Coal Samples Research Articles

Failure precursors recognition method for loading coal and rock using the fracture texture features of infrared thermal images

Early warning of the catastrophic failure of rocks is a challenging rock mechanics problem. This article proposed a failure precursor recognition method based on infrared thermal image texture features for coal and rock. Based on spatiotemporal background noise correction for infrared thermal images, a new thermal image parameter of loading coal and rock, Contrast Texture Feature Values (CTFV), is proposed to extract subtle changes in the thermal image caused by crack evolution based on the Gray Level Co-occurrence Matrix (GLCM). The cumulative Crack Texture Thermal Image (CTTI) is reconstructed using CTFV, which can accurately reflect the spatial evolution process of loading coal and rock cracks. The CTFV remains at 0 in the early stage of loading and gradually increases with stress increase at the unstable crack propagation stage, which can serve as a reference for precursor warning of coal and rock failure. For shale, sandstone, and limestone, the precursor of CTFV at grayscale levels of 7, 8, and 9, can be classified into high failure risk, medium failure risk, and low failure risk, respectively. For coal samples, the CTFV is only applicable for the critical warning of high failure risk when the grayscale level is 7. Then, an adaptive identification method for failure precursors based on the sliding window probability density estimation method is proposed. The research results can enhance the reliability of IR monitoring technology for rock failure and instability early warning and can provide support for the prevention and control of rock engineering and geological disasters.

Characterisation of rare earth element-bearing mineral phases present in South African coal ash using Mineral Liberation analysis

Critical raw materials, including rare earth elements (REE), are the cornerstone of modern technologies. Due to their unique properties, REE are in high demand, leading to the exploration and research of alternative sources. Coal combustion by-products, specifically coal ash, has been identified as a potential alternative source of REE, but more research is required. This study aimed to identify REE’s occurring in power station and laboratory-derived coal ash. Results from Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) and Mineral Liberation Analysis are presented herein, to gain insight into rare-earth-bearing minerals occurring in the ash samples.REEs were determined in both sets of ash samples at values higher than 600 parts per million (ppm) using ICP-MS. Critical REE occur in ranges from 203 to 406 ppm; uncritical REE from 179 to 256 ppm; and excessive REE ranging from 197 to 348 ppm in these samples. Laboratory-derived coal ash and power station-generated ash samples were prepared as polished blocks and coated for examination using a Mineral Liberation Analyser (MLA). A bright phase search mode was employed to detect rare earth-bearing mineral phases using the back-scattered electron detector (BSE).Energy dispersive spectroscopy (EDS) was used to determine elemental compositions in these phases.Four rare earth-bearing mineral phases were determined, namely: monazite-REE, xenotime-REE, Ce-REErium rich, and REE-silicate. The laboratory-derived ash samples had a greater proportion of liberated rare earth-bearing minerals, related to the lower proportion of aluminosilicate glassy phase, related to the low temperature ashing procedure.The detailed mineralogy provides insight into the design of REE extraction processes, targeting specific REE’s hosted in the various minerals. This will be documented in a future paper.

Thermal imaging identification of coal and rock in northern China under thermal excitation——experiment and theoretical model

ABSTRACT The identification of the coal and rock is an important part in the process of coal preparation. Thermal imaging is an effective way to identify coal and rock. However, the current research lacks in-depth studies of the theory behind experimental phenomena. To address the issue, the radiant temperature observation experiments in different coal and rock samples from typical coal mining areas in northern China under thermal excitation were conducted in this paper. The results showed that there was a significant difference in the magnitude of radiant temperature change between coal and rock, the mean radiant temperature of coal was 3.69 K higher than that of rock when heated for 3 min under heat source radiation conditions of 310 W/m2. Second, a quantitative model of coal and rock radiant temperature change was established, and the experimental verification demonstrated that the correlation coefficient between the experimental results and the theoretical inversion results reached 0.988. Finally, combined with the index of temperature sensitivity, a theoretical model for thermal imaging identification of coal and rock was constructed, which lays a theoretical foundation for the application of the technology in coal preparation.

