Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A

energies

Article Macromolecular Structure Controlling Micro Mechanical Properties of Vitrinite and Inertinite in Tectonically Deformed Coals—A Case Study in Fengfeng Coal Mine of Taihangshan Fault Zone (North China)

Anmin Wang 1,2,*, Daiyong Cao 1, Yingchun Wei 1 and Zhifei Liu 1 1 Department of Energy Geology, College of Geoscience & Surveying Engineering, China University of Mining & Technology, Beijing 100083, China; [email protected] (D.C.); [email protected] (Y.W.); [email protected] (Z.L.) 2 Shandong Key Laboratory of Depositional Mineralization & Sedimentary Mineral, Shandong University of Science and Technology, Qingdao 266510, China * Correspondence: [email protected]

Received: 13 November 2020; Accepted: 10 December 2020; Published: 15 December 2020

Abstract: In order to study the evolution of the mechanical properties and macromolecular structures in different macerals of tectonically deformed coal (TDC), vitrinite and inertinite samples were handpicked from six block TDCs in the same coal seam with an increasing deformation degree (unaltered, cataclastic, porphyroclast, scaly and powdery coal). The micro mechanical properties were tested by the nanoindentation experiment and the macromolecular structures were measured using 13C nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). The results show that the range of hardness and elastic modulus of inertinite is 0.373–1.517 GPa and 4.339–12.158 GPa, respectively, which is significantly higher than that of vitrinite with values of 0.278–0.456 GPa and 4.857–7.810 GPa, respectively. From unaltered coal to powdery coal, the hardness of vitrinite and inertinite gradually decreases, with the difference between these macerals becomes smaller and the elastic modulus of vitrinite shows an increasing trend, while that of inertinite was more variable. Both the NMR and FITR results reveal that the macromolecular structure of inertinite has similar structural transitions as vitrinite. As the degree of deformation increases, the aliphatic side chains become shorter and the aromaticity is increasing. Macromolecular alterations caused by tectonic stress is expected to produce defects in the TDCs, therefore there should be more interspacing among the macromolecular groups for the extrusion of macromolecules caused by the indenter of the nanoindentation experiment, thereby reducing the hardness. The elastic modulus of coal is believed to be related to intermolecular forces, which are positively correlated to the dipole moment. By calculating the dipole moments of the typical aromatic molecular structures with aliphatic side chains, the detachment of the aliphatic side chains and the growth of benzene rings can both increase the dipole moment, which can promote elastic modulus. In addition, the increasing number of benzene rings can create more π-π bonds between the molecules, which can lead to an increase in the intermolecular forces, further increasing the elastic modulus.

Keywords: tectonically deformed coal; vitrinite; inertinite; mechanical property; macromolecular structure

1. Introduction The physical and chemical properties of tectonically deformed coal (TDC) are signiﬁcantly diﬀerent from those of unaltered coal [1]. Coal basins in China have generally experienced intensive tectonic

Energies 2020, 13, 6618; doi:10.3390/en13246618 www.mdpi.com/journal/energies Energies 2020, 13, 6618 2 of 23 movements [2,3]; therefore, TDCs are widespread, hindering the safe and effective exploitation of coal resources and coalbed methane [4–7]. Therefore, TDC has been studied extensively [8–10]. The traditional identification of the deformation degree of TDC is usually described qualitatively [1,11–13], because TDC is extremely soft and fragile. Thus, traditional testing methods of mechanical properties, such as triaxial or uniaxial mechanics, are not suitable for TDC, which can only be identified by macroscopic characteristics. TDC can be classified in different types with increasing deformation, such as cataclastic, porphyroclast, scaly, powdery, heterogeneous structure and mylonitic coal [14]. Godyńandand Kožušníková [15] studied the microhardness of TDC using the Vickers hardness test in Silesian Coal Basin, Poland and found the lowest values in cataclastic coal and the highest values in unalerted coal, which was a valuable discovery for TDC research. Therefore, there should be more quantitative descriptions for studies of TDC with different degrees of deformation. For many years, the TDC samples were tested as a whole to analyze the changes in the physical and chemical structures [11,16]; however, few studies have measured the vitrinite and inertinite components separately, which have different physical and chemical characteristics [17–19]. Therefore, the deformation modes of vitrinite and inertinite vary considerably under the action of tectonic stress, thus, the physical and chemical properties of vitrinite and inertinite consequentially would undergo distinct changes. More and more researchers have noted that the coal maceral has influenced the mechanics of the coal. Pan et al. [20] revealed that the coal strength and Young’s modulus are strongly related to coal rank and composition by comparing coals of different vitrinite reflectances and compositions. Hou et al. [21] determined that the hardness and elastic modulus are in the order of exinite < vitrinite < inertinite by employing depth-sensing nanoindentation. Kožušníková [22] revealed that the Vickers microhardness and elastic modulus of coal sample with higher coalification were lower than of coal sample with lower coalification and also, Godyńandet al. [23] found that micro-hardness value decreases with metamorphism degree using Vickers hardness test in Silesian Coal Basin, Poland. Fender et al. [24] suggested that thermally immature liptinite macerals have a lower modal modulus than the inertinites and the modal Young’s modulus of all macerals increases with maturity, based on atomic force microscopy. Therefore, the mechanical properties of each maceral in coal are different, so the study of the mechanical evolution of different macerals has important scientific significance for improving the understanding of TDC. The macromolecular evolution of TDC has gained great interest recently. Ju et al. [25] found that with the increasing degree of coal deformation, the hydrogen-enriched degree and oxygen-enriched degree decrease, while the degree of ring condensation increases. Cao et al. [8] demonstrated that tectonic stress affects the chemical structure of the coal through stress degradation and stress polycondensation; stress degradation is a process in which some chemical bonds of low activation energy are broken up under tectonic stress and the stress polycondensation indicates that the condensed aromatic nuclei tend to be arranged in parallel under tectonic stress. Song et al. [26] revealed that ductile deformation can promote the loss of aliphatic carbons and the degree of macromolecular alignment, while brittle deformation promoted transitions in the aliphatic structure. Liu and Jiang [13] thought that shear tectonic stress caused more stacked aromatic layers and a lower interlayer space. Therefore, there are multiple viewpoints; however, there has been little discussion regarding the macromolecular evolution of a single maceral in TDC. There are at least three unresolved challenges: (1) a quantitative description of TDC classification; (2) the variations in the evolution of the mechanical properties of vitrinite and inertinite in TDC; and (3) how inertinite reacts to tectonic stress. In this study, TDC samples with different degrees of deformation were collected from the same coal seam (No. 2 coal seam) in the Fengfeng coal mine of the Taihangshan fault zone, North China and the vitrinite and inertinite were separated by hand. The mechanical properties of vitrinite and inertinite with different degrees of deformation can be quantitatively described using nanoindentation. In addition, 13C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy (FTIR) were employed to obtain the macromolecular structural evolution of the vitrinite Energies 2020, 13, 6618 3 of 23 Energies 2020, 13, x FOR PEER REVIEW 3 of 21 andevolution inertinite. of the Finally, vitrinite the and controlling inertinite. eff Finally,ect of the the macromolecular controlling effect structural of the macromolecular evolution on the structural changes inevolution the mechanical on the changes properties in arethe discussed.mechanical properties are discussed.

2.2. GeologicalGeological BackgroundBackground TheThe FengfengFengfeng coalcoal mine area is located in in Taihangshan Taihangshan fault fault zone zone [3] [3,], North North China, China, belonging belonging to tonorthern northern China China coal coal occurrence occurrence area area (Figure (Figure 1).1 The). The coal coal-bearing-bearing strata strata in the in theFengfeng Fengfeng mining mining area areais the is Late the Late Paleozoic Paleozoic Carboniferous Carboniferous-Permian-Permian with with the theNo. No. 2 coal 2 coal seam seam of Shanxi of Shanxi Formation Formation being being the themineable mineable coal coal seam seam in the in thewhole whole area area and andthe thecoal coal-bearing-bearing strata strata are unconformable are unconformable contacted contacted with withthe underlying the underlying Ordovician Ordovician limestone limestone strata. strata. In the In western the western part of part the of Fengfeng the Fengfeng mining mining area, area,there thereare magmatic are magmatic rocks develop rocks developeded in the Late in the Jurassic Late Jurassic period. Most period. of the Most magmatic of the magmatic rocks are ultrabasic, rocks are ultrabasic,basic, neutral basic, and neutral partial and alkaline partial rockalkalines [27] rocks. The [27 Fengfeng]. The Fengfeng mining mining area has area undergone has undergone strong strongdeformation deformation since the since Neogene the Neogene and mainly and mainly developed developed NE-SW NE-SW structural structural lines lines[27], includi [27], includingng folds foldsand faults,and faults, which which also also made made the the coal coal seam seam more more severely severely deformed. deformed. These strongly strongly deformed deformed structuresstructures have have seriously seriously aﬀ ected affected the exploration the exploration and development and development of coalbed of coalbedmethane methane in the Fengfeng in the miningFengfeng area mining [28], as area well [28], as theas well safety as productionthe safety production of coal mines. of coal mines.

Figure 1. Geological structure of Fengfeng coal mine area with tectonically deformed coal (TDC) samples. Figure 1. Geological structure of Fengfeng coal mine area with tectonically deformed coal (TDC) 3. Samplessamples and. Methods

3.1.3. Samples Sample Preparation and Methods According to the classification of TDCs by Cao et al. [14] a total of 6 TDC samples, numbered 3.1. Sample Preparation F1-6, including five types of TDCs: unaltered, cataclastic, porphyroclast, scaly and powdery coal (FigureAccording1, Table1), to were the classification collected from of the TDCs No. by 2 coal Cao seam et al. of[14] Fengfeng a total of coal 6 TDC mine samples, of Taihangshan numbered fault F1- zone,6, including North China,five types to ensure of TDCs: that unaltered, the coal rank cataclastic, and maceral porphyroclast, composition scaly of and all thepowdery samples coal were (Figure the same.1, Table The 1), average were collected vitrinite from reflectance the No. (2R coalo) of seam the No. of Fengfeng 2 coal seam coal was mine 0.99% of Taihangshan with a mean fault moisture zone, contentNorth China, of 0.54%, to ensure mean ashthat yieldthe coal of 11.08%rank and and maceral mean volatilecomposition matter of yieldall the of samples 31.35%. were Vitrinite the same. and inertiniteThe average were vitrinite hand sorted reflectance from ( theRo) vitrainof the No. and 2 fusain coal seam in the was collected 0.99% with samples, a mean respectively, moisture content that is, coalof 0.54%, samples mean were ash crushed yield of into 11.08% small and particles mean in volatile size and matter were putyield on of a 31.35%. white paper Vitrinite under and a lamp inertinite and vitrainwere hand particles sorted glistened from butthe fusain vitrain particles and fusain remained in the black. collected So, vitrain samples, particles respectively, and fusain that particles is, coal weresamples separated were crushed with tweezers. into small Each particles sample in wassize strippedand were into put twoon a samples, white paper viz. under vitrinite a lamp (-V) andand inertinitevitrain particles (-I), providing glistened a total but fusain of 12 samples particles (Table remained1). Each black. stripped So, vitrain sample particles was identified and fusain by particles optical were separated with tweezers. Each sample was stripped into two samples, viz. vitrinite (-V) and inertinite (-I), providing a total of 12 samples (Table 1). Each stripped sample was identified by optical

Energies 20202020, 1313, 6618x FOR PEER REVIEW 4 4of of 21 23 microscopy to ensure the correctness of the sample components until the corresponding component ofmicroscopy each sample to ensure reached the more correctness than 90%. of the sample components until the corresponding component of each sample reached more than 90%. Table 1. Sample information.

Table 1. Sample information.Petrographic Analysis (%) Sample Coal Type Vitrinite Inertinite Other Groups Petrographic Analysis (%) F1-V (Vitrinite)Sample Coal Type 93.50 2.10 4.40 F1 Unaltered Vitrinite Inertinite Other Groups F1-I (Inertinite) 1.80 91.60 6.60 F1-V (Vitrinite) 93.50 2.10 4.40 F1F2-V (Vitrinite) Unaltered 95.10 2.30 2.60 F2 F1-I (Inertinite)Cataclastic 1.80 91.60 6.60 F2-I (Inertinite)F2-V (Vitrinite) 95.104.60 2.3090.60 2.60 4.80 F2F3-V (Vitrinite) Cataclastic 94.80 3.30 1.90 F3 F2-I (Inertinite)Porphyroclast 4.60 90.60 4.80 F3-I (Inertinite)F3-V (Vitrinite) 94.803.40 3.3092.60 1.90 4.00 F3 Porphyroclast F4-V (Vitrinite)F3-I (Inertinite) 93.80 3.40 92.605.10 4.00 1.10 F4 F4-V (Vitrinite) Porphyroclast 93.80 5.10 1.10 F4F4-I (Inertinite) Porphyroclast 3.20 94.70 2.10 F4-I (Inertinite) 3.20 94.70 2.10 F5-V (Vitrinite)F5-V (Vitrinite) 96.6096.60 1.201.20 2.20 2.20 F5 F5 ScalyScaly F5-I (Inertinite)F5-I (Inertinite) 2.402.40 94.9094.90 2.70 2.70 F6-V (Vitrinite)F6-V (Vitrinite) 94.3094.30 2.802.80 2.90 2.90 F6 F6 PowderyPowdery F6-I (Inertinite)F6-I (Inertinite) 3.503.50 95.3095.30 1.20 1.20

3.2. Nanoindentation Nanoindentation E Experimentxperiment Nanoindentation isis aa technology technology to to measure measure the the mechanical mechanical properties properties of samplesof samples at microscale at microscale [29] [29]and and the the indentation indentation point point can can be be observed observed and and selected selected under under a a microscopemicroscope with higher higher accuracy. accuracy. As shown shown in in Figure Figure 2,2, an an indenter indenter is is pressed pressed into into the the coal coal sample, sample, forming forming an indentation an indentation profile proﬁle that matchesthat matches the shape the shape of the of the indenter. indenter. Under Under the the maximum maximum pressure pressure load, load, the the distance distance between between the bottom of the indenter and thethe initialinitial surfacesurface isis thethe maximummaximum depthdepth hhmaxmax,, while the displacementdisplacement between thethe bottombottom andand sunken sunken surface surface of of the the coal coal sample sample is theis the contact contact depth depthhc. Thehc. The coal coal surface surface will willnot returnnot return to its to original its origi positionnal position after after unloading. unloading. At this At time, this time, the distance the distance between between the deepest the deepest point pointof the of sunken the sunken surface surface and the and initial the surfaceinitial surface is deﬁned is defined as hf. The as pressure-loadinghf. The pressure-/loading/unloadingunloading process processof the experiment of the experiment was recorded was by recorded a sensor andby a the sensor load-displacement and the load curve-displacement of the nanoindentation curve of the wasnanoindentation obtained (Figure was3 obtained). (Figure 3).

