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

CO2 geological sequestration in coal seams can be carried out to achieve the dual objectives of CO2 emission reduction and enhanced coalbed methane production, making it a highly promising carbon capture and storage technology. However, the injection of CO2 into coal reservoirs in the form of supercritical fluid (ScCO2) leads to complex physicochemical reactions with the coal seam, altering the properties of the coal reservoir and impacting the effectiveness of CO2 sequestration and methane production enhancement. In this paper, theoretical calculations based on ReaxFF-MD were conducted to study the interaction mechanism between ScCO2 and the macromolecular structures of both low-rank and high-rank coal, to address the limitations of experimental methods. The reaction of ScCO2 with low-rank coal and high-rank coal exhibited significant differences. At the swelling stage, the low-rank coal experienced a decrease in aromatic structure and aliphatic structure, and high-rank coal showed an increase in aromatic structure and a decrease in aliphatic structure, while the swelling phenomenon was more pronounced in high-rank coal. At the dissolution stage, low-rank coal was initially decomposed into two secondary molecular fragments, and then these recombined to form a new molecular structure; the aromatic structure increased and the aliphatic structure decreased. In contrast, high-rank coal showed the occurrence of stretches-breakage-movement-reconnection, a reduction in aromatic structure, and an increase in aliphatic structure. The primary reasons for these variations lie in the distinct molecular structure compositions and the properties of ScCO2, leading to different reaction pathways of the functional group and aromatic structure. The reaction pathways of functional groups and aromatic structures in coal can be summarized as follows: the breakage of the O-H bond in hydroxyl groups, the breakage of the C-OH bond in carboxyl groups, the transformation of aliphatic structures into smaller hydrocarbon compounds or the formation of long-chain alkenes, and various pathways involving the breakage, rearrangement, and recombination of aromatic structures. In low-rank coal, there is a higher abundance of oxygen-containing functional groups and aliphatic structures. The breakage of O-H and C-OH chemical bonds results in the formation of free radical ions, while some aliphatic structures detach to produce hydrocarbons. Additionally, some of these aliphatic structures combine with carbonyl groups and free radical ions to generate new aromatic structures. Conversely, in high-rank coal, a lower content of oxygen-containing functional groups and aliphatic structures, along with stronger intramolecular forces, results in fewer chemical bond breakages and makes it less conducive to the formation of new aromatic structures. These results elucidate the specific deformations of different chemical groups, offering a molecular-level understanding of the interaction between CO2 and coal.

Molecular Dynamics Simulation of the Rejuvenation Performance of Waste Cooking Oil with High Acid Value on Aged Asphalt.

Waste cooking oil's (WCO's) potential as a rejuvenator of aged asphalt has received attention in recent years, with the acid value of WCO affecting its rejuvenation effect. This study explored the rejuvenation effect of WCO with a high acid value on aged asphalt by using molecular dynamics simulation. First, the representative molecules of WCO with a high acid value and asphalt were determined. The rejuvenation effect of WCO on aged asphalt was analyzed by adding different contents of WCO to an aged asphalt model. The effect of WCO on the thermodynamic properties of the aged asphalt was analyzed. The results show that WCO can restore the thermodynamic properties of aged asphalt binder to a certain extent. Regarding the microstructure of rejuvenated asphalt, WCO molecules dispersed around asphaltenes weakened the latter's aggregation and improved the colloidal structure of the aged asphalt. In terms of interface adhesion properties, WCO can improve the adhesion properties between asphalt binder and SiO2, but it has limited influence on water sensitivity. The results allowed us to comprehensively evaluate the rejuvenation effect of WCO with a high acid value on aged asphalt and to explore its rejuvenation mechanism.

Molecular Mechanism Study on the Effect of Microstructural Differences of Octylphenol Polyoxyethylene Ether (OPEO) Surfactants on the Wettability of Anthracite.

