Enhancement effect of NaCl solution on pore structure of coal with high-voltage electrical pulse treatment

Fuel 235 (2019) 744–752

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Full Length Article

Enhancement effect of NaCl solution on pore structure of coal with highvoltage electrical pulse treatment

T

⁎

Xiangliang Zhanga,b,c, Baiquan Lina,b,c, , Yanjun Lia,b,c, Chuanjie Zhua,b,c, Jia Konga,b,c, Yong Lia,b,c a

Key Laboratory of Coal Methane and Fire Control, Ministry of Education, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China c State Key Laboratory of Coal Resources and Safe Mining, Xuzhou, Jiangsu 221116, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: High-voltage electrical breakage NaCl solution Pore structure Current

In order to enhance the gas extraction rate of low-permeability coal seams, a study was made on variation characteristics of pore structure of the coal saturated by NaCl solution under the effect of high-voltage electrical pulses (HVEP) by adopting a self-designed experimental system of fracturing and permeability enhancing of the coal with HVEP. In addition, current waveforms in the process of electrical breakdown were also investigated. The results revealed the two types of electrical breakdown of the coal in air environment, namely, surface breakdown and internal breakdown with three forms of breakage, namely, complete comminution, breakage from one side and cracks on the surface. Furthermore, the coal was broken due to internal tension under the influence of HVEP. Based on energy dispersive spectroscopy (EDS) analysis, the coal underwent both physical and chemical changes where the current flowed. Results of scanning electron microscope (SEM) demonstrated that the higher the breakdown voltage, the more the fissures in coal, and the better the breakage effect. Fissures in the coal were observed via the 3D-XRM to extend radially from the center to boundary areas with the breakage effect weakening gradually from anode to cathode when needle electrode discharged. Besides, these fissures were interconnected with each other. The result of liquid nitrogen adsorption suggested that electrical breakdown could effectively boost gas desorption by improving fissures as well as pores and micropores in the coal. Characteristics of the current waveform showed that great thermal expansion stress was caused by huge energy injected into the coal in an instant. During the electrical breakdown of coal with the same voltage, the peak current was different which increased with the growing of breakdown voltage.

1. Introduction As mining depth has increased in recent years, gas extraction faces the challenge of high ground stress, high adsorption and low permeability [1–3]. Thereby, it is of great significance for gas disaster prevention to increase coal permeability by effectively improving its physical structure [4–7]. As a kind of clean energy, gas finds wide application in many fields. Production enhancement of coalbed methane (CBM) can not only achieve maximum energy efficiency but also reduce environmental pollution [8–10]. Conventional methods for improving gas permeability of coal seams, ranging from hydraulic measures (hydraulic fracturing and hydraulic slotting) to deep-hole blasting and long-borehole extraction, have limitations and shortcomings [11,12]. Hydraulic measures, ineffective in soft coal seams, are likely to cause water-locking phenomenon [13–17]. The deep-hole blasting is restricted by the difficulty of

⁎

explosives-feeding [18,19]. Drilling tool sticking and hole collapses are of frequent occurrence during the long-borehole extraction [20]. Some scholars have advocated such techniques as cryogenic liquid nitrogen fracturing, permeability enhancement by microwave and supercritical CO2. However, these techniques have low economic benefits and are inefficient for improving permeability apart from their incapability to solve the root causes of low permeability [21–24]. High-voltage electrical pulse (HVEP) is a technology that allows the load of huge energy in a capacitor on the load in an instant after a sequence of compression and transformation of low energy [25,26]. High-voltage electrical breakage technology began in the 1930s and developed into an independent discipline in the 1960s [27]. The last decades have witnessed rapid development of HVEP with a wide application. Yutkin et al. applied HVEP to break solid materials and Timoshkin et al. used pulsed plasma technology for drilling [28,29]. Numerous scholars in the 1970s have made progresses in adopting

Corresponding authors at: School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China. E-mail address: [email protected] (B. Lin).

