Experimental Investigation for Pore Structure and CH4 Release

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Experimental Investigation for Pore Structure and CH4 Release Characteristics of Coal during Pulverization Process Leilei Si,†,‡ Zenghua Li,*,†,‡ Yongliang Yang,†,‡ Li Xin,†,‡ Zhen Liu,§ Yanan Liu,†,‡ and Xiaoyan Zhang†,‡ †

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China ‡ School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China § College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao, Shandong 266590, China ABSTRACT: Qinshui coalfield is the largest coalbed methane and anthracite production base in China. In the process of mine gas prevention and control, there was a larger amount of gas that can be extracted, but even with this, the residual gas pressure is not up to standard. In this paper, in order to analyze its specific reasons, a series of experiments were carried out to investigate the pore structure, particle size distribution, and CH4 release characteristics of anthracite during the pulverization process. Results show that, compared with bituminous coal, anthracite can continuously release a certain concentration of CH4 during the pulverization process. The SEM test exhibits that anthracite possesses more secondary pores while bituminous shows more mineral pores. With the increase of total pulverizing time, the coal particle size is gradually decreased. Furthermore, compared with the pre-pulverized samples, the porosity, pore volume, and surface area of post-pulverized samples are increased, indicating that the closed pores are destroyed to be open pores during the pulverization process. In addition, by combining the gas release concentration and pore volume evolution, it can be inferred that the anthracite may contain more solid solution gas due to the fact that there is a certain amount of gas that can be released when the pore volume tends to be smooth. The research results can provide the theoretical basis for gas extraction in the anthracite mine. the extraction technique, Ji et al.15,16 investigated the gas adsorption characteristics of different rank coal and deemed that gas could occur in the soluble organic matter of coal in the form of solid solution state. A series of experiments were carried out by Peng et al.,17 who thought that the soluble organic molecules could improve gas adsorption capacity due to the dissolved ability of gas. Pore structure is one of the most significant influencing factors affecting the gas occurrence and release.18,19 For the past few years, more and more test methods were used to analyze the pore structure of coal, such as gas adsorption, scanning electron microscope (SEM), and mercury intrusion porosimetry (MIP). Wang et al.20 used the N2 adsorption and MIP to test the pore structure of middle-high rank coal, considering that the volumes of micropore and mesopore increase with the increase of coal rank. Using the N 2 adsorption, Fu et al.21 tested the pore structure and pore shape of low rank coal and argued that the pores in low rank coal are favorable for gas diffusion and seepage. Li et al.22 used a series of advanced experimental methods to analyze the surface morphology and pore type of anthracite, deeming that anthracite is primarily composed of gas pores and the pore widths range from 10 nm to 2 μm. Using N2 adsorption and nuclear magnetic resonance (NMR) methods, Li et al.23 investigated the pore size distribution and fractal dimension of anthracite, considering that the anthracite possesses more small

1. INTRODUCTION Qinshui coalfield, one of the major coalfields in China, located in the south of Shanxi Province, is the largest coalbed methane and anthracite production base in China. Mine gas disaster is one of the most serious disasters that threaten the safe production of mines in the Qinshui coalfield. However, in the process of mine gas prevention and control, a steady stream of gas can be desorbed from coal pores, which makes it is difficult to reduce the residual gas pressure and the extraction effect is not up to standard. It is generally known that anthracite possesses an extremely complex pore structure and the gas occurrence in anthracite is elusive.1−6 Therefore, it is of great significance to investigate the pore structure characteristics and gas release characteristics of anthracite during the pulverization process. As is known to all, coalbed methane is mainly composed of adsorption gas and free gas.7−11 However, Alexeev et al.12 found that gas also occurs in the coal framework in the form of solid solution state, especially for a high pressure of >2 MPa. The solid solution gas is embedded in the coal macromolecules. Both adsorption gas and free gas can easily diffuse from pore to fracture, but it is difficult for solid solution gas to release from the coal framework. In the process of coalbed methane (CBM) extraction, the pore structure and framework of coal would be destroyed due to the impact of external mining, resulting in the release of solid solution gas, adsorption gas, and free gas. At present, many studies have reported the adsorbed gas and free gas, but few studies focused on the solid solution gas.13,14 This is because the solid solution state gas hardly can be tested by methods other than nuclear magnetic resonance (NMR). Using © XXXX American Chemical Society

Received: July 11, 2017 Revised: November 13, 2017 Published: November 22, 2017 A

DOI: 10.1021/acs.energyfuels.7b01995 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Experimental process.

