Stress response of noncovalent bonds in molecular networks of tectonically deformed coals

Fuel 255 (2019) 115785

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Stress response of noncovalent bonds in molecular networks of tectonically deformed coals Liu Hewua,b, Jiang Boa,b, a b

T

⁎

Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process, Ministry of Education China, University of Mining and Technology, Xuzhou 221008, China School of Resources and Earth Science, China University of Mining and Technology, Xuzhou 221116, China

ARTICLE INFO

ABSTRACT

Keywords: Tectonically deformed coal Molecular structures Tectonic stress Hydrogen bonds πeπ Bonds

Investigations on macromolecular evolution of tectonically deformed coals are of great theoretical and practical significances for the coal safe production and coalbed methane exploitation. Alteration of covalent bonds in rigid coal carbon skeletons was studied extensively, while insufficient attention was paid to the variation of noncovalent bonds in macromolecular networks. In present study, some insights about stress response of noncovalent bonds are given by investigating a primary coal and six typical tectonically deformed coals collected around a fault structure. Self-associated n-mers (n > 3), OH-ether, cyclic OH, COOH dimers, OH-SH and OH-N are all disrupted by tectonic stress, which is partially resulted from dissociation of functional groups. Conversely, amount of OH-π is in an increasing trend, indicating that there is a transformation between OH-π and other hydrogen bonds. In general, the total content of hydrogen bonds generally decreases from primary coal to granulitic coal, and then slightly increases from scaly coal to wrinkle coal, which is ascribed to the increase of OH-π transformed from other hydrogen bonds. Furthermore, −ΔHtotal and −ΔHav both decrease with the increasing deformation intensity even in scaly and wrinkle coals with slightly increasing total content of hydrogen bonds. While, the amount of πeπ bonds increases with enhancement of coal deformation intensity (especially in brittle-ductile and ductile deformed coals), indicating that free molecules liberated by disruption of hydrogen bonds and πeπ bonds are rearranged into a more stable, stacked and ordered configuration accompanied by the formation of new noncovalent bonds.

1. Introduction Tectonically deformed coals (TDCs) have been of great concern for its importance in coal and methane exploration and exploitation [1–3]. It was defined as a kind of coal with regularly changed physical, chemical and optical characteristics [3–5]. Physically, coalbed methane (CBM) reservoir with developed brittle fractures was more favorable for the exploration and exploitation of methane [1,4]. While, areas with developed strong deformed coals (especially the ductile deformed coals) were of higher gas outburst propensity [2]. Chemically, researchers implied that organic maturation of coal was advanced by tectonic deformation [6,7]. Without exception, coal deformed by stress showed the same evolutionary characteristics in experimental studies [8–10]. More specifically, properties of coal macromolecular structures could be significantly changed by tectonic stress [11,12]. Evolution of TDC macromolecules was actually a process of early-metamorphism including stress polycondensation and stress degradation [13]. The stress degradation of aliphatic structures in coal molecules was

⁎

supposed to be a possible origin of the excess CBM of coal and gas outburst [2,10,14,15]. Meanwhile, calculation of molecular mechanics and molecular dynamics showed that adsorption of CBM on coal molecules depended on the distribution of specific structures including aromatic clusters and functional groups. Therefore, adsorption capacity of TDCs was significantly changed with the molecular structure transformation [16]. Additionally, nano-scale pores with aromatic layers surrounding the pore wall was also altered through the evolution of TDC macromolecules (especially the aromatic structures) [4,12], which in turn affected the storage space of CBM [17–19]. It could be concluded that stress-induced alteration of coal macromolecules directly led to the variation of gas content, adsorption capacity and pore structures. Thus, the in-depth understanding of TDC macromolecular evolution provides the coal safe production and CBM exploitation with useful information. Knowing evolution modes and mechanisms of TDC macromolecules is supposed to be based on its specific structures. Conceptual two-phase model of coal molecular structures proposed by Given [20] was

Corresponding author at: School of Resources and Earth Science, China University of Mining and Technology, Xuzhou 221116, China. E-mail address: [email protected] (B. Jiang).

https://doi.org/10.1016/j.fuel.2019.115785 Received 20 March 2019; Received in revised form 4 July 2019; Accepted 7 July 2019 Available online 12 July 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.

Fuel 255 (2019) 115785

H. Liu and B. Jiang

composed of rigid, cross-linking and three-dimension macromolecules (‘host’ molecules) and active phased small molecules (mobile phase or ‘guest’ molecules). While, two-phase model was limited in interpreting molecular substances extracted from coal without breaking covalent bonds [21]. Hence, a composite model of coal with the addition of noncovalent bonds was furtherly presented by researchers [22,23]. In their opinions, noncovalent bonds (four times of covalent bonds) were important in linking small molecules to rigid networks and even in linking rigid phases [24–27]. Composite model was effectively applied to resolve many scientific issues about coal, viz. pyrolysis, oxidation, liquefaction and extraction etc. [28,29]. However, existing studies of molecular evolution of TDCs mostly emphasized the alteration of covalent bonds. Han et al. [30,31] deemed that CeC bonds in aromatic structures could be directly cleaved by brittle deformation of TDCs, and Stone-Wales defects could be generated by rotation of covalent CeC bonds in ductile deformed coals. Liu et al. [10] and Xu et al. [15] proved that elongation and cleavage of CeC bonds of coal molecules led to dissociation of carbonyl and release of CO gases through experimental studies. Li et al. [32] proposed that secondary structural defects of coal molecules were related to alteration of covalent bonds induced by diverse types of tectonic stress. So far there has been no attempt made to define explicitly the role of noncovalent bonds played during evolution of TDC chemical structures. Since covalent bonds (> 50 kcal/mol in strength) were cleaved and rotated by tectonic stress, noncovalent bonds with lower bond strength (∼5 kcal/mol) could be influenced as well [33]. Therefore, we believe that it is extremely necessary to pay much more attention to alteration of noncovalent bonds in TDC molecules, which could help to understand the evolution mechanisms of TDC molecules. Noncovalent bonds in coal were comprised of ionic linkages, hydrogen bonds and π-π bonds [27,34]. Among them, πeπ bonds were described as a kind of interaction between polarized aromatic structures [25,35]. πeπ bonds were sometimes considered containing vander Waal forces and ionic linkages [36]. The amount of πeπ bonds in coal was determined by cluster size and orientations of aromatic layers [37]. Face-to-face aromatic layers were thought to be more beneficial to form π-π bonds. Hydrogen bonds were linkages between molecular segments including small molecules and aromatic structures [21]. Hydrogen bonds with weaker bond strength certainly could be broken easily at 150–200 °C [38,39]. In low rank coals, enriched functional groups increased the importance of hydrogen bonds in cross-link networks. While, π-π bonds gradually took over the control with the increase of coal ranks [25]. Hydrogen bonds and π-π bonds all exist in samples (with Cdaf from 81.59% to 84.91%, Table 2) collected in this study according to the research results proposed by Li et al. [27]. Coal molecules could be liberated by cleaving noncovalent bonds, which allowed them to move locally at low temperatures [40]. Oriented coal molecular configuration with lower free energy was formed through motion of liberated coal molecules and reversible reaction of noncovalent bonds [25,33]. In this work, seven coal samples with various typical deformation characteristics were collected from the same sequence near a fault structure. By quantitatively studying variation characteristics of hydrogen bonds and πeπ bonds, evolution of TDC molecules was estimated and explored from another perspective.

