Coal Structure Research Articles

Permeability evolution of deep-buried coal based on NMR analysis: CO2 adsorption and water content effects

Permeability evolution of deep-buried coal plays a crucial role in CO2 geological storage and coalbed methane (CBM) recovery. In this study, permeability and nuclear magnetic resonance (NMR) tests were conducted on coal specimens under triaxial conditions. Based on the evolution of relaxation time (T2), the effects of CO2 adsorption and water content on permeability, pore structure, and mechanical properties of coal were discussed. Experimental results show that CO2 injection causes adsorption-induced swelling, inhibits flow in the early stage, causes the “expansion” and “complexity” of pore structure, and generates new channels for fluid transport. The presence of water in coal inhibits the flow process, resulting in an overall reduction of permeability by 2 orders of magnitude. Softening occurs in coal specimens, as evidenced by plasticity and low brittle failure and an average strength loss of 29.3 %. Additionally, a complex CO2–H2O coupling effect in coal is observed, causing the dissolution of certain minerals and organic matter, which further amplifies the “weakening effect” of water. Finally, permeability evolution characteristics of coal affected by CO2 and H2O from multiple perspectives were discussed in consideration of NMR fractal analysis. This work provides valuable insights into deep CO2 geological sequestration and CBM recovery.

Quantitative Analysis of Functional Groups in Different Metamorphic Grade Indian Coals using FTIR technique

To enhance the clean and economical feasible utilization of coal and reduce the coal or rock dynamic disaster, one should have an in-depth knowledge of coal macromolecular structures. In view of this, the chemical and physical structural properties of low to high-rank six Indian coals using Fourier transform infrared spectroscopy (FTIR) are presented. Four absorbance bands were identified in the subtracted spectrum for the functional group’s semiquantitative analysis in coal: hydroxyl, aliphatic, oxygen-containing, and aromatic structures associated with absorbance band at 3000–3600 cm-1, 2800–3000 cm-1, 1000–1800 cm-1, and 700–900 cm-1 respectively. Spectrum deconvolution technique was used to determine the FTIR structural parameters of coal samples. To investigate the structural transformation during coal metamorphism, correlations with Vitrinite reflectance (Ro) were formed. The present study will help to predict the conversion of organic groups to valuable liquids and gases from those samples. The macromolecular structure of coal could be useful to design the synthesis methods for carbon-based materials.

Stages of geodynamic development of the territory of the Lviv-Volyn coal basin

The purpose of this work is to analyze geodynamic processes of origination, geological development, and Post-Carboniferous alteration of Carboniferous deposits of the Lviv-Volyn Basin. The methodology is based on historical-geological and facies studies. It includes a comparison of the capacities and distribution of deposits of different ages in the region as well as a dynamic analysis of the formation of coal deposits in the basin. The conducted investigations showed that the territory of the Lviv-Volyn Basin had a special history of geostructural Pre- Carboniferous development that sufficiently influenced its formation and caused geotectonic and morpho-structural alterations. The formation of this region resulted from complex and prolonged, during several tectonic stages, processes of interaction between the zones of tectonic activity and regional fractures connected with it. Activation of these zones happened to be at definite periods of geological time and was manifested in the development of concrete geostructural elements. Tectonic events of all periods of sedimentary cover formation were individual and unique. They became the basis for the formation and changes in the overall structure of the southwestern edge of the East European Platform and, in particular, the Lviv-Volyn Basin, as part of the Lviv-Lublin Trough. With other geostructural relations, the tectonic structure of the basin would be significantly different. In the Paleozoic era, the structural elements of the north-western and south-eastern extension played a decisive role in the formation of this territory. Originality. Based on the analysis of the thickness of the region's deposits of different ages it was shown that forces of horizontal compression affected the coal-bearing unit during the geological formation of the basin. During their influence, there was a depression of the territory, which led to the formation of primary tectonic forms and Post-Carboniferous main tectonic and morphological structures. The coal-bearing deposits have undergone repeated wash-outs, resulting in a shortening of the section and a significant decrease in the volume of the productive part of the coal formation.There is a graphic representation of the change dynamics in the location of crystalline basement surface at different stages of tectonic formation of sedimentary thickness of the basin, whose inclination was different. Practical significance. The conducted research determined stage-by-stage geodynamic development of the territory of the Lviv-Volyn Basin. Five distinct stages can be identified in the formation of coal-bearing deposits, from their origin to their eventual erosion. These stages reflect the unique features of coal formation and geodynamic processes. The geological map of the Pre-Mesozoic deposits of the carboniferous coal-bearing megaformation in the Lviv-Lublin Basin provides insight into the geological structure of the Lviv-Volyn and Lublin Basins, highlighting both similarities and differences. These findings contribute to our understanding of the formation processes of Carboniferous coal-bearing formation in the Lviv-Volyn Basin. They also expand our knowledge of coal distribution, coal seam morphology, and coal thickness structure, which can be used to compare with coal-bearing formations in other basins.

