Improving the Conductivity and Porosity of Coal with NaCl Solution for

Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES

Fossil Fuels

Improving the conductivity and porosity of coal with NaCl solution for high-voltage electrical fragmentation Baiquan Lin, Xiangliang Zhang, Fazhi Yan, Chuanjie Zhu, and Chang Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00535 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 29, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1

Improving the conductivity and porosity of coal with NaCl solution for high-

2

voltage electrical fragmentation

3

Baiquan Lina,b, Xiangliang Zhang a,b,*, Fazhi Yanc, Chuanjie Zhua,b, Chang Guoa,b

4

a

5

Technology, Xuzhou 221116, China

6

b

School of Safety Engineering, China University of Mining and Technology, Xuzhou 221116, China

7

c

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

8

400044, China

9

* Corresponding authors at: School of Safety Engineering, China University of Mining & Technology, Xuzhou

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

10

221116, Jiangsu, China.

11

E-mail address: [email protected]

12

Tel: 15062110193

13

Fax: 83884401

14

Abstract: High-voltage electrical pulse (HVEP) technology is a useful method to improve the pore structure of

15

coal. However, the breakdown voltage of coal is very high; therefore, it is necessary to improve the

16

conductivity of coal first to decrease its breakdown voltage before subjecting it to HVEP. In this work, a

17

solution of NaCl was used for ameliorating the conductivity of Linhua anthracite coal and Hongliu bituminous

18

coal. Both breakdown voltage and energy consumed by the coal sample saturated with the solution of NaCl

19

decreased. The breakdown voltage of Linhua and Hongliu raw coal sample (RCS) was 3.44 and 2.88 times than

20

that of coal saturated with NaCl, respectively. The energy consumed by Linhua and Hongliu RCS was 11.82

21

and 8.21 times than that of coal saturated with NaCl solution, respectively. Scanning electron microscopic

22

(SEM) images showed that the surface cracks on crushed coal samples after saturating with NaCl solution were

23

more than those on RCS. Nuclear magnetic resonance (NMR) results indicate that mesopores and macropores

24

were chiefly enhanced by the electrical breakdown. Chlorine and sodium elements were found on the surface of

25

both Linhua and Hongliu coal through energy-dispersive spectrum (EDS) point scanning, which indicates that

26

these elements play a positive role in electrical breakdown. Fourier transform-infrared spectrometer (FTIR) test

ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

27

showed that the discharge channel left a trail of burning on the surface of the coal sample, and the changes in

28

the functional groups after crushing.

29

Keywords: High-voltage electrical pulse, NaCl solution, Breakdown voltage, Coal conductivity, Pore structure.

30

1. Introduction

31

Coalbed methane (CBM) has now become an important source of energy; additionally, it is eco-friendly1.

32

For over three decades, China has been the leading coal producing country2. However, CBM extraction in

33

China is limited by the low permeability of coal seams3. Therefore, it is necessary to enhance the permeability

34

of coal seams, which is conductive to improve CBM production. Many studies have been performed to enhance

35

coal seam permeability, such as hydraulic fracturing, hydraulic slotting, and deep hole presplitting blasting4–6.

36

However, the coverage and effective influence scope of these methods are limited7,8. In recent years, a novel

37

method for enhancing the permeability of coal seam, called high-voltage electrical pulse (HVEP), was

38

proposed. Correlational research has indicated that the HVEP technology has a good prospect for enhancing the

39

permeability of coal seams9.

40

In the past few decades, rapid developments were made with regard to the HVEP technology10,11. Many

41

scholars proposed high-power pulsed rock breaking technology12. In 2000, the Russian Academy of Sciences

42

developed a high-power pulse generator based on reverse transistor switching according to electrohydraulic

43

fragmentation13. Timoshkin et al14 evaluated the pulsed plasma drilling machine for miniature hole drilling. In

44

recent years, some scholars used HVEP to separate valuable materials in circuit boards15. Since the 1970s,

45

many scholars have applied the electrical pulse technology in petroleum plugging, and achieved satisfactory

46

results16,17. In recent years, some scholars have also applied this technology to enhance coal seam

47

permeability18,19.

48

Over the last several years, numerous scholars have made a great deal of research on the HVEP

49

technology, and demonstrated that the technology is an efficient method to enhance the extraction of CBM20,21.

50

In the HVEP technology, solid materials are crushed by two methods: electrohydraulic fragmentation and

51

electrical fragmentation22. Electrohydraulic fragmentation involves the following steps: prior to the electrical

52

breakdown test, the solid materials are dipped in a liquid medium. A high-voltage electrical pulse is then

53

discharged in the liquid medium, which eventually produces a powerful shock wave. As a result, solids produce

54

cracks under the action of shock waves. By contrast, electrical fragmentation involves crushing the material by


Page 2 of 22

Page 3 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

55

directly applying high voltage to both ends of the solid medium11. The solid medium is eventually crushed by

56

tensional forces. Because the tensional forces produced during rock material damage are far less than the

57

compressional forces produced, the energy needed for electrical fragmentation will be much less than that

58

required for electrohydraulic fragmentation9.

59

According to the theory of solid dielectric breakdown, the process of electrical breakdown can be

60

simplified into three processes23–25, as shown in Fig. 1. Initially, numerous electrons gather near the cathode, a

61

characteristic termed “forerunner of discharge”. In the second stage of discharge, electrons move from cathode

62

to anode. When most electrons have arrived at the anode, a primary discharge channel is produced. Finally,

63

after a major accumulation of electrons on the anode, the solid material is broken by tension, which caused by

64

the electrical current passed26.

65 66

Fig. 1. Electrical breakdown process of rock using the HVEP technology.

67

Yan et al. have demonstrated that coal samples can also be crushed using the HVEP technology in an air

68

environment9, thus demonstrating a novel way of applying the technology and improving energy utilization

69

efficiency. However, a major challenge in applying the technology for crushing coal is that the breakdown

70

voltage of some coal types is very high; for example, the minimum breakdown voltage of a Guhanshan coal

71

sample with a diameter of 10 cm and a height of 1 cm is approximately 10 kV26. Importantly, the higher the

72

voltage, the greater energy consumption27. In addition, the higher the voltage, the more difficult to ensure the

73

safety of the experimenter. Therefore, there is an urgent need to reduce the breakdown voltage of coal samples.

