Structural Evolution Characteristics of MiddleâHigh Rank Coal

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Structural Evolution Characteristics of Middle–High Rank Coal Samples Subjected to High-Voltage Electrical Pulse Fazhi Yan, Baiquan Lin, Jiang Xu, Yihan Wang, Xiangliang Zhang, and Shoujian Peng Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03991 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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Structural Evolution Characteristics of Middle–High Rank Coal Samples Subjected to High-Voltage Electrical Pulse Fazhi Yana,b*, Baiquan Linc, Jiang Xua,b, Yihan Wangc, Xiangliang Zhangc and Shoujian Penga,b a

State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing

University, Chongqing 400044, China b

College of Resources and Environmental Science, Chongqing University, Chongqing

400044, China c

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China

University of Mining and Technology, Xuzhou 221116, China Abstract High-voltage electrical pulse (HVEP) technology has been proposed to increase the gas production of low-permeability coal reservoirs in recent years. In this study, we investigated the variation characteristics of pore structure of coal samples by combining scanning electrical microscopy (SEM) with mercury intrusion porosimetry (MIP) analysis, to better understand the structural evolution characteristic of middle– high rank coal subjected to HVEP. Furthermore, changes in the chemical structure of the coal samples before and after HVEP treatment were investigated by Fourier transform-infrared spectroscopy (FT-IR) analysis. The results show that under the action of HVEP, both anthracite and bituminous coal samples can be crushed into many small pieces. Because the conductivity of anthracite coal samples is better than that of bituminous coal samples, the average breakdown voltage of anthracite coal

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samples is lower than that of bituminous coal samples. It was found that the greater the breakdown voltage, the more the number of particles formed after the coal is broken, and the smaller the particle diameter. At the same time, many pores and cracks were generated in the middle–high rank coal subjected to HVEP. In particular, the mesopores and macropores of coal samples subjected to HVEP are clearly higher than those of raw coal samples. The fractal dimensions of seepage pores of coal samples subjected to HVEP are bigger than those of raw coal samples. The increased pores and cracks are conducive to the release and migration of methane. In addition, oxidation reactions occur on the surface of coal subjected to HVEP. Following HVEP treatment, the surface chemical structure of both anthracite and bituminous coal showed significant changes; importantly, the concentration of oxygen-containing functional groups increased, which benefits methane release. Keywords: Coal; Pore structure; Chemical structure; High-voltage electrical pulse; Coalbed methane. 1. INTRODUCTION Coalbed methane (CBM) is an efficient and clean energy resource that plays an important role in meeting the global demand for energy [1,2]. China’s CBM reserves are estimated at 37 trillion cubic meters, ranking it third in the world [3]. Unfortunately, CBM reservoirs in China are characterized by low permeability, resulting in low CBM production rate [4,5]. One solution to improve the recovery of CBM is to artificially increase the permeability of coal seams [6,7]. Therefore, many methods for enhancing the permeability of CBM reservoirs have been developed,


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such as deep hole presplitting blasting, hydraulic slotting, hydraulic fracturing, and injection of carbon dioxide or nitrogen [8-13]. Although some progress has been made in improving the production of CBM, all of these technologies all have some shortcomings (e.g., small effective influence scope, water blocking effect, water pollution, a complicated process) [14-18]. Therefore, there is an urgent need to develop new technologies to effectively improve CBM production. Research into high-voltage electrical pulse (HVEP) technology began in the 1930s with the use of capacitors to discharge electricity for producing X-rays. The technology developed rapidly since the 1960s and formed an independent discipline [19]. In the HVEP technology, energy is stored for a relatively long period, and then compressed and transformed, eventually releasing huge power instantaneously and efficiently; the technology is characterized by high power, short pulse duration, high voltage, and high current [20,21]. Besides, the technology is environment friendly. After more than half a century of research, this technology is being widely applied in many fields, such as medical (e.g., to breakdown renal calculi), biochemical (e.g., to extract valuable materials from solid waste), engineering (e.g., for hole drilling, to increase the permeability of oil reservoir in oil fields) [22–27]. In particular, HVEP has an important application in the disintegration of solid materials [28,29]. The phenomenon of disintegrating solid materials by HVEP was accidentally discovered by Yutkin while decomposing water into hydrogen and oxygen using HVEP in the 1950s [22]. Subsequently, this novel technology, which uses HVEP to disintegrate solid materials, attracted the attention of many scientific research institutions