Damage characteristics of pore and fracture structures of coal with liquid nitrogen freeze thaw

Liquid nitrogen (LN2) fracturing technology is a novel waterless fracturing technology that has significant potential for application in the development of coalbed methane. However, the changes in the microstructure after coal samples are treated with LN2 freeze thaw are poorly understood. Therefore, a combination of mercury intrusion porosimetry and micro-computed tomography (micro CT) was employed to investigate the evolution of pore and fracture structure of coal samples treated with LN2. The experimental results showed that the pore volume and average pore size of coal samples increase after LN2 freeze thaw. After 12 freeze thaw cycles, the change in pore volume of micropores and minipores of coal samples was not significant, while the pore volume of mesopores and macropores increased significantly before LN2 freeze thaw. The specific surface area of the pores in different size ranges of coal samples increases with the increase in the number of LN2 freeze thaw cycles; the structure of micropores and miniopores were damaged by thermal stress and frost heave force during LN2 freeze thaw; and the pore size gradually increases to form mesopores and macropores. Micro-CT images of coal samples after LN2 freeze thaw indicated the primary fractures of coal sample expanded and generated a large number of secondary fractures. The primary and secondary fractures are interconnected and ultimately form penetrated fracture enhancing the connectivity of fractures, enhancing the connectivity of the fracture structure. The key finding study is expected to provide a theoretical basis for LN2 fracturing.

Occurrence Modes of AAEMs (Na+ and Ca2+) and the Effect on the Molecular Structures of Zhundong Coal via Quantum Chemistry.

Zhundong coal is known for its high content of alkali and alkaline earth metals (AAEMs), which greatly influences coal processing and utilization. To reveal the occurrence modes and the effect of AAEM ions on the molecular structures of Zhundong coal, the previously constructed molecular structure models of vitrinite-rich and inertinite-rich Zhundong coal (ZD-V and ZD-I) were selected to simulate using quantum chemical methods. By focusing on Na+ and Ca2+, the adsorption capacity at different adsorption sites was investigated based on the density functional theory (DFT), and the effects of adsorption of Na+ and Ca2+ on nearby atomic charges, chemical bonds, and molecular orbitals were investigated. Results show that compared with ZD-I, ZD-V contains a more negative electrostatic potential (ESP) distribution and lower bond order, indicating that vitrinite contains more adsorption sites for AAEM ions and exhibits stronger chemical reactivity. Na+ and Ca2+ are easily adsorbed to the most negative ESP with the optimal adsorption site near the carbonyl group (C=O). Compared with adsorbed Na+, Ca2+ has a smaller adsorption distance from the molecule and a higher adsorption energy. Ca2+ can transfer more charge than Na+ and has more affinity with the coal molecule. Ca2+ at all adsorption sites is bound to organic molecules by chemisorption, which also reveals the reason for the low water-soluble Ca content in coal at the molecular level. Adsorption of AAEM ions has a more significant effect on the chemical bond of oxygen-containing functional groups near AAEM ions compared to the overall molecular fragments, which makes the nearby chemical bonds (C-O/C=O) decrease in bond order and increase in bond length. Ca2+ makes the nearby chemical bonds more prone to break than Na+. Additionally, Ca2+ has a more significant impact on the energy gaps (ΔEgap) compared to Na+. Adsorption of Ca2+ near the carbonyl and carboxyl groups leads to a significant decrease in ΔEgap, indicating an enhanced chemical reactivity of coal molecular fragments.