74

The phenomenon of electron injection occurs when the sample is placed in a sufficiently high electrical

75

field, and this is usually shown as the emission of electrons from the metal electrode to the material28. In the

76

initial stage of this process, electrons separate from metal electrodes, which depends on the ability of electrons


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

77

to escape. As soon as an electron separates from the electrode, a contact surface is established between the

78

electrode and the sample, which plays a vital role in electron transport. Finally, the electrons injected into the

79

coal sample will move directly under the action of the electrical field, which depends on the mobility of

80

electrons and the resistivity of coal. Thus, it is a very simple and effective way to reduce the breakdown voltage

81

by changing the resistivity and electron mobility of coal. In this work, 1mol/L NaCl solution was used to

82

saturate coal samples to change their resistivity and electron mobility. It is well-known that a solution of NaCl

83

has very good conductivity29. Therefore, using a solution of NaCl to saturate coal samples is an effective

84

method to reduce their resistivity while increasing electron mobility.

85

Many studies were conducted on the HVEP technology, and their results demonstrated the technology to

86

be efficient for improving the exploitation of CBM; however, many problems still need to be addressed. In the

87

present work, a solution of 1mol/L NaCl was used to improve the conductivity of Linhua anthracite coal and

88

Hongliu bituminous coal, and their breakdown characteristics were analyzed. The results of this study will

89

provide guidance for the application of HVEP in coal mines.

90

2. Experimental analysis

91

2.1 Laboratory equipment

92

The laboratory setup used in this experiment is shown in Fig. 2, and consists of the following: a direct

93

current (DC) power equipment, capacitor, high-voltage (HV) switch, and discharge cavity. The DC power

94

equipment can output an adjustable voltage in the range of 0–50 kV. The capacitor has a capacitance of 8 µF

95

and maximal output energy of 10 kJ. The HV switch performs the function of electrifying a circuit

96

instantaneously. Pin electrodes are installed in the discharge cavity symmetrically.


Page 4 of 22

Page 5 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

97 98

Fig. 2. Laboratory equipment setup.

99

2.2. Sample preparation

100

Two kinds of coal samples were used: anthracite coal samples from Linhua coal mines in Guizhou

101

province and bituminous coal samples from Hongliu coal mines in Shanxi province, China. The raw coal

102

samples (RCS) were molded into a cylindrical mass (diameter 5 cm; height 3 cm; see Fig. 3). Proximate

103

analysis results of the two samples are presented in Table 1.

104

Table 1．． Proximate analysis results of Linhua and Hongliu coal samples.

105 106

Sample

Coal rank

Mad (%)

Ad (%)

Vdaf (%)

FCd (%)

Linhua

Anthracite

2.76

4.42

31.93

60.89

Hongliu

Bituminous

8.68

10.6

35.36

57.79

a

Notes: 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 FCad (air-dried basis).

107 108

Fig. 3. Coal samples: (a) cylindrical samples of Linhua; (b) cylindrical samples of Hongliu.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

109

2.3. Experimental procedure

110

To study the influence of NaCl solution on the conductivity of coal samples, 15 coal samples of each type

111

were prepared. These 15 samples were then divided into three groups (Table 2). Sample numbers from HL-D-1

112

to HL-D-5 and from LH-D-1 to LH-D-5 are RCS, sample numbers from HL-W-1 to HL-W-5 and from LH-W-

113

1 to LH-W-5 are coal samples saturated with deionized water (DW), sample numbers from HL-SC-1 to HL-

114

SC-5 and from LH-SC-1 to LH-SC-5 are coal samples saturated with 1mol/L NaCl solution. Additionally, the

115

meaning of “HL” is Hongliu coal, “LH” is Linhua coal.

116

In brief, the experiment included the following steps: First, electrical breakdown tests were performed in

117

an air environment with coal samples numbered as HL-D-1–HL-D-5 and LH-D-1–LH-D-5. These tests were

118

conducted to achieve the minimum breakdown voltage of RCS. The remaining coal samples were then dried in

119

a 60 °C vacuum environment, which allows them to saturate easily30. After drying the coal samples, the weight

120

of the coal samples was measured and recorded. Coal samples numbered as HL-W-1–HL-W-5 and LH-W-1–

121

LH-W-5 were saturated with DW and those recorded as HL-SC-1–HL-SC-5 and LH-SC-1–LH-SC-5 were

122

saturated with 1mol/L NaCl solution. After saturation, the weight of coal samples was again measured and

123

recorded. The electrical pulse experiment was then performed in an air environment to examine their minimum

124

breakdown voltage.

125

The diameter of the sodium ion with +1 valence and that of chloride ion with –1 valence are 0.204 nm and

126

0.362 nm, respectivey31. However, previous studies show that the pore size in coal seam is in the range of

127

several nanometers32. Therefore, it is possible for sodium and chloride ions to enter pores on the coal surface.

128

NaCl solution was chosen in this experiment because the conductivity of underground rock structures

129

depends on anions and cations in stratum water, and these ions are generally considered to be a combination of

130

sodium and chloride31. Therefore, it can be concluded that the conductivity of underground rock structures

131

mainly depends on the content of chloride and sodium ions in groundwater. Underground coal seams are also a

132

special type of rock, and are in an environment surrounded by groundwater for a long time33. Therefore, this

133

experimental study on the electrical breakdown of coal samples saturated with a solution of NaCl has a guiding

134

significance to the application of the HVEP technology in coal mines.


Page 6 of 22

Page 7 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

135

To enable the samples to achieve the saturation state easily, they were first dried for 24 h in a 60 °C

136

vacuum environment. Coal samples were then saturated with DW and 1mol/L NaCl solution for 24 h,

137

respectively. After 24 h, samples were weighed and recorded. The coal samples were then immersed into the

138

solution of sodium chloride once again, and their weight is measured every 12 h until the difference in weight

139

between successive three measurements is no more than 0.5 g. Once this is achieved, the last weight was

140

recorded as the fully saturated weight. Moisture content (MC) of coal is the saturated water content (Table 2).

141

Table 2．． Sample Parameters and Numbers.