Table 1. Proximate Analysis, Petrographic Composition, and Vitrinite Reflectance of Two Kinds of Samplesa proximate analysis/%

a

petrographic composition/%

sample

Mad

Ad

Vdaf

FCad

V

I

QC QD

1.76 0.56

11.34 13.85

7.20 24.19

82.28 65.32

82.0 80.8

9.6 16.0

L

M

R0/%

rank

8.4 3.2

2.58 1.37

anthracite C bituminous B

Mad, moisture; Ad, ash; Vdaf, volatile content; FCad, fixed carbon; V, vitrinite; I, inertinite; L, liptinite; M, mineral; R0, vitrinite reflectance.

size pores in the range of 50−100 nm. Ouyang et al.24 analyzed the pore structure characteristics of different coal and found that, with the rising rank, the coal reservoir tends to be orderly and uniform. Fu et al.25 investigated the CH4 adsorption capacity and surface area of different coal, considering that it is increased with the coal rank. In summary, the anthracite possesses a more complex pore structure. In addition, there are a lot of works that have been reported about the gas release characteristics. Some study investigated the effect of gas desorption on the coal deformation and proposed a coupling model to describe their relationship.26 Naveen et al.27 constructed a unipore gas kinetic model to estimate the gas diffusion coefficient in coal based on Fick’s law and found that the gas diffusion significantly depends on the pore structure and pore parameters. Keshavarz et al.28 studied the effect of coal rank on the gas diffusion in Australian coals, considering that the gas diffusion in coal pores exhibits 6 orders of magnitude depending on the coal rank. However, these studies primarily focused on the gas diffusion of desorption in the open pores. During the mine gas extraction process, the closed pores would be destroyed to be open pores. Further, the gas in closed pores would be released. Therefore, it is significant to investigate the gas release characteristics of coal during the pulverization process. In summary, to investigate the gas release characteristics of coal during the pulverization process, a series of experiments were performed to obtain the particle size distribution, pore structure, and surface morphology characteristics of coal. Then, by comparing the difference between anthracite and bituminous coal, the gas release mechanism of anthracite was studied, which could provide the theoretical basis for gas control in the anthracite mine of Qinshui coalfield.

coalfield. Compared with the QC samples, QD samples possess lower rank. There is a great difference between these samples. Moreover, the gas content and gas pressure in QD coal mine is much less than that in QC coal mine. All samples were directly collected from the working face and sealed to send to the laboratory. When the samples were received, we first checked and removed the waste rock from samples. Then, all samples were crushed and sieved into the appropriate particle size for subsequent experiments. The specific experimental process is shown in Figure 1. To study the properties of two kinds of coal, the proximate analysis of samples was tested by using a 5E-MAG6600 automatic proximate analyzer following the China national standard GB/T212-2008, the petrographic composition of samples was tested by using an XPF-800 polarizing microscope following the China national standard GB/ T8899-1998, and the rank of samples was classified based on International Organization for Standardization IOS11760:2005. The test results are shown in Table 1. 2.2. SEM Test. The surface morphology of coal was analyzed by using an FEI QuantaTM 250 SEM, which has three modes: high vacuum, low vacuum, and environmental vacuum. The coal samples can be magnified 6−100 million times in the acceleration voltage of 0.2−30 kV. 2.3. CH4 Release Concentration in Coal during Pulverization Process. To avoid the impact of adsorption gas and free gas, the initial coal samples (mass: 210 g, particle size: 1−2 cm) were placed in a sealed chamber for vacuum degassing for 6 h at 60 °C. Then, in the fresh and dry airflow conditions (O2: 21%, N2: 79%), the coal samples were placed in the ball mill to crush for 10 min at the speed of 60 r/ min. Finally, the CH4 concentration in the ball mill was tested by using an FULI9790 gas chromatograph. Furthermore, the coal samples were collected for the next cycle until the CH4 concentration is below 100 ppm. It is important to note that the sealed chamber and ball mill were evacuated before each test to avoid the effect of residual methane. The test system for CH4 release concentration in coal during the pulverization process is shown in Figure 2. 2.4. Particle Size Distribution Test. During the coal pulverization process, to obtain the coal samples particle size distribution, we used the screens to sieve pulverized coal samples. Then, the sieved coal samples were weighed to analyze particle size distribution. However, with the rising total pulverizing time (TPT), the particle size is too small to get accurate data by manual sieving. The pulverized coal samples were tested by using a Winner 2000 Laser particle size analyzer (LPSA), which can measure the emission spectrum of particle swarm to get the coal samples’ particle size distribution based on the optical principle.