https://doi.org/10.1016/j.fuel.2018.08.049 Received 2 May 2018; Received in revised form 22 June 2018; Accepted 11 August 2018 Available online 23 August 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

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HVEP for oil unplugging [30,31]. Recently, several scholars have conducted theoretical research and field tests on the application of HVEP for permeability improvement [32,33]. There are two main application forms of HVEP in CBM extraction, namely, electrohydraulic fragmentation and electrical fragmentation [34]. The former allows the discharge electrode surrounded by liquid to discharge, breaking down the liquid. The strong shock waves generated in the liquid repeatedly act on the coal seam to increase its permeability. In essence, it is the pressure of shock waves that contributes to the improvement of permeability, which is inefficient in utilizing electric energy. The later relies on the direct contact between the discharge electrode and the coal seam inside of which the current flows. Under the effect of expansion stress, the coal seam undergoes tensile failure. Relevant studies prove that the energy for rock failure by tensile stress is only 4%-5% of that by compression, so electrical fragmentation conserves more energy [35]. Yan et al. have reported the feasibility of electrical breakdown of the coal in air environment [35]. However, the breakdown voltage for coal samples is too high, for instance, anthracite with a height of 10 mm and a diameter of 100 mm from the Guhanshan mine can be broken down with voltage of approximately 10 kV [36]. The relative reference mentions that the formation water mainly contains salt of NaCl, KCl, CaCl2 and NaSO4, of which NaCl is account for 70–95%, therefore, the formation water can be equivalent to NaCl solution in general [37–39]. The coal seam, a special kind of underground rock, is surrounded by stratum water for a long time [40]. Hence, it is very meaningful to study the breakage of coal samples saturated by NaCl solution with HVEP. In this paper, an experiment was carried out on Shanxi Hongliu bituminous coal saturated by 1 mol/L NaCl solution to study the electrical breakdown phenomenon in air environment by adopting a selfdesigned experimental system of fracturing and permeability enhancing of the coal with HVEP. In addition, SEM, EDS analysis and 3D-XRM were combined to investigate the breakage effect of the coal with HVEP. Meanwhile, discussion was made on the relationship between peak current and breakdown voltage in the process of electrical breakdown. The results provide a guide for the application of HVEP in coal mines.

Fig. 2. Experimental coal samples.

voltage is produced. After the differential calculation is conducted on the measured voltage, the current value can be obtained and displayed on the oscilloscope. The maximum energy output of this experimental system is 10 kJ, with the capacitance of the HV capacitor of 8 µF and the maximum voltage of 50 kV. Besides, this experiment adopts the rogowski coil with scaling ratio of 1:1000 and the RIGOL digital oscilloscope. 2.2. Coal samples Coal samples from Shanxi Hongliu coal mine were processed into cylindrical cores of 50 mm in diameter and 30 mm in length by rock coring machine. The surface of each core was polished, as presented in Fig. 2. The proximate analysis and maceral composition of samples in Table.1 reveal that Hongliu coal is low-grade with large moisture content, low content of fixed carbon and R0 of only 0.56%. It was reported that vitrinites, inertinites and liptinites mainly comprised micropores, mesopores, and macropores, respectively [41]. Furthermore, basic parameters of experimental coal sample are shown in Table. 2. 2.3. Experimental procedure Prepared cores were dried at the temperature of 60 °C in a vacuum environment for 24 h before weighing, in order to reach their full saturation. Then, they were saturated by a pre-prepared 1 mol/L NaCl solution for 24 h in a concrete vacuum water saturation instrument. Next, the weight of cores was recorded after water on their surfaces was wiped by wet rag. Afterwards, they were saturated in the vacuum water saturation instrument for another 12 h before getting weighed. The above steps were repeated until the weight difference before and after measurements was less than 0.5% in three consecutive times when cores were considered to be fully saturated. The final weight was recorded as the quality of the core after saturation. Electrical breakdown was carried out on cores after they were fully saturated. In the experiment, the center of the upper and lower ends of cores was aligned with needle-shaped anode and cathode, respectively, as shown in Fig. 1. The HV capacitor was charged until voltage reached the preset value. Then, the discharge switch was flipped on to witness huge energy flowed through the coal to break it when the expansion stress caused by the current surpassed the tensile strength of the coal. Furthermore, different voltage applied to the coal to produce various

