Pore distribution characteristics of various rank coals matrix and their influences on gas adsorption

Journal Pre-proof Pore distribution characteristics of various rank coals matrix and their influences on gas adsorption Jiwei Yan, Zhaoping Meng, Kun Zhang, Huifang Yao, Haijin Hao PII:

S0920-4105(20)30135-2

DOI:

https://doi.org/10.1016/j.petrol.2020.107041

Reference:

PETROL 107041

To appear in:

Journal of Petroleum Science and Engineering

Received Date: 30 March 2019 Revised Date:

21 November 2019

Accepted Date: 5 February 2020

Please cite this article as: Yan, J., Meng, Z., Zhang, K., Yao, H., Hao, H., Pore distribution characteristics of various rank coals matrix and their influences on gas adsorption, Journal of Petroleum Science and Engineering (2020), doi: https://doi.org/10.1016/j.petrol.2020.107041. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

1

Pore distribution characteristics of various rank coals matrix and

2

their influences on gas adsorption

3

Jiwei Yana, Zhaoping Menga,b,∗, Kun Zhanga, Huifang Yaoc, and Haijin Hao

4 5 6 7 8 9

b

a

College of Geosciences and Surveying Engineering, China University of Mining and Technology

(Beijing), Beijing, 100083, PR China b

State Key Laboratory of Coal and CBM Co-mining, Shanxi Jincheng Anthracite Mining Group

Company, Ltd., Jincheng 048000, PR China c

Department of Earth Science and Engineering, Taiyuan University of Technology, Taiyuan

030024, PR China

10

Abstract: Methane is primarily stored in coal matrix pores and pore size

11

distribution has an important effect on gas adsorption/desorption. Investigation of the

12

relationship between pore size distribution and adsorption performance of coal is of

13

significance for the understanding of the evolution of coalbed methane reservoirs. In

14

this study, a series of laboratory experiments (nitrogen, carbon dioxide and methane

15

adsorption) were carried out to determine the pore size distribution and adsorption

16

capacity of coal samples of various coal ranks. The relationship between pore size

17

distribution, Langmuir volume and the metamorphic degree of coals were established.

18

The factors influencing methane adsorption of coals were also analyzed. The results

19

show that the matrix pores are mainly cylindrical, thin bottleneck-, ink bottleneck-

20

and parallel plate-shaped. With the increase in coal rank, both the total pore volume

21

and the specific surface area initially decrease and then increase. Matrix pores,

22

including micro-, transitional- and mesopores, show a similar asymmetric U-shaped

23

trend in the pore volume and the specific surface area with an increase in coal rank.

24

The percentages by volume and by specific surface area increase for the micropores

25

and declines for the transitional pores and mesopores. In the low-pressure zone,

26

micro- and transitional pores play a dominant role in methane adsorption; however, in

27

the high-pressure zone, the influence of mesopores on methane adsorption is

28 29

pronounced. Key words: various rank coals, pore size distribution, controlling mechanism,

30

methane adsorption

31

1 Introduction

32

Coal experiences a complex evolution processes after its formation, which

33

results in great heterogeneity and complexity in coal structure (Meng et al., 2015;

34

Mishra et al., 2018; Xu et al., 2019; Zhao et al., 2019). The pore size distribution in

35

the coal matrix has an important influence on gas adsorption, desorption and

36

migration. Therefore, it is essential to identify the evolutionary characteristics of coal

37

matrix pore structure and their influences on gas adsorption (Bustin et al., 2016; Meng

38

and Li, 2017; Meng et al., 2016). Fluid invasion and radiation methods are widely

39

used to determine the pore size distribution (PSD) of coals. The fluid invasion

40

methods, such as mercury intrusion porosimetry (MIP) and low-temperature nitrogen

41

adsorption (LT-NA), can provide information about pore morphology and

42

connectivity (Peng et al., 2017; Rijfkogel et al., 2019). The radiation method can

43

reveal the spatial distribution characteristics of pores within the coal (Clarkson et al.,

44

2013). For example, both microfocus X-ray computed tomography (Micro-CT) and

45

focused ion beam scanning electron microscopy (FIB-SEM) can characterize the PSD

46

of coal matrix in two-dimensional plane and in three-dimensional space (Karimpouli

47

et al., 2017; Liu et al., 2017a); small angle X-ray scattering (SAXS) and small angle

48

neutron scattering (SANS) can determine the volume percentages of different types of

49

pores in coal (Melnichenko et al., 2012; Nakagawa et al., 2000; Okolo et al., 2015).

50

Atomic force microscopy (AFM) is a powerful tool for the characterization of

51

macromolecular structure and 3-D spatial structure of coal (Liu et al., 2019a,b Pan et

52

al., 2015b). The above-mentioned characterization methods provide insight into the

53

pore structure of coals. However, a single technology cannot characterize the

54

full-scale pore structure, and it is necessary to combine various methods to study

55

coals' pore characteristics.

56

According to the pore classification method proposed by Hodot (1966), coal

57

matrix pores can be divided into four categories: micropores (pore diameter d≤10nm),

58

transitional

59

macropores (d>1000nm). Previous studies have found that the proportion of macro-

60

and mesopores is relatively large for low-rank coals, while the proportion of

61

transitional pores and micropores is relatively large for medium- and high-rank coals.

62

The total pore volume and specific surface area (SSA) show a U-shaped variation law

63

with the increase in coal rank (Pan et al., 2015a). For the coals in the eastern margin

64

of Ordos, with the increase in coal rank, the proportion of micropores first decreases

65

and then increases and the proportion of transitional pores follows an opposite trend

66

(Chen et al., 2015). Pores less than 5nm in diameter contribute significantly to the

67

SSA, while the pores greater than 10nm in diameter make a major contribution to the

68

pore volume for the high-rank coals in the Sichuan Basin (Shan et al., 2015). The

69

fractures in low-rank coals are short and irregular while they are well-developed and

70

regular in high-rank coals (Chen et al., 2015), which, to some extent, results in an

71

increase in the total pore volume of high-rank coals.

pores

(10nm
mesopores

(100nm
and

72

The coal macerals (vitrinite, inertinite and exinite) also affect the pore size

73

distribution. The SSA, volume, and diameter of pores are generally higher in the

74

inertinite than in the vitrinite (Berbesi et al., 2009; Shan et al., 2015), while Teng et al.

75

(2017) concluded that vitrinite was more porous than inertinite in the Illinois Basin

76

(U.S.) coal. The macerals vary in the different coal regions and basins (Sakurovs et al.,

77

2018). The coal macerals are key factors that influence the behavior of CH4 adsorption.

78

Many studies have shown that vitrinite has a significant effect on the CH4 sorption

79

capacity (Bustin and Clarkson, 1998; Dutta et al., 2011; Moore, 2012). Kiani et al.

80

(2018) held that inertinite contains a large number of pores with a diameter of 8-50nm

81

and has significant influence on the rate of gas sorption, but the maximum adsorption

82

capacity of coals is basically independent of inertinite content. Clarkson and Bustin

83

(2000) conducted isothermal adsorption experiments on CH4 and CO2 and found that

84

the coal with the highest adsorption capacity does not have the highest content of

85

vitrinite nor inertinite, and vitrinite and inertinite coexist in the studied coal.