2. EXPERIMENTAL METHODS 2.1. Sampling. Two kinds of samples were sampled from two coal mines of China, which are from Qincheng (QC) coal mine located in Shanxi province, and Qingdong (QD) coal mine located in Anhui province, respectively. QC coal mine is located in the south of Qinshui coalfield. During the process of mine gas extraction in the QC coal mine, a lot of gas was extracted, while the residual gas content and gas residual cannot be reduced. Further, the QD samples were chosen as the reference samples. QD coal mine was located in the Huaibei B


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3. RESULTS AND ANALYSIS 3.1. SEM Analysis. Figure 4 shows the SEM images of coal samples. According to the formation cause of pores, Zhang et al.29 recommends the pores to be classified into 4 types: primary pores, secondary pores, mineral pores, and exogenous pores. Observing the image of the scanning electron microscope, we can find various sizes of pores. The pores of QC samples are mainly composed of secondary pores (also called gas pores), which is usually relatively small and the pores size is from 0.01 to 2 μm. The connectivity of secondary pores is poor, so it is bad for gas seepage but good for gas adsorption.30 The QC samples are anthracite and the rank is relative high. In the process of coal metamorphism, a large amount of gas is produced. Then, the migration and production of gas results in substantial secondary pores, which in turn provide occurrence space for gas, which is why the Qinshui coalfield contains a lot of CBM. The QD samples show a more complex pore structure and surface morphology. The QD samples are mainly controlled by the mineral pores. A large amount of minerals are attached to the surface of the QD samples. The mineral pores size is affected by the mineral type, so we can find various sizes of pores in QD samples. In addition, QD samples also contain a certain amount of secondary pores, but the proportion of secondary pores is much smaller than that of QC samples. 3.2. CH4 Release Concentration Analysis. Figure 5 shows the CH4 release concentration in coal during the pulverization process. With the increase of TPT, the CH4 release concentration gradually decreases. The largest concentration of QC samples is 1738 ppm, while that of QD samples is 1354 ppm. It is significant to note that, when TPT is 70 min, the CH4 release concentration of QD samples is only 45 ppm, while that of QC samples is 725 ppm. Especially, the CH4 release concentration of QC samples can still reach 75 ppm when TPT is 170 min. The results show that QC samples contain a lot of closed pores, which would be destroyed in the process of coal pulverization, resulting in the gas release. In addition, the storage of gas in coal not only is in the free or adsorption state in pores but also occurs in the solid solution state. During the coal pulverization process, in addition to the

Figure 2. Test system for CH4 release concentration in coal during pulverization process. 2.5. Pore Structure Test. Further, the samples of different TPT were used to test the pore structure, and the pore structure test methods are shown in Figure 3. In order to acquire the pore structure

Figure 3. Pore structure test methods.

characteristics of coal samples, the MIP test was carried out by using the Autopore IV 9500 mercury porosimeter, which can measure the pore diameter in the range from 100 to 100 000 nm. Then, the pores below 100 nm was tested by using a 3H-2000PS2 N2 adsorption analyzer, which can accurately calculate the pores in the range from 2 to 100 nm.