142 143

Weight (g) D (g)

S (g)

MC (%)

HL-W-1

77.01

80.41

4.42

78.57

HL-W-2

78.05

81.45

HL-D-3

78.62

HL-W-3

77.42

HL-D-4

78.18

HL-W-4

HL-D-5

78.42

LH-D-1

Sample

W (g)

Sample

HL-D-1

78.45

HL-D-2

Sample

Weight (g)

MC (%)

D (g)

S (g)

HL-SC-1

75.24

79.18

5.24

4.36

HL-SC-2

75.98

79.97

5.25

81.09

4.74

HL-SC-3

76.26

80.37

5.39

77.18

80.76

4.64

HL-SC-4

76.71

80.68

5.18

HL-W-5

79.42

83.40

5.01

HL-SC-5

76.01

80.05

5.32

82.24

LH-W-1

80.31

81.17

1.07

LH-SC-1

77.07

78.65

2.05

LH-D-2

79.88

LH-W-2

79.79

80.72

1.17

LH-SC-2

77.85

79.46

2.07

LH-D-3

78.88

LH-W-3

77.41

78.34

1.20

LH-SC-3

76.19

77.73

2.02

LH-D-4

79.15

LH-W-4

78.46

79.38

1.17

LH-SC-4

76.35

77.94

2.08

LH-D-5

79.67

LH-W-5

79.53

80.36

1.04

LH-SC-5

76.21

77.86

2.17

a

Notes: W is the weight of RCS; D is the weight of coal sample after drying; S is the weight of coal

sample after saturation; MC is the water content.

144

To study the variations in the fracture and pore structure of coal samples crushed by HVEP, environmental

145

scanning electron microscope (SEM; Quanta 250, FEI Company, USA) and Nuclear magnetic resonance

146

(NMR) were used in our experiment34,35. In addition, an energy-dispersive spectrum (EDS) point scanning test

147

was performed to investigate the effects of NaCl solution on electrical breakdown36, and a Fourier transform-

148

infrared spectrometer (FTIR; Bruker) was used to study variations of various functional groups on the coal

149

surface37,38.

150

The center of coal samples was fixed with a pin electrode (see Fig. 2). The capacitor was charged from 1

151

kV and the high-voltage discharge switch was then turned on. If the coal sample cannot be broken down, the

152

capacitor is charged again at step sizes of 0.3 kV until the coal sample could be broken down. Finally, the


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 22

153

voltage was recorded and treated as the minimum breakdown voltage of the coal sample with a diameter of 5

154

cm and a height of 3 cm.

155

3. Results and discussion

156

3.1 The breakdown voltage, energy consumed, and breakdown field strength

157

The breakdown voltage of coal samples is presented in Table 3. Five samples were prepared for each

158

group, and the standard deviation (SD) breakdown voltage of each group was calculated. The SD values for the

159

six groups are 1.43, 0.96, 0.33, 0.38, 0.29 and 0.15, respectively, with the maximum SD value being 1.43.

160

Therefore, it can be concluded that there is not much difference between the breakdown voltages of coal

161

samples in each group, demonstrating that these voltage values are dependable. In Table 3, it can be seen that

162

the average breakdown voltages of LH-D, LH-W, LH-SC, HL-D, HL-W, and HL-SC are 18.03 kV, 13.30 kV,

163

5.24 kV, 29.38 kV, 18.36 kV, and 10.19 kV, respectively. The average breakdown voltage of Linhua RCS is

164

1.36 times than that of coal samples saturated with DW, and 3.44 times than that of coal samples saturated with

165

1mol/L NaCl solution [Fig. 4 (a)]. The average breakdown voltage of Hongliu RCS is 1.6 times and 2.88 times

166

than that of coal samples saturated with DW and 1mol/L NaCl solution, respectively [Fig. 4 (b)].

167

The complex permittivity of coal samples can be described using the following equation: = − ,

168

where is the real dielectric constant and is the loss factor36. In this experiment, the NaCl solution changed

169

the dielectric properties of coal samples; moreover, the value of coal samples decreased and the value

170

increased after saturation (i.e., after treatment with NaCl solution)39. Thus, it is clear that the NaCl solution

171

reduced the permittivity of coal samples and enhanced their conductivity simultaneously. Furthermore, the W–

172

S conductivity model established by Waxman and Smits showed that the coal samples saturated with NaCl

173

solution can be considered a parallel conduction model of NaCl solution and coal sample40. According to the

174

literature, the resistivity of NaCl solution is much less than that of the coal sample29, and therefore, the

175

resistivity of the coal sample saturated with NaCl is smaller than that of the RCS.

176

Table 3．． Breakdown voltage of coal samples Sample

V (kV)

HL-D-1

28.53

HL-D-2

31.76

HL-D-3

29.44

A (kV) 29.38

SD 1.43

Sample

V (kV)

HL-W-1

20.00

HL-W-2

17.68

HL-W-3

18.26

A 18.36

SD 0.96


Sample

V (kV)

HL-SC-1

10.26

HL-SC-2

10.10

HL-SC-3

10.57

A

SD

10.19

0.33

Page 9 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

177 178

Energy & Fuels

HL-D-4

29.12

HL-W-4

18.26

HL-SC-4

9.69

HL-D-5

28.05

HL-W-5

17.62

HL-SC-5

10.34

LH-D-1

18.34

LH-W-1

13.64

LH-SC-1

5.20

LH-D-2

18.41

LH-W-2

13.33

LH-SC-2

5.31

LH-D-3

17.49

LH-W-3

12.97

LH-SC-3

5.02

LH-D-4

18.05

LH-W-4

13.52

LH-SC-4

5.43

LH-D-5

17.86

LH-W-5

13.06

LH-SC-5

5.22

a

18.03

0.38

13.30

0.29

5.24

0.15

Notes: A is the average breakdown voltage of each group; SD is the standard deviation of the breakdown

voltage.

179

According to Formula 1, the total energy consumed by the capacitor can be calculated. In this

180

experimental system, the capacitance is 8 µF and the average breakdown voltage of LH-D, LH-W, LH-SC, HL-

181

D, HL-W, and HL-SC is 18.03 kV, 13.30 kV and 5.24 kV, 29.38 kV, 18.36 kV, and 10.19 kV, respectively.