86

CH4 adsorption capacity greatly depends on the degree of coal metamorphism

87

and PSD characteristics (Naveen et al., 2018). Low-rank coals always exhibit a weak

88

methanogenic capacity and a weak methane adsorption capacity. The methanogenic

89

capacity increases gradually from lignite to anthracite (Scott, 2002). The bituminous

90

coals, high-rank bituminous coals and anthracite coals always have a relatively high

91

cumulative gas production and a high adsorption capacity. The pores <2nm in coals

92

provide a large amount of SSA and play a dominant role in gas adsorption at low

93

pressure, and there is a positive correlation between gas content and pore (<2nm)

94

volume (Mastalerz et al., 2008). With the rise of gas pressure, the transitional-, meso-

95

and macropores tend to have an important influence on gas adsorption. Thus,

96

exploring the PSD characteristics is helpful to understanding the behavior of gas

97

adsorption in various rank coals.

98

At present, the following questions need to be further explored about coal pore

99

structure: systematical analysis of the morphology and evolution mechanism of pores

100

of various rank coals; the influence and controlling mechanism of different pore

101

structures on CH4 adsorption, especially the distribution and evolution characteristics

102

of micropores and their influences on gas adsorption. In this work, low temperature

103

N2 and CO2 adsorption experiments were carried out to determine the pore structure

104

parameters of various rank coals. Methane isotherm adsorption tests were carried out

105

and correlations between adsorption capacity, pore size distribution and Ro,max

106

(maximum vitrinite reflectance with oil) with various rank coals were explored.

107

Finally, the influences of pore morphology and distribution characteristics on CH4

108

adsorption capacity were elucidated, and the evolution characteristics of micropores

109

(in the range of 1.4-10nm in diameter) and transitional- pores and their effects on CH4

110

adsorption were analyzed, and the relationship between pore size distribution and

111

Ro,max was established. The outcomes help the understanding of gas flow and

112

adsorption behaviors in coal mines, and it can also provide a reference for the CBM

113

well drainage.

114

2 Materials and methods

115

2.1 Description of coal samples

116

According to the degree of coal metamorphism (Ro,max, following ISO

117

7404–5:2009),

118

(Ro,max=0.65%-2.0%) and high-rank coals (Ro,max>2.0%) (ISO 11760-2005; Zhang et

119

al., 2018). In this research, coal samples were collected from eastern Yunnan and

120

Inner Mongolia (low-rank coals, Ro,max=0.23%-0.57%), the northern Qinshui Basin

121

and the eastern Ordos Basin (medium-rank coals, Ro,max=0.69%-1.80%) and the

122

southern Qinshui Basin (high-rank coals, Ro,max=2.16%-3.45%) in China. The results

123

of proximate analysis (following ISO 17246: 2010 and ISO 17247-2013) and

124

petrographical determination (following ISO 7404-1:2016) of the coal samples are

125

listed in Table 1. The results indicate that the contents by volume for vitrinite,

126

inertinite and exinite are in the range of 54.1-95.02%, 0.48-22.68% and 0-10.8%,

127

respectively. The exinite largely disappears when Ro,max>1.1%. The mineral content is

128

approximately 5%, except for Baode No.8 coal sample (with a high mineral content of

129

22.60%).

130

2.2 Experimental methods

coals

can

be

divided

into

low-

(Ro,max<0.65%),

medium-

131

Many technologies have been used to determine the PSD of coals, such as MIP

132

(Okolo et al., 2015), LT-NA (Labani et al., 2013), CO2 adsorption (Song et al., 2017),

133

Micro-CT (Ramandi et al., 2016), SEM or FIB-SEM (Gaboreau et al., 2016), AFM

134

(Misra et al., 2019), nuclear magnetic resonance (NMR) (Yao et al., 2010). The

135

principles and the range of apertures tested by different methods are quite different

136

(Fig.1). N2 and CO2 adsorption methods are convenient and effective and are widely

137

used for the determination of PSD.

138

The low temperature nitrogen adsorption (LT-NA) experiments were carried out

139

with the TriStar II3020M automatic multi-station SSA and pore size analyzer made by

140

Micromeritics Instrument, USA. The samples were broken to 40-60 mesh in size, with

141

a weight of 2-3 g for each sample was used for this experiment. Samples were ground

142

and carefully sieved to avoid damaging matrix pores. Because the size of coal

143

particles is three orders of magnitude larger than the pore size of coal matrix, the

144

influence of the coal sample preparation process on the pore structure of coal matrix is

145

insignificant and can be neglected. Before the LT-NA experiments, all samples were

146

dried in a vacuum oven at 378.15K for 24 hours. After vacuum degassing, the samples

147

were placed in the analyzer. Temperature of the container was kept constant with

148

liquid nitrogen (77.15K). The adsorption-desorption experiments were started

149

according to the preset pressure. The adsorption-desorption quantities under different

150

pressures were obtained. The multipoint Brunauer-Emmett-Teller (BET) (Brunauer et

151

al., 1938) and the Barrett-Joyner-Halenda (BJH) methods were used to calculate SSA

152

and pore volume by the instrument software, respectively. The test range in pore

153

aperture is between 1.7nm and 300nm (Li and Meng, 2016).

154

The experimental instrument and procedure of CO2 adsorption experiment are

155

the same as those of the LT-NA experiment. These two experiments were carried out

156

with the same samples. While the experimental temperature for CO2 adsorption was

157

set to 273.15K. The instrument automatically carries out the adsorption experiment

158

and calculates the parameters of PSD by the D-A method according to preset pressure

159

conditions. The aperture measured by CO2 adsorption falls in the range of <2nm

160

(Gaucher et al., 2011).

161

The isothermal adsorption experiment is carried out using the ISO-300

162

isothermal adsorption-desorption apparatus made by Terra Tek Company, USA. The

163

experimental procedure and the apparatus have been described in detail (Zhang et al.,

164

2011). According to the national standard GB/T 19560-2008 (Experimental method of

165

high-pressure isothermal adsorption to coal), the coal samples are broken into

166

particles of 60-80 mesh (0.25-0.18mm). The weight of each sample used for the

167

experiment was 100-120g. Moisture equilibration of the coal samples was performed

168

according to the national standard and then the sample was quickly placed into the

169

sample cell. Sample containers are placed in a humidity-balanced dryer. A sufficient

170

amount of solution, supersaturated with potassium sulfate, is placed at the bottom of

171

the dryer. The weight of the coal sample is measured every 24 hours until the weight

172

variation does not exceed 2%. Helium gas is used to calibrate the volumes of the

173

sample and reference cells and check the system for any leaks. The system

174

temperature is equal to 303.15K, similar to the reservoir temperature. The maximum

175

equilibrium pressure is ~8MPa. Seven pressure points were measured in the

176

isothermal adsorption experiment for each sample. When the gas pressure in the

177

sample cylinder remains unchanged for over 12 hours, the adsorption equilibrium is

178

considered to be reached. Then the isothermal adsorption experiment steps to the next

179

pressure point.

180

3 Results

181

3.1 The low temperature N2 adsorption experiment

182

Experimental results of LT-NA are shown in Fig.2 and Table 2. The adsorption

183

and desorption curves of various rank coals are significantly different (Fig.2a-f).

184

According to the morphological characteristics of the adsorption and desorption

185

curves, the relative pressure can be divided into three zones: low-pressure zone

186

(p/p0<0.1), medium-pressure zone (p/p0=0.1-0.9) and high-pressure zone (p/p0>0.9).

