Coal Pore Research Articles

Evaluating coal pore structure and gas sorption-diffusion behavior alteration induced by ultrasound stimulation using sorbing tests and matrix diffusion modeling

Coalbed methane development has great significance to ensure the safe and efficient coal mining, clean energy supplementation and carbon emission reduction. Ultrasound stimulation can effectively modify the coal pore/fracture structure and change the gas ad/de-sorption process and diffusion kinetics characteristics, thus altering gas production for coal deformations. This study estimated the pore structure damage and gas sorption-diffusion of coal before and after ultrasound stimulation with gas sorption tests and gas sorption-diffusion modeling. The results show that the maximum nitrogen adsorption capacity increases by 181.2%–218.9% in coal after ultrasound stimulation, and the pore volume and specific surface area of the ultrasound stimulated coals are 1.5–2.0 times and 5.6–7.6 times of those of the raw coals, respectively. Ultrasound stimulation (20 kHz) has a significant modification effect on pores with size of 2∼50 nm. Fractal analysis of pore structure indicates that the pore surface roughness reduces after the ultrasound treatment, and the development of coal pores has become more complex, and the pore heterogeneity has increased with ultrasound stimulation. The sorption tests and gas sorption-diffusion modeling indicate that gas sorption and diffusive transportation increases after the ultrasound treatment, especially in the initial period. In the initial pressure boosting stage (0∼0.5 MPa), the cumulative methane adsorption amount increases by 20.94%, the diffusion coefficient K increases by 17.32%, and the gas adsorption constant b increases by 62.66%, which is attributed to the “pore-forming” and “pore-expanding” effect caused by ultrasound vibration and cavitation. Ultrasonic stimulation technology has a special improvement performance on coal seams with poor matrix gas deliverability, which is beneficial for the life cycle production enhancement of coalbed methane wells.

The multi-scale pore structure of superfine pulverized coal. Part 2. Mesopore and closed pore morphology

Superfine pulverized coal exhibits extraordinary properties in thermal conversion processes like pyrolysis and combustion. The exploration of closed pores is of great significance for studying of the macromolecular structure, and developing advanced clean combustion technologies. In this work, the influences of coal rank and particle size on the mesopore characteristics (i.e., pore size, volume, and specific surface area), especially closed pores, were obtained by combining nitrogen adsorption and synchrotron-based small-angle X-ray radiation (SAXS). In addition, combined with the closed pore volume measured by He density and SAXS, research focuses on the formation and opening mechanisms of closed pores in different rank coals at a molecular level. The results indicate that both the bituminous (NMG) and anthracite coal (HN) have a few large open pores (>100 nm), with large amount of small mesopores and micropores (<2 nm). The HN coal has more open mesopores which have a smaller pore size than NMG coal. The closed pore proportion of HN coal in the studied pore size range is between 2 and 52 %, while that of NMG coal is between 47 and 67 %. In addition, both coals have a certain amount of closed pores formed by shear force and ash blockage, while the matrix compression also contribute to certain types of closed pores in NMG. These findings contribute to illustrating the closed pore structure in coal at the molecular level, which sheds light on constructing comprehensive macromolecular pore models of coal and thus promoting the development of clean coal utilization technologies.

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 &gt; 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.

Simulation of the activation process for an activated carbon production plant

Charcoal activation is the process by which the pores of the charcoal open. As a result, its absorption capacity is greatly increased. Opening of pores in coal occurs upon contact with a coolant having a high temperature of 800-900 °C. The temperature regimes of the activation process are described by differential heat transfer equations. Mass transfer during activation is described by differential mass transfer equations for coal and the resulting activation gases. Modeling the activation process and designing equipment for coal activation is a complex process that requires many factors to be taken into account. For an energy- and resource-saving continuously operating plant for the production of activated carbon, a zone of coal activation by superheated steam obtained in the process of cooling coal and flue gases has been developed. Water to create steam is separated as a result of mass transfer processes occurring in the activated carbon production unit. An engineering method for calculating a shaft-type activation chamber used for dispersed particles has been developed. A mathematical model of the activation process is presented, which determines the duration of the coal activation process. Quantitative characteristics of superheated steam are calculated from the initial productivity of coal. Relationships are given for calculating the geometric dimensions of the activation chamber in an activated carbon production unit.

