Characterization of the Chemical Structure of Tectonically Deformed

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Characterization of the chemical structure of tectonically deformed coals Wu Li, Bo Jiang, Tim A Moore, Geoff G. X. Wang, Jie-gang Liu, and Yu Song Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 3, 2017

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Characterization of the chemical structure of tectonically deformed coals Wu Lia*, Bo Jianga*, Tim A. Mooreb,c, Geoff Wangd, Jie-gang Liua, Yu Songa a

Key Laboratory of Coalbed Methane Resource & Reservoir Formation Process, Ministry of

Education (China University of Mining & Technology), Xuzhou, P. R China

b

Cipher Consulting, 6 Stardust Street, Kenmore, Brisbane QLD 4069, Australia

c

School of Earth, Environment and Biology, Queensland University of Technology, Brisbane,

QLD 4000, Australia

d

School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane, QLD 4072,

Australia

Abstract: Tectonic deformation damages the macromolecular structures of coal. No

evidence can be found of tectonically deformed coal (TDC) structures in which traces

of lattice defects exist. In this study, the mechanisms of ductile deformation and the

evolution of molecular structure have been investigated using coal samples collected

from one thrust fault in the Qinan Coal Mine of China. Various characterization

methods, such as Raman spectroscopy (Raman), Fourier transform infrared

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spectroscopy (FTIR), X-ray diffraction (XRD) and nuclear magnetic resonance (NMR)

imaging were employed to explore the macromolecular structure parameters and

related information that can be derived from coal subjected to ductile deformation.

The results show that the full width at half maximum (FWHM parameter) for D1 of

ductile deformation coal is greater than that of brittle deformation coal, because of its

high content of defects in the graphitic structure. The difference in the ratio of the

-CH2- and -CH3 functional groups indicates that coals with that undergo brittle

deformation have a much higher content of aliphatic chains compared to coal that

undergo ductile deformation. Defects may result from the breaking of C=O bonds in

coal macromolecules, given that the maximum energy of broken C=O bonds is less.

Hence, the content of C=O functional groups is less in TDCs than in primary structure

coals. Evidently, high shearing stress leads to strong ductile deformation, while brittle

deformation is related to the breaking of a series of bonds due to tensile stress and

direct bond breakage. Ductile deformation, which is more likely to be related to point

and line defects, leads to the assessed changes in the carbon structure.

1. Introduction


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Tectonic deformation can cause significant changes in the physical and chemical structures of coal.1 Tectonically deformed coal (TDC), defined as coal affected by

superimposed reformations caused by tectonic stress, is also called deformed coal and

is characterized by tectonically induced features, notably a granular or mylonitic texture.2 Primary coal is defined as coal with primary structure and no influence from

tectonic stress. Because of structural deformation, TDC is distinctly different from

primary coal in terms of macro-coal lithotypes, banded structure, fracture type, and

the degree of crumpling and fragmentation. On the whole, the primary structure and

bedding of coal that has been tectonically deformed is destroyed; the coal is most often sheared and displays a directional arrangement of grains.3 TDC has been

divided into 3 sequences: the brittle deformation sequence, the ductile deformation sequence, and the brittle-ductile transition deformation sequence.4 Locally, abnormal

thickening of a shear zone and extensive compressive structures where ductile fabrics frequently occur are closely related with the site of coal and gas outbursts.5 China will continue to be one of the largest coal producers and users in the world.6 TDC is

currently extracted from structurally complex areas in the eastern part of China, where


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it formed as a result of superimposed tectonic stresses of different types having

different orientations. As a result, the formation of the coal and coalbed methane in eastern China is complicated.2 Previous studies have considered tectonic deformation

and the microstructure of the TDC to be important factors that cause coal gas outburst.5, 7-8 The results show that the outbursts are associated in many places with

anthracite and are found in association with such deformational and depositional

structures as folds, faults, rolls and slips; the outbursts are particularly common anywhere there are rapid fluctuations in the seam thickness.7

