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From their perspective the optimal tapping process would be when slag and metal are drained from the furnace at the same rate as being produced and separated in the post tap-hole process. Tangstad et al I . i discussed the optimal tapping process for manganese ferroalloy production in submerged arc furnaces (SAFs) in their paper "Conceptual tapping model of Mn-ferroalloy furnaces". J. D. Steenkamp and Q. G. Reynolds are the Guest Editors for the Furnace Tapping topic in the November 2022 issue of JOM. [Extracted from the article]

Among all methods for investigating the complex phenomenon of the coal and gas outburst, numerical modelling is the most comprehensive one that can consider all involved parameters and simulate the complex multi-physical phenomenon. The main aim of this paper is to review recent numerical modelling studies concerning this disastrous phenomenon observed in coal mining. The paper aims to analyse these studies comprehensively and identify any existing research gaps that could be addressed in future research endeavours. To this end, the concepts and equations that have been used in previous studies to simulate the solid and fluid behaviour during outburst initiation and how these equations are coupled to consider multiphysical interactions were reviewed. Both strengths and deficiencies in past outburst modelling work were highlighted. Finally, potential research topics for future studies were discussed in three categories; looking into the outburst phenomenon in the scale of the particles which is mostly focused on the discrete element method, conservation of the energy as a method to quantitively assess outburst initiation, and the application of statistics in both applying the heterogeneity in the models and assessing the possibility of the outburst occurrence. [ABSTRACT FROM AUTHOR]

In recent years, coal and gas outburst disasters are still occurring and difficult to prevent, seriously endangering the safety of coal mine production. It is well known that the transporting and crushing of outburst coal is the main pathway of energy dissipation during the coal and gas outburst process. However, a consensus regarding how much gas involves in outburst and affects energy dissipation is still lacking. Quantitative study on the gas effect on migration and fragmentation characteristics of outburst coal in restricted roadway space can improve the energy model and guide the prevention and control of gas outburst. In this paper, an improved visual coal and gas outburst dynamic effect simulation experiment system was used to conduct outburst simulation experiments at different gas pressure conditions. The results showed that the movement of outburst coal in the roadway has experienced various flow patterns. In the initial stage of the outburst, under low gas pressure condition, the motion of the outburst coal was dominated by stratified flow. However, as the gas pressure increases, the initial acceleration increases, and the outburst coal mainly move forward rapidly in the form of plug flow. The average velocity at 0.3, 0.5, and 0.8 MPa gas pressure condition were 6.75, 22.22 and 35.81 m/s, respectively. Gas also has a crushing effect on outburst coal. With increasing gas pressure, the number of coal powder particles of the same mass increased significantly, and the range of the particle size distribution of the particles decreaed, and the median particle size decreased. As the gas pressure increases, the outburst intensity gradually increases, and the total energy involved in the outburst work also increases. However, the energy dissipation pathways are different. At 0.3 MPa, the energy dissipation is dominated by crushing energy, which is about six times the ejection energy. As the gas pressure increased to 0.8 MPa, the proportion of the ejection energy gradually increases to about twice that of the crushing energy. Under the experimental conditions, 2.71–13.43% of the adsorbed gas involves in the outburst (AGIO) through rapid desorption, and the proportion increases with increasing gas pressure. This paper improves the energy model of coal and gas outburst, which is applicable to risk assessment and prevention of outburst disasters. [ABSTRACT FROM AUTHOR]

Saturation 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. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is extensively utilized for the prevention and control of gas outbursts and rockbursts in the deep sections of coal mines. The determination of fracturing construction parameters based on the coal seam conditions and stress environments merits further investigation. This paper constructs a damage analysis model for coal under hydraulic loads, factoring in the influence of the intermediate principal stress, grounded in the octahedron strength theory analysis approach. It deduces the theoretical analytical equation for the damage distribution of a coal medium subjected to small-flow-rate hydraulic fracturing in underground coal mines. Laboratory experiments yielded the mechanical parameters of coal in the study area and facilitated the fitting of the intermediate principal stress coefficient. Leveraging these datasets, the study probes into the interaction between hydraulic loads and damage radius under assorted influence ranges, porosity, far-field crustal stresses, and brittle damage coefficients. The findings underscore that hydraulic load escalates exponentially with the damage radius. Within the variable range of geological conditions in the test area, the effects of varying influence range, porosity level, far-field stress, and brittle damage coefficient on the outcomes intensify one by one; a larger hydraulic load diminishes the impact of far-field stress variations on the damage radius, inversely to the influence range, porosity, and brittle damage. The damage radius derived through the gas pressure reduction method in field applications corroborates the theoretical calculations, affirming the precision of the theoretical model. These findings render pivotal guidance for the design and efficacy assessment of small-scale hydraulic fracturing in underground coal mines. [ABSTRACT FROM AUTHOR]

To study the effects of gas hydrates on the prevention and control of coal and gas protrusions, this paper reports the results of acoustic emission experiments on coal bodies containing gas hydrates with different saturation levels. The results showed that few acoustic emission events were generated in the elasticity stages of coal bodies containing gas hydrates, and the first sudden increase in the number of ringing counts generally occurred before and after the yielding point. Additionally, the acoustic emission events in the yielding stage were more active, and the cumulative number of ringing counts increased faster. The peak ringing counts appeared around the damage point, and a small number of acoustic emission events were still generated after destruction of the coal samples. The cumulative ringing counts decreased linearly with increasing saturation. The effect of saturation on the cumulative ringing counts in the elasticity stage of the gas hydrate-containing coal samples was small, but the difference between the cumulative ringing counts in the yielding stage and those in the destruction stage was larger. The total cumulative ringing counts and the cumulative ringing counts during each stage for the gas hydrate-containing coal samples decreased with increasing enclosure pressure. The energy and amplitude of the loading process were consistent with the trend for the ringing counts. The acoustic emission ringing counts of gas-containing coals were greater than those of gas hydrate-containing coals in the yielding and destructing stages. [ABSTRACT FROM AUTHOR]

