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Close-distance coal seams (CDCS) are widely distributed, and the layout of the upper and lower panels can be divided into "=" type and "+" type. The "+" superposition of upper and lower coal pillars in CDCS caused strong mine pressure, but there are few studies on the panel crossing residual coal pillars (RCP) when the upper and lower coal seams are "+" type layout. In view of the special spatial position ("+" type layout), this paper takes the typical panel 4-301 of a particular mine as the project indagation background and studies mining and crossing the overlying coal pillars by dint of field measurement, numerical simulation, indoor test, and engineering application. Compared with vertical stress or horizontal stress alone, the indexes of deviatoric stress and plastic zone can reflect the failure evolution of surrounding rock more comprehensively. Hence, this paper analyzes the expansion form of the plastic zone and the variation law of deviatoric stress before and after mining influence in the underlying mining roadway. The research results show that: (1) There is a sub-peak zone of deviatoric stress under the RCP. The deviatoric stress is bimodal in the range of 9 m below. After the peak value decays to 7.4 MPa, it changes to a single peak located in the area directly below the middle of the RCP. (2) The maximum plastic zones of the roof and two ribs of the roadway below the RCP are 3.4 m and 5 m, respectively. The crest value of deviatoric stress reaches 10 MPa. As the distance between the panel and the RCP decreases, the shape of the high deviatoric stress area presents the evolution law from the "ellipse" of the roof → the "crescent" of two ribs → the "cochlea" of the tips of the ribs. (3) When the mining of the underlying panel is 10 m, 0 m, or − 10 m away from the RCP (without passing through the RCP). The crest value of deviatoric stress within 5–10 m in advance of the roadway increases in turn. However, the peak value is significantly reduced when it is − 20 m away from the RCP (through the RCP). The crest value of deviatoric stress of two ribs decreases in turn along the panel rib → section coal pillar rib → solid coal rib. Based on this, the underlying 45 m of the RCP is divided into area I (10 m), area II (overlapping area 20 m), and area III (15 m) based on the degree of disturbance. And propose the technical scheme of asymmetric combined control in different zones by using asymmetric channel steel truss anchor cable for the top-ribs of areas I and III, and top-ribs asymmetric channel steel truss anchor cable + door-type support in area II. On-site project practice shows that the partitioned control technology successfully resisted the roadway instability and failure caused by the dynamic-static superimposed stress disturbance under the RCP and realized the primary support of the sectional coal roadway. The conclusion provides technical support and scheme design for the partitioning support of roadways under similar "+" type cross‐panels. [ABSTRACT FROM AUTHOR]

The level of phosphorus must be carefully monitored for proper and effective utilization of coal and coal ash. The phosphorus content needs to be assessed to optimize combustion efficiency and maintenance costs of power plants, ensure quality, and minimize the environmental impact of coal and coal ash. The detection of low levels of phosphorus in coal and coal ash is a significant challenge due to its complex chemical composition and low concentration levels. Effective monitoring requires accurate and sensitive equipment for the detection of phosphorus in coal and coal ash. X-ray fluorescence (XRF) is a commonly used analytical technique for the determination of phosphorus content in coal and coal ash samples but proves challenging due to their comparatively weak fluorescence intensity. Fourier Transform Infrared spectroscopy (FTIR) emerges as a promising alternative that is simple, rapid, and cost-effective. However, research in this area has been limited. Until now, only a limited number of research studies have outlined the estimation of major elements in coal, predominantly relying on FTIR spectroscopy. In this article, we explore the potential of FTIR spectroscopy combined with machine learning models (piecewise linear regression—PLR, partial least square regression—PLSR, random forest—RF, and support vector regression—SVR) for quantifying the phosphorus content in coal and coal ash. For model development, the methodology employs the mid-infrared absorption peak intensity levels of phosphorus-specific functional groups and anionic groups of phosphate minerals at various working concentration ranges of coal and coal ash. This paper proposes a multi-model estimation (using PLR, PLSR, and RF) approach based on FTIR spectral data to detect and rapidly estimate low levels of phosphorus in coal and its ash (R 2 of 0.836, RMSE of 0.735 ppm, RMSE (%) of 34.801, MBE of − 0.077 ppm, MBE (%) of 5.499, and MAE of 0.528 ppm in coal samples and R 2 of 0.803, RMSE of 0.676 ppm, RMSE (%) of 38.050, MBE of − 0.118 ppm, MBE (%) of 4.501, and MAE of 0.474 ppm in coal ash samples). Our findings suggest that FTIR combined with the multi-model approach combining PLR, PLSR, and RF regression models is a reliable tool for rapid and near-real-time measurement of phosphorus in coal and coal ash and can be suitably modified to model phosphorus content in other natural samples such as soil, shale, etc. [ABSTRACT FROM AUTHOR]

A two-dimensional unsteady seepage model for coal using a finite element program is developed, and the temporal variations of key factors such as water pressure and hydraulic gradient are analyzed in this paper. Additionally, the triaxial rock mechanical experiment and utilized pneumatic fracturing equipment on raw coal samples to investigate both hydraulic and pneumatic fracturing processes are conducted. Through these experiments, the relationship between pressure and crack formation and expansion are examined. The analysis reveals that the pore pressure gradient at the coal inlet reaches its peak during rapid surges in water pressure but diminishes over time. Conversely, the pore pressure gradient at the outlet side exhibits a gradual increase. Hydraulic fracturing is most likely to occur at the water inlet during sudden increases in water pressure. Besides, as the permeability of coal decreases, the duration for seepage stabilization prolongs due to the intensified pore pressure gradient resulting from sudden increases in water pressure. Moreover, an extended period of high hydraulic gradient further increases the risk of hydraulic fracturing. The experimental findings indicate that coal samples initially experience tensile failure influenced by water and air pressure. Subsequently, mode I cracks form under pressure, propagating along the fracture surface and becoming visible. The main types of failure observed in hydraulic and pneumatic fracturing are diametrical tensile failure, and the development of fractures can be categorized into three distinct stages, which contains the initial stage characterized by slight volume changes while water pressure increases, the expansion stage when pressure reaches the failure strength, and the crack closure stage marked by little or even decreasing volume changes during pressure unloading. The acoustic emission signal accurately corresponds to these three stages. [ABSTRACT FROM AUTHOR]