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worldwide. The mechanism underlying HVEP disintegration of solid mainly includes the following steps: first, the plasma channel will form inside the solid material under a sufficient high electric field. Second, a large amount of energy will be injected into the plasma channel in a very short period, which releases a huge amount of heat and causes the plasma channel to expand sharply. Finally, the shock wave produced by the expanding channel is converted into an elastic wave with an expressed region of tensile tangential stresses that can stimulate radial crack nucleation, eventually breaking down the solid and generating many pores and cracks on its surface [30,31]. Given the advantages of HVEP technology in cracking solid materials, there is an increased interest in applying it to fracture coal for enhancing the permeability of CBM reservoirs [32-34]. In fact, some studies have used this technology in the field of mining already and achieved good results [35-37]. Although anthracite coal can be disintegrated by HVEP in an air environment [34], very few studies have evaluated the effect of HVEP in improving the pore structure of middle–high rank coal, and the structural evolution characteristics of coal (including anthracite) subjected to HVEP are still not completely clear. To gain a better insight into the structural evolution characteristics of middle– high rank coal subjected to HVEP, an experimental system was developed and the cracking mechanism of coal samples (anthracite and bituminous coal) was studied. The variation characteristics of pore structure of coal subjected to HVEP were investigated by combining scanning electrical microscopy (SEM) with mercury intrusion porosimetry (MIP). Furthermore, changes in the surface chemical structure


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of coal samples before and after HVEP treatment were investigated by Fourier transform-infrared spectroscopy (FT-IR) analysis. 2. EXPERIMENTAL ANALYSIS 2.1. Sample Preparation To study the structural evolution characteristics of middle–high rank coal samples subjected to HVEP, anthracite and bituminous coal samples were collected from Jiaozuo coalfield (JZ) in Henan province of China and Xinbei coalfield (XB) in Gansu province of China, respectively. Proximate and ultimate analyses were performed on these samples and their results are presented in Table 1. Maceral analysis of these samples are presented in Table 2. Spectral analysis of these coal samples was performed using energy-dispersive spectrometer (EDS; Bruker QUANTAX 400-10) and their elemental composition are shown in Fig. 1. Both anthracite and bituminous coal samples contained iron and aluminum, which are highly conductive metal elements that can aid in the electrical breakdown of coal body [38]. Prior to HVEP, samples were processed into cylinders with a diameter of 50 mm and a height of 10 mm. We included five samples from each of the two types of coal [Fig. 2(b)]. Table 1. Properties of coal samples Proximate analysis (%)

Ultimate analysis (%)

Ro, max

Coal rank

Sample location Mad

Aad

Vad

FCad

Oad

Cad

Had

Nad

(%)

Jiaozuo coalfield

2.49

8.56

7.68

81.27

2.36

82.41

2.88

0.90

3.09

Anthracite

Xinbei coalfield

9.32

6.94

30.08

53.66

12.98

65.96

3.77

0.79

0.52

Bituminous


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Note: Mad is the moisture content (air-dried basis), Aad is the ash content (air-dried basis), Vad is the volatile matter content (air-dried basis), and FCad is the fixed carbon (air-dried basis). Table 2. Maceral analysis of coal samples Sample location

Vitrinite (vol %)

Inertinite (vol %)

Exinite (vol %)

Minerals (vol %)

Jiaozuo coalfield

90.26

8.41

0

1.33

Xinbei coalfield

57.24

41.44

1.32

3.80

Fig. 1. EDS spectra of the coal samples: (a) JZ anthracite coal sample; (b) XB bituminous coal sample.