With the continuous increase of mining depth, coal and gas outburst poses a significant threat to mining safety. Conducting research on the mechanisms of coal and gas outbursts contributes to understanding the evolutionary process of such incidents, thus enabling accurate prediction and prevention of coal and gas outbursts during mining operations. This paper has developed a comprehensive visual experimental system that is specifically tailored to simulate diverse coal body conditions, ground stress and gas pressures. By monitoring and analyzing the real-time progression of coal fissures during the outburst process, we can obtain valuable insights into the evolution and mechanisms of coal and gas outbursts. Additionally, this study introduces a method to determine the critical threshold for predicting coal and gas outbursts, and the critical gas pressure threshold for Jiulishan Coal Mine (Jiaozuo City, Henan Province, China) is established at 0.6 MPa. [ABSTRACT FROM AUTHOR]

The shock wave of a coal and gas outburst is a high-pressure and high-speed impact airflow formed rapidly after the outburst. The propagation destroys the ventilation facilities and causes the destruction of the ventilation system. The theoretical research on the outburst shock wave is of great significance. In order to deeply understand the formation mechanism of the outburst shock wave, this paper draws on the shock wave theory to theoretically analyze the microscopic formation process of the outburst shock wave. The main difference between the formation process of a coal and gas outburst shock wave and the formation process of a general shock wave is that the outburst shock wave has a solid–gas flow zone in the high-pressure zone. The calculation formulas of pressure, density, temperature and other parameters before and after the outburst shock wave are derived. After the outburst shock wave passes through, the pressure, temperature and density of the roadway air will change suddenly. The relationship expression between outburst gas pressure and outburst shock wave intensity is derived, which can reflect the role of pulverized coal in the formation process of a shock wave. In order to facilitate the understanding and calculation, the concept of equivalent sound velocity of coal-gas flow is proposed, and the direct calculation of the impact strength of a coal and gas outburst is attempted. This paper is helpful to improve the understanding of the essence of a coal and gas outburst shock wave. It is also of great significance to outburst disaster relief. [ABSTRACT FROM AUTHOR]

Highlights: What are the main findings? The 3D stress distributions around the gob after mining are numerically calculated. What is the implication of the main finding? The propagations of hydraulic fractures in different disturbed areas caused by mining-induced stress are compared. The influence of the geological conditions that often appear in situ in hydraulic fracturing patterns in a hard roof is examined. A hard roof can cause serious issues corelated with the stability of the panel including large deformation of the roadway in the gob, rock burst, coal and gas outbursts, etc. Currently, hydraulic fracturing has ever-increasingly been used to help control these above-mentioned issues in many engineering cases. This paper presents a series of numerical simulations for hydraulic fracturing with a recently developed model to examine the weakening effect of this pre-conditioning measure on a hard roof. The results show that large deflections in the principal stress direction occur above the coal seam after mining, which can be progressively enhanced as the working face continuously advances. This further could significantly affect the hydraulic fracturing pattern and result in arc-shaped fracture propagation, especially for the hydraulic fracture in stress-descending areas. The obtained results suggest that the hydraulic fracturing operation in a hard roof is preferable in the areas close to the middle of the gob where the created fractures would be deflected more. At last, sensitivity analysis shows that geological conditions have great influence on the hydraulic fracturing pattern. Among the factors analyzed in this paper, the difference between the maximum and minimum stress has the largest influence and should be fully considered. This study could provide a theoretical basis for the practical hydraulic fracturing operation in a hard roof. [ABSTRACT FROM AUTHOR]

Featured Application: The XGBoost–GR–stacking gas outburst early warning model established in this article demonstrates high accuracy and practical performance, making it suitable for gas outburst risk warning in mining safety. To improve the accuracy of gas outburst early warning, this paper proposes a gas outburst risk warning model based on XGBoost–GR–stacking. The statistic is based on gas outburst data from 26 mines and establishes a data generation model based on XGBoost. The obtained virtual datasets are analyzed through visualization analysis and ROC curve analysis with respect to the original data. If the augmented data has an ROC area under the curve of 1, it indicates good predictive performance of the augmented data. Grey correlation analysis is used to calculate the grey correlation degrees between each indicator and the "gas emission". The indicator groups with correlation degrees greater than 0.670 are selected as the main control factor groups based on the sorting of correlation degrees. In this study, SVM, RF, XGBoost, and GBDT are selected as the original models for stacking. The original data and virtual data with correlation degrees greater than 0.670 are used as inputs for SVM, RF, XGBoost, GBDT, and stacking fusion models. The results show that the stacking fusion model has an MAE, MSE, and R2 of 0.031, 0.031, and 0.981. Comparing the actual and predicted values for each model, the stacking fusion model achieves the highest accuracy in gas outburst prediction and the best model fitting effect. [ABSTRACT FROM AUTHOR]