Figure 2. Schematic and parameters of nanoindentation. h max isis the the maximum maximum depth; depth; hc isis the the contact contact depth; P is the loading pressure.

Energies 2020, 13, 6618 5 of 23 Energies 2020, 13, x FOR PEER REVIEW 5 of 21

Figure 3. TypicalTypical load load-displacement-displacement curve. hff isis the the distance distance between between the the deepest deepest point point of of the the sunken sunken P h surface and the initial surface; Pmax isis the the maximum maximum loading loading pressure; pressure; h isis the the depth. depth.

Because the TDC is extremely soft, it must be conglutinated by epoxy resin so that the TDC can Because the TDC is extremely soft, it must be conglutinated by epoxy resin so that the TDC can be tested, which is a common approach by many researchers because the “side-e ect” of glue to the be tested, which is a common approach by many researchers because the “side-effect”ff of glue to the measurement results can be negligible [21]. The experimental equipment is a nanoindentation tester measurement results can be negligible [21]. The experimental equipment is a nanoindentation tester µ produced by Swiss CSM Instrument Co., Co., Ltd Ltd,, Peseux Peseux,, Switzerland Switzerland,, with with a a minimum minimum load load of of 25 25 μN,N, µ a maximum load of 350 mN and a maximum indentation depth of 3100 μm,m, with a displacement resolution of 0.0004 nm. In In this this study, study, the maximum maximum pressure pressure load was set to 100 mN to to ensure ensure there could be be indentation indentation in in the the coal coal sur surfaceface after after unloading unloading based based on onthe the laboratory’ laboratory’ advice. advice. Actually, Actually, the hardnessthe hardness and and elastic elastic modulus modulus do donot not change change with with the the maximum maximum pressure pressure which which has has been been confirmed confirmed by Sun et al. [30] [30],, so the maximum pressure load setting could could not affect affect the testing results as the hardness and elastic modulus are the only two results we want. The The indentation indentation points of the vitrinite vitrinite samples werewere specifiedspecified in in telocollinite telocollinite and and those those of theof the inertinite inertinite samples samples were were placed placed in fusinite. in fusinite. Each Eachsample sample was subjected was subjected to 5 nanoindentation to 5 nanoindentatio experimentsn experiments to ensure to the ensure accuracy the of accuracy the experiment of the experimentaccording to according Sun et al.’s to researchSun et al.’s [30 ].research [30]. TThen,hen, the hardness and modulus of the samples could be calculated using the Oliver-PharrOliver-Pharr method [31]: [31]: The unloading curve can be fitted fitted with a power function as:as:

mm PP =α(()h h hf ) ,, (1) − f (1) where α and m are the fittedfitted parameters andand hhff is the indentation depthdepth afterafter unloading,unloading, nm.nm. The contact stiffnessstiffness S can be obtained by the differentialdifferential computing of Equation (1) at the maximum load: dP m 1 S = = αm(hmax h f ) − , (2) dhdPmax − m1 S  m() hmax  hf , (2) where hmax is the maximum depth (nm).dh Themax contact depth hc (nm) can be calculated as: where hmax is the maximum depth (nm). The contact depthεPmax hc (nm) can be calculated as: hc = hmax , (3) −  PS h h  max , (3) where Pmax is the maximum load, µN, and εc is amax constantS referring to the shape of the indenter. For the Berkovich indenter used in this experiment, the value is 0.75. The contact area A can be calculated by whereEquation Pmax (4): is the maximum load, μN, and ε is a constant referring to the shape of the indenter. For the Berkovich indenter used in this experiment, the value2 is 0.75. The contact area A can be calculated A = 24.5hc , (4) by Equation (4):

2 A 24.5 hc , (4)

Then, the hardness (H, GPa) of the samples can be obtained by Equation (5):

Energies 2020, 13, 6618 6 of 23

Then, the hardness (H, GPa) of the samples can be obtained by Equation (5):

P H = max , (5) A The elastic modulus can be calculated using Equations (6) and (7):

2 1 1 v2 1 v = − + − i , (6) Er E Ei

√π S Er = , (7) 2β √A where E is the elastic modulus (GPa). v is Poisson’s ratio, which is 0.3 for the coal in this study. Ei and vi are the elastic modulus and Poisson’s ratio of the indenter, respectively, for the Berkovich indenter in this experiment, Ei = 1141 GPa and vi =0.07. β is a constant related to the indenter’s shape, for the Berkovich indenter, β = 1.034.

3.3. 13C NMR Experiment Samples were crushed below 80 mesh and were demineralized by hydrochloric acid and hydrofluoric acid to exclude the mineral effect on the test results and the 13C solid-state nuclear magnetic resonance experiment was carried out using the Agilent 600 M equipment (Agilent Technologies Co. Ltd., Palo Alto, CA, USA). The instrument parameters were as follows: double-resonance probe head, ZrO2 rotor with outer diameter of 4 mm and magic angle spinning speed of 8 KHz. The experimental data were recorded by the probe on an Agilent DSX-300 (Agilent Technologies Co. Ltd., Palo Alto, CA, USA) spectrometer at ambient temperature. The radio frequency field strength of the 13C was set to 600 MHz and the cross-polarization contact time was 2 ms.

3.4. FTIR Experiment The samples were crushed to below 80 mesh and were demineralized by hydrochloric acid and hydrofluoric. After drying to remove the water, 1–2 mg of each sample powder was mixed with 200 mg potassium bromide. After being tableted by a tablet machine, the samples were placed into infrared 1 spectrometer chamber to be scanned with a spectral resolution of 4 cm− The Fourier transform infrared spectroscopy (FTIR) curves were obtained using a computer and were processed by the OMINC software. According to previous studies on the peak attribution of different infrared spectrum bands, the infrared spectrum curves were divided into different bands for further analysis, as shown in Table2 shows [32,33].

Table 2. The band assignments for functional groups in coal Fourier transform infrared (FTIR) spectra [32,33].

1 Band Position (cm− ) Assignments 3600–3200 -OH stretching 3000–2800 Aliphatic CH stretching 1650–1520 Aromatic C=C ring stretching 1460–1350 Aliphatic CH2 and CH3 deformation 1200–1000 C-O-C stretching 900–700 Aromatic C-H (out-plane bending modes)

The obtained infrared spectrum curves were analyzed by the curve-ﬁtting method [13] and macromolecular structures can be characterized by the following parameters [34–37]:

1 1 1. CH2/CH3 = A (2940–2900 cm− )/A (2940–3000 cm− ), representing the length of aliphatic side-chains; Energies 2020, 13, 6618 7 of 23

Energies 2020, 13, x FOR PEER REVIEW 7 of 21 1 1 2. I = A (900–700 cm− )/A (3000–2800 cm− ), representing the aromatic degree; −1 1 −1 1 3. 3. DOCDOC= = A ((900–700900–700 cm cm)/A− ) /(1600A (1600 cm cm), representing− ), representing the polycondensation the polycondensation degree of the degree aromatic of the aromaticrings; rings; 4. Har / Hal = A (1650–1520 cm−11/A (3000–2800 cm−11), representing the relative abundance of aromatic 4. Har/Hal = A (1650–1520 cm− /A (3000–2800 cm− ), representing the relative abundance of aromatic toto aliphatic aliphatic hydrogen. hydrogen.

4. Results4. Results

4.1.4.1. Nanoindentation Nanoindentation Experiment Experiment Results Results TheThe nanoindentation nanoindentation pictures pictures are are shown shown in Figure in Figure4. The 4. vitrinite The vitrinite exhibits exhibits a conspicuous a conspicuous concave indentationconcave indentation after unloading, after while unloading, the inertinite while the displays inertinite only displays a shallow only indentation, a shallow indicating indentation, that indicating that inertinite has stronger resistance to pressure than vitrinite. For the vitrinite, the inertinite has stronger resistance to pressure than vitrinite. For the vitrinite, the indentation depth indentation depth increases from F1-I to F6-I, indicating the hardness decreases with the increasing increases from F1-I to F6-I, indicating the hardness decreases with the increasing deformation degree. deformation degree.

Figure 4. Typical nanoindentation image of each sample (the indentation is in the red box). Figure 4. Typical nanoindentation image of each sample (the indentation is in the red box).

Comparing the hmax and hf of the vitrinite with those of the inertinite (Figure 5), the indentation Comparing the hmax and hf of the vitrinite with those of the inertinite (Figure5), the indentation depth of the inertinite is smaller than that of the vitrinite group. However, as the deformation degree depth of the inertinite is smaller than that of the vitrinite group. However, as the deformation degree of TDC increases, the hmax and hf of both vitrinite and inertinite increase, with the difference of TDC increases, the h and h of both vitrinite and inertinite increase, with the diﬀerence decreasing, decreasing, indicatingmax that withf the increase of deformation degree, the resistance of both vitrinite indicating that with the increase of deformation degree, the resistance of both vitrinite and inertinite and inertinite becomes weaker and the difference between the two becomes smaller. becomes weaker and the diﬀerence between the two becomes smaller.

Energies 2020, 13, 6618 8 of 23

Energies 2020, 13, x FOR PEER REVIEW 8 of 22

(a)

(b)

Figure 5. hmax and hf of tectonically deformed coal samples. (a) hmax of samples (b) hf of samples. hmax is Figure 5. hmax and hf theof maximum tectonically depth; hf is deformed the distance between coal the samples. deepest point of (a the) h sunkenmax of surface samples and the initial (b) hf of samples. hmax surface. is the maximum depth; hf is the distance between the deepest point of the sunken surface and the

initial surface.

The load–displacement curves obtained are shown in Figure6. The displacement of vitrinite is noticeably greater than that of inertinite and the load-displacement curves of vitrinite are closer to each other, while those of inertinite are more scattered. From unaltered coal to powdery coal, the load-displacement curves of vitrinite and inertinite also become closer, indicating that as the deformation degree increases, the diﬀerence in the mechanical properties of vitrinite and inertinite diminishes. Energies 2020, 13, x FOR PEER REVIEW 8 of 21

(a) (b)

Figure 5. hmax and hf of tectonically deformed coal samples. (a) hmax of samples (b) hf of samples. hmax is the maximum depth; hf is the distance between the deepest point of the sunken surface and the initial surface.

The load–displacement curves obtained are shown in Figure 6. The displacement of vitrinite is noticeably greater than that of inertinite and the load-displacement curves of vitrinite are closer to each other, while those of inertinite are more scattered. From unaltered coal to powdery coal, the load-displacement curves of vitrinite and inertinite also become closer, indicating that as the deformation degree increases, the difference in the mechanical properties of vitrinite and inertinite Energies 2020, 13, 6618 9 of 23 diminishes.

FigureFigure 6. Load 6. Load-displacement-displacement curves curves of of samples (ﬁve(five measure measure points points for for each each sample). sample). The calculated hardness and modulus are listed in Table3. Overall, the H of the inertinite is

higher than that of the vitrinite. As the degree of deformation increases, the standard deviation of E and H of inertinite decreases from F1-I to F6-I, indicating that the hardness and elastic modulus of inertinite are more homogeneous and the hardness of both inertinite and vitrinite gradually decreases. The reduction in H of the inertinite is signiﬁcantly higher than that of the vitrinite with increasing deformation, so that the diﬀerence between the two lessens (Figure7a). The elastic modulus E of inertinite is also higher than that of vitrinite. As the degree of deformation is increased, the elastic modulus of the inertinite decreases and then increases, showing no obvious regularity but that of the vitrinite gradually increases (Figure7b). Energies 2020, 13, x FOR PEER REVIEW 9 of 21

The calculated hardness and modulus are listed in Table 3. Overall, the H of the inertinite is higher than that of the vitrinite. As the degree of deformation increases, the standard deviation of E and H of inertinite decreases from F1-I to F6-I, indicating that the hardness and elastic modulus of inertinite are more homogeneous and the hardness of both inertinite and vitrinite gradually decreases. The reduction in H of the inertinite is significantly higher than that of the vitrinite with increasing deformation, so that the difference between the two lessens (Figure 7a). The elastic Energies 2020, 13, 6618 10 of 23 modulus E of inertinite is also higher than that of vitrinite. As the degree of deformation is increased, the elastic modulus of the inertinite decreases and then increases, showing no obvious regularity but that of the vitrinite gradually increasesTable 3. (FiCalculationgure 7b). results of E and H.