Figure 4. SEM images of samples. C


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Figure 5. (a) The CH4 release concentration in QC coal during pulverization process. (b) The CH4 release concentration in QD coal during pulverization process.

LPSA test results agreed with the results obtained by manual sieving. In summary, it can be concluded that, as the TPT increases, the particle size of samples is gradually decreased, while it would be concentrated in the range of 10−100 μm. 3.4. Pore Structure Analysis. 3.4.1. Pore Structure Analyses Using MIP Method. Figure 8 shows the mercury intrusion−ejection curves of post-pulverized (70 min) and prepulverized samples. Compared with the pre-pulverized samples, the intrusion mercury volume of post-pulverized samples shows a notable increase, which is agreement with the literature reported by Jin et al.31 It is important to note that the mercury intrusion−ejection curves of post-pulverized and pre-pulverized samples exhibit a remarkable difference. Post-pulverized samples: At the low pressure stage, the intrusion mercury curve exhibits a sharp rise. Then, at the high pressure stage, the curve gradually tends to be smooth, and the curves of intrusion−ejection are almost coincident, indicating that the pores of post-pulverized samples are mainly composed of open pores, which possess better connectivity. Pre-pulverized samples: At the low pressure stage, the intrusion curve shows a relative smooth trend. Then, at the high pressure, the intrusion curve exhibits a steep rise. In addition, unlike the intrusion−ejection curve of post-pulverized samples, at the high pressure, the intrusion−ejection curve of pre-pulverized samples shows an obvious misalignment, indicating that the pores of pre-pulverized samples are mainly composed of dead end pores (especially the ink-bottle-shaped pores), which possess poorer connectivity.32,33 Figure 9 shows the pore size distribution of post-pulverized and pre-pulverized samples by MIP test. With the rising intrusion pressure, the pore structure of coal samples would be destroyed due to the excessive pressure. Therefore, the pores below 100 nm cannot be accurately measured based on the MIP test. In this paper, MIP test results are used to analyze the pores above 100 nm. Observing the pore size distribution curve, we can find that the pore volume of post-pulverized samples increases obviously, especially the pores in range of 1000−100 000 nm. In addition, compared with the pre-pulverized samples, the pore size distribution curve of post-pulverized samples exhibits a remarkable bimodal feature, one of the peaks located at 3000 nm and the other located at 10 000 nm, indicating that the pores of post-pulverized samples are concentrated in these two locations.

destroyed pore structure, the tight coal framework would be broken, leading to the release of solid solution gas in the coal framework. For QC samples, there is still a certain amount of released CH4 when the coal samples have been crushed for 170 min. Compared with the QD samples, it can be inferred that most closed pores have been destroyed, indicating that QC samples may contain more solid solution gas. 3.3. Particle Size Distribution Analysis. Figure 6 is the particle size distribution by manual sieving. With the increase of

Figure 6. Particle size distribution by manual sieving (QC samples).

TPT, the particle size is gradually decreased. As the TPT increases from 30 to 50 min, the quality of coal samples below 74 μm exhibits a steep increase, while that above 74 μm shows a corresponding decrease. With the increase of TPT from 50 to 70 min, the quality of coal samples below 74 μm rises slightly, that in the range from 74 to 250 μm is relatively stable, and that above 250 μm shows a corresponding reduction. In order to acquire more accurate data, the LPSA was used to test the samples particle size distribution. As is shown in Figure 7, the samples particle size is mainly concentrated in the range of 10− 100 μm. With the increase of TPT, the samples below 10 μm exhibit a remarkable increase, while that in the range of 100− 700 μm shows a notable reduction. After the TPT is 50 min, the particle size distribution shows a relatively stable trend. The D


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Figure 7. (a) The particle size distribution by LPSA test when TPT is 30 min (QC samples). (b) The particle size distribution by LPSA test when TPT is 50 min (QC samples). (c) The particle size distribution by LPSA test when TPT is 70 min (QC samples).