Gas extraction quantity from coal seam is the key index to evaluating the effect of eliminating coal and gas outbursts. The prerequisite for increasing the gas extraction quantity and improving the effect of gas control is to make clear the influencing factors of gas extraction. The main factors affecting gas extraction are permeability, gas pressure, and buried depth of coal seam in this paper. The qualitative and quantitative analyses of gas extraction quantity by three factors are carried out according to the test data. The research results show that: (1) In the areas with high permeability and gas pressure, the gas extraction volume shows random characteristics, indicating clear regional differences. (2) The increase of buried depth of coal seam will reduce the fluctuation of gas extraction quantity, resulting in a lower extreme value of gas extraction quantity. The possibility of gas extraction at a lower level will be significantly increased. (3) The correlation analysis shows the change law of the positive correlation coefficient between gas extraction quantity, gas pressure, and permeability, and shows the opposite trend with the increase of the buried depth of coal seam. The results show an important guiding significance for improving the efficiency of gas control. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The coal mine in deep mining can easily form a compound dynamic disaster with the characteristics of rock burst and gas outburst. In this paper, the analytic hierarchy process (AHP) and the entropy weight method (EWM) are combined, and the fuzzy comprehensive evaluation (FCE) secondary evaluation model of compound dynamic disaster is proposed to evaluate the risk of compound dynamic disaster, which avoids the problems of the imperfect evaluation index system and strong subjectivity of index weight. Based on the statistical analysis of typical compound dynamic disaster cases in China, three first-level indicators were established, and sixteen second-level indicators were developed. The verification results show that the accuracy and weight are better than the traditional evaluation methods. Combined with geological and mining data, the compound dynamic disaster risk assessment was carried out on the second mining area of mine B, in the Pingdingshan mining area, and the result was grade II (weak risk). Corresponding prevention measures and parameters were implemented, and no compound dynamic disaster occurred during the working face excavation. [ABSTRACT FROM AUTHOR]

The surrounding rock during a coal mine excavation is prone to significant engineering disasters such as considerable deformation and rock bursts. Pressure release can improve the stress field of a deep rock mass and prevent the occurrence of dangers such as roadway collapse and coal and gas outbursts. This paper uses the ANSYS 19.0/LS-DYNA finite element software to simulate the crush area and fracture zone of a detonation charge with different diameters under in situ stress. The stability of the surrounding rock was analyzed based on the impact stress and velocity, and was verified by field tests. The research results show that the blasting load primarily affects the damaged area near the borehole, while the in situ stress affects far-field crack propagation. The crack propagates in the direction of high surrounding rock pressure. When the uncoupling index is 1.5, the impact pressure of a 60 mm diameter cartridge is eight times that of a 20 mm diameter cartridge. The impact speed can reach two times that of the 20 mm diameter cartridge. The high-energy event at the roof is transferred to the front of the working face, the distribution is no longer concentrated, and a better pressure-relief blasting effect is achieved. The research results can help guide the prevention and control measures of rock bursts and other mining disasters. [ABSTRACT FROM AUTHOR]

Coal and gas outbursts are some of the most serious coal mine disasters, and effective prediction of coal and gas outbursts can reduce the likelihood of accidents and fatalities. Previously conducted studies have established that machine learning has achieved results in the prediction of coal and gas outbursts, but there is a problem that the available accident data of coal and gas outbursts are diminished or missing. This paper proposes a prediction model based on multiple filling of chained equations for random forests (miceforest) and the Harris Hawk optimization algorithm with Piecewise chaos mapping (PHHO) to optimize the kernel extreme learning machine (KELM) to solve the problem of missing data in coal and gas outburst prediction and to improve prediction accuracy in the case of missing data. Firstly, the miceforest algorithm was adopted to fill missing values in the salient samples, and then the PHHO algorithm was used to optimize the parameters of KELM. Finally, the datasets before and after filling were input into the PHHO–KELM model for experimentation and comparison with other models. The results show that miceforest filling is effective in improving the salient sample accuracy and overall accuracy of predictions, but the improvement is not significant for non-salient samples. The use of the PHHO–KELM model can effectively avoid falling into a local optimum and further improve the prediction accuracy of the KELM algorithm. The salient sample accuracy and overall accuracy of the miceforest–PHHO–KELM model prediction are 96.77% and 98.50%. And an effective coal and gas outburst model has been proposed, which is the miceforest–PHHO–KELM model. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Tectonically deformed coal (TDC) is closely related to gas outbursts. Since TDC exploration is an essential objective for coalfield exploration, it is of great significance to study the petrophysical properties of TDCs and explore their differences. This study collected 17 TDCs and undeformed coal samples from the Huaibei coalfield and ultrasonically tested their petrophysical parameters, including densities, P- and S-wave velocities, and their derived petrophysical parameters (VP/VS ratio, P- and S-wave impedances). Undeformed coal and TDCs with different deformation types (brittle, shear, and plastic deformations) show significant differences in their petrophysical parameters, and cross-plot analysis can directly differentiate them. As with traditional geological methods, acoustically measured petrophysical parameters are good indicators to determine the type of coal deformation. However, the TDCs with the same deformation type have similar petrophysical parameters; it is not easy to distinguish them directly. Instead, the proposed method incorporating principal component analysis and clustering can accurately distinguish up to five classes of TDCs. Different types of tectonic deformation environments and their intensities are highly correlated with the clustering results. This paper also provides essential petrophysical parameters for undeformed coal and TDCs in the Huaibei coalfield, and these parameters can help interpret undeformed coal and TDCs using wireline logs and seismic data. [ABSTRACT FROM AUTHOR]