Changes in physical and chemical structure and Full-stage oxidation characteristics of coal caused by igneous intrusion

The influence of igneous intrusion on entire coal oxidation and its mechanism remains unclear, impacting mining operations. To investigate spontaneous combustion characteristics, this study analyzed the composition and physicochemical structure of igneous intrusion coal (II coal) using various experimental methods: low-temperature oxidation, low-pressure N2 adsorption, XPS, FTIR, and TGA-DSC. Results show that II coal has developed pores favourable for coal-oxygen interaction, but increased moisture and ash relative content, and decreased volatile and reactive functional groups, implying reduced low-temperature oxidation and combustion reactivity. The decrease in CO and CO2 production and the increase in XPT and apparent activation energy confirm this result. Notably, II coal releases more CO and CO2 below 400 K and the gases are lower than raw coal after 400 K, suggesting native CO and CO2 exist in coal and abundant native active sites which are rapidly consumed during this stage. These findings provide insights into predicting spontaneous combustion in mines extracting coal seams affected by igneous intrusion.

Construction of macromolecular structure in KunNing coal and analysis of Macro-Micro oxidation characteristics

To explore the macro–micro coal oxidation characteristics for the purpose of preventing coal spontaneous combustion and achieving efficient coal conversion and utilization, the enclosed coal oxidation characteristics of KunNing coal sample (KN) under 25–70 °C were investigated. KN coal molecular models were constructed based on analysis and testing results of X-ray Photoelectron Spectroscopy (XPS), Carbon-13 Nuclear Magnetic Resonance Spectroscopy (13C NMR), Fourier Transform Infrared Spectroscopy (FTIR) and other techniques. The pyrolysis processes of coal-oxygen reaction systems with different oxygen contents were simulated using ReaxFF force field. The results show that the oxygen consumption rate, CO release rate, CO2 release rate and oxidation heat release intensity of coal increase exponentially with rising temperature. The ratio of aromatic bridge carbon to peripheral carbon in KN coal molecular structure is 0.08, and the molecular formula is C149H76O15N2S2. In the later stage of the experimental and molecular dynamics simulation processes, the CO content decreased while the growth rate of CO2 increased. Different indicator gases exhibit varied responses to oxygen content. The C-atom labeling and traceback results for the four indicator gas molecules demonstrate that CO2 mainly comes from aldehyde, hydroxyl, carboxyl and methoxy groups in coal molecules, CO is primarily generated from esters, phenols, carbonyls, C2H2 is predominantly derived from ring-opening and cracking of cycloalkanes, and the source of C2H4 is related to cycloalkanes and alcohols.

Evolution characteristics of pore structure of coal under freeze-thaw cycles combined with scanning electron microscope instrument and nitrogen adsorption experiment

ABSTRACT In this paper, anthracite in Shanxi region is taken as a research object, and the pore structure of coal under different freeze-thaw times is characterized and analyzed by scanning electron microscope and nitrogen adsorption experiment. The SEM experimental results show that the micron-scale cracks on the surface of anthracite gradually connect after freeze-thaw, and the secondary crack channels are formed around the pores and air holes, so that the pores also begin to connect. Further, the quantitative information of the picture was extracted, and it was found that the surface porosity and fractal dimension of the coal sample changed significantly after three freeze-thaw cycles, which increased by 163.3% and 15.9% respectively, and increased with the increase of freeze-thaw cycles. The results of low-temperature nitrogen adsorption experiments show that the coal samples after freeze-thaw are mainly crack, and the pore structure changes from simple to complex. In addition, the maximum nitrogen adsorption capacity, specific surface area and pore volume increased by 125.7%, 69.3% and 72.6%, respectively. With the increase of freeze-thaw times, the growth rates of maximum nitrogen adsorption capacity, specific surface area and pore volume are significantly different. The results of joint characterization show that the number of pores and cracks increases to different degrees after freezing and thawing. Comparing the increase in the degree of fracturing (163.3%) with the increase in the total pore volume (30.68%), we find that the degree of change of the crack after thawing is higher than that of the pore. In this paper, the damage effect of coal samples after freezing and thawing with liquid nitrogen is investigated from an experimental point of view, which accelerates the great leap of cyclic cryogenic fracturing technology from basic theory to technical practice.