Tectonic deformation will damage the macrostructure and impact the macromolecular structures of coal.9 Tectonic deformation has also been linked, in rare

cases, to enhancement of rank, especially where shear stresses and thermal effects from frictional heating along thrust faults are found.10 It has been shown that “easy-slip” deformation is responsible for the enhanced deformation of coal seams.11

The nanoscale structure and the variations in the characteristics of closed pores in coal under tectonic deformation have also been investigated by many researchers.12-15

Tectonic deformation can also have an important influence on the macromolecular


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structure of coal.16 A study examined the effects of stress on the composition of the

macromolecular structure of TDCs. The results show that middle and high rank coals,

including different kinds of TDCs, display remarkable changes in terms of the Hfa/Hfal

ratio (Hfa/Hfal, the ratio of width at the half height of the aromatic carbon and aliphatic

carbon peaks). Variations from 2.427 to 13.800 in the Hfa/Hfal ratio were noted and are

related to carbon aromaticity, carbon aliphaticity and the composition of the macromolecular structure, compared with low rank coals.17

Many methods can be used to characterize coal structure, including both their

physical and chemical structures. These methods include X-ray scattering (XRS),

mercury intrusion porosimetry (MIP), small-angle X-ray scattering (SAXS),

small-angle neutron scattering (SANS), scanning electron microscopy (SEM),

transmission electron microscopy (TEM), XRD, Raman, FTIR, and NMR, as well as others.18 Based on SAXS, SANS, SEM, and TEM, much research has focused on the physical structure, particularly the pore structure, of TDCs.12 However, very few

attempts have been made to characterize the chemical structure of TDCs, especially

the deformation mechanism of the TDCs, using those methods.


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It is generally accepted that the main microscopic deformation mechanisms seen

in ductile mylonite in rock involve lattice defects and recrystallization caused by

lattice slip. However, there is no evidence found within TDC structures that indicates

traces of lattice defects. In this study, a series of samples from one location that goes

from unaffected samples to highly deformed samples were collected. The ductile

deformation mechanisms of coal will be investigated in TDC samples using structural

characterization methods and by examining the evolution of the molecular structure.

The study will explore how coal is deformed and what kind of macromolecular

structure parameter-related information can be derived from ductile deformation.

2. Methods and Experiments

2.1 Geological Setting

The coals investigated in this study were collected from the Qinan Coal Mine,

which is located in the southern part of Huaibei coalfield and covers an area of 55 km2. The Permo-Carboniferous Huaibei Coalfield of the North China plate, which is

located in the northern part of Anhui province, China, is one of the major coalfields in China and contains 23 active underground coal mines.19 It is in the Xu–Su depression


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in the central-southern Luxi–Xuhuai uplift. The Lower Shihezi Formation is a delta

plain deposit, which is regionally distributed and conformable with the underlying Shanxi Formation.20 A major coal-bearing stratum, approximately 250 m thick on

average, contains coal seams Nos. 4, 5, 6, 7, 8 and 9. The coal seams occur at

relatively shallow depths, due to the strong tectonic deformation that led to the

development of the TDCs. Of note, the movement of the NW-trending Xisipo normal

fault and the formation of the Sunan syncline has caused the texture of the coal to be severely disrupted.2

2.2 Samples

To avoid influences of other factors that may affect coal deformation, several

coal samples N20, N24-2, N25, N26-1, N27 and N32 were collected from the face of

the No. 6 coalbed in the Qinan Coal Mine in the Huaibei Coalfield of China,

following Chinese Standard Method GB/T 482-2008. The sampling position is

located in the No. 6125 coal face (Figure 1), where there is a thrust fault. Sample N25

were collected from the hanging wall of the fault, while samples N24-2, N26-1, and

N27 were collected from the footwall (Figure 2). The final maceral analysis data from


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the coal samples are shown in Table 1. TDC has been divided into 3 sequences and 10

types. The brittle deformation sequence includes cataclastic coal, mortar coal,

granulated coal, fine coal, callis, and flake coal. The ductile deformation sequence

includes wrinkle coal, mylonitic coal, and heterogeneous coal. Additionally, the

brittle-ductile transition deformation sequence is scaled coal.