2. Experiment 2.1. Experimental system As illustrated in Fig. 1, the experimental system consists of three subsystems, namely, charge and energy storage system, discharge system and measure system. Charge and energy storage system which contains a transformer and a high-voltage (HV) energy storage capacitor allows the conversion of 220 V alternating current to direct voltage ranging freely from 0 kV to 50 kV. The HV capacitor can store tremendous energy generated after a sequence of compression and transformation of low one. Discharge system includes discharge cavity and discharge switch that allows the instant application of enormous energy to the coal. Measure system contains rogowski coil and digital oscilloscope. When vertical current passes through the coil, induced

Diode 220V ~

Capacitor

Transformer

Charge and energy storage system

Discharge switch

Table 1 Proximate analysis and maceral composition of Hongliu coal.

Discharge cavity

Coal sample

Rogowski coil Discharge system

Digital oscilloscope Measure system

Hongliu

Proximate analysis (%)

R0,

Mad

Ad

Vdaf

FCd

7.4

15.29

35.36

54.75

max

0.56

(%)

Coal maceral composition (%) V

I

L

55.73

43.48

0.79

Note: V, vitrinite; I, inertinite; L, liptinite; Mad, moisture content (air-dried basis); Ad, ash content (dry basis); Vdaf, volatile matter content (dry and ash free basis); FCd, fixed carbon (dried basis).

Fig. 1. The circuit diagram of the laboratory equipment. 745

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Table 2 Basic parameters of experimental coal sample. Sample

HL-1 HL-2 HL-3 HL-4 HL-5

Size (mm)

Weight (g)

MC (%)

Diameter

Length

Dried

Saturation

50 50 50 50 50

30 30 30 30 30

78.05 77.42 77.18 78.35 74.68

81.45 81.09 80.51 81.97 78.09

Sample

4.35 4.74 4.31 4.62 4.57

HL-6 HL-7 HL-8 HL-9 HL-10

Size (mm)

Weight (g)

MC (%)

Diameter

Length

Dried

Saturation

50 50 50 50 50

30 30 30 30 30

75.13 80.20 80.13 78.62 76.05

78.52 83.93 83.56 82.22 79.66

4.51 4.65 4.28 4.58 4.74

Note: MC is the moisture content of coal sample.

(a)

(b)

(c)

(d) Discharge on the surface of coal sample

(e)

(f)

(g)

(h) Discharge in internal coal sample

Fig. 3. Breaking state of coal samples with HVEP.

breakage effects.

contact between the coal surface and the electrode.

3. Results and discussion

3.2. EDS analysis results

3.1. Electrical breakdown characteristics of coal in air environment

In this experiment, EDS analysis was conducted on coal surfaces before and after electrical breakdown by adopting Bruker QUANTAX400-10 electric refrigeration spectrometer. Studies were carried out on the coal with burned marks on the surface for accurate measurement of the impact of electrical breakdown on the surface, as presented in Fig. 4. The four main elements in Hongliu coal are C, O, Ca and Al, among which Al is conducive to the electrical breakdown because it can increase the conductivity of the coal.Fig. 5. Statistical analysis conducted on the variation of elements on the coal surface before and after the electrical breakdown indicates an increase of C and a decrease of O after electrical breakdown. The current flowing through the coal witnesses the occurrence of oxidation reaction on the surface induced by the growing temperature. Besides, Na and Cl, non-existent in Hongliu raw coal, were detected in samples saturated by 1 mol/L NaCl solution, which suggested that Na+ and Cl− in the solution entered into samples to connect areas with poor conductivity, enhancing the coal’s vulnerability to breakdown.