Experimental investigation of gas diffusion kinetics and pore-structure characteristics during coalbed methane desorption within a coal seam

The desorption-diffusion properties of coalbed methane (CBM) are not only the theoretical basis for investigating gas outburst hazards in a coal mine, but also impose a non-negligible impact on CBM recovery performance. In this paper, the pore structure of coal with five different metamorphism degrees was qualitatively and quantitatively characterized with the scanning electron microscopy (SEM) and low-temperature nitrogen gas adsorption (LTNGA) measurements. The isothermal methane ad/desorption experiments were performed with four initial gas desorption pressure scales. Subsequently, the matching degree between eight empirical correlations (ECs) and the measured methane desorption data was determined, and the methane diffusion coefficient (Dfg) was numerically calculated combining with an FGDG (free gas density gradient) diffusion model. Importantly, we evaluated and discussed the sensitivity of pore structure parameters, initial gas desorption pressure, and volatile fraction to Dfg. The results show that pore density distribution differs among coal samples and the pore structure, especially the micropores, is developed better in high-rank coals than in low- and medium-rank ones. Initial gas release velocity increases with an increase in coal rank and decreases negatively with the increasing particle size in an exponential manner. Among the eight ECs, EC-VIII can be applied to more accurately describe gas desorption process during the whole time scale, and its correlation coefficient with the measured data is larger than 0.9900. Dfg increases with an increase in pore volume and specific surface area (SSA), and it is negatively correlated with the average pore diameter. Additionally, Dfg exhibits an asymmetric U-shaped pattern with an increase in the volatile fraction, which is mainly related to the switch between the dominant roles of mesopores and micropores during coal metamorphism. At a lower volatile fraction, the diffusion coefficient is more sensitive to micropores, while, at a higher volatile fraction, mesopores contribute more to the diffusion coefficient.

A comparative study of molecular structure and combustion behavior of coal and its separated vitrains

A systematic study has been conducted on two coking coals and their separated vitrains to understand the macromolecular structure and their thermal behavior. The structural parameters have been determined by using XRD (X-ray diffraction), Raman spectroscopy, FTIR, Solid-state 13C NMR and TG/DTG–DSC. The proximate and ultimate analyses results indicate that vitrinite is the major organic component of coal and it is type III kerogen having H/C ratio in the range of 0.58–0.72 and O/C ratio in the range of 0.09 to 0.16. The inter-layer spacings, crystallite size, the average number of the aromatic layers, aromaticity, and rank of demineralized coal and vitrain as obtained from XRD studies vary in the range of 3.49–3.70 Å, 13.24–16.72 Å, 3.60–4.74, 0.72–0.80 and 2.62–3.98 respectively. The values of aromaticity, rank and inter-layer spacing of vitrain are higher however the crystallite size and the average number of the aromatic layer are lower than that of respective demineralized coal. Raman spectra results show that vitrain is more graphitic and stable in nature compared to their bulk coal and the structure of coal gets disordered during demineralization. The FTIR spectrum shows that carboxylic (–COOH), carbon–Carbon double bonds (–CC–), carbonyl/carboxyl (CO), methyl and methylene (–CH3, –CH2–), and a hydroxyl group (-OH) are the major functional groups. The structural parameters as obtained from FTIR studies such as aromaticity (fa)FTIR, degree of condensation of aromatic rings and aliphatic to aromatic ratio are higher in vitrain than their respective coals, while the aliphatic hydrogen to total hydrogen ratio (Hal/H) of demineralized coal is higher than the raw coal and vitrain. The 13C NMR spectra show the fraction of unsaturated carbon is higher in vitrain than the respective raw coal. The thermal analysis result shows the reactivity of the samples increases in the order of demineralized coal > vitrain > raw coal and the activation energy increases in the order of raw coal > demineralized coal > vitrain.

