Macromolecular Structure Of Coal Research Articles

An investigation into the modification of microwave-assisted oxidation in the macromolecular structure of coal via XRD and Raman spectroscopy

The macromolecular structure of coal governs the generation and expansion of micro/mesopores, contributing significantly to the storage and transport of coalbed methane (CBM). To reveal the modification mechanism of microwave-assisted oxidation at molecular scale, this paper presents a comprehensive analysis of the evolutionary characteristics of macromolecular structure of coal using X-ray diffraction and Raman spectroscopy tests. Results show that microwave irradiation triggers the conversion of ordered graphitic crystalline carbon to amorphous carbon, as well as the condensation and heterogeneous spatial arrangement of aromatic clusters. The oxidation of NaClO or Na2S2O8 destroys the initial aromatic structure and reduces the aromaticity of coal, whereas Fenton's reagent oxidation causes the cyclocondensation of macromolecular structure and contributes to the graphitization and crystallization of coal. Microwave-assisted oxidation not only facilitates the directional growth of aromatic rings and the cross-linking of aliphatic chains, but also induces the rotation, displacement, stacking and gradual parallel alignment of aromatic lamellae, thus improving the integrity of three-dimensional lattice of graphite structure and the orderliness of macromolecular structure. As the average size of microcrystalline structural units increases and align directionally to form molecularly oriented domains, the pore size and volume subsequently increases, thereby enhancing the gas transport capacity of coal and the extraction of CBM. This study provides theoretical guidance for the optimisation of microwave-assisted oxidant modification and the paramount consideration of CBM development strategy.

Potential effect of carbon dioxide injection on the functional groups of medium volatile bituminous coals analysed using in-situ diffuse reflectance Fourier-transform infrared spectroscopy

CO2 sequestration with enhanced coalbed methane recovery (CO2–ECBM) is a promising technology for mitigating CO2 emissions. To implement CO2–ECBM, complex interactions must be elucidated between CO2 and coal during CO2 sequestration, but the mechanism through which coal and CO2 interact remains controversial mainly because coal contains a complex macromolecular structure. Therefore, we used in-situ high-pressure diffuse reflectance Fourier-transform infrared spectroscopy to investigate the potential impact of the CO2 injection on the functional groups of medium volatile bituminous (MVB) coals. The results of this study showed that CO2 physisorption could be found near the MVB coal macromolecular CH bonds, and the interactions between the MVB coal macromolecular CH bonds and CO2 kept strengthening with increasing CO2 injection pressure. Injecting CO2 into the MVB coal disrupted the weak interactions between aromatic layers, and supercritical CO2 (ScCO2) could disrupt the physical association between small molecular compounds and the coal macromolecular structure, which extracted hydrocarbon and oxygen-containing compounds, and thus reduced the content of oxygen-containing functional groups and -CH2- in the MVB coal. The coal macromolecular CO bonds were not conducive to methane adsorption on the coal surfaces, injecting high-pressure CO2 (P = 7 MPa, 8 MPa) into the MVB coal matrix irreversibly damaged the coal macromolecular CO bonds, which enhanced the CH4 adsorption capacity and was not conducive to CH4 desorption. The results of this study showed that injecting ScCO2 into the MVB coal could change the coal macromolecular structure and reduce the content of oxygen-containing functional groups and -CH2-, thus affecting the properties of MVB coal reservoir, such as porosity, permeability, and mechanical characteristics, which could advance knowledge of the theoretical usefulness for CO2-ECBM recovery and sequestration in the MVB coal reservoirs.

Enhancement of multi-gas transport process in coalbed methane reservoir by oxidation treatment: Based on the change of the interaction force between coal matrix and gas molecules and knudsen number

Anthracite coal of Zhijin Block was taken as the coal samples and NaClO as the oxidants to investigate enhancement of multi-gas transport process in coalbed methane reservoir by oxidation treatment. Industrial microscope observation, SEM observation, low-temperature N2 sorption measurement and NMR test were used to comprehensively characterize the multiscale pore structure of coal. The change of macromolecular structure of coal before and after oxidation treatment was tested by 13C NMR test, and the force between coal matrix and gas molecules was estimated. Based on the pore size distribution of coal obtained by NMR analysis, the Knudsen number was calculated. Oxidation treatment effectively promoted the desorption of methane in coal, increased the content of free methane in coal. It also led to the CBM gas well entering the stable production period earlier, and shortens the gas breakthrough time of the gas well. Oxidation treatment improved micropores to larger pores by dissolving the coal matrix, and the porosity and permeability of coal were increased. Oxidation treatment led to the widening of pore diameter, and the effect of Knudsen diffusion on the core seepage of the matrix was gradually enhanced. Macroscopically, the gas slippage effect was more significant and the permeability was enhanced. The mechanism of oxidation treatment on multi-gas transport process was mainly reflected in two aspects: increasing the content of free methane and enhancing the gas supply capacity of coal.

