Coal Samples Research Articles

Microstructure Imaging and Characterization of Rocks Subjected to Liquid Nitrogen Cooling

Liquid nitrogen (LN2) fracturing is a potential stimulation method in unconventional hydrocarbon recovery, showing its merits in being water free, creating low formation damage and being environmentally friendly. The microstructure evolution of rocks subjected to LN2 cooling is a fundamental concern for the engineering application of LN2 fracturing. In this paper, pore-scale imaging and characterization were performed on two rocks, i.e., tight sandstone and coal specimens subjected to LN2 cooling using computed tomography scanning. The digital core technique was employed to reconstruct the microstructures of rocks and give a quantitative analysis of the pore structure evolution of both dry and water-saturated rocks. The results indicate that LN2 cooling has a great effect on the pores’ morphology and their spatial distribution, leading to a great improvement in pore diameter and aspect ratio. When compared to the sandstone, coal is more sensitive to LN2 cooling and thermal stresses, having a more noticeable growth in pore–throat size. The porosity growth of coal is 291% higher than that of sandstone. There is a growing trend in the irregularity and complexity of pore structures. After LN2 cooling, the fractal dimensions of the pores of sandstone and coal grow by 11.7% and 0.87%, respectively, and the proportion of pores with a shape factor > 100 increases. More bundle-like and strip-shape pores with multiple branches are generated, which causes a significant growth in the throat size and the proportion of connected pores with a coordination number ≥ 1, enhancing the complexity and connectivity of pore structures dramatically. Additionally, pore water plays an important role in aggravating rock damage during LN2 cooling, enhancing the pore space and connectivity. The porosities of the saturated sandstone and coal samples grow by 22.6% and 490.4%, respectively, after LN2 cooling, which are 5.6% and 186.6% higher than dry samples. The generation of macropores ≥ 70 μm is the primary contributor to porosity growth during LN2 cooling, although such pores account for only a small proportion of the total. These findings contribute to our understanding of the microscopic mechanism of LN2 cooling on rock damage and may provide some guidance for the engineering application of LN2 fracturing.

Experimental study on resistivity evolution law and precursory signals in the damage process of gas-bearing coal

The occurrence of elevated gas pressure leads to the distortion of the internal pore structure within the coal matrix, thereby exerting a substantial influence on the destabilization of the coal body. This disturbance further contributes to intricate changes in coal resistivity. To delve into the process of gas-induced damage and its implications for coal resistivity, this research employs nuclear magnetic resonance (NMR) technology as a means to characterize coal samples. Moreover, a controlled loading experimental equipment is devised for gas-bearing coal, while concurrently gathering data on resistivity and acoustic emission (AE). The result shows that with increasing applied stress, the change in resistivity within gas-bearing coal exhibits a progressive intensification, manifesting a notable “V”-shaped pattern. This pattern is characterized by an initial rapid decline, succeeded by a subsequent swift increase during the failure stage. The AE signals exhibit a synchronized consistency with the observed resistivity change. With increasing gas pressure, the coal's resistivity exhibits heightened fluctuations, and notably, the range of resistivity at failure under a pressure of 3 MPa is 2.27 times greater compared to that at 0 MPa. Furthermore, the damage variable and constitutive equation for gas-bearing coal based on resistivity is established. The critical value for damage (DE) occurs 1.75 s before the critical value of the resistivity change rate, and 3.88 s earlier than the actual occurrence of damage in the coal body. The correlation between the verified damage and measured stress exceeds 0.8, demonstrating a strong correlation at different stages. The research results contribute to understanding the evolution mechanism of gas instability and hold considerable significance for preventing coal and gas outburst disasters.

Experimental study on mechanical damage characteristics of water-bearing tar-rich coal under microwave radiation