Figure 8. (a) The mercury intrusion−ejection curve of post-pulverized (70 min) and pre-pulverized QC samples. (b) The mercury intrusion− ejection curve of post-pulverized (70 min) and pre-pulverized QD samples.

Figure 9. (a) Pore size distribution of post-pulverized (70 min) and pre-pulverized QC samples by MIP test. (b) Pore size distribution of postpulverized (70 min) and pre-pulverized QD samples by MIP.

E


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Figure 10. (a) The N2 adsorption−desorption curve of post-pulverized (70 min) and pre-pulverized QC samples. (b) The N2 adsorption− desorption curve of post-pulverized (70 min) and pre-pulverized QD samples.

Figure 11. (a) Pore size distribution of post-pulverized (70 min) and pre-pulverized QC samples by N2 adsorption test. (b) Pore size distribution of post-pulverized (70 min) and pre-pulverized QD samples by N2 adsorption test.

3.4.2. Pore Structure Analyses Using N2 Adsorption Method. Figure 10 shows the N2 adsorption−desorption curves of post-pulverized and pre-pulverized samples. Though the gas adsorption volume of post-pulverized samples is significantly larger than that of pre-pulverized samples, the adsorption curve types of post-pulverized and pre-pulverized samples are similar. IUPAC recommendations are for physisorption isotherms to be grouped into 8 types.34 In this paper, the isotherms of samples can be classified as a combination of type IV(a) and type II, indicating that coal samples are given by micro-macroporous adsorbents. There is a notable uptake at low relative pressure (P/P0 < 0.02), where the gas adsorption follows the monolayer adsorption theory. The point B, the beginning of the middle almost linear section, represents the end of the monolayer adsorption and the beginning of the multilayer adsorption. Then, the adsorption isotherms show a steep uptake at high relative pressure (P/P0 ≈ 0.8), resulting in pore condensation at high P/P0. The traditional isotherms of type IV(a) remain nearly horizontal over the upper range of P/P0, but the isotherms of samples in this paper are replaced by an unlimited growth trend, which is the feature of type II isotherms, indicating that the macropores in coal increase the gas adsorption at P/P0 ≈ 1 following the capillary condensation theory. In summary, the results show

that coal samples exhibit a wide range pore size, including micropores, mesopores, and macropores. Figure 11 shows the pore size distribution of post-pulverized and pre-pulverized samples by N2 adsorption test. As we can observe, the pore sizes of all samples are concentrated in 3 nm. The pore volume increment of QC samples is obviously larger than that of the QD samples, indicating that the QC coal samples contain more closed pores. In the process of coal pulverization, the closed pores are transformed into open pores, result in the rising pore volume. Furthermore, the evolution of pore volume was analyzed during the coal pulverization process. As shown in Figure 12, the pore volume is increased with the rising TPT. In the initial stage, the pore volume shows a notable increase, which is called the steep stage. This is because, in the initial stage, the coal samples contain more closed pores, resulting in the remarkable increment of pore volume. Then, with the increase of TPT, more and more closed pores are destroyed to be open pores, leading to the stable pore volume in the later stage, which is called the smooth stage. In summary, before 100 min, the pore volume is increased sharply due to the destroyed closed pores. However, the pore volume tends to be smooth because all the closed pores are destroyed. In addition, the pore volume curve F


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theory to define the complexity of pore structure.35−37 However, the commonly used fractal theory can only analyze the N2 adsorption results and MIP results individually. In order to study the fractal dimension of full pore size, we adopt the Menger theory to analyze the results of the combination of N2 adsorption and MIP.38,39 On the basis of the Menger theory, the pores of coal can be assumed to be spherical.16 Then, we obtain N = Cr −d f

