.267 1.153 9.921 8.612 7.713 6.946 6.353 5.839 4.143 3.203
10 106 106 106 106 106 106 106 107 107 107 107 107 107 107 107
2.186 1.628 1.374 1.127 9.588 9.016 8.029 7.264 6.198 5.228 4.586 4.152 3.679 3.329 2.299 1.739
10 106 106 106 107 107 107 107 107 107 107 107 107 107 107 107
The gas desorption characteristics of the two unpulverized coal samples with different particle sizes were tested and compared with those of pulverized coal under the same conditions. The results are shown in Figs. 6 and 7. Figs. 6 and 7 indicate that the desorption capacity and the initial average gas desorption rate of pulverized coal are higher than those of the two unpulverized coal samples. The degree of metamorphism is known to have a significant effect on the gas desorption capacity of coal. Generally, the higher the degree of metamorphism, the stronger the gas desorption capacity of coal, which is consistent with the gas desorption results of anthracite coal from the Wolonghu Coal Mine and bituminous coal from the
0.1e0.2 mm 1.174 8.535 7.056 5.814 5.157 4.518 4.068 3.735 3.155 2.772 2.500 2.245 2.038 1.867 1.321 1.018
6
10 107 107 107 107 107 107 107 107 107 107 107 107 107 107 107
0.074e0.1 mm 5.254 4.171 3.377 2.738 2.266 1.975 1.795 1.591 1.363 1.189 1.054 9.539 8.551 8.021 5.514 4.251
7
10 107 107 107 107 107 107 107 107 107 107 108 108 108 108 108
0.01e0.074 mm 1.646 1.212 9.754 8.037 6.918 6.029 5.354 4.852 4.120 3.492 3.047 2.712 2.451 2.243 1.546 1.179
7
10 107 108 108 108 108 108 108 108 108 108 108 108 108 108 108
<0.01 mm 2.664 1.911 1.531 1.248 1.069 9.447 8.459 7.673 6.451 5.686 5.080 4.568 4.173 3.833 2.714 2.097
109 109 109 109 109 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010 1010
Linhuan Coal Mine. However, Figs. 6 and 7 show that the gas desorption law of pulverized coal is irregular, which indicates that the degree of metamorphism is not the primary factor affecting the gas desorption ability of pulverized coal. To a large extent, it is the decrease of the coal matrix size and damage to the coal pore structure during the formation of pulverized coal that determine the coal gas desorption properties. As a result, pulverized coal has both a higher gas desorption capacity and initial gas desorption rate; this is the reason that the value of Dh2 is still greater than 350 Pa, which is much higher than the threshold value when the gas pressure and content decreased to less than 0.15 MPa and 2 m3/ t, respectively, after a long period of gas drainage in the No. 7 coal
Fig. 6. Gas desorption curves of pulverized coal and unpulverized coal.
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Fig. 7. Initial average gas desorption rate of pulverized coal and unpulverized coal.
seam of the Haizi Coal Mine. The diffusion coefficient is recognized as a critical parameter for evaluating the methane diffusion properties in coal matrices re et al., 2010; Jin et al., 2015; Lito et al., 2015; Lu et al., (Charrie 2015a; Walker and Mahajan, 1978; Xu et al., 2015). In our research, the diffusion coefficient of pulverized coal was also compared with that of unpulverized coal. The results are shown in Fig. 8. Fig. 8 shows that the diffusion coefficient of pulverized coal is greater than that of unpulverized coal in the initial stage. Over time, the difference in the diffusion coefficient between unpulverized coal and pulverized coal becomes increasingly smaller, and the diffusion coefficient of unpulverized coal eventually becomes equal to or greater than that of pulverized coal. As mentioned above, the coal matrices shrink when the pore gas pressure decreases due to gas diffusion. The strength and hardness of the pulverized coal mass are lower than those of unpulverized coal; consequently, the pulverized coal skeleton is more easily affected. Thus, for pulverized coal, over time, the decreasing magnitude of the diffusion coefficient is more obvious than for unpulverized coal.
pores have a significant effect on gas storage, desorption and transport in coal. When pulverized coal was formed, many closed pores were transformed into open pores, and some semi-enclosed pores, such as the spherical and ink bottle types, might have deformed into cylindrical, conical or other types of pores. In addition, many interconnected pores were likely divided into many simple pores, and some long pores were possibly divided into several parts as the coal matrices were severely damaged. In other words, the pores in coal matrices become increasingly simpler during the generation of the pulverized coal. These changes caused the paths for gas entering or leaving pores to become short and caused the resistance to decrease significantly. Thus, for pulverized coal, the gas desorption capacity and initial gas desorption rate are extremely high, and the desorption and diffusion properties are quite different from those of unpulverized coal. If mining activities break the gas adsorption equilibrium state, compared with unpulverized coal, the gas in pulverized coal could much more rapidly be desorbed and diffuse from coal pores to fractures and then flow out of the coal. There is no doubt that the uncontrollable emission of such a large quantity of gas would quite easily initiate the coal and gas outburst.