syncline situate in underlying and overlying systems of Xisipo thrust fault respectively (Fig. 1b). Southwestern movement of overlying system of Xisipo thrust fault leads to a strong tectonic reformation of East Suzhou syncline. Compared with underlying system, more developed TDCs occur in overlying system of Xisipo thrust fault. Therefore, Zhuxianzhuang colliery lied in East Suzhou syncline was selected as sampling area. Reverse faults are more developed than normal faults in Zhuxianzhuang colliery (Fig. 1c). East Suzhou syncline, secondary folds Bojia anticline and Gaojia anticline control the macroscopic distribution of TDCs in Zhuxianzhuang colliery [1]. While, the distribution of TDCs in local area is mainly controlled by fault structures (especially reverse faults). Herein, a primary coal sample and six typical TDC samples were collected around a reverse fault structure according to the GB482-1995 standard and GB/T19222-2003 standard (Fig. 1d). Coal seam 8 as the sampling sequence belongs to lower Shihezi Formation of middle Permian (Fig. 2). Lower Shihezi Formation is an important coal-bearing stratum of Zhuxianzhuang colliery. As fluvialdominated delta facies, lower Shihezi Formation is mainly composed of sandstone, mudstone and coal. Selected coal samples with Ro, max in the range of 0.876%–1.15% (Song et al. [41]) belong to bituminous coal. 2.2. Samples Seven selected samples with volatile matter yield in the range of 30.22–36.48% and Cdaf lower than 86% belong to high volatile bituminous coals (Table 1). TDCs were classified with a structural-genetic classification scheme based on the concept of tectonite [2]. Three subdivided sequences including brittle, brittle-ductile and ductile deformation types reflect corresponding stress–strain environments [32,41] (Table 2). Brittle deformed coals including cataclastic, mortar and granulitic coals are commonly developed in a deformation environment with higher strain rates, which is proved by the uniaxial compression experiments [10,15]. However, the mechanic mechanism of brittle deformed coal formed is diversified, it could be formed under the action of compressive, tensile and even shearing stress. With increase of deformation intensity (from cataclastic coal to granulitic coal), particle sizes of brittle deformed coals decrease at both macro and micro scales, which illustrates the catholicity of strong mechanical crushing and grinding of tectonic stress. In this way, brittle deformation of coal transformed into frictional heat and kinetic energy [2,3,32]. As transition type, brittle-ductile deformation sequence including schistose and scaly coals usually indicates a shear stress environment. A set of parallel fractures led by shear stress dislocate coal primary structures. As for ductile deformation sequence, wrinkle coal with plastic flow is formed in a strong shear stress environment with low strain rates. During the long-term deformation process, the ductile deformation mainly converts into strain energy with dislocation and creep of inner molecular nucleus [15]. 3. Experiments Observation of micro deformation characteristics of coal samples followed the steps proposed by Drelich et al. [42,43]. Firstly, samples were consolidated by injecting epoxy. Secondly, treated samples were polished by using sandpapers and Al2O3 colloidal solution. Thirdly, morphologies of polished samples were observed utilizing Polarizing Microscope Eclipse LV100N Pol (Nikon) in the Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education. The polarizing microscope is equipped with illumination lamphouse (LV-LH50PC). Ultimate analysis was performed on the Vario Macro Cube element analyzer with an analysis precision of 0.05–0.15%. Detection range of the analyzer is 0.03–100%. Proximate analysis of coal samples followed

2. Geological settings and samples 2.1. Geological settings Suxian coal mine located in the southeast of Suzhou city, Anhui province, China is one of the most important coal production bases (Fig. 1a). The coal mine adjacent to Tanlu fault is on the southeastern margin of North China Craton. South Suzhou syncline and East Suzhou

2

Fuel 255 (2019) 115785

H. Liu and B. Jiang

K-E

Suzhou

24

Ea Su

o si p

st

Xi

zh

nc

F2

l in e

ic lin e

F 22

u S y nc

l in e

F

0

F 5-

-1

2

10

F

1 km

F

5Km

(b )

S am pl in g si tes

(d)

Fa u l

Tanl u

Beijing

50°

t

(a)

Taib ei

0

2m

Guan gzhou

Sou

th

S

Ch i na

E

Se a

Sha nghai

N W

F

5

fa ul t

P2s s

Suzhou

D F2

23

13

P3sq

uz ho E as t S 11

isi po

6

F

X

Syncline

Sout h Suzhou

a Ant

F1 0

P1

Sy

So u A nt t h S uz i cli n ho u e

G ao j i

ou

lt f au

P1s

0

F2 1

Bo j i a A nt icl i ne

C-O

F3

P3sq

(c)

F

140< 60°

Legend s Normal Fault

Thrust Fault

Fault Zone

Anticline

Sync line

Zhuxianzhuang colliery

Outcrop of coal seam 8

Uncorformity boundary

Suxian coal mine

Zhuxianzhuang Colliery

Sampling location

Fig. 1. Distribution of sampling sites, a. Location of Suxian coal mine, b. Structure outline of Suxian coal mine and location of Zhuxianzhaung colliery, c. Structure outline of Zhuxianzhaung colliery and location of specific sampling site, d. Sampling transact of TDCs (modified from Jiang et al. [1]).

the GB/T 212-2008 standard. All detection results for TDC samples were listed in Table 2. Fourier Transform infrared spectroscopy (FT IR) was a promising method to characterize hydrogen bonds in coal [44]. Samples were firstly ground with 80 mg of KBr for 20 min in an agate mortar. Then ground mixture of KBr and coal samples was molded into a disc for final FT-IR spectra detections. Adsorption of water molecules is in the same region of FT IR spectrum as that of coal hydrogen bonds. To remove the interference of water molecules, molded discs were vacuum-dried at the temperature of 105 °C [38,45]. During the detection process, dried disks were completely isolated from water molecules in air by using Vertex 80 FT IR spectrometer equipped with vacuum optical platform. A mirror used as background was believed not to adsorb water [46]. Detections were performed in a resolution of 4 cm−1 and sample scans of 32. Pure ground KBr after being dried was used to obtain a reference spectrum. X-Ray Photoelectron spectroscopy (XPS) contained important information about π-π bonds in coal [47,48]. Coal samples (under 200 mesh) were examined on an Escalab 250Xi (Thermo Fisher) with a monochromatic Al target Kα radiation. The spot size was 900 μm, the number of scans was 30, the energy step size was 0.05 eV and the number of energy steps was 361. Raman spectroscopy (Raman) was constantly applied to characterize molecular structures of TDCs [31,49]. Powder coal samples under 200 mesh were detected by using Raman spectroscopy Senterra (Bruker) in the Advanced analysis and computation center of China university of mining and technology (λ0 = 532 nm). The laser power of the incident beam on the experimental sample surfaces was maintained at 5 mW and the spectral resolution was in the range of 9–18 cm−1. X-ray diffraction (XRD) was a powerful analytical technique to characterize crystallinity of coal macromolecules [50,51]. To remove the interference of minerals, powder TDC samples was primarily digested by hydrochloric and hydrofluoric acids repeatedly. The specific

