Simulation of Gas Production Mechanisms in Shear Deformation of Medium-Rank Coal.

The discovery of mechanochemical action provides a theoretical basis for revealing gas production from coal under stress degradation. The research on gas production in such a manner is conducive to revealing mechanisms of coal and gas outburst and excess coalbed methane (CBM). By selecting a model of a macromolecular structure of Given medium-rank coal, its structure was optimized based on molecular mechanics, molecular dynamics, and quantum chemistry, and the six optimized models were constructed into a coal polymer cell. The coal polymer cell was loaded to shear deformation through large-scale atomic/molecular massively parallel simulator (LAMMPS) software. The Given model was optimized by quantum chemistry software Gaussian and the frequency was calculated to obtain the bond strength and average local ionization energy (ALIE). The following understanding was reached: under shear, bridge bonds of a ring structure, and large π-bonds are subjected to shear and tensile action, and atoms (atomic clusters) in the outermost region of coal macromolecules tend to be sheared by surrounding molecules. The shear action shortens a molecular chain of medium-rank coal with a cross-linked structure and promotes the evolution of the coal macromolecular structure. The shear action can lead to the formation of free radicals, such as H• and •CO from macromolecules of medium-rank coal, thus producing many small gas molecules, such as H2 and CO. Moreover, the shear action can not only break chemical bonds but also can produce new chemical bonds. The research on gas production mechanisms under shear deformation of medium-rank coal provides a certain reference for studying mechanochemistry.

Introduction

Mechanochemical phenomena have long since been discovered: until the end of the 19th century, when Matthew Carey Lea proved that Ag and HgCl2 can produce Cl2 by mechanical grinding, while heating HgCl2 can only sublimate powder, mechanochemistry was established as an independent branch of chemistry.[1] In 1919, Wilhelm Ostwald proposed the concept of mechanochemistry.[2] Force directly changes chemical bonds, leading to reactions,[3] which reduces obstacles to, and accelerates, chemical reactions.[4] Stress can even change reaction pathways by changing reaction barriers. Different from the way that stress affects the reaction,[5] heating can increase the energy of reactants and make more reactants cross the reaction barriers, but heating cannot reduce the reaction barriers. The latest research shows that force can directly act on molecular structures to make them change chemically, that is, mechanochemical action.[6−8]

Due to the early coal forming time and the influence of plate movement, most coal reservoirs in China are affected by different degrees of tectonism. Tectonism makes coal subject to different degrees of stress and various types of deformation. Based on mechanochemical action, it is speculated that structural deformation can also result in chemical changes in the macromolecular structure of coal. As early as the 1920s, White explained that the metamorphism of coal in the eastern USA is mainly caused by structural stress.[9,10] This is particularly true for unconventional coalification near thrust faults, typically shown in ① Rocky Mountains;[11] ② Cordillera Mountains and Kandersteg Region in south-eastern Canada,[12] and ③ the Swiss Alps.[13] Graphitization, the end stage of coalification, may occur on carbon-rich faults.[14,15] In the research into graphitization, it is generally believed that shear stress plays an important role in this process,[16,17] which may promote the gradual alignment of basic structural units[18,19] and reduce the activation energy of coal graphitization.[20] Graphitization can occur in shear tests at a temperature as low as 873.15 K, while it is realized at above 2273.15 K in simple heating tests. Using electron paramagnetic resonance (EPR), researchers have found that more free radicals are induced by bond breaking under stress/mechanical force.[21] The results of Fourier transform infrared (FTIR) spectroscopy and carbon-13 nuclear magnetic resonance (13C NMR) show that tectonically deformed coal (TDC) has lower aliphatic carbon atoms and higher aromatic carbon atoms compared with the adjacent undeformed coal.[22] TDC and undeformed coal are sampled from contiguous areas, so they show the same thermal evolution and differences in influences of stress. The effects of stress on TDC are summarized as stress degradation and stress condensation,[23] indicating that stress can promote the early evolution of coal.