Experimental investigation of the fracture and damage evolution characteristics of flawed coal based on electric potential and acoustic emission parameter analyses

Flawed coal is the most common construction object in the mining process, and understanding its mechanical behavior and damage evolution is crucial for monitoring the stability of underground projects. In this work, uniaxial compression experiments on flawed coal were conducted, and the electric potential (EP) and acoustic emission (AE) signals were recorded synchronously. An in-depth study of the fracture mode and damage evolution characteristics of coal samples based on EP and AE responses was conducted. The results showed that (1) as loading proceeds, the coal body stress increases, the EP and AE values increase, and EP and AE are excited whenever the stress drop phenomenon occurs. With the gradual expansion of the initial flaws, the EP field and AE event distributions show regional differences. High-intensity EP anomaly bands are generated in the path of the main crack formation, and many high-energy AE events are generated. (2) The fractured behavior of the coal sample is dramatically altered by flaw inclination. When the flaw inclination increases, the AE signals show a shift from the common distributions of the six types to the concentrated distributions of the low amplitude–low frequency (LA–LF) and high amplitude–low frequency (HA–LF) types, the coal sample destruction modes are transformed from dominantly tensile to mixed tensile and shear, and the destruction mode is increasingly complicated. (3) It is possible to evaluate and analyze the damage evolution characteristics of the coal samples by defining the coefficient of variation (CV value) of the EP and the b value of the AE. Both a sudden increase in the CV value and a sudden decrease in the b value can be used as precursor information for coal failure. A two-parameter joint monitoring approach can be used to precisely and promptly warn about coal instability damage. The research results can provide theoretical support for the stability assessment of coal structures during the deep coal seam mining process and early warning of the occurrence of dynamic disasters.

Periodic action mechanism of soaking on the evolution of lignite pore structure and microactive groups

As the mining of the overlying coal seam advances, the risk of spontaneous combustion (SC) of water-immersed and air-dried coal in the overlying goaf increases. The structure of functional groups in coal is the fundamental factor for the occurrence of oxidation and heat production, and the surface pores are the main sites of oxidation reactions. In order to explore the mechanism of action of water immersion on the macrostructure and micro-group of lignite, the water-immersed coals (WIC) of 30 days (I30), 60 days (I60), 90 days (I90), 120 days (I120) and 150 days (I150) were prepared. The surface pore characteristics of coal and the evolution of active functional group structure were studied by nitrogen adsorption and infrared spectroscopy. The adsorption capacity of pores and surface structure of coal were quantitatively characterized by pore adsorption kinetic equations and fractal methods. The results showed that the fine-necked bottle-shaped pore structure of water-immersed lignite decreased, and the wedge-shaped pores and parallel plate slits increased. After water-soaking, the specific surface area of lignite increased first and then decreased, the pore volume and pore diameter increased gradually, D1 decreased gradually, D2 increased gradually, and the pore adsorption potential increased gradually, which characterized the gradual pore structure complexity and enhanced adsorption capacity. Water-soaking triggered the conversion of active carbon-hydrogen bonds at the α-position of lignite molecules into active carbon–oxygen single bonds with alcohol-oxygen bonds. I90 and I120 have the highest content of active functional groups, indicating that the oxidation activity of lignite immersed in water for 90–120 days is the highest. This study provides a new method for the quantitative characterization of coal adsorption and has reference value for the study of SC mechanism of water-immersed, air-dried coal in the overlying goaf of close distance coal seam.