The macro-characteristics, microscopic textures, and strength properties of

selected samples are shown in Figure 3. There are three series of coals: primary coal

(sample N32), ductile deformation coals (samples N24-2, N25, and N26-1), and

brittle deformation coals (samples N20 and N27). Scaled coal is formed by coal seam

fracture after pressing and show foliated structure. The original banded structure of

scaled coal is well preserved and the bedding is clearly visible. Wrinkle coal is

suffering from strong ductile deformation. The primary structure of coal seam is

seriously damaged. During the deformation of the coal seam, a strong small fold is

formed in the coal band. Cataclastic coal is formed by crushing of coal seam under

tectonic stress. When the coal is loaded, it produces cracks in different directions and

splits into fragments along the fracture surface. There is no large displacement


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between the fragments. The fragments still maintain sharp edges and corners.

However, there is few coal being grinded into fine powder the coal on some shear

fracture surfaces. The ductile deformation coals were affected mainly by ductile shear

to varying degrees. The brittle deformation coals were affected mainly by

compressive and tensile stresses.

Figure 1 Structural geology and distribution of mining areas in the Qinan Coal Mine,

Huaibei coalfield. Shaded area in the inset is the larger area shown to the right. Note

the position of the No. 6125 coal face referred to in the text.


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Figure 2 The distribution of sample collection locations.


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Figure 3 Photographs of tectonically deformed coal.

Table 1 Key measured parameters of coal samples.

Coal

Ro, max

(%)

Proximate analysis (wt %)

Wa

Ad

Vdaf

Coking

Elemental analysis (wt %)

Fcd

St.d


Odaf

Cdaf

Hdaf

Ndaf

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index

N20

0.96

1.62

23.07

34.49

4

50.40

0.20

9.74

83.36

5.07

1.56

N24-2

0.90

1.32

17.66

35.70

6

52.94

0.25

11.90

81.91

4.65

1.23

N25

0.88

1.51

20.44

35.18

6

51.57

0.20

8.30

84.71

5.26

1.48

N26-1

0.90

1.56

18.33

36.47

6

51.89

0.21

10.86

82.26

5.15

1.47

N27

0.87

1.30

10.78

36.34

6

56.80

0.20

8.14

84.68

5.38

1.57

N32

0.91

1.15

17.75

34.70

6

53.71

0.14

8.92

84.55

5.04

1.32

a W, moisture; A, ash; V, volatile matter; S d, sulfur; C, carbon; H, hydrogen; N, nitrogen; a, air basis; t

d, dry basis; and daf, dry and ash-free basis.

2.3 Characterization of coals

X-ray diffraction (XRD). XRD measurements of coal samples were conducted

using a D8 ADVANCE (Bruker) instrument with a Cu target and Kα radiation at the

Advanced Analysis and Computation Center in the Chinese University of Mining and

Technology. The operating conditions of the X-ray tube are U=40 kV and I=30 mA.

The top size of the powdered sample is 300 mesh.


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Fourier transform infrared spectroscopy (FTIR). Powdered coal (0.9 mg, size

less than 200 mesh) was initially ground with 80 mg of potassium bromide (KBr) for

20 min in an agate mortar. The mixture was molded into a disc, and this powder was

pressed into a transparent sheet for 10 min. using a tablet machine. Pure ground KBr

was used to obtain a reference spectrum. The discs were analyzed by FTIR (model

VERTEX-70, made by Bruker in Germany), and the spectra were recorded in the range of 400 to 4000 cm-1 at a resolution of 4 cm-1.