Mudstone

>690m

Shangshihezi

1coal

256 m

3coal 4coal 5coal 6coal 7coal 8coal 9coal 10coal

156 m

Xiashihez i formation

2coal

Shanxi formation

Lower Permian

Middle

Permian

series

Sequence Thickness Lit hology Coal

Silty Gritstone mudstone

Medium sandstone

Siltstone

Sampling sequence

Fig. 2. Detailed stratigraphic column of coal-bearing Zhuanxianzhuang colliery (modified from Jiang et al. [1]).

strata

in

3

Fuel 255 (2019) 115785

H. Liu and B. Jiang

Table 1 Ultimate analysis and proximate analysis results of TDC samples. Sample number

1 2 3 4 5 6 7

Deformation types

Primary coal Cataclasitc coal Schistose coal Mortar coal Granulitic coal Scaly coal Wrinkle coal

Proximate analysis/%

Ultimate analysis/% (wt)

Mad

Ad

Vdaf

FCd

St.d

Odaf

Cdaf

Hdaf

Ndaf

1.70 1.94 1.95 0.88 0.96 1.99 1.92

6.65 5.66 5.12 6.08 9.77 11.17 13.44

35.55 36.04 30.22 36.48 34.80 33.28 33.52

60.16 60.34 66.21 59.65 58.82 59.27 57.54

0.16 0.22 0.12 0.29 0.30 0.28 0.29

9.94 2.71 8.75 11.30 11.03 9.96 10.22

83.16 82.63 84.91 81.59 81.94 83.09 83.00

5.01 5.21 4.69 5.29 5.24 5.19 5.09

1.71 1.61 1.53 1.51 1.45 1.45 1.35

Note: Mad: Inherent moisture content with air-dried basis; Ad: Ash yield with dry basis; Vdaf: Volatile matter yield with dry-ash-free basis; FCd: Fixed carbon content with dry basis; ad: air-dried basis; d: dry basis; daf: dry-ash-free basis.

three-step demineralization process (viz. HCl-HF-HCl) was described by Strydom et al. [52] in detail. It is worthy note that crystallinity of coals is considered to be not influenced by the acid treatment [53]. XRD spectrums for demineralized samples were subsequently derived utilizing the Bruker D8 Advance instrument (Cu target, Kα radiation) equipped with 0.6 mm divergence slit and 8 mm anti-scatter slit systems. Operating conditions of the X-ray tube were U = 40 kV and I = 30 mA. The sampling step was 0.019450° and the angle reproducibility was ± 0.0001°.

4. Results and discussion 4.1. Hydrogen bonds 4.1.1. Calculation Firstly, Gaussian distribution functions were applied to resolve FT IR spectrums by using software Origin 7.5 (Fig. 3), which was precise enough for parameters calculation [46]. Seven types of hydrogen bonds (P1–P7) clarified in Fig. 3 are listed in Table 3 in detail. Sub-peaks of ‘CH’ in ranges of 2900–3000 cm−1 and 3100–3200 cm−1 represented stretch vibration of aliphatic CH and aromatic CH respectively [27].

Table 2 Deformation characteristics and mechanisms of diverse types of coal samples (according to Jiang and Ju, 2004; Ju and Li, 2009; Pan et al., 2015). Macrographs and micrographs of coal samples

Deformation types

Fractures and structures

Fragmentation degrees

Deformation mechanisms and stressstrain environments

Primary coal

Coal complete bulk was cut by few fractures without obvious displacement.Maceral bands are preserved intact

It could be hardly crumbled into angular fragments with sizes > 10 cm

Less affected by tectonic stress

Cataclastic coal

It was cut by ≧2 sets of sparse and straight fractures into fragments without obvious displacement.Maceral bands could be clearly observed

It could be crumbled into angular fragments with sizes of ∼3 cm

Commonly developed under the action of weak extensional, compressive and shearing stress with higher strain rates.

Schistose coal

It was cut by a set of sparsely parallel fractures with relative sliding shift.Maceral bands are preserved intact

It could be crumbled into flat particles with width of 1–5 cm

Formed in a shear stress environment

Mortar coal

It was cut by multidirectional dense and mussy fractures. Grains with two obvious size fractions could be observed microscopically.Maceral bands could be vaguely observed

It could be crumbled into sub-angular particles with sizes of 3–1 cm

Granulitic coal

It was cut by multidirectional and more dense and mussy fractures. Grains less than 1 mm were of significant directivity.Layered structure disappeared

It could be easily crumbled into particles with well psephicity less than 1 cm

Commonly developed under the action of strong extensional, compressive and shearing stress with higher strain rates Commonly developed under the action of strong compressive and shearing stress with high strain rates

Scaly coal

It was densely cut into willow leaf shapes by a set of dense arc fracturesLayered structure totally disappeared

It could be easily crumbled into fragments or flakes with width of 0.5–1 cm

Formed in a strong shear stress environment

Wrinkle coal

It was intensively arc crumpled maceral bands.The primary structure disappeared

It could be easily crumbled into powder and grains less than 1 cm

Formed in a strong shear stress environment with low strain rate

4

Fuel 255 (2019) 115785

H. Liu and B. Jiang

0.04

Confidence Coefficient = 0.997 Sample 4

of hydrogen bond (from P1 to P7, Table 3) in i type of TDC (from sample 1 to sample 7, Table 1) respectively. σij could be obtained from Miura et al. [46]:

Experimental curve Fitting curve Sub-peak curve

Absorbance / a.u.