Study on the Destructive Effect of Small Faults on Coal Pore Structure

Small fault area in coal mine has great risk of coal and gas outburst. However, the damage effect of small faults on coal pore structure still lacks systematic research. Taking a small reverse fault under Pingmei No. 2 mine as an example, this paper collects the hanging wall coal samples within 5 m from the fault plane and the coal samples 50 m from the fault plane, where there is no fault development. The pore structure characteristics of the two coal samples were analyzed and compared by means of a scanning electron microscope, mercury injection, liquid nitrogen adsorption, and Raman spectroscopy. The test results show that compared with the coal samples without faults, the coal samples at faults form micron-scale fracture zones with constant spacing, stable occurrence, and flat two walls. The pore volume and pore specific surface area of different pore sizes increase, the pore peak of mesopore (100~1000 nm) shifts from 1000 nm to 200~400 nm, and its connectivity is enhanced. The number of micropores (<10 nm) increased significantly, and the ink bottle type pores developed. Faulting also has a certain impact on the macromolecular structure of coal, which shows that the D peak area decreases and the area of G peak increases, indicating that the order degree of macromolecular structure of coal increases.

The molecular dynamics simulation of lignite combustion process in O2/CO2 atmosphere with ReaxFF force field

Pressurized oxy-fuel combustion is a complex process consisting of free radical collisions and reactions. ReaxFF reactive Molecular Dynamics (MD) simulation method is a useful tool to investigate the chemical events in coal combustion at atomistic level. In this work, the impact of operating conditions on the distribution of main fragments and the reaction mechanisms were examined using a lignite coal model with ReaxFF force field to simulate the process of oxy-fuel combustion under pressures. The ReaxFF MD simulations found that the decomposition of coal molecular structure would be enhanced by increasing the pressure/density, oxygen concentration and temperature. The variety of initial reactions increased with temperature. Increasing pressure promoted the effective collision of molecules and the coal structure was more likely to be attacked by oxidants, so the chemical bonds in coal molecule broke earlier and the consumption of O2 increased at higher pressure. The dehydrogenation reaction on the macromolecular structure of coal and small fragments was also promoted after increasing pressure/density. However, the number of fragments decreased as the pressure/density continued to grow, because large fragments further decomposed or some small molecular benzene ring structures with less than 6C atoms were re-polymerized. The dehydrogenation reaction of coal molecule and small fragments was promoted after increasing the pressure. O2 attacked the benzene ring structure, the C atom on the outer side of the coal, and the carbon-containing fragments in the system, indicating that the increase of O2 concentration promoted the ring opening and oxidation reactions, thereby promoting the decomposition and combustion of coal. The work in this paper has confirmed the findings obtained by other oxy-fuel experimental work from the microscopic point of view.

Experimental Study on the Microstructure of Coal with Different Particle Sizes

In the study of coal pore structure, the traditional test method does not consider the influence of coal particle size. During the crushing process, coal samples are affected by crushing stress. While the particle size changes, the change characteristics of pore structure and macromolecular structure are a matter for which systematic research is still lacking. In this paper, mercury injection and liquid nitrogen were used to characterize the pore structure of coal. It was found that the porosity, total pore volume and total specific surface area of the coal increased with the decrease of particle size. However, during this process, the pore volume of macropores and mesopores decreases, while the micropores and transition pores increase significantly, indicating that while the particle size decreases, macropores and mesopores are broken into micropores and transition pores. In addition, the pore structure of samples with a particle size less than 200 mesh changes significantly. With the decrease of coal particle size, the areas of the D peak and G peak of the Raman spectrum increase, indicating that the ordering degree of coal increases. Finally, the statistical results of the peak area of the Fourier infrared spectrum show that alcohol, phenol, ammonia hydroxyl and fatty hydrocarbon CH2 and CH3 are greatly reduced, while the out-of-plane deformation vibration of alkyl ether and aromatic structure C–H are significantly increased, which also indicates the transformation of the coal macromolecular structure to an aromatic structure with strong stability.