182

Therefore, the average energy consumption of HL-D, HL-W, HL-SC, LH-D, LH-W, and LH-SC is 3.45 kJ,

183

1.35 kJ, 0.42 kJ, 1.30 kJ, 0.71 kJ, and 0.11 kJ, respectively. As shown in Fig. 5, the electrical breakdown

184

energy required for Linhua RCS is 1.83 times than that of coal sample saturated with DW, and 11.82 times than

185

that of coal sample saturated with the 1mol/L NaCl solution; by contrast, the electrical breakdown energy

186

required for Hongliu coal samples is 2.56 times than that of coal sample saturated with DW, and 8.21 times

187

than that of coal sample saturated with 1mol/L NaCl solution. Thus, it is safe to assume that a great amount of

188

energy can be saved by saturating coal samples with NaCl solution. 1 W = CU 2 2

189 190 191

(1)

where W is the energy stored in the capacitor, J; C is the capacitance, F; and U is the average voltage stored in capacitor, V.


Energy & Fuels

12

13.30

3.44

14

10 8 6 4

5.24

2 0

30

(b)

27

29.38

24

HL-D HL-W HL-SC

2.88

18.03

16

LH-D LH-W LH-SC

1.60

(a)

Average breakdown voltage (kV)

18

33

1.36

Average breakdown voltage (kV)

20

21 18 18.36

15 12 9

10.19

6 3 0

192

LH-W

LH-D

LH-SC

HL-D

HL-W

HL-SC

193

Fig. 4. Average breakdown voltage: (a) average breakdown voltage of Linhua coal samples; (b) average

194

breakdown voltage of Hongliu coal samples. 4.0 LH-D LH-W LH-SC

0.8 0.71

0.6 0.4

2.5 2.0 1.5

1.35

0.5

0.11

0.0

195

3.45 3.0

1.0

0.2

HL-D HL-W HL-SC

8.21

11.82

1.0

(b) 2.56

1.30

3.5

Energy (kJ)

1.2

(a) 1.83

1.4

Energy (kJ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 22

LH-D

LH-W

0.42 0.0

LH-SC

HL-D

HL-W

HL-SC

196

Fig. 5. Average electrical breakdown energy: (a) average electrical breakdown energy of Linhua coal samples;

197

(b) average electrical breakdown energy of Hongliu coal samples.

198

Previous studies have indicated that deionized water is almost nonconductive41; however, the breakdown

199

voltage of coal saturated with DW is lower than that of RCS, which can be attributed to the minerals in coal

200

that are dissolved in deionized water. These dissolved minerals will ionize free moving ions, and migrate

201

directionally under the action of electrical field, which helps enhance the electrical conductivity of coal42. This

202

is thus an effective way to decrease the breakdown voltage, although the effect is not obvious enough.

203

The breakdown voltage of Linhua and Hongliu RCS is 3.44 and 2.88 times than that of coal saturated with

204

1mol/L NaCl solution. The electrical breakdown energy consumed by Linhua and Hongliu RCS is 11.82 and


Page 11 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

205

8.21 times higher than that of coal saturated by 1mol/L NaCl solution. These improvements in the breakdown

206

voltage and electrical breakdown of coal may be caused by the following reasons: (a) NaCl dissolved in water

207

ionizes sodium and chloride ions, and these ionized ions can enter into the pores of the coal sample. The

208

chloride and sodium ions then migrate directionally under the action of electrical field, converting core, which

209

has very poor electrical properties, to a region having fine electrical conductivity42. (b) It is possible that the

210

water in the sample is first broken, and the strong shock wave acts on the coal sample, which provides an

211

additional force to break the sample43,44. (c) Some of the minerals in coal will also dissolve in NaCl solution, in

212

turn providing free migrating ions to convert the coal into a conductor. By comparing the average breakdown

213

voltage of coal saturated with DW and NaCl solutions, it can be concluded that saturation with NaCl solution is

214

more conducive to enhancing the conductivity of coal, which indicates that both chloride and sodium ions play

215

a dominant role in the electrical breakdown process.

216

The breakdown field strength of coal samples was calculated according to equation (2), in which L is 3 cm.

217

As shown in Fig. 6, it can be seen that the average electrical breakdown field strength of Linhua and Hongliu

218

RCS is 6.01 kV/cm and 9.79 kV/cm, respectively. The average electrical breakdown field strength of Linhua

219

and Hongliu coal samples saturated with DW is 4.43 kV/cm and 6.12 kV/cm, respectively. The average

220

electrical breakdown field strength of Linhua and Hongliu coal samples saturated with 1mol/L NaCl solution is

221

1.75 kV/cm and 3.40 kV/cm, respectively. Thus, it can be concluded that the breakdown field intensity required

222

by coal saturated with NaCl solution is far less than that of raw coal.

223 224 225

E=

U L

where E is the electrical field strength, kV/cm; U is the voltage stored in the capacitor, kV; and L is the length of coal sample, cm.


(2)

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

(b)

LH-D LH-W LH-SC

-4

226

Page 12 of 22

-3

6.01

9.79

4.434.43

6.12

HL-D HL-W HL-SC

3.40

-2

-1 0 1 2 Voltage field (kV/cm)

3

4 -6

-5

-4

3.40

-3

-2 -1 0 1 2 3 Voltage field (kV/cm)

4

5

6

227

Fig. 6. Breakdown field strength: (a) breakdown field strength of Linhua; (b) breakdown field strength of

228

Hongliu.

229

3.2 Macroscopic fracture characteristics of coal electrical breakdown

230

To investigate the difference in surface cracks between raw coal and coal crushed by HVEP, an

231

environmental scanning electron microscope (SEM; Quanta 250, FEI Company, USA) was used; on SEM

232

images, surface fractures and elemental changes of coal samples were observed. The surface microscopic

233

features of Linhua and Hongliu coal samples can be directly observed in Figs. 7 and 8, respectively. Figs. 7

234

(a)–(d) (for Linhua coal samples) and 8 (a)–(d) (for Hongliu coal samples), respectively, show the surface

235

characteristics of raw coal, RCS crushed by HVEP, coal sample crushed by HVEP after saturating with DW,

236

and coal sample crushed by HVEP after saturating with DW (magnification: 1000×). It can be seen from Figs.