0.03

ij

0.02 CH

P1

CH P4

P3 CH

P2

P5

nij =

P6

P7

2600

2800

3000

3200

3400

3600

-1

Wavenumber / cm

Number

Types of hydrogen bonds

Band positions/cm−1

P1

SH-OH

∼2508 ∼2739

P3

OH-N

∼3006

P4

Cyclic OH

∼3200

P5

OH-ether

∼3300

P6

Self-associated nmers (n > 3) OH-π

∼3406

P7

ij

0 (1

vOH , ij)

+ 0.0147 ×

ij

0 (1 + 0.0147 × vOH , ij )

where i and j refer to diverse types of TDCs and hydrogen bonds respectively, and Aij and σij are integral intensity and absorptivity of j type

0.28

Content of hydrogen bonds / mol/kg

OH-SH COOH dimers OH-N Cyclic OH OH-ether Self-associated n-mers

0.35

0.21 0.14 0.07

3

4

5

The number of samples

6

vOH , ij )

(5)

(0.067 ×

(0.067 ×

vOH , ij

vOH , ij

+ 2.64)

(6)

+ 2.64)

Aij

(7)

0 (1 + 0.0147 × vOH , ij)

4.1.2. Content of hydrogen bonds According to Eqs. (3) and (5), content of hydrogen bonds in TDCs was calculated (Fig. 4a and b). OH-π occupies the first position in hydrogen bonds of TDCs, self-associated n-mers (n > 3), OH-ether and cyclic OH come second, and the third are COOH dimers, OH-SH and OH-N. With increase of coal deformation intensity, the latter six types of hydrogen bonds are all basically in a decreasing trend, which indicates that deformation of coal obviously leads to cleavage of six types of hydrogen bonds. However, content of OH-π gradually increases from primary coal to granulitic coal, and relatively rapid increasing shows in scaly and wrinkle coals. Variation trend of OH-π contrary to other hydrogen bonds illustrates that other cracked hydrogen bonds could connect to aromatic structures to form new OH-π. Furthermore, total content of hydrogen bonds decreases with the increasing deformation intensity from primary coal to granulitic coal, while, a slight increasing trend

(1)

Content of hydrogen bonds / mol/kg

ij

ij

∼3506

2

+ 0.0147 ×

Aij

Hav =

(a)

(4)

+ 2.64

Aij

ij

1

vOH , ij

0 (1

Htotal =

Aij

0.00

(3)

vOH , ij)

Aij

ntotal =

Configurations of hydrogen bonds

According to Lambert-Beer law, the amount of hydrogen bonds (nij) was calculated as following:

nij =

+ 0.0147 ×

Accordingly, total amount (ntotal), average strength (−ΔHav) and total strength (−ΔHtotal) of hydrogen bonds could be calculated as follows:

Table 3 Assignment of FT IR spectrum in range of 2400–3600 cm−1 (According to Painter et al. [24], Miura et al. [46] and Li et al. [54]).

COOH dimers

(2)

vOH , ij)

Aij 0 (1

Hij = 0.067 ×

Fig. 3. Curve fitting of FT IR spectrum ranging from 2400 cm−1 to 3600 cm−1.

P2

+ 0.0147 ×

Strength of hydrogen bonds is estimated by enthalpy (ΔH) of hydrogen bond formation reactions (it is worthy note that ΔH < 0). In the light of research supported by Miura et al. [46], strength of i type of hydrogen bond in j type of TDC, −ΔH,ij, is given by:

0.00 2400

0 (1

where σ0 is absorption coefficient of free OH groups, ΔvOH,ij refers to wavenumber shift of j type hydrogen bond relative to that of free OH groups (Li et al. [27]), then Eq. (1) is transformed into:

CH

0.01

=

7

(b)

Total content of hydrogen bonds Content of OH-π

2.0 1.5 1.0 0.5 0.0

1

2

3

4

5

The number of samples

6

7

Fig. 4. Content of hydrogen bonds, a. Content of OH-SH, COOH dimers, OH-N, cyclic OH, OH-ether and self-associated n-mers, b. Content of OH-π and total content of hydrogen bonds. 5

Fuel 255 (2019) 115785

-∆Η of hydrogen bonds / KJ/mol

H. Liu and B. Jiang

45

increases π-π bonds interactions, while πeπ bonds are less introduced into aromatic systems with other types of configurations. There is, then, a good reason to believe that aromatic structures in TDCs are rearranged into a more stable form with more formation of π-π bonds. Besides, it is worthy note that π-π bonds are more accumulated in schistose coal compared with mortar coal, and those in scaly and wrinkle coals are much more than those in granulitic coal, which is believed to closely correlate with various stress-strain environments of TDCs (detailed discussion could be found in Section 4.5).

Total strength of hydrogen bonds Average strength of hydrogen bonds

40 35 30 25 20 15

1

2

3

4

5

The number of samples

6

4.2.3. Functional groups Fig. 8a shows that eCeO (ether and hydroxyl) occupies the largest part of oxygen functional groups, eC]O (carbonyl) takes second place followed by eCOOH (carboxyl). Content of those three types of oxygen functional groups all decreases with the increase of deformation intensity generally, which illustrates that coal deformation promotes dissociation of oxygen functional groups. Nitrogen functional groups in selected coal samples are composed of pyrrole and pyridine (Fig. 8b). Pyrrole with ratios higher than 80% occupies the dominant position between the two. Content of Ndaf decreasing from primary coal to wrinkle coal illustrates that tectonic stress impels the depletion of nitrogen atoms, which is consistent with previous research results (Fig. 8c) [59]. Correspondingly, ratios of pyrrolic nitrogen in TDCs slightly decrease with the increase of deformation intensity. Previous studies indicated that pyrrole the most abundant form in coal was the most unstable ones, which could be degraded more easily with the action of metamorphism [56,60–62]. Therefore, it could be speculated that decreasing nitrogen content was ascribed to the stress degradation of pyrrole. Apparently, dissociation of oxygen and nitrogen functional groups is one of the important reasons that lead to content decrease of hydrogen bonds [46].

7

Fig. 5. Total and average strength of hydrogen bonds.

shows in scaly and wrinkle coals, which is ascribed to variation rates of hydrogen bonds in different types of TDCs. Namely, increasing rate of OH-π is lower than decreasing rates of other six types of hydrogen bonds in brittle deformed coals and schistose coal, while variation rates are opposite in scaly and wrinkle coals, which leads to a slight increasing of total content of hydrogen bonds in those two types of deformed coals. Although content of OH-π is in an increasing trend, the strength of OH-π is at the same level of that of other hydrogen bonds, which implies that OH-π could also be dissociated at initial stage of tectonic stress alteration and then could be reformed to new ones or be transformed from other cleaving hydrogen bonds. 4.1.3. Strength of hydrogen bonds Total and average enthalpy for the formation reactions of hydrogen bonds were calculated through Eqs. (6) and (7) respectively. Fig. 5 shows that −ΔHtotal of hydrogen bonds basically decreases with the increasing deformation intensity, especially in scaly and wrinkle coals. The decrease of −ΔHtotal of brittle deformed coals and schistose coal is mainly ascribed to the reduction of ntotal resulted from stress cleavage of hydrogen bonds. Unlike brittle deformed coals and schistose coal, scaly and wrinkle coals with higher ntotal are of lower −ΔHtotal, which is believed to be caused by the transformation of dissociated hydrogen bonds (viz. dissociated hydrogen bonds are transformed into OH-π with higher stability etc.) [27,46]. Besides, it is worthy note that content of OH-π slightly increases with rising deformation intensity, which indicates that hydrogen bonds transformation is another reason for the decrease of −ΔHtotal in brittle deformed coals and schistose coal. −ΔHav in brittle deformed coals shows a very slight decreasing trend with fluctuation, which is resulted from rearrangement of hydrogen bonds. Obvious reduction of −ΔHav showed in scaly and wrinkle coals suggests that there is a high-frequency rearrangement of hydrogen bonds.