Pore Evolution Law and Gas Migration Characteristics of Acidified Anthracite in Liquid CO2‐ECBM: An Experimental Study

The high gas pressure, low permeability, and strong gas absorption of coal seams in China complicate gas extraction, severely restricting the efficient development of coalbed gas. Liquid CO2 (LCO2) has a dual effect of cracking and enhancing the permeability of coal rock, thus, enhancing gas recovery. In this study, experimental testing and comparative analyses were performed to analyze the LCO2 acidification of antireflection coal, for which reference and variable experimental groups were designed. The acidification effect was quantitatively analyzed by examining changes in the pH value of the reaction solution, mineral content of coal, and pore structure during the experimental process. The experimental results indicated that a higher pressure resulted in a greater amount of CO2 being dissolved and a stronger acidity of the reaction water sample. As the reaction time increased, the minerals in the coal gradually dissolved and more H+ ions in the solution were consumed. The calcite (carbonate) and illite (clay mineral) contents significantly decreased, which is the main reason for the change of coal pore structure. The minerals on the pore surface of coal react with (CO2–H2O) weak acid, which increases the development of pore branches, improves the complexity of coal pores, and roughens the pore surface. Acidification significantly increases the number of micropores (&lt;10 nm) and small pores (10–100 nm) in coal; furthermore, pore development is simple, the surface is smooth, and the pore connectivity is improved, which makes this part of pores have fractal characteristics. CO2–H2O–coal acidification increases the pore volume corresponding to the CO2/CH4 gas slippage flow in coal and strengthens the mass and energy transfer of CO2/CH4 in coal. CO2 can enter more pores than CH4 and displace the adsorbed and free CH4 in the pores.

Влияние активированного угля на эффективность сорбционного выщелачивания золотосодержащих руд (на примере обогатительной фабрики ГОКа «Кубака»)

Activated granular coal is widely used in processing plants, both in Russia and abroad. There are two problems at Kubaka mining and processing plant (MPP) during hydrometallurgical processing of gold-containing raw materials: loss of coal during sorption due to partial abrasion, decrease in sorption properties and, as a result, deterioration of coal quality as a result of clogging of pores with dissolved compounds in the pulp. This in turn leads to a decrease in the efficiency of sorption leaching of gold-bearing ores and an increase in the cost of gold. The aim of the study is to improve the quality of activated carbon of the GOLGCARB 207C 6X12 brand based on the use of reduction processes in order to increase the sorption capacity. Research objectives are as follows: development of a research methodology; determination of parameters of changes in the kinetic properties of activated carbon; step-by-step assessment of changes in the properties of activated carbon in the technological process; identification of the effect of coal treatment with alternative reagents (NaOH, hydrochloric acid, sulfuric acid, lime) on improving the quality of activated carbon. The object of the study is activated carbons of the GOLGCARB 207C 6X12 brand used at the Kubaka MPP (Magadan region). Research methodology and methods are as follows: a comprehensive methodology for the study of activated carbon, laboratory studies and pilot tests. The effect of activated carbon on the efficiency of sorption leaching of gold-bearing ores in laboratory conditions with the approbation of a new technological solution in semi-industrial tests has been studied. The effect of acid treatment and reactivation of coal on its sorption properties has been experimentally proven. Optimal conditions for cleaning the pores of coal from impurities of organic, carbonate and sulfur-containing compounds and an increase in sorption activity were achieved by acid treatment (replacement of hydrochloric acid with sulfuric acid) and pH control.

Molecular simulation study of microstructural evolution during low-temperature oxidation of coal

In this study, industrial analysis, 13C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), and in-situ infrared spectroscopy were employed to determine the elemental composition and structure of Tingnan coal. The peak area, pore volume, and specific surface area were used as the main parameters to investigate the structural changes of coal molecules and pore evolution during low-temperature coal oxidation, using an automated surface area and pore size analyzer. The analysis revealed that with increasing oxidation degree, the CH groups in the aromatic structure gradually decreased, while the total content of oxygen-containing functional groups such as alcohols, phenols, carboxyls, and carbonyls remained relatively constant. However, the relative content of hydroxyl groups and ether linkages increased with increasing oxidation temperature. Additionally, coals with different oxidation degrees exhibited well-developed micropores, and the specific surface area gradually decreased while the pore size increased with temperature. Molecular models of Tingnan coal in its original state and oxidized at temperatures of 50 °C, 70 °C, and 110 °C were constructed using Materials Studio software. Based on these models, pores with diameters of 2.79 nm, 3.07 nm, 3.07 nm, and 3.11 nm and a length of 4.5 nm were constructed using the coal molecular structure as the pore walls.