237

7 and 8 that the surface of coal samples has changed significantly before and after electrical breakdown. There

238

are almost no cracks on the surface of both Linhua and Hongliu RCS. Linhua coal sample has a smooth surface

239

layer, whereas Hongliu coal sample has a fibrous structure, which may be due to its low degree of

240

metamorphism. By contrast, after saturation, many tiny cracks can be observed on both coal samples.

241

Additionally, the fracture number and size of crushed coal samples are different from each other, which may be

242

due to unequal breakdown voltage. The breakdown voltage of RCS is higher than that of coal saturated with

243

NaCl solution; however, Figs. 7 (a)–(d) and 8 (a)–(d) show that the number of cracks on the crushed coal

244

samples saturated with NaCl solution is no less than that of RCS, which were crushed by HVEP. NaCl

245

treatment improved the conductivity of coal samples, thus making them more prone to cracks under high


Page 13 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

246

voltage, which indicates that saturating coal sample with NaCl solution before electrical breakdown is a better

247

way to form cracks in the coal.

248 249

Fig. 7. SEM images of Linhua coal samples before and after crushing by HVEP: (a) surface of RCS; (b)

250

surface of RCS crushed by HVEP; (c) surface of coal sample crushed by HVEP after saturating with DW; (d)

251

surface of coal sample crushed by HVEP after saturating with NaCl solution.

252 253

Fig. 8. SEM images of Hongliu coal samples before and after crushing by HVEP: (a) surface of RCS; (b)

254

surface of RCS crushed by HVEP; (c) surface of coal sample crushed by HVEP after saturating with DW; (d)

255

surface of coal sample crushed by HVEP after saturating with NaCl solution.

256

3.3 Microscopic pore characteristics of coal electrical breakdown

257

NMR test was used to investigated to the pore structure of coal broken down by electrical pulse. The T2

258

spectroscopy is to make the transverse relaxation time of 1H in water proportional to the pore radius of coal.

259

Different T2 values correspond to diverse pore structure, 0-10 ms (micropores), 10-100 ms (mesopores)

260

and >100 ms (macropores & microfractures).

261

NaCl solution not only can improve the conductivity of coal sample, but it can also enhance the porosity

262

of coal sample, the variaions of porosity characteristics are shown in Fig. 9. It is obviously to be seen that

263

before electrical breakdown, Linhua anthracite coal has two peaks (micropores and macropores), whereas

264

Hongliu bituminous coal has three peaks, which indicates that Hongliu coal porosity develops better than

265

Linhua coal porosity. Compared with the raw coal, the numbers of different-sized pores in both kinds of coal


Energy & Fuels

266

samples grow to various degrees after the electrical breakdown. Furthermore, mespores and macropores

267

appeared in the Linhua coal sample treated with electrical pulse, which will greatly multiply channels of gas

268

diffusion and permeation and thereby promote the extraction of CBM. ∆φ, the total porosity increase resulted

269

from electrical breakdown, is calculated by subtracting the total porosity of raw coal samples from that of the

270

electrically broken samples. The ∆φ of Linhua coal sample and Hongliu coal sample are 108.14%, 34.10%

271

respectively, demonstrating that markable increases in total porosities of electrically broken samples occurred,

272

which provides a fine channel for CBM exploitation.

0.1

1

10

100

1000

10000

Porosity percentage (%)

∆ϕ=108.14%

Macropores & microfractures

0.01

273

Mespores

Cumulative porosity (%)

Micropores

Raw coal Electrically broken coal Raw coal Electrically broken coal

(a)

Micropores

0.01

(b) ∆ϕ=34.10%

Mespores Macropores & microfractures

0.1

T2 /ms

1

10

100

1000

Cumulative porosity (%)

Raw coal Electrically broken coal Raw coal Electrically broken coal

Porosity percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 22

10000

T2 /ms

274

Fig. 9. T2 spectroscopy characteristics of coal samples before and after the electrical breakdown: (a) Linhua

275

anthracite coal; (b) Hongliu bituminous coal.

276

Figure. 10 presents the changes of different-sized pores in coal samples before and after the electrical

277

breakdown. It can be easily seen that different-sized pores of both Linhua anthracite coal and Hongliu

278

bituminous coal increase greatly in varying degrees. The growth rate of micropores, mesopores and macropores

279

& microfractures of Linhua coal grow substantially by 86.09%, 1167.15% and 339.16%, respectively, while

280

those of Hongliu coal increase by 24.61%, 71.49% and 210.86%, respectively. It can be concluded that the

281

changes of mesopores and macropores & microfractures are very remarkable, which is benefical to enhancing

282

the permeability of coal seam.


Page 15 of 22

30000

12000

(a) 86.09%

10000 8000 6000 4000

500 400 300 1167.15% 200 100 0 Mesopores

2000

25000

Raw coal Electrically broken coal

24.61%

750 600

20000

450

210.86%

300

15000

150 0

10000


71.49%

339.16%

5000 0

0 Micropores

283

(b)

Raw coal Electrically broken coal

Amplitude

14000

Amplitude

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Mesopores


Micropores

Mesopores


284

Fig. 10. Capacities of different-sized pores in coal samples before and after the electrical breakdown: (a)

285

Linhua anthracite coal; (b) Hongliu bituminous coal.

286

Fig. 11 shows the EDS spectral analysis results of both Linhua and Hongliu coal samples before and after

287

electrical breakdown. In Fig. 11, the abscissa represents the level of the energy, with different elements

288

appearing in different locations; the peak height indicates the relative content of the element45. Valuable

289

information such as content and composition of elements can be easily obtained from Fig. 11. A comparison of

290

Figs. 11 (a) and (b) shows that the contents of chlorine and sodium on the surface of Linhua coal sample

291

saturated with NaCl solution increased, demonstrating that sodium and chloride can enter into the pores of coal

292

during the saturation process. These elements could migrate directionally under a strong electrical field, thereby

293

making the coal sample a fine conductor. Besides, there are changes in the composition of other elements (such

294

as carbon and oxygen) on the coal surface before and after electrical breakdown, which may be caused by the

295

high-temperature current generated during electrical breakdown. In addition, mental elements such as

296

aluminum, sodium, silicon, and titanium can be found on the surface of Linhua coal samples, which play a