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2.2. Experimental System An HVEP generation equipment was developed in this work. The equipment consists of the following parts: a high-voltage power, which can output a maximum voltage of 50 kV; a capacitor with a capacitance of 8 µF; a discharge switch; a breakdown cavity; an anode and a cathode, which are installed symmetrically in the breakdown cavity, as shown in Fig. 2(a). The maximum energy of a discharge can reach up 10 kJ. A high-speed video camera (Phantom, USA) was used to record the breakdown process of coal sample subjected to HVEP. The maximum resolution of the camera is 1024 × 1024 pixels with a maximum shooting speed of 140,000 frames per second. 2.3. Experimental Scheme 2.3.1. Coal sample breakdown by HVEP JZ anthracite and GS bituminous coal samples (five coal samples each) were used for the HVEP breakdown test, respectively. First, a cylindrical coal sample was placed between the anode and the cathode, with one end of the sample in contact with the anode and the other in contact with the cathode. The coal samples have significant anisotropic characteristics [39-41], which will affect the position of plasma channel [38]. The anisotropic characteristics of coal, thickness of coal between the anode and cathode tips, and the amount of energy injected into the coal will all affect the structure evolution of coal during electrical breakdown. By contrast, the orientation of electrodes to the cylindrical coal sample has no effect on the structure evolution of the coal body. Second, the high-voltage power charge to the capacitance is applied. When the voltage reaches the set breakdown voltage, the capacitor charging is stopped and


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the discharge switch is turned on. Finally, the energy stored in the capacitor breaks down the cylindrical coal sample placed between the anode and the cathode. 2.3.2. Structural evolution characteristics of the coal samples To investigate the structural evolution characteristics of JZ anthracite and XB bituminous coal samples during HVEP treatment, SEM and MIP analyses were performed. Surface morphology changes of coal samples were investigated using Quanta 250 SEM (FEI, USA). The MIP analysis was performed using Micromeritics AutoPore IV 9500 (test pressure, 0.1–60,000 psia, with the mercury contact angle assumed to be 130°). Changes in the surface chemical structure of the coal samples were investigated by FT-IR analysis (For the FT-IR analyses, a Bruker VERTEX 80v Fourier transform-infrared spectrometer was used and the spectrum was obtained with a resolution 4 cm-1, between 4000 and 400 cm-1; the samples were scanned 32 times.).

Fig. 2. Experimental system: (a) schematic diagram of the HVEP generation equipment; (b) coal samples. 3. Experimental results


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3.1. Breakdown Process and Macroscopic Breakage Characteristics A high-speed video camera (Phantom, USA) was used to record the breakdown process of coal sample subjected to HVEP, and the captured images are presented as Fig. 3. A burning phenomenon that lasted about 65667 µs was seen during the breakdown process. This phenomenon can be explained as follows: First, during the process of electrical breakdown, a plasma channel is formed inside the coal body. A large amount of energy is injected into the plasma channel in a short period, causing a large thermal expansion stress that breaks the coal around the plasma channel. Under the action of thermal expansion stress, fine pulverized coal is produced; furthermore, a large amount of methane is released from the broken coal body. The fine pulverized coal and methane released are burnt at high temperatures under the condition of air, which is captured in the images. To avoid methane explosion during the practical implementation of HVEP to crush the coal for increasing CBM production, we can draw on the experience of deep hole blasting technology for enhancing the permeability of coal seam [42]. First, a cross-borehole is drilled from rock roadway to the coal seam. Second, the electrodes connected to the high-voltage power are put into the cross-borehole. Third, the cross-borehole is sealed with a sealing material to avoid methane’s contact with air. Importantly, in this method, the duration of the discharge is very short (in the microsecond range) during the process of high-voltage breakdown and lacks enough oxygen in the cross-borehole, which prevent methane explosion.