It has also been reported that coal can produce gas molecules under the action of external force. As early as 1966, Juntgen and Karweil found that, when bituminous coal is ground at 473.15 to 573.15 K, gases, such as CO, H2, CO2, and CH4, can be generated.[24] A team led by professor Hou Quanlin also detected CO gas through creep experiments at a low temperature and high pressure on high-rank coal[25] and studied gas-production processes and mechanisms during coal deformation through quantum mechanical calculation.[26] The external force on coal can be divided into tensile, compressive, and shear actions. Combined with the previous research results, it is found that compression cannot chemically change the macromolecular structure of coal, while tension can degrade its macromolecules into relatively small molecular fragments.[27] Therefore, it is considered that only shearing action can cause the coal macromolecular structure to produce small gas molecules. Using the widely recognized medium-rank coal to represent macromolecular structures of Given[28] coal, the corresponding coal polymer model was constructed based on a molecular dynamics simulation and studied by loading the coal to shear deformation. The aim is to reveal mechanisms of gas production in the deformation of the macromolecular structure of coal.

Simulation and Calculation Methods

Construction and Optimization of Coal Polymer Cells

The macromolecular structure of Given medium-rank coal, with chemical formula C102H84O10N2, was mainly dominated by cross-linked structures. It included structures, such as benzene rings, naphthalene rings, common five-membered carbon rings, common six-membered carbon rings, common seven-membered carbon rings, oxygen-containing functional groups, nitrogen-containing functional groups, and methyl groups. To understand the broken chemical bonds more intuitively, the line mode was used in the macromolecular structure model of the coal, and part atomic-number information was added to select the best angle for output, as shown in Figure .

Figure 1

Macromolecular structure model of Given coal (gray, red, blue, and green separately indicate C, O, N, and H).

The macromolecular structure of Given coal was plotted by Materials Studio (Accelrys Inc.) software, and six macromolecular structures of Given coal were optimized and loaded to assemble polymer cells. The specific optimization details are demonstrated in the method provided by Yang,[27] as shown in Figure .

Figure 2

Polymer cell model of Given coal.

Simulation of the Shear Process in Coal Polymer Cells

The optimized polymer model of Given coal was transformed into a data format recognized by LAMMPS. The three-dimensional periodic boundary conditions and ReaxFF force fields were used.[29] The energy of the model was minimized by sd and cg methods, and the temperature was kept at 298 K by the temp/rescale method in the NVE system. The coal polymer model was simulated by applying shear in the XY-, XZ-, and YZ-directions at the real strain rate of 1.5 × 10–3, and the calculation was carried out in 4000 steps.

By observation using visualization software Ovito,[30] it is found that each coal macromolecule is gradually extended and adjusted during the shearing process and finally broken. Statistical analysis was conducted by taking the XY-direction as an example.

Results

Chemical Bonds Broken (Generated) in Coal Macromolecules under the Shearing of Medium-Rank Coal

By sorting the bond.reaxc file output by LAMMPS, it is found that there are 40 broken chemical bonds in total, and six chemical bonds are newly generated through observation combined with the Ovito visualization process. Statistics pertaining to each broken chemical bond and the numbers thereof are listed in Table and those pertaining to newly generated chemical bonds in Table .

Table 1

Statistical Table of Broken Chemical Bonds and Times About 6 Given Coal Macromolecules During Shearing

chemical bonds	bond breaking times	chemical bonds	bond breaking times	chemical bonds	bond breaking times
C17–C18	1	C43–C44	2	C81–C86	1
C18–C27	2	C44–C45	1	C81–H170	1
C19–N24	3	C50–H141	2	C82–C87	1
C25–C28	1	C55–H144	1	C87–H175	1
C26–C27	1	C58–H153	1	C94–C95	1
C26–C34	1	C61–C62	1	C94–C99	2
C29–C30	1	C64–C65	1	C95–C103	1
C36–C42	1	C65–H155	1	C96–C97	3
C37–C39	2	C67–C68	1	C97–C98	2
C39–C40	2	C72–C73	1	O112–H198	4
C39–C99	2	C73–H159	1	O1–H115	1
C41–C42	2	C76–H166	1	O93–H183	4
C41–C47	2	C80–C81	3
C42–C98	2	C80–H169	1