Copyright of Coal Geology & Exploration is the property of Xian Research Institute of China Coal Research Institute and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In this study, coal and gas outbursts are the "biggest killer" of mine safety production. With the deepening of mining, the mine gas disaster is becoming more and more serious. From the perspective of big data, the establishment of a dynamic visualization system of gas geology integrating gas geology, gas drainage, dynamic outburst prevention, and other information can effectively improving the defects of gas disaster prevention and control in the coal industry, such as insufficient advance, lack of systematic identification methods and backward information collection methods, which is of great significance to improve the ability of gas hazard identification. Based on the precise detection of geology, structure, and gas, this paper proposes to use information technology, the Internet of things, big data analysis, and other technologies to comprehensively analyze the changes in gas occurrence and coal seam occurrence on the basis of the causes of mine gas geological outburst, fully consider the logical relationship between different factors and outburst and adopt the disciplinary advantages of grey theory, fault tree theory, BP neural network, and so on. The tree of coal and gas outburst accidents with general significance is constructed by 24 relatively independent factors. The input vector is determined as the matrix composed of eight main factors affecting and controlling outburst, including gas pressure, coal mechanical strength, comprehensive characteristic coefficient of coal fragmentation, the permeability coefficient of coal, comprehensive characteristic coefficient of coal seam bifurcation and combination, comprehensive characteristic coefficient of coal thickness and coal thickness change, fault complexity coefficients and interlayer sliding comprehensive characteristic coefficient. The geological data affecting coal and gas outbursts are analyzed and calculated scientifically so that the gas geological data can be updated in time, and the change of gas geological laws is presented dynamically, so as to guide the mine to predict the gas disaster more scientifically and reliably. [ABSTRACT FROM AUTHOR]

The overburden rock mining fissures are the main cause of coal spontaneous combustion, gas pooling, and mine water inrush caused by goaf air leakage. Rapid and accurate determination of the development and evolution law of mining fissures have great significance for the application of coal spontaneous combustion prevention and control, gas disaster prevention and control, and water damage prevention and control measures. In this paper, a preliminary judgment of the development height of the water-conducting fracture zone is made based on the theoretical analysis, and the physical model size of the numerical simulation is determined according to its judgment result. It is judged that the development height of its water-conducting fracture zone is between 49 and 64.2 m, which is in line with the actual results. Based on this, a three-dimensional solid model was established in PFC (Particle Flow Code) software to analyze the fissure development pattern of the overburden rock and the development height of the water-conducting fracture zone when the main key stratum of the rock seam is in different positions by simulating the excavation process of the coal seam. The results show that when the main key stratum is located in the "original crack belt boundary," the development of water-conducting fracture zone is significantly inhibited; when the main key stratum is located in the "original caving zone," the water-conducting fracture zone is fully developed, and the crack belt finally develops to the top of the model. In order to verify the accuracy of the numerical simulation, similar material simulation experiments were performed under the same scheme. The results are consistent with the numerical simulation conclusions, effectively verifying the accuracy of the numerical simulation. Finally, the extraction of porosity of the goaf was carried out based on numerical simulation, and the permeability zoning of the goaf was performed; the results show that the development of the water-conducting fracture zone has a significant influence on the permeability of the mining area, and the more fully developed the fissure is, the greater is its permeability. In this paper, the fissure development law in the goaf under different key stratums is explored by various research stratums, and the results show a good consistency, which provides a scientific basis for the prevention and control of disasters such as water inrush and coal and gas outburst in mines, and provides theoretical guidance for safe mining. [ABSTRACT FROM AUTHOR]

The rock masses that occur in nature are damaged and unstable due to the impact of rock burst, coal and gas outbursts, and other human mining activities, posing a major threat to human life and safety. In the light of the early warning of the danger of the loaded rock mass, this paper adopts acoustic emission (AE) device to analyze the AE signal characteristics and damage laws of the loaded rock under different stress levels. Then, based on the AE signal characteristics of the loaded rock, data mining technology is used to construct a model to predict the failure and instability of the loaded rock mass and, finally, verify the reliability of the prediction model based on data mining. The results show that the AE signal characteristics of red sandstone under uniaxial load are related to the magnitude of the bearing stress. Before the plastic deformation stage, the AE energy and the cumulative count per second are both small. After the loaded rock enters the plastic deformation stage, the AE energy and the cumulative count per second both increase sharply. After the AE energy is greater than 500 mV ∗ ms and the cumulative count per second is greater than 150, the loaded rock mass will issue an early warning signal. The research results can provide a reference value for the safe production of the project site and the dangerous early warning of the loaded rock mass. [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 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]

High intensity near infrared femtosecond laser is a promising tool for three-dimensional processing of biological materials. During the processing of cells and tissues, long lasting gas bubbles randomly appeared around the laser focal point, however physicochemical and mechanical effects of the gas bubbles has not been emphasized. This paper presents characteristic behaviors of the gas bubbles and their contact effects on cell viability. High-speed imaging of the gas bubble formation with various additives in physiological medium confirms that the gas bubble consists of dissolved air, and amphipathic proteins stabilize the bubble surface. This surface protective layer inhibits interactions of gas bubbles and cell membranes. Consequently, the gas bubble contact does not cause critical effects on cell viability. On the other hands, burst of gas bubbles stimulated by an impact of femtosecond laser induced cavitation can lead to liquid jet flow that might cause serious mechanical damages on cells. These results provide insights for the parameter of biological tissue processing with intense fs laser pulses. [ABSTRACT FROM AUTHOR]