Vitrinite Inertinite Sample Table 3. Calculation results of E and H. F1-V F2-V F3-V F4-V F5-V F6-V F1-I F2-I F3-I F4-I F5-I F6-I Vitrinite Inertinite Sample Data: 1F1 0.443-V F2 0.402-V F3 0.404-V F4 0.414-V F5 0.390-V 0.308F6-V 0.572F1-I 0.695F2-I 0.574F3-I 0.542F4-I 0.486F5-I 0.373F6-I Data:Data: 1 20.443 0.441 0.402 0.426 0.404 0.390 0.414 0.400 0.390 0.387 0.2910.308 0.7800.572 0.8300.695 0.5430.574 0.6730.542 0.5550.486 0.4000.373 Data: 3 0.456 0.417 0.402 0.414 0.399 0.283 0.612 0.623 0.544 0.450 0.454 0.413 Data: 2 0.441 0.426 0.390 0.400 0.387 0.291 0.780 0.830 0.543 0.673 0.555 0.400 H (GPa) Data: 4 0.428 0.443 0.450 0.404 0.361 0.278 1.378 0.679 0.617 0.615 0.494 0.426 Data: 3 0.456 0.417 0.402 0.414 0.399 0.283 0.612 0.623 0.544 0.450 0.454 0.413 Data: 5 0.409 0.446 0.422 0.402 0.367 0.378 1.517 0.841 0.871 0.481 0.446 0.429 H (GPa) Data: 4 0.428 0.443 0.450 0.404 0.361 0.278 1.378 0.679 0.617 0.615 0.494 0.426 Mean 0.435 0.427 0.414 0.407 0.381 0.308 0.972 0.734 0.630 0.552 0.487 0.408 Data: 5 0.409 0.446 0.422 0.402 0.367 0.378 1.517 0.841 0.871 0.481 0.446 0.429 Standard deviation 0.016 0.016 0.021 0.006 0.014 0.037 0.397 0.087 0.124 0.083 0.039 0.020 Mean 0.435 0.427 0.414 0.407 0.381 0.308 0.972 0.734 0.630 0.552 0.487 0.408 StandardData: deviation 1 0.016 5.108 0.016 5.227 0.021 5.261 0.006 5.261 0.014 5.494 6.1240.037 7.3470.397 7.5880.087 6.9320.124 4.8390.083 4.8300.039 6.7940.020 Data:Data: 1 25.108 4.857 5.227 5.552 5.261 5.291 5.261 5.152 5.494 5.325 6.7316.124 8.2157.347 7.7717.588 7.0536.932 12.1584.839 5.1004.830 6.6866.794 Data:Data: 2 34.857 5.180 5.552 5.383 5.291 5.267 5.152 5.309 5.325 5.520 7.8106.731 7.3848.215 6.7707.771 7.2527.053 5.02112.158 5.2955.100 6.8456.686 E (GPa) Data:Data: 3 45.180 4.935 5.383 5.721 5.267 5.155 5.309 5.007 5.520 4.891 6.1917.810 9.3097.384 6.7356.770 6.8197.252 6.4435.021 5.3725.295 6.6436.845 Data: 5 4.884 5.383 5.340 5.529 5.337 5.674 9.707 7.070 6.570 4.339 5.160 6.619 E (GPa) Data: 4 4.935 5.721 5.155 5.007 4.891 6.191 9.309 6.735 6.819 6.443 5.372 6.643 Mean 4.993 5.453 5.263 5.252 5.313 6.506 8.392 7.187 6.925 6.560 5.151 6.717 Data: 5 4.884 5.383 5.340 5.529 5.337 5.674 9.707 7.070 6.570 4.339 5.160 6.619 Standard deviation 0.128 0.169 0.061 0.173 0.226 0.733 0.971 0.423 0.228 2.885 0.187 0.088 Mean 4.993 5.453 5.263 5.252 5.313 6.506 8.392 7.187 6.925 6.560 5.151 6.717 Standard deviation Note:0.128 Data0.169 1 to 50.061 are the 0.173 ﬁve measure 0.226 points0.733 of0.971 each sample.0.423 0.228 2.885 0.187 0.088 Note: Data 1 to 5 are the five measure points of each sample.

(a) (b)

Figure 7. DistributionsDistributions of of HH andand EE ((HH isis the the hardness; hardness; E isis the elastic modulus modulus;; ( (aa)) and ( b) are the distributions of H and E, respectively).respectively).

4.2. NMR E Experimentxperiment R Resultsesults The NMR curves obtained are shown shown in FigureFigure8 . 8. According According to to previous previous studies, studies, the the NMR NMR spectrum can can be be divided divided into into three three parts parts [38 [38,39]:,39 Aliphatic]: Aliphatic carbons carbons (0–90 (0–90 ppm), ppm), aromatic aromatic carbons carbons (90– (90–165165 ppm) ppm) and and carbonyl carbonyl carbons carbons or orcarboxyl carboxyl carbons carbons (165 (165–240–240 ppm). ppm). As As the the degree degree of of deformation deformation increases, thethe aliphaticaliphatic carboncarbon peaks peaks of of both both the the vitrinite vitrinite and and inertinite inertinite gradually gradually become become flat, flat, while while the peakthe peak shapes shapes of the of aromatic the aromatic carbon peakscarbon are peaks not significantly are not significantly different, indicating different, that indicating the deformation that the deformationof TDC has a of stronger TDC has influence a stronger on aliphaticinfluence carbon on aliphatic than aromatic carbon th carbon.an aromatic carbon.

Energies 2020, 13, 6618 11 of 23 Energies 2020, 13, x FOR PEER REVIEW 10 of 21 Energies 2020, 13, x FOR PEER REVIEW 10 of 21

(a) (b)

Figure 8. NuclearNuclear magnetic magnetic resonance resonance ( (NMR)NMR) spectra of vitrinite ( a) and inertinite ( b).

The NMR spectraspectra ofof thethe samples samples were were analyzed analyzed using using the the curve-ﬁtting curve-fitting method method (Figure (Figure9). Based9). Based on onthe the widely widely used used values values for for the the chemical chemical shifts shifts related related to ditoﬀ differenterent functional functional groups groups of coalof coal[40 –[4042–], 42], fal (aliphatic carbons, 0–90 ppm), fa (aromatic carbons, 90–165 ppm),H faH (protonated aromatic 42],fal (aliphatic fal (aliphatic carbons, carbons, 0–90 ppm), 0–90 ppm),fa (aromatic fa (aromatic carbons, carbons, 90–165 90ppm),–165f a ppm),(protonated faH (protonated aromatic aromatic carbons, carbons, 100–129B ppm), faB (aromatic bridgehead carbons, 129–135 S ppm), faS (alkylation aromatic 100–129carbons, ppm), 100–129fa ppm),(aromatic faB (aromatic bridgehead bridgehead carbons, carbons,129–135 ppm), 129–135fa ppm),(alkylation faS (alkylation aromatic aromatic carbons, carbons, 135–150 ppm)P and faP (non-protonated aromatic carbon constituted by phenolic, 150–165 135–150carbons, ppm) 135–150 and ppm)fa (non-protonated and faP (non-protonated aromatic carbon aromatic constituted carbon constituted by phenolic, by 150–165 phenolic, ppm) 150 have–165 ppm) have been calculated. In addition, the XBP (the ratio of aromatic bridge carbon to aromatic ppm)been calculated. have been calculated. In addition, In the addition,XBP (the the ratio XBP of(the aromatic ratio of bridge aromatic carbon bridge to aromatic carbon to peripheral aromatic peripheralcarbon), which carbon), is an which indicator is an of indicator the basic of structural the basic unitstructural (BSU) unit size (BSU) [9], was size calculated [9], was calculated following followingB XHBP = fPaB / (SfaH+ faP+ faS). In this study, fa / fal was also calculated to reveal the degree of XfollowingBP = fa / (fXa BP+ =f afaB+ /f a (fa).H+In fa thisP+ fa study,S). In thisfa/fal study,was also fa /calculated fal was also to calculatedreveal the degree to reveal of enrichment the degree of enrichmentaromatic carbons of aromatic to aliphatic carbons carbons. to aliphatic carbons.

Figure 9. Curve-ﬁtting method (Taking Sample F1-V as an example). Figure 9. Curve-fitting method (Taking Sample F1-V as an example). The calculated results are shown in Figure 10. The deformation degree of the TDC increases from samplesThe F1calculated to F6, with results evident are shown changes in as Figure the deformation 10. The deformation degree is degree increased, of the indicating TDC increases that tectonic from samplesstress can F1 induce to F6, with the macromolecular evident changes structuralas the deformation evolution degree of coal. is Inincreased, particular, indicating the macromolecular that tectonic stressstructure can ofinduce inertinite the macromolecular also changes with structural deformation evolution degree, of coal. implying In particular, that inertinite the macromolecular is not inert structureto tectonic of stress. inertinite Each also NMR changes parameter with deformation of vitrinite degree, and inertinite implying changes that inertinite with the is deformation not inert to tectonic stress. Each NMR parameter of vitrinite and inertinite changes with the deformation degree tectonicdegree of stress. the coal. Each The NMRfal of parameter vitrinite andof vitrinite inertinite and decreases inertinite with changes the increasing with the deformation deformation degree of the coal. The fal of vitrinite and inertinite decreases with the increasing deformation degree (Figure (Figureof the coal. 10a) The so itfal canof vitrinite be inferred and thatinertinite tectonic decreases stress can with reduce the increasing aliphatic carbons.deformation Compared degree with(Figurefal, 10a) so it can be inferred that tectonic stress can reduce aliphatic carbons. Compared with fal, the fa of 10a)the f asoof it vitrinite can be inferred and inertinite that tectonic gradually stress increases can reduce (Figure aliphatic 10b), suggesting carbons. Compared that the aliphatic with fal, carbonsthe fa of vitrinite and inertinite gradually increases (Figure 10b), suggesting that the aliphatic carbons have a vitrinitehave a process and ine ofrtinite transforming gradually intoincreases aromatic (Figure carbons 10b), thatsuggesting can also that be the triggered aliphatic by carbons tectonic have stress. a process of transforming into aromatic carbons that can also be triggered by tectonic stress. The fal processThe fal decreases of transforming and fa increases, into aromatic so the carbonsfa/fal consequentially that can also increasesbe triggered with by the tectonic deformation stress. degreeThe fal decreases and fa increases, so the fa/fal consequentially increases with the deformation degree (Figure (Figuredecreases 10 g),and leading fa increases, to an increaseso the fa/f inal consequentially the relative aromatic increases carbons, with which the deformation is similar to degree the ﬁndings (Figure of 10g), leading to an increaseH B inS the relativeP aromatic carbons, which is similar to the findings of Song 1Song0g), leading et al. [26 to]. Thean increasefa , fa , infa theand relativefa show aromatic no obvious carbons, regularities which is with similar change to the in findings the deformation of Song H B S P et al. [26]. The faH, faB, faS and faP show no obvious regularities with change in the deformation degree etdegree al. [26]. (Figure The f10a c–f)., fa , f Ina and addition, fa showXBP noalso obvious shows regularities a negative relationship with change with in the the deformation deformation degree degree (Figure 10c–f). In addition, XBP also shows a negative relationship with the deformation degree of the (Figure 10c–f). In addition, XBP also shows a negative relationship with the deformation degree of the

Energies 2020, 13, 6618 12 of 23 Energies 2020, 13, x FOR PEER REVIEW 11 of 21

TDC of(Figure the TDC 10h), (Figure indicating 10h), indicating that tectonic that tectonic stress had stress destroyed had destroyed the original the original basic basic structural structural unit unit (BSU) (BSU) in the coal, decreasing the size of the BSU with the deformation degree. in the coal, decreasing the size of the BSU with the deformation degree.

(a) (b)

(e) (f)

(g) (h)

HH BB S PP FigureFigure 10. NMR 10. NMR structure structure parameters parameters of ofsamples samples (( ((aa––hh)) are are the the distributions of offal fal, ,f af,a,f afa ,,f faa ,, ffaa ,, ffa ,, fa/fal, H and XfaBP/fal, ,respectively) and XBP, respectively).. fal is aliphaticfal is aliphaticcarbons; carbons; fa is aromaticfa is aromatic carbons; carbons; faH is protonatedfa is protonated aromatic aromatic carbons; B S P B f S f P f fa is carbons;aromatica bridgeheadis aromatic bridgeheadcarbons; fa carbons; is alkylationa is alkylation aromatic aromatic carbons; carbons; fa is nona -isprotonated non-protonated aromatic aromatic carbon constituted by phenolic; and XBP is the ratio of aromatic bridge carbon to aromatic carbon constituted by phenolic; and XBP is the ratio of aromatic bridge carbon to aromatic peripheral peripheral carbon. carbon. 4.3. FTIR Experiment Results 4.3. FTIR Experiment Results The infrared spectrum curves of vitrinite and inertinite are shown in Figure 11. There are obvious changesThe infrared in the spectrum curves along curves with of the vitrinite diﬀerent and deformation inertinite degree, are shown once in again Figure proving 11. There that under are obvious the changesaction in ofthe tectonic curves stress, along the with inertinite the different is not inert. deformation For both vitrinite degree, and once inertinite, again proving the variation that degreeunder the actionof of aliphatic tectonic C-H stress, is noticeably the inertinite stronger is not than inert. that ofFor aromatic both vitrinite C=C, in and agreement inertinite, with the the variation NMR data. degree of aliphatic C-H is noticeably stronger than that of aromatic C=C, in agreement with the NMR data.