Prediction of HGI of South African coalfields: a comparative application of ANN, SVR and LSTM models

ABSTRACT Hardgrove grindability index (HGI) is a significant index used to determine a mill’s capability and overall efficiency in the grinding of coal. There are several factors that affect HGI due to the complex nature of coal. The influence of proximate analysis, calorific value, total sulfur, and pyrite content on the HGI values of 292 coal samples from South African coalfields were examined. Predictive models of HGI are developed by using soft computing techniques such as Long Short-Term Memory (LSTM), support vector regression (SVR), and Artificial Neural Network (ANN). This study shows that ANN is the most effective predictive model for all the three coalfields, with SVR models being second and LSTM models being the least effective. The correlation between the predicted value and the input data is established by using the cosine amplitude method (CAM). It was found that HGI is most influenced by the fixed carbon content, calorific value, ash content and volatile matter content while pyrite has the least influence.

Investigation into hydraulic fracture propagation behavior during temporary plugging and diverting fracturing in deep coal seam

In this study, we conducted indoor temporary plugging and diverting fracturing (TPDF) experiments on samples of natural deep and shallow coal seams using a small-scale true triaxial fracturing simulation system. By integrating high-precision CT scanning technology and crack reconstruction techniques, and relying on the quantitative characterization of the crack complexity coefficient, we investigated the differences in internal structures between deep and shallow coal seam samples and the variations in the expansion of TPDF cracks. Furthermore, the study primarily focused on exploring the influence of temporary plugging agent (TPA) parameters (quantity and particle size) on the expansion patterns of TPDF cracks in deep coal seam coal-rock samples. The experimental results reveal that in shallow coal samples, artificially induced fractures are notably longer and extend to various surfaces of the samples. Conversely, in deep coal samples, the expansion of artificially induced fractures is influenced by well-developed cleavage and cleats. The crack complexity coefficient of artificial fractures in deep coal samples is 1.54 higher than that in shallow coal samples, with an increased crack width of 98 μm. However, the expansion distance of the fractures is shorter and does not extend to the S3 and S6 surfaces. Increasing the dosage of the TPA is advantageous for inducing frequent redirection of fractures and communication with previously untouched areas. When the dosage of the TPA is increased from 30 g/L to 60 g/L, the expansion distance of artificial fractures significantly increases, extending to various surfaces of the samples. The crack complexity coefficient increases by 1.1, and the crack width enlarges by 84 μm. Appropriate particle sizes of the TPA can effectively form seals. However, small particle sizes (200/400 mesh) struggle to seal initial fractures, while large particle sizes (10/20 mesh) can lead to excessive wellhead blockage, causing rapid shut-in pressure, destructive fracturing in deep coal samples, and the generation of a large amount of coal fines. This study holds significant guidance for the optimal selection of process parameters in the temporary plugging hydraulic fracturing of deep coal rock.

Effect of hydrofluoric acid on Guizhou anthracite structure modification and response to enhanced hydrolysis and permeability of coal

ABSTRACT To investigate the structural modification of Guizhou anthracite through the application of hydrofluoric acid and its corresponding response in terms of enhanced gas desorption and permeability, we utilized Guizhou anthracite from a mining area as the focal point of our study. We conducted a series of experiments encompassing X-ray diffraction (XRD), initial gas release velocity measurements, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR). Our investigation identified hydrofluoric acid as the primary acid solution, with an optimal concentration of 6% and a soaking duration of 12 hours. Furthermore, our theoretical analysis unveiled the physicochemical structural evolution of Guizhou anthracite under the influence of hydrofluoric acid and its subsequent response to enhanced gas desorption and permeability within the coal. Regarding chemical structure, hydrofluoric acid effectively dissolves inorganic minerals in anthracite and reduces the condensation of its regular basic structural units. Peak fitting of functional groups on the surfaces of the experimental coal samples revealed that hydrofluoric acid induces side chain reactions in the molecular structure of anthracite, promoting the gradual ordering of the aromatic structure. Simultaneously decreasing the content of cyclically closely bonded and self-associated hydroxyl hydrogen bonds. These alterations in irregular basic structural units can diminish coal’s adsorption capacity for gas, thereby enhancing gas desorption. Regarding the physical structure, treatment with hydrofluoric acid leads to the dissolution of minerals within specific pores and fissures of the coal samples, generating numerous dissolution pores. These pores and fissures expand and interconnect. The surface morphology quantified by fractal dimension has decreased by about 25.8%, indicating that pore and fracture structures often become relatively simple, which is conducive to gas flow and desorption in coal. Furthermore, the effective porosity and total porosity of the coal samples increase, as does the total movable fluid porosity. This enhances pore connectivity within the coal seam. Calculations reveal that after acidification, the permeability of the experimental coal samples increases by approximately 1.37 times, confirming the permeability-enhancing effect of hydrofluoric acid on anthracite.