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Fig. 3. Breakdown process of a coal sample subjected to HVEP. The destructive characteristics of coal samples subjected to HVEP are shown in Fig. 4. It can be seen that both anthracite and bituminous coal samples are broken into many pieces and some extremely fine particles after the electrical pulse breakdown. In this experiment, to crush the samples, the average breakdown voltage of JZ anthracite coal was 14.8 kV, whereas that of XB bituminous coal was 17.8 kV. The average breakdown voltage of XB bituminous coal samples is higher than that of JZ anthracite coal samples, because anthracite coal is a kind of semiconductor, and thus its conductivity is better than that of bituminous coal [43,44]. By analyzing the fracture characteristics of the coal samples subjected to HVEP, it can be found that the greater the breakdown voltage, the greater the number of particles formed after the coal is broken, and the smaller the particle diameter.


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Fig. 4. Destructive characteristics of coal samples subjected to HVEP. 3.2. SEM Results The surface morphology changes of the raw coal samples and those subjected to HVEP were analyzed by SEM (Fig. 5). The surface of both raw anthracite and bituminous coal sample is flat and smooth, with nearly no pores and cracks on it [Fig. 5(a) and (c)]. By contrast, the surface of anthracite and bituminous coal subjected to HVEP had burn marks and was filled with ravines; besides, many pores and cracks were noted on their surface [Fig. 5(b) and (c)]. The surface morphology of the coal subjected to electrical breakdown was similar to that of the heated coal [45,46], which is because during electrical breakdown, large amounts of energy are injected into the plasma channel in the coal body instantaneously, forming a very-high-temperature environment. As a result, the surface morphology of coal around the plasma channel presents traces of burn. The SEM images show the presence of multiple pores and cracks on coal samples subjected to HVEP, which are beneficial for migration of methane and effectively improve coalbed methane extraction efficiency.


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Fig. 5. SEM images of coal samples (magnification 3000×): (a) JZ raw coal; (b) JZ coal sample subjected to HVEP; (c) XB raw coal; (d) XB coal sample subjected to HVEP. 3.3. Result of MIP Analysis The MIP analysis was performed to gain a clear insight into the nature of the pore structure, such as pore geometry, pore size distribution, and connectivity [47]. The test also provides information about the influence of HVEP treatment on pore properties of middle–high rank coal samples. Considering the pore characteristics of coal samples, the Hodot classification method of pore size was applied as follows: micropores (pore radius < 10 nm), transition pores (pore radius between 10 and 100 nm), mesopores (pore radius between 100 and 1000 nm), and macropores (pore radius > 1000 nm) [48,49]. The cumulative pore volume of both JZ anthracite and XB


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bituminous coal increased after HVEP treatment (Fig. 6). HVEP also had a significant influence on the pore size distribution of both JZ anthracite and XB bituminous coal samples (Fig. 7), which is supported by data presented in Table 3. Table 3. Pore properties of the coal samples determined by MIP. Coal

Pore volume (mm3 g-1)

Total pore

Porosity

samples

Micro- Transition Meso- Macro- Total

area (m2·g-1)

(%)

JZ1

18.6

8.1

0.9

1.8

29.4

17.3

3.7

JZ2

15.4

8.0

2.5

7.8

33.7

14.5

4.7

XB1

52.6

26.7

3.2

5.3

87.8

46.6

10.1

XB2

43.1

42.1

39.3

14.6

139.1

40.8

15.8

Note: JZ1 is JZ raw coal sample; JZ2 is JZ coal subjected to HVEP; XB1 is XB raw coal sample; XB2 is XB coal subjected to HVEP. Thus, both micropores and transition pores have a dominant role in determining the pore volume of both JZ anthracite and XB bituminous coal. This result is consistent with the well-known viewpoint that micropores and transition pores comprise most of the pore volume in the middle–high rank coal [50]. Micropores and transition pores are favorable for methane adsorption and storage, whereas mesopores and macropores are seepage pores, which are favorable for methane transport [51]. Compared with raw coal samples, the number of micropores of JZ anthracite and XB bituminous coal samples decreased after HVEP treatment, whereas that of mesopores and macropores increased. This can be explained as follows: during the electrical breakdown process of coal, a large amount of energy was injected into the coal body,