The problem of boreholes deflection in coal mines is common, especially in soft and hard composite coal seams. As a result, the distribution of boreholes is uneven and forms a blank zone in the drainage area, which fails to achieve the regional pre-draining, causing gas explosion or coal and gas outburst accidents. Aiming at the above problems, this paper developed the technology of anti-inclination for drilling based on the causes of borehole deflection in detail. The anti-inclination advice integrates straight keeping, slag removal and smooth back drilling with reasonable technical parameters. Full hole drilling was adopted to realize straight drilling. Three spiral grooving, vertical grooving and three-edge short-circuit were adopted for collaborative slag removal. Three alloy pieces were welded at the rear end of anti-inclination device to act as reverse drill bit, realizing smooth drilling withdrawal. For sticking problems, we improved the anti-inclination device. Three 20 mm millimeters wide vertical grooves were milled in each 120° direction around the device. After using the improved anti-inclination device, the average deviation of dip and azimuth was only 4.8 and 4.5 degrees, respectively, which were reduced by 51 and 44%. The borehole offset was reduced from 9.9 m to 2.9 m, a decrease of 71%, which improved the gas drainage effect and ensured the safe production of the mine. [ABSTRACT FROM AUTHOR]

Small fault area in coal mine has great risk of coal and gas outburst. However, the damage effect of small faults on coal pore structure still lacks systematic research. Taking a small reverse fault under Pingmei No. 2 mine as an example, this paper collects the hanging wall coal samples within 5 m from the fault plane and the coal samples 50 m from the fault plane, where there is no fault development. The pore structure characteristics of the two coal samples were analyzed and compared by means of a scanning electron microscope, mercury injection, liquid nitrogen adsorption, and Raman spectroscopy. The test results show that compared with the coal samples without faults, the coal samples at faults form micron-scale fracture zones with constant spacing, stable occurrence, and flat two walls. The pore volume and pore specific surface area of different pore sizes increase, the pore peak of mesopore (100~1000 nm) shifts from 1000 nm to 200~400 nm, and its connectivity is enhanced. The number of micropores (<10 nm) increased significantly, and the ink bottle type pores developed. Faulting also has a certain impact on the macromolecular structure of coal, which shows that the D peak area decreases and the area of G peak increases, indicating that the order degree of macromolecular structure of coal increases. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Mine Automation is the property of Industry & Mine Automation Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Rock burst is one of the most sophisticated and threatening problems in the process of mining activities of underground coal mines, which can evolve into phenomena such as gas outburst, violent ground pressure behavior of surrounding rock of stope roadway, and high dynamic load energy event of roof overburden fracture through the interaction between various influencing factors. This paper deeply studies the causes, consequences, and disaster mechanism of "2–2 rock burst accident" in Xinjulong coal mine, gives the network diagram of the relationship between the main influencing factors of fault type rock burst and high-energy events, establishes a mechanical model on this basis, and explores the influence and evolution law of fault influencing factors on the elastic strain energy field of surrounding rock of stope roadway. [ABSTRACT FROM AUTHOR]

Coal mining has to develop to deeper strata with the increasing demand for coal, which leads to frequent coal and gas outburst disasters. Especially, when there is a deformed coal in the coal seam, the outburst disasters are easier and more intense. Coal and gas outburst disasters can be promoted by the fragmentation. In order to clearly explain the reasons for the impact of fragmentation on deformed coal and gas outburst, mercury intrusion porosimetry, low-temperature liquid N2 adsorption test, and scanning electron microscope were carried out in this paper. The differences in pore characteristics of intact coal and deformed coal under different particle size conditions were analyzed from pore shapes, pore size distributions, and fractal dimensions. The damage paths of fragmentation on various kinds of pores in intact coal are as follows: macropores, mesopores, minipores, and micropores, and those on various kinds of pores in deformed coal are as follows: macropores, micropores, mesopores, and minipores. According to the fractal dimension theory and experimental results, the calculative results of fractal dimensions show that the fragmentation and tectonism reduce the surface roughness and structural complexity of macropores and mesopores and increase those of minipores and micropores in deformed coal. Finally, combined with the coupling effect of fragmentation and tectonism on pores, the reason of deformed coal with high crushing degree in fields of outburst was explained. The research provides the basis for the prevention of coal and gas outburst. [ABSTRACT FROM AUTHOR]

In the water-rich karst regions, high water and mud outbursts are common geological disasters in tunnel construction. To ensure the safe and smooth construction of tunnel projects, it is necessary to consider anti-water pressure, water inrush prevention and geological disasters during the design of tunnels. Based on the Yongfutun Tunnel Project, this paper studies the application and effect of radial grouting and curtain grouting, which involves those in high-water-pressure tunnels under double-layer support conditions. To obtain the effects and parameters of radial grouting and curtain grouting, the influences of different grouting ranges on the tunnel's surrounding rocks and supporting structures were analyzed and the finite difference method was adopted. The results show that the radial grouting of the surrounding rock can notably improve the initial support of the tunnel, but the impact is less obvious when the grouting range exceeds 4 m. The design of radial grouting is recommended to be 4.0 m to 4.5 m. Curtain grouting can effectively reduce the external water pressure of the tunnel lining. The external water pressure of the grouting area is 23% greater than that without curtain grouting. Curtain grouting can slow down the infiltration of external water pressure. This is beneficial to the stress of the tunnel lining structure, but the improvement in initial support force is slight. Moreover, curtain grouting improves the safety reserve of the secondary lining by strengthening the self-stability ability of the surrounding rock. Meanwhile, the double-layer primary support can effectively share the external water pressure and surrounding rock pressure. This study provides a certain reference for the lining design of high-water-pressure tunnels. [ABSTRACT FROM AUTHOR]