The low loading rate of the thin coal seam shearer drum is a severe obstacle to the efficient mining of thin seam resources, and the auxiliary drum loading through the cowl is an effective measure to alleviate this situation. However, the working mechanism of the coal cowl still remains unclear. In this paper, with the help of the discrete element method and the modeling experiment method, the effects of coal cowl's offset distance, tilt angle and wrap angle on the coal loading rate under different loading modes of the drum are investigated; and the significance of various factors and their interactions to the drum coal loading rate is explored by designing response surface experiments. The findings show that a monotonous negative correlation between the offset distance of the coal cowl and the coal loading rate is identified, and that a smaller offset distance can effectively improve the coal loading rate of the drum. The conveying torque is significantly increased, easily inducing the drum choking, coal recycling coal over-crushing. Along with the increasing tilt angle, the rate of ejection loading decreases monotonically, and the rate of pushing loading increases first and then decreases. Coal loading rate is weakly affected by changes in coal cowl's wrap angle. The results of response surface analysis reveal that the most significant factors affecting the drum's coal loading rate are tilt angle and offset distance in ejection and pushing loading modes, respectively. The conclusions drawn here offer implications for improving the coal loading performance of the thin coal seam shearer drum, as well as certain guidance on the optimal design of coal cowl parameters. [ABSTRACT FROM AUTHOR]

Carbazole compounds are widely used in determining the direction of petroleum migration, but the effect of thermal maturity on carbazoles is still ambiguity. In this paper, using compaction pyrolysis simulation experiments, artificial mature samples with vitrinite reflectance (Ro) range from 0.38 to 3.0% were acquired. And the content and composition change characteristics of carbazole compounds were analyzed in coal source rocks. The experimental results showed that thermal maturity controls the generation of a large amount of carbazole compounds in coal rocks. Compared with the low mature stage, the content of carbazole compounds was about 10–100 times higher in the mature stage. With the increasing maturity, in the coal sample, the content of carbazole compounds showed a trend of first increasing and then decreasing. In derivatives of carbazole, the corresponding maturity for the maximum generation of ethylcarbazole (EC), dimethylcarbazole (DMCA), methylcarbazole (MCA), carbazole (CA) and benzocarbazole (BCA) performed the increasing sequence. With the increasing maturity, the relative abundance of 2-MCA, 1,7-DMCA and benzo[a]carbazole increased with the increasing maturity, while 4-MCA, 1,4-DMCA and benzo[c]carbazole gradually decreased. Benzocarbazole ratio [a]/[a] +[ c] varies only in a narrow range 0.36–0.61 in the entire maturity range, suggesting limited maturity dependence. The experimental conclusion provides more theoretical basis for future geochemical analysis using carbazole compounds. [ABSTRACT FROM AUTHOR]

Currently the main method of mine solid waste disposal and utilization is to apply gangue to underground filling. For the grading characteristics of coal gangue in the hollow zone in underground filling, the state of pressure and the mechanical properties of coal gangue filling materials under different particle sizes. In this paper, through laboratory tests and numerical simulations, the bearing characteristics and deformation damage of gangue are deeply investigated and analyzed from both macroscopic and microscopic perspectives. The results of the study show that when the axial pressure reaches a certain threshold, the strain increases accordingly with the increase in grain size. Under the axial pressure condition of 15MPa, the bearing capacity of gangue with different grain sizes under the lateral pressure of steel cylinder showed obvious positive correlation. In the software PFC3D simulation, as the particle size of gangue particles increases, it has a significant effect on the natural stacking porosity of the specimen, and gangue particles are more prone to force chain instability and failure. Mixed particle size gangue can significantly reduce the natural accumulation porosity of the specimen, thus improving its overall stability. Significant displacement triangles existed in the top and lateral directions of the B1 (0–5 mm), B2 (5–10 mm), and B5 (0–20 mm) specimens under the same circumferential pressure conditions. The results of the study are of great significance for further selection of reasonable gangue gradation and determination of its mechanical properties. [ABSTRACT FROM AUTHOR]

Shenfu Dongsheng coal field is a cross-century energy base which is developed and constructed in China. In recent years, some mines have successively entered to the coal seam of the second layer. Due to the reasons of early mining, many coal pillars are left in the coal seam of the first layer, resulting in the phenomenon of strong ore pressure in the mining range before and after the coal pillar in the lower coal seam and even causing the buckling accident. In order to solve such safety problems, this paper takes the 22,307 working face in Bulianta coal mine as the research object, adopts physical similarity simulation experiment and theoretical analysis to systematically study the overlying rock characteristics and abnormal ore pressure manifestation mechanism of shallow and close coal seam in different working stages. The results show that the roof overburden of the key layer in the lower group bends and sinks when the coal pillar is mined, resulting in the activation and instability of the "masonry beam" structure formed by the roof of the upper coal seam. When the coal pillar is discharged, the residual concentrated coal pillar and the room type coal pillar are unstable under the action of high supporting stress, resulting in shear failure of the inter-layer rock in the upper part of 22,307 working face, causing the strong dynamic pressure of the working face to appear and then leading to the buckling accident. The working resistance of the support in each stage is obtained by establishing the structure diagram of the overlying rock under each stage and the corresponding mechanical structure model. Finally, the working resistance required by the support in the mining stage under the goaf is 16,692.6 kN, the working resistance required by the support in the coal pillar stage is 19,692.6 kN, the working resistance required by the support in the mining stage under the concentrated coal pillar is 13,150.6 kN, and the working resistance required by the support in the coal pillar stage is 19,215.6 kN. [ABSTRACT FROM AUTHOR]

