Research on the Interaction Mechanisms between ScCO2 and Low-Rank / High-Rank Coal with ReaxFF-MD Force Field

1. Introduction

Environmental protection and energy scarcity stand as the two major challenges faced today. The growing awareness of CO₂ as a greenhouse gas has attracted increasing attention. The CO₂-enhanced coal bed methane (CO₂-ECBM) projects not only enhances coal bed gas recovery rates but also facilitates the geological sequestration of CO₂ [1,2,3]. The injected CO₂ into coal reservoirs exists in a supercritical fluid state (ScCO₂) [4,5], leading to intricate physicochemical reactions with the coal, resulting in changes of coal structure and properties [6,7,8,9]. The nanoscale pores within coal serve as the primary space for coalbed methane accumulation [10,11]. Alterations in these nanoscale pores can affect the adsorption/desorption capabilities of coal, thereby influencing coalbed methane production and the safety of CO2 storage. The interaction between coal and ScCO₂ is fundamentally a molecular phenomenon [12,13]. The alterations in coal pore structure arise from changes in the molecular structure of coal [14,15]. Hence, it is imperative to undertake research on coal structure at the nanoscale and molecular level to understand the response characteristics and mechanisms of coal structure to ScCO₂. To provide an effective theoretical basis for the widespread application of CO₂-ECBM technology.

Currently, scholars have conducted numerous studies on the changes in coal pore structure after ScCO₂ exposure. Zhang et al found that after ScCO₂ treatment, micropores in coal transform into mesopores, leading to a reduction in micropores and an increase in mesopores in different ranks of coal [16]. Chen et al found that the only mesopore volume increases in medium-rank coal, while both micropore and mesopore volumes increase in high-rank coal [17]. Liu et al found that minimal changes in micropores, with an increase in mesopore volume in both low-rank and high-rank coal, while medium-rank coal exhibited a decrease in mesopore volume [18]. Cheng et al found that both micropore and mesopore volumes increase in medium-rank coal [19].

The modifications in coal pore structure induced by ScCO₂ primarily arise from the interaction between ScCO₂ and coal molecules, resulting in the removal, aggregation, and rearrangement of aromatic structures and functional groups. Wang et al, Sampath et al, and Cao et al shown that following the interaction of ScCO₂ with low-rank coal, there is an increase in the content of aromatic structures and oxygen-containing functional groups within the coal, after interaction with medium- to high-rank coal, there is a decrease in the content of aromatic structures and oxygen-containing functional groups within the coal [20,21,22]. However, there are also different research conclusions. Wang et al suggest that following the interaction with ScCO₂, there is an increase of aliphatic structures and a decrease of aromatic structures in high-rank coal, with almost no change observed in low-rank coal [23]. Li et al propose that following the interaction with ScCO₂, partial aliphatic branches detach in low-rank coal, oxygen-containing functional groups break, forming more C=C bonds, leading to a decrease in aliphatic content and an increase in aromatic content in low-rank coal [24].

It is evident that the impact of ScCO₂ on the changes in pore structure and macromolecular structure in coals of different ranks is not consistent. Experimental methods cannot precisely analyze the essence of ScCO₂ on coals of different ranks. The reaction mechanism of ScCO₂ with coals of different ranks should be established through theoretical methods. Currently, methods such as Molecular Dynamics (MD), Grand Canonical Monte Carlo (GCMC), and Density Functional Theory (DFT) are widely employed to investigate the adsorption and diffusion characteristics of CO₂ in coal, as well as the structural changes of coal pores and fractures after CO₂ injection [25,26,27,28]. The results shown that, after CO₂ adsorption, not only does the non-covalent bond energy decrease, but changes also occur in bond angle energy, torsion energy, etc., leading to alterations in the macromolecular structure of coal, thereby inducing changes in pore structure [29,30,31,32].

In addition, the reactive force field molecular dynamics (ReaxFF-MD) theoretical method can simulate the in-situ reaction process of molecular structures, with clear identification of its reaction pathways under extreme conditions (such as high temperature and high pressure), coupled with low cost and short processing time. Currently, the ReaxFF-MD method is widely used to investigate the pyrolysis mechanism of coal. Zhang et al demonstrated that there are three predominant pathways for the initial decomposition of TAGP: the breakage of the C-N bond, the scission of the N-N bond, and the rupture of another N-N bond [33]. Liu et al shown that the pyrolysis process of low-rank coal was divided into three critical stages: First, the initial pyrolysis process where weak-bridge bonds and macromolecular-network; Second, some tar undergoing cracking reactions to generate gaseous products and other tar undergoing condensation and polymerization reactions to generate coke; Third, the pyrolysis process basically ended [34]. However, there has been no analysis of the interaction mechanism between ScCO₂ and coal.