Effect and mechanism of ultrasonic mechanical vibration on methane adsorption

Ultrasonic stimulation technology with mechanical vibration effect and thermal effect offers unique advantages for promoting coalbed methane desorption and improving reservoir permeability. Recently extensive studies have been conducted on the effects of ultrasound on reservoir physical characteristics and methane adsorption behavior. However, most studies have neglected the influence of each effect as an independent variable on methane adsorption. Particularly, there has been inadequate attention given to the adsorption behavior of methane by coal under ultrasonic mechanical vibration. The coupling relationship among coal-water-methane molecules under ultrasonic mechanical vibration has yet to be revealed. Here, anthracite and bituminous coal were used as research objects, and the variable separation of the ultrasonic mechanical vibration was carried out using a constant temperature device. The effect and mechanism of ultrasonic mechanical vibration on methane adsorption were revealed by isothermal adsorption experiments. The results showed that ultrasonic mechanical vibration had a certain promotion effect on methane adsorption, which was opposite to the comprehensive effect. However, this promotion effect was influenced by the coal rank, moisture condition, and pressure. The effect of ultrasonic mechanical vibration on the molecular structure of the coal was an internal mechanism that enhanced methane adsorption. Among them, the methyl structure of the aliphatic side chain, the hydroxyl structure, the aromatic hydrogen structure, and the oxygen-containing heteroatom structure were sensitive to ultrasonic mechanical vibration. This influence improved the orientation degree of the coal molecular structure and increased the coal's surface free energy and adsorption potential, resulting in the ability of the coal to adsorb more methane molecules under the same pressure conditions. The content of sensitive functional groups was the main reason for the difference in promotion effect. There were more sensitive functional groups in bituminous coal, leading to a more significant promotion of methane adsorption by ultrasonic mechanical vibration. Furthermore, the moisture content would also affect the promotion effect. Moisture in coal strengthened the effect of ultrasonic mechanical vibration on sensitive functional groups by facilitating ultrasonic propagation through the coal. Therefore, in moisture-equilibrated samples, ultrasonic mechanical vibration had a greater effect on methane adsorption.

Research on coal matrix pore structure evolution and adsorption behavior characteristics under different thermal stimulation

The technique of thermal stimulation for coalbed methane extraction is considered an important approach for increasing coalbed methane production capacity in the future. This study focuses on the impact of the thermal evolution of pore structure in coal on gas adsorption characteristics. The low-pressure CO2 and N2 physical adsorption data were analyzed using classical thermodynamic methods and density functional theory. The results showed a decrease in pore volume and an increase in average pore size. The fractal dimensions obtained from the Frenkel-Halsey-Hill and Sierpinski fractal models indicate that the micro-scale spatial structure of coal becomes simpler, and the surface becomes smoother. The high-pressure volumetric method was employed to determine the gas adsorption parameters of the coal, and a comparison was made between the calculated gas adsorption parameters based on the micro-pore structure parameters of the coal samples. It was found that the variation pattern of micro-pores in the coal samples correlated highly with the methane adsorption capacity. The decrease in adsorption capacity increases the proportion of free-phase methane, which helps enhance the early-stage methane extraction rate. Improving gas migration pathways avoids the limitation imposed by diffusion in the later stages of coalbed methane extraction.

Mechanical properties of coal-rock combined bodies under the action of ScCO2: A study of damage and failure mechanism

Injecting CO2 into deep unrecoverable coal seams can help achieve carbon sequestration targets and improve the coalbed methane output rate. However, CO2 reaches a supercritical in situ storage state that impacts the coal storage structure. Based on the independently developed ScCO2 soak system, RFPA3D numerical simulation, and theoretical calculations, the mechanical damage characteristics and failure mechanism of RCR samples with three types of coal seam thicknesses and three types of top and bottom plate lithologies under the action of ScCO2 are investigated. The study reveals that (1) under the action of ScCO2, with the increase in the coal thickness, the deterioration degree of compressive strength and modulus of elasticity of the RCR combined body gradually increases and decreases, respectively, (2) ScCO2 will promote the plastic failure of the coal mass and exacerbate the tendency of the RCR combined body to change from tensile splitting failure to shear plastic failure, and the degree of plastic failure of the RCR combined body has a positive correlation with the coal thickness and the rock-coal strength ratio, and (3) ScCO2 accelerates the failure of the RCR combined body, and the coal mass enters the plastic failure stage earlier, while the rock mass ends the elastic strain energy accumulation and unloads and releases energy earlier, reducing the accumulation and release of energy in the combined body. The research results can provide a theoretical reference for the geological storage of CO2 deep inaccessible coal seams.