Table 2

Statistical Table of New Chemical Bonds and Times About 6 Given Coal Macromolecules During Shearing

chemical bonds	C17–C19	N24–C80	C40–C47	C94–C98	C96–C103	C96–O112
forming bond times	1	1	1	1	1	1

Based on data in Table and connection of chemical bonds in the macromolecular structure of the Given coal (Figure ), more broken chemical bonds are located at the connection of each part, such as chemical bonds C18–C27, C19–N24, C37–C39, C39–C40, C39–C99, C41–C42, C41–C47, C42–C98, C43–C44, C80–C81, C94–C99, C96–C97, and C97–C98. The others are at the periphery of molecules, such as C50–H141, C76–H166, C50–H141, O112–H198, and O93–H183. There are also various types of broken chemical bonds, including various chemical bonds containing H (hydroxyl, methyl, and methylene), C–C single bonds, and large π-bonds.

The generated chemical bonds comprise five-membered rings formed by breaking one bond in six-membered rings and bonds formed by the combination of free radicals, such as N24–C80 and C96–O112.

Bond Breaking Order of Coal Macromolecules under Shear

Under shear, the chemical bonds of each coal macromolecule are broken in different orders. By taking the XY-direction as an example, the shearing action causes the Y-axis to deform in the X-direction. The broken chemical bonds of six molecules and their order are summarized in Table .

Table 3

Chemical Bond Breaking (Generation) Sequence of Given Coal Macromolecules During Shearinga

bonds broken sequence	no. 1	no. 2	no. 3	no. 4	no. 5	no. 6
1	O112–H198	C19–N24	O112–H198	O93–H183	C58–H153	O112–H198
2	O93–H183	C81–H170	C73–H159	C19–N24	O93–H183	C65–H155
3	C96–C97	C18–C27	O93–H183	C17–C18	O112–H198	C43–C44
4	C43–C44	C80–C81	C42–C98	C17 + C19	C19–N24	C55–H144
5	C50–H141	C82–C87	C39–C40	C37–C39	C64–C65	O1–H115
6	C41–C42	C80 + N24	C94–C99	C80–C81	C41–C42	C41–C47
7	C94–C95	C42–C98	C94 + C98	C36–C42	C18–C27	C87–H175
8	C39–C99	C39–C99	C96–C97	C26–C27	C39–C40	C97–C98
9	C97–C98	C96–C97	C96 + O112	C50–H141	C80–H169	C94–C99
10	C72–C73	C95–C103	C61–C62	C44–C45		C25–C28
11	C80–C81	C96 + C103	C81–C86			C26–C34
12			C41–C47			C29–C30
13			C40 + C47
14			C37–C39
15			C67–C68
16			C76–H166

+ represents new forming bonds.

As demonstrated in Table , under shear, the overall trend is as follows: C–H and O–H bonds are most easily broken and then large conjugate π-bonds and C–C cross-linked bonds are broken alternately. The C–H and O–H bonds are mostly located in atoms (atomic group) in the periphery of basic structural units of coal macromolecules. The C–H bonds are mainly located on methylene, such as C76–H166 and C58–H153 connected by bridge bonds and methyl (C50–H141). The O–H bonds are mainly found on hydroxyl, such as O93–H183 and O112–H198. Furthermore, C–C bonds mostly reside on bridge bonds (C37–C39, C39–C40, C39–C99, C41–C42, and C42–C98), chemical bonds on seven-membered rings (C41–C47 and C43–C44), and large π-bonds (C19–N24, C94–C99, and C96–C97).

Products of Given Medium-Rank Coal under Shearing Action

The macromolecular structure model of coal is mainly dominated by a cross-linked ring structure and only contains some structures, such as ketone bonds, hydroxy, methyl, and bridge-bound methylene. Under shear, only a small number of H• free radicals are separated from the macromolecular structure, and a ring-opening reaction tends to occur (Figure ). These H• free radicals combine with each other to form small stable H2 molecules, and •CO is produced through some ring-opening reactions.