Electrical breakdown characteristics of coal samples saturated by 1 mol/L NaCl solution with different breakdown voltage in Fig. 3 reflects the two main forms of breakdown in air environment, namely, internal breakdown and surface breakdown. These two forms depend on the current flowing through or on the coal, respectively. For clear trace of the current flowing on the surface, a layer of insulating flexible glue was spread on the coal before the electrical breakdown experiment, as shown in Fig. 3 (a), (b), (c) and (d). The current passing through the surface causes strong impact force which makes the glue burst and the coal break, as given in Fig. 3 (d). The internal breakdown of coal can be grouped into three types, namely, breakage from one side (Fig. 3 (e) and (f)), complete comminution (Fig. 3 (g)), and increasing cracks on the surface of unbroken coal (Fig. 3 (h)), due to different pore structures in the coal that exert an impact on the current channel. It is worth noting that cracks were coated with a layer of white paint for clarity in Fig. 3 (f), (h). In addition, visible cracks on the fracture plane in Fig. 3 (e) reveal that under the effect of HVEP, the coal is broken from within due to the internal tension, differing essentially from that by external compression [42]. The remarkable sign of burns in Fig. 3 (b) and (e), suggests that with current flowing through the coal, high temperature triggers chemical reaction, thus affecting functional groups on coal surfaces [43]. With huge voltage directly applied to both ends of the coal, great energy is released into current channels formed within the coal. The temperature of channels upsurges, producing a great expansion stress and leading to the implosion of coal. Surface breakdown occurs when internal resistance exceeds that of the surface for the current. No noticeable damage is caused on the coal due to the escape of most energy. Factors affecting surface breakdown include surface roughness, conductive particles on the surface, surface moisture and the

3.3. Analysis of the pore-fracture structure of coal under the effect of HVEP 3.3.1. Analysis of adsorption results of low-temperature liquid nitrogen Low-temperature nitrogen adsorption method is commonly applied to measure pores and micropores in the coal based on BET absorption, that is, the multilayer adsorption of gas molecules on the solid surface. The second layer adsorption can be formed on the first one, by the same token, the third layer on the second one. Layers accumulate until the adsorption reaches balance when evaporation rate equals to coagulation speed of each layer, as shown in Eq. (1) [44]. Test and analysis were performed on the coal surface before and after electrical breakdown by employing the 3H-2000PS4 high-pressure adsorption instrument made by Beishide Instrument Technology Co., Ltd. In this 746

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cps/eV p /

cps/eV p / 9 8 7 6 O 5 CCO Al 4 CaCa Al 3 2 1 0 2

(a) Element Atomic (%)

C O Ca Al Na Cl

Ca Ca

4

6

10

8

Element Atomic (%)

4

48.62 33.59 16.87 0.91 0 0

C O Ca Al Na Cl

Cl

3 ClCa 2

Al CaO Al Cl C Na Na Cl Ca Ca C

1

14

12

(b)

5

16

18

0

4

2

6

8

10

80.85 17.27 1.31 0.16 0.18 0.23

12

14

18

16

Fig. 4. EDS analysis results of Hongliu coal before and after electrical breakdown.

70

Percentage (%)

60 50

current channels before and after electrical breakdown. Under the same pressure, the adsorption and desorption of the coal after breakdown both exceed that of the raw coal, proving an increase of open pores in the coal due to electrical breakdown. The relationship between cumulative pore volume and pore surface area and pore size are illustrated in Fig. 6 (b) and (c). The cumulative pore volume and pore surface area increase with the rise of pore size. Specifically, they experience fast and slow change corresponding to the pore size in the range of 0 nm–10 nm and 10 nm–50 nm, respectively. The cumulative pore volume and pore surface area of the electrically broken coal exceed that of the raw coal. The Na+ and Cl− in NaCl solution which enter into pores turn the original non-conductive or poorly conductive areas into that of excellent conductivity. As each of current branches flows through pores in the process of electrical breakdown, the joule heat it carries causes thermal expansion stress to break pores [45,46]. Therefore, electrical breakdown can not only cause cracks in the coal but also make closed and semi-closed pores open to effectively improve the pore and micropore structure for accelerating the gas desorption from the coal.