297

positive role in the process of electrical breakdown9. Less number of metal elements are found in Hongliu coal

298

sample, compared with Linhua coal sample, which could be one of the reasons that the breakdown voltage of

299

Hongliu coal sample is higher than that of Linhua.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cps/eV cps/eV cps/eV

10 Element C O Na Al Si K Ti Cl

3.5 3.0

2.0

cps/eV

cps/eV

(b) (b)

(a)

4.0

2.5 2.5

Page 16 of 22

Ti K O Na Si C Al

K

Ti

Weight(%) 54.78 31.85 0.61 6.93 5.22 0.52 0.15 0

Atomic(%) 64.82 28.29 0.37 3.65 2.64 0.19 0.04 0

Element C O Na Al Si K Ti Cl

8

6

4

Cl Ti K O Al C Na Si

Cl

K

Ti

Weight(%) 66.89 11.55 6.93 0.55 0.56 0.28 0.15 13.1

Atomic(%) 79.42 10.29 4.3 0.29 0.28 0.1 0.04 5.27

1.5 1.0

2

0.5

0

0 2 2

300

4 4 4

6 6

8 8 8

10 10 10

12 12

14 14 14

16 16 16

18 18 18

2

44

2

20 20 20

6

6

8

8

keV cps/eV cps/eV

12 12

14 14

16 16

18 18

cps/eV cps/eV

(c)

4.5

(d)

5

4.0

Element C O Na Al Ca Cl

3.5 3.0 2.5 2.0

10 10 keV

CaO Na C Al

Ca

1.5

Weight(%) Atomic(%) 62.22 73.23 25.00 22.09 0.27 0.27 0.63 0.33 11.89 4.19 0 0

Element C O Na Al Ca Cl

4

3

Cl CaO Al C Na

Cl

Ca

Weight(%) 73.76 20.99 0.32 0.32 3.99 0.62

Atomic(%) 80.85 17.27 0.18 0.16 1.31 0.23

2

1.0

1

0.5

0

0

301

2 2

4 4

6 6

8 8

10 10 keV

12 12

14 14

16 16

18 18

2

4

6

8

10 keV

12

14

16

18

302

Fig. 11. EDS spectral analysis results: (a) spectral analysis of Linhua RCS; (b) spectral analysis of Linhua coal

303

sample crushed by HVEP after saturating with NaCl solution; (c) spectral analysis of Hongliu RCS; (d) spectral

304

analysis of Hongliu coal sample crushed by HVEP after saturating with NaCl solution.

305

3.4 Breakage properties and FTIR analysis results for coal samples

306

The differences between raw coal and coal saturated with DW and NaCl solution crushed by HVEP are

307

obvious, as can be seen in Fig. 12. Figs. 12 (a) and (d) show Linhua and Hongliu raw coal samples crushed by

308

HVEP, respectively. Figs. 12 (b) and (e) show Linhua and Hongliu coal samples crushed by HVEP after

309

saturating with DW. Figs. 12 (c) and (f) show coal samples crushed by HVEP after saturating with NaCl

310

solution. The high-temperature current will flow through the coal surface when the coal samples are crushed by

311

HVEP, and the current flowing will produce large quantities of heat, which will cause some chemical changes

312

on the surface of coal sample46. As can be seen in Figs. 12 (a)–(f), the traces of the discharge channel on the

313

surface of coal fragments crushed by HVEP are different. It is very difficult to find the traces of the discharge

314

channel on the surface of RCS crushed by HVEP. By contrast, there are also few traces of the discharge

315

channel on the surface of coal samples crushed by HVEP after saturating with DW. However, the coal samples

316

saturated with NaCl solution show obvious distinction, with many obvious traces of burning on the coal surface,


Page 17 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

317

which indicates that these are more likely to produce high-temperature current on the surface of coal sample

318

saturated with the NaCl solution.

319

The ability of adsorbing gas by coal not only depends on the pore, but also depends on the surface

320

functional group47, therefore, FTIR spectra of different coal samples were analyzed to study the effects of

321

HVEP on the surface of coal sample. Fig. 12 shows the FTIR spectra analysis result of Linhua and Hongliu

322

coal samples crushed by HVEP, including RCS (Curves a and d), coal saturated with DW (Curves b and e) and

323

NaCl solution (Curves c and f). The peak values at different wave numbers represent the content of different

324

functional groups. In Fig. 12, it can be seen that the trend of peak value for the different curves is consistent

325

with each other. However, obvious changes in each functional group occurred during the process of electrical

326

breakdown, which may be caused by high-temperature current during discharge, and these changes may have

327

some influence on the process of gas adsorption and desorption48.

328 329

Fig. 12. FTIR spectral analysis: (I) Linhua anthracite coal; (II) Hongliu bituminous coal.

330

4. Conclusions

331 332

In this work, a solution of 1mol/L NaCl was used to improve the conductivity of two different coal samples with different metamorphic grade. The major conclusions from this study are as follows:

333

(1) The average breakdown voltage of raw coal is the greatest among all samples tested and that of coal

334

saturated with NaCl solution is the minimum. These results demonstrate that saturating coal with NaCl solution

335

reduces the permittivity of coal samples and enhances their conductivity simultaneously, thereby making

336

breakdown easier.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

337

(2) SEM images indicate that the number of cracks on the surface of the coal sample crushed by HVEP is

338

much more than that of RCS, which indicates that the HVEP technology is a good method to increase fractures.

339

Additionally, the number of cracks on the surface of sample saturated with NaCl solution is no less than that of

340

sample saturated with DW and raw coal. This indicates the saturation process before electrical breakdown does

341

not affect the breakdown effect, but saves energy consumption.

342

(3) NMR results show that the porosity of mesopores and macropores are enhanced by electrical

343

breakdown, which provide a dominant role in gas extraction. Therefore, it is a very good method for the

344

improvement of porosity to saturate coal with NaCl solution prior to electrical breakdown.

345

(4) FTIR spectra of results show significant changes in functional groups during the process of electrical

346

breakdown, which may be caused by the high-temperature current during discharge, and these changes may

347

have some influence on the process of gas adsorption and desorption.

348

Acknowledgements

349

This work financed by the National Natural Science Foundation of China (Grant No. 51474211), and A

350

Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions

351

(PAPD). The authors would like to thank SWAN Editorial Service ([email protected]) for

352

editing this manuscript.