Investigation on the Gas Emission Law of Water-Containing Coal across the Rank Range.

Water commonly occurs in coal reservoirs, and it can block the gas flow channels. This has a significant influence on methane transportation within coal. To reveal the gas emission law of water-containing coal across the rank range, three typical coal samples with different coal ranks covering lignite to anthracite were selected in this work. The initial velocity of gas emission (ΔP) under the effect of moisture was measured, and the combination of scanning electron microscopy and mercury injection method was adopted to study the pores and fracture characteristics within coal. Distribution features of oxygen-containing groups in coal were explored by X-ray photoelectron spectroscopy. The microscopic influence mechanism of the water content on ΔP in coal was also comprehensively elucidated. The experimental results show that the moisture content has an obvious inhibitory effect on the ΔP of coal, but the degree of influence on different coal rank samples was different. As the pore space of anthracite (sample XJ) is developed with numerous gas transportation channels, the ΔP has less changes at the lower moisture content (<4.36%). When the moisture content is >4.36%, a large number of water molecules will band together to form water clusters, hindering the gas release, thus greatly reducing the ΔP. However, the change of lignite (sample SL) shows an inverse trend to that of anthracite. Its ΔP is sensitive to the moisture content due to the small number of pores and low porosity. In addition, a great number of oxygen-containing groups in lignite can also provide good surface hydrophilicity for water molecules, and even a small amount of the moisture content (<3.21%) can block most of the pore and facture channels within coal, leading to the remarkable decrease in ΔP. For bituminous coal (sample ML), the distribution of pores and oxygen-containing groups is the most uniform, and the ΔP decreases linearly with the increase in the moisture content.

Numerical simulation study on the evolution law of mechanical properties of different metamorphic coals after heat treatment.

In order to study the effect of temperature on the structure and mechanical properties of coal with different metamorphic degree. Three coal samples with varying degrees of metamorphism were chosen for analysis. The discrete element software PFC2D is used to simulate the heat treatment and compression of coal. The findings indicate that during the heating process, low-order coal exhibits noticeable thermal cracks at an early stage, while thermal crack development in middle-order coal is concentrated in the later stages. In contrast, high-order coal demonstrates a more stable macroscopic structure. The strength and stiffness of low rank coal show the lowest value and decrease significantly within 135°C. However, the strength and stiffness of medium rank coal decrease significantly after 135°C. The changes of mechanical properties and damage modes of coal caused by thermal damage are often ignored, which may lead to the deviation of design and research results from the actual situation. Therefore, this study is of great significance to the prevention and control of coal mine disasters.

Acoustic emission and failure characteristics of coal and rock under single-free-face true triaxial loading

The airside roadway is frequently identified as the predominant site for dynamical disasters involving coal and rock. For the assurance of safety in mining zones, it becomes imperative to understand the mechanical response and energy characteristics of the coal and rock system within the airside roadway during mining processes. A triaxial static and dynamic load test system was used to conduct compression experiments on coal and rock samples under single-free-face true triaxial loading at different loading rates. The research findings are summarized as follows: (1) The damage and failure of the samples gradually propagate from the free face to the deeper sections of the samples and evident stage characteristics. Revealed that the damage mechanism was “microfracture localization – localized region development through – overall instability.” (2) A rising loading rate impedes the coal and rock's ability to make adaptive adjustments, thereby amplifying the sample's resistance against crack development, and playing a positive role in stimulating coal and rock failure, and the coal and rock system changed from progressive stable failure to sudden instability failure. The mesoscopic failure pattern gradually evolves into inadequacy and irregularity. (3) Throughout the entire failure episodes of coal and rock samples, the predominant mode of failure is tensile. However, influenced by the loading rate, the principal failure mechanism transitions from tensile to shear during the Ⅳ stage of failure. (4) There is noteworthy congruence between the macrofracture sites and the spatial positioning of high-energy fracture episodes. This furnishing pivotal insights for preempting and control deep impact disasters.