187

For some low-rank coals, the adsorption capacity increases slowly with

188

increasing pressure in low-pressure zone (Fig.2a), indicating that the micropores are

189

poorly interconnected. While for other low-rank coals, the adsorption capacity

190

increases rapidly with increasing pressure in the low-pressure zone (Fig.2b), meaning

191

that the micropores are well developed and interconnected. This difference in gas

192

adsorption is mainly related to the formation environment of the coal and the

193

coal-forming materials (e.g., coal petrographic composition, moisture content) (Fu et

194

al., 2017). The above results show that the micropore distribution of low-rank coal

195

varies in different regions, and the micropores mainly exist in the coal

196

macromolecules and are closely related to the degree of coal metamorphism (Song et

197

al., 2017) and the morphology of these micropores is mainly cylindrical. The

198

adsorption capacity of the BZDB coal samples hardly rises with increasing pressure in

199

the medium-pressure zone (Fig.2a), indicating that the transitional pores have poor

200

interconnectivity and there are many fine bottleneck-shaped transitional pores. In

201

contrast, the adsorption capacity of SCH2 increases significantly with increasing

202

pressure and there is an obvious hysteresis loop in the desorption curve (Fig.2b),

203

indicating that there are some ink bottle-shaped pores in this coal sample. The

204

adsorption capacity increases rapidly with increasing pressure in the high-pressure

205

zone, and capillary condensation effects mainly occur in the mesopores in this stage.

206

Most of the adsorption/desorption isotherms overlap in the high-pressure zone, which

207

means that mesopores are well-developed with a good interconnectivity and are

208

mainly composed of parallel plate-shaped pores or wedge-shaped pores with one side

209

closed.

210

In the low-pressure zone, the adsorption capacity of medium-rank coals is

211

generally greater than that of low-rank coals, meaning that the quantity of micropores

212

in medium-rank coals increases with increasing metamorphism. The adsorption

213

capacity increases relatively slowly with increasing pressure in the medium-pressure

214

zone (Fig.2c, d), reflecting that the pore interconnectivity is very poor at this scale.

215

For some samples the desorption curves have an obvious hysteresis loop, indicating

216

that there are many fine or ink bottleneck-like pores in these coals. Some desorption

217

curves almost have no hysteresis loops in this region, indicating that there are many

218

cylindrical pores with one end closed. The N2 adsorption capacity increases rapidly in

219

the high-pressure zone, capillary condensation mainly occurs in the mesopores, and

220

the desorption curve has no hysteresis loops, indicating that the pores are primarily

221

wedge-shaped pores with one end closed.

222

For high-rank coals, nitrogen adsorption increases rapidly with increasing

223

pressure in the low-pressure zone, and the desorption curve has an obvious hysteresis

224

loop (Fig.2e, f), indicating that gas desorption from micropores is relatively difficult.

225

The micropores in high-rank coals are mainly cylinder-shaped metamorphic pores.

226

The adsorption capacity rises relatively slowly in the medium-pressure zone,

227

indicating that the transitional pores are poorly interconnected. The adsorption

228

capacity increases significantly in the high-pressure zone, and the corresponding

229

desorption curve has a large hysteresis loop, indicating that the mesopores are

230

partially interconnected fine bottleneck-shaped pores, and that the nitrogen molecules

231

are difficult to desorb from these pores due to capillarity.

232

In summary, low-rank coals contain more cylindrical or wedge-shaped pores

233

with one side closed, medium-rank coals contain more wedge-shaped pores, and

234

high-rank coals contain a large number of cylindrical- or fine bottleneck-shaped

235

pores.

236

3.2 Carbon dioxide adsorption experiment

237

The CO2 adsorption isotherm curves (Fig.3) belong to Type I as characterized by

238

IUPAC (Sing et al., 1984), which reveals the gas adsorption characteristics of the

239

pores with 1.4-1.6nm in diameter. The parameters of CO2 adsorption isotherm and

240

pores with size 1.4-1.6nm show similar variation laws while the adsorption capacity

241

varies greatly in various rank coals (Table 3). The SSA and volume of pores with size

242

1.4-1.6nm can be arranged in a descending order: high-rank coals > low-rank coals >

243

medium-rank coals, respectively. The adsorption isotherms of CO2 and N2 are

244

significantly different from each other for the coal samples, which means that the

245

adsorption mechanism and the pore size range measured by CO2 and N2 adsorption

246

methods are different. The adsorption capacity of CO2 in pores with size 1.4-1.6nm

247

are much higher than that of N2 in pores with size 1.7-300nm, and the SSA of pores

248

with size 1.4-1.6nm are very high. The main reasons include that the diameter of CO2

249

molecule is relatively small and the molecules can easily enter the pores with size

250

1.4-1.6nm and/or that the CO2 molecules have a strong affinity to the pore surface of

251

coal so that much more CO2 can be adsorbed (Okolo et al., 2015; Song et al., 2017).

252

3.3 Experimental results of CH4 adsorption

253 254

CH4 adsorption in coal can be described by a Langmuir adsorption model (Langmuir, 1918):

V=

255

PVL P + PL

1

256

where V is the adsorbed gas amount per unit mass of coal, cm3/g; VL is the

257

Langmuir volume of coal, cm3/g; P is the CH4 pressure, MPa; PL is the Langmuir

258

pressure, MPa. The CH4 adsorption capacities of 12 coal samples are presented in

259

Table 3. CH4 adsorption isotherms of various rank coals (Fig.4) show that: the VL

260

values of low-, medium- and high-rank coals are 11.17~17.83cm3/g, 11.81~

261

27.25cm3/g, and 28.49~52.63cm3/g, respectively. The PL values of low-, medium- and

262

high-rank coals are 3.84~9.78MPa, 1.14~3.25MPa, and 2.05~4.64MPa, respectively.

263

Generally, with the increase in coal rank, VL increases and PL first decreases and then

264

increases. The adsorption amount increases rapidly with increasing pressure at a

265

pressure of less than 3MPa and thereafter increases slowly. Within the initial stage of

266

adsorption isotherms, the coal contains abundant adsorption spaces and the pore

267

surface has a strong adsorption force to CH4. Therefore, the CH4 molecules can be

268

adsorbed easily as the adsorption pressure increases, and as the adsorption saturation

269

rises and the available adsorption space decreases in the coal matrix, and the

270

cumulative adsorption quantity increases slowly with further increasing pressure.

271

4 Discussion

272

As the degree of coalification rises, both the pore structure and the

273

methane-generated quantity vary. Pore evolution is one of the key factors that

274

influence the CH4 adsorption of coals.

275 276

4.1 Variation of pore structure in various rank coals 4.1.1 Variation characteristics of pore volume with metamorphism degree

277

Both the total pore volume and the micro-, transitional- and mesopores volumes

278

first decrease and then increase as the coal rank increases, and they decrease rapidly at

279

Ro,max<1.5% and then increase slowly at Ro,max>1.5% (Fig.5a,c,e,g). The relationship

280

between pore volume, SSA and Ro,max can be generalized as follows: y = Ae

281 282

BRo ,max

(2)

where y is the pore volume or specific surface area; A and B are the regression

283

coefficients; Ro,max is the maximum vitrinite reflectance. The detailed results are

284

shown in Figs 5 and 7.