Inhalable coal dust poses a serious threat to coal mining safety, air quality, and the health of miners. Therefore, the development of efficient dust suppressants is crucial for addressing this issue. This study evaluated the ability of three high-surface-active OPEO-type nonionic surfactants (OP4, OP9, and OP13) to improve the wetting properties of anthracite via extensive experiments and a molecular simulation and determined the micro-mechanism of different wetting properties. The surface tension results show that OP4 has the lowest surface tension (27.182 mN/m). Contact angle tests and wetting kinetics models suggest that OP4 exhibits the strongest wetting improvement ability on raw coal with the smallest contact angle (20.1°) and the fastest wetting rate. In addition, FTIR and XPS experimental results also reveal that OP4-treated coal surfaces introduce the most hydrophilic elements and groups. UV spectroscopy testing shows that OP4 has the highest adsorption capacity on the coal surface, reaching 133.45 mg/g. The surfactant is adsorbed on the surface and pores of anthracite, while the strong adsorption ability of OP4 results in the least amount of N2 adsorption (8.408 cm3/g) but the largest specific surface area (1.673 m2/g). In addition, the filling behavior and aggregation behavior of surfactants on the anthracite coal surface were observed using SEM. The MD simulation results indicate that OPEO reagents with overly long hydrophilic chains would produce spatial effects on the coal surface. Under the influence of the π-π interaction between the hydrophobic benzene ring and the coal surface, OPEO reagents with fewer ethylene oxide quantities are more prone to adsorb onto the coal surface. Therefore, after the adsorption of OP4, both the polarity and the water molecule adhesion ability of the coal surface are greatly enhanced, which helps to suppress dust production. These results provide important references and a foundation for future designs of efficient compound dust suppressant systems.

The Molecular Model of Organic Matter in Coal-Measure Shale: Structure Construction and Evaluation Based on Experimental Characterization.

To investigate the molecular structure and micropore structure of organic matters in coal-measure shale, the black shale samples of the Shanxi formation were collected from Xishan Coalfield, Taiyuan, and a hybrid experimental-simulation method was used for realistic macromolecular models of organic matter (OM). Four experimental techniques were used to determine the structural information of OM, including elemental analysis, state 13C nuclear magnetic resonance (13CNMR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). With structural parameters, two-dimensional (2D) average molecular models of OM were established as C177H160O8N2S with a molar weight of 2474, which agreed well with the experimental 13C-NMR spectra. A realistic three-dimensional (3D) OM macromolecular model was also reconstructed, containing 20 2D molecules with a density of 1.41 g/cm3. To determine the connectivity and spatial disposition of the OM pores, focused ion beam microscope (FIB-SEM) and transmission electron micrographs (TEM) were utilized. The 3D OM pores models were developed. The results show that whether the OM pores varied from 20 to 350 nm as obtained from FIB-SEM images or less than 10 nm as observed in the TEM images, both were of poor connectivity. However, the ultra-micro pores from the 3D OM macromolecular model varied from 3Å to 10 Å and showed certain connectivity, which may be the main channel of diffusion. Furthermore, with the pressure increased, the methane adsorption capacity of the 3D OM model increased with a maximum value of 103 cm3/g at 7 MPa, indicating that OM pores less than 1 nm have a huge methane adsorption capacity. Therefore, our work provides an analysis method that is a powerful and superior tool in further research on gas migration.

HRTEM Analysis of the Influence of Non-stick Coal's Oxidation Degree on Aromatic Fringe Morphology.

The purpose of this research was to explore the parameters of the aromatics lattice fringes by using high resolution transmission electron microscopy (HRTEM) patterns, combined with ArcGIS and MATLAB methods, to quantitatively evaluate and analyze the coal samples oxidized by different concentrations of H2O2, and to explore the changes in the morphology and spatial distribution of the aromatic system under oxidation. As the degree of oxidation increased, the orientation of the aromatic lattice fringes became more disordered, and the distortion degree increased. The distribution range of Y and T type dislocation structures, which were widely distributed in short (<0.59 nm) lattice fringes, increased, while that of spiral type dislocation structures, which were distributed in medium (0.59-0.99 nm) and long (1.00-2.49 nm) lattice fringes, decreased. In addition, the collapse and condensation of aromatic slices caused by continuous oxidation further weakened the π-π stacking effect between aromatic rings, resulting in a decrease in the interlayer distance and stacking height. The advantages of HRTEM analysis were confirmed by XRD, SEM and FTIR analysis. This provides a new perspective on the oxidation phenomenon and enriches the examination of the low-temperature oxidation mechanism of coal.