As a recognized special resource, tar-rich coal can extract the country's scarce oil and gas resources and generate semi-coke that can replace anthracite and coking coal. The tar-rich coal in northern Shaanxi is prominent, but due to the dense structure and high strength of tar-rich coal, it is easy to cause frequent dynamic disasters in coal mining. Therefore, the realization of pressure relief and disaster reduction has become the primary problem in mining tar-rich coal. There are many shortcomings in conventional pressure relief methods, so a new method of microwave-weakening coal is proposed. Through different water saturation treatments of tar-rich coal samples, the longitudinal wave velocity degradation trend and surface crack expansion law of water-bearing coal after microwave irradiation were analyzed, and the strength softening characterization and energy evolution relationship under the combined action of microwave and water were studied. Fractal dimension and its internal correlation based on the equivalent side length-mass of coal sample fragments. The experimental results show that: (1) Under the same microwave radiation condition, with the increase of water saturation, the deterioration trend of physical and mechanical parameters such as longitudinal wave velocity and peak strength is obvious. (2) After microwave radiation, the uniaxial compression results show that the coal sample is damaged by load, there is still a high residual strength, the ratio of elastic energy to dissipation energy decreases, and the possibility of rockburst of the coal sample decreases. The strength softening degree of coal specimen under the degradation of microwave and water is the highest, followed by microwave and water. (3) The fractal dimension is inversely proportional to the moisture content and microwave radiation intensity, and the fractal dimension has a significant positive correlation with the peak intensity and longitudinal wave velocity. The mechanical damage law of water-bearing tar-rich coal under microwave action is revealed, which aims to solve the problem of weakening and reducing the impact of hard coal on-site to a certain extent, ensure the safety of working face, and improve the mining efficiency of tar-rich coal. It provides basic theoretical support for microwave-assisted hydraulic fracturing technology and effective weakening measures for hard roof treatment.

Effect of SiO2 nanofluid concentration on micro-mechanical weakening behavior of coal.

Coalbed methane is a crucial component of unconventional natural gas, essential for energy strategies. However, its extraction is often impeded by coal and gas outbursts, which pose significant safety and operational challenges. Coal seam water injection is an effective method for preventing these outbursts, and SiO2 nanofluids can enhance this method by improving coal seam wettability. Nevertheless, the microscopic mechanical properties of SiO2 nanofluid-wetted coal samples remain unclear, complicating the understanding of the enhancement mechanism against outbursts. This study employed nanoindentation tests, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) to analyze the effects of SiO2 nanofluids at concentrations of 0.5 wt%, 1.0 wt%, and 1.5 wt%, comparing them to untreated coal samples. The results reveal that SiO2 nanofluid-modified coal samples exhibit an increased degree of softening, with hardness reduced by up to 21.51% and elastic modulus decreased by up to 27.97%. SEM images and EDX analysis show that lower concentration SiO2 nanofluids weaken coal samples by enhancing their water absorption capacity and reducing cohesion, while higher concentration nanofluids strengthen coal samples by filling and covering microcracks, thereby limiting moisture entry. These findings provide a quantitative reference for the application of SiO2 nanofluids and guide the selection of optimal nanofluid concentrations.

Energy Dissipation and Fractal Properties of Coal under Cyclic Impact Loading

To investigate the periodic disturbance and damage characteristics of coal under different impact pressures, the stress‒strain failure and damage trends of weak, impact‐prone coal samples were analysed under different impact pressures and different numbers of impacts. Split‐Hopkinson pressure bar (SHPB) tests were conducted to study the stress‒strain curve, dynamic compressive strength, elastic modulus, failure strain, and absorbed energy responses with different numbers of impacts and different strain rates. Fractal theory and methods were applied to analyse the influence of different impact pressures and different numbers of impacts on coal sample failure to reveal the relationship between the number of impacts and the fractal size characteristics of coal fragmentation. The energy modes and failure modes of coal samples under different impact effects were analysed under dynamic loading. Regarding the energy absorption corresponding to different numbers of impacts, the energy absorption per unit volume (EAPUV) of three impacts was calculated to be 0.46 J/cm3. The number of impacts was directly correlated with the energy absorbed by the coal sample. Furthermore, the number of impacts was directly correlated with the severity of the deformation and damage of the coal sample and was inversely correlated with the fragmentation of the coal sample. The degree of coal sample fragmentation was directly proportional to the number of impacts, and the fractal dimension increased with the number of impacts. The study of the failure and fractal characteristics of coal samples under different impact pressures and different numbers of impacts provides important information for the prevention and control of coal mine rock bursts.