Spatial evolution of pore and fracture structures in coal under unloading confining pressure: A stratified nuclear magnetic resonance approach

The inherent spatial complexity and strong heterogeneity characterise the pore and fracture structures (PFS) in coal. Understanding the spatial evolution of PFS in coal under unloading confining pressure is crucial for ensuring the safety of coal mining operations. In this study, the stratified nuclear magnetic resonance (NMR) technique and a triaxial mechanical loading system were combined to realise real-time observation of the spatial evolution of PFS in coal during the unloading confining pressure process. A conceptual model depicting the spatial evolution of PFS in coal under unloading confining pressure was formulated. Based on our experimental findings, the mesoscopic mechanical behaviour of coal subjected to unloading confining pressure was simulated using PFC 2D. Our research reveals that throughout the unloading confining pressure process, the PFS within coal samples simultaneously exhibits characteristics of both compacted and fractured states, with mutual transformation occurring in the adsorption and seepage pores. A reduction in confining pressure results in a notable escalation of observed damage within the coal samples and intensified development of PFS. Furthermore, our numerical simulation results closely align with NMR test results, providing additional validation to our findings.

Effect of Resveratrol on physicochemical structure evolution of lignite during spontaneous combustion

Studying the evolution of the physicochemical structure of coal during coal spontaneous combustion (CSC) by antioxidants is of great significance for revealing the mechanism of CSC and preventing it. The effects of three concentrations of Resveratrol (RES) on the physicochemical structure evolution of coal during the low-temperature oxidation (L-TO) process of CSC were comprehensively analyzed through low-field nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) experiments. The results showed that the pore cracks in the coal develop more fully with the increase in oxidation temperature. Besides, there was an increase in porosity and permeability, and a full development of pore throat. Based on the T1-T2 2D fluid identification results, it was found that as the oxidation temperature increased, the adsorption capacity and permeability of the coal were stronger. RES can effectively inhibit the CSC process of coal, with a maximum reduction of 23 % in porosity and a maximum reduction of 82 % in permeability. The hydroxyl groups and CH3/CH2 of coal samples treated with RES decreased by a maximum of 42 % and 43 %, respectively. The oxygen-containing functional groups increased by a maximum of 77 %. RES can not only cover on the coal surface to prevent oxygen from seeping into the coal, but also infiltrate into the pores of coal to inhibit the combination of the active functional groups and oxygen molecules.

Evolution law study of pore and fracture of coal gangue cemented filling body under axial compression using PFC2D

Filling body is an important structure to support the safety of coal mine goaf. The cemented gangue backfill (CGB) is a multiphase heterogeneous brittle material composed of coal gangue, hydration reactants, pores and microcracks. According to Micromechanics studies, the accumulation and development of meso-structural damage in heterogeneous brittle materials is the direct cause of macroscopic failure. In this paper, particle flow numerical software PFC2D is used to simulate the meso-structure evolution of CGB. Considering the particle size of filling material, the coal gangue particles are drawn according to the real shape by vector software, then import it into PFC2D by FISH language and constructed as Clump unit. The relationship between micro-characteristics such as particle force chain distribution, cracks and pores and mechanical properties of CGB is analyzed. The results show that the stress-strain and crushing mode of the numerical simulation model have a good correspondence with the laboratory test results. The internal force chain distribution and fracture-pore evolution law of the model particles are analyzed. It is found that most of the force chains are concentrated near the larger particle units, forming the 'skeleton force chain' phenomenon. The initial cracks are mainly produced around the coal gangue particle unit. With the increase of axial pressure, the cracks grow obliquely from 30° to 45°, and finally macroscopic cracks are generated. Due to the hydration product generated, the internal porosity is gradual decrease. Other than that, during the uniaxial compression process，the porosity of the same curing age decreases first and then increases, and the pores are sudden increase after the complete crushing stage.

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.