Figure 3

Small molecules formed by shearing about Given coal. (a) Molecule no. 1 underwent a ring-opening reaction, shedding •CO, H atom transfer from O93–H183 and O112–H198. (b) Molecule no. 3 underwent the complex ring-opening and binding reaction, shedding the H atom, and H atom transfer from O93–H183 and O112–H198. (c) Molecule no. 6 underwent ring-opening reaction, shedding the H atom.

It can be seen from Table that H atoms are separated first, followed by •CO under the shear action of medium-rank coal. Therefore, H2 and CO are successively produced from this medium-rank coal under shear. According to the statistical results, the numbers of H• and •CO are 19 and one, and two H free radicals can form a H2 molecule. The amount of gas produced can be calculated by molecular weight, as shown in Formula .where G represents the production (m3/t) of f gas (f represents the type of gas, including H2, CO); n denotes the amount of gas f (the amount of H2 is n/2); Mf and Mt denote the mass fractions (g/mol) of gas f and coal polymer, respectively.

By substituting 19/2 and 1 into formula , the production of H2 and CO from this Given coal polymer under shear was found to have reached 23.71 and 2.50 m3/t, respectively.

Discussion

H atoms are produced by breaking of chemical bonds containing H atoms under shear. H atoms on methyl (C50–H141 bonds of nos 1 and 4 molecules), methylene (C73–H159 and C76–H166 bonds of no. 3 molecule; C58–H153 and C80–H169 of no. 5 molecule; C55–H144, C65–H155, and C87–H175 of no. 6 molecule), methylidyne (C81–H170 bonds of no. 2 molecule), and hydroxyl (O93–H183 on nos 1, 3, 4, and 5 molecules; O112–H198 on nos 1, 3, 5, and 6 molecules and O1–H115 on no. 6 molecule) can be separated under shear. •CO is generated by separating H atoms on O112–H198 hydroxyl of no. 1 molecule, breaking large π-bonds C96–C97 on benzene rings, and finally breaking C97–C98 bonds. The ring-opening reaction mainly happens to two categories of chemical bonds, namely, rings formed by common single rings (double rings) and rings formed by large π-bonds. They are mainly concentrated in three large areas: ① C17–C18, C18–C27, C19–N24, C25–C28, C26–C27, C26–C34, and C29–C30; ② C36–C42, C37–C39, C39–C40, C39–C99, C41–C42, C41–C47, C42–C98, C43–C44, C44–C45, C94–C95, C94–C99, C95–C103, C96–C97, and C97–C98; and ③ C61–C62, C64–C65, C67–C68, C72–C73, C80–C81, C81–C86, and C82–C87. The breaking of bonds propagates from both ends to the middle.

Effects of Position of Chemical Bonds in Molecules on Breaking of Chemical Bonds under the Shear Action