Study on the Extraction Mechanism of Metal Ions on Small Molecular Phase of Tar-Rich Coal under Ultrasonic Loading

This study aims to elucidate the mechanism by which the ultrasonic loading of metal affects the extraction of small molecular phase substances (low molecular compounds) in tar-rich coal. Tar-rich coal samples were collected from the Huangling mining area in the southeastern Ordos Basin, China. The coal, the leaching solution of the coal, the extraction products, and the extraction residual coal samples with different metal ions loaded by ultrasound were analyzed using field emission scanning electron microscopy, pH detection, gas chromatography–mass spectrometry, a Fourier transform infrared spectrometer, and an X-ray diffractometer. The obtained results indicated that the ultrasonic loading of coal samples with different metal ions (Mn2+, Co2+, Cu2+, Fe2+, and Ni2+) promoted the extraction of small molecular phase substances in coal and increased the proportion of extracted aliphatic hydrocarbons, alkylbenzene, naphthalene, phenanthrene, and other compounds. The extraction rate of Mn2+ was the highest. Compared with the control group, the extraction rate was increased by 212%. After the ultrasonic loading of metal ions, the physical structure of the coal was loose and the contact area of the solvent increased; the degree of branching and the hydrogen enrichment of the residual coal decreased, the aromaticity increased, the interlayer spacing and stacking layers decreased, and the stacking degree and ductility increased. Metal ions exchanged with hydrogen ions in the coal molecules. At the same time, the metal ions were adsorbed in the coal molecules and effectively combined with free electrons in the coal molecules to catalyze; thus, the extraction effect of the small molecular phase of tar-rich coal was improved. This provides a new method for the clean and efficient utilization of tar-rich coal.

Mechanical Activation as a Stage of Coal Sample Preparation in the Analysis of Rare Earth Elements Content by Inductively Coupled Plasma Mass Spectrometry

Background: Brown coals are recognized as promising sources of rare earth elements (REEs). Rare earths are present in both the mineral and organic parts of brown coal. Objectives: This study was conducted to investigate the influence of preliminary mechanical activation in the process of sample preparation prior to analyzing the concentrations of rare earth elements in brown coal samples of various origins and compositions Methods: Four coal samples from different deposits in Russia were selected for the study. Samples were treated with mechanical activation, without reagents, or mechanochemical activation, with humic acids added externally as reagents. X-ray phase analysis was carried out with the selected samples. The quantities of rare-earth elements present in the samples were studied by the method of high-sensitivity inductively coupled plasma mass spectrometry (ICP-MS). Results: It was found that the mechanical activation of coal before dissolution in a mixture of nitric and hydrofluoric acids leads to an increase in the determined concentration of rare earth elements. For this study, the expediency of using only nitric acid as an optimal solvent for the elemental analysis of coal samples was shown. The total concentration of all REE after dissolution of nitric acid and mechanochemical activation with humic acid reached 2456 g/t in Vanchin coal, 968 g/t in Azeysky coal, and 24 g/t and 150 g/t in Itatsky and Spetsugli coals, respectively. Conclusion: Mechanical activation and mechanochemical treatment can greatly help to facilitate sample preparation of natural objects, such as coals for elemental analysis, but in some cases, only a change of solvent is sufficient. When developing technology for concentrating rare earth elements from coal involving grinding, it is necessary to take into account the fact that mechanical activation of coal changes its tendency to dissolve, which may affect the results of the analysis and should be taken into account during experiments.

ЛАБОРАТОРНІ ДОСЛІДЖЕННЯ ДВОВИМІРНОЇ ФІЛЬТРАЦІЇ ВОДНОГО РОЗЧИНУ ТВЕРДОГО АНТИПІРОГЕНУ «PIROCOOL» ЧЕРЕЗ ВУГІЛЛЯ

Purpose. Study of filtration parameters of an aqueous solution of solid anti-pyrogen "Pirocool" and assessment of its influence on methane migration from the coal seam to the preparatory production. Method. The research was carried out on a stand for two-dimensional filtration of liquid with anti-pyrogen "Pirocool" through a coal sample. The sample is 40x40x40 mm with a hole located in the middle of the sample. Results. The obtained dependence of the relative permeability of the aqueous solution of the antipyrogen "Pirocool" on its concentration compared to the permeability of pure water. Obtained experimental data on the relative change in gas permeability of a coal sample before hydrotreatment, after hydrotreatment with water, and after repeated treatment with an aqueous solution of the anti-pyrogen "Pirocool". Scientific novelty. It consists of obtained experimental data. In addition, the wavy character of the dependence of the relative permeability of the aqueous solution of the anti-pyrogen "Pirocool" deserves attention, which may turn into a well-founded regularity in further more advanced research. Practical meaning. The obtained results prove the possibility of combining two separate safety measures in the development of coal seams - hydro loosening to prevent sudden emissions during preparatory work, and injection of aqueous solutions of anti-pyrogens in order to prevent ignition of the seam. This allows you to reduce costs and increase the productivity of mandatory preventive measures. Key words: anti-pyrogen, coal seam, liquid filtration, solution concentration, sudden emissions.