4.3. Crystallinity of coal molecules 4.3.1. Characterization of coal crystallinity utilizing Raman Raman spectrums of coal samples were fitted by using the software Origin 7.5 (Fig. 9a). In the first order Raman spectrum ranging from 1000 cm−1 to 1800 cm−1, D sub-peak (at ∼1350 cm−1) was ascribed to various types of structural disorder including amorphous carbons, structural defects and the hetero atoms etc., while G sub-peak (at ∼1600 cm−1) represented the ideal graphitic lattice vibration mode with E2g symmetry [49,63,64]. ID/IG ratio was a useful parameter to characterize defective carbon materials including coals [65]. Decrease of ID/IG indicated the increase of crystallinity of coal molecules, viz. the increase of orientation, stacking order and even cluster sizes of aromatic structures [64,66,67]. Fig. 9b shows that ratios of ID/IG basically increase from primary coal to cataclastic coal, then fall from granulitic coal to wrinkle coal, indicating that crystallinity of TDCs decreases from primary coal to granulitic coal and then rises in scaly and wrinkle coals. It is worthy note that ID/IG ratios of schistose, scaly and wrinkle coals are lower than those of cataclastic, mortar and granulitic coal, which implies that there are different alteration mechanisms and efficiency between brittle deformation sequence and other sequences. In brittle deformed sequence, the transformed kinetic energy and frictional thermal energy usually dominate the evolution process [3,15], and the increase of ID/IG indicates that efficiency of mobile molecules generation is higher than that of structural rearrangement under the action of transformed energy. While, in brittle-ductile and ductile deformed sequences, strong shear stress and strain energy are better in favor of rearrangement of mobile molecules generated by cleavage of noncovalent bonds, which leads to the decrease of ID/IG ratios [2–4].

4.2. π-π bonds and functional groups 4.2.1. Curve fitting of XPS spectrums XPS spectrums of coal samples involve important information about coal macromolecular structures [55]. In this study, spectrums were deconvoluted by software Origin 7.5 (Fig. 6) in ranges of 280–292 eV (C1s) and 392–406 eV (N1s) involving useful information about π-π bonds, oxygen and nitrogen functional groups [47,48,56,57]. Assignment of sub-peaks was listed in Table 4 in detail. 4.2.2. πeπ bonds Content of πeπ bonds in TDCs increases with the increasing deformation intensity generally, especially in scaly and wrinkle coals (Fig. 7). Configuration of aromatic structures determines the content of πeπ bonds. Face-to-face configuration of aromatic layers significantly

4.3.2. Characterization of coal crystallinity utilizing XRD A very broad hump at ∼25° and a relatively weak sub-peak around 40° representing graphite diffraction were observed in XRD spectrums 6

Fuel 255 (2019) 115785

H. Liu and B. Jiang

(a)

400

Confidence Coefficient = 0.999

300

16000

Counts / s

C=C vs CH

500

Experimental curve Fitting curve Sub-peak curve

12000

Counts / s

20000

Sample Z15

8000

-C-O

4000

-C=O

-COOH

(b)

Experimental curve Fitting curve Sub-peak curve

Pyrrolic nitrogen

Confidence Coefficient = 0.999 Sample 4

Pyridine nitrogen

200 100 0

π-π bonds

-100

0 280

282

284

286

288

290

292

392

394

396

Binding energy / eV

398

400

402

404

406

Binding energy / eV

Fig. 6. Curve fitting of XPS spectrums, a. Curve fitting of XPS spectrum ranging from 280 eV to 292 eV, b. Curve fitting of XPS spectrum in the range of 392–406 eV.

ranging from 10° to 50° [17]. To calculate the crystallinity parameters, XRD spectrums were primarily deconvoluted using software Origin 7.5 and three feature bands were clarified including γ-band around 20°, πband (002 reflection) around 25° and (10) band around 42° (Fig. 10a) [68,69]. Accordingly, crystallinity parameters interlayer spacing (d002), lateral size (La), stacking height (Lc) and average effective number of aromatic layers (N) were furtherly determined by using Bragge and Debye-Scherrer formulas as follows,

Table 4 Assignment of C(1 s) and N(1 s) peaks (According to Perry and Grint [47], Shi et al. [48], Kelemen and Kwiatek [57] and Pietrzak [58]). Binding energy (eV)

Assignment

∼285 ∼286.3 ∼286.6 ∼289.2 ∼290.5 and ∼291.5 ∼398.7 ∼400.3

Aromatic or aliphatic carbons Ether, hydroxyl Carbonyl Carboxyl π-π bonds between aromatic species Pyridinic nitrogen Pyrrolic nitrogen

Mole content of π-π bonds / %

5

d 002 =

π-π bonds

2 1 0 3

4

5

6

The number of samples

7

Fig. 7. Distribution of π-π bonds.

-C-O -C=O -COOH

30

(b) The number of samples

Mole content / %

25 20 15 10 5 0

10 cos 10

Lc = 0.89 /

002 cos 002

Pyrrolic nitrogen

Pyridinic nitrogen

(c)

7

2

3

4

5

The number of samples

6

7

Ndaf

1.7

6 5 4 3 2 1

1

(10) (11)

Content of Ndaf / %

(a)

(9)

La = 1.84 /

where λ refers to the wavelength of the Kα radiation (0.15418 nm), β10 and β002 are the full width at half maximum height of sub-peaks 10 and 002 respectively, and θ10 and θ002 are the corresponding scattering angles. As shown in Fig. 10b, the average effective numbers of aromatic layers associated in stacked clusters and the stacking heights perpendicular to the aromatic sheets all decrease from primary coal to mortar coal, though decrease from granulitic coal to wrinkle coal (especially for scaly and wrinkle coal). Conversely, the average interlayer spacing firstly increases from primary coal to mortar coal, then decrease from granulitic coal to wrinkle coal. Therefore, stacked aromatic layers are inclined to be expanded by brittle deformation (except for granulitic coal). The expansion of associated aromatic layers preferentially starts with weaker bonds [3], namely noncovalent bonds much weaker than covalent bonds [33]. Although πeπ bonds associating aromatic layers were formed in brittle deformed coals resulted from the frictional heat

3

2

(8)

002

N = Lc / d 002

4

1

/2sin

0

5

10

15

90

92

94

96

Ratios of pyridine and pyrrolic nitrogen / %

98

100

1.6

1.5

1.4

1.3

1

2

3

4

5

The number of samples

6

7

Fig. 8. Distribution of various functional groups, a. Distribution of oxygen functional groups, b. Distribution of nitrogen functional groups, c. Distribution of Ndaf.

7

Fuel 255 (2019) 115785

H. Liu and B. Jiang

(a)

0.70

G

600 D

400

ID/IG

Sample 4

Ratio of ID/IG

Intensity / a.u.