353

References:

354

(1) Moore, T. A. Coalbed methane: a review. International Journal of Coal Geology. 2012, 101, 36-81.

355

(2) Yang, W.; Lin, B. Q.; Qu, Y. A.; Zhao, S.; Zhai, C.; Jia, L. L.; Zhao, W. Q. Mechanism of strata

356

deformation under protective seam and its application for relieved methane control. International Journal of

357

Coal Geology. 2011, 85, 300-306.

358

(3) Hong, Y. D.; Lin, B. Q.; Zhu, C. J.; Li, H. Influence of microwave energy on fractal dimension of coal

359

cores: Implications from nuclear magnetic resonance. Energy Fuels. 2016, 30 (12), 10253-10259.

360

(4) Javadpour, F.; McClure, M.; Naraghi, M. E. Slip-corrected liquid permeability and its effect on hydraulic

361

fracturing and fluid loss in shale. Fuel. 2015, 160, 549-559.

362

(5) Lu, T. K.; Yu, H.; Zhou, T. Y.; Mao, J. S.; Guo, B. H. Improvement of methane drainage in high gassy coal

363

seam using waterjet technique. International Journal of Coal Geology. 2009, 79, 40-48.

364

(6) Wang, F. G.; Tu, S. H.; Yuan, Y.; Feng, Y. F.; Fang, F.; Tu, H. S. Deep-hole pre-split blasting mechanism


Page 18 of 22

Page 19 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

365

and its application for controlled roof caving in shallow depth seams. International Journal of Rock Mechanics

366

and Mining Sciences. 2013, 64, 112-121.

367

(7) Li, H.; Lin, B. Q.; Chen, Z. W.; Hong, Y. D.; Zheng C. S. Evolution of coal petrophysical properties under

368

microwave irradiation stimulation for different water saturation conditions. Energy Fuels, 2017, 31 (9), 8852–

369

8864.

370

(8) Zhai, C.; Qin, L.; Liu, S. M.; Xu, J. Z.; Tang, Z. Q.; Wu. S. L. Pore Structure in Coal: Pore evolution after

371

cryogenic freezing with cyclic liquid nitrogen injection and its implication on coalbed methane extraction.

372

Energy Fuels. 2016, 30, 6009−6020.

373

(9) Yan, F. Z.; Lin, B. Q.; Zhu, C. J.; Guo, C.; Zhou, Y.; Zou, Q. L.; Liu, T. Using high-voltage electrical

374

pulses to crush coal in an air environment: An experimental study. Powder Technology. 2016, 298, 50-56.

375

(10) Andres, U. Electrical disintegration of rock. Mineral Processing and Extractive Metallurgy Review. 1994,

376

14, 87-110.

377

(11) Cho, S. H.; Cheong, S. S.; Yokota, M.; Kaneko, K. The dynamic fracture process in rocks under high-

378

voltage pulse fragmentation. Rock Mechanics and Rock Engineering. 2016, 49, 3841-3853.

379

(12) Inoue, H.; Lisitsyn, I.; Akiyama, H.; Nishizawa, L. Drilling of hard rocks by pulsed power. IEEE

380

Electrical Insulation Magazine. 2000, 16, 19-25.

381

(13) Tisack, M.; Sacks, R. Dielectric breakdown characteristics of a high current metal vapor plasma.

382

Pergamon Journals. 1987, 42, 959-972.

383

(14) Timoshkin, I. V.; Mackersie, J. W.; MacGregor, S. J. Plasma channel miniature hole drilling technology.

384

IEEE Transactions on Plasma Science. 2004, 32, 2055-2061.

385

(15) Zhao, Y. M.; Zhang, B.; Duan, C. L.; Chen, X.; Sun, S. Material port fractal of fragmentation of waste

386

printed circuit boards (WPCBs) by high-voltage pulse, Powder Technology. 2015, 269, 219–226.

387

(16) Scott, H. W. US Patent, 1978 (US 1978/4074758).

388

(17) Wesley, R. H. US Patent, 1982 (US 1982/4997044).

389

(18) Yao, W. B.; Zhang, Y. M.; Tang, J. P.; Guo, J. M.; Cheng, L. Cathode plasma integral spectroscopy

390

diagnosis on high current pulsed electron beam diode. IEEE Transactions on plasma Science. 2014, 42, 3264-

391

3267.

392

(19) Shi, Q. M.; Qin, Y.; Li, H. L.; Qiu, A. C. Zhang, Y. M.; Zhou, X. T.; Zheng, S. J. Response of pores in


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

393

coal to repeated strong impulse waves. Journal of Natural Gas Science and Engineering. 2016, 34, 298-304.

394

(20) Eric, W.; Shi, F. N.; Emmy, M. Mineral liberation by high voltage pulses and conventional comminution

395

with same specific energy levels. Minerals Engineering. 2012, 27-28, 28-36.

396

(21) Andres, U.; Jirestig, J.; Timoshkin, I. Liberation of minerals by high-voltage electrical pulses. Powder

397

Technology. 1999, 104, 37-49.

398

(22) Sperner, B.; Jonckheere, R.; Pfänder, J. A. Testing the influence of high-voltage mineral liberation on

399

grain size, shape and yield, and on fission track and 40Ar/39Ar dating. Chemical Geology. 2014, 371, 83-95.

400

(23) Boev, S.; Vajov, V.; Levchenko, B.; Jgun, D.; Muratov, V.; Peltsman, S.; Adam, A.; Uemura, K.

401

Electropulse technology of material destruction and boring. International Conference of pulsed power. 1997,

402

220-225.

403

(24) Cho, S. H.; Mohanty, B.; Nakamiya, Y.; Owada, S.; Kubota, S.; Ogata, Y.; Tsubayama, A.; Yokota, M.;

404

Kaneko, K. Dynamic fragmentation of rock by high-voltage pulses. American Rock Mechanics Association.

405

2006.

406

(25) Macgregor, S. J.; Turnbull, S. M. EP Patent, 2011 (EP 2011/1474587B1).