The spatial distribution of each Given coal macromolecule in the polymer is different. The atoms (atomic clusters) on the periphery of molecules are the first to contact, and the relative movement between the molecules will produce the shearing action. In this case, the chemical bonds (chemical bonds on the periphery of molecules) connected by atoms (atomic groups) at the shearing site are subjected to shearing action, and the connected parts are stretched by the shearing point. Therefore, tensile action occurs at the earliest and affects each molecule in the shear process. Coal molecules, being soft molecules, are stretched to some extent under tensile action. When chemical bonds rotate clockwise around the point of action in the direction of the applied shear (the positive X-direction), the rotation angle is between 0 and 90° (or between 270 and 360°) so that chemical bonds are easily broken under tensile action. When shear is applied at shear sites to a certain extent, chemical bond threats are broken. For the macromolecular structure of coal, there are obvious cross-linked structures. The cross-linked structure contains bridge bonds of low strength and lies in the middle of molecules, making any direct effect difficult to realize, but an indirect effect on the surrounding chemical bonds arises, therefore, the cross-linked part is protected. Therefore, the ring-opening reaction of the macromolecular structure involves tensile breaking. The chemical bonds in the long-chain ring structure break from both ends and failure thereof gradually develop to the middle. The shear action is manifested in two ways: chemical bonds connected to atomic groups in the outermost regions of molecules are broken under shear and H atoms are mainly separated. They include O1–H115 (hydroxyl), O93–H183 (hydroxyl), O112–H198 (hydroxyl), C50–H141 (methyl), C55–H144 (methylene), C58–H153 (methylene), C65–H155 (methylene), C73–H159 (methylene), C76–H166 (methylene), C80–H169 (methylene), C81–H170 (methylidyne), and C87–H175 (methylene). Chemical bonds in the middle of the structure are readily subjected to tensile action, leading to tensile breaking. The chemical bonds mainly include C–C and C–N bonds, such as C17–C18 (a large π-bond), C19–N24 (a large pyridine π-bond), C26–C27 (a large π-bond), C26–C34 (a carbon–carbon single bond), C36–C42 (a carbon–carbon single bond), C37–C39 (a carbon–carbon single bond), C39–C40 (a carbon–carbon single bond), C39–C99 (a carbon–carbon single bond), C41–C42 (a carbon–carbon single bond), C41–C47 (a carbon–carbon double bond), C42–C98 (a carbon–carbon single bond), C43–C44 (a carbon–carbon single bond), C44–C45 (a carbon–carbon single bond), C64–C65 (a carbon–carbon single bond), C80–C81 (a carbon–carbon single bond), C82–C87 (a carbon–carbon single bond), and C94–C99 (a large π-bond). The position of chemical bonds in the whole molecule determines whether chemical bonds are subjected to tensile action or shear action in the shear process and ultimately determines the manner in which the chemical bonds break.

Under either tensile or shear action, the angle between a chemical bond and the shear direction constantly changes throughout the shear process. To determine the change of the angle between the chemical bond and the shear direction under shear, the coordinates of all atoms are output using a step length of 50 steps in LAMMPS software. The broken chemical bonds of no. 1 molecule in this coal polymer are not only present in significant quantity but are also of various types (separated H atoms and CO). The angle between the broken chemical bond and the X-direction is calculated by taking the data pertaining to no. 1 molecule as an example, as shown in Figure .

Figure 4

Angle between the broken chemical bonds and shear direction about no. 1 molecule in Given coal polymer varies with the shear process.

Because of the complexity of the cross-linked structure in the coal model, the trends shown in Figure cannot clearly show tensile and shear breaking, therefore, a parameter ΔR is introduced to measure the amount, and angle, of changes in chemical bonds for auxiliary judgmentwhere ΔR represents the ratio of changes in the length of chemical bonds and rotation angle of chemical bonds and the unit is Å/°; R1 and R2 indicate the bond lengths (Å) when the angles between the chemical bond and the shear direction are θ1 and θ2, respectively; θ1 and θ2 denote the angles (°) between the chemical bond and the shear direction under different shear steps.

A smaller absolute value of ΔR suggests that the bond length of the chemical bond changes little as the bond rotates by 1° and the chemical bond is mainly subjected to shearing action; a larger absolute value of ΔR implies that the bond length of the chemical bond changes significantly per degree angle of rotation and the chemical bond is mainly subjected to tensile action. ΔR is calculated from the final two data points before breaking of no. 1 molecule and the key statistical data are listed in Table .

Table 4

ΔR Data Statistical Table of No. 1 Molecule in Given Coal

bonds	ΔR (Å/°)	bonds	ΔR (Å/°)	bonds	ΔR (Å/°)	bonds	ΔR (Å/°)
O112–H198	0.058	C39–C99	0.167	C94–C95	0.451	C41–C42	–1.792
C50–H141	0.092	C97–C98	0.254	C43–C44	0.855	C96–C97	–54.897
O93–H183	0.100	C80–C81	0.340	C72–C73	–0.829

In accordance with ΔR data in Table , chemical bonds are broken under shearing action when |ΔR| ≤ 0.100 and under tensile action when |ΔR| > 0.100. While breaking, when the data points selected show that the length of chemical bonds is two to three times the initial length, the chemical bonds are considered to be completely broken.