Raw coal Electrically broken coal

Oxidation reaction

80

0.8

+

Na

0.6

40

Cl

0.4

30

0.2

20

0.0

Al

Na

-

Cl

10 0 C

O

Ca

Al

Na

Cl

Elements Fig. 5. Variation characteristics of elements before and after electrical breakdown.

3.3.2. SEM analysis To study the variation of microscopic characteristics of the coal surface before and after electrical breakdown, an observation was made on coal samples under different breakdown voltage by adopting the FEI Quanta 250 scanning electron microscope (SEM) with the magnification of 2000 X, as illustrated in Fig. 7. Fig. 7 (a) shows the surface of the raw coal, while (b)–(f) are that of samples saturated by 1 mol/L of NaCl solution before broken down with 10 kV, 13 kV, 16 kV, 19 kV and 22 kV voltage, respectively. The surface of the raw coal is slightly rough with regular fibrous structure, which may be due to the low grade of Hongliu coal [21]. After electrical breakdown, the surface becomes rough and

experiment, particles of coal samples were 60–80 mesh, and the temperature was 77.3 K.

V=

Vm CP (Ps−P )[1 + (C −1)(P / Ps )]

(1)

where V is gas adsorption, ml/g; Vm is maximum adsorption of monolayer, ml/g; P is adsorbate pressure, MPa; Ps is adsorbate saturated vapor pressure, MPa; and C is a constant related to adsorbate. Fig. 6 (a) displays results of adsorption and desorption of liquid nitrogen by the fracture plane of broken Hongliu coal with clear trace of

Desorption

(a)

12 10 8 6 4 2 0

(b)

12 8

Raw coal Electrically broken coal

4

0 0.0192 Volume (ml/g)

Quantity adsorbed (ml/g)

14

16

Raw coal Raw coal

2

Electrically broken coal Electrically broken coal

Area (m /g)

16

Adsorption

0.0

0.2

0.4

0.6

0.8

0.0144 0.0096

0.0000

1.0

Relative pressure (P/P0)

(c)

Raw coal Electrically broken coal

0.0048 0

5

10

15

20 25 30 Pore size (nm)

35

40

45

50

Fig. 6. Low-temperature liquid nitrogen adsorption isotherms of Hongliu coal samples before and after electrical breakdown: (a) adsorption and desorption curve of liquid nitrogen; (b) relationship between cumulative pore surface area of coal and pore size; (c) relationship between cumulative pore volume of coal and pore size. 747

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Raw coal

(a)

10 kV

(b)

13 kV

(c)

Crack

2000X 16 kV

(d)

2000X

(e)

19 kV

2000X

(f)

22 kV

Crack

Crack

2000X

2000X

Cracks

2000X

Fig. 7. Microscopic characteristics of the coal surface with different breakdown voltage.

Upper surface

Anode

Middle part Lower surface

Cathode

Fig. 8. Fissures structure of 3D-XRM slice image.

beam emitted from cathode bombards the anode target to produce wide-spectrum X-rays which passes through the rotating sample and pauses at different angles. Then, two-dimensional (2D) projection images are collected by a receiver before analyzed by 3D analysis software to form a 3D reconstruction of the sample. This system can eliminate the diffuser circle caused by X-rays by dual reference [47,48]. Scanning the sample when the turntable rotates 0.9°, a total of 1004 2D CT slices with 1004 × 1024 pixel and the resolution of 50 μm can be obtained. Gas extraction can be reduced to the following steps: (1) gas desorbs from the coal matrix; (2) gas diffuses in pore structure (micro-, meso-

uneven because it enhances cracks whose number is limited with low voltage and increases gradually with the voltage building up. The greater the voltage, the more energy of a single injection, and the more the joule heat converted from electric energy. Hence, there is an increase in the temperature of discharge channels and in expansion stress, leading to more cracks on the surface. 3.3.3. High Solution 3D X-ray Microanalyser The High Solution 3D X-ray Microanalyser (3D-XRM) of Carl Zeiss, German, a nondestructive testing technology, can make direct and accurate detection on the internal structure of coal samples. The electron 748