Based on the greening and low interference disposal requirements of coal gangue in high-yield and high-efficiency mines in Inner Mongolia and Shaanxi, and by integrating the existing theories and technologies such as underground filling technology of coal gangue, mine yellow mud grouting technology, and the evolution law of mining overburden fractures, a technology of filling goaf with coal gangue slurry for green disposal of coal gangue is proposed. The principle and technical framework of the technology of filling goaf with coal gangue slurry are clarified. This paper expounds on the technological process of technology of filling goaf with coal gangue slurry, establishes three types of slurry filling systems, such as centralized ground layout, ground and underground coordinated layout, and centralized underground layout, and constructs three slurry filling methods, including low-level grouting, adjacent level grouting, and high-level grouting, forming seven kinds of technology models of filling goaf with coal gangue slurry, including ground centralized pulping + high-level grouting, ground centralized pulping + adjacent grouting, ground centralized pulping + low level grouting, ground and underground coordinated pulping + adjacent grouting, ground and underground coordinated pulping + low level grouting, underground centralized pulping + adjacent grouting, and underground centralized pulping + low level grouting, and gives the selection process of technology models of filling goaf with coal gangue slurry. Based on the different conditions and requirements of Haidaze Coal Mine and Huangling No. 2 Coal Mine, engineering tests were carried out on two different technology modes, namely, ground and underground coordinated pulping + low level grouting and ground and underground coordinated pulping + adjacent grouting, based on simulation tests of low level grouting and adjacent grouting. The tests prove the feasibility of filling goaf with coal gangue slurry and explore the way for the theoretical research of the technology of filling goaf with coal gangue slurry and greening and low interference disposal of coal gangue. [ABSTRACT FROM AUTHOR]

Under double-seam mining, the main roadway surrounding rock is affected by the superposition of the advanced stress of the two-seam coal working faces. The stress superposition mode and degree are of great significance to the width calculation of the protective coal pillar and the determination of the critical control direction of the surrounding rock. This paper uses theoretical analysis, numerical simulation, and site engineering practice to carry out targeted research. The conclusions are as follows: Under different lateral pressure coefficients, the superposition evolution law of maximum principal stress direction of two coal seams with different offsets; Two developmental trends and three types of evolution models of J2 peak zone (the critical area of the stress increase and deflection changes) under different superimposed loading modes are summarized. Based on the typical asymmetric evolution model of the J2 peak zone, an asymmetric truss-cable co-anchoring method is proposed aimed at the J2 critical zone. The field monitoring results show that the main roadway surrounding rock is stable after support when the upper coal seam protective coal pillar is left 80 m, and the lower one is 60 m wide. It is of great reference importance for similar engineering practices. [ABSTRACT FROM AUTHOR]

Understanding the molecular structure characteristics of coal at the molecular level is of great significance to realize the rational utilization and efficient conversion of coal. This paper gives insights into the acquisition of characterization parameters of coal molecular microstructure by testing and analyzing the long flame coal from Qinggangping (QGP) Coal Mine through proximate analysis, ultimate analysis, vitrinite reflectance determination, fourier transform infrared Spectroscopy test (FTIR), X-ray photoelectron epectroscopy test (XPS), carbon nuclear magnetic resonance (13C-NMR) and X-ray diffraction (XRD). The results show that benzene rings in the QGP coal are mainly connected in a disubstituted way, accounting for 36.48%. Oxygen atoms mainly exist in the oxygen-containing functional groups such as the ether C–O, C=O and –COO. Aliphatic hydrocarbons in the aliphatic group are mainly of symmetrical -CHx stretching vibration. Hydroxyl groups are mainly composed of OH–OH and OH–O hydrogen bonds, accounting for 29.21% and 21.53%, respectively. Nitrogen atoms exist in the form of C4H5N. The coal molecular is mainly of aromatic carbon structure, where the ratio of bridge aromatic carbon to peripheral carbon is 0.198. There are benzene, naphthalene and anthracene in the coal molecular structure, and the former two chemicals play a dominating role. According to the analysis results, the molecular formula of the QGP coal is finally determined as C205H181O29N3S. On this basis, the two-dimensional and three-dimensional macromolecular models are constructed with the assistance of simulation software. In addition, the 13C-NMR spectra and densities of the constructed molecular models are calculated, which verifies the rationality of the models. The macromolecular structure model of bituminous coal constructed in this study provides a theoretical model basis for the optimal surfactant. [ABSTRACT FROM AUTHOR]

Under double-seam mining, the main roadway surrounding rock is affected by the superposition of the advanced stress of the two-seam coal working faces. The stress superposition mode and degree are of great significance to the width calculation of the protective coal pillar and the determination of the critical control direction of the surrounding rock. This paper uses theoretical analysis, numerical simulation, and site engineering practice to carry out targeted research. The conclusions are as follows: Under different lateral pressure coefficients, the superposition evolution law of maximum principal stress direction of two coal seams with different offsets; Two developmental trends and three types of evolution models of J2 peak zone (the critical area of the stress increase and deflection changes) under different superimposed loading modes are summarized. Based on the typical asymmetric evolution model of the J2 peak zone, an asymmetric truss-cable co-anchoring method is proposed aimed at the J2 critical zone. The field monitoring results show that the main roadway surrounding rock is stable after support when the upper coal seam protective coal pillar is left 80 m, and the lower one is 60 m wide. It is of great reference importance for similar engineering practices. [ABSTRACT FROM AUTHOR]