Microscopic investigation of the effect of uniaxial stress on the structure of pore-fissure system and methane adsorption in lean coal

In order to study the mechanism of interaction between stress, pore-fissure system, and methane adsorption. Adopting a novel combination of experimental and simulation methods. Through uniaxial fracture experiments and X-ray micro-CT scanning experiments, it was found that the pore-fissure system of coal blocks first closed and then sharply increased with the increase of uniaxial stress. Based on relevant experiments, unit cell models under different uniaxial stresses were constructed. Through simulation calculations using Zeo++-0.3 code and Materials Studio, it was found that the pore system of unit cells first decreases and then increases with the increase of uniaxial stress, and the maximum adsorption capacity of methane first decreases and then increases. The experimental and simulation results for the evolution characteristics of the coal pore system under different uniaxial stresses were consistent. Stress affects methane adsorption in coal by altering the pore-fissure system characteristics. Stress has a significant impact on the macroscopic and molecular microscopic systems of coal pores. In front of the mining and excavation working face, the gas pressure and content increase, the coal body is damaged, and the coal body stress increases. This also explains that this area is prone to coal and gas outburst.

Exploring the factors influencing the low temperature CO gas production characteristics of low-metamorphic coal

CO as an important indicator gas for predicting early spontaneous combustion of coal mainly comes from the oxidation of residual coal in goaf. Low-metamorphic coal has a stronger tendency for spontaneous combustion and is more prone to oxidation to produce CO gas at low temperatures. For example, there are no signs of natural combustion in certain goaf areas of mines, but the CO concentration in the working face continues to exceed the limit, which misleads early natural combustion prediction and warning work. Therefore, taking low metamorphic coal as the main research object, the CO gas production characteristics of different coal samples at low temperatures were determined through temperature-programmed experiments. It was found that the CO gas production rate and concentration of CYM at low temperatures were much higher than those of the other two metamorphic coal samples (approximately 2 times RNM and 3–10 times QM); Determination of content and proportion changes of hydroxyl, aliphatic, Aromatic hydrocarbon and oxygen-containing functional groups in different coal samples before and after heating by Fourier transform infrared spectroscopy. Based on the analysis of CO gas production characteristics of different coal samples, it was found that the intermolecular hydrogen bonds in the hydroxyl groups of CYM and the C-O bonds in the oxygen-containing functional groups decreased significantly. The strongest correlation between intermolecular hydrogen bonds in hydroxyl groups and C-O bonds in oxygen-containing functional groups and the low-temperature CO gas production characteristics of coal is demonstrated; Finally, scanning electron microscopy was used to study the differences in apparent characteristics of different coal samples before and after heating, to determine more complex apparent structures that increase oxygen adsorption sites and promote low-temperature oxidation and CO production of coal during the low-temperature stage. Further guidance will be given to the research on the suppression of intermolecular hydrogen bonds and C-O bonds, as well as the sealing of coal micro pores and crack structures, to improve the targeted application and fire prevention efficiency of fire prevention materials.

Controlled mechanism of dynamic behavior of water-based ultrasonic cavitation in coal pore microstructure

Coal pores are sealed as a result of water lock effect, which in turn inhibits coalbed methane (CBM) from desorbing and diffusing. It seriously restricts the efficient development of CBM in the complex geological environment. Relieving pore water lock (RPWL) in coal reservoirs is a scientific problem that puzzles CBM development. Currently, the widely-interested technology of enhancing coalbed methane recovery (ECBM) can not effectively break through the pore water lock. Ultrasonic cavitation can generate shaped charge micro-jets, high temperature, and high pressure, which may be a potential strategy to realize RPWL in coal reservoirs. It has been maturely applied in numerous fields such as medicine, chemical industry, and manufacturing industry. Therefore, water-based ultrasonic cavitation-enhanced coalbed methane recovery (WUC-ECBM) was proposed in this study. This technology has enormous application potential for the efficient and green development of CBM. However, as the theoretical key to the successful application of WUC-ECBM, its RPWL mechanism is still blank. In this paper, X-ray nondestructive testing and fluid invasion methods were used to reconstruct the digital model of coal pore microstructure. Through ultrasonic cavitation numerical simulation, the dynamic behavior mechanism of water-based ultrasonic cavitation was studied in coal pore systems, revealing the microscopic RPWL mechanism of WUC-ECBM. Results show that low metamorphic coal has a larger and fuller pore system, which reflects a stronger topological advantage. Small pores and throats are the main locations of ultrasonic cavitation. This spatial feature is consistent with pore water locking phenomenon. During the formation of a cavitation bubble, ultrasonic energy instantly weakens the surface tension of the water film and activates RPWL. When the cavitation bubble collapses, the micro-jets, high temperature, and high pressure wreak mechanical damage and thermal shock to pore water, accelerating the RPWL process. Ultrasonic frequency affects the cavitation threshold. Increasing ultrasonic power will significantly boost cavitation intensity in pores, particularly in high metamorphic coal. The research results contribute to the green, efficient and intelligent development of CBM in the harsh geological environment, and provide theoretical guidance for WUC-ECBM.