Confining pressure is closely related to the deformation and failure characteristics of gas-containing coal and is one of the key factors for gas outburst-rockburst coupled dynamic disaster. Firstly, this paper briefly summarized the triaxial compression test results of gas-containing coal under different confining pressures, including deformation and stiffness and failure and strength. Then, based on the second developed MatDEM gas-solid coupling program, the macro- and microdeformation and failure process of coal samples under different confining pressures and constant gas pressure were simulated, and the distribution of particle stress, pore stress, and crack, as well as energy conversion characteristics, was obtained. Finally, the influence mechanism of confining pressure on deformation and failure of gas-containing coal was discussed, and the occurrence conditions of coal and gas outburst, rockburst, and their coupled dynamic disasters were distinguished. The results show that the peak strength and residual strength after failure of gas-containing coal both increase approximately linearly with the increase of confining pressure, while the elastic modulus does not change much or increases continuously, which is closely related to the number of microdefects in the coal sample. The strengthening effect of confining pressure on rock stiffness and strength mainly comes from external support and compaction. The total number of cracks decreases with the increase of confining pressure, while the number of shear cracks increases, and the inclination angle between the formed macroscopic main crack and the horizontal plane gradually decreases. The maximum cumulative elastic energy and residual elastic energy both increase with the increase of confining pressure, which leads to the increase of the impact tendency index of coal samples. [ABSTRACT FROM AUTHOR]

The analysis of natural hazards, including gas-geodynamic phenomena, requires study of the basic physical processes that take place at each stage of an event. This paper focuses on analysing the transport of fragmented rock material during rock and gas outbursts. Our theoretical considerations and experiments have allowed us to specify and verify the significant forces acting on fragmented rock during its transport, thus determining the speed of grains of each grain class in the stream of expanding gas. The above study may serve as a preface to a wide-ranging quantitative and qualitative energy analysis of the movement of material ejected during Gas-geodynamic phenomena. [ABSTRACT FROM AUTHOR]

Understanding the dynamics of mild explosive activity is a fundamental tool for hazard assessment at open conduit volcanoes. This is a particularly critical task for Etna volcano. Etna is in fact characterized by frequent, mild explosive activity, punctuated by lava flows and paroxysmal events ('lava fountains'), which, because of their greater impact, have been the main target for hazard studies, whereas more frequent Strombolian activity has been overlooked. As a result, their impact and associated hazards have been never quantified, despite the extensive monitoring and surveillance activities carried out on this volcano. In this paper, we analyze video recordings of a sequence of Strombolian explosions occurring at the summit craters of Mt. Etna, in Italy, in February 2020. Data were also integrated with a petrographic analysis of collected samples, and drone surveys were performed at the same time as the video recordings. We estimate the frequency of explosions (20–12 per min); particle exit speeds (1–50 m/s), and erupted mass (100–102 kg) of those explosions. A very regular, small-scale activity (marked by a single burst of gas breaking the magma free surface into bombs and lapilli fragments) was occasionally punctuated by larger explosions, (at least one every 5 min), with a longer duration, fed by larger magma volumes, and consisting of two to three distinct pulses followed by a stationary phase. We found that the repose times between explosions follows a log logistic distribution, which is in agreement with the behavior of open vent explosive activity. The four largest explosions of the sequence were analyzed in detail: they emitted particles with median diameters (Mdphi) ranging from −10.1 to −8.8 phi, with bimodal distributions. [ABSTRACT FROM AUTHOR]

The strong disturbance of upward mining of the short-distance multiseam results in frequent fractures in the coal seam to be mined, jeopardizing the operation safety due to the instability of the surrounding rock of the overlying stope and risks of coal and gas outburst. A lucrative alternative is the downward mining of multiseam, which wide implementation is limited by the lack of reliable data on stress evolution characteristics. In this paper, the evolution, nonlinear interaction, and superposition characteristics of mining-induced stress after multiple pressure relief in the short-distance multi-seam were investigated by the numerical simulation and physical simulation, taking the endowment and mining technology characteristics of group B short-distance multiseam in Pan'er Coal Mine in Anhui Province of China as an example. The numerical and physical models of multiple (2—5 times) mining activities in pressure relief multiseam (with five main coal layers) under downward mining are constructed and implemented with the FLAC3D software and 1 : 100 scaling, respectively. The results showed that a pressure relief arch, which bears and transfers stresses, is formed on the coal seam's roof and floor. In downward mining, the local arch expands to the long arch, which bearing capacity within the influence range of multiple mining disturbances is weak, and the arch structure is prone to instability. If a coal pillar is left in the upper coal seam after its mining, the lower coal seam mining will switch from the large-scale low-stress area under the goaf to the small-scale high-stress area of nonlinear interaction of the multiple mining stresses with the stress under the coal pillar. [ABSTRACT FROM AUTHOR]