285

A large number of thermogenic pores are produced in the process of thermal

286

metamorphism. This phenomenon is especially obvious in medium- and high-rank

287

coals (Wang et al., 2014), leading to an increase in micropore volume (Fig.5b, h). The

288

volume of pores with size 10-100nm and 100-300nm decreases significantly at

289

Ro,max<1.5% and then increases slowly (Fig.5c, e); the volume percentage of pores

290

with size 10-100nm decreases gradually (Fig.5d) and that of pores with size

291

100-300nm decreases rapidly (Fig.5f). It is possibly due to the variation of pressure

292

and coal composition during coalification. Cai et al. (2018) presented that the coal

293

matrix compressibility decreases more rapidly for low-rank coals than for the

294

medium- and high-rank coals due to mechanical compaction, dehydration and

295

degassing during coalification, with the wetting effect of water molecules on coals

296

seeming to weaken the link between coal particles. The pores >2nm are sensitive to

297

compaction induced by effective stress changes in the coal matrix, whereas the pores

298

<2nm are less effected. The content of moisture and porosity of pores >2nm in

299

low-rank coals are higher than those in the medium- and high-rank coals (Cai et al.

300

2018). Therefore, the influence of pressure on pore and bulk volumes is stronger for

301

low-rank coals than for medium- and high-rank coals. On the other hand, previous

302

research revealed that there are a large number of pores >2nm in inertinite (Giffin et

303

al., 2013). Inertinite content declines quickly in low- and medium-rank coals, which

304

can account for the rapid decrease in mesopore volume in the low- and medium-rank

305

coals.

306

The results of CO2 adsorption experiments show that the volumes of pore with

307

size 1.4-1.6nm of low-, medium- and high-rank coals are in the range of

308

0.051~0.058cm3/g, 0.022~0.042cm3/g and 0.039~0.083cm3/g, respectively. The

309

volumes of pore with size 1.4-1.6nm first decrease and then increase with an

310

inflection point near Ro,max=1.5%. While the volumes of pore with size 1.4-1.6nm of

311

low- and medium-rank coals changes slowly with increasing Ro,max and that of the

312

high-rank coals increases quickly. There are good correlations between the volumes,

313

SSAs of pore with size 1.4-1.6nm and Ro,max (Fig.5h, Fig.7h). The number of aliphatic

314

structures decreases and the number of aromatic structures increases during

315

coalification, resulting in the continuous variation of the coal macromolecules and the

316

evolution of pores <2nm (Liu et al., 2018). Based on thermal simulation and natural

317

metamorphism of coal, Liu et al. (2017b, 2018) found that the volume of pore <2nm

318

decreases to the lowest value near Ro,max=1.4% and then increases with the rise of

319

Ro,max, which is in accordance with our experimental results. The development of

320

pores <2nm is mainly controlled by the aliphatic parts of the chemical structure before

321

Ro,max<1.4%, and by aromatic rings when Ro,max=1.4%~4.0%, and the number of

322

micropore rises with the increasing aromatic carbon content.

323

The macromolecular structure model of the Xiaolongtan coal sample (Fig.6a) has

324

a large quantity of small molecular substances (-CH3, -OH or CH3-O-CH3) and they

325

are the active structures in the experiment of low-temperature oxidation (Meng et al.,

326

2017). The small molecular substances are sensitive to temperature and stress, leading

327

to a rapid change in the pore structure of low-rank coals during the evolution process.

328

For the medium-rank Malan8 coal sample, the macromolecular structure model

329

contains many cyclohexanes and benzenes (Fig.6b) (Si, 2014). The breaking off of

330

small molecular functional groups (-CH3, -OH, -COOH) and the condensation of the

331

aromatic cluster structure leads to the decrease of pores <2nm. For high-rank coals,

332

more methyl groups are lost because of the pervasive conversion from hydroaromatic

333

methyl structure to aromatic rings (Ahamed et al., 2019; Liu et al., 2019c), and the

334

proportion of C=C groups and C=O groups rises with the rise of coal rank (Sonibare

335

et al., 2010; He et al., 2019). The significant increase in aromatic cluster size results in

336

an increase in the volume of pore <2nm (Cao et al., 2013). The macromolecular

337

structure of the Chengzhuang coal (Ro.max=3.21%) comprises 2-5 polycyclic aromatic

338

hydrocarbons and the structural arrangement becomes relatively regular, with only a

339

few small molecular substances (-CH3, -OH) (Fig.6c) (Xiang et al., 2013). At the

340

same time, a large number of thermogenic pores are produced at high temperatures.

341

The above factors result in the rapid increase in micropore volume in high-rank coals.

342 343

4.1.2 Variation characteristics of pore specific surface area with metamorphism degree

344

The SSA of micro-, transitional- and mesopores, total SSA(BET) decline with

345

increasing Ro,max at Ro,max<1.5% and rise at Ro,max>1.5% (Fig.7a,c,e,g,h). It’s clear that

346

the SSA is greater for pores with size 1.4-1.6nm than for pores with a diameter of

347

1.7-300nm (Fig.7h). There is a non-linear relationship between the pore SSA and

348

Ro,max, and the regression coefficients are obtained on the basis of Eq.2 (Fig.7). The

349

SSA percentage of pores with size 1.7-10nm increases nonlinearly as Ro,max increases

350

(Fig.7b), while the SSA percentage of pores with size 10-100nm and 100-300nm

351

decrease nonlinearly with increasing Ro,max (Fig.7d, f). The SSAs of low- and

352

high-rank coals change quickly while that of the medium-rank coals changes slowly.

353

The mechanical strength and metamorphism of low-rank coals are low and sensitive

354

to pressure and temperature. Thus, the mechanical compaction and dehydration have a

355

great influence on the pore structure, resulting in SSA that varies quickly. The number

356

of small molecular functional groups (-CH3, -OH, CH3-O-CH3 and -COOH) in

357

high-rank coals decreases, and the number of aromatic cyclic structures with abundant

358

pores <2nm increases, causing a sharp increase in the SSA of BET and D-A (Wei et

359

al., 2019). The effects of compaction and thermal evolution on pore structure of

360

medium-rank coals are relatively strong, resulting in SSAs that are smaller for

361

medium-rank coals than for low- and high-rank coals.

362

4.1.3 Variation of pore diameter with metamorphism degree

363

For the LT-NA and CO2 adsorption experiments, the average pore diameter of the

364

coal matrix was calculated by BJH and D-A methods, respectively (Fig.8a, b). The

365

average pore diameter measured by LT-NA is mainly in the range of 10-60nm and the

366

variation range is relatively large. The size of micropores measured by CO2

367

adsorption is in the range of 1.49-1.59nm, which is much smaller than that measured

368

by LT-NA adsorption. The average pore diameter decreases rapidly with increasing

369

coalification degree in the lignite stage, and it changes relatively slowly with

370

increasing coal metamorphism for bituminous coals, and increases gradually for the

371

anthracite coals. The variation rule of both SSA and pore volume with metamorphism

372

degree is similar to that of average aperture with metamorphism degree.