Therefore, this study employs the ReaxFF-MD method to calculate the kinetic behavior of low-rank coal (YZ) and high-rank coal (CZ) interacting with ScCO₂, obtaining the reaction pathways and changes in chemical bonds during the reaction process between coal and ScCO₂, analyzing the reaction mechanisms of ScCO₂ with low-rank coal and high-rank coal. The objective of this research is to provide a theoretical basis to advance the sustainable development of CO₂-ECBM technology and enhance the efficiency of energy reserves and utilization.

2. Simulation Process

The simulation process can be summarized as follows: Selecting YZ [35] and CZ [36] coal molecular structures as research models (Figure 1), the coal industry analysis and elemental analysis are shown in Table S1. Next, optimizing the coal macromolecular structure model is performed, followed by the construction of supramolecular structures, then ScCO₂ is adsorption into the system. Finally, the ReaxFF-MD force field is applied to calculate the changes in small molecule products, reaction pathways, and chemical bond dynamics following the injection of ScCO₂. The flowchart is shown in Figure 2.

2.1. Simulation Details

2.1.1. Model Construction

(1) Coal macromolecular structure optimization

Both the model construction and energy optimization of YZ and CZ molecules were performed using Materials Studio software. The geometry optimization task in the Forcite module was employed to optimize the structure of the model. The final configuration was an optimized coal model with the lowest energy. Based on energy fluctuations, Metropolis operation rules were employed to either accept or reject changes, thereby facilitating the formation of a new configuration (Figure 2a).

(2) Construction of Supramolecular Structure Models

The simulation parameter contained 50 YZ or CZ molecules are built by Amorphous Cell module. These super molecule models should be optimized before calculations, which can be performed by Forcite Module, the simulation parameter are COMPASS II force field [37], temperature of 298 K and NVT ensemble [38]. The size of the box is 6.77 nm×6.77 nm×6.77 nm. The optimized models are shown in Figure 2b.

2.1.2. ScCO₂ Injection Process

The Adsorption Locator module from Materials Studio was used to place some CO₂ molecules in the super cells, according to the average adsorption in 8MPa pressure adsorption, shown in Figure 2. The simulation used the isothermal isobaric ensemble (NPT), selecting Andersen temperature control method (50℃) and Berendsen pressure control method. The total time period of the simulation was 250 ps, the step size was 1 fs, and the sampling interval was 100 fs. The last 50 ps of data were collected for analysis [39] (Figure 2c).

2.2. ReaxFF Force Field Calculate

The ReaxFF force field employs distance-dependent bond order functions to characterize the contribution of chemical bonds to the potential energy. The calculation formula for the original bond order function is shown in Equation (1) [40]:

$${BO}_{ij}^{\mathrm{\text{'}}}={BO}_{ij}^{\mathrm{\text{'}}\mathsf{\sigma }}+{BO}_{ij}^{\mathrm{\text{'}}\mathsf{\pi }}+{\mathrm{B}\mathrm{O}}_{ij}^{\text{'}\pi \pi }=exp\left[p_{\mathrm{b}\mathrm{o}1}{\left(\frac{r_{ij}}{r_0^{\mathsf{\sigma }}}\right)}^{p_{bo2}}\right]+exp\left[p_{\mathrm{b}\mathrm{o}3}{\left(\frac{r_{ij}}{r_0^{\mathsf{\pi }}}\right)}^{p_{bo4}}\right]+exp\left[p_{\mathrm{b}\mathrm{o}5}{\left(\frac{r_{ij}}{r_0^{\mathsf{\pi }\mathsf{\pi }}}\right)}^{p_{bo6}}\right]$$

(1)

In the equation, ${BO}_{ij}^{\mathrm{\text{'}}\mathsf{\sigma }}$ , ${BO}_{ij}^{\mathrm{\text{'}}\mathsf{\pi }}$and ${BO}_{ij}^{\mathrm{\text{'}}\mathsf{\pi }\mathsf{\pi }}$ represent the bond orders for single, double, and triple bonds, respectively. $r_0^{\sigma }$, $r_0^{\pi }$ and $r_0^{\pi \pi }$ are the equilibrium distances for single, double, and triple bonds, respectively. The symbol $p_{bo1}$-$p_{bo6}$ denotes the regression empirical parameters for the ReaxFF force field.