Nuclear magnetic resonance (NMR). The

13

C NMR spectra of coal samples

were obtained on a Bruker Avance III 400 spectrometer. All experiments were run in

double-resonance probe heads using 4-mm sample rotors. Semi-quantitative

compositional information was obtained with good sensitivity by using a

13

C

CP/MAS NMR in conjunction with the total sideband suppression (TOSS) technique

(MAS=4 kHz, contact time=1 ms, recycle delay=1 s).

Raman spectroscopy (Raman). The Raman spectra of the coal samples were

recorded with a Bruker Sentera Raman spectrometer. The operating conditions of the


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Raman spectrometer include a wavelength of 532 nm, a power of 5 mV, an integration time of 2 s, and a resolution of 9-18 cm-1. Each measurement was repeated 10 times.

3. RESULTS AND DISCUSSION

3.1 Raman spectra The coal samples all show two characteristic peaks that appear at ~1360 cm-1 (D peak) and ~1590 cm-1 (G peak) in the first-order Raman spectra. The G band is higher

than the D2 band, implying that graphite-like structures dominate over turbostratic structures.21 The D band position wave numbers of brittle deformed coal samples N20 and N27 are 1345 cm-1 and 1342 cm-1, which is smaller than those of ductile

deformed coal. This was considered to result from the resonance effect involved in Raman scattering from carbons.22 The G band of the coal is a result of aromatic ring quadrant breathing with contributions from graphitic structures.23 Many disordered

and amorphous carbons are also indicated by additional peaks appearing at ~1180 (D4), ~1500 (D3), and ~1620 cm-1 (D2), which are related to sp3 carbon, amorphous

carbon, and disordered carbon, respectively. The D3 band was further studied as a way to evaluate the amorphous carbon in the coal samples.24


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Table 2 Parameters obtained from Raman spectra of the coals.

Coal types

Sample

ID1/IG

D1 band

D1 FWHM

G FWHM

center cm-1

cm-1

cm-1

AD1/AG

Brittle deformation

N20

0.68

0.88

1345

147.74

113.65

Ductile deformation

N24-2

0.76

1.23

1346

180.49

111.59

Ductile deformation

N25

0.89

1.85

1353

174.90

83.80

Ductile deformation

N26-1

0.73

1.18

1353

173.40

107.86

Brittle deformation

N27

0.83

1.40

1342

150.87

89.40

Primary structure

N32

0.73

1.16

1350

149.33

94.02

Note: height ratio of D1 to G (ID1/IG); area ratio of D1 to G (AD1/AG); D1 full width at

half-maximum (D1 FWHM); G full width at half-maximum (G FWHM).

To interpret the Raman spectra of coal, the spectral information is typically fitted

with curves to extract quantitative spectral features using Gaussian distributions

(Figure 4). The height and area ratios of the D to the G bands and the widths and

positions in the D and G bands have been used to describe the carbon structural


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information. The intensity ratio of the D and G band (ID/IG) can be used to indicate the crystallite dimension.24

Figure 4 Results of fitting curves to Raman spectra from coal samples.

(a-N20, b-N24-2, c-N25, d-N26-1, e-N27, f-N32)

Although there is no appreciable variation in the positions of the D and G bands

among the coal samples, there are two patterns observed among different degrees of

deformation. One pattern is that the D1 FWHM parameter of the ductile deformation


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coal (N24-2, N25, N26-1) is greater than that of the brittle deformation coal (N20,

N27) as a result of its high content of defects in the graphitic structures, as observed

in Figure 5. In this case, the D1 FWHM is in good agreement with the ductile

deformation, even though the differences relative to the G FWHM are not statistically

significant. One possible reason is that, during the process of ductile deformation,

mechanical energy transforms into strain energy, and the strain energy is released via

the rotation of C−C bonds, which produces defects. Then, the stress may finally

fragment the macromolecular structure by breaking the defect structure and old bonds and forming new bonds.25-26 Another possible reason is that the ID/IG ratio in coal

samples N25 and N27 (0.8861 and 0.8298, respectively; Table 2). An increase in ID/IG

indicates the growth of basic graphene structural units (BSUs). Ductile deformation leads to changes in carbon structure.22


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Figure 5 Comparison of Raman parameters: (a) the height ratio of D1 to G (ID1/IG); (b)

the area ratio of D1 to G (AD1/AG); (c) D1 FWHM; and (d) G FWHM.