373

The PSD of coal is closely related to the evolution of macromolecular structure

374

of coal during coalification. For the low-rank coals within the first stage of

375

coalification, the coal structure is relatively loose, and the content of primary pores is

376

high; therefore, the low-rank coals contain more meso- and transitional pores than the

377

medium- and high-rank coals. Compaction and dehydration have an effect on PSD

378

during coalification; therefore, the average pore diameter declines rapidly with

379

increasing coalification. During the transition from the first to the second stage of

380

coalification (Ro,max=0.6~1.2%) (Liu et al., 2010), the geological process promotes the

381

aromatization of coals and the coal structure becomes compact and the pore volume

382

declines as coalification continues. Particularly, the decrease of the meso- and

383

transitional pore number leads to the decrease of the average pore diameter for

384

medium-rank coals. The high-rank coals contain a large number of gas pores formed

385

by the action of high temperature. To some extent, the generation of endogenetic

386

fractures in the process of coalification also lead to an increasing trend in the average

387

pore diameter. In general, the condensed aromatic rings and the gradual regularity of

388

the molecular arrangement of coal lead to an increase in micropore quantity (Chen,

389

2001). Therefore, the content of micropores in high-rank coals further increases with

390

the average pore diameter displaying a slowly increasing trend.

391 392

4.2 Effect of pore structure on gas adsorption 4.2.1 Adsorption properties of various rank coals

393

Fig.9 shows that the VL increases linearly with the increasing coal rank for all

394

samples (R2=0.85). The adsorption capacity of CH4 is closely related to the physical

395

and chemical properties of coal, which is mainly controlled by coal composition and

396

structure. During the evolution process from low-, via medium- to high-rank coals,

397

the content of aromatic carbon in coal rises (Fig.6), and the vitrinite in coal increases

398

while inertinite and exinite decreases (Table 1), and the CH4 adsorption capacity also

399

increases. The -COOH and -OH show weak affinity to CH4 in low-rank coals, while

400

-CH3 and C=C functional groups in medium- and high-rank coals have a significant

401

influence on CH4 adsorption (Merkel et al., 2015). The influence of coal maceral of

402

various rank coals on methane adsorption is mainly manifested in the following

403

aspects: The methane adsorption capacities of various coal macerals can be arranged

404

in a descending order: vitrinite > inertinite > exinite (Beamish and Crosdale, 1998;

405

Karacan, 2003; Moore, 2012). It is verified that the vitrinite has a strong adsorption

406

force on methane molecules, and there are a large number of micropores and a big

407

SSA in vitrinite, which leads to a strong adsorption force on methane molecules.

408

Therefore, the methane adsorption capacity of coals increases with increasing vitrinite

409

content (Bustin and Clarkson, 1998). Inertinite contains a lot of irregular macropores,

410

and the distance between macropore walls can be relatively large, which results in

411

inertinite having a relatively low methane adsorption capacity and the methane

412

adsorption capacity of coal displays a decreasing trend with increasing inertinite

413

content (Chen, 2001; Lamberson and Bustin, 1993; Moore, 2012). Exinite is rare in

414

coals and it contains less pores and therefore it has no obvious influences on CH4

415

adsorption capacity of coals (Crosdale et al., 1998; Harris and Yust, 1979).

416

In terms of the results of previous studies (Merkel et al., 2015; Zhang et al.,

417

2011), the adsorption capacity of CH4 varies by coal rank. At Ro,max<1.5%, the total

418

pore volume of coal is relatively large, whereas the volume proportion of micropores

419

is small and the gas adsorption capacity of coal is relatively weak. When

420

Ro,max=1.5-3.45%, the coal is mainly composed of vitrinite and contains a large

421

quantity of micropores and is favorable for gas adsorption (Fig.10).

422

4.2.2 Effect of pore structure on CH4 adsorption

423

The relationship between the pore structure parameters and the adsorption

424

capacity of various rank coals shows that, there is a positive correlation between SSA,

425

volume of pores with 1.4-1.6nm in diameter and VL, respectively (Fig.11a, b). There is

426

also a linear correlation between the SSA percentage, pore volume percentage of

427

pores with a diameter of 1.7-100nm and VL (Fig.11c, d), while the total SSA (BET)

428

and the total pore volume (BJH) of low- and medium-rank coals are non-linearly

429

correlated with the VL (Fig.11e, f). Comparing the above relationships, it can be seen

430

that the SSA and volume of pores with size 1.4-1.6nm and 1.7-100nm in coal have an

431

important controlling effect on the CH4 adsorption capacity. The smaller the pore size

432

in the coal the larger the SSA and the higher the CH4 adsorption potential.

433

In the initial stage, the adsorption amount increases quickly with increasing

434

pressure, which is mainly controlled by the presence of relatively small pores, such as

435

micro- or transitional pores. The micropores have a greater SSA and provide a major

436

space for CH4 adsorption (Gensterblum et al., 2013; Li et al., 2018), the filling of

437

micro- and transitional pores is basically completed with the increase of gas pressure,

438

while the CH4 molecules in the meso- and macropores are adsorbed on the pore wall

439

surface under a strong equilibrium pressure (Wang et al., 2015). The influences of the

440

meso- and macropores on methane adsorption capacity of coal increase continuously

441

with increasing pressure. Fig.11e, f shows that, for some samples, the SSA and pore

442

volume are large, but the VL is relatively small. These coals are low- and medium-rank

443

coals, which indicates that for low- and medium-rank coals, the adsorption capacity is

444

not only affected by PSD, but also by other factors, such as, the coal composition

445

and/or coal rank. The molecular structure of low- and medium-rank coals contains

446

more hydrogen- and oxygen-bearing functional groups which have a relatively weak

447

adsorption force on methane molecules. The simulations on the coal molecular

448

structure and CH4 adsorption also show that the presence of moisture, results in a

449

decline in the adsorption capacity of CH4 (Zhang et al., 2014).

450

The pore volume and SSA affect the adsorption capacity, which is closely related

451

to the polycondensation of coal molecular structure during coalification. In the

452

process of coalification, the chemical reactions of dehydrogenation and deoxidation

453

continue and the number of aromatic ring structures increase continuously as well

454

(Fig.12a-c). The number of relatively large intergranular pores in coal decreases, but

455

the number of micropores increases (Fig.12d). These micropores result in a strong

456

methane adsorption capacity of coal. That is, the VL increases with the rise of the

457

Ro,max (Harris and Yust, 1979; Meng and Li, 2016).

458

5 Conclusion

459

N2, CO2 and CH4 adsorption experiments were carried out with various rank

460

coals, and the PSD characteristics and their effects on the CH4 adsorption capacity of

461

coals were investigated. The main conclusions are as follows:

462

(1) The coal matrix mainly contains cylindrical, thin bottle neck-shaped, ink

463

bottle-shaped, and parallel plate-shaped pores. The micropores in low- and

464

medium-rank coals are mainly cylindrical pores with one end closed. The meso- and

465

transitional pores are well interconnected and are primarily parallel plate pores with

466

one end closed. The high-rank coals mainly contain a large number of micropores

467

which are primarily cylindrical pores, the meso- and transitional pores in high-rank

468

coals are mainly open parallel plate pores.

469

(2) The distribution of total pore volume and SSA follow a similar exponential

470

law as the rise of coal metamorphic degree. The average pore diameter of coal matrix

471

first decreases and then increases with the rise of coal rank, with an inflection point

472

nearly at Ro,max=1.5%. The volume and SSA of micro-, transitional- and mesopores

473

have the similar variation law (asymmetric ‘‘U” shape) with Ro,max. Both SSA

474

percentage and pore volume percentage of micropores increase continuously, while

475

those of the transitional- and mesopores vary inversely with increasing Ro, max.

476

(3) The adsorption capacity of CH4 increases as the coal rank increases. There

477

are linear relationships between VL and Ro,max less than 3.5%, the SSA and the volume

478

of pores with a diameter of 1.4-1.6nm, SSA percentage and pore volume percentage

479

of pores of 1.7-100nm in diameter.