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which broke down the coal under the action of electric stress and thermal expansion. This formed many pores, with some of the primary micropores also having evolved into mesopores and macropores. This further results in the total pore area of the coal subjected to HVEP being less than that of the raw coal, as shown in Table 3. The decrease of the total pore area will reduce the methane adsorption, which favors the release of methane. At the same time, the porosity of coal subjected to HVEP is higher than that of raw coal, which improves the connectivity of coal pores. In sum, the results of the MIP analysis indicated that HVEP can effectively improve the pore structure and permeability of both anthracite and bituminous coal samples, which will benefit the efficient exploitation of CBM.

Fig. 6. Cumulative pore volume of coal samples: (a) JZ anthracite coal; (b) XB bituminous coal.


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Fig. 7. Effect of HVEP treatment on pore size distribution of coal samples from MIP: (a) JZ anthracite coal; (b) XB bituminous coal. 3.4. Fractal Dimension Analysis Fractal geometry is a widely used method to quantitative describe irregular structures. And previous studies have shown that the pore structure of coal exhibits fractal characteristics and that the fractal dimension can characterize the roughness of the pore surface [49,52]. In this study, we analyzed the fractal characteristics of pore


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structure by the MIP analysis. The relationship between mercury intrusion pressure and pore volume in the MIP analysis was established based on the Washburn equation [53]

⁄ ∝

(1)

where is pore volume, is intrusion pressure, and D is fractal dimension. A double logarithmic equation is established between pore volume and intrusion pressure based on equation (1) as follows:

lg ⁄ ∝ − 4lg

(2)

where is the cumulative mercury intrusion volume when the intrusion pressure is

. Using lg ⁄ and lg, a scatter diagram is drawn, and the straight line is fitted according to the scatter diagram, where the slope of the straight line is equal to − 4 . Therefore, the fractal dimension D can be calculated using the form

= 4 + . In this experiment, when lg = 1, the scatter plots show obvious transitions. Therefore, two straight lines were fitted, as shown in Figs. 8 and 9. When lg < 1, fractal dimension 1 corresponded to seepage pores and when lg > 1 , fractal dimension 2 corresponded to adsorption pores. When lg > 1 , the correlation coefficients of fitting lines of JZ and XB raw coal samples were very small, indicating that the fractal features of the adsorption pores of JZ and XB raw coal samples are not obvious. This could be attributed to the compressibility of coal: the higher mercury intrusion pressure may compress the coal matrix or even destroy the pore structure [53]. The fractal dimension 1 of both middle–high rank coal samples increased


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after they were broken down by HVEP. This can be explained as follows: the seepage pore volume of coal samples subjected to HVEP is obviously greater than that of raw coal sample, and coal compressibility increases with increasing pore volume [53,54]. Thus, the fractal dimension 1 of middle–high rank coal samples is much bigger than that of raw coal samples. In general, the larger the fractal dimension, the rougher the pore surface; therefore, the surface of seepage pores of the coal samples subjected to HVEP is rougher than that of raw coal samples.

Fig. 8. Fractal dimension of raw coal samples by MIP: (a) JZ anthracite coal; (b) XB bituminous coal.