3.2 FTIR

Figure 6 shows the FTIR spectra of five coal samples which were determined to

have undergone deformation (samples N20, N24-2, N25, N26-1, N27) and one

primary structure coal (i.e. coal with no deformation; sample N32). These samples

displayed similar patterns as a result of the small organic functional groups in the coal

structure and because the coal samples were collected from one tectonic position. The broad band near 3400 cm-1 in all spectra, which indicates that the ductile deformation

coal has oxygen-containing functional groups and secondary bonding within its


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structure, the result of O-H and N-H stretching. The peak at 3040 cm-1 is related to aromatic sp2-bonded C−H stretching. These coals contain aliphatic structures in the region of 3000−2800 cm−1,

indicating strong aliphatic C-H stretching. The coal samples exhibited bands at 2920 cm-1 and 2850 cm-1, which could be assigned to symmetric stretching vibrations and asymmetric stretching vibrations of -CH2- methylene groups, respectively.23 In addition, the intensity of the peak at 2920 cm-1 was greater than the intensity of the peak at 2850 cm-1, indicating the presence of long aliphatic chains in the ductile

deformation coal and the primary structure coal. As observed in the FTIR of ductile deformation coal, the absorption at 1700-1735 cm-1 is very weak, which may be due

to carbonyl (C=O) groups. Compared with the carbonyl group intensity of ductile

deformation coal, the primary structure coal represented by sample N32 exhibits a

higher absorption band intensity. The coal sample N32 has probably more

oxygen-containing functional groups in its structure because of its low degree of

coalification, while ductile deformation coals have less oxygen-containing functional

groups as a result of ductile deformation. The peak related to the stretching vibration


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of the aromatic C=C band is of moderate intensity and occurs at approximately 1620 cm-1. The absorption of the asymmetric bending of -CH3 and symmetric bending of -CH2- is centered at 1444 cm-1 decrease with temperature. Bands were observed between 900 and 700 cm-1 regions in all the samples, which may be attributable to the

aromatic C-H out-of-plane bending vibrations.

Figure 6 FTIR spectra of coal samples.

Figure 7 XRD patterns of coal samples.

The coal samples that have experienced strong ductile deformation, N24-2, N25

and N26-1, which were all collected close to the fault, display less aromatic C=C

band stretching vibration. Noticeable differences were observed in the intensity of


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bands of functional groups corresponding to O-H stretching and absorption because of

carbonyl stretching –C=O (Figure 6). Furthermore, the primary structure coal sample

(N32) and the brittle deformation coal samples (N20, N27) show greater aromatic

C=C band stretching than the ductile deformation coals. As evidenced by the higher

intensity of the C=C band in samples N32, N20, and N27, ductile deformation can

lead to the breaking of the C=C functional groups. The greater intensity of the C=O

band in sample N32 than in samples N20 and N27 indicates that brittle deformation

must cause the decrease in oxygen-containing functional groups. The spectral region at 3000−2800 cm−1, which represents aliphatic C−H stretching, was curve-fitted to derive A2920 cm−1 and A2950 cm−1 for calculation of the CH2/CH3 ratio. The spectral region at 1800−1000 cm−1, representing C＝O and C ＝C bending, was then curve-fitted to derive A1800−1650 cm−1, A1650−1520 cm−1, A3000−2800 cm−1, and A1800−1520 cm−1 to calculate the A factor (3000-2800 cm-1/3000−2800 cm-1 + 1650−1520 cm-1) and C factor (1800−1650 cm-1/1800−1650 cm-1 + 1650−1520 cm-1), which represents the intensity of aliphatic relative to

aromatic peaks, as well as the Al/C＝C, Al/OX, Aar/Aal, and C＝O/C＝C ratios. The


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values of these structural parameters are listed in Table 3. The Aar/Aal ratio of sample

N32 (0.95) is less than that of the other coal samples, which indicate that sample N32

contains a smaller amount of the aromatic C-H functional groups. The higher

C=O/C=C ratio reflects the high C=O content in sample N32. Additionally, the values

of CH2/CH3 in samples N20 and N27 are very high, up to 7.65 and 12.89, respectively,

which reflect the fact that the content of aliphatic chains in brittle deformation coals is

much greater than that in ductile deformation coals.