High-efficiency rock breaking technology is an important problem to be solved urgently in deep mining. The existing auxiliary rock breaking technology for coal mine excavation has problems such as polluting coal, high requirements for tool materials, and difficulty in subsequent washing. In this paper, an active excitation cutting system for rotary cylinder of cantilever roadheader based on alternating valve is proposed. According to the classical calculation formula of cutting load of roadheader, considering the stiffness-damping characteristics of cutting part and cutting cylinder, the simulation algorithm of cutting load is compiled based on MATLAB simulation analysis software. The excitation cutting experiments of different cutting depths are carried out on the cutting test bench, and compared with the simulation algorithm of cutting load. Taking the cutting load as the discriminant index, the influence of oil supply pressure and excitation frequency of rotary cylinder on the cutting load is analyzed based on the simulation algorithm of cutting load. The results show that the error between the simulation results and the experimental results of the active excitation state is less than 12%, and the two are in good agreement. Under the condition of 30 HZ, when the excitation amplitude is 8 Mpa, 10 Mpa, 12 Mpa, 14 Mpa, when the rotary cylinder excitation amplitude is 10 Mpa, the minimum is 131.42 KN. Compared with the rotary cylinder excitation amplitude under the condition of 10 Mpa, when the excitation frequency is 30 HZ, 35 HZ, 40 HZ, 45 HZ, when the excitation frequency is 40 HZ, the minimum is 83.08 KN, indicating that changing the excitation amplitude or oil supply pressure of the rotary cylinder is helpful to adjust the cutting performance. [ABSTRACT FROM AUTHOR]

Understanding the molecular structure characteristics of coal at the molecular level is of great significance to realize the rational utilization and efficient conversion of coal. This paper gives insights into the acquisition of characterization parameters of coal molecular microstructure by testing and analyzing the long flame coal from Qinggangping (QGP) Coal Mine through proximate analysis, ultimate analysis, vitrinite reflectance determination, fourier transform infrared Spectroscopy test (FTIR), X-ray photoelectron epectroscopy test (XPS), carbon nuclear magnetic resonance (13C-NMR) and X-ray diffraction (XRD). The results show that benzene rings in the QGP coal are mainly connected in a disubstituted way, accounting for 36.48%. Oxygen atoms mainly exist in the oxygen-containing functional groups such as the ether C–O, C=O and –COO. Aliphatic hydrocarbons in the aliphatic group are mainly of symmetrical -CHx stretching vibration. Hydroxyl groups are mainly composed of OH–OH and OH–O hydrogen bonds, accounting for 29.21% and 21.53%, respectively. Nitrogen atoms exist in the form of C4H5N. The coal molecular is mainly of aromatic carbon structure, where the ratio of bridge aromatic carbon to peripheral carbon is 0.198. There are benzene, naphthalene and anthracene in the coal molecular structure, and the former two chemicals play a dominating role. According to the analysis results, the molecular formula of the QGP coal is finally determined as C205H181O29N3S. On this basis, the two-dimensional and three-dimensional macromolecular models are constructed with the assistance of simulation software. In addition, the 13C-NMR spectra and densities of the constructed molecular models are calculated, which verifies the rationality of the models. The macromolecular structure model of bituminous coal constructed in this study provides a theoretical model basis for the optimal surfactant. [ABSTRACT FROM AUTHOR]

In this paper, the development law of residual deformation of coal gangue subgrade filler is analyzed through large scale triaxial test, and the residual deformation model of coal gangue mainly sandstone and limestone is established. The purpose is to provide research basis for the applicability of coal gangue as subgrade filler. The results show that the deformation of coal gangue filler increases first and then tends to be constant under cyclic load of multiple vibration times. It is found that the Shenzhujiang residual deformation model cannot accurately predict the deformation law, and the corresponding modification is made to the residual deformation model of coal gangue filling body. Finally, according to the calculation of grey correlation degree, the influence degree of main factors of coal gangue filler on its residual deformation is sorted. Combined with the actual engineering situation represented by these main factors, it can be analyzed that the effect of packing particle density on residual deformation is greater than that of packing particle size composition. [ABSTRACT FROM AUTHOR]

Coal mines may change from non-outburst mines into coal and gas outburst mines with increasing mining depth. Therefore, scientific and rapid prediction of the coal seam outburst risk and effective prevention and control measures could ensure coal mine safety and production. This study aimed to propose a solid–gas–stress coupling model and assessed its applicability in predicting the coal seam outburst risk. Based on a large amount of outburst case data and the research results of previous scholars, coal and coal seam gas constitute the material basis of outbursts, and gas pressure is the energy source of coal seam outbursts. A solid–gas–stress coupling model was proposed, and a solid–gas–stress coupling equation was established via regression. Among the three major outburst factors, the sensitivity to the gas content during outbursts was the lowest. The causes of coal seam outbursts with a low gas content and the effect of the structure on outbursts were explained. It was theoretically revealed that the coupling of the coal firmness coefficient, gas content and gas pressure determined whether coal seams could experience outbursts. This paper provided a basis for assessing coal seam outbursts and classifying outburst mine types and listed application examples of solid–gas–stress theory. [ABSTRACT FROM AUTHOR]