(b)0.75

Confidence Coefficient = 0.998

Experimental curve Fitting curve Sub-peak curve

800

200

0.65

0.60

0

1000

1200

1400

-1

1600

0.55

1800

Raman shift / cm

1

2

3

4

5

The number of samples

6

7

Fig. 9. Curve fitting results of Raman spectrums, a. Curve fitting of Raman spectrum ranging from 1000 cm−1 to 1800 cm−1, b. Distribution of ratios of ID/IG.

Fig. 10. Distribution of crystallinity parameters, a. Deconvolution of XRD spectrum ranging from 5° to 35°, b. Distribution characteristics of d002, Lc and N, c. Distribution characteristic of La.

and kinetic energy, the higher cleavage rates of hydrogen bonds continually dislocated aromatic structures. It should be noted that schistose coal formed in shear stress environment is characterized by a little higher La and N, indicating that shear stress is more supportive in rearrangement of aromatic layers to more stacked structures. More frictional heat is generated in granulitic coal with strongest brittle deformation intensity, which leads to the increase of macromolecular crystallinity. With regard to brittle-ductile and ductile deformed coals, strong shear stress and transformed strain energy help to rearrange the coal macromolecules to a more stacked configuration with lower d002 and higher Lc and N (Fig. 10b). The fluctuation of lateral sizes of various TDC aromatic layers indicates that coal deformation has negligible effects on the polycondensation of aromatic structures.

deformation (Fig. 11, stage 2). Breaking of those noncovalent bonds leads to generation of more active reaction sites, liberation of coal molecules and relaxation of network structures of TDC samples [33,40], which provides stress-induced macromolecular rearrangement with favorable conditions. 4.4.2. Stress-induced rearrangement TDCs in brittle, brittle-ductile, ductile deformed sequences develop in different stress-strain environments (Table 1), which leads to various changing efficiency of coal macromolecular structures. In brittle deformed sequence, particle sizes of TDCs are reduced and psephicity of particles is increased by mechanical crushing and grinding of tectonic stress accompanied by generation of kinetic energy and frictional heat, which helps liberated molecules to overcome motion activation barrier and allows partial molecules to be rearranged into a more stable configuration that contains more π-π bonds [2,15,25]. (Fig. 11, stage 3). More stacked aromatic layers and lower interlayer spacing in brittleductile deformed coals (schistose and scaly coals) are ascribed to the shear stress environment formed in (Figs. 7 and 9b). On the one hand, shear stress parallel to basal planes of laminated molecules promotes layer dislocation and break of noncovalent bonds that releases more molecules, on the other, shear stress is more conducive to local motion and orientated alignment of liberated molecules. Ductile deformation properties developed in winkle coal indicate a strong shear stress environment with low strain rate [3]. Mechanical stress inclines to be transformed into strain energy that could change coal macromolecular structures more efficiently [10]. Accordingly, liberated molecules are moved and rearranged into a more stacked and ordered configuration (face-to-face configuration) with lower free energy by shear stress with lower strain rate. A face-to-face configuration results in the formation of more new πeπ bonds (Fig. 7). Transformed strain energy assists liberated molecules to overcome motion energy barrier easily, which makes the motion of liberated molecules more frequent. In addition, significant increasing band position of D peak

4.4. Rearrangement of coal chemical structures 4.4.1. Cleavage of noncovalent bonds Hydrogen bonds and πeπ bonds are important cross-links of coal macromolecular networks, which limit the mobility of molecules including rigid phases and mobile phases [70]. Quantum chemistry calculations showed that mechanical energy produced by tectonic stress satisfied the energy of carbonyl dissociation (1199 KJ/mol, 1328 KJ/ mol) and Stone-Wales defects generation (303.4 KJ/mol) required [15,31]. −ΔHtotal and −ΔHav of hydrogen bonds of selected samples are in ranges of 26.33–41.46 KJ/mol and 17.79–26.20 KJ/mol respectively (calculated by Eq. (4)). And strength of πeπ bonds is no > 41.84 KJ/ mol [25]. Therefore, noncovalent bonds with lower strength compared with covalent bonds could be easily dissociated by different types of tectonic stress. Direct cleavage or dissociation of diverse types of functional groups both results in disruption of hydrogen bonds in various TDCs (Fig. 11, stage 2). Although the amount of π-π bonds increases with the rise of coal deformation intensity (Fig. 7), they could also be disrupted by tectonic stress inevitably at initial stage of coal 8

Fuel 255 (2019) 115785

H. Liu and B. Jiang

Carbon atoms Oxygen atoms Hydrogen atoms Y

π- π b onds Y

X Z

Y

X Z

Stage 1 : Proposed configuration of primary coal.

Z

Stage 2 : Cle avage o f hydrogen b onds a nd π-π bonds.

X

S tage 3: Rearrangement of molecules

Hydrogen bonds Proposed moving direction

Fig. 11. Rearrangement of coal molecular structures.

1380

increasing amount of OH-π. −ΔHtotal and −ΔHav both decrease with the increasing deformation intensity even in scaly and wrinkle coals with slightly increasing total content of hydrogen bonds. The amount of πeπ bonds increases with coal deformation intensity (especially in schistose, scaly and wrinkle coals), indicating that an ordered configuration with more πeπ bonds is generated. Combined with variation of coal macromolecular crystallinity, tectonic stress induced evolution of coal molecules with the addition of noncovalent bonds is furtherly proposed as: cleavage of noncovalent bonds, liberation of molecules, motion of free segments and rearrangement to a more ordered, stacked and stable configuration.

WD

1375

WD / cm-1

1370 1365 1360 1355 1350

Acknowledgements 1

2

3

4

5

The number of samples

6

7

The research is sponsored by the National Natural Science Foundation of China (No. 41672147, 41430317), and the Scientific Research Foundation of Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education (China University of Mining and Technology) (No. 2017-004).

Fig. 12. Distribution of band position of D sub-peaks.

indicates the more developed structural defects in ductile deformed coals [49], which increases initial reactant energy and reduces motion energy barrier of certain molecular to a lower level subsequently [31,32] (Fig. 12). More importantly, long-term action of ductile deformation supplies motion and adjustment of molecules with enough time. Besides, mechanism of tectonic stress induced hydrocarbon generation from coal mentioned by Hou et al. [14] is still under discussion. Proposed rearrangement process of TDC molecules might give useful insights (Fig. 11). Composite model suggested that coal molecules were composed of macromolecular part (‘host molecules’) with weight of 1300–3500 amu and small molecules in the range of ∼100 to 500 amu (‘guest molecules’) [21]. At evolution stage 2 (Fig. 11), breakage of noncovalent bonds not only liberates small molecules (∼100 to 500 amu) that could be moved locally, but also increases pore structures (especially nano-scale pores) by opening laminated molecules [4,12,33,71]. If these two conditions hold, mobile molecules with appropriate sizes will be no more trapped in macromolecular networks, and could be adsorbed on the surfaces of nano-scale pores [17] or even be released into the air.