Table 3 FTIR indexes deduced from FTIR spectra.

Sample

A factor

C factor

CH2/CH3

Aar/Aal

Al/OX

Al/C=C

C=O/C=C

N20

0.43

0.02

7.65

1.33

0.74

0.75

0.02

N24-2

0.49

0.06

3.28

1.06

0.89

0.94

0.06

N25

0.50

0.04

4.91

1.01

0.95

0.99

0.04

N26-1

0.44

0.02

7.17

1.29

0.76

0.77

0.02

N27

0.46

0.02

12.89

1.17

0.84

0.86

0.02

N32

0.51

0.12

3.65

0.95

0.93

1.06

0.14

3.3 XRD


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Information on interlayer spacing, stacking height and distance between

crystallite units can be investigated by XRD with a careful assignment of the peak positions, FWHM values, and intensities of the diffraction peaks.27-28 Figure 7 shows

the XRD curves with peak characteristics for the six coal samples. The peak (002) is

located at a 2θ equal to 25° reflection and is called the π-band, and the (10) band was near 40°.23 The intensities of the (002) peaks for the six coal samples are similar. The

spectra exhibit peaks at 20°, 35°, and 38°. In contrast, peaks at ~30° appeared in

samples N25, 26-1, and N27. This is possibly the result of deformation, as reflected in

the coal samples. In addition, sample N24-2 displays two new diffraction peaks at 47°

and 48°. The (002) band is correlated with the stacking of aromatic layers. Theoretically, the (002) band is symmetric and sharp.29

The interlayer spacing of aromatic ring layers (d002) and average stacking height

(Lc) values of the coals are presented in Table 4. For these coal samples, the d002

values varied from 3.42 to 3.55, indicating a low degree of ordered crystallite units

with respect to pure graphite (3.36-3.37). The Lc values of the tectonically deformed

coal samples and primary structure coal samples ranged from 20.35 to 26.52 and


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27.14, respectively. Lc is a good indicator of the graphitic structure of the samples and

shows that the coal sample with primary structure (N32) is more ordered compared to

the tectonically deformed coals.

Table 4 XRD indexes deduced from XRD spectra.

Sample

N20

N24-2

N25

N26-1

N27

N32

2θ002 (°)

25.20

25.30

25.06

25.17

25.02

25.38

d002 (angstrom)=λ/2sinθ002

3.53

3.52

3.55

3.42

3.42

3.51

Lc (angstrom)=0.9λ/β002cosθ002

26.52

20.35

20.35

20.38

20.38

27.14

3.4 NMR

The NMR spectra of the coal samples measured in this study are shown in Figure

8. Distinct aromatic and aliphatic groups could be identified in these spectra. Shifts of

0−90 ppm are assigned to aliphatic carbon, shifts of 0-20 ppm are assigned to CH3 or

quaternary C, shifts of 25-60 ppm are assigned to CH or CH2, shifts of 60-90 ppm are

assigned to aliphatic C bonded to oxygen, and shifts of 90−160 ppm are assigned to

aromatic carbon. The latter range includes shifts of 130-135 ppm for bridgehead

aromatics, 135-150 ppm for alkylated aromatic C, 150-165 ppm for aromatic C−O,


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90-130 ppm for protonated aromatic C−H, and 130-165 ppm for non-protonated aromatic C−C.30 There is no remarkable difference in the patterns of the NMR spectra

for the coal samples studied (Figure 8). The peaks at 154 ppm, 140 ppm, and 15 ppm