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Due to the severity and great harm of coal and gas outbursts accidents, outbursts prediction becomes very necessary; the paper presents a hybrid prediction model of feature extraction and pattern classification for coal and gas outbursts. First, discrete wavelet transform (DWT) is utilized as a preprocessing technique to decompose subseries and extract the features with different frequencies and the optimal feature components are retained; second, in order to eliminate the redundancy between the features and uncorrelation between features and outbursts, we use the fast independent component analysis (FICA) to obtain each independent component, obtaining the global information in the feature; then, the obtained features are input into linear discriminant analysis (LDA), under the guidance of class labels, then the local information in features is obtained; finally, the projected features are input into the deep extreme learning machine (DELM) classifier based on the optimal parameters by quantum particle swarm optimization (QPSO) for training and classification. The experimental results on the dataset of coal and gas outbursts show that compared with other models in the current prediction of coal and gas outbursts, this method has significant effect on various indicators. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Vibrations caused by downhole operations often induce coal and gas outburst accidents in tectonic zone coal seams. To clarify how vibration affects the pore structure, gas desorption, and diffusion capacity of tectonic coal, isothermal adsorption-desorption experiments under different vibration frequencies were carried out. In this study, high-pressure mercury intrusion experiments and low-pressure liquid nitrogen adsorption experiments were conducted to determine the pore structures of tectonic coal before and after vibration. The pore distribution of vibration-affected tectonic coal, including local concentration, heterogeneity, and connectivity, was analyzed using multifractal theory. Further, a correlation analysis was performed between the desorption diffusion characteristic parameters and the pore fractal characteristic parameters to derive the intrinsic relationship between the pore fractal evolution characteristics and the desorption diffusion characteristics. The results showed that the vibration increased the pore volume of the tectonic coal, and the pore volume increased as the vibration frequency increased in the 50 Hz range. The pore structure of the vibration-affected tectonic coal showed multifractal characteristics, and the multifractal parameters affected the gas desorption and diffusion capacity by reflecting the density, uniformity, and connectivity of the pore distribution in the coal. The increases in the desorption amount (Q), initial desorption velocity (V0), initial diffusion coefficient (D0), and initial effective diffusion coefficient (De) of the tectonic coal due to vibration indicated that the gas desorption and diffusion capacity of the tectonic coal were improved at the initial desorption stage. Q, V0, D0, and De had significant positive correlations with pore volume and the Hurst index, and V0, D0, and De had negative correlations with the Hausdorff dimension. To a certain extent, vibration reduced the local density regarding the pore distribution in the coal. As a result, the pore size distribution was more uniform, and the pore connectivity was improved, thereby enhancing the gas desorption and diffusion capacity of the coal. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Gas outbursts pose a significant threat in the mining of deep low‐permeability coal seams. As mining depth increases, there is a rise in coal and gas dynamic disasters, rendering the gas outburst prevention technology from shallow mining areas unsuitable for deep coal seam exploitation. Addressing the challenges posed by deep, highly gassy coal seams, this study introduces a gas outburst prevention technology integrating destressing in the close distance underlying roadway with gas drainage. The article investigates the relevant technical parameters and assesses the feasibility of this approach. First, it establishes destressing model for the underlying roadway to derive an analytical solution for stress distribution in the coal seam. Second, appropriate technical parameters are designed considering the seepage characteristics of gas in the coal seam. Finally, the gas drainage technical scheme is implemented at Qujiang coal mine to verify its on‐site effectiveness. Field test results indicate an increase in gas permeability within the coal seam following destressing in the underlying roadway, accompanied by significant improvements in the efficiency of gas drainage and the advancement speed of the coal roadway. Specifically, the driving speed of the coal roadway has escalated from 40 m/month to 150 m/month. These outcomes demonstrate the potential of this technology as a promising approach for preventing gas outbursts in the challenging context of deep, low‐permeability coal seams, facilitating swift coal roadway excavation. [ABSTRACT FROM AUTHOR]

The coal and gas outburst and rockburst coupling disaster is becoming increasingly serious due to deep mining. To clarify the mechanism inducing the outburst–rockburst coupling disaster, a true triaxial single-sided unloading mechanical test was conducted with the aid of a true triaxial solid–thermal–gas coupling test device, an industrial computed tomography (CT) system, and an acoustic emission system. Through this test, the mechanical characteristics, meso crushing characteristics, and acoustic characteristics in the disaster formation process were obtained. Additionally, the outburst–rockburst coupling instability disaster law was verified by numerical simulation. The results demonstrated that the stress unloading degree of the coal body was negatively correlated with the initial gas pressure in the outburst–rockburst coupling disaster. The time domain parameter count and energy of acoustic emission exhibited a "bimodal" distribution pattern in the instability stage. The rockburst would occur when the peak value was in a "low-count and high-energy" state, while coal and gas outburst would occur when the peak value was in a "high-count and low-energy" state. The meso slice revealed that gas degradation promoted the development of microcracks in the coal body, and the penetration of cracks resulted in the main cracks of structural instability during rockburst. The coal and gas outburst was mainly attributed to the "cross" shear failure pattern of the coal body. These research findings may lay a foundation for the effective prevention and control of outburst–rockburst coupling disasters. [ABSTRACT FROM AUTHOR]