(1)

where r is the effective radius (m), N is the number of pores when pore radius is r, C is a constant, and df is the fractal dimension. Then, taking the logarithm of both sides of eq 1, we obtain ln(N ) = ln(C) − df ln(r )

(2)

In the process of calculation, the pores below 100 nm were tested by N2 adsorption, while the pores above 100 nm were tested by MIP. Figure 14 and Table 3 show the results of fractal dimension. As we can observe, the fitting curve can match well experimental data and R2 is above 0.99. All df of post-pulverized samples exhibit a notable reduction. Compared with the prepulverized samples, df of QC samples is reduced by 0.52, and that of QD samples is reduced by 0.45. This is because the complexity of the pore structure is reduced, resulting in a more uniform pore size. Especially, during the coal pulverization process, the closed pores and dead end pores are changed with open pores, which improves the pore connectivity, leading to the release of gas. In addition, Δdf of QC samples is larger than that of QD samples, indicating that pre-pulverized QC samples possess more closed pores and dead end pores, which is in agreement with the MIP test results and closed porosity test results. Further, the data from Alexeev’s literature40 were used to analyze the relationship between coal rank and closed porosity, which is shown in Figure 15. As we can observe, with the rising rank, the open pore is decreased while the closed pore is increased. In this paper, the QC samples show lower volatile content (7.20), which is a lot smaller than the volatile content of QD samples (24.19). By combining Tables 2 and 3 and Figure 15, it can be concluded that the QC samples contain more closed porosity. During the CBM extraction of Qincheng coal mine, the pore structure would be constantly destroyed due to the influence of external mining, causing that the residual pressure cannot be reduced. As is known to all, in addition to the adsorption gas and free gas, the gas also occurs in the coal framework in the form of solid solution state. Alexeev et al.,41 using the 1H HMR to test the content of solid solution gas, argued that the gas can dissolve in the coal framework especially at high pressure (>2 MPa). As described in section 3.1, QC samples exhibit

Figure 12. Evolution of pore volume during QC samples pulverization process.

is similar to the gas release concentration, which would be discussed in detail. 3.4.3. Full Pore Size Analysis. Table 2 lists the pore volume and specific surface area of full pore size. To accurately investigate the pore characteristics, the pore size below 100 nm was tested by N2 adsorption, while that above 100 nm was tested by MIP. As we can observe, after the samples are pulverized, the pore volume and specific surface area exhibit a notable increase. This is because, during the coal pulverization process, the closed pores are transformed into open pores, resulting in the rising pore volume and specific surface area. Further, as we all know, the porosity is controlled by the pore volume and coal matrix volume. During the coal pulverization process, the total pore volume of the QC sample is increased by 0.5650 cm3·g−1, while that of the QD sample is only increased by 0.4754 cm3·g−1, indicating that the QC sample contains more closed pores. During the pulverization process, there are more closed pores that can be transformed to be open pores.

4. DISCUSSION During the process of CBM extraction of the Qincheng coal mine, though a steady stream of gas can be extracted from the coal seam, the residual gas pressure cannot be effectively reduced. In this paper, to analyze its specific reasons, the coal samples were constantly pulverized to release the gas in closed pores and the gas in the form of solid solution state. Figure 13 shows the change of pore structure and gas content during the coal pulverization process. Pore structure is one of the significant factors affecting the storage and migration of gas. Many studies used the fractal

Table 2. Pore Volume and Specific Surface Area of Full Pore Sizea N2 adsorption (100 nm)

SN2/m ·g 2

−1

1.43 0.16 1.60 0.27

VMIP/cm ·g 3

0.577 0.015 0.481 0.011

−1

total pore volume −1

SMIP/m ·g 2

0.429 0.011 0.727 0.008

VN2 + VMIP 0.5811 0.0161 0.4877 0.0123

a

VN2, pore volume by N2 adsorption test; SN2, pore specific surface area by N2 adsorption test; VMIP, pore volume by MIP test; SMIP, pore specific surface area by MIP test. G


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Figure 13. Change of pore structure and gas content during coal pulverization process.