References [1] Jiang B, Qu Z, Wang GGX, Li M. Effects of structural deformation on formation of coalbed methane reservoirs in huaibei coalfield, China. Int J Coal Geol 2010;82:175–83. [2] Hou Q, Li H, Fan J, Ju Y, Wang T, Li X, et al. Structure and coalbed methane occurrence in tectonically deformed coals. Sci China Earth Sci 2012;55:1755–63. [3] Ju Y, Li X. New research progress on the ultrastructure of tectonically deformed coals. Prog Nat Sci-Mater 2009;19:1455–66. [4] Pan J, Zhu H, Hou Q, Wang H, Wang S. Macromolecular and pore structures of Chinese tectonically deformed coal studied by atomic force microscopy. Fuel 2015;139:94–101. [5] Wang Z, Pan J, Hou Q, Niu Q, Tian J, Wang H, et al. Changes in the anisotropic permeability of low-rank coal under varying effective stress in Fukang mining area, China. Fuel 2018;234:1481–97. [6] Teichmüller M, Teichmüller R. Geological causes of coalification. Washington, DC, Advances in Chemistry. Am Chem Soc 1966:133–55. [7] Bustin RM. Heating during thrust faulting in the Rocky mountains: friction or fiction? Tectonophysics 1983;95:309–28. [8] Ross JV, Bustin RM. The role of strain energy in creep graphitization of anthracite. Nature 1990;343:58–60. [9] Bustin RM, Ross JV, Rouzaud JN. Natural graphitization of anthracite: experimental considerations. Carbon 1995;33:679–91. [10] Liu H, Jiang B, Liu J, Song Y. The evolutionary characteristics and mechanisms of coal chemical structure in micro deformed domains under sub-high temperatures and high pressures. Fuel 2018;222:258–68. [11] Cao YX, Mitchell GD, Davis A, Wang DM. Deformation metamorphism of bituminous and anthracite coals from china. Int J Coal Geol 2000;43:227–42. [12] Pan J, Wang S, Ju Y, Hou Q, Niu Q, Wang K, et al. Quantitative study of the macromolecular structures of tectonically deformed coal using high-resolution transmission electron microscopy. J Nat Gas Sci Eng 2015;27:1852–62. [13] Cao D, Li X, Zhang S. Influence of tectonic stress on coalification: stress degradation mechanism and stress polycondensation mechanism. Sci China Ser D 2007;50:43–54. [14] Hou Q, Han Y, Wang J, Dong Y, Pan J. The impacts of stress on the chemical structure of coals: a mini-review based on the recent development of mechanochemistry. Sci Bull 2017;62:965–70. [15] Xu R, Li H, Guo C, Hou Q. The mechanisms of gas generation during coal deformation: preliminary observations. Fuel 2014;117:326–30. [16] Song Y, Zhu Y, Li W. Macromolecule simulation and CH4 adsorption mechanism of coal vitrinite. Appl Surf Sci 2017;396:291–302. [17] Pan J, Lv M, Hou Q, Han Y, Wang K. Coal microcrystalline structural changes related to methane adsorption/desorption. Fuel 2019;239:13–23. [18] Duber S, Rouzaud JN. Calculation of reflectance values for two models of texture of carbon materials. Int J Coal Geol 1999;38:333–48.

5. Conclusions To furtherly clarify stress response of noncovalent bonds in TDCs, and the role they played in macromolecular evolution, a primary coal sample and six typical TDC samples collected around a fault structure were investigated utilizing FT IR, XPS, Raman and XRD. Several cognitions are obtained as follows: Amounts of Self-associated n-mers (n > 3), OH-ether, cyclic OH, COOH dimers, OH-SH and OH-N all decrease with the increase of coal deformation intensity. Dissociation of six types of hydrogen bonds is caused by direct disruption and fallen of oxygen and nitrogen functional groups. On the contrary, amount of OH-π is in an increasing trend, indicating the dissociated hydrogen bonds are inclined to be transformed into more stable OH-π. Total content of hydrogen bonds generally decreases from primary coal to granulitic coal, then slightly increases from scaly coal to wrinkle coal, which is ascribed to 9

Fuel 255 (2019) 115785

H. Liu and B. Jiang

[47] Perry DL, Grint A. Application of xps to coal characterization. Fuel 1983;62:1024–33. [48] Shi Q, Qin B, Liang H, Gao Y, Bi Q, Qu B. Effects of igneous intrusions on the structure and spontaneous combustion propensity of coal: a case study of bituminous coal in daxing mine, China. Fuel 2018;216:181–9. [49] Pan J, Lv M, Bai H, Hou Q, Li M, Wang Z. Effects of metamorphism and deformation on the coal macromolecular structure by laser Raman spectroscopy. Energy Fuels 2017;31:1136–46. [50] Roberts MJ, Everson RC, Neomagus HWJP, Okolo GN, Van Niekerk D, Mathews JP. The characterisation of slow-heated inertinite- and vitrinite-rich coals from the south african coalfields. Fuel 2015;158:591–601. [51] Zhang Y, Kang X, Tan J, Frost RL. Structural characterization of hydrogen peroxideoxidized anthracites by X-ray diffraction, fourier transform infrared spectroscopy, and Raman spectra. Appl Spectrosc 2014;68:749–57. [52] Strydom CA, Bunt JR, Schobert HH, Raghoo M. Changes to the organic functional groups of an inertinite rich medium rank bituminous coal during acid treatment processes. Fuel Process Technol 2011;92:764–70. [53] Van Niekerk D, Pugmire RJ, Solum MS, Painter PC, Mathews JP. Structural characterization of vitrinite-rich and inertinite-rich permian-aged South African bituminous coals. Int J Coal Geol 2008;76:290–300. [54] Li W, Bai Z, Bai J, Guo Z. Decomposition kinetics of hydrogen bonds in coal by a new method of in-situ diffuse reflectance Ft-ir. J Fuel Chem Technol 2011;39:321–7. [55] Geng W, Kumabe Y, Nakajima T, Takanashi H, Ohki A. Analysis of hydrothermallytreated and weathered coals by X-ray photoelectron spectroscopy (XPS). Fuel 2009;88:644–9. [56] Ding D, Liu G, Fu B, Yuan Z, Chen B. Influence of magmatic intrusions on organic nitrogen in coal: a case study from the zhuji mine, the huainan coalfield, China. Fuel 2018;219:88–93. [57] Kelemen SR, Kwiatek PJ. Quantification of organic oxygen species on the surface of fresh and reacted argonne premium coal. Energy Fuels 1995;9:841–8. [58] Pietrzak R. Xps study and physico-chemical properties of nitrogen-enriched microporous activated carbon from high volatile bituminous coal. Fuel 2009;88:1871–7. [59] Song Y, Jiang B, Liu H, Li F, Shao P, Yan G. Variations in stress-sensitive minerals and elements in the tectonic-deformation early to middle permian coals from the zhuxianzhuang mine, Anhui Province. J Geochem Explor 2018;188:11–23. [60] Boudou J, Schimmelmann A, Ader M, Mastalerz M, Sebilo M, Gengembre L. Organic nitrogen chemistry during low-grade metamorphism. Geochim Cosmochim Ac 2008;72:1199–221. [61] Valentim B, Guedes A, Boavida D. Nitrogen functionality in “oil window” rank range vitrinite rich coals and chars. Org Geochem 2011;42:502–9. [62] Kelemen SR, Freund H, Gorbaty ML, Kwiatek PJ. Thermal chemistry of nitrogen in kerogen and low-rank coal. Energy Fuels 1999;13:529–38. [63] Xu R, Li H, Hou Q, Li X, Yu L. The effect of different deformation mechanisms on the chemical structure of anthracite coals. Sci China Earth Sci 2015;58:502–9. [64] Potgieter-Vermaak S, Maledi N, Wagner N, Van Heerden JHP, Van Grieken R, Potgieter JH. Raman spectroscopy for the analysis of coal: a review. J Raman Spectrosc 2011;42:123–9. [65] Eckmann A, Felten A, Mishchenko A, Britnell L, Krupke R, Novoselov KS, et al. Probing the nature of defects in graphene by raman spectroscopy. Nano Lett 2012;12:3925–30. [66] Mochida I, Korai Y, Fujitsu H, Takeshita K, Komatsubara Y, Koba K, et al. Aspects of gasification and structure in cokes from coals. Fuel 1984;63:136–9. [67] Seong HJ, Boehman AL. Evaluation of Raman parameters using visible raman microscopy for soot oxidative reactivity. Energy Fuels 2013;27:1613–24. [68] Sonibare OO, Haeger T, Foley SF. Structural characterization of nigerian coals by xray diffraction, raman and Ftir spectroscopy. Energy 2010;35:5347–53. [69] Baysal M, Yürüm A, Yıldız B, Yürüm Y. Structure of some western anatolia coals investigated by ftir, raman, 13c solid state nmr spectroscopy and x-ray diffraction. Int J Coal Geol 2016;163:166–76. [70] Shui H, Wand Z, Cao M. Effect of pre-swelling of coal on its solvent extraction and liquefaction properties. Fuel 2008;87:2908–13. [71] Pan J, Zhao Y, Hou Q, Jin Y. Nanoscale pores in coal related to coal rank and deformation structures. Transport Porous Med 2015;107:543–54.