Quality management in a 3D geological model – reliability of predicted hard coal quality parameters

This paper presents some of the results of the project undertaken in JSW SA, which aim was to create a three-dimensional model of the deposits that make up the company and schedule company’s production. The assessment of the quantity of coals without analysis of qualitative data, i.e. physicochemical parameters, coking parameters, and optical petrographic analysis is not suitable for obtaining commercial contractors. To obtain information on the quality of the coal seam, the geological service of the mine takes coal samples. In the stratigraphic model and quality model, dedicated interpolators are used for interpolation and extrapolation. In the seam quality model, the most optimized interpolators are Inverse and Height. Modelled parameters such as volatile parts content and random vitrinite reflectivity were analysed in detail. The Height interpolator looks for both a random and a linear dependency. It extracts random changes locally while searching for linear dependencies and extrapolates them to a deposit area that does not have qualitative data. There is a risk of extrapolating a given value to infinity. Nevertheless, the amount of data and the area modelled allows you to close its scope at an acceptable level. A separate POLYGON interpolator based on mxl express surfaces was created to map coal type range. It uses interpolated quality parameters at a given location, generating a range of a particular type of coal. Setting the trend of variability makes it possible to predict higher coal types in deeper, unrecognized batches of deposits according to documented variability in parameters.

Reliable exploration of the deposit as the first step in the process of forecasting the quality of commercial coal

The article presents the basics of the deposit exploration process carried out at Jastrzębska Spółka Węglowa, which aims to ensure a rational exploitation of resources while maintaining high product quality. It showcases the problems associated with the small mass of samples coming from the exploration wells, which make determining the key quality parameters of coke impossible and proposes solutions to assess the quality of coals in terms of producing coke with high parameters, as determined by the Nippon Steel Corporation (NSC) test, i.e. coke reactivity (CRI) and post-reaction strength (CSR). Jastrzębska Spółka Węglowa, in initiating the “Quality Program” in 2018, covering the processes of deposit modeling, production planning and scheduling, and commercial product quality monitoring, set the goal of improving production processes by enabling the control of the production planning process. Such efforts are aimed at enabling the dynamic adjustment of product quality to the changing business environment, particularly at a time of increased decarbonization efforts in the European steel industry. Knowing the quality of deposits is a milestone in the process of predicting the quality of a commercial product. The primary tools for exploration of the deposit are both surface and underground drilling and sampling of seam coal during ongoing access and mining operations. In addition, analysis of qualitative data, i.e. physicochemical parameters, coking parameters and petrographic analysis from previous mining operations is carried out for the purpose of assessing coal quality and its suitability for coking.

Analysis of common reservoir damage problems in coalbed methane wells and study on acid plugging removal agent system

Reservoir damage is an important factor limiting the productivity of coalbed methane wells. In this paper, the coal seam in a coalbed methane producing area in the southeast edge of Ordos Basin is taken as the research object, and the types of reservoir damage in this block are analyzed through the composition test of coal samples and the comparison of production conditions, and the targeted solutions are given. The results show that the main types of reservoir damage in this block are solid plugging and water locking damage. The dissolution rate of acidic plugging removal working fluid with hydrochloric acid, hydrofluoric acid, acetic acid and hydrogen peroxide as main components for pulverized coal in this block reached 13.73%. After acidification, the permeability increased by 1.8~6.9 times, and the porosity increased by 0.35%~2.87%, indicating that the plugging removal working fluid system can effectively dredge the liquid seepage channel, reduce the seepage resistance and achieve the plugging removal effect.