480

(4) Both the coal rank and PSD have an important influence on the CH4

481

adsorption. At pressures smaller than 3 MPa, the CH4 adsorption capacity is mainly

482

controlled by micropores and transitional pores. At pressure greater than 3 MPa, the

483

mesopores have an important influence on the adsorption capacity.

484

Acknowledgments

485

This work was financially supported by the Shanxi Province Science and

486

Technology Major Project (Grants 20191102001 and 20181101013), the National

487

Science and Technology Major Project of the Ministry of Science and Technology of

488

China

489

2016ZX05065). The authors thank the reviewers and the editor for their constructive

490

comments.

491

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679

Rank Coals: Implications for CO2 Sequestration in Coal Seams. Energy Fuels,

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29(6), 3785-3795.

681

Wei, Q., Li, X., Zhang, J., Hu, B., Zhu, W., Liang, W., Sun, K., 2019. Full-size pore

682

structure characterization of deep-buried coals and its impact on methane

683

adsorption capacity: A case study of the Shihezi Formation coals from the Panji

684

Deep Area in Huainan Coalfield, Southern North China. J. Petrol. Sci. Eng. 173,

685

975-989.

686

Xiang, J., Zeng, F., Bin, L., Zhang, L., Li, M., Liang H., 2013. Construction of

687

macromolecular structural model of anthracite from Chengzhuang coal mine and

688

its molecular simulation. J. Fuel Chem. Techn. 41(4), 391-400.

689

Xu, X., Meng, Z., Wang, Y., 2019. Experimental comparisons of multiscale pore

690

structures between primary and disturbed coals and their effects on adsorption

691

and seepage of coalbed methane. J. Petrol. Sci. Eng. 174, 704-715.

692

Yao, Y., Liu, D., Che, Y., Tang, D., Tang, S., Huang, W., 2010. Petrophysical

693

characterization of coals by low-field nuclear magnetic resonance (NMR). Fuel

694

89(7), 1371-1380.

695

Zhang, D., Cui, Y., Liu, B., Li, S., Song, W., Lin, W., 2011. Supercritical Pure

696

Methane and CO2 Adsorption on Various Rank Coals of China: Experiments and

697

Modeling. Energy Fuels 25, 1891-1899.

698

Zhang, M., Fu, X., Wang, H., 2018. Analysis of physical properties and influencing

699

factors of middle-rank coal reservoirs in China. J. Nat. Gas Sci. Eng. 50,

700

351-363.

701

Zhang, J., Clennell, M.B., Dewhurst, D.N., Liu, K., 2014. Combined Monte Carlo and

702

molecular dynamics simulation of methane adsorption on dry and moist coal.

703

Fuel 122, 186-197.

704

Zhao, J., Tang, D., Qin, Y., Xu, H., 2019. Fractal characterization of pore structure for

705

coal macrolithotypes in the Hancheng area, southeastern Ordos Basin, China. J.

706

Petrol. Sci. Eng. 178, 666–677.

707

Zhao, Y., Liu, S., Elsworth, D., Jiang, Y., Jie, Z., 2014. Pore Structure

708

Characterization of Coal by Synchrotron Small-Angle X-ray Scattering and

709

Transmission Electron Microscopy. Energy Fuels 28(6), 3704–3711.

710

711 712 713

Fig.1 Comparison of pore size measured by different methods (Modified from Zhao et al., 2014).

adsorption desorption

1.5 1 0.5 0

0.2 0.4 0.6 0.8 Relative Pressure (P/P 0) (a)BZDB (Ro,max=0.43%)

adsorption desorption

8 6 4 2

1

adsorption desorption

1.5 1.0 0.5 0.0

1.2

0.2 0.4 0.6 0.8 Relative Pressure (P/P 0) (c)BD2 (Ro,max=0.77%)

0.6 0.4 0.2 0.0 0.2 0.4 0.6 0.8 Relative Pressure (P/P 0) (e)YC3 (Ro,max=2.57%)

1

0.2 0.4 0.6 0.8 Relative Pressure (P/P 0) (b)SCH2 (Ro,max=0.23%)

1

adsorption desorption

0.4 0.3 0.2 0.1 0.0 0

0.8

0

0.5

1

adsorption desorption

1.0

0

Adsorption volume (cm3/g)

Adsorption volume (cm3/g)

2.0

0

Adsorption volume (cm3/g)

10

0

0

715

Adsorption volume (cm3/g)

2

Adsorption volume (cm3/g)

Adsorption volume (cm3/g)

714

1.4

0.2 0.4 0.6 0.8 Relative Pressure (P/P 0) (d)DQ8-1 (Ro,max=1.8%)

1

adsorption desorption

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

0.2 0.4 0.6 0.8 Relative Pressure (P/P0) (f)SH3-1 (Ro,max=2.91%)

Fig.2 Low-temperature nitrogen adsorption-desorption curves.

1

716 Ro,max=0.47 Ro,max=0.77 Ro,max=1.56 Ro,max=2.57

Adsorption volume (cm3/g)

35 30

Ro,max=0.57 Ro,max=1.10 Ro,max=1.80 Ro,max=2.67

Ro,max=0.69 Ro,max=1.47 Ro,max=2.16 Ro,max=2.91

25 20 15 10 5 0 0

717 718

0.01 0.02 Relative pressure (p/p0)

0.03

Fig.3 CO2 adsorption isotherms in coal.

0.04

35

Ro,max=0.47% Ro,max=0.77% Ro,max=0.57% Ro,max=0.69% Ro,max=1.10% Ro,max=1.47% Ro,max=1.56% Ro,max=1.80% Ro,max=2.16% Ro,max=2.57% Ro,max=2.67% Ro,max=2.91%

Adsorption volume(cm3 /g)

30 25 20 15 10 5 0 0

719 720 721

2

4

6 Pressure(MPa)

8

Fig.4 Adsorption capacity of CH4 in various rank coals.

10

722 Ro,max<1.5%

4

Ro,max>1.5%

3 2 y = 0.7757e-1.687x R² = 0.12

1

y = 0.0205e0.9715x R² = 0.40

0 0

1

2

3

60

Percentage of micropores (1.7-10nm) volume (%)

Micropores (1.7-10nm) volume (×10-3 cm3 /g)

5

50 40 30 20 10 0

4

0

1

Ro,max(%)

Ro,max>1.5%

6 y = 0.0393e1.0051x R² = 0.75

4

y = 7.0883e-2.379x R² = 0.55

2 0 0

1

2 Ro,max(%)

3

Percentage of transitional (10-100nm) pores volume (%)

Transitional pores (10100nm) volume (×10-3 cm3 /g)

Ro,max<1.5%

40 30 20 10 0

4

0

1

Percentage of mesopores (100-300n m) volume (%)

Mesopores (100-300nm) volume (×10-3 cm3 /g)

Ro,max<1.5% Ro,max>1.5%

5 4 y = 5.3017e-2.392x R² = 0.59

2

y = 0.0506e0.6691x R² = 0.46

1 0 1

2

3

50 40 30 20 10 0

4

0

Micropores (1.4-1.6nm) volume (×10-2 cm3 /g)

Total pore volume (BJH) (×10-3 cm3 /g)

Ro,max<1.5% Ro,max>1.5%

15 14.52e-2.388x

y= R² = 0.55

y = 0.092e0.9886x R² = 0.84

5 0 2

Ro,max(%) (g)

1

2 Ro,max(%)

3

4

(f)

(e)

20

1

2 Ro,max(%)

60

Ro,max(%)

723 724 725 726

4

50

(d)

6

0

3

60

(c)

7

10

4

70

8

0

3

(b)

(a)

10

3

2 Ro,max(%)

3

4

10

Ro,max<1.5%

8

Ro,max>1.5% y = 7.4692e-0.95x R² = 0.75

6 4

y = 0.9511e0.7409x R² = 0.84

2 0 0

1

2

3

Ro,max(%) (h)

Fig.5 The variation of volume of (a) micropores (1.7-10nm), (b) percentage of micropores (1.7-10nm), (c) transitional pores, (d) percentage of transitional pores, (e) mesopores (100-300nm), (f) percentage of mesopores (100-300nm), (g) total pores (1.7-300nm) tested by low-temperature nitrogen adsorption, (h) micropores (1.4-1.6nm) tested by CO2 versus rank for all coals

727 728

investigated.