Structural Model Construction and Optimal Characterization of High-Volatile Bituminous Coal Molecules

The structural characteristics of coal at the molecular level are important for its efficient use. Bituminous coal from the Baozigou Coal Mine is investigated, using elemental analysis, 13C nuclear magnetic resonance, X-ray photoelectron spectroscopy, and Fourier transform infrared. The molecular structure was determined. The aromatic compounds of bituminous coal molecules are primarily two- and three-ring structures, and the aliphatic structures are primarily in the form of methyl, ethyl side chains, and naphthenic hydrocarbons. The ratio of aromatic bridge carbon to peripheral carbon in the molecular structure is 0.279. Oxygen atoms in the form of carbonyl, phenolic hydroxyl and C–O, and nitrogen atoms in pyrroles. Thus, the average structure model of bituminous coal macromolecules was constructed; the molecular formula was C169H128O10N2S, and the molecular weight was 2378. The aromatic structural units in the macromolecular structure of coal include four naphthalenes, three anthracenes, two tetracenes, and heteroatoms in the form of three carbonyl groups, one phenolic hydroxyl group, one pyrrole, and one pyridine. The structure optimization and annealing kinetic simulation of a single macromolecular structure model were performed. Chemical bonds such as bridge bonds and aliphatic bonds were found to be twisted, and π–π interactions between the aromatic sheets in the molecule produced adjacent aromatic sheets. This arrangement tends to be approximately parallel, and the total energy decreases from 6713.401 to 2667.595 kJ/mol, among which the bond stretching energy and van der Waals energy dominate. We used 20 bituminous coal macromolecular models to construct aggregated structural models. After optimization by molecular dynamics simulation, the macromolecules were constrained by the surrounding molecules, and the sheet-like aromatic carbon structures that were originally approximately parallel were distorted. The macromolecular structure model of bituminous coal constructed in this study provides a theoretical model basis for the optimal surfactant.

Direct evidence of CO2 softening effects on coal using nanoindentation

Sequestration of carbon dioxide (CO2) in deep coal seams has been identified as a promising technique to mitigate global warming with added benefit of enhancing coalbed methane recovery. Understanding coal structure and its reactivity with injected CO2 is important for CO2 sequestration in coal. Despite the long-standing hypothesis, there is no direct evidence of softening effects of CO2 on coal due to CO2-coal interactions. Here, we used optical microscopy and nanoindentation to study changes in microstructures and nanoscale mechanical properties of untreated and CO2-treated anthracite coal. Microscopic images indicated coal surface cracked after short-term treatment and then became highly wrinkled and distorted after long-term treatment. These changes reflect that CO2 dissolves in the macromolecular network and acts as a solvent allowing a rearrangement of the network. Nanoindentation directly confirmed the softening effects of CO2 on nanoscale mechanical properties, including Young's modulus, hardness, and fracture toughness. Interestingly, all these changes were reversible to some extent after removing CO2 from coal. These findings provide new evidence for clarifying the polymeric macromolecular structure of coal and directly demonstrated the softening effects of CO2 on the macromolecular structure.

Study on the spontaneous combustion and oxidation mechanism of low molecular ketone compounds in coal

Many scholars at home and abroad mainly focus on the macromolecular structure of coal and the change of chemical bonds connecting its basic structural units in the process of coal spontaneous combustion. The effect of low molecular compounds in coal on coal spontaneous combustion and the mechanism of low molecular spontaneous combustion have not been reported. In order to solve these problems, the chemical reaction process and mechanism of low molecular compound 3-pentanone in coal spontaneous combustion were studied by using the theory of quantum chemistry. The results show that the active sites of low molecular compound 3-pentanone in coal are C3 and C10 atoms in the process of spontaneous combustion. The oxidation mechanism of 3-pentanone in coal spontaneous combustion is also revealed, and the priority order of each reaction channel is determined: Path 5 > Path 2 > Path 1 > Path 3 > Path 4. It lays a theoretical foundation for the prevention and control of coal spontaneous combustion and guides the development of low molecular spontaneous combustion inhibitors.