The recognition of cutting state of coal-rock is the key technology to realize "unmanned" mining in coal face. In order to realized real-time perception and accurate judgment of coal-rock cutting state information, this paper combined the field test sampling, construction technology of complex coal seam, virtual prototype technology, bidirectional coupling technology, data processing theory, image fusion method, and deep learning theory to carry out multi domain deep fusion experimental research on multi-source heterogeneous data of coal and rock cutting state. The typical complex coal seam containing gangue, inclusion, and minor fault in Yangcun mine of Yanzhou mining area was taken as the engineering object. The high-precision three-dimensional simulation model of the complex coal seam that can update and replace particles was constructed. Based on the simulation results of Discrete Element Method-Multi Flexible Body Dynamics (DEM-MFBD), the one-dimensional original vibration acceleration signals of the key components of the shearer cutting part were determined, including spiral drum, rocker arm shell, and square head. After transforming one-dimensional original signal data into two-dimensional time–frequency images by Short-time Fourier Transform, morphological wavelet image fusion technology was used to realize the effective fusion of characteristic information of spiral drum, rocker arm shell, and square head under different working conditions. Based on the deep learning theory, the DCGAN-RFCNN (Deep Convolutional Generative Adversarial Networks-Random Forest Convolutional Neural Networks) coal and rock cutting state recognition network model was constructed. Combining convolution neural network with random forest recognition classifier, RFCNN coal and rock cutting state recognition classification model was constructed, and the recognition network model was trained to obtain the model recognition results. Through the comparative experimental analysis of the RFCNN network model with different recognition network models and different synthetic sample numbers in the recognition network, the effectiveness of the recognition network model was verified. The results show that: When synthetic samples are not included in each working condition in the RFCNN model, the average recognition rate is 90.641%. With the increase of the number of synthetic samples, the recognition rate of coal and rock cutting state increases. When the number of synthetic samples added to each working condition reaches 5000, the recognition effect is the best, and the average recognition rate reaches 98.344%, which verifies the superiority of enriching the data set by using the improved DCGAN network. Also, the RFCNN outperformed the other variants: it obtained higher recognition accuracy by 25.085, 21.925 and 19.337%, respectively, over SVW, CNN, and AlexNet. Also, the experimental platform of shearer cutting coal and rock was built, where the coal and rock cutting state recognition network was trained and tested based on the migration learning theory. Through the statistical test results, the accuracy of coal and rock cutting state recognition is 98.64%, which realizes the accurate recognition of coal and rock cutting state. [ABSTRACT FROM AUTHOR]

The mechanical properties of coal containing fluid are an important factor affecting the safe mining of soft coal seams. In particular, for class III–V coal, coal and gas outbursts and other dynamic phenomena are prone to occur due to the influences of gas pressure and groundwater, which seriously threaten the safety and lives of field workers. However, briquette samples are usually used in place of raw coal in laboratory tests conducted on class III–V coal samples. Whether the research conclusions for briquette and raw coal are consistent and whether briquette coal can replace raw coal in research on strength characteristics need to be further verified. In this paper, the evolution of the strength characteristics of fluid-bearing briquette coal and raw coal is studied. The strength characteristics, instability failure characteristics, and acoustic emission characteristics of raw coal and briquette coal under uniaxial and triaxial compression are analyzed in detail. In addition, the influence of the water content and pore pressure on the strength characteristics of class III–V raw coal and briquette coal is further studied. The results show that the failure characteristic of raw coal is overall brittle failure, mainly axial splitting failure, whereas that of briquette is overall ductile failure, mainly cone-shaped continuous spalling. The strength parameters of the raw coal and briquette coal improve under confining pressure, but the internal difference in the raw coal is significantly reduced. The cohesion of the raw coal sample initially increases and then decreases with increasing water content, and the internal friction angle increases with increasing water content. In addition, it is verified that the strength, cohesion, elastic modulus, and deformation modulus of the briquette decrease with increasing pore pressure under different pore pressures, but the strength difference of the class III–V coal decreases under increasing pore pressure. Based on the abovementioned results, the strength parameters of a coal body are estimated using the Hoek–Brown (H–B) criterion. Based on a comparison of the strength parameters of the coal sample and coal body, the estimated strength parameters of the coal body are closer to the actual values on site. [ABSTRACT FROM AUTHOR]

Nowadays the complex international political situation has caused the shortage of coal supply in the European region. Scholars have done a lot of research on supplier evaluation. However, these studies don't reflect the variability of the indicators, such as interruption caused by recent war. Coal-electricity-integrated companies have a large demand for thermal coal and high requirements for stable supply, so they need to conduct timely and effective short-term evaluation of suppliers. This paper improves the CRITIC method and uses short-term transaction data for a coal-electricity-integrated firm to evaluate its coal suppliers. The results show that the improved CRITIC method effectively avoids the problem of weight changes caused by conflicting value ranges of indicators, and its evaluation results are more consistent with the actual situation, which can meet the requirements of large coal enterprises for evaluating suppliers. [ABSTRACT FROM AUTHOR]

The fusion of X-ray fluorescence spectroscopy (XRF) and visible near infrared spectroscopy (visNIR) has been widely used in geological exploration. The outer product analysis (OPA) has a good effect in the fusion. The dimension of the spectral matrix obtained by OPA is large, and the Competitive Adaptive Reweighted Sampling (CARS) cannot cover the whole spectrum. As a result, the selected variables by the method are inconsistent each time. In this paper, a new feature variable screening method is proposed, which uses the Least Angle Regression (LAR) to select the high dimensional spectral matrix first, and then uses CARS to complete the secondary selection of the spectral matrix, forming the LAR-CARS algorithm. The purpose is to make the sampling method cover all the spectral data. XRF and visNIR tests were carried out on three cores in two boreholes, and a cross-validation set, validation set and a test set were established by combining the results of wavelength dispersion X-ray fluorescence spectrometer and ITRAX Core scanner in the laboratory. The quantitative model was established with the Extreme Gradient Boosting (XGBoost) and LAR-CARS was compared to these other algorithms (LAR, Successive Projections Algorithm, Monte Carlo uninformative variables elimination and CARS). The results showed that the RMSEP values of the models established by the LAR-CARS for six rock-forming elements (Si, Al, K, Ca, Fe, Ti) were relatively small, and the RPD ranges from 1.424 to 2.514. All these results show that the high-dimensional matrix formed by XRF and visNIR integration combined with LAR-CARS can be used for quantitative analysis of rock forming elements in in-situ coal seam cores, and the analysis results can be used as the basis for judging lithology. The research will provide necessary technical support for digital mine construction. [ABSTRACT FROM AUTHOR]