729 730

(a)

731 732

(b)

733 734 735 736

(c) Fig.6 Macromolecular structure of various rank coal, (a)xiaolongtan (adopted from Meng et al., 2017); (b)Malan8 (adopted from Si, 2014); (c)Chengzhuang (adopted from Xiang et al., 2013).

737 100

Ro,max<1.5% Ro,max>1.5%

y = 0.0222e0.9974x R² = 0.43 y = 0.5266e-1.338x R² = 0.08

0

1

2

3

4

Percentage of micropores (1.7-10nm) specific surface area (%)

Micropores (1.7-10nm) specific surface area (m2 /g)

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

80 60 40 20 0 0

Ro,max(%)

1

Ro,max<1.5%

1.2

Ro,max>1.5%

1.0 0.8 y = 0.9118e-2.359x R² = 0.48 y = 0.0049e1.0567x R² = 0.82

0.6 0.4 0.2 0.0 0

1 2 Ro,max(%) (c)

3

0

y = 0.1297e-2.334x R² = 0.53

0.05

y = 0.0015e0.6014x R² = 0.41

0.00 1

2 Ro,max(%)

3

Percentage of mesopores (100-300n m) specific surface area (%)

Mesopores (100-300nm) specific surface area (m2 /g)

0.10

0

10 5 0 0

4

7 6

y = 0.0047e2.0658x R² = 0.94

5 4

y = 2.0668e-1.685x R² = 0.24

1

250

2 Ro,max(%) (g)

2 Ro,max(%)

3

4

Ro,max<1.5% Ro,max>1.5% y = 25.356e0.7375x R² = 0.85

200 150 100 50

y = 187.32e-0.908x R² = 0.76

0

0 1

1

(f)

Ro,max<1.5% Ro,max>1.5%

Micropores (1.4-1.6nm) specific surface area (m2 /g)

Total pore specific surface area (BET) (m2 /g)

4

15

(e)

738 739 740 741 742

2 3 Ro,max(%)

(d)

Ro,max>1.5%

0.15

0

1

20 Ro,max<1.5%

2

4

80 70 60 50 40 30 20 10 0

4

0.20

3

3

(b)

1.4

Percentage of transitional (10-100nm) pores specific surface area (%)

Transitional pores (10100nm) specific surface area (m2 /g)

(a)

2 Ro,max(%)

3

4

0

1

2 Ro,max(%)

3

(h)

Fig.7 The variation of specific surface area of (a) micropores (1.7-10nm), (b) percentage of micropores (1.7-10nm), (c) transitional pores, (d) percentage of transitional pores, (e) mesopores (100-300nm), (f) percentage of mesopores (100-300nm), (g) total pores tested by low-temperature nitrogen adsorption (BET), (h) micropores (1.4-1.6nm) tested by CO2 adsorption (D-A) versus rank for all coals investigated.

70

Average micropores (1.41.6nm) diameter (nm)

Average pore diameter tested by N2 adsorption (nm)

743 60 y = 5.52x 2 - 25.675x + 41.152 R² = 0.36

50 40 30 20 10 0

1.60 1.58

y = 0.0242x2 - 0.0989x + 1.6047 R² = 0.43

1.56 1.54 1.52 1.50 1.48

0

1

2

3

4

0

1

Ro,max(%)

(a)

2 Ro,max(%)

3

(b)

744

Fig.8 The variation of (a) average pore diameter (BJH) tested by low-temperature nitrogen

745

adsorption, (b) micropores (1.4-1.6nm) (D-A) tested by CO2 adsorption versus rank for all coals

746

investigated.

747

Langmuir volume (cm3 /g)

50 y = 13.04 x + 2.15 R² = 0.85

40 30 20 10 0 0

748 749

1

2 Ro,max(%)

3

Fig.9 The Langmuir volume of various rank coals.

4

35

16

30

14 12

25

10

20

8

15

6

Total pore volume

10

4

5

2

0

0 0

750 751

×10-3 cm3 /g

18

Langmuir volume

Total pore volume

Langmuir volume(cm3 /g)

40

0.5

1

1.5

2

2.5

3

3.5

Ro,max(%)

Fig.10 The variation of Langmuir volume and total pore volume versus Ro,max.

60 y = 0.21x - 0.08 R² = 0.67

50 40 30 20 10 0 0

Langmuir volume (cm3 /g)

Langmuir volume (cm3 /g)

752

50 100 150 200 250 Micropores (1.4-1.6nm) specific surface area (D-A) (m2 /g)

60 y = 535.24x + 0.68 R² = 0.64

50 40 30 20 10

0 0.00 0.02 0.04 0.06 0.08 0.10 Micropores (1.4-1.6nm) volume (DA) (cm3 /g) (b)

50

y = 3.43x - 304.83 R² = 0.50

40 30 20 10 0 90

92 94 96 98 100 Percentage of specific surface area (BET) with 1.7-100n m in diameter (%)

Langmuir volume (cm3 /g)

60

60

cm3 /g)

Langmuir volume (cm3 /g)

(a)

60

y = 0.97x - 42.12 R² = 0.51

50 40 30 20 10 0 50

60

Langmuir volume

40

Low-rank coals Medium-rank coals High-rank coals Fitting of High-rank coals

50

30 20

(d)

y = 7.70x + 32.0 R² = 0.41

10 0 0 1 2 3 4 5 6 2 Specific surface area (BET) (m /g) (e)

753 754 755 756 757

Langmuir volume

(cm3 /g)

(c)

60 70 80 90 Percentage of volume (BJH) with 1.7-100nm in diameter(%)

50 40 30 20

Low-rank coals Medium-rank coals High-rank coals Fitting of High-rank coals y = 15202x + 24.38 R² = 0.67

10 0 0.000

0.005 0.010 0.015 0.020 Pore volume (BJH) (cm3 /g) (f)

Fig.11 The variation of (a) micropore (1.4-1.6nm) specific surface area (D-A), (b) micropore (1.4-1.6nm) volume (D-A), (c) percentage of specific surface area (BET) with 1.7-100nm in diameter, (d) percentage of volume (BJH) with 1.7-100nm in diameter, (e) total specific surface area (BET), (f) total pore volume (BJH) versus Langmuir volume for all coals investigated.

758 759

Fig.12 Schematic diagram of the evolution of pore structure in coal.