Impact of change in coal quality on operating parameters of thermal power plant Stanari and its risk assessment

The technical and technological characteristics of the Stanari Power Plant block were designed based on the results of the analysis of coal samples from the Raskovac opencast mine. There haven't been any notable variations in the coal quality during the Stanari thermal power plant's present operating term from the project's stated values. For the expected lifetime of mine, i.e., the thermal power plant operation until 2050, it is necessary to unearth the total remaining available coal reserves from the Stanari basin. The thermal power plant's operation may be significantly impacted by variations in coal quality at the Ostruznja opencast mine. The use of coal with quality parameters lower than designed can cause problems in the operation of the boiler, stoppages in operation, and an increased volume of maintenance and overhaul of the boiler. Coal quality control in the process of sampling, mining, transportation, and depositing of coal must be at a level that enables smooth operation of the thermal power plant. This paper presents test results during the operation of the thermal power plant burning coal of different quality, (LCV in range from 9,100 to ?7,500 kJ/kg), a risk assessment of long-term planning, and the effectiveness and economy of the thermal power plant operation. The potential economic effects and reliability of the operation of thermal power plants due to the increased consumption of coal, limestone and self-consumption in operating modes with significantly lower coal quality than designed were analyzed in particular.

Experimental study of cryogenic treatment of Karaganda coal samples

Karaganda Coal Basin bears the largest undeveloped reserve of coalbed methane (CBM) in Kazakhstan, which lacks water resources for implementing large-volume hydraulic fracturing. Cryogenic fracturing utilizing liquid nitrogen (LN2) has been trialled in fields and is a waterless fracturing technique under intensive research these days. This study aimed to evaluate the cryogenic treatment efficacy of Karaganda coal samples as well as to understand the coal permeability evolution during the thawing period. X-ray fluorescent spectrometry (XRF) and microscope imaging identified the compositional and structural heterogeneities of coal specimens mined from different interlayers. Acoustic emission test, permeability measurement, and microscope imaging comparatively characterized the dry coal structure alteration before and after immersion into LN2. Cryogenic treatment slowed down the S-wave velocity through coal specimens, enhanced permeability by over 65 % after temperature recovery as well as created new fractures, enlarged existing ones, and spalled coal particles. Dynamic permeability evolution against temperature rise during the thawing process has been successfully captured for the first time. Overall, the experimental measurements support that the LN2 cryogenic fracturing technique would be effective in stimulating coalbeds for CBM production in Karaganda Coal Basin.

MULTIFRACTAL CHARACTERIZATION OF THE INHOMOGENEOUS STRAIN EVOLUTION OF THE DEHYDRATED COAL: INSIGHT FROM COAL MICROSTRUCTURE

Underground coal mining in China has gradually moved into deeper seams in recent years, which results in a higher ambient temperature in the mining space and significantly affects the mechanical behavior of coal. In this study, dehydrated coal samples were obtained at different temperatures ranging from 30[Formula: see text] to 70[Formula: see text], and the mechanical behavior of the dehydrated coal was investigated through compressive loading tests. The digital image correlation (DIC) method was used to acquire the strain field of coal, and a multifractal analysis was conducted to characterize the strain evolution of coal. The findings suggest that the increasing temperatures result in higher moisture desorption rates and greater volumetric contraction strain in coal. Furthermore, coal with higher moisture desorption exhibits higher peak stress and peak strains when subjected to compressive loading. The multifractal analysis of the inhomogeneous strain evolution indicates a gradual decrease in the parameter [Formula: see text] under compressive loading, followed by a sudden increase before reaching the failure point due to strain localization. The multifractal mechanism was further investigated, revealing that the inhomogeneous strain field of coal is inherently affected by the microstructure of coal. In addition, a mathematical model was proposed to elucidate the relationship between the inhomogeneous coal strain and the microstructure of coal. The result indicates that the inhomogeneity of the coal strain is directly associated with the multifractal singularity of the coal microstructure. Finally, the feasibility of using the multifractal parameter [Formula: see text] to identify coal strain localization has been demonstrated, indicating its potential value in aiding engineers to determine the SLZ in deep coal mines.

Maturity Parameter of Carbonaceous and Coaly Matter Revealed through Raman Spectroscopy