In the ReaxFF force field, the total system energy is represented by Equation (2) [41]:

$$E_{system}=E_{bond}+E_{over}+E_{under}+E_{val}+E_{pen}+E_{tors}+E_{conj}+E_{vdWaals}+E_{Coulomb}$$

(2)

In the equation, $E_{bond}$ is bond energ, $E_{over}$ is over-coordination energy, $E_{under}$ is under-coordination energy, $E_{val}$ is bond angle energy，$E_{pen}$ is loss energy, $E_{tors}$ is torsional energy，$E_{vdWaals}$ is van der Waals forces，$E_{Coulomb}$ is Coulombic forces.

The Reaxff force field is used in MAPS program to describe bond formation/breaking and reaction path of coal and ScCO₂ interaction(Figure 2e,f).

3. Results and Discussion

3.1. The Deformation Characteristics of Coal Molecular Structure

The changes in the coal supramolecular structure before and after the interaction with ScCO₂ are show in Figure 3. At 0-70 ps, the entanglement between coal molecules gradually loosens, leading to a decrease in intermolecular distance, while CO₂ molecules diffuse from the outside to the inside. At 70-200ps, chemical bonds within the molecules break, leading to the formation of more small radicals (·H, ·OH, ·CH₃, etc.), and the coal molecular structure gradually diffuses towards the edges. At 200-250ps, there is no significant alteration in the supramolecular structure of the coal. The main difference between high-rank coal and low-rank coal lies in the degree of molecular structural looseness and the quantity of chemical bonds broken. In high-rank coal, the degree of molecular looseness is greater. In low-rank coal, there is a greater occurrence of chemical bond fractures and the generation of small molecular compounds.

To further analyze the changes in coal structure after ScCO₂ adsorption, coal macromolecular structures were isolated from the supramolecular structure to observe their deformation characteristics. The coal macromolecular structures before and after the reaction are shown in Figure 4, and the deformation characteristics of coal macromolecular structures at different reaction times are shown in Figures S1 and S2. During the reaction process, both low-rank coal and high-rank coal molecules tend to flatten, which is consistent with a previous study on high-volatile bituminous coal [24]. However, molecular simulation methods enable the observation of intermediate processes in the reaction.

For low-rank coal (Figure 4a,b): At 0.00 to 36.00 ps, some thiophene, oxygen-containing functional group and aliphatic structures are broken, leading to H⁺ ions are released, the deformation of the molecular structure is subtly(Figure S1.(b)). From 36 to 48 ps, more aliphatic structures detach from the macromolecular structure, leading to an increase in small molecular products, and the deformation of the molecular structure is obviously(Figure S1.(c)). From 48 to 69.75 ps, the anthracene in the molecules is broken, leading to the decomposition of the macromolecular structure into two secondary molecular structures (Fragment 1: C₁₀₄H₈₀O₇S, Fragment 2: C₇₆H₅₉O₇N₃)(Figure S1.(d)). From 69.24 to 204.00 picoseconds, the two secondary molecular structures continue to react with ScCO₂ independently(Figure S1.(e),(f)). At 204.75 ps, the two secondary molecular fragments recombine into a macromolecular structure(Figure S1.(g)). From 204.75 to 250 picoseconds, there is no significantly change in the coal molecular structure.

For high-rank coal (Figure 4c,d): From 0 to 31 ps, the aromatic rings connected to oxygen atoms occur deformation, and some aliphatic structures transform into aromatic structures due to the dehydrogenation of cyclohexane(Figure S2(b)). From 64.25 ps, aromatic structures in the coal undergo twisting-breakage-movement-reconnection(Figure S2(c),(d),(e),(f),(g)). At 116.25 ps, the flattening of the coal molecular structure reaches its maximum(Figure S2(e)), and by 142.25 ps, the mobile aromatic structures return to their original positions, then there is no further movement of the aromatic structure within the coal molecule(Figure S2(f)). At 202.25ps, only a minority of chemical bonds break and form in the molecular structure, with no significant changes occurring.