[19] Alexeev AD, Ulyanova EV, Starikov GP, Kovriga NN. Latent methane in fossil coals. Fuel 2004;83:1407–11. [20] Given PH. Structure of bituminous coals: evidence from distribution of hydrogen. Nature 1959;184:980–1. [21] Marzec A. Towards an understanding of the coal structure: a review. Fuel Process Technol 2002;77:25–32. [22] Marzec A. Macromolecular and molecular model of coal structure. Fuel Process Technol 1986;14:39–46. [23] Qin K, Guo S, Li S. New concept on coal structure and new consideration for the generation mechanism of oil from coal. Chinese Sci Bull 1998:2025–35. [24] Painter PC, Sobkowiak M, Youtcheff J. Ft-i.r. study of hydrogen bonding in coal. Fuel 1987;66:973–8. [25] Nishioka M, Larsen JW. Association of aromatic structures in coals. Energy Fuels 1990;4:100–6. [26] Larsen JW, Baskar AJ. Hydrogen bonds from a subbituminous coal to sorbed solvents. An infrared study. Energy Fuels 1987;1:230–2. [27] Li D, Li W, Li B. Hydrogen bonds in coal. The influence of coal rank and the recognition of a new hydrogen bond in coal. Chem Res Chinese U 2003;19:70–5. [28] Zubkova V. Some aspects of structural transformations taking place in organic mass of ukrainian coals during heating. Part 1. Study of structural transformations when heating coals of different caking capacity. Fuel 2005;84:741–54. [29] Fan X, Wei X, Zong Z. Application of gas chromatography/mass spectrometry in studies on separation and identification of organic species in coals. Fuel 2013;109:28–32. [30] Han Y, Xu R, Hou Q, Wang J, Pan J. Deformation mechanisms and macromolecular structure response of anthracite under different stress. Energy Fuels 2016;30:975–83. [31] Han Y, Wang J, Dong Y, Hou Q, Pan J. The role of structure defects in the deformation of anthracite and their influence on the macromolecular structure. Fuel 2017;206:1–9. [32] Li W, Jiang B, Moore TA, Wang G, Liu J, Song Y. Characterization of the chemical structure of tectonically deformed coals. Energy Fuels 2017;31:6977–85. [33] Vasireddy S, Morreale B, Cugini A, Song C, Spivey JJ. Clean liquid fuels from direct coal liquefaction: chemistry, catalysis, technological status and challenges. Energy Environ Sci 2011;4:311–45. [34] Niekerk DV, Mathews JP. Molecular representations of permian-aged vitrinite-rich and inertinite-rich south african coals. Fuel 2010;89:73–82. [35] You C, Fan X, Zheng A, Wei X, Zhao Y, Cao J, et al. Molecular characteristics of a chinese coal analyzed using mass spectrometry with various ionization modes. Fuel 2015;155:122–7. [36] Iino M. Network structure of coals and association behavior of coal-derived materials. Fuel Process Technol 2000;62:89–101. [37] Quinga EMY, Larsen JW. Noncovalent interactions in high-rank coals. Energy Fuels 1987;1:300–4. [38] Chen C, Gao J, Yan Y. Observation of the type of hydrogen bonds in coal by Ftir. Energy Fuels 1998;12:446–9. [39] Li D, Li W, Chen H, Li B. The adjustment of hydrogen bonds and its effect on pyrolysis property of coal. Fuel Process Technol 2004;85:815–25. [40] Nishioka M. Molecular mobility of high-volatile bituminous coals accompanying drying. Fuel 1994;73:57–62. [41] Song Y, Jiang B, Han Y. Macromolecular response to tectonic deformation in lowrank tectonically deformed coals (TDCs). Fuel 2018;219:279–87. [42] Drelich J, Laskowski JS, Pawlik M. Improved sample preparation and surface analysis methodology for contact angle measurements on coal (heterogeneous) surfaces. Coal Prep 2000;21:247–75. [43] Drelich J, Laskowski JS, Pawlik M, Veeramasuneni S. Preparation of a coal surface for contact angle measurements. J Adhes Sci Technol 1997;11:1399–431. [44] Zubkova V, Czaplicka M. Changes in the structure of plasticized coals caused by extraction with dichloromethane. Fuel 2012;96:298–305. [45] Solomon PR, Carangelo RM. Fti r analaysis of coal. I. Techniques and determination of hydroxyl concentrations. Fuel 1982;61:663–9. [46] Miura K, Mae K, Li W, Kusakawa T, Morozumi F, Kumano A. Estimation of hydrogen bond distribution in coal through the analysis of oh stretching bands in diffuse reflectance infrared spectrum measured by in-situ technique. Energy Fuels 2001;15:599–610.

10