As shown in Figure , for the ring structure, the broken chemical bonds are subjected to shearing action, under the influences of which the angle (θ) between the chemical bond and the shear direction gradually increases. Such a process is evinced by changes in the bond angle of chemical bonds, corresponding to the rotation of chemical bonds, while the bond length changes slightly. As the angle (θ) between the chemical bond and the shear direction begins to exceed 90°, significant strain energy is accumulated and the force changes from rotation to rotation and tension. With the increase of the distance between atoms, chemical bonds are rapidly broken, and the action mechanisms are shown in Figure . Atoms shown in red are subjected to the shear action, and the force can be decomposed into compression along the direction of the bond length (due to the relationship between the bond length and bond energy, the compressibility of bond length is very limited), and rotation occurs perpendicular to the direction of the bond length. When the chemical bonds rotate more than 90°, the force on those atoms shown in red can be decomposed into a stretching action along the bond length direction and a rotation perpendicular to the bond length direction. When the angle between the chemical bond and the shear action exceeds 30° while breaking, |ΔR| ≤ 0.100, such as for chemical bonds O112–H198 and C50–H141. The larger the angle between the chemical bond and the axis of the applied shear, the stronger the shearing action, so the chemical bonds are more easily broken thereunder. When the angle between the chemical bond and the applied shear is less than 30° while breaking, |ΔR| > 0.100. The smaller the angle, the stronger the tensile action, so the chemical bonds are easily broken under tensile action.

Figure 5

Schematic of the shear action mechanism. Adapted with permission from ref (31). Copyright 2021 Elsevier B.V.

Effects of Bond Strength and Bond Angle on Breaking of Chemical Bonds under Shear

When the force constant[32] was used to measure the strength, different types of bonds can be compared. The model was optimized, and the frequency was calculated at the B3LYP level of density functional theory,[33−37] using the Def2-SV(P) basis set[38] with the DFT-D3 dispersion correction[39] in Gaussian software. After verifying that the optimized structure is at the minimum point by checking the calculated results, the file of calculated wave functions is imported into the compliance software[32,40] to output the relaxed force constant of chemical bonds (bond angle). The results are listed in Table .

Table 5

Statistics of Relaxed Force Constant about Bonds Broken by Shear Action in Given Coal

bonds	relaxed force constant	bonds	relaxed force constant	bonds	relaxed force constant
C41–C42	2.611	C44–C45	4.505	C17–C18	5.917
C42–C98	3.534	C37–C39	4.587	C18–C27	5.952
C36–C42	3.610	C65–H155	4.785	C96–C97	6.135
C39–C40	3.846	C81–H170	4.878	C61–C62	6.173
C82–C87	4.016	C80–H169	4.902	C19–N24	6.250
C80–C81	4.098	C95–C103	4.926	C94–C99	6.289
C43–C44	4.115	C87–H175	5.000	C94–C95	6.803
C81–C86	4.132	C25–C28	5.155	C67–C68	6.944
C64–C65	4.237	C76–H166	5.208	C29–C30	7.519
C39–C99	4.292	C50–H141	5.263	O1–H115	7.576
O112–H198	4.292	C55–H144	5.263	C26–C27	7.634
C72–C73	4.310	C73–H159	5.376	C41–C47	7.874
O93–H183	4.367	C58–H153	5.405
C26–C34	4.386	C97–C98	5.747

It can be observed from Table that the overall trend of the bond strength of broken chemical bonds of this coal macromolecules is as follows: C–C single bonds, hydroxyl O–H on the benzene ring (affected by ketonic bonds), bridge-bound C–H or C–H on the side chain, large π-bonds, hydroxyl O–H on the benzene ring, and C=C double bonds (ranked thus in ascending order by bond strength). By combining the data in Tables and 5, chemical reactions allowing tensile breaking easily occur in the shear process at a high frequency and the broken chemical bonds are mostly concentrated on bridge bonds or ring structures. For instance, chemical bonds including C19–N24, C26–C27, C37–C39, C39–C40, C39–C99, C41–C42, C41–C47, C42–C98, C43–C44, C64–C65, C80–C81, C82–C87, and C94–C99 are distributed on both sides of the ring structure and as bridges for force transmission, they are easily broken under tensile action.