Abstract This article presents the results of ten coal samples of sub- bituminous rank collected from working mines in Chhattisgarh, India, and subjected to proximate and ultimate analysis, organic petrology and Raman spectroscopic study to reveal their maturity parameters and structural characteristics of carbonaceous materials. Vitrinite reflectance measurement was also performed to understand the maturity parameter, with values ranging from 0.35% to 0.57%. The research is unique as it combines petrological, geochemical and spectroscopic technique on coals of Hasdeo basin. Conventional parametei; volatile matter ranges from 21.83 and 33.68% suggesting low maturity. The ash yield values range from 12.04 and 26.61%. Petrographically, the coals are vitrinite rich ranging from 53.95 to 63.47% while the inertinite group range from 17.9 to 29.2%. The liptinite concentration the samples remain below 10%. The maceral constitution of the coals indicates carbonaceous materials undergoing early diagenetic phase. The Raman results showed the presence of the D and G bands, two prominent spectral signatures, which are typical of natural materials rich in carbon. The Raman shift recorded for the D band ranges from 1348 to 1385 cm-1 and for the G band, the Raman shift was recorded between 1607 to 1616 cm-1. The ratio of the various parameters derived from the two bands aid to understand the material response to the changing physical and chemical variables in the geological setup. The intensity ratio of the ID/IG band ranged from 0.15 to 0.79 (average 0.50), while the (G-D1) parameter ranges from 225 to 260 indicating heterogeneity in the samples. However; the ratio of AD1/AG did not show any regularity, with values ranging from 0.07 to 1.85. The investigation elucidates that by integrating Raman spectroscopy analysis with vitrinite reflectance (VRo), we can discern distinctive structural attributes characteristic of immature kerogen. Furthermore, the petrological, proximate, and ultimate data collectively corroborate the carbonaceous, coaly material in its early stages of coalification.

Performance Comparison of Waste Cooking Oil on Coal Slime Flotation with Sunflower Oil and Gas Oil

This study explores the potential use of waste cooking sunflower oil (WSO) as an eco-friendly collector for coal slime flotation. WSO and coal slime are both wastes and are hazardous to human health and the environment, if not disposed of properly. In this study, co-disposal of the two wastes was investigated; a gas oil (petroleum derived oil) and crude sunflower oil (CSO) were used for collector efficiency comparisons. This study also presents a green, low-cost and environmentally friendly alternative. Kinetic flotation tests were carried out to study the flotation selectivity, flammability and combustible recovery. Contact angle measurements were performed with 3 different oils (CSO, WSO and gas oil) by sessile drop technique to determine the hydrophobicity and surface properties of coal. Fourier-transform infrared (FTIR) spectroscopy was utilized to analyze the chemical composition of both waste oil and coal samples. 
 Keywords: Sunflower oil, waste cooking oil, gas oil, coal slime, flotation

Coal facies of the Middle Permian Baruunnaran deposit, South Mongolia

The Baruunnaran coal deposit is located in the northeastern part of the South Gobi Basin, southern Mongolia, and hosted in the middle Permian Tavantolgoi Formation. In this paper we present indices of coal facies determined from 34 coal samples obtained from three seams in the lower and upper part of the formation (III, IXG and X), by studying their organic petrography and the geochemistry of the coal ash. The results of coal petrography revealed that seams III, IXG and X are composed of 46.9-80.9 vol.% vitrinite, 11.6-47.5 vol.% inertinite and 1.2-18.2 vol.% liptinite. In samples from seams III and X the average content of mineral matter is low at 11 and 13.4 vol.%, respectively, and 6.3 vol.% in seam IXG, and consists of clay, silica, pyrite, carbonate, and other minerals. The inorganic content mostly occurs as fillings of cell cavities, cracks, and fissures of vitrinite and inertinite macerals. The vitrinite random reflectance values range from 0.81-1.07%. The Gelification Index and Tissue Preservation Index suggest the peats accumulated in wet forest swamp environments with high tree density. The majority of the seams accumulated in mildly oxic to anoxic conditions with good tissue preservation. The peat mire water ranged from weakly to strongly acidic. Further, it was determined by Al2O3/TiO2 ratios that the clastic sediments were probably sourced from volcanic basement characterized by intermediate felsic composition.

Fractal character of coal nanopore and effect of deviation corrected, coal rank, and gas adsorption

Pore structure in coal is fundamental to determining the flowability properties and mechanical functions. Various insights into pore structure, including pore size distribution, specific area, and pore volume, have been obtained in experiments with many methods. Systematic research is lacking on how to characterize pore structure with more detailed information using fractal theory. Here, using the small-angle X-ray scattering (SAXS) method, we studied the fractal characters of four coal samples and showed how the deviation corrected, coal rank, and gas adsorption affected the fractal dimension of the pore structure. The average pore diameters of four samples obtained by the gyration radius using Guinier curves were 50.25–52.95 nm. The surface fractal of coal samples was the dominant feature, and the fractal features altered with the q value. Scattering information of simple bodies, such as pores and fractures, could superimpose the non-integer nature of the fractal dimension. The Porod and Debye laws guided the deviation correction process in the high q region. Pore fractals might be changed into surface fractals through the fractal dimension after the deviation correction, and the SAXS data obtained by Debye law correction maintained the originality and validity better than Porod law correction. The irregular shape of M was observed on the curve of fractal dimension as the maximum reflectance of vitrinite increases, resulting in three stages. More adsorption sites per unit projection area could be found in a larger fractal dimension of the pore surface, which results in an increase in Langmuir adsorption volume. The increase in the adsorption pressure reduced the fractal dimension. Our findings enhance the experimental support for comprehending the structure of coal nanopores.