760

Table 1 Proximate analysis and petrographical data of coals. Proximate analysis

761 762 763

No.

Sample information

Ro,max(%)

1

Ha’erwusu No.6(HEWS6)

2 3

Coal composition

Mad (%)

Aad (%)

Vad (%)

V(%)

I(%)

E(%)

M(%)

0.47

3.71

8.03

33.24

57.6

22

10.8

9.5

Shendong 5-2(SD5-2) Shichaohe No.1(SCH1)

0.57 0.29

4.52 26.41

3.49 5.01

29.38 33.91

76.26 81

20.43 17

1.167 0

2.15 2

4

Shichaohe No.2(SCH2)

0.23

25.02

20.46

30.52

80.2

15

0.2

4.6

5 6

Buzhaodongbang(BZDB) Xiaolongtan No.1(XLT1)

0.43 0.23

29.3 34.45

5.19 7.14

35.58 31.42

80.8 83

17.2 12

0.4 2

1.6 3

7 8 9 10 11 12 13

Puyang No.1(PY1) Shanxincun No.1(SXC1) Baode No.2(BD2) Baode No.8(BD8) Malan No.8(ML8) Malan No.2(ML2) Dongqu No.8-1(DQ8-1)

0.32 0.36 0.77 0.69 1.47 1.1 1.80

20.24 47.95 3.65 2.47 0.69 0.86 0.82

9.61 15.83 8.40 5.49 10.71 7.71 8.03

34.19 24.05 32.77 35.81 17.56 24.21 14.18

84.2 80.6 83.27 54.1 74.39 91.04 88.52

12.4 16 6.57 16.5 22.68 3.87 6.42

0.4 0.1 3.59 6.7 0 0 0

3 3.3 6.58 22.60 2.93 5.09 5.06

14 15

Dongqu No.2(DQ2) Dongqu No.8-2(DQ8-2)

1.56

0.80 0.89

9.12 2.97

15.66 13.67

87.54

4.55

0

7.91

16 17

Zhaozhuang No.3(ZZ3) Shihe No.3(SH3-1)

2.16 2.91

0.89 1.92

10.63 8.62

10.68 6.25

87.92 94.3

5.941 0.475

0 0

6.14 5.23

18 19

Changping No.3(CP3) Yuecheng No.3(YC3)

2.67 2.57

1.12 1.88

8.09 10.96

8.61 6.41

94.93 95.02

0.48 0.657

0 0

4.59 4.22

20

Shihe No.3-2(SH3-2)

3.45

0.71

13.39

12.01

Note: Mad, Aad and Vad are the moisture, ash and volatile matter content on air-dried basis, respectively; V, I, E, M represent the content of vitrinite, inertinite, exinite and mineral, respectively.

764

Table 2 The pore parameters tested by low-temperature nitrogen adsorption for coal samples. BET (m2/g)

Avd(nm)

Vt(cm3/g)

HEWS6 SD5-2 SCH1

2.2101 4.9558 0.3711

31.847 15.756 45.484

SCH2

5.8303

BZDB XLT1

Samples

765 766 767 768 769

Percentage of pore volume (%)

Percentage of SSA (%)

V1

V2

V3

S1

S2

S3

0.015629 0.016776 0.002656

7.07 21.96 2.30

50.96 43.40 52.45

41.97 34.64 45.25

33.62 72.27 13.32

56.75 23.60 74.93

9.63 4.13 11.75

11.180

0.014169

28.35

52.34

19.31

75.62

23.07

1.32

0.2670

61.086

0.002722

1.10

42.30

56.61

12.60

68.01

19.39

0.3042

52.200

0.002993

2.11

43.86

54.03

24.61

59.89

15.51

PY1

2.5319

15.097

0.007381

21.66

53.13

25.20

66.73

31.02

2.25

SXC1

1.0653

30.217

0.008007

6.96

41.81

51.24

56.61

32.45

10.94

BD2

0.9263

15.203

0.002826

21.00

58.02

20.98

63.09

34.90

2.01

BD8 ML8

0.4957 0.5169

24.337 14.910

0.002120 0.000483

10.37 16.09

58.83 43.80

30.79 40.10

42.73 75.69

52.41 20.60

4.86 3.70

ML2 DQ8-1

0.2279 0.0986

29.663 16.426

0.000439 0.000664

5.54 14.53

52.27 50.86

42.19 34.61

45.26 68.08

46.50 28.19

8.24 3.73

DQ2

0.2003

19.493

0.000495

11.47

48.03

40.50

65.44

29.49

5.07

DQ8-2 ZZ3 SH3-1 CP3

0.1509 0.2133 2.2343 1.2956

36.676 9.650 5.895 10.924

0.002786 0.000537 0.001676 0.000830

4.56 29.87 55.91 27.41

55.49 41.37 31.61 49.07

39.95 28.76 12.48 23.52

32.89 84.56 91.81 78.27

59.21 13.61 7.74 20.17

7.89 1.83 0.45 1.56

YC3 SH3-2

1.2298 5.3582

6.500 25.231

0.001482 0.003414

48.27 5.57

34.69 62.71

17.04 31.72

90.25 45.47

9.10 49.91

0.65 4.62

Avd-average pore diameter (BJH) (nm); Vt-total pore (1.7-300nm) volume (cm3/g); V1-micropores (1.7-10nm) volume; V2- transitional pores volume; V3-mesopores (100-300nm) volume; S1-micropores (1.7-10nm) specific surface area; S2-transitional pores specific surface area; S3- mesopores (100-300nm) specific surface area.

770

Table 3 Calculation parameters of CO2 and CH4 adsorption. Samples

771 772

CO2 adsorption

CH4 adsorption

SD-A

VD-A

Avds

VL

PL

HEWS6

129.03

0.051

1.59

11.17

3.84

SD5-2

146.35

0.058

1.58

17.83

9.78

BD2

86.83

0.033

1.51

11.81

3.25

BD8

78.66

0.030

1.51

12.5

3.21

ML8

57.33

0.022

1.51

18.55

1.98

ML2

59.09

0.023

1.54

17.42

2.14

DQ8-1

109.44

0.042

1.53

27.25

2.03

DQ2

74.82

0.028

1.50

18.45

1.14

ZZ3

104.36

0.039

1.49

28.49

2.05

SH3-1

199.44

0.074

1.49

44.05

4.32

CP3

169.96

0.065

1.53

41.15

2.96

YC3

217.91

0.083

1.53

52.63

4.64

SD-A-amicropores (1.4-1.6nm) specific surface area, m2/g; VD-A-micropores (1.4-1.6nm) volume, cm³/g; Avds-Average diameter of micropores (1.4-1.6nm), nm; VL- Langmuir volume, cm3/g; PL-Langmuir pressure, MPa.

Highlights 1.Variation of pore parameters of coal matrix with coal rank are analyzed quantitatively. 2.Volume and SSA of pores initially decrease and then increase with increasing Ro,max. 3.Influence of different-type pores on gas adsorption capacity is analyzed. 4.Methane adsorption is controlled by micro-, transitional- and mesopores successively.

The author contributions are as follows: Jiwei Yan: Collection and assembly of data, writing the article, N2, CO2 adsorption experiments. Zhaoping Meng: Research concept and design, data analysis and interpretation. Kun Zhang: Critical revision of the article. Huifang Yao: Collection samples. Haijin Hao: CH4 adsorption experiment.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

No conflict of interest.