Comparison of three methods for the sequential extraction of lignite pretreated by ionic liquid

AbstractIn order to improve the extraction rate of coal by solvent extraction, Lianhuatang lignite (LHT) was pretreated with 1‐butyl‐3‐methylimidazolium hexafluorophosphate ionic liquid([Bmim]PF6) firstly. Then, the Soxhlet extraction (SE), ultrasound‐assisted extraction (UAE), and microwave‐assisted extraction (MAE) were carried out to obtain extracts (ES1‐3, EU1‐3, and EM1‐3) and residues (RS1‐3, RU1‐3, and RM1‐3) from the Lianhuatang lignite pretreated with ionic liquid (BLHT), respectively, and CS2, acetone, and ethanol were used as solvents. The coal samples before and after the ionic liquid pretreatment were characterized by scanning electron microscope (SEM), Fourier‐transform infrared spectroscopy (FTIR), and thermogravimetric analyzer (TG). The residues were characterized by FTIR, and the extracts were characterized by a gas chromatograph/mass spectrometer (GC/MS). According to the results, ionic liquid can crack coal particles, increase the contact area of coal surface with solvent, and destroy part of the hydrogen bonds and intermolecular forces in coal, enabling easy extraction of the active small molecules. The macromolecular structure of coal is not destroyed in the three sequential extraction experiments. The types of extracts are similar under different methods, but the specific substances are very different. The total extraction rate by MAE was the highest, which was 48.70%. Microwave radiation can promote the extraction of oxygen‐containing substances, especially esters.

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.

The Characteristics of Closed Pores in Coals With Different Ranks

The closed pores in coal seams influence the storage of coalbed methane. The investigation of closed pores characteristics for coals is of great significance in improving the production of coalbed methane and revealing the mechanism of coal and gas outburst. However, due to limitations in analytical techniques, the characteristics and evolution mechanism of closed pores in coals with different ranks are not sufficiently understood. In this paper, eight coal samples with different ranks were collected and characterized by small-angle X-ray scattering (SAXS) and low-temperature nitrogen adsorption (LTNA). The open and closed pores of coals with various ranks were studied, and the mechanism for evolution of closed pores during coalification was proposed. The results show that among eight coal samples with different ranks, the closed porosity of low-metamorphic coals is relatively lower, the closed porosity of medium-metamorphic coals is in the middle, and the closed porosity of high-metamorphic coals is relatively higher. The change in closed porosity for coals with different ranks may be related to varieties of the molecular structure of coals. The low-metamorphic coals have more disordered arrangement of molecular structure and easily form connected pores. Therefore, the closed porosity in low-metamorphic coals is low. The aromatization of medium-metamorphic coals turns aliphatic chains into closed aromatic rings, and the closed porosity of these coals also increases. When coals reach a high degree of metamorphism, polycondensation compacts the coal macromolecular structure, providing for easy formation of closed pores between aromatic condensed rings, so the closed porosity is obviously increased in high-metamorphic coals. This study has dual significance in advancing the understanding of open and closed pores in coals and the mechanism of coal and gas outburst.

The roles of low molecular compounds on the light aromatics formation during different rank coal pyrolysis

Light aromatic hydrocarbons are an important by-product of the coal chemical industry. The structure and properties of low molecular compounds (LMCs) embedded in coal are different from those of coal macromolecular skeleton structure, they have important effects on the coal pyrolysis. In this paper, the roles of LMCs on the formation of light aromatics during coal pyrolysis are investigated. The results show that LMCs have little effect on the thermal weight loss behavior of the three coals, but they can influence the carbon structure distribution of coals. The main source of light aromatics from coal pyrolysis is derived from thermal cracking of the coal macromolecular framework. And also the LMCs will be pyrolyzed to generate light aromatics. The total amount of BTEXN from extracts of Shengli coal (SL), Pingshuo coal (PS) and Hexi coal (HX) pyrolysis was 15%, 91%, and 12% of that of raw coal. The alkylbenzenes, naphthalene series, and PAHs can be cracked to form light aromatics. The long chain and cyclic aliphatic compounds can not only provide hydrogen for the free radical fragment of aromatics but also generate light aromatics. And the content of these compounds in the extracts of PS is higher than that of the other two coals. Due to the complexity of the combination between LMCs and coal structure, the extracts are easier to generate light aromatics than LMCs embedded in coal during pyrolysis. Because the composition and content of low molecules in coal with different metamorphic degrees are different, the effects of LMCs on the pyrolysis of low rank coal are more obvious compared with high rank coal.