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600

600 500

(a)

400 200

300

10.15 kV

512 A

100 0

Afterwinds

10.26 kV

100 0

-400 0

18 ȝs

25

50

75 Time (ȝs)

Afterwinds

-300

-300 100

125

500

0

40

80 120 Time (ȝs)

160

200

600

400

500

(c)

Current

200

Current (A)

9.95 kV

464 A

(d)

400

300

Current (A)

560 A

200

-200

21 ȝs

-200

100

300

Current 10.06 kV

528 A

200 100 0 -100

0

-200

19 ȝs

-100

-300

Afterwinds 0

15

30

600

45 Time (ȝs)

60

90

Afterwinds 30

60

10.0 kV

150 0 17 ȝs

-300 Afterwinds

-450 80

120 Time (ȝs)

160

200

240

90 Time (ȝs)

120

150

180

Peak value of current

600

624 A

40

0

Current

450

0

17 ȝs

630

(e)

300

75

-400

Peak value of current (A)

-200

Current (A)

Current

-100

-100

-150

(b)

400

Current (A)

Current (A)

300

500

Current

The average breakdown current

(f) 570 540 510

537.6 A

480 450

10.15

10.26

9.95

10.06

10.0

Electrical breakdown voltage (kV)

Fig. 9. Current waveforms with the same breakdown voltage.

central part of coal breaks first with fissures extending outward under the influence of current and shock waves until the energy of current branch fails to make cracks. In addition, cracks on the upper surface is greater than that on the lower surface and in the middle part in number and size, indicating that the current flows from the upper anode through the sample to the lower cathode. Since energy of the current is first applied to the upper surface, the lower one witnesses poor fragmentation effect due to the consumption of part energy. Based on 3DXRM slice images, interconnected cracks are produced in the electrically broken coal. Non-conductive or poorly conductive areas are connected to form continuous conductive areas in the coal saturated by NaCl solution. Na+ and Cl− in the coal migrate directionally under strong electrical field and the conducting mode of coal is transformed from electronic conductivity to ionic conductivity. The directional flow of huge carriers within the coal forms strong current with tremendous joule heat. Cracks occur when thermal expansion stress induced by

and macropore) of the coal; (3) gas seepages into the extraction tube under the effect of extraction pressure [49,50]. Hence, cracks in the coal play a vital role in gas extraction. To study the variation characteristics of fissure structure of the sample saturated by NaCl solution with HVEP, scans were conducted on the electrically broken sample via 3D-XRM of Carl Zeiss, as displayed in Fig. 8. Three 3D-XRM slice images each of the upper surface, central part and lower surface of the coal sample were taken, respectively, to analyze the characteristics of pore structure in the coal. The upper and lower surfaces corresponded to anode and cathode, respectively. Fig. 8 shows numerous fissures in the coal saturated by NaCl solution with HVEP, suggesting that NaCl solution connects the poorly conductive areas. When branch-shaped current flows through the coal, cracks are generated under the effect of thermal expansion stress [9]. From three slice images of the upper surface, fissures spread radially from the center with more larger-scaled cracks to boundary areas. As the needle electrode discharges, the 749

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600

700

500

(a)

400 200 100 0 21 ȝs

-200

13.12 kV

608 A

200 100

-100

Afterwinds

0

25

50

75 Time (ȝs)

100

-300

125

Afterwinds

22 ȝs

-200

-300

0

15

30

45

60 75 Time/ȝs

90

105

120

900

900 750

(d)

750

(c)