Fig. 9. Fractal dimension of coal samples subjected to HVEP: (a) JZ anthracite coal; (b) XB bituminous coal. 3.5. FT-IR spectrum analysis


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FT-IR is an effective nondestructive technique to study the chemical structure of coal [55]. Fig. 10(a) shows the FT-IR spectrum of JZ anthracite coal samples before and after HVEP treatment, and Fig. 10(b) is the FT-IR spectrum of XB bituminous coal samples before and after HVEP treatment. The results showed that the surface chemical structure of both anthracite and bituminous coal changed after the HVEP treatment. In Fig. 10(a), the peak between 3000 cm–1 and 3600 cm–1 corresponds to the OH group [56,57]; the intensity of the absorption peak of the OH group (3432 cm– 1

) of coal subjected to HVEP was significantly higher than that of raw coal. However,

the aromatic methyl CH group (3034 cm–1) disappeared in samples subjected to HVEP; thus, the intensity of the absorption peak of aromatic structures (809 and 742 cm–1) of the coal subjected to HVEP is less than that of raw coal. The relative strength of the FT-IR spectrum peak reflects the concentration of functional groups [58], which indicates that a large amount of heat is released during the process of coal breakdown. This causes an oxidation reaction on the surface of anthracite coal and reduces the CH content of anthracite coal. In Fig. 10(b), the FT-IR peaks appear at 3410 cm–1, which represent the OH group of the water or peroxide, and this peak disappeared in the bituminous coal subjected to HVEP. This may be because there are many primary pores in the bituminous coal, and the large amount of heat released during the electrical breakdown makes the water volatilize through these primary pores, thus reducing the OH concentration of water [59,60]. The FT-IR peaks appearing at 3620 cm–1 and 3693 cm–1 represent the free OH group of alcohols and phenols. These new emerging peaks


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suggest that an oxidation reaction had occurred in bituminous coal during HVEP treatment. The peak between 1000 cm–1 and 1800 cm–1 represents the oxygen-containing functional groups. Besides, new peaks at 1036, 1116, and 1226 cm-1 appear for the bituminous coal subjected to HVEP, which mainly represent ether oxygen and phenolic hydroxyl [58,61]. These new emerging peaks can also be attributed to oxidation during HVEP treatment. Numerous investigations have shown that the oxygen-containing functional groups have negative effects on methane adsorption [62,63]. The intensity of these peaks increased after the bituminous coal was subjected to HVEP, indicating that the concentration of these oxygen-containing functional groups increased, which benefits methane release.


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Fig. 10. FT-IR spectra of raw coal and coal subjected to HVEP: (a) JZ coal samples; (b) XB coal samples. 4. CONCLUSIONS In this study, the structural evolution characteristics of middle–high rank coal subjected to HVEP treatment were analyzed. Major conclusions from this study are as follows: First, under the action of HVEP, both anthracite and bituminous coal can be crushed into many small pieces. Because the conductivity of anthracite coal samples is better than that of bituminous coal, the average breakdown voltage of anthracite coal samples is lower than that of bituminous coal samples. At the same time, the greater the breakdown voltage, the more the number of particles formed after the coal is broken, and the smaller the particle diameter. Second, many pores and cracks were generated on the surface of middle–high rank coal subjected to HVEP treatment. The mesopores and macropores of the coal


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subjected to HVEP were clearly greater than those of raw coal. The fractal characteristic of adsorption pores of raw coal samples was not obvious, and the fractal dimension of seepage pores of coal samples subjected to HVEP is bigger than that of raw coal. The increased pores and cracks are conducive to the release and migration of methane. Third, oxidation reactions occur on the surface of coal treated with HVEP. The surface chemical structure of both anthracite and bituminous coal changed after the HVEP treatment. In addition, the concentration of oxygen-containing functional groups increased, which is beneficial for methane release. Author information Corresponding Author E-mail: [email protected]. Tel: +86-18223093865. Acknowledgments We acknowledge the financial support of the National Natural Science Foundation of China (Grant No. 51474211, No. 51474040) and the National Science and Technology Major Project (Grant No. 2016ZX05044002). The authors also thank SWAN Editorial Services for editing this paper. REFERENCES [1] Moore, T. A. Coalbed methane: A review. International Journal of Coal Geology 2012, 101, 36–81. [2] Keshavarz, A.; Badalyan, A.; Carageorgos, T.; Bedrikovetsky, P.; Johnson, R. Stimulation of coal seam permeability by micro-sized graded proppant placement


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