As demonstrated in Table , the bond angle of the broken chemical bonds of the coal macromolecule shows the following overall trend: chemical bonds with H atoms at the periphery, bridge bonds (for connecting conjugate rings), large π-bonds, and bridge bonds or chemical bonds on the common ring structure, and large conjugate π-bonds are ranked thus (in ascending order by bond angle). Therefore, H atoms at the periphery of the molecular structure are mostly easily sheared off, and they are O1–H115 (hydroxyl), O93–H183 (hydroxyl), O112–H198 (hydroxyl), C50–H141 (methyl), C55–H144 (methylene), C58–H153 (methylene), C65–H155 (methylene), C73–H159 (methylene), C76–H166 (methylene), C80–H169 (methylene), C81–H170 (methylidyne), and C87–H175 (methylene).

Table 6

Statistics of Bond Angle Relaxed Force Constant About the Bond Broken by Shear Action in Given Coal

angle	relaxed force constant	angle	relaxed force constant	angle	relaxed force constant
C72–C73–H159	0.738	C45–C44–C49	1.626	C42–C98–C97	2.299
C62–C73–H159	0.745	C43–C44–O49	1.639	C96–C97–O112	2.315
C59–C58–H153	0.762	C37–C39–C99	1.650	C28–C29–C30	2.410
C47–C55–H144	0.766	C40–C43–C44	1.656	C67–C68–C69	2.427
C64–C65–H155	0.767	C44–C45–C46	1.672	C19–N24–C25	2.445
C52–C65–155	0.772	C60–C80–C81	1.704	C68–C67–C72	2.475
C53–C58–H153	0.776	C41–C47–C46	1.706	C94–C99–C98	2.494
C2–O1–H115	0.778	C36–C42–C98	1.730	C6–C61–C62	2.577
C60–C80–H169	0.803	C81–C82–C87	1.736	C29–C30–C31	2.577
C75–C76–H166	0.805	C44–C45–C51	1.757	C25–C26–C27	2.584
C62–C73–C72	0.816	C36–C42–C50	1.764	C61–C62–C63	2.584
C77–C76–H166	0.824	C41–C42–C98	1.764	C18–C27–C26	2.611
C42–C50–H141	0.840	C44–C43–O48	1.786	C16–C17–C18	2.625
C86–C81–H170	0.845	C61–C87–C82	1.789	C95–C94–C99	2.653
C61–C87–H175	0.847	C81–C86–C88	1.789	C17–C18–C19	2.674
C81–C80–H169	0.861	C30–C29–C34	1.845	C97–C98–C99	2.695
C82–C87–H175	0.910	C42–C41–C47	1.848	C95–C103–C102	2.710
C80–C81–H170	0.915	C25–C28–C33	1.855	C18–C19–N24	2.717
C94-O93–H183	1.055	C39–C40–C43	1.890	C96–C97–C98	2.717
C52–C65–C64	1.124	C81–C86–C85	1.901	C39–C99–C98	2.725
C97–O112–H198	1.131	C27–C26–C34	1.901	C39–C40–C41	2.732
C67–C72–C73	1.149	C40–C41–C47	1.905	C36–C37–C39	2.740
C71–C72–C73	1.198	C18–C17–O23	1.946	C94–C95–C96	2.747
C61–C62–C73	1.304	C38–C37–C39	1.969	C95–C96–C97	2.755
C41–C47–C55	1.342	C98–C97–O112	1.996	C40–C41–C42	2.755
C83–C82–C87	1.377	O93–C94–C99	2.045	C96–C95–C103	2.770
C66–C67–C68	1.383	C80–C81–C82	2.079	C19–C18–C27	2.786
C41–C42–C50	1.420	N24–C25–C28	2.088	C94–C95–C103	2.809
C63–C64–C65	1.477	C35–C36–C42	2.141	C37–C36–C42	2.809
C40–C39–C99	1.481	C95–C103–O113	2.160	C42–C98–C99	2.865
C80–C81–C86	1.504	C20–C19–N24	2.174	C97–C96–C100	2.899
C43–C44–C45	1.520	O93–C94–C95	2.179	C25–C26–C34	3.165
C59–C64–C65	1.536	C17–C18–C27	2.183	C25–C28–C29	3.311
C50–C42–C98	1.587	C39–C99–C94	2.193	C26–C25–C28	3.333
C37–C39–C40	1.605	C62–C61–C87	2.252	C26–C34–C29	3.390
C36–C42–C41	1.610