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW 11 of 21

TDC (Figure 10h), indicating that tectonic stress had destroyed the original basic structural unit (BSU) in the coal, decreasing the size of the BSU with the deformation degree.

(a) (b)

(e) (f)

(g) (h)

Figure 10. NMR structure parameters of samples ((a–h) are the distributions of fal, fa, faH, faB, faS, faP, fa/fal,

and XBP, respectively). fal is aliphatic carbons; fa is aromatic carbons; faH is protonated aromatic carbons; faB is aromatic bridgehead carbons; faS is alkylation aromatic carbons; faP is non-protonated aromatic carbon constituted by phenolic; and XBP is the ratio of aromatic bridge carbon to aromatic peripheral carbon.

4.3. FTIR Experiment Results The infrared spectrum curves of vitrinite and inertinite are shown in Figure 11. There are obvious changes in the curves along with the different deformation degree, once again proving that under the Energiesaction of2020 tectonic, 13, 6618 stress, the inertinite is not inert. For both vitrinite and inertinite, the variation degree13 of 23 of aliphatic C-H is noticeably stronger than that of aromatic C=C, in agreement with the NMR data.

Energies 2020, 13, x FOR PEER REVIEW 12 of 21 (a) (b)

Figure 11. FTIR spectrum of vitrinite (a) and inertinite (b). Figure 11. FTIR spectrum of vitrinite (a) and inertinite (b).

TheThe calculated calculated macromolecular macromolecular parameters parameters are are shown shown in in Figure Figure 12.12. (1) (1) The The CHCH2 2// CH3 of vitrinite vitrinite andand inertinite inertinite decreases decreases from from Sample Sample F1 F1 to to F6 (Figure 12a),12a), indicating that the aliphatic side side-chains-chains graduallygradually detached withwith thethe increasingincreasing deformation deformation degree degree of of the the TDC, TDC, which which is inis agreementin agreement with with the theresults results of the of NMR.the NMR. (2) The (2) aromaticThe aromatic degree de (Igree) of vitrinite(I) of vitrinite gradually gradually ﬂuctuates fluctuates upward upward from Sample from SampleF1 to F6 F1 (Figure to F6 12 (Figureb), suggesting 12b), suggesting that the aromatic that the degree aromatic is positively degree is correlated positively with correlated the deformation. with the deformation.(3) The polycondensation (3) The polycondensation degree of the aromaticdegree of rings the aromatic (DOC) of rings vitrinite (DOC) and of inertinite vitrinite doesand inertinite not show doesany obviousnot show relation any obvious with the relation deformation with the degree deformation (Figure 12 degreec), indicating (Figure that 12c), the indicating polycondensation that the polycondensationdegree of aromatic degree rings does of aromatic not increase rings with does the not increasing increase with deformation. the increasing (4) The deformation. relative abundance (4) The relativeof aromatic abundance to aliphatic of aromatic hydrogen to (aliphaticHar/Hal) of hydrogen vitrinite and(Har / inertinite Hal) of vitrinite increases and from inertinite Sample increases F1 to F6 from(Figure Sample 12d), implyingF1 to F6 (Figure that the 12d), aliphatic implying carbon that decreases the aliphatic but aromatic carbon carbon decreases increases, but aromatic which coincide carbon increases,with the NMR which results coincide in Section with the 4.2 NMR. results in Section 4.2.

(a) (b)

FigureFigure 12. 12. FTIRFTIR structure structure parameters parameters of ofsamples samples ((a– ((da),– dare), arethe thedistributions distributions of CH of2 /CH CH2/3CH, I, DOC3, I, DOC, and , Handar / HHalar, /respectively)Hal, respectively).. CH2/CH3 represents the length of aliphatic side-chains; I represents the aromatic degree; DOC represents the polycondensation degree of the aromatic rings; Har/Hal represents CH2 / theCH3 relative represents abundance the length of aromatic of aliphatic to aliphatic side-chains; hydrogen. I represents the aromatic degree; DOC represents the polycondensation degree of the aromatic rings; Har / Hal represents the relative abundance of aromatic to aliphatic hydrogen.

5. Discussion

5.1. Macromolecular Structure Controlling Effect on Mechanical Property of TDCs

5.1.1. Macromolecular Structure Effect on the Hardness (H) of TDC

Energies 2020, 13, 6618 14 of 23

5. Discussion

5.1. Macromolecular Structure Controlling Eﬀect on Mechanical Property of TDCs

5.1.1. Macromolecular Structure Eﬀect on the Hardness (H) of TDC

There are exponentially positive relationships between the fal and mean H values (Figure 13a) and exponentially negative correlations between the fa and H (Figure 13b) of both vitrinite and inertinite, H indicating that more aliphatic and aromatic carbons suggest a greater hardness of the TDC. The fa B decreases logarithmically with increasing H (Figure 13c), while fa increases logarithmically with increasing H (Figure 13d), suggesting that increasing the protonated aromatic and decreasing aromatic Energies 2020, 13, x FOR PEER REVIEW 13 of 21 bridgehead carbons could reduce the hardness of the TDC. There are exponentially positive correlations correlationsbetween XBP betweenand H and XBP negativeand H and correlations negative correlations between fa/f betweenal and H, f witha/fal and a high H, with correlation a high correlation in vitrinite inand vitrinite low correlation and low in correlation inertinite (Figure in inertinite 13e–f), (Figure implying 13e that–f), a implying smaller BSU that size a smaller and higher BSU f sizea/fal ratio and 2 highercould reducefa/fal ratio the could hardness reduce of the the hardness TDC. The of R thevalues TDC.of The inertinite R2 values in of Figure inertinite 14c,d in are Figure relatively 14c,dlow, are relativelyso the inferences low, so abovethe inferences are more above suitable are for more vitrinite. suitable for vitrinite. The macromolecular evolution evolution of of both vitrinite and inertinite also can be described by the increasingincreasing deformation deformation degree degree of of the the TDC. TDC. The The tectonic tectonic stress stress leads leads to to a a decrease in the aliphatic carbons and an increase inin ffaa and the ratio of ffaa/f/falal.. T Thehe tectonic tectonic stress also decreases the XXBPBP andand all all the changes inin thethe macromolecularmacromolecular structure structure could could reduce reduce the the hardness hardness of theof the TDC. TDC. In addition, In addition, this isthis clear is clearevidence evidence that, in that, addition in additionto thermal to metamorphism, thermal metamorphism, tectonic stress tectonic can change stress the can macromolecular change the macromolecularstructure of coal. structure of coal.

(a) (b)

(e) (f)

HH BB Figure 13. RelationshipsRelationships between between NMR parameters ((((aa––ff)) are are the the falfal, , faf,a, fafa ,, ffaa , XBP,, and fa/f/fal,, respectivelyrespectively)) and H. (Note: Here, Here, H isis the the mean mean value value for each sample) sample)..

There are exponentially positive correlations between CH2/CH3 and H and between I and H (Figure 14 a,b), suggesting that with more aliphatic side chains and a higher aromatic degree, the H of the TDC is higher. The hardness does not show any correlations with DOC (Figure 14c) but is exponentially correlated with Har/Hal, with R2 values of 0.86 (vitrinite) and 0.59 (inertinite) (Figure 14d), indicating that the increase in the relative abundance of aromatic to aliphatic hydrogen will reduce the hardness of the TDC. Therefore, the process in which the macromolecular structure of vitrinite and inertinite of TDC changes with the increase in deformation can be described as follows: As the degree of deformation increases, the aliphatic side chains gradually detach and the aromatic degree gradually increases, representing that the relative content of aromatic carbons increase and the aliphatic carbons decrease, reducing the hardness of the TDC.

Energies 2020, 13, 6618 15 of 23

There are exponentially positive correlations between CH2/CH3 and H and between I and H (Figure 14a,b), suggesting that with more aliphatic side chains and a higher aromatic degree, the H of the TDC is higher. The hardness does not show any correlations with DOC (Figure 14c) but is 2 exponentially correlated with Har/Hal, with R values of 0.86 (vitrinite) and 0.59 (inertinite) (Figure 14d), indicating that the increase in the relative abundance of aromatic to aliphatic hydrogen will reduce the hardness of the TDC. Therefore, the process in which the macromolecular structure of vitrinite and inertinite of TDC changes with the increase in deformation can be described as follows: As the degree of deformation increases, the aliphatic side chains gradually detach and the aromatic degree gradually increases, representing that the relative content of aromatic carbons increase and the aliphatic carbons decrease, reducing the hardness of the TDC. Energies 2020, 13, x FOR PEER REVIEW 14 of 21

(a) (b)

Figure 14. 14. RelationshipRelationship between between FTIR FTIR parameters parameters (( ((aa––f)d ) are are the the CHCH2 /2 /CHCH33, ,I,I ,DOCDOC, , and and HHarar / /H Halal, respectively)respectively) andand H.H. (Note:(Note: Here, H isis thethe meanmean valuevalue forfor eacheach sample).sample).

5.1.2. Macromolecular StructureStructure Eﬀffectect onon thethe ElasticElastic ModulusModulus (E)(E) ofof TDCTDC There is nono correlationscorrelations betweenbetween the NMRNMR parametersparameters of inertinite and E (mean(mean valuevalue forfor eacheach sample) butbut thethe NMRNMR parametersparameters ofof vitrinitevitrinite showshow obviousobvious relationshipsrelationships withwithE E(Figure (Figure 15 15).). TheThef falal isis negatively correlated with E inin exponentialexponential formform (Figure(Figure 1515a),a), indicating that a decrease of aliphaticaliphatic carbons producesproduces aa higherhigher elasticelastic modulus,modulus, whilewhile thethe ffaa is positively correlated with E in exponentialexponential formform (Figure(Figure 1515b),b), implyingimplying anan increaseincrease inin thethe ffaa suggestssuggests aa higherhigher elasticelastic modulus,modulus, soso thethe ffaa//ffalal ofof H vitrinitevitrinite isis positivelypositively correlatedcorrelated withwith EE (Figure(Figure 15 15e).e). ThereThere isis anan positivepositive correlationcorrelation betweenbetweenf afaH and B EE (Figure(Figure 1515c)c) andand aa negativenegative correlationcorrelation betweenbetween ffaaB and E (Figure(Figure 1515d),d), suggestingsuggesting thatthat increasedincreased protonatedprotonated aromatics aromatics could could promote promote the theelastic elastic modulus, modulus, while while increased increased aromatic aromatic bridgehead bridgehead carbons couldcarbons reduce could the reduce elastic the modulus elastic ofmodulus the TDC. of The theX TDC.BP of The vitrinite XBP of is positivelyvitrinite is correlated positively to correlatedE (Figure 15tof), E suggesting(Figure 15f), that suggesting when the that BSU when size isthe larger, BSU size the elasticis larger, modulus the elastic is lower. modulus is lower.

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW 14 of 21

(a) (b)

Figure 14. Relationship between FTIR parameters ((a–f) are the CH2 / CH3, I, DOC, and Har / Hal, respectively) and H. (Note: Here, H is the mean value for each sample).

5.1.2. Macromolecular Structure Effect on the Elastic Modulus (E) of TDC There is no correlations between the NMR parameters of inertinite and E (mean value for each sample) but the NMR parameters of vitrinite show obvious relationships with E (Figure 15). The fal is negatively correlated with E in exponential form (Figure 15a), indicating that a decrease of aliphatic carbons produces a higher elastic modulus, while the fa is positively correlated with E in exponential form (Figure 15b), implying an increase in the fa suggests a higher elastic modulus, so the fa/fal of vitrinite is positively correlated with E (Figure 15e). There is an positive correlation between faH and E (Figure 15c) and a negative correlation between faB and E (Figure 15d), suggesting that increased protonated aromatics could promote the elastic modulus, while increased aromatic bridgehead Energiescarbons2020 could, 13, 6618reduce the elastic modulus of the TDC. The XBP of vitrinite is positively correlated16 to of 23E (Figure 15f), suggesting that when the BSU size is larger, the elastic modulus is lower.

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW(c) (d) 15 of 21

(e) (f)

H B FigureFigure 15.15. Relationship betweenbetween NMR NMR parameters parameters (( ((a–af–)f are) are the thefal ,falfa, ,faf,a faH,,f afaB,, ffaa//ffalal, and XBPBP,, respectively respectively)) andand E.E. (Note:(Note: Here, E isis thethe meanmean valuevalue forfor eacheach sample).sample).

BasedBased on FTIR results, therethere cancan bebe foundfound thatthat thethe allall thethe FTIRFTIR parametersparameters ofof inertiniteinertinite showshow nono correlationscorrelations with with elastic elastic modulus modulus (Figure (Figure 16), which 16), which is the same is the with same NMR with parameters, NMR parameters, so a conclusion so a canconclusion be inferred can be that inferred the macromolecular that the macromolecular structure variation structure of variation inertinite of cannot inertinite change cannot the change elastic regularlythe elastic with regularly the increasing with the deformationincreasing deformation degree. degree. TheThe macromolecular structurestructure variationvariation of of inertiniteinertinite can can changechange thethe elasticelastic modulus.modulus. (1)(1)CH CH22//CHCH33 isis inverselyinversely proportionalproportional toto E with a negative-indexnegative-index exponential function (Figure 1616a),a), indicating that thethe detachmentdetachment ofof aliphaticaliphatic side side chains chains increases increases the the elastic elastic modulus. modulus. (2) (2)I isI is linearly linearly proportional proportional to toE (FigureE (Figure 16 b),16b), indicating indicating that that the the increase increase in the in aromaticthe aromatic degree degree increases increases the elastic the elastic modulus. modulus. (3) There (3) isThere no obvious is no obvious correlation correlation between betweenDOC and DOCE (Figure and E 16 (Figurec). (4) There 16c). is (4) a linearlyThere ispositive a linearly correlation positive betweencorrelationH ar between/Hal and EHar(Figure/Hal and 16 Ed), (Figure which implies16d), which that the implies increase that in the the increase relative in abundance the relative of aromaticsabundance to of aliphatic aromatics hydrogen to aliphatic increases hydrogen the elastic increases modulus. the elastic modulus.