3.4. Evolution mechanism of the pulverized coal pore structure Generally, the shape of pores in coal matrices is irregular. Based on the pore shapes, the types of pores in coal matrices can be classified into cylindrical, conical, spherical or ink bottle, slits and interstices, which are illustrated in Fig. 9(a) (Rouquerol et al., 1999; Wang et al., 2014a). According to the differences in pore connectivity, the types of pores in coal matrices can also be classified as passing, interconnected, dead end and closed pores, which are illustrated in Fig. 9 (b). The former three types (passing, interconnected and dead end) are also referred to as open pores. Open
4. Formation causes of pulverized coal A coal and gas outburst is a process in which the energy stored in the coal seam is released unsteadily, resulting in large amounts of coal (rock) and gas being expelled in an extremely short period of time (Beamish and Crosdale, 1998; Briggs, 1921; Hargraves, 1983; Lama and Bodziony, 1996; Shepherd et al., 1981). The energy is primarily provided by the gas stored in the coal, and the premise is that there is a large amount of gas rapidly desorbing from the coal
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Fig. 8. Diffusion coefficients of pulverized coal and unpulverized coal.
Fig. 9. Types of pores in coal matrices.
mass that could provide enough energy in an extremely short time. According to a laboratory experiment and site investigation, pulverized coal from the No. 7 coal seam in the Haizi Coal Mine possesses all of the conditions. Nowadays, many scholars are studying the reasons for the formation of tectonic coal or pulverized coal, among which the most important reason is the complex set of geological conditions.
4.1. Influence of regional tectonic movement The Haizi Coal Mine is located in the Huaibei Coalfield. The complex geological structure in the Huaibei Coalfield is primarily affected and controlled by the plate movement. The continuous sedimentation of strata began in the middle Carboniferous before successively experiencing three tectonic epochs, i.e., the Indosinian,
Yanshan and Himalayan movements (Wang et al., 2014c). The overlap of multiple periods of tectonic movements resulted in the reticular fault tectonic framework of the Huaibei Coalfield, which is shown in Fig. 10. Different grades of the tectonic movements and stress fields control not only the range and intensity of the tectonism but also the occurrence and distribution of CBM. Furthermore, they also determine the migration conditions of the coal seam and the degree of fragmentation of the coal. Fig. 10 indicates that the Haizi Coal Mine is located in the fault block enclosed by the Subei, GuangwueGuzhen, XiayieGushi and Fengwo faults and lies in the northwest portion of the Tongting anticline. This region is a general monoclinal structure with approximately an EeW strike and tilting northwards. According to the statistical results, there are 54 faults per square kilometer in the No. 7 coal seam of the Haizi Coal Mine, which are mainly normal
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Fig. 10. Geological structure distribution of the Huaibei Coalfield.
faults that suffered from extension tectonism. The Subei fault strikes EeW and cuts the Fengwo and XiayieGushi faults, which strike approximately NeS and are located in the eastern and western portion of the Haizi Coal Mine, respectively. The direction of the horizontal stress is approximately eastewest. Therefore, the relative movement between the roof and floor of the coal seam can easily occur when there is unbalanced tectonic movement in the strata (Li et al., 2011). In the long geological period since the deposition of coal, the large horizontal stress resulted in a substantial layereslide movement between the upper strata and underlying strata of the No. 7 coal seam in the Haizi Coal Mine. As a result, the raw coal was severely damaged and numerous damaged faces, e.g., the layered structure, were formed. For all of these reasons, the strength of the coal mass was substantially reduced and is easily further broken with additional stress. 4.2. Damage caused by the magmatic intrusion In the long geological history, the Huaibei Coalfield not only
experienced multiple periods of tectonic movement but was also greatly influenced by magmatic intrusion accompanying the tectonic movement. The magmatic intrusion that occurred in the Yanshanian of the Mesozoic was recognized to be the strongest, which plays an important role in damaging the coal mass. The magmatic rock in the Haizi Coal Mine was primarily squeezed into the strata, and the profile of the magmatic intrusion direction is shown in Fig. 11. Along with the magma squeezed into the strata with an average thickness of 120 m, the coal mass of the No. 7 coal seam under extremely thick magmatic rock was affected by both the gravity stress of the overlying rocks and the large extrusion stress caused by the magmatic intrusion (Stewart et al., 2005; Wang et al., 2014b), and the extrusion stress proportionally increased as the thickness of the overlying magmatic rocks increased. Geological exploration indicates that the thickest of the overlying magmatic rocks in the No. 7 coal seam of the Haizi Coal Mine approaches 170 m. The large extrusion stress intensifies the layereslide movement between the upper strata and underlying strata of the No. 7 coal seam. Under the combined effect of multiple
Fig. 11. Profile of the magma intrusion direction in the Haizi Coal Mine.