3.2. The Pathways of Functional Groups and Aromatic Structures

The functional groups in coal can be classified into four types: hydrocarbon structures, aliphatic structures, oxygen-containing functional groups, and aromatic structures. The action of CO₂ on coal molecular structure can be categorized into two main types: Firstly, CO₂ directly react with functional groups; Secondly, during the reaction process, H⁺ and CO generated from decomposition react with functional groups.

3.2.1. The Pathways of Functional Groups

Hydroxyl groups pathway (-OH): The CO₂ attacks the H⁺ atom in the hydroxyl group, inducing a dehydrogenation reaction. The remaining oxygen-containing groups subsequently break, releasing CO molecules(Figure 5: Pathway 1).

Carbonyl groups pathways (-C=O-): (1) Hydrogenation: Carbonyl groups combine with free H⁺ ions to form hydroxyl groups(Figure 5: Pathway 2). (2) Aromatization: The H⁺ reacts with CO₂ to produce CO, which replaces the oxygen in the carbonyl group, subsequently forming aromatic rings with surrounding aliphatic structures(Figure 5: Pathway 3).

Carboxyl groups pathways (-COOH): The H⁺ attacks the oxygen atom in the carboxyl group (C-OH), causing the detachment of the OH- and the formation of a H₂O molecular. Additionally, the C-C bond breaks, resulting in the formation of CO molecules(Figure 5: Pathway 4).

Aliphatic structures pathways(-CH₃): (1) Decomposition reactions: The breaking of C_al-C_al or C_al-H bonds in coal results in the formation of free radical ions such as H⁺ and C⁴⁺ or smaller hydrocarbon compounds(Figure 5: Pathway 5).(2) Condensation reaction: The free -CH₃/-CH₂ groups detached from the coal macromolecular structure, occur condensation reactions with carbonyl groups or other aliphatic structures in the coal to form long chain alkenes. From the reaction process diagram, it can be observed that aliphatic structures at the edges of coal macromolecules are more prone to occur decomposition reactions: reactions, while aliphatic structures connected to aromatic structures or oxygen functional groups are more prone to occur addition reactions(Figure 5: Pathway 6).

3.2.2. The Pathways of Aromatic Structures

During the interaction between ScCO₂ and coal, not all aromatic structures in coal changed. Overall, aromatic rings at the edges of macromolecular structures and those connected to side chains are more susceptible to deformation. The changes in aromatic structures mainly include the following forms: (1) Aromatic rings are directly broken, gradually changing from cyclic to linear or irregular shapes(Figure 6: Pathway 7); (2)Aromatic structures polymerize with free C⁴⁺ and H⁺ ions to form C_ar-C_ar bonds(Figure 6: Pathway 8). (3) Aromatic rings change from a six-membered aromatic ring to seven-membered ring, and then be damaged(Figure 6: Pathway 9); (4) During the reaction, aromatic rings first break and then recombine. At the end of the reaction, there is no change in the aromatic rings(Figure 6: Pathway 10);

3.2.3. The Reaction Pathways of Low-Rank and High-Rank Coal

In low-rank coal(Figure 7): hydroxyl groups (A, B) occur dehydrogenation(Pathway 1). Aliphatic structures (F, G) aromatize to form aromatic structures(Pathway 6), while aliphatic structures (D, E) decompose to form small hydrocarbon compounds(Pathway 5). The decomposed aliphatic structures combine with carbonyl groups (D, E) to form aromatic structures(Pathway 3). Carbonyl groups (C) occur substitution reactions only. Carboxyl groups (F, G) break to form carbonyl groups. In aromatic structures, benzene and naphthalene (C, D, F) occur Pathways 7 and 8, while polycyclic aromatic hydrocarbons (A, B) occur Pathways 9 and 10.

In high-rank coal(Figure 8): carbonyl groups (③, ④, ⑦ Pathway 2) occur hydrogenation to form hydroxyl groups, while a small amount of aliphatic structures (① Pathway 5) broken. Similar to low-rank coal, benzene and naphthalene in aromatic structures (②, ③, ⑤) occur Pathways 7 and 8. Polycyclic aromatic hydrocarbons (①, ④, ⑥, ⑦) occur Pathways 9 and 10.

The molecular fragments located at the edges of the molecular structure and those connected to side chains in both low-rank and high-rank coal are prone to deformation. Compared to high-rank coal, low-rank coal exhibits some differences in reaction pathways. Low-rank coal occurs more decomposition of aliphatic structures, resulting in the generation of new aromatic structures. High-rank coal occurs only a small amount of decomposition of aliphatic structures, and there is no generation of new aromatic structures.