(a) (b)

Figure 16. Relationship between FTIR parameters ((a–f) are the CH2 / CH3, I, DOC, and Har / Hal, respectively) and E. (Note: Here, E is the mean value for each sample).

5.2. Mechanisms of Mechanical Property Evolution Controlled by Macromolecular Structure Based on the results above, it can be concluded that the tectonic stress can change the macromolecular structure and mechanical properties of both vitrinite and inertinite. Aliphatic and aromatic carbons are the two primary components of the macromolecular structure in coal and the aliphatic side chains and benzene rings are strongly influenced by tectonic stress according to the NMR and FITR experimental results (Figures 10 and 12). The aliphatic side chains become detached, decreasing the aliphatic carbons and the benzene rings are expanded by the aromatization process,

Energies 2020, 13, x FOR PEER REVIEW 15 of 21

(e) (f)

Figure 15. Relationship between NMR parameters ((a–f) are the fal, fa, faH, faB, fa/fal, and XBP, respectively) and E. (Note: Here, E is the mean value for each sample).

Based on FTIR results, there can be found that the all the FTIR parameters of inertinite show no correlations with elastic modulus (Figure 16), which is the same with NMR parameters, so a conclusion can be inferred that the macromolecular structure variation of inertinite cannot change the elastic regularly with the increasing deformation degree. The macromolecular structure variation of inertinite can change the elastic modulus. (1) CH2/CH3 is inversely proportional to E with a negative-index exponential function (Figure 16a), indicating that the detachment of aliphatic side chains increases the elastic modulus. (2) I is linearly proportional to E (Figure 16b), indicating that the increase in the aromatic degree increases the elastic modulus. (3) There is no obvious correlation between DOC and E (Figure 16c). (4) There is a linearly positive Energiescorrelation2020, 13 between, 6618 Har/Hal and E (Figure 16d), which implies that the increase in the relative17 of 23 abundance of aromatics to aliphatic hydrogen increases the elastic modulus.

(a) (b)

Figure 16. RelationshipRelationship between between FTIR FTIR parameters parameters (( ((a–af–)d are) are the the CHCH2 /2 /CHCH33, ,I,I ,DOCDOC, , and and HHar ar/ / Hal,, respectively)respectively) and E.E. (Note:(Note: Here, E is the mean value for each sample).sample).

5.2. Mechanisms of M Mechanicalechanical P Propertyroperty E Evolutionvolution ControlledControlled by MacromolecularMacromolecular StructureStructure Based on the results above, it can be concluded that the tectonic stress can change the macromolecular structure and mechanical properties of both vitrinite and inertinite. Aliphatic and aromatic carbons are the two primary components of the macromolecular structure in coal and the aliphaticaliphatic side chains and benzene rings are strongly influencedinfluenced by tectonic stress according to the NMR and FITR experimental results (Figures 10 and 1212).). The aliphatic side chains become detached, detached, decreasing the aliphatic carbons and the benzene rings are exp expandedanded by the aromatization process, resulting in the increase in aromatic carbons. According to the conclusions obtained, the mechanism of the mechanical property evolution controlled by the macromolecular structure can be analyzed as follows. At the nanometer scale, when the cone-shaped rigid indenter is pressed into the surface of the coal, a denser macromolecular structure is less likely to be deformed because there is not enough space surrounding the macromolecular groups in contact with the cone-shaped indenter to release the induced pressure (Figure 17a). Conversely, the compactly arranged macromolecular groups resist more strongly, resulting in high hardness. It has been confirmed that all aliphatic side chains are an critical part of macromolecules in coal [8]. As the aliphatic side chains detach, there will be many structural defects in the macromolecular structure in the TDC [26], gradually causing gaps among macromolecular groups to emerge. From unaltered coal to powdery coal, the CH2/CH3 values of vitrinite and inertinite decrease by 25% and 24%, respectively and when the cone-shaped rigid indenter is applied, there should be more space among the macromolecular groups for the extrusion of the macromolecular groups caused by the indenter, thus reducing the hardness of the tectonic coal (Figure 17b,c). In addition, there is always interspace among the BSUs, which was confirmed by Ju et al. [1] using high-resolution transmission electron microscopy. From Samples 1 to 6, the XBP value of vitrinite decreased by 34.88%, while that of inertinite decreased by 37.17%, indicating that the distribution density of the interspace among the BSUs of the TDC will increase by more than a third from unaltered coal to powdery coal, so TDC with more interspace among the BSUs exhibit a lower hardness. Energies 2020, 13, x FOR PEER REVIEW 16 of 21

resulting in the increase in aromatic carbons. According to the conclusions obtained, the mechanism of the mechanical property evolution controlled by the macromolecular structure can be analyzed as follows. At the nanometer scale, when the cone-shaped rigid indenter is pressed into the surface of the coal, a denser macromolecular structure is less likely to be deformed because there is not enough space surrounding the macromolecular groups in contact with the cone-shaped indenter to release the induced pressure (Figure 17a). Conversely, the compactly arranged macromolecular groups resist more strongly, resulting in high hardness. It has been confirmed that all aliphatic side chains are an critical part of macromolecules in coal [8]. As the aliphatic side chains detach, there will be many structural defects in the macromolecular structure in the TDC [26], gradually causing gaps among macromolecular groups to emerge. From unaltered coal to powdery coal, the CH2/CH3 values of vitrinite and inertinite decrease by 25% and 24%, respectively and when the cone-shaped rigid indenter is applied, there should be more space among the macromolecular groups for the extrusion of the macromolecular groups caused by the indenter, thus reducing the hardness of the tectonic coal (Figure 17b,c). In addition, there is always interspace among the BSUs, which was confirmed by Ju et al. [1] using high-resolution transmission electron microscopy. From Samples 1 to 6, the XBP value of vitrinite decreased by 34.88%, while that of inertinite decreased by 37.17%, indicating that the distribution density of the interspace among the BSUs of the TDC will increase by more than a third Energiesfrom unaltered2020, 13, 6618 coal to powdery coal, so TDC with more interspace among the BSUs exhibit a lower 18 of 23 hardness.

(a) (b) (c)

FigureFigure 17. SketchSketch map map of ofmacromolecules macromolecules with with aliphatic aliphatic side chains side chains detaching detaching in TDC (( ina) TDC is unalerted ((a) isunalerted coal;coal; ( (bb)) is is tectonicallytectonically deformed deformed coal; ( (cc)) is tectonically deformed deformed coal coal with with greater greater deformation deformation degree thandegree (b )).than (b)).

The elastic modulus is a measure of the ability of an object to resist elastic deformation. From a The elastic modulus is a measure of the ability of an object to resist elastic deformation. From a microscopic point of view, the elastic modulus is a reflection of the bonding strength between atoms, microscopic point of view, the elastic modulus is a reﬂection of the bonding strength between atoms, ions or molecules and is positively related to the intermolecular forces [43]. As is well known [44], ionsthe or intermolecular molecules and force is is positively positively related related to to the polar intermolecular molecules. The forces polarity [43 ]. of As a ispolyatomic well known [44], themolecule intermolecular is not only force related is positively to the polarity related of tothe polar chemical molecules. bond but The also polarity depends of aon polyatomic the spatial molecule isconfiguration not only related of the to the molecule. polarity Therefore, of the chemical the polarity bond of but a polyatomicalso depends molecule on the spatialcan usually conﬁguration be of thecharacterized molecule. by Therefore, the dipole themoment polarity [45]. ofAccording a polyatomic to the results molecule of the can NMR usually and FTIR be characterizedexperiments, by the dipolethe most moment prominent [45]. features According of the to macromolecular the results of thestructur NMRal andevolution FTIR of experiments, TDC are the thedetachment most prominent featuresof aliphatic of the side macromolecular chains and the increase structural in the evolution relative content of TDC of are aromatic the detachment molecules. ofTherefore, aliphatic this side chains andstudy the uses increase the in evolution the relative of typical content molecular of aromatic formulas molecules. to characterize Therefore, thisthese study two uses evolution the evolution characteristics to explain the variation in the elastic modulus of TDC. of typical molecular formulas to characterize these two evolution characteristics to explain the variation The dipole moment calculation can be carried out through the Visualizer module and Dmol3 in the elastic modulus of TDC. module of the Materials Studio software and the calculation parameter setting adopted the method of GeThe and dipole Zhang moment[46]. The Visualization calculation canmodule be carriedwas used out to build through the model the Visualizer with the parameter module andof: Dmol3 moduleFunctional: of the GGA, Materials PW91; Properties: Studio software Frequency; and theConvergence calculation Tolerance: parameter Energy: setting 2.0 adopted 10−5 Hartree; the method of Ge and Zhang [46]. The Visualization module was used to build the model with the parameter of:

Functional: GGA, PW91; Properties: Frequency; Convergence Tolerance: Energy: 2.0 10 5 Hartree; × − Max. force: 0.0004 Hartree; Max. displacement: 0.0005 nm; max. step size: 0.03 nm. The Dmo13 module was used for density-functional calculations with the parameters of: Properties: Electrostatics: Electrostatics moments; Grid interval: 0.025 nm; Border: 0.3 nm; Properties: Population analysis: Mulliken analysis: Atomic Charge; Hirshfeld analysis: Charge. The calculation results are listed in Table4. From isobutylbenzene to methylbenzene, representing the detachment of aliphatic side chains, the dipole moment value increases and the experimental values collected from Dean [47] also support the increasing trend. From naphthalene to phenanthrene, representing the increase in aromatic rings, the dipole moment value increases. From 1-Methyl naphthalene to 1-Methyl phenanthrene and from 2-Ethylnaphthalene to 2-Ethylphenanthrene, representing the increase in aromatic rings with branched aliphatic groups, the dipole moment also increases, the length of the branch is longer and the dipole moment value increases when the aromatic ring is added. Therefore, it can be inferred that the detachment of aliphatic side chains and the increase in aromatic rings may cause an increase in the molecular dipole moment in coal, thereby increasing the intermolecular forces and elastic modulus. In addition, there are π-π bonds between the unconnected benzene rings, which have been described as a type of interaction between polarized aromatic structures [48,49] and are sometimes considered to contain van der Waals forces and ionic linkages [50]. According to Liu and Jiang [13], the content of π-π bonds in TDC increased with a higher deformation degree, increasing the intermolecular forces and, consequently, the elastic modulus. Energies 2020, 13, x FOR PEER REVIEW 17 of 21 Energies 2020, 13, x FOR PEER REVIEW 17 of 21 Energies 2020,, 13,, xx FORFOR PEERPEER REVIEWREVIEW 17 of 21 Max.Energies force: 2020, 1 3 0.0004, x FOR PEER Hartree; REVIEW Max. displacement: 0.0005 nm; max. step size: 0.03 nm. The Dmo1317 of 21 Max.moduleEnergies force: 20 20 was, 13 0.0004, x usedFOR Hartree;PEER for REVIEW density Max. -functional displacement: calculations 0.0005 nm; with max. the step parameters size: 0.03 nm. of: The Properties: Dmo1317 of 21 Max.moduleEnergies force: 20 20 was, 1 3 0.0004, x usedFOR PEER Hartree; for REVIEW density Max. -functional displacement: calculations 0.0005 nm; with max. the step parameters size: 0.03 nm. of: The Properties: Dmo1317 of 21 moduleElectrostatics:Max. force: was 0.0004 Elec usedtrostatics Hartree; for density moments; Max.- functional displacement: Grid interval: calculations 0.0005 0.025 nm; nm; with max.Border: the step 0.3 parameters size:nm; Properties: 0.03 nm. of: The Population Properties: Dmo13 Electrostatics:Max.Energies force: 20 20 0.0004, Elec13, x trostaticsFOR Hartree; PEER REVIEWmoments; Max. displacement: Grid interval: 0.0005 0.025 nm; nm; max.Border: step 0.3 size:nm; Properties: 0.03 nm. The Population Dmo1317 of 21 Electrostatics:analysis:module Mulliken was Elec usedtrostatics analysis: for density moments;Atomic-functional Charge; Grid interval: Hirshfeld calculations 0.025 analysis: nm; with Border: Charge. the 0.3 parameters nm; Properties: of: Population Properties: Electrostatics:analysis:Max.moduleEnergies force: Mulliken was20 20 0.0004 , Elec13 used, x trostaticsFOR analysis: Hartree; for PEER density REVIEWmoments; Atomic Max. - functionaldisplacement: Charge; Grid interval: Hirshfeld calculations 0.0005 0.025 analysis: nm; nm; with max.Border: Charge. the step 0.3 parameters size:nm; Properties: 0.03 nm. of: The Population Properties: Dmo1317 of 21 analysis:Electrostatics:The Mulliken calculation Elec trostaticsanalysis: results moments;Atomic are listedCharge; Grid in interval: Hirshfeld Table 0.025 4. analysis: From nm; Border: isobutylbenzene Charge. 0.3 nm; Properties: to methylbenzene, Population analysis:moduleMax.EnergiesThe force:Mulliken was20 calculation20 , 1 used3 0.0004, x FORanalysis: for resultsPEER Hartree; density REVIEWAtomic are Max. -functional listed Charge; displacement: in Hirshfeld Table calculations 4. 0.0005 analysis: From nm; with isobutylbenzene Charge. max. the step parameters size: to 0.03 methylbenzene, of: nm. Properties: The Dmo1317 of 21 analysis:Electrostatics:The Mulliken calculation Elec trostaticsanalysis: results moments;Atomic are listed Charge; Grid in interval: Hirshfeld Table 0.025 4. analysis: From nm; Border: isobutylbenzene Charge. 0.3 nm; Properties: to methylbenzene, Population representingElectrostatics:Max.moduleThe force: calculation was t heElec 0.0004 detachmenttrostatics used results Hartree; for moments; density of are Max. aliphatic listed - functional displacement:Grid in side interval: Table chains, calculations 0.025 4. 0.0005 the From nm; dipole nm; Border: isobutylbenzene with max. moment the0.3 step nm; parameters valuesize: Properties: to 0.03 increases methylbenzene, nm. of: Population The and Properties: Dmo13 the representinganalysis:The Mulliken calculation the detachment analysis: results Atomic of are aliphatic listed Charge; inside Hirshfeld Table chains, 4. analysis: the From dipole isobutylbenzene Charge. moment value to increases methylbenzene, and the experimentalrepresentinganalysis:moduleElectrostatics:Max. force:Mulliken was tvalueshe 0.0004 Electrostatics detachment usedanalysis: collected Hartree; for Atomic densityfrom of moments; Max. aliphatic Dean Charge; -functional displacement: [47]Grid side Hirshfeld also interval: chains, support calculations 0.0005 analysis: the0.025 the dipole nm;nm; increasing withCharge. max.Border: moment the step trend. 0.3 parameters valuesize:nm; From Properties: 0.03 increases naphthalene nm. of: The andPopulation Properties: Dmo13 the to experimentalrepresentingThe calculation tvalueshe detachment collected results from of are aliphatic Dean listed [47] inside also Table chains, support 4. the From the dipole increasing isobutylbenzene moment trend. value From to increases methylbenzene, naphthalene and the to phenanthrene,experimentalElectrostatics:analysis:moduleThe calculation Mulliken was values representing Electrostatics used collected analysis: results for the densityfrom aremoments;Atomic increase listedDean-functional Charge; in [47]Grid in aromatic also Table interval: Hirshf support calculations rings, 4.eld 0.025 From analysis: thethe nm; increasingdipole isobutylbenzene with Border: Charge. mome the trend. 0.3 parametersnt nm; value From toProperties: methylbenzene,increases. naphthalene of: Population Properties: From to phenanthrene,experimentalrepresenting tvalueshe representing detachment collected the from of increase aliphatic Dean in [47] sidearomatic also chains, support rings, the the the dipole increasingdipole moment mome trend. nt value value From increases increases. naphthalene and From the to 1phenanthrene,representing-Methylanalysis:Electrostatics:The naphthalene Mulliken calculation the representing Electrostatics detachment analysis: results to the of 1moments;Atomic -increase Methyl aliphatic are listed Charge; in Grid phenanthrene sidearomatic in interval: chains,Hirshf Table rings,eld the0.025 4. analysis: and the From dipole nm; dipole from Border: isobutylbenzene momentCharge. mome 2 -0.3Ethylnaphthalene nt valuenm; value Properties: increases toincreases. methylbenzene, andPopulation to From the 2- 1phenanthrene,experimental-Methyl naphthalene values representing collected to the from 1 -increaseMethyl Dean in [47] phenanthrene aromatic also support rings, andthethe increasingdipole from mome 2 -trend.Ethylnaphthalenent value From increases. naphthalene to From 2to- Ethylphenanthrene,1experimental-Methylrepresentinganalysis:The naphthalene Mulliken calculationvalues the representingcollected detachment analysis: results to from 1Atomic- Methyl ofthe are Dean aliphatic increase listed Charge; [47] phenanthrene inalso sidein Hirshf Tablearomatic support chains,eld 4. analysis:andringsthe From increasing dipole with from isobutylbenzene Charge. branched moment 2 trend.-Ethylnaphthalene valueFromaliphatic to naphthalene increases methylbenzene, groups, to and the to 2 - the Ethylphenanthrene,1phenanthrene,-Methyl naphthalene representing representing to the 1 -increaseMethyl the increase in phenanthrene aromatic in aromatic rings, rings andthe dipole with from branched mome 2-Ethylnaphthalenent value aliphatic increases. groups, to From the 2- dipoleEthylphenanthrene,phenanthrene,representingexperimental momentThe calculation representing thealsovalues representing detachment increases, collected results the increasethe from ofthe are aliphaticlength increase listedDean in aromatic of[47] in sidein the also Tablearomatic chains,branc rings, su pport 4.h rings theis the From longerthe dipole with increasing isobutylbenzene and branchedmome moment the nttrend. dipole value valuealiphatic Fromto increases. moment increases methylbenzene, naphthalenegroups, valueFrom and the the to dipoleEthylphenanthrene,1-Methyl moment naphthalene also representing increases, to 1 - theMethyl the length increase phenanthrene of in the aromatic branc h rings and is longer with from andbranched 2 -theEthylnaphthalene dipole aliphatic moment groups, to value the 2- increasesdipole1-Methylexperimentalphenanthrene,representing moment when naphthalene thealsovalues representing aromaticdetachment increases, collected to ring1 the-the Methylfrom of increaseis aliphaticlength added.Dean phenanthrene ofin[47] side Therefore,aromatic the also chains,branc su pportrings, h it and theis can longerthe the dipole be from increasingdipole inferred and moment 2 moment -theEthylnaphthalene trend. that dipole value thevalue From momentdetachment increases increases. naphthalene to value and From of2 - the to increasesdipoleEthylphenanthrene, moment when the also aromaticrepresenting increases, ring the the is length increase added. of Therefore,in the aromatic branc h it rings is can longer bewith inferred andbranched the that dipole aliphatic the detachmentmoment groups, value the of aliphaticincreasesEthylphenanthrene,phenanthrene,1experimental-Methyl side when chains naphthalene thevalues representing representingand aromatic collected the increase to ring the from 1the -increase isMethyl in increase added.Dean aromatic in[47] phenanthrene Therefore,inaromatic alsoringsaromatic su may pportrings, it rings cause can andthethe bewith increasing andipole inferred fromincrease branched moment 2 trend.- that Ethylnaphthalenein aliphaticthe thevalue From molecular detachment increases. naphthalenegroups, dipole the to From of to 2- aliphaticincreasesdipole moment side when chains thealso and aromatic increases, the increase ring the is lengthin added. aromatic of Therefore, the rings branc may h it iscause can longer be an inferred increaseand the thatin dipole the the molecular momentdetachment dipole value of momentaliphaticdipole1Ethylphenanthrene,phenanthrene,-Methyl moment inside coal, chains naphthalene t alsohereby representing and increases, representin increasingthe increase to the the 1 - gincrease theMethyl length thein intermoleculararomatic increase ofin phenanthrene aromatic the ringsin branc aromatic mayforces rings,h is cause and longerandringsthe anelasticdipole withfrom increaseand modulus.branched momentthe 2- Ethylnaphthalenein dipole the value aliphatic molecular moment increases. groups, dipole value to From the 2- momentaliphaticincreases inside when coal, chains t thehereby and aromatic increasingthe increase ring the is in added. intermoleculararomatic Therefore, rings forcesmay it cause can and be elastican inferred increase modulus. thatin the the molecular detachment dipole of momentincreasesEthylphenanthrene,dipole1-Methyl in when moment coal, naphthalene Table thethereby also aromatic 4. representin Calculation increasing increases, to ring 1 -results g theisMethyl the added. intermolecular length increase of dipole phenanthrene Therefore, of inmoment the aromatic branchforces itfor can typicaland andrings is be longer elastic inferredwith fromorganic andmodulus.branched 2molecule - thattheEthylnaphthalene dipole the aliphatic. detachment moment groups, to value of the 2- momentaliphatic inside coal, chains Tablethereby and 4. Calculation increasingthe increase results the in intermoleculararomatic of dipole rings moment mayforces for cause typicaland elastican organic increase modulus. molecule in the molecular. dipole aliphaticdipoleincreasesEthylphenanthrene, side moment whenchainsTable thealso and 4. representin aromaticCalculation increases,the increase ring resultsg the thein is lengtharomatic increase added.of dipole of rings Therefore,inmoment the aromatic branchmay for cause it typicalrings is can longer an bewithorganic increase inferred andbranched molecule thein thatthe dipole aliphatic .molecular the momentdetachment groups, dipole value the of Energiesmoment2020 in, 13coal,, 6618 Tablethereby 4. Calculation increasing results the intermolecular of dipole moment forces forCalculated typicaland elastic organic modulus. molecule . 19 of 23 momentincreasesaliphaticdipole in moment coal, side when Table tchainshereby thealso 4. and aromaticCalculation increasing increases, the increase ring results the isintermolecular lengthin added.of aromatic dipole of Therefore, moment the rings branchforces mayfor Calculated it typicaland is cause can longer elastic beorganic an inferred increase andmodulus. molecule Experimentalthe that in dipole the. the molecular momentdetachment Dipole dipole value of CalculatedDipole Experimental Dipole aliphaticmomentincreases in side when coal,Table chains thethereby 4. and aromaticMolecularCalculation increasingthe increase ring results isthe in Molecular added.of intermoleculararomatic dipole Therefore, moment rings mayforcesfor Calculated it typical Dipolecause can and beorganic anelastic inferred increase moleculemodulus.ExperimentalM oment thatin the. the fromolecular detachmentm D Deanipole dipole of Table 4. MolecularCalculation results Molecularof dipole moment forCalculated typical organic moleculeExperimentalMoment. from D Deanipole aliphaticName side chains and the increase in aromatic rings mayMDipole causeoment an increase in the molecular dipole momentName in coal,Table thereby 4.MolecularFCalculationormula increasing results theMolecularS intermoleculartructure of dipole moment forcesCalculatedM foroment and typical elastic organic modulus.ExperimentalMoment(1987) molecule. fro [47]m D Dean ipole Name TableMolecular F4.ormula Calculation resultsMolecularStructure of dipole moment(unit: MforDipoleoment typical debey, organic ExperimentalM molomentecule(1987) fro. [47]m D Dean ipole momentName in coal, therebyMolecularFormula increasing theMolecularS intermoleculartructure forces(unit:CalculatedMoment and debey, elastic modulus.M(unit:oment(1987) debey, fro [47]m Dean D) Name Table F4.ormula Calculation resultsStructure of dipole moment(unit: forMDipoleoment typical debey, organic Experimental mol(unit:ecule(1987) debey,. [47] D D)ipole Molecular Molecular (unit:DipoleCalculatedD) debey, Moment froExperimentalm Dean Dipole Name TableMolecular F4.ormula Calculation resultsMolecularStructure of dipole moment forMoment typical organicM mol(unit:omentExperimentalecule(1987) debey, fro. [47]m Dean D) D ipole MolecularFormula MolecularStructure Calculated(unit:0.33D) debey, Dipole Moment(1987)Moment [47] from Dean NameName (unit:MCalculatedoment0.33 Dipoledebey, (unit: debey, D) FormulaFormulaMolecular SStructuretructureMolecular (unit:0.33D) debey, D) ExperimentalM(1987)oment [47] from(1987) D Deanipole [47] IsobutylbenzeneName C10H14 (unit:Calculated0.33M Dipoledebey,oment (unit: 0.31debey, D) Isobutylbenzene CMolecular10FHormula14 MolecularStructure 0.33D) (unit:ExperimentalMoment (1987)0.31debey, (unit:from [47] D) D debey,Dean ipole D) IsobutylbenzeneName C10H14 (unit:D)MDipoleoment debey, 0.31 Isobutylbenzene CMolecular10FHormula14 MolecularStructure 0.33 M(unit:oment(1987)0.31 debey, from [47] Dean D) IsobutylbenzeneName C10H14 (unit:0.33MomentD) debey, 0.31 F ormula Structure — (1987) [47]

— (unit: debey, D) Isobutylbenzene C10H 14 (unit: debey, 0.31 CH 0.33D) IsobutylbenzeneIsobutylbenzene CC10HH14 — 0.330.31 0.31 10 14 CH

— (unit: debey, D) D) CH 0.33 2 — 10 14 CH Isobutylbenzene C H 0.31 2 10 14 detachmen Sec-butylbenzene C H 0.31 0.37 — detachmen 10 14 CH 0.33 Sec-butylbenzene C H 2 0.31 0.37

10 14 Isobutylbenzene C H 2 0.31 — detachmen —

Sec-butylbenzene C10H 14 CH 0.31 0.37 Sec-butylbenzene C10H14 detachmen 0.31 0.37 10 14 2

Isobutylbenzene C H CH 0.31

CH detachmen Sec-butylbenzene C10H14 — 0.31 0.37 2

10 14 detachmen Sec-butylbenzene C H CH 0.31 0.37

Sec-butylbenzene C H 2 0.31 0.37 — 10 14 2

detachmen 10 14 detachment SecSec-butylbenzene-butylbenzene C HC10H 14 t 0.31 0.31 0.37 0.37

CH t 2

detachment Sec-butylbenzene C10H14 t 0.31 0.37

8 10 2 Ethylbenzene C H 0.37 / 8 10 Ethylbenzene C H 10 14 detachment 0.37 / Sec-butylbenzene C H t 0.31 0.37 Ethylbenzene C8H 10 0.37 /

Ethylbenzene C8H10 t 0.37 /

EthylbenzeneEthylbenzene CCH8H10 t 0.37 0.37 / /

8 10 Ethylbenzene C8H10 0.37 / Ethylbenzene C8H 10 0.37 / EthylbenzeneEthylbenzene C8H C108H 10 0.37 0.37 / /