407

(26) Yan, F. Z.; Lin, B. Q.; Zhu, C. J.; Zhou, Y.; Liu, X.; Guo, C.; Zou, Q. L. Experimental investigation on

408

anthracite coal fragmentation by high-voltage electrical pulses in the air condition: Effect of breakdown voltage.

409

Fuel. 2016, 183, 583-592.

410

(27) Burkin, V. V.; Kuznetsova, N. S.; Lopatin, V. V. Dynamics of electro burst in solids: I. Power

411

characteristics of electro burst. Journal of Physics D: Applied Physics. 2009, 42, 1-6.

412

(28) Zhao, W. B. Phenomena and Mechanism of Surface Flashover across Semiconducting Materials under

413

Pulsed Voltage, Ph.D. dissertation, Xi'an Jiaotong University, Xi'an, 2007.

414

(29) Chen, L. M.; Cheng, M. X.; Xiao, X. F.; Huang, Z. H. Measurement of the relationship between

415

conductivity of salt solution and concentration and temperature. Research and Exploration in Laboratory. 2010,

416

29, 39-42.

417

(30) Deng, J.; Deng, Y.; Zhang, Y. T.; Zhang, Y. N. Experimental research on coal sample secondary oxidation

418

and spontaneous combustion properties with moisture contents. Journal of Xi'an University of Science and

419

Technology. 2016, 36, 451-456.

420

(31) Liu, H. Q. Ionic conductive and dielectric properties in rocks, China Science Publishing and Media Ltd,


Page 20 of 22

Page 21 of 22 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

421

Beijing, 2015.

422

(32) Tang, Z. Q.; Zhai, C.; Zou, Q. L.; Qin, L. Changes to coal pores and fracture development by ultrasonic

423

wave excitation using nuclear magnetic resonance. Fuel. 2016, 186, 571-578.

424

(33) Gu, F. J. Experimental study of methane adsorption and seepage characteristics of coal inTaoyuan Colliery

425

after long-term water immersion, MA.Eng. Thesis, China University of Mining and Technology, Xuzhou, 2016.

426

(34) Zhu, C. J.; Lu, X. M.; Gao, Z. S.; Yan, F. Z.; Guo, C.; Zhang, X. L. Effect of high-voltage thermal

427

breakdown on pore characteristics of coal. International Journal of Mining Science and Technology. 2017.

428

(35) Wu, H. W.; Bryant, G.; Benfell, K.; Wall, T. An Experimental Study on the Effect of System Pressure on

429

Char Structure of an Australian Bituminous Coal. Energy Fuels. 2000, 14 (2), 282–290.

430

(36) Meisam, P.; Shekhar, R. M.; Manoj, K. M.; Liu, J. Bioelectrochemical treatment of acid mine drainage

431

(AMD) from an abandoned coal mine under aerobic condition. Journal of Hazardous Materials. 2017, 333,

432

329-338.

433

(37) Sahoo, B. K.; De, S.; Meikap, B. C. Artificial neural network approach for rheological characteristics of

434

coal-water slurry using microwave pre-treatment. International Journal of Mining Science and Technology.

435

2017, 27, 379-386.

436

(38) Backstrom, D.; Johansson, R.; Andersson, K.; Johnsson, F.; Clausen, S.; Fateev, A. Measurement and

437

Modeling of Particle Radiation in Coal Flames. Energy fuels. 2014, 28 (3), 2199–2210.

438

(39) Pickles, C. A.; Gao, F.; Kelebek, S. Microwave drying of a low-rank sub-bituminous coal. Minerals

439

Engineering. 2014, 62, 31-42.

440

(40) Zhou, F.; Cheng, J.; Liu, J. Z. Wang, Z. H.; Zhou, J. H.; Cen, K. Improving the permittivity of Indonesian

441

lignite with NaCl for the microwave dewatering enhancement of lignite with reduced fractal dimensions. Fuel.

442

2015, 162, 8-15.

443

(41) Liu, J. D. On-line measurement conductivity of deion-water. The Administration and Technique of

444

Encironmental Monitoring. 1997, 9, 48.

445

(42) Gao, C. Q.; Mao, Z. Q.; Li, J. F. The electrieal effieiency of rocks and its relationship with water

446

saturation. Geophysical Prospecting for Petroleum. 1998, 37, 130-136.

447

(43) Zhou, H. B.; Zhang, Y. M.; Han, R. Y.; Jing, Y.; Wu, J. W.; Liu, Q. J.; Ding, W. D.; Qiu, A. C. Signal

448

analysis and waveform reconstruction of shock waves generated by underwater electrical wire explosions with


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

449

piezoelectric pressure probes. Sensors. 2016, 16, 1-18.

450

(44) Liu, Q. J.; Ding, W. D.; Han, R. Y.; Wu, J. W.; Jing, Y.; Zhang, Y. M.; Zhou, H. B.; Qiu, A. C. Fracturing

451

effect of electrohydraulic shock waves generated by plasma-ignited energetic materials explosion. IEEE

452

Transactions on Plasma Science. 2017, 45, 423-431.

453

(45) Zhu, J. F.; Wang, P.; Zhang, W. B.; Li, J. G.; Zhang, G. H. Polycarboxylate adsorption on coal surfaces

454

and its effect on viscosity of coal-water slurries. Powder Technology. 2017, 315, 98-105.

455

(46) Sang, H. C.; Mitsuhiro, Y.; Mayumi, I.; Satoru, K.; Soo, B. J.; Byoung, K. K.; Katsuhiko, K. Electrical

456

disintegration and micro-focus X-ray CT observations of cement paste samples with dispersed mineral particles.

457

Minerals Engineering. 2014, 57, 79-85.

458

(47) Si, L. L.; Li, Z. H.; Yang, Y. L.; Zhou, J.; Zhou, Y. B.; Liu, Z.; Liu, L. W. Modeling of gas migration in

459

water-intrusion coal seam and its inducing factors. Fuel. 2017, 210, 398-409.

460

(48) Crosdale, P. J.; Moore, T. A.; Mares, T. E. Influence of moisture content and temperature on methane

461

adsorption isotherm analysis for coals from a low-rank, biogenicallysourced gas reservoir. International Joural

462

of Coal Geology. 2008, 76, 166–174.


Page 22 of 22

Improving the Conductivity and Porosity of Coal with NaCl Solution for

Recommend Documents