3.3. Chemical Bond Change Characteristics

The changes in the coal molecular structure are primarily caused by the breaking and formation of crosslinks and aromatic rings. Crosslinks in coal can be primarily categorized into C_al-C_al bonds constituting aliphatic side chains, C-O bonds in oxygen-containing functional groups, and C-H bonds. The aromatic structures are primarily composed of C_ar-C_ar bonds.

3.3.1. Low-Rank Coal Chemical Bond Change Characteristics

The curve of chemical bond changes in low-rank and high-rank coal are shown in Figure 9. At 0-48 ps, there is a significant decrease in C-H bonds, C_ar-C_ar bonds rapidly decrease initially and then decrease at a slower rate, C_al-C_al bonds initially increase followed by stabilization, and a small decrease in C-O bonds. The reduction in aromatic bonds in low-rank coal is main due to the breakage of aromatic rings such as thiophene and pyridine. The increase in C_al-C_al bonds is due to the loosening of the molecular structure in low-rank coal, which stretches the C_ar-C_ar bonds in the aromatic ring, transforming them into C_al-C_al bonds.

At 48-69.75 ps, the reduction in C-H bonds accelerates, and there is a slight decrease in C_ar-C_ar bonds and C_al-C_al bonds. This indicates that the coal molecular structure no longer continues to loosen, leading to the fragmentation of aromatic structures. The breakage of the C-H bonds around the aromatic structure, forming more free radical ions, and some aromatic structures are decomposed. At 69.25-204 ps, the decrease in C-H bonds continues rapidly, and the reduction in aliphatic structures as well as the increase in aromatic structures both intensify. This is because during this stage, the condensation effect strengthens, some aliphatic structures transform into aromatic structures, and aromatic structures that fragmented in the previous stage recombine. After 204.75 ps, C-H bonds remain essentially unchanged, and the rate of decrease in aliphatic bonds as well as the rate of increase in aromatic bonds slow down.

3.3.2. High-Rank Coal Chemical Bond Change Characteristics

The curve of chemical bond changes in low-rank and high-rank coal are shown in Figure 10. At 0-31 ps, the increase in C_ar-C_ar bonds and the decrease in C_al-C_al and C-H bonds are attributed to the dehydrogenation of cyclohexane in high-rank coal, resulting in the formation of aromatic structures. Due to the low sulfur content in high-rank coal, more CO₂ can reacts with oxygen-containing functional groups in the coal, resulting in a faster decrease in C-O bonds compared to low-rank coal.

At 31-116.25 ps, more CO₂ diffuses into the interior of high-rank coal, leading to a stronger decomposition effect on high-rank coal and the formation of numerous free radical ions. These free radical ions then disrupt the coal molecular structure through chain reactions. As high-rank coal contains a higher proportion of aromatic structures and a lower proportion of aliphatic structures, more aromatic structures are disrupted, leading to the formation of aliphatic structures.

At116.25-250 ps, although there is movement and reconnection of aromatic structures, the decomposition effect of CO₂ on coal molecules weakens, there is not a significant change in chemical bond variations.

3.4. Reaction Mechanism of Low and High Rank Coal

Based on the previous discussion, it is evident that there are significant differences in the structural changes, functional group transformation pathways and chemical bond broken during the reaction process of high-rank and low-rank coal reaction with ScCO₂. The main reasons for these differences are related to the reaction mechanisms between ScCO₂ and coal molecular structures, and the differences in the molecular structures of low and high rank coal. The interaction between ScCO₂ and coal can be divided into two processes: swelling and dissolution.

Stage I Swelling (Figure 11): During the swelling process, the molecular structure to expand, and the entanglement between molecules becomes looser. The main phenomenon involves the breakage of non-covalent bonds between molecules, accompanied by the breaking of weak bridging bonds within the macromolecular network. Hydrogen bonds, π...π stacking, C_al-O, C_al-C_al, and C_al-H bonds broken, leading to the dissociation of complex system structures and the release of small molecular gases. At this process, the molecular center remains unchanged. Although ScCO₂, as a nonpolar solvent, exhibits slightly inferior swelling effects compared to polar solvents, the aromatic structures within coal possess strong adsorption capabilities for nonpolar solvents. Highly aromatic high-rank coals can adsorb more CO₂. Therefore, the swelling phenomenon of high-rank coal molecules is more pronounced. High-rank coal contains fewer aliphatic structures and aromatic structure stability, with only a partial breaking of C-H bonds in some cyclohexane molecules, leading to the formation of aromatic C_ar-C_ar bonds. Low-rank coal contains more aliphatic structures, and its aromatic structures are unstable. Therefore, during the swelling stage, both C_al-C_al and C_ar-C_ar decrease.