Modes of occurrence of gallium in Al-Ga-rich coals in the Jungar Coalfield, Ordos Basin, China: Insights from LA-ICP-MS data

Owing to the concern about supply of Ga, it has been attracted much attention for the modes of occurrence of Ga in Al-Ga-rich coal-hosted deposits. However, the modes of occurrence of Ga in coal were mainly investigated using indirect methods, which may cause uncertainties. In this study, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) spot and mapping analysis were used to directly characterize the concentration of Ga within the host minerals including kaolinite and boehmite, and to investigate the distribution of Ga in organic matter in the Carboniferous Al-Ga-rich coal from the Haerwusu Mine in the Jungar Coalfield of the Ordos Basin, North China. Microscope observation and scanning electron microscope - energy dispersive spectrometry (SEM-EDS) analysis were performed, along with petrographical and mineralogical investigations. Eight types of kaolinite and three types of boehmite particles from coal, erosional-sourced mudstone, and tonstein were identified, classified, and selected for the determination of in-situ elemental compositions.The results showed that the concentration of Ga in the selected minerals varies, with the following order from highest to lowest: colloidal boehmite (90.38 ppm), low-Al (Al2O3/SiO2) clastic kaolinite (84.67 ppm), high-Al clastic kaolinite (55.49 ppm), Na-bearing kaolinite (49.94 ppm), fine-grained boehmite (47.13 ppm), high-Al cryptocrystalline kaolinite (40.25 ppm), low-Al cryptocrystalline kaolinite (38.78 ppm), and low-Al vermicular kaolinite (29.67 ppm), high-Al vermicular kaolinite (28.37 ppm), K-bearing kaolinite (19.44 ppm), and clastic boehmite and cell-filling kaolinite (below detection limit). LA-ICP-MS mapping demonstrated that a certain proportion of Ga could be associated with organic matter, concentrating on vitrinite. Notably, the concentration of Ga in clastic kaolinite and colloidal boehmite was found to be substantially higher compared to the whole-rock coal and erosional-sourced mudstone samples, indicative of their dominant host minerals for Ga. Conversely, the concentration of Ga in K-bearing or Na-bearing kaolinite which had experienced transformation from primary kaolinite is much lower than that in clastic kaolinite, while the concentration of Ga in fine-grained boehmite, which had altered from kaolinite, surpassed that of its corresponding kaolinite layer. These findings suggest that gallium could have undergone migration during the evolution or alteration of clay minerals.

Quantitative characterization and analysis of pore-fractures in tar-rich coal under high-temperature pyrolysis based on micro-CT imaging

This study investigates pore distribution and permeability behavior of tar-rich coal following high-temperature pyrolysis at 500°C using X-ray computed tomography (CT) scanning. Coal samples post-pyrolysis were CT scanned, generating 1755 cross-sectional slices for three-dimensional reconstruction. An axial algorithm extracted pore distribution features, and geometric parameters were computed. An Equivalent Pore Network Model analyzed permeability characteristics. The results show that Post-pyrolysis pore distribution in tar-rich coal exhibited nonuniformity with a significant range in pore size distribution. Pores displayed concentrated spatial patterns. Total porosity was 14.24%, with 12.34% being connected. Most pores in Representative Elementary Volume (REV) regions fell within 10–50 μm in width and 20–60 μm in length, constituting over 40% of the total. Pore surface area peaked between 200–100 μm2, also comprising over 40% of the total. The Pore Network Model showed distinct characteristics in two REV regions: REV-1 demonstrated an early stage of development with poor connectivity, while REV-2 displayed a well-developed network with a bimodal coordination number histogram. The study highlights nonuniform post-pyrolysis pore distribution and significant pore size variations in tar-rich coal. This study is crucial for understanding permeability behavior in tar-rich coal after high-temperature pyrolysis.