Stress degradation mechanism of coal macromolecular structure: Insights from molecular dynamics simulation and quantum chemistry calculations

The shear action caused by tectonic stresses deforms and destroys the coal macrostructure and significantly affects the coal macromolecular structure. To study the degradation mechanism of the macromolecular structure of low-rank coal during shearing, the macromolecular structure model of Wender coal was selected in this study. Forty molecules of this structure were used to construct a polymer cell. Shearing simulations were performed using Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software, and bond strength was determined using Gaussian software. Two chemical bond breakage types were observed: tensile break and shear break. The former is controlled by the bond's position and strength, while the latter is controlled by the bond's position and angle strength. The mechanism of tensile break is different from that of shear break. The former tends to occur when the bond rotates clockwise around the point of action, towards the shear action direction (X axis positive direction), and the rotation angle is between 0° and 90°. The latter easily occurs when the bond rotates clockwise around the point of action, towards the shear action direction (X axis positive direction), and the rotation angle is between 90° and 180°. Small molecules, such as CH4, are produced by the shear action, partially revealing the excess coalbed methane eruption mechanism of coal and gas outburst. The shear action increases the order degree and stratification of the coal molecular structure by breaking and cutting side chains and bridge bonds, promoting the evolution of the coal rank.

Stress Sensitivity for the Occurrence of Coalbed Gas Outbursts: A Reactive Force Field Molecular Dynamics Study

Coalbed gas outburst processes commonly occur in ductile shear zones and release gas quantities that far exceed the maximum gas adsorption capacity of coal seams. Although outburst accidents pose a serious safety hazard, this process has long remained poorly understood at a molecular level. In this work, we use reactive force field molecular dynamics simulations to analyze the influence of stress on the mechanochemical decomposition reactions and products of coal macromolecular models. The results indicate that a maximum of reaction amounts occurs upon changing the external force for the mechanochemical reactivity of bituminous coal under working conditions. This implies that gas generation readily occurs over a suitable stress-sensitive range, which can be used to qualitatively explain why coalbed gas outbursts typically occur in ductile shear zones. When a force is too large, it can cause brittle fracture and is less likely to initiate mechanochemical reactions that generate gas, whereas a too small of a force is insufficient to trigger chemical changes of the coal macromolecular structure. The anisotropic properties of coal must also be considered in terms of mechanochemical reactions. Coal molecules can generate different kinds of gases when driven by external forces in different orientations. The simulation results provide important insights into coalbed gas outburst characteristics and the anisotropic nature of coal. Only by comprehensively considering stress factors will it be possible to understand the occurring conditions of coalbed gas outbursts.

Macromolecular Structural Responses of Coking Coal to Stress-Strain Environments Based on High-Resolution Transmission Electron Microscopy.

In China, most coal seams have experienced multiple phases of tectonics, which increase the complexity of the coal porosity. The responses of macromolecular structures to stress-strain environments are the key to understanding the complexity of coal porosity caused by tectonism. This study investigated the macromolecular structural response of coking coal to different stress-strain environments. The calculation and analysis of high-resolution transmission electron microscopy (HRTEM) test results for six coking coals with different deformation types and degrees were performed. The results showed that the macromolecular structure of coking coal has different responses to stress- strain environments. The lattice fringe length, curvature, and d002 parameters are not prominently governed by certain principles increasing shear and toughness. Furthermore, the lattice fringe tends to be consistent with the deformation. However, the lattice fringe during ductile deformation in the main direction is arranged nearly parallel on the plane. In shear deformation, the lattice fringes are arranged parallel in the principal direction. This phenomenon is caused by the directional screening of the macromolecular structure of coking coal by stress. Under the action of tectonic stress, the structure hindering tectonic stress is destroyed, and the structure grows in other directions gradually under the promotion of tectonic stress. As a result, the direction of the whole lattice fringe gradually converges under stress screening after tectonic stress is applied. Therefore, the direction tends to be consistent with the strengthening of the deformation. The response of the other parameters is slow, showing the coexistence of the old and new orders of direction. The directivity of the macromolecular structure of coal is an important index for reflecting the types and degrees of tectonic deformation.