Analysis of the influence of different pore ratios on methane adsorption characteristics of coal

The diverse pore structures in coal are pivotal in coalbed methane storage and transportation, making it highly significant to explore methane adsorption characteristics across varying pore sizes for effective prevention and control of coal mine gas disasters. Using the proposed graphene nanotube filling method, eight coal molecule models with varying pore size ratios were constructed, including four different pore radii: 0.35 nm, 0.55 nm, 0.75 nm, and 2 nm. Methane adsorption amount, methane density, and methane concentration distribution were obtained through Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods. The research results show that coal consisting of multiple pore sizes had a better methane adsorption capacity than a single pore size. Combining pore sizes of 0.35 nm, 0.55 nm, and 0.75 nm in a 1:2:1 ratio resulted in the highest excess methane adsorption capacity in coal, reaching 5.623 mmol/g. The single pore size of 0.55 nm in coal had the lowest excess adsorption amount of methane, which was 2.236 mmol/g. The presence of mesopores weakened the methane adsorption capacity of coal, and this weakening effect increased with the increase of mesopore proportion. In the model composed of single pore size micropores, the methane concentration inside the pores was uniformly distributed. The coexistence of multiple micropores may lead to methane filling in the micropores in a monolayer adsorption form. The coexistence of micropores and mesopores does not alter methane filling in the micropores and monolayer adsorption in the mesopores. The research results provide new insights into our understanding of the impact of pores on methane adsorption in coal.

Digital rock characterization and CO2 flow simulation of high-volatile bituminous coal: An application to carbon geosequestration

The permeability of CO2 sequestration is influenced by the characteristics of pore–fracture structures in coal, yet the flow simulation via the pore network model (PNM) and the controls of topology and morphology on permeability remain to be understood. First, the pore–fracture structure of high-volatile bituminous coal (HVBC) is characterized using micro-computed tomography, and the flow characteristics of single-phase CO2 in the connected pore–fracture network are simulated by the PNM. Second, the surface roughness of the connected pore–fracture is extracted by fractal characterization, and morphological algorithms are applied to accurately present the pore–fracture network structure. Finally, the implications of structural parameters (pore or throat diameter, coordination number, tortuosity, and sphericity) on CO2 transport are discussed, and the mechanism of pore–fracture structure response due to mineral dissolution in CO2 injection is analyzed. The results shows that the HVBC sample provides significant pore–fracture space (porosity of 10.87%) and flow path (connectivity of 66.50%) for hydrogen and carbon storage. CO2 flow simulation results demonstrate anisotropic flow, with higher CO2 permeability observed in the Z-axis direction (0.061 × 10−3 μm2) compared to the X- and Y-axis directions (0.044 × 10−3 μm2 and 0.059 × 10−3 μm2, respectively). Moreover, the fractures perpendicular to the coal bedding plane in the coal seam with a large tectonic dip strongly influence the flow of CO2 injection. Fractal analysis reveals a positive correlation between fractal dimension and porosity, indicating that structures with higher surface roughness are not convenient for CO2 transport. Significant pore angle characteristics (sphericity average of 0.58), high pore–throat connectivity (coordination number average of 4.31), and low capillary resistance (tortuosity average of 1.19) collectively affect the flow of CO2. Overall, the strongly anisotropic pore–fracture structure contributes to the inhomogeneous flow pattern in CO2 geosequestration. Changes in pore–fracture structure resulting from mineral dissolution during the early stage of CO2–coal matrix interaction in the HVBC reservoir can significantly enhance storage potential. This study contributes to the existing understanding of flow characteristics and provides insights for optimizing CO2 injection efficiency in carbon geosequestration.