In this paper, in the deformation and damage process under different confining pressures, the energy evolution characteristics and damage mechanism of coal–rock combinations with different inclination angles are studied. Based on the brittleness indexes of coal rock combinations, the evolution rules between brittleness indexes and the inclination are explored, as well as the confining pressure of coal rock combinations; then, the influence mechanism of the inclination angle of coal rock combinations on the plastic yielding degree, energy dissipation level, crack extension and fracture speed in the pre-peak stage is revealed. The composite specimens are mainly damaged due to oblique shear and accompanied by tensile damage; In the deformation and damage, various energies of coal rock composites are distributed as a negative exponential function of the inclination angle, which is significantly affected by the change of the confining pressure. [ABSTRACT FROM AUTHOR]

Longwall top coal caving technology is one of the main methods of thick coal seam mining in China, and the classification evaluation of top coal cavability in longwall top coal caving working face is of great significance for improving coal recovery. However, the empirical or numerical simulation method currently used to evaluate the top coal cavability has high cost and low-efficiency problems. Therefore, in order to improve the evaluation efficiency and reduce evaluation the cost of top coal cavability, according to the characteristics of classification evaluation of top coal cavability, this paper improved and optimized the fuzzy neural network developed by Nauck and Kruse and establishes the fuzzy neural network prediction model for classification evaluation of top coal cavability. At the same time, in order to ensure that the optimized and improved fuzzy neural network has the ability of global approximation that a neural network should have, its global approximation is verified. Then use the data in the database of published papers from CNKI as sample data to train, verify and test the established fuzzy neural network model. After that, the tested model is applied to the classification evaluation of the top coal cavability in 61,107 longwall top coal caving working face in Liuwan Coal Mine. The final evaluation result is that the top coal cavability grade of the 61,107 longwall top coal caving working face in Liuwan Coal Mine is grade II, consistent with the engineering practice. [ABSTRACT FROM AUTHOR]

The reserved thickness of top coal has an important influence on the stability of a large section open-off cut under gob in the thick seams slicing mining. The destabilization extremum conditions of the open-off cut top coal were derived from by elastic–plastic theory, and the optical fibre sensing technology was utilized to monitor the top coal deformation law with different thicknesses (3, 3.5, and 4 m) in the physical similar simulation experiment in the paper. The results show that the top coal thickness is greater than 3.4 m without tension cracks. In the vertical direction, the top coal of the large open-off cut is divided into mining and excavation disturbance zones under the influence of the upper slice coal mining and the excavation disturbance. In the direction of the span of the top coal can be divided into the roof fall risk zone and the warning zone. The deformation changes from exponential to linear to logarithmic in the roof fall risk zone, and it changes from linear to logarithmic in the roof fall warning zone as the number of excavations increases. The sinking amount in the two zones is smaller as the thickness of the top coal becomes larger. It is comprehensively determined that the thickness of the top coal of open-off cut is set as 3.5 m, the stability is moderate, and the field application shows that the integrity of the top coal is good after support, and the maximum off-layer value is 6 mm, which can satisfy the production requirements. [ABSTRACT FROM AUTHOR]

This paper analyzes the influence of the overlying extremely thick primary key stratum on the strong mine pressure hazard at the large mining face in Gaojiapu coal mine. The analysis of the distribution characteristics of the primary key stratum in the Gaojiapu coal mine reveals the bow-shaped structural characteristics of the overlying thick primary key stratum. An elastodynamic model was developed using the variational method to calculate and analyze the influence of the movement of the primary key stratum on the stress and energy of the underlying weak rock. The results show that the arch structure of the overlying extremely thick primary key stratum can significantly affect the distribution pattern of stress and strain energy in the coal body, and the stress and strain energy in the coal body are transferred to the middle of the coal column, and the middle region of the coal column enters a high stress state. These results suggest that the change in thickness of the overlying primary key stratum at Gaojiapu in the coal column area is a major factor in the frequent occurrence of impact ground pressure events at the mine. This study explains the causes of frequent impact ground pressure in the lower coal rock mass of the extremely thick primary key stratum, and provides a reference for the prevention and control of impact hazards in the extremely thick primary key stratum. [ABSTRACT FROM AUTHOR]

In order to reduce the risk of spontaneous combustion of coal left after long-term flooding in the goaf of the mine, in this paper, the inhibitory properties of different inhibitors on two kinds of water-immersed coals with different metamorphic degrees were studied in depth. The experiment selected Pingzhuang brown coal and Shaqu coking coal as research objects. The raw coal and water-immersed coal samples were compared and analyzed by thermogravimetric experiment method and Fourier transform infrared spectroscopy experiment method. The study showed that the activation temperature of brown coal and coking coal decreased by 7.91 and 2.25 °C respectively, and the activation energy decreased by 43.18 kJ/mol and 20.58 kJ/mol respectively. The natural tendency of coal was enhanced after water immersion, and water immersion had a greater impact on low-metamorphic brown coal. After adding four kinds of inhibitors, MgCl2, TEMPO, TPPI and PA to the two water-immersed coals, it was found that TPPI could significantly reduce the heat release rate of water-immersed brown coal, and the reduction value was 10.49 W/mg. The dry cracking temperature of water-immersed brown coal increased by 11.75 °C, and PA greatly increased the combustion activation energy of water-immersed coking coal by 25.77 kJ/mol. Meanwhile, it was found from the microscopic active groups that TTPI increased the content of water-immersed brown coal ether bonds by 4.84%. The absorption peak intensity of oxygen-containing functional groups such as C=O was significantly weakened. Similarly, PA also produced a large number of stable ethers in water-immersed coking coal, whose content increased by 5.21%, and the hydroxyl content decreased most significantly. The decomposition of TPPI into phosphoric acid after heating can inhibit the growth of active groups such as a large number of oxygen-containing functional groups in the water-immersed brown coal, thereby reducing the risk of spontaneous combustion. As a metal chelator, PA can reduce the catalytic effect of metal ions in water-immersed coking coal with fewer active groups, and inhibit coal spontaneous combustion by generating stable metal complexes to increase activation energy. This indicated that TTPI had the best inhibitory effect on water-immersed brown coal, while PA was more suitable for water-immersed coking coal. [ABSTRACT FROM AUTHOR]