are shoulder peaks, and the 15 ppm peaks are not obvious in samples N24-2, N25,

N26-1, and N27. The aromaticity value of samples N20, N24-2, N25, N26-1, N27,

and N32 is approximately 0.67 (Table 5). The contents of alkylated aromatic carbons (faS) were higher than those of bridged aromatic carbon (faB) in the TDCs, indicating

that the contents of monocyclic aromatic structures were greater than those of polycyclic aromatic structures.31 The ratio of aromatic bridge carbon to aromatic

peripheral carbon (XBP) of naphthalene, which has two condensed aromatic rings, is

0.25. The XBP values of the TDCs were less than 0.24, indicating two rings.


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Figure 8 13C NMR spectra of the six coal samples. Table 5 13C CP/MAS solid state NMR structural parameters of the coal samples.

Sample

N20

N24-2

N25

N26-1

N27

N32

Aromatic carbon rate, fa

0.679

0.675

0.643

0.683

0.670

0.656

Aliphatic carbon rate, fal

0.321

0.325

0.357

0.317

0.330

0.344

0.08

0.08

0.09

0.09

0.09

0.10

0.22

0.22

0.24

0.21

0.22

0.24

Protonated and aromatic carbon rate,

fal*

Methylene rate, falH


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Energy & Fuels

Oxy-aliphatic carbon rate, falO

0.02

0.02

0.02

0.02

0.02

0.01

Protonated and aromatic carbon, faH

0.37

0.37

0.36

0.37

0.38

0.35

Bridged aromatic carbon rate, faB

0.12

0.13

0.12

0.13

0.11

0.13

Alkylated-substituted carbon rate, faS

0.15

0.14

0.14

0.15

0.15

0.14

0.04

0.04

0.03

0.03

0.04

0.03

0.16

0.39

0.20

0.03

0.24

0.48

0.68

0.68

0.67

0.67

0.67

0.69

0.21

0.23

0.22

0.23

0.19

0.24

2.08

2.40

2.63

2.09

2.24

2.54

Aromatic carbon bonded to hydroxyl

or ether oxygen rate, faP

Carbonyl carbon rate, faC (10-2)

Methylene percentage of aliphatic

carbon, Ai

Aromatic cluster size, XBP 

Average carbon number of the

methylene chain, Cn

3.5 Deformation mechanisms of coal

Chemical differences between ductile and brittle deformation. Both of the

two kinds of deformation coals investigated in this study were in the same coal rank

and had many oxygen functional groups, side chains, bridge bonds and hydrogen


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bonds. Figure 9 shows that the Aar/Aal value of sample N20 is highest, and sample

N25 (located in the shear zone) has the maximum ID1/IG value. The former implies

that coal sample N20 contains more aromatic carbon structures, while the latter means

there are more defects in coal sample N25. Normally, the change in Lc in the TDCs

displaying ductile deformation is greater than the TDCs showing brittle deformation.17 Brittle deformation could alter the macromolecular structure of coal and can enhance coal metamorphism to a certain degree.32

Figure 9 Structure parameters of six samples.

Ductile deformed coals are thought to have formed through point defect

dislocations of single carbon atoms. Brittlely deformed coals are caused by line defects and stacking faults involving many carbon atoms.33 The defect types seen in


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the molecules of carbon materials (e.g., graphene) include one-dimensional lines of defects and point defects.34 The difference between ductile and brittle deformation is

tectonic behavior, which is influenced by stress type, stress direction, temperature and

strain rate. It has been shown by the Raman results that the tectonic deformation is

probably a result of macromolecular defects. The defects have been strongly

influenced by factors such as stress, time, temperature and Td (the minimum energy

which has to be transferred to a carbon atom to cause it to leave its lattice position

without immediate recombination with the vacancy). Some bonds have been stretched,

while others may have been compressed.