Study on the kinetics of chemical structure reaction in coal catalyzed by OH free radicals

This study investigated the influence of OH free radicals on the reactivity of aromatic rings and their side chain active groups. Quantum chemistry theory and in situ diffuse reflectance infrared Fourier transform spectroscopy were applied to perform simulations and experiments. Density functional theory was used to calculate the electrophilic substitution of aromatic hydrocarbons of six model structures by OH free radicals. The reactivities of the key elementary reaction of the side chain active group before and after the reaction were compared. The results showed that the reaction energy barriers of the addition and elimination of OH radicals of aromatic hydrocarbons were lower than 40 kJ/mol, indicating that the reactions can occur at room temperature and emit large amounts of heat. The electronic effect of phenolic hydroxyl and benzene rings further improved the reactivity of the aromatic ring C–H. Combining these results with the change rule of the phenolic hydroxyl structure in coal under isothermal oxidation conditions, we conclude that OH radicals participate in the formation of the phenolic hydroxyl structure and further catalyze the oxidation reaction of coal at low temperatures. This finding is significant for the heat storage stage of low-temperature oxidation in coal.

An Experimental Study on the Effect of Magmatic Thermal Evolution on the Molecular Structure of Low-Rank Coal

Low-rank coal accounts for over half of China’s proven coal reserves. The possibility of coal and gas outbursts in low-rank coal is higher, especially in the m·agmatic thermal evolution area. The complexity of coal’s molecular structure is one of the reasons for problems during the process of mining operations. Different analysis techniques, including XRD, FTIR and Raman spectroscopy, were used to obtain the molecular characteristics of magmatic thermal evolution coal samples and normal coal samples, so that a comparative study could be carried out to investigate the influence of the magmatic thermal evolution effect on the molecular structure of low-rank coal. The ranges of the aromatic interlayer spacing (d002), average stacking heights (Lc) and stacking layer number (Nave) of the thermally evolved coal samples are 3.41–3.51 Å, 22.76–27.02 Å, and 6.68–7.70, respectively. The ranges of the full width at half maximum ratio (FD1/FG) and the peak integral intensity ratio (ID1/IG) are 2.16–2.19 and 1.55–1.84, respectively. Compared with the normal coal samples, those affected by magmatic thermal evolution have smaller d002, ID1/IG, and FD1/FG values, but larger Lc values. The results indicate that the thermally evolved coal samples have more ordered structures and more developed microcrystalline structure sizes than normal coal samples.

Effect of Inhibitors on Hydrogen Bond Reduction and Kaolinite Dissolution for Enhanced CO2 Adsorption in Coal.

The application of an inhibitor to the remaining coal in the goaf not only prevents spontaneous combustion of the coal seam in the mining area but also greatly enhances the capacity of coal to adsorb CO2. To investigate the mechanism by which inhibitors improve the CO2 adsorption capacity of the coal seam in the goaf, we conducted swelling experiments, infrared spectroscopy, scanning electron microscopy, and X-ray diffraction analyses to examine the microstructural changes in the adsorption of CO2 before and after inhibition. The results indicate that after inhibition, the number of hydrogen bonds between coal macromolecules decreased, and the samples exhibited approximately 5% swelling. This swelling of the coal macromolecular structure and the increased distance between coal particles create additional space for CO2 sequestration, which is a critical factor contributing to the enhanced CO2 adsorption capacity of coal. The mineral composition of coal consists of 75.6% kaolinite, and inhibition leads to a reduction in kaolinite content by 0.8-7.9%. After inhibition, the swelling and disintegration of kaolinite cause uneven stress, resulting in changes to the pore structure. Closed pores filled with kaolinite transform into open pores, and the original pores crack, forming new pores and pore channels. The dissolution of kaolinite particles increases the porosity of the coal, further facilitating gas adsorption. Among the three inhibitors tested, the most effective in enhancing CO2 sequestration by bituminous coal in the mining area was the urea solution. This study holds significant importance in improving the CO2 sequestration capacity of residual coal in goaves.