Effect of Water on Effective Pore Structures for Medium-Rank Coal: Based on the Prefreezing Nitrogen Adsorption-Desorption Experiment.

Water is ubiquitous in coal reservoirs, and its distribution can have a remarkable influence on the effective pore space of methane. This study conducted the combination experiments of moisture equilibrium and prefreezing nitrogen adsorption-desorption to explore the adsorption behavior of water in coal pores and thus to reveal the distribution characteristics of water in pores with different scales as well as the influence of water on pore structures. The results showed that the adsorption mechanism of water vapor undergoes a transition from monolayer to multilayer to condensation with the increase in relative humidity (RH). The occurrence characteristics of adsorbed water in coal pores are controlled by the RH and pore size. When the RH is increased from 0 to 98%, the nitrogen adsorption capacity, specific surface area, and effective pore volume of the samples were all decreased significantly due to the different adsorption modes of water, which is more significant in pores with d < 10 nm. Additionally, the relative pressure corresponding to the branching position of the nitrogen adsorption-desorption curve will be changed with the increase in moisture content. Based on this, it is calculated that the adsorbed water will change the smoothness of the pore wall and the complexity of the pore structure.

Pore and permeability changes in coal induced by true triaxial supercritical carbon dioxide fracturing based on low-field nuclear magnetic resonance

Supercritical carbon dioxide (ScCO2) fracturing is a green, clean, waterless extraction technique that has gained widespread attention. Coal pore properties, such as porosity, pore size distribution, connectivity, and permeability, are critical for fracturing and efficient coalbed methane production. This study examines the effects of ScCO2 fracturing on coal reservoir pore modification by conducting true triaxial ScCO2 fracturing experiments on high-rank coal samples under various stresses and injection rates. Low field nuclear magnetic resonance technique was used to compare and analyse the pore permeability characteristics of the samples before and after ScCO2 fracturing. The research discusses the influence of stress and ScCO2 fracturing fluid injection rate on coal pore modification and its controlling mechanisms. The results show a significant impact on coal pore modification, with a 59.85 % increase in porosity, 60 % increase in pore volume, 56 % increase in pore throat volume, and 47.5 % increase in permeability. Under the same injection rate and fixed temperature (40 °C), higher stress differences (8 MPa) benefit large pore modification and connectivity, while lower stress differences (4 MPa) are more favourable for micropore and transition pore modification and connectivity. These findings contribute to a deeper understanding of the microscopic mechanisms of ScCO2 fracturing in modifying coal pores.

Characteristics and Influence Factors of Natural Desorption in Coal Bodies from Fukang Mining Area, Xinjiang, China.

Coal body desorption characteristics are one of the key factors that influence the development of coalbed methane (CBM). In this study, 91 coal core samples from 11 CBM wells in the Fukang mining area were collected from Xinjiang, China, and the coal quality, high-pressure mercury compression, gas content, and natural desorption characteristics measurements were launched. With the detailed analyses of the differences in cumulative desorption volume, desorption ratio, and on-site average desorption rate for the coal samples with different body structures and macrolithotypes, the influence of the maximum reflectance of vitrinite, microscopic coal rock composition, and coal quality and pore characteristics on CBM desorption characteristics were discussed. The results showed that the cumulative desorption volume, desorption ratio, and desorption rate of cataclastic structure-bright coal are higher than those of primary structure-semibright coal. With the increase of RO,max and vitrinite content, the adsorption capacity of coal increases, and the increased methane concentration difference during desorption leads to an increase in cumulative desorption volume and on-site average desorption rate. The higher contents of moisture and ash yield would occupy the adsorption sites and hinder gas diffusion, which would decrease the desorption of coalbed methane. The greater porosity/pore volume ratio of medium and large pores can enhance the connectivity of pores, which increases the desorption ratio and the average desorption rate, while the higher micropore porosity/pore volume ratio can increase the gas adsorption space and the cumulative desorption volume. The pore characteristics have the most significant effect on the cumulative desorption volume and desorption ratio. The results of the study can help guide coal mine gas management and CBM development from middle-and low-rank coal reservoirs in Xinjiang.