With the increasing depth of coal mining, the geological stress and structure becomes more and more complex. The elevation of No.8 coal floor significantly undulates in the studied coal mine. Even though a lot of boreholes have been drilled, it remains difficult to predict the spatial distribution features and it becomes challenging to plan the tunnel for the working face. Consequently, there has been a great loss of production in the coal mine, therefore, it is of great significance to study the prediction method for coal floor elevation of the working face. The surface spline function can be regarded as an infinite flat plate deformation in pure bending. The fluctuation of the coal floor can be considered to be the result of multi-period tectonic stress applied on the coal seam, so, the prediction of elevation of the coal floor with surface spline method is feasible. An advantage of surface spline method is that any order differentiable smooth surface can be obtained without regular known lattice and boundary derivatives. In the paper, the complete expression of surface spline function is derived, which is used to predict the elevation of No.8 coal floor of the coal mine. The results show that the trend of elevation of the coal floor can be extrapolated by known data, and the maximum error is 20 m, the minimum error is 0 m, and the error of 80% of the data is less than 3 m. The general trend of the coal floor has been predicted well. However, local peaks and valleys could not be predicted correctly, therefore, the first and second order derivative are projected to predict the peaks and valleys in the head of the tunneling. [ABSTRACT FROM AUTHOR]

The study of water-conducting fracture zone development height is key to the scientific prevention and control of water damage in mines. Based on the geological conditions of the Wenjiapo coal mine in Binchang, China, this paper investigates the development of water-conducting fracture zone in overlying bedrock during mining under large buried depth and huge thick aquifer by combining on-site well-location microseismic monitoring and laboratory similar material simulation. To overcome the limitation of the " limited outlook " of water-conducting fracture zone investigation, the spatial development characteristics of roof fissures in coal seam mining were determined by on-site " the underground - ground" combined microseismic monitoring and follow-up monitoring, and the development of overlying rock fracture under the large depth of burial was concluded. The fractures were mainly distributed in the upper part of the protective coal pillar on both sides of the working face, but less in the upper part of the working face, and primarily distributed in the protective coal pillar on the side of the working face and the adjacent mining area. To verify the accuracy of the conclusion, the overlying bedrock movement and deformation characteristics and the development process of the hydraulic fracture zone during coal seam mining were analyzed by simulating similar materials in the laboratory, using the monitored area as a prototype. The results show that the development height of the mining fracture zone obtained from microseismic monitoring is basically consistent with the simulation results of similar materials. The research finding have significant implications for the study of fracture distribution characteristics and the evolution law of mining overburden, and provide a foundation for scientific prevention and control of water damage on the roof. [ABSTRACT FROM AUTHOR]

The stressed environment of the inclined coal seam roadway is complex and changeable, and the damage degree of surrounding rock increases, threatening the safe mining of coal mines. In order to take targeted support measures to control the stability of roadway surrounding rock, it is very important to study the stress and deformation characteristics of roadway surrounding rock in inclined coal seam. Therefore, this paper analyzes the deformation and failure law of inclined coal seam roadway according to the theory of complex variable function. It optimizes the solution process and accuracy of the mapping function coefficient and deduces the analytical solution of surrounding rock stress and deformation inclined coal seam roadway. The deformation and failure mechanism of surrounding rock in inclined coal seam roadway is revealed theoretically, and further use numerical simulation and physical simulation tests for supplementary analysis and verification. The results show that the stress and deformation of roadway surrounding rock in inclined coal seam show obvious asymmetric distribution characteristics. The stress and deformation of roadway surrounding rock on the right side are greater than on the left side. The two sides of the roadway, the right side of the roof and the roof angle of the right side, are the key positions of roadway stress concentration and deformation. According to the variation law of stress and deformation distribution of roadway surrounding rock, roadway cyclic deformation and failure theory is put forward. The numerical simulation and physical simulation tests show that the deformation and failure law of roadway is consistent with the theoretical analysis results, and there are differences in numerical values. The cyclic deformation and failure mechanism of roadway in inclined coal seam is verified, which can provide theoretical guidance for roadway support design. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study to assess the behaviour of coal samples under tensile loadings to better understand the failure mechanisms and the interactions with the coal characteristics. A set of Brazilian splitting tests were carried out using disk specimens obtained from Tashan Coal Mine in China. The digital speckle correlation method and acoustic emission (AE) were used to capture the deformation localisation and AE characteristics of each specimen during the loading process. The precursor characteristics of AE and the failure mechanism are discussed. It was found that the entire loading process mainly consists of compaction, elastic and post-peak dropping stage without an obvious yielding stage. Two kinds of deformation localisation were observed: central symmetry and axis symmetry. The corresponding AE evolution patterns have different phases, including gradual rise, step rise, transient rise and steady rise. During the subcritical failure stage, AE counts demonstrate a "rapidly increasing + flatten" intermittent feature. The results provide a reference for a better understanding of the damage process of the brittle coal material and its application in ground control design. [ABSTRACT FROM AUTHOR]