Study on the Effect of Small Molecule Compounds on the Structure and Properties of Coal

In this research, small molecular compounds are extracted from Shenfu coal using solvent extraction method, then through analysis and comparison, the structure and properties of coal before and after extraction of coal in order to study small molecular compounds affect the structure and properties of coal. The results show that they have been affected to varying degrees and the core of the coal structural unit aromatic ring has not been reduced, the macromolecular structure of coal has not changed after the extraction of coal. And these studies surely help to study the reaction mechanism of coal liquefaction and guide the study of coal macromolecular.

Variation of Sulfur Content in Coking Coal as Function of Its Particle Size

To understand quality of coking coal possessing different particle sizes, coal samples from Gao Yang (GY) and Shui Yu (SY) mines were systematically studied. The samples had wide particle size variations with six distinct ranges. The main emphasis was placed on analyzing sulfur and coke contents as function of particle sizes. X-ray photoelectron spectroscopy (XPS) was used to analyze sulfur content as well as its chemical state. Sulfur coordination and group geometry was analyzed by Fourier transform infrared spectroscopy (FTIR). Scanning electron microscopy (SEM) was applied to characterize surface morphology. Sieving of the coal samples increased ash and sulfur contents in all fractions. Organic sulfur was mostly present as thiophene for all particle size ranges. Macromolecular structure of coal was almost the same for all samples. Changes in total sulfur content of different coal fractions were mainly caused by the variations in inorganic sulfur concentrations because organic sulfur content remained almost constant. Understanding particle size distribution in different coals is important for selecting and optimizing appropriate desulfurization methods.

Analysis of the three-phase state in biological hydrogen production from coal

The production of biohydrogen from coal is a new research direction in the bioengineering of coalbed methane. In order to study the transformation process and mechanism of the gas-liquid-solid tripe phase in biohydrogen production from coal, a biohydrogen production experiment from low-rank coal is carried out under laboratory conditions. The results show that: The daily gas production of hydrogen reaches a peak value of 1.23 mL/g on the fifth day. The cumulative hydrogen production is 6.24 mL/g. The pH of the liquid products gradually decreases to 5, and the Eh gradually increased from −180 to −50 mV during the experiment. The peak value of lignin degradation rate is 0.42% on the 7th day. The chemical oxygen demand (COD) first increases and then decreases. The highest COD is 4068 mg/L, and the final COD degradation rate is 46.3%. The peak values of cellulase are 0.021 mg/(mL h) and 0.223 mmol H2/(min mg) at 3 d later than that of hydrogenase. The absorbance of bacterial turbidity first increases and then decreases, with the community structure proving that the hydrolytic bacteria are dominated by Acinetobacter, Comamonas, Intestinimonas, and some fermentation bacteria, including Macellibacteroides. The changes of carbon, oxygen, nitrogen, sulfur and functional groups of carbon and oxygen in coal are obvious, with aliphatic carbon (methoxy, carbonyl, and so on) representing the main part of the biochemical reaction in the macromolecular structure of coal. The analysis of the three-phase state in the process of coal hydrogen production is helpful to study the mechanism of hydrogen production by coal fermentation from different perspectives, and also provides a reference for the promotion of coal hydrogen production by fermentation in the next step.

Potential impact of CO2 injection into coal matrix in molecular terms

Coal seams are one of the targeted geological bodies for CO2 sequestration. Coal structure deformation is a significant feature of the interaction between CO2 and coal seams. The rearrangement of macromolecular structure is a recognized deformation mechanism whose process has not been quantified. We used a high-order coal macromolecular model to reveal specific processes of coal macromolecular rearrangement caused by CO2 injection through molecular dynamics. In addition, the potential impact of this injection on the sequestration and mechanical properties of CO2 in the coal matrix was investigated. The rearrangement of the coal macromolecular structure is accomplished through the mutual coupling of the pore structure and macromolecular ordering. The compression of closed pores and the expansion of open pores together cause the swelling of macromolecular volumes. The concentration of closed pores was an important indicator for the quantification of the rearrangement of macromolecular structure. The greater the content of closed pores in the coal, the higher the potential for macromolecular structure transformation after CO2 injection. Furthermore, the more disordered the direction of the coal macromolecular layer, the lower the peak strength of the coal matrix. The amount of CO2 sequestration in the coal matrix is gradually saturated near the critical pressure point. Our findings contribute to the study of CO2 sequestration in coal seams and CO2 leakage risk prediction.