Response of Macroscopic Coal Oxidation and Pyrolysis Parameters to Different Extractants

ABSTRACT The active structure of coal molecules is one of the key factors affecting the characteristics of coal spontaneous combustion (CSC). By actively stripping or reducing the key active structure of coal molecules, and then studying the influence of different structures on the oxidation characteristics of coal, it is helpful to accurately and efficiently inhibit CSC. Herein, using coal samples from Caojiatan Mine in Shaanxi Province, China, the CSC characteristics of 10 coal groups treated with different solvents and the response of the macroscopic coal oxidation and pyrolysis parameters to each extractant were investigated. The extractant type considerably affected CSC, and the oxidization of coal was most responsive to biomass extraction agents, which had the best weakening effect on CSC. The order of CSC characteristics of extracted coal from strong to weak is AE> YM> CYC> AOT> CYH> EG> MT> TP> THF> CYC + AOT + AE> EG + TP. In general, all extraction reagents in each group have a certain weakening effect on the CSC activity of coal samples. The extraction of aromatic hydrocarbon structures can effectively delay the oxidation of coal. The study provides new insights for the development of CSC mitigation technology and have important practical significance for active prevention and control of CSC risks.

The multi-dimensional joint response of the electric potential time–frequency-field in the fracture process of flawed coal under dynamic load

The fracture failure of flawed coal has always been a problem in underground engineering. Therefore, it is necessary to monitor crack propagation and damage evolution during coal fracture. In this study, the split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) were combined to analyze the impact failure electric potential (EP) of flawed coal samples. The multi-dimensional joint response characteristics of coal EP time–frequency-field under dynamic load were studied and the differential expression and multi-fractal characteristics of EP responses at different measuring points were analyzed. Based on the EP signals, the evolution of damage to the coal was calculated. The results show that the time series evolution of EP and stress–strain curves during coal failure is consistent. The generation of fracture zones will excite the high-value area of local EP, and the spectral distribution of EP signals in this area shows the characteristics of low frequency and high amplitude. Each channel’s EP signal and the degree of deformation at the corresponding place are perfectly matched. The multi-fractal spectrum of the EP signal close to the fracture zone has a higher Δα and a lower Δf, which demonstrates the dominance of the large-scale EP signal produced in this region by the destruction of sample. Theoretical study indicates that the main causes of the anomalous distribution of the EP field are crack tip potential, diffusion potential, etc. The results can provide theoretical basis for the evaluation of coal fracturing by EP technology.

Microcrack monitoring and fracture evolution of coal and rock using integrated acoustic emission and digital image correlation techniques.

The mechanical properties of a coal-rock body were examined through uniaxial compression tests, and the rupture process of the coal-rock body was monitored in real time using a combined acoustic emission (AE) monitoring system and a digital image correlation (DIC) full-field strain measurement system. From a comparison of the mechanical properties of coal and sandstone, clear differences are apparent regarding the uniaxial compressive strength, deformation characteristics, and damage mode; the brittle failure characteristics of the coal samples are also more evident. The change in AE energy reflects the accumulation and release of elastic energy during the rupture process, and the evolution of AE localization points under different stress levels can effectively reflect rupture propagation. Further, the DIC full-field strain measurement method can quantitatively monitor the evolution of the displacement and strain fields at the marking point and surface simultaneously, thereby overcoming the limitations of traditional empirical and qualitative rupture processes. During monitoring, the AE focuses on the internal rupture of the specimen and the DIC focuses on the surface deformation. These complement each other and reflect the rupture process more comprehensively.

Study on pore-crack evolution and connectivity of coal subjected to controlled electrical pulse based on CT scanning technology

Controlled electric pulse (CEP) technology is a novel method for enhancing coal seam permeability. To study the pore-crack evolution and connectivity of the coal body by CEP, a self-built CEP cracking and enhancing permeability of coal rock test system was used to test five groups of coal samples under different breakdown voltages (BVs). We established a three-dimensional crack structure and equivalent pore network model for before and after penetration using high-precision micro-CT technology to quantitatively study the evolution and connectivity of the pore and crack structures of the coal body. The results showed that with rising BV, coal body crushing intensifies, leading to an increased cracking ratio and fractal dimension. This summarised the three stages of coal sample crushing by CEP. In addition, the proportion of larger equivalent pores and throat radii in the breakdown coal body was significantly increased compared to that of raw coal, and the connectivity structure was improved to different degrees compared to that of raw coal. Eventually, a three-dimensional crack network structure formed inside the coal samples, which was broken at both ends and complex in the middle, providing a transport channel for coalbed methane mining.