Breaking Mechanisms of Chemical Bonds on Ring Structures (Including Large π-Bonds) under Shear

Ring structures in the coal macromolecules can generate a ring-opening reaction under tensile action (applied by the shear action) and can be easily attacked by surrounding molecules, leading to ring-opening reactions under tensile action. The average local ionization energy (ALIE) can be used to investigate reaction activity and sites of molecules. By combining data with Visual Molecular Dynamics (VMD), the ALIE of the coal macromolecules was obtained using Multiwfn software[41] for reduced density gradient (RDG) analysis.

Figure shows the nine areas with the minimum ALIE in the coal macromolecules, namely, ① C2, C5, C11, and C14; ② C17, C20, N24, and C27; ③ C30, C33, C35, and C38; ④ C40, C41, C43, C44, C45, C46, and C47; ⑤ C51, C52, C53, and C54; ⑥ C59, C60, C61, C62, C63, and C64; ⑦ C68, C70, and C72; ⑧ C85, C86, C88, C89, C90, and C91; and ⑨ C95 (only white point), C96 (only white point), and C98 (only white point). They appear near the conjugate ring, on which some atoms and electrons are weakly bound. When other molecules act, the ring-opening reaction readily occurs to generate free radicals. Based on the number of bonds broken (Table ), the majority of large π-bonds connected to these atoms are broken many times, such as C17–C18, C18–C27, C19–N24, C26–C27, C29–C30, C61–C62, C67–C68, C94–C95, C96–C97, and C97–C98. In addition, the bridge bonds connected to atoms that approach the minimum ALIE, such as C39–C40, C41–C42, C41–C47, C42–C98, C43–C44, C44–C45, C64–C65, C72–C73, C81–C86, and C95–C103, are easily broken to undergo ring-opening reactions.

Figure 6

ALIE distribution on the surface of Given coal macromolecules (blue represents the minimum area).

The tensile breaking under shear is more severe than the breaking under simple tensile action because of different interactions. In the case of simple tensile action, breaking is realized only by force propagation from both ends to the middle. The atoms on molecules can be easily affected and they are readily subjected to tensile action in molecules due to their location on the bridge for force propagation. Under shear, the tensile action comes from the mutual attack of molecules. As shearing continues and molecules move relative to each other, each chemical bond may undergo interactions with different molecules and there are many potential sites at which such action can occur; in particular, the effects are strongest under complex cross-linking of ring structures so that the probability of breaking at each site increases to a significant extent.

Conclusions

By applying the shear action on the polymer model of medium-rank coal with a cross-linked structure by LAMMPS software, the response of the coal macromolecular structure in the shear process was obtained. The mechanisms were analyzed through quantum chemistry software. The following conclusions can be drawn:

The bridge bonds of the ring structure and large π-bonds are readily subjected to shear and tensile actions, finally experiencing tensile breaking. Atoms (atomic clusters) in the outermost of coal macromolecules tend to undergo shear as applied by surrounding molecules, resulting in shear breaking.

The shear action can not only break chemical bonds but also generate new chemical bonds. The shear stress does not destroy all of the chemical bonds but only the chemical bonds with low bond energy in coal.

As the angle between the bond and the action direction exceeds 90°, the early shear action on bonds shows the effect of both shear and tension. The chemical bonds connected to atoms nearest to the area with the minimum ALIE are easily broken due to the shear action.

Due to the effects of the shearing action, small molecules (such as H• and •CO) are separated from the Given macromolecules and the coal molecular chain is thus shortened, however, these free radicals combine with each other to produce many small gas molecules, such as H2 and CO.