As noted in Table 3, the content of C=O functional groups in TDCs is higher

than in the coal with primary structure. It has been reported that one mode of CO

generation is through the breaking of chemical bonds (one carbonyl) within the molecular structures of coal.26 Hence, it can be concluded that defects may be easily

formed by the breaking of C=O bonds in coal macromolecules, given that the

maximum energy of broken C=O bonds is less. Strain energy can promote

deformation and breakage of the coal’s molecular units.


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Mechanism model of coal deformations. As illustrated in Figure 10, brittle coal

structure, which shows continuous bending deformation characteristics, may be

broken with development of some macropore and mesopores, However, the ductile

deformation of coal structure is a process of plastic deformation acting on successive

bedding. In summary, ductile deformation is related to the growth of defects as a

result of shear stress, while brittle deformation is related to a series of bonds broken

because of tensile stress. Thus, tectonic deformation plays an important role in the

ultrastructure of coal. Because tectonic deformation not only transforms the

macrostructure of coal and enhances the degree of coal deformation, but also slightly

improves the degree of coal metamorphism and significantly affects the macromolecular structures of coal.18 The secondary structural defect can make ductile

deformed coal.


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Figure 10 Model of the mechanism of coal deformation under different stresses.

4. CONCLUSIONS

In this study, the deformation mechanism of coal from one tectonic structure was

investigated with Raman, FTIR, XRD, and NMR techniques. The results provide an

understanding of coal structure, in particular the evolution of its molecular structure.

Several important macromolecular structural parameters of coal have been analyzed,

which can be correlated to the ductile deformation of the coal.

According to the Raman measurements, the D1 FWHM parameter of ductile

deformation coal is greater than that of brittle deformation coal because of the high

content of defects in ductile deformation coal. Coal sample N25 (located in the shear

zone) exhibits the maximum ID1/IG value, implying the presence of a greater number


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of defects in the coal. The high shearing stress evidently leads to strong ductile

deformation, which further results in the assessed changes in the carbon structure. In

contrast, the brittle deformation of coal is related to a series bonds broken as a result

of tensile stress.

As further revealed in this study, the difference in the ratio of the -CH2- and

-CH3 functional groups indicates the content of aliphatic chains in the brittle

deformed coals is much greater than in the ductile deformed coals. There is no

remarkable difference in the total aromaticity among these coal samples. Defects may

cause atoms to migrate easily within macromolecules, given that the maximum

energy of broken C=O bonds is less, which consequently influences the content of the

C=O functional group in TDCs.

Finally, a model has been developed as an outcome of this work, which suggests

that ductile deformation is more likely to be related to point defects, and brittle

deformation usually results from line defects.

AUTHOR INFORMATION Corresponding Author


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*Wu LI. E-mail: [email protected]; Bo Jiang. E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support of the National Natural

Science Foundation of China (Grants No. 41430317, No. 41672147 and No.

41472135), the Natural Science Foundation of Jiangsu Province (Grant No.

BK20160243) and the China Postdoctoral Science Foundation (Grant No.

2015M581878).

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Figure 1 Structural geology and distribution of mining areas in the Qinan Coal Mine, Huaibei coalfield. Shaded area in the inset is the larger area shown to the right. Note the position of the No. 6125 coal face referred to in the text.


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Figure 2 The distribution of sample collection locations.


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Figure 3 Photographs of tectonically deformed coal.


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Figure 4 Results of fitting curves to Raman spectra from coal samples. (a-N20, b-N24-2, c-N25, d-N26-1, e-N27, f-N32)


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Figure 5 Comparison of Raman parameters: (a) the height ratio of D1 to G (ID1/IG); (b) the area ratio of D1 to G (AD1/AG); (c) D1 FWHM; and (d) G FWHM.


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Figure 6 FTIR spectra of coal samples.


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Figure 7 XRD patterns of coal samples.


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Figure 8 13C NMR spectra of the six coal samples.


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Figure 9 Structure parameters of six samples.


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Figure 10 Model of the mechanism of coal deformation under different stresses.


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Characterization of the Chemical Structure of Tectonically Deformed

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