The methods of fire risk assessment in its early phase employed so far used to take into account only the temperature of the heating of coal. The research works reported so far in this field have been conducted at ambient temperatures, without considering the rock mass primary temperature occurring in a particular coal seam and neglecting the depth of coal seams. The method for the determination of the composition of gases emitted from coal samples, employed in the study presented here, takes into account the effect of rock mass pressure, and therefore the depth of the coal seam, as well as the temperature typical for the rock mass in a particular coal mine. In the paper the results of the experimental research on the effects of the primary temperature of a coal seam on the content of gases emitted from coal, performed with the use of a specially designed experimental stand are presented. The method may be useful in developing fire hazard predictions for longwalls with residual coal in goafs. The relations between the self-ignition characteristics as well as selected physical properties of coal samples and carbon monoxide emission were also given. The research results proved that the accurate fire hazard assessment requires considering other than just self-heating of coal causes of carbon monoxide emission, including the rock mass primary temperature, which is expected to improve the existing methods of fire risk assessment in coal mines and post-mining areas. [ABSTRACT FROM AUTHOR]

The gas emission zone is an important parameter for the space–time effect of coal excavation and gas emission. In this paper, according to the effect of roadway excavation, a numerical model of gas emission zone based on the evolution of stress and permeability was established to obtain the width of gas emission zone with different pressure and permeability coefficient. Then the numerical simulation results were verified by measuring the gas content at different depths. Through numerical simulation and field measured data, the theoretical calculation formula is established on the basis of comprehensive consideration of the influencing factors of gas emission zone. The results showed that the gas emission zone increases with the increase of coal seam gas pressure and permeability coefficient when the roadway section and exposure time are the same. The measured gas emission zone, when taking gas content as the index with the same logistic function growth curve, matches the measured results with a relative error of 1.3 to 6%. The validity of the model is also verified by field experiments. The results can provide guidance for mine gas emission and gas drainage design. [ABSTRACT FROM AUTHOR]

The "geochemical effect of lanthanides" is a new concept proposed by the authors during the past decade. This concept reflects lanthanide shrinkage in elemental geochemistry, and it is statistically quantifiable. However, the geological significance of the various parameters of the equations obtained following quantization is not entirely clear. Cooperation and discussions from scholars in related research fields of rare earth elements in geology are required. In the present paper, from the perspective of coal geochemistry, the metamorphic coal seam of C2 series in Fengfeng Mine of Handan Coalfield in Hebei Province, China was used as an example. The geochemical significance of parameters in geochemical effect regression equation for lanthanides was evaluated, and two new formulas (regression equations) that characterized the geochemical behaviors of lanthanides were proposed. On this basis, concepts related to the geochemical effect of lanthanides, such as "individual parameters", "parameters in common", "two-sided parameters", and the "deviation value" of lanthanides, were proposed. In this study, it was proved that the goodness of fit for all types of function regression equations for lanthanides and the radii of their trivalent ions, and the "deviation value" of lanthanides, were all "individual parameters" that could indicate the post-modified geological environment of C2 coal seam, such as the influence from magmatic-hydrothermal fluids. A covariant figure was constructed according to these individual parameters and other indexes, and the C2 coal seam in Handan was effectively divided into two different metamorphic series of A (C2 coal seam uninfluenced or slightly influenced by magmatic-hydrothermal fluids) and B (C2 coal seam strongly influenced and evidently changed by magmatic-hydrothermal fluids). Consequently, the scientific significance of all the parameters for lanthanides in an identifying series of metamorphic coals within the geochemical effect regression equation was further clarified. [ABSTRACT FROM AUTHOR]

This paper addresses the issue of the sorption process on coal concerning propylene released from the source of coal heating in the deposit. In this study, the interaction between Polish coals and propylene molecules, as well as three other hydrocarbons (ethylene, ethane, and propane) with the application of a fixed-bed column, was investigated. The experimental results show that propylene adsorption was measurable under the experimental conditions. The differences in the amount of adsorbed propylene were predominately caused by various gas diffusion rates within the pore network associated with the molecular sieving effect. According to the experimental results, the influence of mesopores on propylene adsorption was significantly stronger than the share of micropores of the explored coals. The column tests demonstrated that the largest amount of propylene was adsorbed by coal with the highest value of pore diameter (6.48 nm) determined by nitrogen adsorption at 77 K. Under the experimental conditions, the influence of other hydrocarbons and a surface area on the quantity of the adsorbed gas was unnoticeable. This study provides an understanding of the behavior of some of the fire gases during the flow of the mixture through a heterogeneous structure of coal in the mine environment. The sorption of gases from multi-component mixture released during the self-heating of coal on carbon materials such as bituminous and lignite coals is poorly understood which provides the rationale for the topic of this work. [ABSTRACT FROM AUTHOR]

An amendment to this paper has been published and can be accessed via a link at the top of the paper. [ABSTRACT FROM AUTHOR]

To achieve further insight into the pore characteristics, the coal specimens with different bursting proneness before and after uniaxial compression failure are tested and compared in this paper. The data of mercury intrusion test is corrected by that of low-temperature nitrogen adsorption and desorption test (LTNAD). The pore size distribution and pore volume of specimens are obtained. The pore compressibility coefficient is determined based on the fractal dimension of pore. Scanning electron microscope (SEM) and computed tomography (CT) are combined to evaluated the pore connectivity. The value of pore compressibility coefficient of specimens with high bursting proneness is larger than that of medium bursting proneness. It means more compressibility and abrupt failure under stress. The researches of both SEM and CT indicate that the pore connectivity of specimens with medium bursting proneness is better. The results show that great differences exist in the pore characteristics of specimens with high and medium bursting proneness, and uniaxial compression failure exacerbates the complexity of pore characteristics. [ABSTRACT FROM AUTHOR]