Ethylbenzene C8H10 0.37 / Methylbenzene C7H8 0.41 0.45 Methylbenzene C7H 8 0.41 0.45 Methylbenzene C7H 8 0.41 0.45 MethylbenzeneMethylbenzeneMethylbenzene CC7H7H8C87 H8 0.410.41 0.410.450.45 0.45 Methylbenzene C7H8 0.41 0.45 Methylbenzene C7H8 0.41 0.45 Methylbenzene C7H 8 0.41 0.45

Methylbenzene 7 C87H8 B 0.41 0.45 Methylbenzene C H 0.41 0.45 enzene increase

B B B enzene increase

10 8 enzene increase Naphthalene C H enzene increase 0.00 / 10 8 B Naphthalene 10 C8 H B 0.00 / Naphthalene C H B 0.00 / enzene increase enzene increase NaphthaleneNaphthalene CC1010H H8 8 enzene increase 0.00 0.00 / / NaphthaleneNaphthalene C10HC108 H8 B 0.000.00 / / enzene increase 10 8 B NaphthaleneNaphthalene C HC10 H8 0.000.00 / / enzene increase Naphthalene C10 H8 B 0.00 0.05 / Naphthalene C10 H8 enzene increase 0.00 / 0.05 0.05 Phenanthrene C14H10 / 0.05 0.05 Phenanthrene C14H10 0.05 /

14 10

Phenanthrene 14 C 10H / PhenanthrenePhenanthrene CC14HH10 0.05 0.05 / /

Phenanthrene C14H 10 /

14 10 PhenanthrenePhenanthrene C14HC10H 0.05 / / 0.53

Phenanthrene C14H10 B / 0.05 enzene increase Phenanthrene1- Methyl C14H 10 0.53 / C11H10 B

Phenanthrene C14H10 / 1- Methyl enzene increase 0.53 naphthalene B 0.53

B 0.53

C11H10 B 1-Methyl enzene increase 1-Methyl1- Methyl enzene increase 0.53 naphthalene enzene increase 1-Methyl CC1H1H10 B 0.53 0.53 1111 C101110H10 B Energiesnaphthalenenaphthalene1 20-Methyl20, 13, x FOR PEER REVIEWC H enzene increase 18 of 21 Energies 201-naphthalene20Methyl, 13, x FOR PEER REVIEW enzene increase 0.53 18 of 21 naphthalene C11H 10 B C11H10 0.53 enzene increase naphthalene1-Methyl1-Methyl B naphthalene C11H1510 12 0.53 C H enzene increase 0.60

1-Methyl B naphthalenephenanthrene1-Methyl 11 10

C H enzene increase 1-Methyl C15H12 0.60 naphthalene1-Methyl C11H10 1-Methyl1phenanthrene-Methyl1-Methyl 15 12

naphthalene C15 CH1215H 12 0.600.60 phenanthrene CC1515HH1212 0.60 0.60 phenanthrenephenanthrenephenanthrene

Energies 2020, 13, x FOR PEER REVIEW 18 of 21

B B B B

0.69 enzene increase 0.690.69 enzene increase

enzene increase 0.69 enzene increase

2-Ethylnaphthalene2 -Ethylnaphthalene C CH12H12 0.69 2-Ethylnaphthalene2-Ethylnaphthalene C1212H12C1212 H12

2- 2- 2- C16H14 2.41 16 C1416H14 2.41

2-Ethylphenanthrene CC16HH14 2.41 2.41 C H 2.41 EthylphenanthreneEthylphenanthreneEthylphenanthrene 16 14

InIn addition, addition,In addition, there there there are are areππ--π ππ bonds-bondsπ bonds between between between the the the unconnected unconnected unconnected benzene benzene benzene rings, rings, rings, which which which have have have been been been 5.3.describeddescribed Recommendationsdescribed asas aaas typetype a type ofof interaction interactionof interaction betweebetwee betweennn polarizedpolarized polarized aromaticaromatic aromatic structuresstructures structures [48[48 ,49][48,49] ,49] andand and areare are sometimessometimes sometimes consideredconsideredconsidered to to contain containto contain van van van derder derWaals Waals Waals forces forces forces and and and ionic ionic ionic linkages linkages linkages [50] [50] [50]. .According According. According to to Liu Liuto Liuand and and Jiang Jiang Jiang [13] [13] [13], , , thethe the It content content can content be of of realized π ofπ--π π π bonds- bondsπ from bonds in in the TDC inTDC above TDC increased increased conclusions increased with with with athat a higher higher a in higher the deformation deformation process deformation of increasingdegree, degree, degree, increasing increasing deformation increasing the the the degree ofinterminterm TDCs,intermolecularolecularolecular the changes forces forces forces and, and, in and, consequently, theconsequently, consequently, mechanical the the theelastic propertieselastic elastic modulus. modulus. modulus. of vitrinite and inertinite are different but the difference between the hardness and the elastic modulus of the two is getting smaller and smaller. 5.3. Recommendations In5.3. addition, 5.3.Recommendations Recommendations the micro mechanical properties of vitrinite and inertinite are both controlled by the macromolecularItIt can canIt can be be realized be realized structure. realized from from from Some the the the above above recommendations above conclusions conclusions conclusions that that for that in in future the thein the process process work process ofin of increasing field increasingof increasing of TDCs deformation deformation deformation are as follows: degreedegreedegree of of TDCs, TDCs,of TDCs, the the thechanges changes changes in in the thein themechanical mechanical mechanical properties properties properties of of vitrinite vitriniteof vitrinite and and and inertinite inertinite inertinite ar aree aredifferent different different but but but thethe differencethe difference difference between between between the the hardnessthe hardness hardness and and and the the elasticthe elastic elastic modulus modulus modulus of of the the of twothe two two is is getting getting is getting smaller smaller smaller and and and smaller. smaller. smaller. InIn addition, addition,In addition, the the the micro micro micro mechanical mechanical mechanical properties properties properties ofof vitrinite vitriniteof vitrinite and and and inertiniteinertinite inertinite are are are both both both controlled controlled controlled byby theby the the macromolecularmacromolecularmacromolecular structure. structure. structure. Some Some Some recomm recomm recommendationsendationsendations for for future forfuture future work work work in in field fieldin field of of TDCs TDCsof TDCs are are areas as follows: follows:as follows: (1)(1) The (1)The The tectonically tectonically tectonically deformed deformed deformed coal coal coal cannot cannot cannot be be simply simplybe simply studied studied studied as as a a aswhole. whole. a whole. The The The coal coal coal macerals macerals macerals should should should bebe studied studiedbe studied separately separately separately to to reveal revealto reveal the the theevolution evolution evolution of of mechanical mechanicalof mechanical properties properties properties of of TDCs. TDCs.of TDCs. (2)(2) The (2)The The coalificatio coalificatio coalificationnn process process process of of TDCs TDCsof TDCs is is different different is different from from from that that tha of of tunalerted unalertedof unalerted coal coal coal and and and it it may may it may be be more morebe more meaningfulmeaningfulmeaningful to to study studyto study coalification coalification coalification process process process of of TDCs TDCsof TDCs from from from the the theperspective perspective perspective of of a aof single single a single coal coal coal maceral. maceral. maceral. (3)(3) The (3)The The micro micro micro mechanical mechanical mechanical properties properties properties of of vitrinite vitriniteof vitrinite and and and inertinite inertinite inertinite of of TDCs TDCsof TDCs have have have obvi obvi obviouslyouslyously different different different evolutionevolutionevolution characteristics. characteristics. characteristi As cs.As a Asa coal coal a coal reservoir, reservoir, reservoir, the the physicalthe physical physical properties properties properties of of vitrinite vitriniteof vitrinite and and and inertinite inertinite inertinite should should should alsoalsoalso be be different, different,be different, which which which has has hasimportant important important directive directive directive significances significances significances in in coalbed coalbed in coalbed methane methane methane exploration exploration exploration and and and the the the coalcoalcoal mine mine mine gas gas gasdrainage. drainage. drainage.

6.6. Conc Conc6. Conclusionslusionslusions InIn thisIn this this study, study, study, the the the micromechanical micromechanical micromechani cal properties properties properties and and and macromolecular macromolecular macromolecular structural structural structural evolution evolution evolution of of of vitrinitevitrinitevitrinite and and and inertinite inertinite inertinite in in TDC inTDC TDC with with with different different different degrees degrees degrees of of deformation ofdeformation deformation were were were quantified quantified quantified through through through nanoindentationnanoindentationnanoindentation experiments, experiments, experiments, NMR NMR NMR and and and FTIR FTIR FTIR and and and the the thefollowing following following concl concl conclusionsusionsusions are are areobtained: obtained: obtained: ((1)1) The(The1) The evolution evolution evolution of of the theof the mechanical mechanical mechanical properties properties properties of of vitrinite vitriniteof vitrinite and and and inertinite inertinite inertinite in in TDC TDCin TDC have have have been been been quantitativelyquantitativelyquantitatively describeddescribed described atat the theat the microscale.microscale. microscale. TheThe The hardnesshardness hardness andand and elasticelastic elastic modulusmodulus modulus ofof the theof the inertiniteinertinite inertinite werewere were higherhigherhigher than than than those those those ofof the theof thevitrinite. vitrinite. vitrinite. FromFrom From un unaltered alteredunaltered coal coal coal to to powdery powderyto powdery coal, coal, coal, the the thehardness hardness hardn of essof the theof thevitrinite vitrinite vitrinite andandand inertiniteinertinite inertinite graduallygradually gradually decreaseddecreased decreased andand and thethe the differencedifference difference betweenbetween between thethe the twotwo two becamebecame became smaller.smaller. smaller. TheThe The elasticelastic elastic modulusmodulusmodulus ofof thetheof the vitrinitevitrinite vitrinite showedshowed showed anan increasinganincreasing increasing trendtrend trend butbut but thatthat that ofof the theof the inertiniteinertinite inertinite showedshowed showed nono obviousnoobvious obvious rregularities.egularities.regularities. ((2)2) The (The2) The inertinite inertinite inertinite did did didnot not notshow show show the the thecharacteristics characteristics characteristics of of inertness inertnessof inertness under under under the the theaction action action of of tectonic tectonicof tectonic stress stress stress butbut but likelike like the the the vitrinite, vitrinite, vitrinite, as as the theas the degree degree degree of of deformation deformationof deformation increased, increased, increased, the the the aliphatic aliphatic aliphatic side side side chains chains chains gradually gradually gradually detacheddetacheddetached and and and the the thearomatic aromatic aromatic degree degree degree increased increased increased.. .In In addition, addition,In addition, the the thesize size size of of the theof theBSU BSU BSU was was was also also also reduced. reduced. reduced. (3) For both vitrinite and inertinite, the decreases in the aliphatic carbons and the XBP and the (3) F(3)or Fbothor both vitrinite vitrinite and and inertinite, inertinite, the thedecreases decreases in the in thealiphatic aliphatic carbons carbons and and the theXBP XandBP and the the increase in the ratio of fa/fal could reduce the hardness of the TDC. Because the detachment of aliphatic increaseincrease in the in ratiothe ratio of fa of/fal fcoulda/fal could reduce reduce the hardnessthe hardness of the of TDC.the TDC. Because Because the detachmentthe detachment of aliphatic of aliphatic sidesideside chains chains chains could could could result result result in in structural in structural structural defects, defects, defects, ttherehere there should should should be be bemore more more interspace interspace interspace among among among the the the macromolecularmacromolecularmacromolecular groups groups groups for for the forthe extrusionthe extrusion extrusion of of the theof macromolecularthe macromolecular macromolecular groups groups groups caused caused caused by by the bythe indentationthe indentation indentati of onof of thethe thesubulate subulate subulate indenter, indenter, indenter, leading leading leading to to a ato reduction reduction a reduction in in the thein thehardness hardness hardness of of th thofee theTDC. TDC. TDC. (4) For vitrinite, the decrease in aliphatic carbons and the increase in the fa/fal could promote the

elastic modulus because with the increasing deformation degree of the TDC, the detachment of aliphatic side chains and the addition of aromatic rings could increase the dipole moment of the macromolecules in the TDC, resulting in the increase in the intermolecular forces and elastic

Energies 2020, 13, 6618 20 of 23

(1) The tectonically deformed coal cannot be simply studied as a whole. The coal macerals should be studied separately to reveal the evolution of mechanical properties of TDCs. (2) The coalification process of TDCs is different from that of unalerted coal and it may be more meaningful to study coalification process of TDCs from the perspective of a single coal maceral. (3) The micro mechanical properties of vitrinite and inertinite of TDCs have obviously different evolution characteristics. As a coal reservoir, the physical properties of vitrinite and inertinite should also be different, which has important directive significances in coalbed methane exploration and the coal mine gas drainage.