600

Current

600

16.0 kV

752 A

300 150 0

Current

832 A

450

450

Current/A

Current/A

300

0

-100

19.10 kV

300 150 0 -150 -300

-150 Afterwinds

19 ȝs

-300 0

20

40

60 Time/ȝs

80

100

16 ȝs Afterwinds

-450 -600

120

0

30

60

90 120 Time/ȝs

150

180

960

(e)

750 600

22.02 kV

450

912 A

300 150 Afterwinds

0

Peak value of current

900

Current Peak value of current (A)

900

Current/A

Current

400

10.15 kV

512 A

(b)

500

Current (A)

Current (A)

300

600

Current

Linear fitting (10-16 kV) Linear fitting (16-22 kV) Linear fitting (10-22 kV)

840 780 720

(f) R2 I max

I max

43.33V 52.46 R2

660

0.999

26.57V 326.10

0.983 I max

600

35 09V 156.85 35.09 R2 0. 0.979

540

-150

19 ȝs 0

15

480

30

45 Time/ȝs

60

75

90

10

12

14

16

18

20

22

Electrical breakdown voltage (kV)

Fig. 10. Current waveforms with different breakdown voltage.

maximum value at the first peak before quickly disappearing. Therefore, electrical breakdown is considered to occur at the inflection point of the first peak current [36] which is that generated in the current circuit during electrical breakdown. The time of the current growing from 0 to the first peak value is that of electrical breakdown. Since electrical breakdown takes place at the first peak, the rear peaks are of afterwinds generated in the circuit. Fig. 9 suggests that the five samples have slightly different time of electrical breakdown which is in the range of 17 μs–21 μs with the voltage of 10 kV. The value of peak current reaches several hundred A with the same breakdown voltage, demonstrating that the huge energy is instantly injected into the coal. The accumulated energy converts to great joule heat, generating tremendous thermal expansion stress. Besides, the peak current of five samples are 512 A, 560 A, 464 A, 528 A, and 624 A, respectively, due to differences in their pore structure, mineral composition and moisture content, which results in various conductivity. Despite the difference in peak current, the breakdown current

joule heat exceeds the tensile strength of coal. Under the action of thermal expansion stress, cracks extend to witness the breakage of coal from within when the number and size of cracks reach a certain degree.

3.4. Current waveform of Hongliu coal samples during the electrical breakdown Test and analysis have been conducted on the current waveform of Hongliu coal with the same breakdown voltage to study the influence of coal structure on the electrical breakdown mechanism, as presented in Fig. 9. Fig. 9 (a), (b), (c), (d), and (e) correspond to breakdown voltage of 10.15 kV, 10.26 kV, 9.95 kV, 10.06 kV, 10.0 kV, respectively. It can be observed from the relationship between peak current and breakdown voltage during the electrical breakdown process in Fig. 9 (f) that current waveform consists of multiple peaks with the value of peak current topping at the first peak and then gradually falling to zero. The plunge of current after the first peak indicates that the energy reaches the 750

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Fig. 11 displays the system diagram of the application of hydraulic fracturing and HVEP for permeability enhancing in a surface CBM well. The system mainly includes the following parts: 1) Two surface CBM wellbore (for installation of water injection pipe, anode and cathode, and CBM extraction tube); 2) A HVEP generator (including HV capacitor, HV switch, anode and cathode, and HV wire); 3) CBM extraction system (including the CBM extraction pump and tube); 4) High pressure water injection system (High pressure water injection pump, drainage pump and water injection pipe). The specific application procedure is as follows: Step 1 When drilling is completed, conventional methods were used to seal the wellbores. Inject the conductive ionic solution (such as NaCl solution) with a certain pressure to fracture the coal seam for enhancing the permeability and to increase the conductivity for facilitating the electrical breakdown. Step 2 Apply drainage pump to expel the solution from wellbores before the anode and cathode are fixed on corresponding positions of the coal seam in two CBM wellbores, respectively. Then, charge the HV capacitor until the voltage reaches the preset value. Turn on the HV switch to discharge for breaking the coal seam down. Repeat the discharge step to achieve higher permeability according to the field situation. Step 3 Repeat Steps (1) and (2) before the CBM extraction device is accessed for maximum efficiency of CBM extraction.