Accurate characterization method of pores and various minerals in coal based on CT scanning

Pores, fractures, and mineral compositions in coal have a significant impact on the mechanical properties, seepage characteristics, and water–rock interaction of coal. This paper proposes a method to determine the threshold value of various mineral components and pores inside the coal sample through computed tomography (CT) scanning, X-ray diffractometer (XRD) test and nuclear magnetic resonance (NMR) technology. This method determines the porosity of the coal by NMR, and then determines the pore reconstruction threshold. The type and composition of coal sample minerals were determined by XRD. Combined with the difference in the density of each mineral, the classification threshold of each type of mineral can be determined. We first determined the representative elementary volume through pore and mineral reconstruction of coal with different sizes. It was found that when the edge length of the selected cube is greater than 140 pixels (5 mm) and less than 400 pixels (13.6 mm), the pore threshold is stable. While for the mineral reconstruction, the range of threshold stability is 140 pixels (5 mm) to 580 pixels (20.7 mm). On this basis, we selected three cubes with a side length of 300 pixels and different positions for pore and mineral reconstruction. The reconstruction results show that the coal sample measurement area has pore and mineral composition thresholds of 19804 and 24533, respectively. The pores in coal samples can be divided into connected pores and isolated pores. The equivalent diameter and shape factor of a connected pores are much larger than any isolated pore. The shape of the connected pores is close to a circle or a square. The shapes of isolated pores are close to needles. The threshold values of amorphous, feldspar, quartz, illite and chlorite are 24533, 25740, 25927, 26826 and 28748, respectively. The three-dimensional characterization results show that illite and chlorite are distributed in flakes, and the rest of the minerals are distributed in granular form. The extraction of different mineral components is helpful for the research on the evolution of various minerals before and after water immersion.

Study on the inhibitory effect of DL‐malic acid on the phase structure and combustion characteristics of coal

AbstractDL‐malic acid (DMA), as a recognized antioxidant inhibitor and chelating agent in the food and chemical industries, has good application prospects in coal fire prevention and control. This paper investigates the impact of a DMA solution on the surface properties and phase structure of coal through a range of analytical techniques, including electron microscope scanning, particle size analysis, and x‐ray diffraction. The thermal behavior characteristics of DMA‐treated coal were analyzed using thermogravimetric experiments. The results show that after adding malic acid, the particle size of coal sample is reduced, the sphericity and roundness are reduced, and the pore size is increased. The hydroxyapatite and calcite components in coal were dissolved and consumed, and the calcite components were gradually depleted with the increase of DMA concentration. The flocculent structure produced by the condensation of the crushing residue and broken small particles are attached to the internal pores of coal, which reduced the coal oxygen contact sites and increases the content of the sealed solution, resulting in the increase of the thermal characteristic temperature of the coal and the inhibition of the oxidation heat generation process. DMA has obvious inhibitory effect on fresh coal and oxidized coal, and the inhibitory effect is stronger on high‐temperature oxidized coal. Therefore, DMA can be effectively applied to the prevention and control of spontaneous combustion of residual coal in goaf. The research results provide a reference for the research of coal fire prevention technology and the preparation of environmentally friendly flame retardant materials.

Investigation on coal floatability and pore characteristics using acidification method

This paper investigated the relationship between coal floatability and pore characteristics from the perspective of froth flotation. Three types of coal samples with different ash contents were pretreated with HCl and acetic acid solutions to alter the pore features. The X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) measurements were conducted to specify the reactions that happened in acidification. The batch flotation and contact angle measurements were conducted to evaluate the floatability of the coal samples. The liquid nitrogen adsorption and mercury intrusion porosimetry (MIP) measurements were applied to measure the pore characteristics of the coal. The acidification treatment partially dissolved mineral components and organic functional groups on the coal surface but did not change the types of the major functional groups. The sedimentation of liberated fine minerals on coal pores caused the decrease in pore volume. The BET specific surface area of the three coal samples after acidification was found to have consistent negative relations with the flotation recovery and combustible recovery rate. This indicated the adverse impact of porosity in coal flotation. The floatability of coal is mainly influenced by the pores at the size fraction of 1–20 μm by pore volume. The nano-scale size pore and fine mineral inside the micron pore increased the water-wettability and resulted in the low floatability of coal.