6. Conclusions In this study, the micromechanical properties and macromolecular structural evolution of vitrinite and inertinite in TDC with different degrees of deformation were quantified through nanoindentation experiments, NMR and FTIR and the following conclusions are obtained: (1) The evolution of the mechanical properties of vitrinite and inertinite in TDC have been quantitatively described at the microscale. The hardness and elastic modulus of the inertinite were higher than those of the vitrinite. From unaltered coal to powdery coal, the hardness of the vitrinite and inertinite gradually decreased and the difference between the two became smaller. The elastic modulus of the vitrinite showed an increasing trend but that of the inertinite showed no obvious regularities. (2) The inertinite did not show the characteristics of inertness under the action of tectonic stress but like the vitrinite, as the degree of deformation increased, the aliphatic side chains gradually detached and the aromatic degree increased. In addition, the size of the BSU was also reduced. (3) For both vitrinite and inertinite, the decreases in the aliphatic carbons and the XBP and the increase in the ratio of fa/fal could reduce the hardness of the TDC. Because the detachment of aliphatic side chains could result in structural defects, there should be more interspace among the macromolecular groups for the extrusion of the macromolecular groups caused by the indentation of the subulate indenter, leading to a reduction in the hardness of the TDC. (4) For vitrinite, the decrease in aliphatic carbons and the increase in the fa/fal could promote the elastic modulus because with the increasing deformation degree of the TDC, the detachment of aliphatic side chains and the addition of aromatic rings could increase the dipole moment of the macromolecules in the TDC, resulting in the increase in the intermolecular forces and elastic modulus. However, for inertinite, there is no obvious correlation between the macromolecular structures and elastic modulus.

Author Contributions: Original draft preparation, A.W.; Supervision, D.C.; Resources, Y.W.; Methodology, Z.L. All authors have read and agreed to the published version of the manuscript. Funding: This study was financially supported by the National Natural Science Foundation of China (No. 41902170, 42072197, 41972174) and Youth Science and Technology Research Project in Natural Science Foundation of Shanxi Province (201801D221355). Conflicts of Interest: The authors declare no conflict of interest.

References

1. Ju, Y.; Li, X. New research progress on the ultrastructure of tectonically deformed coals. Prog. Nat. Sci. 2009, 19, 1455–1466. [CrossRef] 2. Wang, A.; Cao, D.; Li, J.; Jiang, A.; Yang, C. A New Discovery on the Deformation Behavior of Shale Gas Reservoirs Aﬀecting Pore Morphology in the Juhugeng Coal Mining Area of Qinghai Province, Northwest China. Acta Geol. Sin. 2017, 91, 1932–1933. [CrossRef] 3. Cao, D.; Wang, A.; Ning, S.; Li, H.; Guo, A.; Chen, L.; Liu, K.; Tan, J.; Zheng, Z. Coalﬁeld structure and structural controls on coal in China. Int. J. Coal Sci. Technol. 2020, 7, 220–239. [CrossRef] 4. Wei, Y.; Cao, D.; Yuan, Y.; Zhu, X.; Zhang, X.; Yao, Z.; Zhou, J. Characteristics of Pulverized Coal during Coalbed Methane Drainage in Hancheng Block, Shaanxi Province, China. Energy Explor. Exploit. 2013, 31, 745–757. [CrossRef] Energies 2020, 13, 6618 21 of 23

5. Wei, Y.; Li, C.; Cao, D.; Wang, A.; Zhang, A.; Yao, Z. The effects of particle size and inorganic mineral content on fines migration in fracturing proppant during coalbed methane production. J. Pet. Sci. Eng. 2019, 182, 106355. [CrossRef] 6. Wang, A.; Wei, Y.; Yuan, Y.; Li, C.; Li, Y.; Cao, D. Coalbed methane reservoirs’ pore-structure characterization of different macrolithotypes in the southern Junggar Basin of Northwest China. Mar. Pet. Geol. 2017, 86, 675–688. [CrossRef] 7. Wang, A.; Li, J.; Wei, Y.; Yang, C.; Nie, J.; Cao, D. Gas migration for terrestrial gas hydrates in the Juhugeng mining area of Muli basin, Qilian Mountains, Northwest China. Energy Explor. Exploit. 2020, 38, 989–1013. [CrossRef] 8. Cao, D.; Li, X.; Zhang, S. Influence of tectonic stress on coalification: Stress degradation mechanism and stress polycondensation mechanism. Sci. China Ser. D: Earth Sci. 2007, 50, 43–54. [CrossRef] 9. Wang, X.; Chen, T.; Xu, H. Thickness Distribution Prediction for Tectonically Deformed Coal with a Deep Belief Network: A Case Study. Energies 2020, 13, 1169. [CrossRef] 10. Lu, J.; Wang, Y.; Chen, J. Detection of Tectonically Deformed Coal Using Model-Based Joint Inversion of Multi-Component Seismic Data. Energies 2018, 11, 829. [CrossRef] 11. 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. [CrossRef] 12. Chen, Q.; Jing, J.; Liu, J.; Long, J.; Zhang, S. Productivity Evaluation of Coalbed Methane Well with Geophysical Logging-Derived Tectonically Deformed Coal. Energies 2019, 12, 3459. [CrossRef] 13. Liu, H.; Jiang, B. Stress response of noncovalent bonds in molecular networks of tectonically deformed coals. Fuel 2019, 255, 115785. [CrossRef] 14. Cao, D.Y.; Zhang, S.R.; Ren, D.Y. The influence of structural deformation on coalification: A case study of carboniferous coal measures in the northern foothills of the Dabie orogenic belt. Geological Review 2002, 48, 313–317. 15. Godyń,K.; Kožušníková, A. Microhardness of Coal from Near-Fault Zones in Coal Seams Threatened with Gas-Geodynamic Phenomena, Upper Silesian Coal Basin, Poland. Energies 2019, 12, 1756. [CrossRef] 16. Li, W.; Liu, H.; Song, X. Multifractal analysis of Hg pore size distributions of tectonically deformed coals. Int. J. Coal Geol. 2015, 138–152. [CrossRef] 17. Stach, E.; Mackowsky, M.-T.H.; Techmuller, M. Stach’s textbook of coal petrology, 3rd ed.; Gebrüder Borntraeger: Berlin, Germany, 1982; pp. 66–107. 18. Wang, A.; Cao, D.; Wei, Y.; Nie, J.; Qin, R. Comparison of nanopore evolution in vitrinite and inertinite in coalbed methane reservoirs during coalification. J. Nat. Gas Sci. Eng. 2020, 78, 103289. [CrossRef] 19. Qin, R.; Wang, A.; Cao, D.; Wei, Y.; Ding, L.; Li, J. Effect of peat mire evolution on pore structure characteristics in thick coal seam: Examples from Xishanyao Formation (Middle Jurassic), Yili Basin, China. Energy Explor. Exploit. 2020, 38, 1484–1514. [CrossRef] 20. Pan, J.; Meng, Z.; Hou, Q.; Ju, Y.; Cao, Y. Coal strength and Young’s modulus related to coal rank, compressional velocity and maceral composition. J. Struct. Geol. 2013, 54, 129–135. [CrossRef] 21. Hou, C.; Jiang, B.; Liu, H.; Song, Y.; Xu, S. The differences of nanoscale mechanical properties among coal maceral groups. J. Nat. Gas Sci. Eng. 2020, 80, 103394. [CrossRef] 22. Kožušníková, A. Determination of microhardness and elastic modulus of coal components by using indentation method. Geolines 2009, 22, 40–43. 23. Godyń,K.; Dutka, B.; Chuchro, M.; Młynarczuk, M. Synergy of Parameters Determining the Optimal Properties of Coal as a Natural Sorbent. Energies 2020, 13, 1967. [CrossRef] 24. Fender, T.; Rouainia, M.; Van Der Land, C.; Jones, D.M.; Mastalerz, M.; Hennissen, J.A.; Graham, S.P.; Wagner, T. Geomechanical properties of coal macerals; measurements applicable to modelling swelling of coal seams during CO2 sequestration. Int. J. Coal Geol. 2020, 228, 103528. [CrossRef] 25. Ju, Y.; Jiang, B.; Hou, Q.-L.; Wang, G.-L. [FTIR spectroscopic study on the stress effect of compositions of macromolecular structure in tectonically deformed coals]. Guang Pu Xue Yu Guang Pu Fen Xi 2005, 25, 1216–1220. 26. Song, Y.; Jiang, B.; Qu, M. Macromolecular evolution and structural defects in tectonically deformed coals. Fuel 2019, 236, 1432–1445. [CrossRef] Energies 2020, 13, 6618 22 of 23

27. Cao, D.Y.; Zhan, W.F.; Zhang, J.; Yang, S.C.; Zhang, L.S.; Fan, H.P.; Huang, P.; Qiao, J.; Liu, D.M.; Chang, M.H. Neotectonic character of Handan-Fengfeng mining area and its significance for coal resource exploitation. J. China Coal Soc. 2007, 32, 141–145. 28. Lv, D.W.; Lu, C.; Wen, Z.; Song, H.; Yin, S. Coal-bed methane geology of the No. 2 coal seam in Fengfeng Coalfield, North China. Arab. J. Geosci. 2019, 12, 529. [CrossRef] 29. Bao, Y.; Wang, W.; Zhou, Y. Investigation of the relationship between elastic modulus and hardness based on depth-sensing indentation measurements. Acta Mater. 2004, 52, 5397–5404. [CrossRef] 30. Sun, C.L.; Li, G.C.; Elgharib, G.M.; Xu, J.H.; Rong, H.Y. Experimental investigation on the mechanical properties of crushed coal samples based on the nanoindentation technique. J. China Coal Soc. 2020. [CrossRef] 31. Oliver, W.; Pharr, G. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J. Mater. Res. 1992, 7, 1564–1583. [CrossRef] 32. Painter, P.; Starsinic, M.; Coleman, M. Determination of functional groups in coal by Fourier transform interferometry. Fourier Transform. Infrared Spectra 1985, 169–241. [CrossRef] 33. Ibarra, J.; Muñoz, E.; Moliner, R. FTIR study of the evolution of coal structure during the coalification process. Org. Geochem. 1996, 24, 725–735. [CrossRef] 34. Ibarra, J.V.; Munoz, E.R.; Bonet, A.J. FT-IR investigation on char formation during the early stages of coal pyrolysis. Fuel 1994, 73, 918–924. [CrossRef] 35. Guo, Y.; Bustin, R. Micro-FTIR spectroscopy of liptinite macerals in coal. Int. J. Coal Geol. 1998, 36, 259–275. [CrossRef] 36. Dutta, S.; Hartkopf-Fröder, C.; Witte, K.; Brocke, R.; Mann, U. Molecular characterization of fossil palynomorphs by transmission micro-FTIR spectroscopy: Implications for hydrocarbon source evaluation. Int. J. Coal Geol. 2013, 115, 13–23. [CrossRef] 37. Song, Y.; Jiang, B.; Mathews, J.P.; Yan, G.; Li, F. Structural transformations and hydrocarbon generation of low-rank coal (vitrinite) during slow heating pyrolysis. Fuel Process. Technol. 2017, 167, 535–544. [CrossRef] 38. Li, W.; Zhu, Y. Structural Characteristics of Coal Vitrinite during Pyrolysis. Energy Fuels 2014, 28, 3645–3654. [CrossRef] 39. Liu, Y.; Zhu, Y.; Chen, S. Effects of chemical composition, disorder degree and crystallite structure of coal macromolecule on nanopores (0.4–150 nm) in different rank naturally-matured coals. Fuel 2019, 242, 553–561. [CrossRef] 40. Suggate, R.; Dickinson, W. Carbon NMR of coals: The effects of coal type and rank. Int. J. Coal Geol. 2004, 57, 1–22. [CrossRef] 41. Erdenetsogt, B.-O.; Lee, I.; Lee, S.K.; Ko, Y.-J.; Bat-Erdene, D. Solid-state C-13 CP/MAS NMR study of Baganuur coal, Mongolia: Oxygen-loss during coalification from lignite to subbituminous rank. Int. J. Coal Geol. 2010, 82, 37–44. [CrossRef] 42. Malumbazo, N.; Wagner, N.; Bunt, J.; Van Niekerk, D.; Assumption, H. Structural analysis of chars generated from South African inertinite coals in a pipe-reactor combustion unit. Fuel Process. Technol. 2011, 92, 743–749. [CrossRef] 43. Dong, J.; Cheng, Y.; Hu, B.; Hao, C.; Tu, Q.; Liu, Z.-D. Experimental study of the mechanical properties of intact and tectonic coal via compression of a single particle. Powder Technol. 2018, 325, 412–419. [CrossRef] 44. Liu, F.; Yang, H.; Yang, M.; Wu, J.; Yang, S.; Yu, D.; Wu, X.; Wang, J.; Gates, I.D.; Wang, J. Effects of molecular polarity on the adsorption and desorption behavior of asphaltene model compounds on silica surfaces. Fuel 2021, 284, 118990. [CrossRef] 45. Al-Malah, K.I. Aqueous solubility of a simple (single-carbon) organic molecule as a function of its size & dipole moment. J. Mol. Model. 2011, 17, 1029–1034. [CrossRef][PubMed] 46. Ge, T.; Zhang, M.X. Dielectric properties of the model compounds of thiophene sulfur in coals and its relation to molecular polarity. J. China Univ. Min. Technol. 2016, 45, 1245–1250. 47. Dean, J.A. Hand Book of Organic Chemistry; McGraw-Hill: New York, NY, USA, 1987; pp. 450–500. 48. Nishioka, M.; Larsen, J.W. Association of aromatic structures in coals. Energy Fuels 1990, 4, 100–106. [CrossRef] 49. You, C.-Y.; Fan, X.; Zheng, A.-L.; Wei, X.-Y.; Zhao, Y.-P.; Cao, J.-P.; Zhao, W.; Zhou, C.-C.; Zhu, J.-L.; Chen, L.; et al. Molecular characteristics of a Chinese coal analyzed using mass spectrometry with various ionization modes. Fuel 2015, 155, 122–127. [CrossRef] Energies 2020, 13, 6618 23 of 23

50. Iino, M. Network structure of coals and association behavior of coal-derived materials. Fuel Process. Technol. 2000, 62, 89–101. [CrossRef]

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).