Fig. 11. The application of HVEP for fracturing and permeability enhancing of coal in a CBM well.

of different samples fluctuates up and down the average value with the same voltage. In addition, current waveforms of five samples are basically the same during electrical breakdown with HVEP. The peak current of them descends progressively, suggesting that the first peak witnesses the occurrence of electrical breakdown with the largest energy. Furthermore, the following shockwaves may be due to the lack of damping resistance in the circuit [33,51]. In order to investigate the relationship between breakdown current and breakdown voltage, test and analysis are performed on the current with different breakdown voltage. Current waveforms of Hongliu coal is illustrated in Fig. 10. Fig. 10 (a), (b), (c), (d), and (e) correspond to the breakdown voltage of 10.15 kV, 13.12 kV, 16.0 kV, 19.10 kV and 22.02 kV respectively. The peak current corresponding to the five voltages is 512 A, 608 A, 752 A, 832 A, and 912 A, respectively, suggesting that the higher the breakdown voltage, the greater the injected energy, and the higher the peak current through the coal. Hence, the coal is easier to be broken down. The electrical breakdown time ranges from 16 μs to 22 μs with different breakdown voltages, which shows that there is no direct relationship between the electrical breakdown time and the voltage and that the breakdown current depends on the device. Fig. 10 (f) presents the liner relationship between peak current and breakdown voltage in the range of 10 kV–22 kV. The change rate of peak current is relatively large and small with the voltage in the range of 10 kV–16 kV and of 16 kV–22 kV, respectively. With high breakdown voltage, the vaporization of ionized water due to the high temperature of discharge channels lowers the conductivity of the coal, which has negative influence on the formation of current channels, thus slowing current rise.

5. Conclusions This paper conduct studies on variation characteristics of pore and fissure structure of the coal saturated by 1 mol/L NaCl solution under the effect of HVEP and on that of current during electrical breakdown process, with main conclusions as follows: (1) The two breakdown forms of coal with HVEP in air environment are surface breakdown with basically intact coal surface and internal breakdown with three types of breakage, namely, complete comminution, breakage from one side and cracks on the surface. In addition, the coal is broken from within due to expansion stress under the influence of HVEP, which is essentially different from that by compression. (2) Results of liquid nitrogen adsorption, SEM and 3D-XRM indicate that the coal saturated by 1 mol/L NaCl solution with HVEP finds both the occurrence of cracks and the improvement of internal pore structure which is conducive to CBM extraction. Besides, the higher the voltage, the greater the injected energy, and the more the fissures in the coal. (3) In the process of discharge, the energy is injected into the coal in an instant (about 20 μs ), resulting in huge thermal expansion stress. The peak current of coal samples is different with the same breakdown voltage, and it increases with the rise of the breakdown voltage. (4) Based on the feasibility of electrical breakdown of the coal saturated by NaCl solution in air environment, this paper proposes a new method for fracturing and permeability enhancing based on the combination of hydraulic fracturing and HVEP.

4. Potential application In light of the successful application of HVEP for oil unplugging [52–53], a new method of permeability-enhancement for CBM wells was proposed based on the combination of hydraulic fracturing and HVEP. The coal seam is first filled with conductive ionic solution by adopting hydraulic fracturing to improve the conductivity of the coal. Then, the coal between anode and cathode is broken down due to the huge energy produced by HVEP to enhance permeability. During the electrical breakdown process, between anode and cathode forms discharge channels through which tremendous energy from the HV capacitor passes. Strong explosion stress and shock waves generated contribute to the formation of fracture network by new cracks and the extension of original ones, which is conducive to the gas extraction.

Acknowledgements This work was financed by the National Natural Science Foundation of China (Grant No. 51474211), the State Key Research Development Program of China (2016YFC0801402), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). 751

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