Effect of ionic liquid-surfactant compound solutions on the wettability and functional groups of coal

In order to investigate the feasibility of ionic liquid-surfactant compound solutions for coal dust suppression, three ionic liquids and sodium alpha-olefin sulfonate (AOS) were selected to be compounded respectively. The compound solutions were optimized by surface tension tests and coal dust settling experiments, and FTIR and SEM tests were carried out on coal samples treated with the optimum wetting agent. The results show that when the concentration is 0.3 wt%, and the ratio of [Bmim][Cl]/AOS is 4:6, the wetting rate of compound solution is the maximum. The content of hydrophilic functional groups OH…O and C-O in the coal samples increases significantly, and the degree of benzene ring substitutions is elevated; while hydrophobic functional groups –CH, aromatic ring C = C, and benzene ring 3 substitutions (3H) are destroyed by dissolution. The micromorphology of the coal surface changes with the appearance of many cracks and pores and an increase in surface roughness. [Bmim]+ can adsorb on the drain absorption points on the coal surface by electrostatic effect, turning them into water absorption sites and attracting the surrounding AOS molecules to arrange the composite adsorption layer more closely. These results can contribute to the development of novel wetting agents with specific structures and provide theoretical guidance for the reduction of coal dust concentrations.

Creep characteristics and long-term strength of underground water reservoirs’ coal pillar dam specimens under different osmotic pressures

Overlying rock pressure and stored water seepage notably affect coal pillar dam creep and long-term strength in underground water reservoirs. However, the current understanding of the seepage-creep coupling effects on coal pillar dam behavior is unclear, necessitating targeted research. Therefore, This Paper conducted experiments on coal pillar dam samples under various osmotic pressures to study their creep behavior and long-term strength. The results indicate that seepage fluids, with their softening, dissolving, lubricating, and flow carrier effects, significantly notably promote the creep behavior of coal samples. Viscoelastic creep in coal decreases with increasing osmotic pressure, whereas the viscoplastic creep strain increases with increasing time and stress. This Paper proposes employing the critical stress at the steady state viscoplastic creep rate threshold as the long-term strength of coal. These findings indicate an inverse link between long-term strength and osmotic pressure, with osmotic pressure going from 0 to 6 MPa, strength drops from 23.2 to 8.5 MPa. Therefore, when designing and ensuring the long-term stability of coal pillar dams, it is crucial to consider the impact of stored water seepage.

Studies on increased utilization of high-ash Indian coal in coke-making

ABSTRACT Due to low-ash coking coal scarcity, India depends on high-cost imported coal for coke making. The present work aims to increase the Indian coal input in the original coal blend for coke making. One typical Indian high-ash and one imported low-ash coal were used as reference materials, presently used in coal blend for coke production in IISCO Steel Plant Burnpur (ISP), West Bengal, India. Coal samples were ground, changing various parameters. The ash distribution in different particle sizes was evaluated. It is observed that decreasing the particle size up to −88 µs can reduce ash content from (18–23%) to (12–14%), which is generally considered suitable for the blended coal mixture used for coke making in the coke-oven battery in ISP. Reduced ash containing fraction of high- ash coal was utilized to evaluate the dilatation behavior and economics of the coking process by considering blending with different percentages of beneficiated Indian coal. From the behavioral study, it was observed that 20% blending would be economical for coke making without hampering the original blend property. The improved dilatation behavior was observed by incorporating 20% dry beneficiated Indian coal in the original blend. The CRI, CSR, and Micum index properties are identical with the normal blend and within the acceptable limit.

Spectral band selection and ANIMR-GAN for high-performance multispectral coal gangue classification

Low-energy and efficient coal gangue sorting is crucial for environmental protection. Multispectral imaging (MSI) has emerged as a promising technology in this domain. This work addresses the challenge of low resolution and poor recognition performance in underground MSI equipment. We propose an attention-based multi-level residual network (ANIMR) within a super-resolution reconstruction model (ANIMR-GAN) inspired by CycleGAN. This model incorporates improvements to the discriminator and loss function. We trained the model on 600 coal and gangue MSI samples and validated it on an independent set of 120 samples. The ANIMR-GAN, combined with a random forest classifier, achieved a maximum accuracy of 97.78% and an average accuracy of 93.72%. Furthermore, the study identifies the 959.37 nm band as optimal for coal and gangue classification. Compared to existing super-resolution methods, ANIMR-GAN offers advantages, paving the way for intelligent and efficient coal gangue sorting, ultimately promoting advancements in sustainable mineral processing.