Stage II Dissolution(Figure 12)：The first stage(Figure 12(a)): the chain segments of coal macromolecules exhibit enhanced mobility. Due to the internal rotation of the main chain σ bonds, certain segments of the molecule can move relative to others while the molecular center of mass remains unchanged (no plastic deformation occurs). This leads to a reduction in side chains and the fracture of aromatic structures linked to oxygen functional groups in the macromolecular structure of coal. The second stage (Figure 12(b)): the movement of chain segments reaches the movement of the entire macromolecular chain (also known as coordinated segmental motion). This stage is primarily characterized by decomposition and condensation reactions, including the breakage of C_ar-O, C_ar-C_al, C_ar-H, and C_ar-C_ar bonds. During the coordinated segmental motion, the impact of ScCO₂ on the coal molecular structure is most pronounced. The original functional groups and aromatic structures in coal are broken, leading to the detachment of free radical ions, which occur addition or substitution reactions with the coal macromolecular structure. Due to the weak intra-molecular forces in low-rank coal molecules, they are more prone to broken and decomposition into smaller molecular units. At this stage, the low-rank coal is decomposed into two second-order molecular fragments, and the aromatic structure of the high-rank coal is migration. The third stage (Figure 12(c)): under supercritical conditions, along with an increase in molecular collision frequency, coupled with the weak acidity of ScCO₂, the fragmentation of coal molecules could interact with free radical ions, so the combination reaction occurs and resulting in the formation of new bonds, molecular structural reorganization. Hence, the phenomenon of molecular recombine in low-rank coal and the reconnection of aromatic structures in high-rank coal are observed in the later stages of the reaction. After the dissolution reaction concludes, both low-rank coal and high-rank coal show no significant changes.

4. Conclusion

(1)

The interaction between coal and ScCO₂ leads to various chemical reactions, including the breakage of aliphatic side chains and oxygen-containing functional group side chains, removal of heteroatoms, and ring-opening and polymerization reactions between aliphatic and aromatic structures. The process of structural changes in low-rank coal molecules can be summarized as stretches-breakage-recombine, while in high-rank coal be summarized as stretches-migration-reconnection.

(2)

Functional groups and aromatic structures in coal exhibit various reaction pathways. The O-H bond in hydroxyl groups and the C-OH bond in carboxyl groups break. The carbonyl group may occur hydrogenation to form a hydroxyl group or aromatization with surrounding aliphatic structures to create aromatic ring structures. Aliphatic structures can decompose to form smaller hydrocarbon compounds or condensate to form long-chain alkenes. The reaction pathways of aromatic structures are more complex and involve processes such as breakage, rearrangement, and recombination.

(3)

The transformation of coal molecular structure is governed by changes in chemical bonds within the coal. The content of C-H bonds and C-O bonds decreases in both low-rank and high-rank coals. In low-rank coal, the C_ar-C_ar bonds initially decrease and then increase, while the C_al-C_al bonds initially increase and then decrease. Conversely, in high-rank coal, the C_ar-C_ar bonds initially increase and then decrease, while the C_al-C_al bonds initially decrease and then increase. Overall, in high-rank coal, both C_ar-C_ar and C_al-C_al bonds decrease, while in low-rank coal, C_ar-C_ar bonds increase and C_al-C_al bonds decrease.

(4)

The responses of high-rank and low-rank coal are related to the structural differences and the interaction mechanisms between ScCO₂ and coal molecules. Due to the stronger adsorption affinity of aromatic structures for CO₂, the changes in high-rank coal during the swelling stage are more pronounced compared to low-rank coal. At dissolution stage, chemical bonds in low-rank coal are weaker, the bonds are more prone to breaking after exposure to ScCO₂, the disrupted aromatic structures or carbonyl groups combine with detached aliphatic side chains to form larger aromatic structures in low-rank coal. Conversely, in high-rank coal, there are stronger intramolecular forces, with fewer aliphatic structures and carbonyl groups, new aromatic structures are not formed, leading to a reduction in aromatic structure content.