Figure 14. (a) The fitting curve of fractal dimension of QC samples based on the full pore size test results. (b) The fitting curve of fractal dimension of QD samples based on the full pore size test results.

Table 3. Calculation Results of Fractal Dimensiona samples QC QD a

post-pulverized pre-pulverized post-pulverized pre-pulverized

df

Δdf

ln(C)

R2

2.46 2.98 2.51 2.96

0.52

−13.40 −23.99 −14.12 −23.41

0.992 0.999 0.991 0.998

0.45

primarily from the framework of coal. In the initial stage, the released gas is primarily controlled by the closed porosity. However, in the later stage, the released gas is mainly affected by the solid solution gas due to the fact the all the closed pores have been destroyed. As described in Figure 13, unlike the adsorption gas and free gas, it is difficult for solid solution gas to release from the coal framework. However, when the framework is destroyed, the solid solution gas can be released easily. In summary, it can be concluded that a larger amount of closed pores and dead end pores is the primary reason that the residual gas pressure of coal mines in Qinshui coalfield cannot be reduced, and the solid solution gas in the coal framework also plays a significant role.

Δdf, the difference of df; R2, fitting coefficient.

relatively high rank and a lot of secondary pores (gas pores), indicating that generous gas was generated during the process of coal metamorphism. It can be deduced that there must be a certain amount of gas stored in the coal framework in the form of solid solution state. In addition, according to the results of section 3.2, compared with the QD samples, QC samples can constantly release a certain amount of gas, indicating that QC samples may contain more solid solution gas. Further, combining Figures 5 and 12, as we can observe, in the initial pulverization stage, the CH4 release amount and pore volume show a steep increase. As the TPT increases, the closed pore is gradually decreased, and the CH4 concentration is reduced, indicating that the gas in closed pores can be released during the coal pulverization process. For the QC samples, although the pore volume tends to be constant after 100 min, there is a certain amount of CH4 that can be released, indicating that none of the new open pores were generated, and this gas is

5. CONCLUSIONS In this paper, to investigate the reason that the residual pressure cannot be reduced in coal mines of Qinshui coalfield, a series of experiments were carried out to study the CH4 release concentration, particle size distribution, and pore structure. Specific conclusions are as follows: (1) The pore types were analyzed by SEM test. QC samples are controlled by the secondary pores, while the QD samples are composed of mineral pores and secondary pores. Compared with the QD samples, during the process of coal metamorphism, a large amount of gas is H


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Figure 15. (a) Value of relation of opened porosity to total porosity depending on stage of metamorphism. (b) Value of relation of closed porosity to total porosity depending on stage of metamorphism (reported by Alexeev et al.40).

■

generated, resulting in numerous secondary pores, which in turn provide storage space for gas. (2) By continuously pulverizing coal samples, the CH4 release concentration was tested. With the rising TPT, the CH4 release concentration is gradually decreased. Compared with the QD samples, QC samples can constantly release a certain amount of CH4, indicating that QC samples contain more closed pores and solid solution gas. During the coal pulverization process, the pore structure and coal framework are destroyed, resulting in the release of gas. (3) A series experiments were performed to investigate the particle size distribution and pore structure characteristics. With the increase of TPT, the particle size is gradually decreased. Compared with the pre-pulverized samples, the pore volume and specific surface area of post-pulverized samples exhibit a notable increase, indicating that a larger number of new pores are generated and the closed pores are transformed into open pores. Using the full pore size results, the fractal dimension was calculated. The fractal dimension of postpulverized samples shows a notable reduction, indicating that the pore structure is simplified, leading to the release of gas.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zenghua Li: 0000-0003-4331-4256 Yongliang Yang: 0000-0003-0293-1194 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (2017CXNL02), the program for Innovative Research Team in University of Ministry of Education of China (IRT13098), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). I


Article

Energy & Fuels

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