H. Guo et al. / Journal of Natural Gas Science and Engineering 33 (2016) 867e878
stresses, the coal experienced additional serious destruction. The result is that the activity of organic components in the coal mass was intensified. Then, tectonic coalification occurred in the coal mass and pulverized coal was eventually formed. For pulverized coal, the extreme decrease in the coal matrix size and the heavy damage to the coal pore structure caused by fragmentation determine its increased gas desorption and diffusion properties, which results in an unprecedented high potential for coal and gas outbursts in the pulverized coal seam. This research can provide a reference for the prediction of coal and gas outbursts in similar coal seams and lead to the improvement of safety conditions in the coal mining industry in the future. 5. Conclusions 1) The degree of fragmentation of pulverized coal is extremely high and very similar to coal powder produced after a coal and gas outburst. For pulverized coal, proximate analyses vary with the decreasing particle size, the pore size distribution with the different particle size is similar, and the total pore volume and specific surface area increase continually as the particle size decreases. According to the analysis for pore types, pores in the coal matrices became increasingly simple as pulverized coal was generated. 2) The gas desorption ability decreases with the increasing particle size of pulverized coal, and the diffusion coefficient decreases as the particle size decreases. For pulverized coal with the same particle size, the diffusion coefficient decreases rapidly at the initial stage and then tends to stabilize after decreasing to some extent, and the decreasing amplitude is more significant over time compared with unpulverized coal. 3) Under the combined effect of the large horizontal stress and magmatic intrusion, raw coal could easily turn into pulverized coal. Pulverized coal not only has an extremely decreased matrix size but also severely damaged pore structures, which leads to an unprecedented high potential for coal and gas outbursts. Acknowledgments The authors are grateful to the financial support from the sponsorship of Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents, the Qing Lan Project, the National Natural Science Foundation of China (No. 51574229, No. 51404260, No. 51304204), the Fundamental Research Funds for the Central Universities (2014XT02) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). References Airey, E.M., 1968. Gas emission from broken coal. An experimental and theoretical investigation. Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 5, 475e494. Barenblatt, G.I., Zheltov, I.P., Kochina, I.N., 1960. Basic concepts in the theory of seepage of homogeneous liquids in fissured rocks [strata]. J. Appl. Math. Mech. 24, 1286e1303. Barron, K., Kullmann, D., 1990. Modelling of outbursts at #26 Colliery, Glace Bay, Nova Scotia. Part 2: proposed outburst mechanism and model. Min. Sci. Technol. 11, 261e268. Beamish, B.B., Crosdale, P.J., 1998. Instantaneous outbursts in underground coal mines: an overview and association with coal type. Int. J. Coal Geol. 35, 27e55. Briggs, H., 1921. Characteristics of outbursts of gas in mines. Trans. Inst. Min. Eng. 61, 119e146. Busse, J., Scheuermann, A., Galindo-Torres, S.A., Bringemeier, D., Li, L., 2014. In-situ and Laboratory Measurements of Coal Matrix and Cleat Permeability. Crc PressTaylor & Francis Group, Boca Raton. Cao, Y., Davis, A., Liu, R., Liu, X., Zhang, Y., 2003. The influence of tectonic deformation on some geochemical properties of coalsda possible indicator of outburst potential. Int. J. Coal Geol. 53, 69e79. re, D., Pokryszka, Z., Behra, P., 2010. Effect of pressure and temperature on Charrie
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