Influence of plasma breakdown on pores and fractures in bituminous coal: A new characterization method of breakdown energy

Plasma technology has a potential application prospect in the field of coal seam permeability enhancement. However, the relationship between plasma breakdown energy and the improvement effect of coal pores and fractures is lack of in-depth research. This study proposed a novel method for characterizing the plasma breakdown energy, and three testing techniques, namely, particle size distribution, mercury intrusion porosimetry (MIP) and low-field nuclear magnetic resonance (NMR), were utilized to examine the pore and fracture distribution of coal samples obtained from Jiulong Mine under varying breakdown energy conditions. The influence between breakdown energy and pores and fractures development is revealed. The results show that: (1) The degree of coal sample fragmentation increases with the increase of plasma breakdown energy. (2) Different breakdown energy induced the development of different pore distribution patterns. At lower breakdown energy, an increase in the volume of macropores dominates. As the breakdown energy increases, there is a rapid increase in the volume of mesopores, a slight increase in minipores, and the least increase in micropores, indicating that plasma treatment primarily improves pores with diameters greater than 100 nm. The test results of the three techniques were mutually verified. (3) Compared with capacitance energy and voltage, the effective breakdown energy obtained by the integral method has a more significant linear relationship with the particle size distribution, total pore volume, and average pore diameter, indicating that the effective breakdown energy is more suitable for characterizing the crushing effect of plasma. During the plasma breakdown process, the expansion stress and thermal stress formed by the effective breakdown energy exceeded the local tensile strength of coal, leading to the generation and development of pores and fractures, which in turn led to the rupture of the coal body. The larger the breakdown energy, the larger the crushing circle formed around the ion channel, the finer the coal is crushed, and the more pore fractures are developed. These nascent pores and fractures provide channels for gas transport, which is extremely favorable for coal mine gas extraction. The research conclusions provide theoretical support for the application of plasma in fracturing and permeability enhancement in coal seams.

Biogenic Degradation of Lignite Coal from Pakistani Coal Mine and Extraction of Humic Acid through Pretreatment Strategies by Fungal Isolate

The utilization of white rot fungal strain for degradation of low rank coal from coal environments is considered to be the best choice for converting raw coal into different kinds of valuable by products. NF-1, a fungal strain was isolated from coal mine of Thar region of Pakistan. For enhancing the process of biodegradation, the indigenous fungal isolate was used in the study. The lignite (low rank) coal sample degradation initiated by NF-1 strain were undergone through optimization process and had shown considerable production of organics at 1.5% Glucose, 1% coal loading ratio and 7 days of incubation period. The FTIR analysis of untreated coal and treated coal after solubilization was performed in order to observe changes in structural pattern by NF-1 fungal isolate. NF1 has the ability to break down the aromatic side linkages and attacking the amine and carboxyl groups. Scanning electron microscopy (SEM) of the coal treated by NF-1 strain was analyzed to investigate the attachment and adherence of the fungal mycelium on the coal surface. At 200µm the NF-1 isolate hyphae has clearly entrapped the coal particles showing erosion and up to 50µm colonization and visible changes on the coal surface can be observed clearly. Moreover, the residual coal after fungal pretreatment was processed for extraction of humic acid as a byproduct for defining the effect of such biological pretreatments. The produced humic acid was analyzed on the basis of E4/E6 ratio and FTIR. The fungal pretreated humic acid has shown more improvement in E4/E6 ratio as well as presence of functional groups related to humic acid through FTIR. This explains that the isolate NF1 have ability to speed up the biodegradation process and shows an aptitude of producing valuable by products i.e. humic acid as an alternative fuel.

Water saturation characteristics of coal of different ranks and its effect on capillary pressure

AbstractSaturation characteristics of water and capillary pressure are important for hydrofracturing or water flooding, which is widely adopted in dust suppression, methane displacement, and gas and coal outburst elimination. In this paper, we have studied the water saturation characteristics of coal samples with different coal ranks and their effect on capillary pressure. It was found that the isothermal adsorption of water vapor from coal samples can be divided into three processes: monolayer adsorption, multilayer adsorption, and capillary condensation. The dent isothermal adsorption model can well describe water vapor adsorption of coal samples, indicating water molecules were first adsorbed on the first‐order adsorption site of pore surfaces at low relative humidity and then on the second‐order adsorption site with increasing relative humidity. An obvious hysteresis occurred in the process of water imbibition and drainage. A capillary pressure of ~325 MPa or even larger was required to displace the water from the coal. Besides, micro‐ and mesopores play a more important role in capillary pressure. The capillary pressure of coal samples was positively correlated with wettability at lower water saturation, but it is not significant at higher capillary pressures. Therefore, the article's results provide a new direction for solving the water‐locking effect in coal seams.