Addressing common challenges such as limited indicators, poor adaptability, and imprecise modeling in gas pre-warning systems for driving faces, this study proposes a hybrid predictive and pre-warning model grounded in time-series analysis. The aim is to tackle the effects of broad application across diverse mines and insufficient data on warning accuracy. Firstly, we introduce an adaptive normalization (AN) model for standardizing gas sequence data, prioritizing recent information to better capture the time-series characteristics of gas readings. Coupled with the Gated Recurrent Unit (GRU) model, AN demonstrates superior forecasting performance compared to other standardization techniques. Next, Ensemble Empirical Mode Decomposition (EEMD) is used for feature extraction, guiding the selection of the Variational Mode Decomposition (VMD) order. Minimal decomposition errors validate the efficacy of this approach. Furthermore, enhancements to the transformer framework are made to manage non-linearities, overcome gradient vanishing, and effectively analyze long time-series sequences. To boost versatility across different mining scenarios, the Optuna framework facilitates multiparameter optimization, with xgbRegressor employed for accurate error assessment. Predictive outputs are benchmarked against Recurrent Neural Networks (RNN), GRU, Long Short-Term Memory (LSTM), and Bidirectional LSTM (BiLSTM), where the hybrid model achieves an R-squared value of 0.980975 and a Mean Absolute Error (MAE) of 0.000149, highlighting its top performance. To cope with data scarcity, bootstrapping is applied to estimate the confidence intervals of the hybrid model. Dimensional analysis aids in creating real-time, relative gas emission metrics, while persistent anomaly detection monitors sudden time-series spikes, enabling unsupervised early alerts for gas bursts. This model demonstrates strong predictive prowess and effective pre-warning capabilities, offering technological reinforcement for advancing intelligent coal mine operations. [ABSTRACT FROM AUTHOR]

For deep coal mines, geological disturbances or mining activities can cause damage to coal and rock masses, easily inducing dynamic disasters such as rock bursts or coal and gas outbursts, which seriously threaten the safety production of coal mine. In this paper, first, the Liyazhuang Coal Mine introduced the SOS MS (Sensor Observation Service Microseismic) monitoring system from the Polish Research Institute and verified the accuracy of its positioning. Then, to study the characteristics of MS signals induced by different mining activities, based on the field monitoring results, the waveform and frequency spectrum characteristics of MS signals at different energy levels induced by different mining activities are analyzed, and the energy levels of MS signals caused by different mining activities are classified. Studies have shown that there are large differences in the waveform and spectral characteristics of MS signals at different energy levels. High-level energy seismic signals are characterized by large amplitudes, low frequencies, relatively concentrated distribution, long vibration duration, and slow attenuation. In addition, the relationship between mining activities and MS events is also statistically analyzed based on the spatial and temporal distribution of MS events with the advancement of working face, and the results show that the increase of vibration frequency and energy can be precursor information for roof pressure. With the advance of the working face, the MS energy has the characteristic of periodic distribution, which is consistent with the periodic weighting revealed by the working resistance of the support. This study provides a reference for further revealing the energy release mechanism of rock burst. [ABSTRACT FROM AUTHOR]

Magma intrusion has an important influence on the physical and mechanical properties of coal and rock. In the area of magma intrusion, disasters such as gas outburst are prone to occur. Revealing its invasion law will be conducive to disaster management and energy development. For this purpose, changes in industrial analysis components of coal, mineral composition, major oxides, trace elements, and rare earth elements of coal under the thermal metamorphism of magma intrusion were analyzed. It is found that the moisture and volatile matter contents of the thermally affected coals in the mining face are generally lower than that of normal coals, while moisture and volatile matter contents are reduced towards to the magma intrusion contact. For example, the moisture and volatile matter of coal sample M01 decreased by 64.6% and 38.6% respectively compared with coal sample M05. During magma intrusion, some minerals remain on the surface of the coal body, resulting in changes in the mineral composition of the coal body. The decrease in carbon atom net spacing, the increase in crystallite aggregation and ductility, and aromaticity in thermally affected coals have a positive impact on the improvement of coal metamorphism. Due to the influences of magmatic intrusion, the variation rules of major oxides in coal are different, and the closer to the magmatic intrusion zone, the easier the major oxides are to be depleted. However, magma intrusion will not lead to the loss of all major oxides in thermally affected coals, such as content of CaO is 54.8%, which is higher than that of coal not affected by magmatic hydrothermal fluid. Most of the trace elements in the thermally affected coals of the No. 9 coal seam are depleted. The contents of rare earth elements are low on the whole coalbasis, with an average of 29.48 μg/g, and the distribution pattern towards to magmatic intrusion shows a wide and gentle "V" curve with left high and right low, showing the characteristics of enrichment of light rare earth elements. [ABSTRACT FROM AUTHOR]

China's coal mines have to extend to greater depths for the exploitation of more mineral resources, and they have suffered catastrophic mining-induced disasters, such as rockbursts, water inrushes, coal and gas outbursts, and roof fall accidents. The microseismic monitoring technique is a practical tool for mine safety management, which is extensively utilized in many Chinese coal mines. Microcracks of coal/rock masses are recorded as microseismicities in the field, and the potential mining-induced instabilities can be assessed by in-depth analysis of the microseismic parameters. This study provides a state-of-the-art review of the achievements and developments of the microseismic monitoring technique in coal mines. It also presents some prospects for improving the location accuracy of microseismicity, efficient and intelligent processing of the microseismic data, comprehensive assessment of coal/rock instabilities, and development of new microseismic monitoring equipment. This study is valuable for mine safety management and may contribute to improving the deep mining production. [ABSTRACT FROM AUTHOR]

The finite difference model and particle flow model were constructed to reproduce the process of coal and gas outburst in front of the tunnel. The reliability of the particle flow model was verified by studying the internal stress, gas pressure, and related parameters of gas and coal two-phase flow. Then the process of coal and gas outburst is described from a microscopic perspective. The results show that the particle flow numerical model of coal and gas outburst is in line with reality and can intuitively and conveniently monitor relevant parameters; The concentrated stress caused by excavation of tunnels can reach 1.2 times the in-situ stress; High stress causes the adhesion between coal particles to break down, which is the main reason for the initiation of coal and gas outbursts; The intermittent breakthrough of gas on the coal particle collection that blocks the release channel has caused pulsating characteristics of coal and gas outbursts. [ABSTRACT FROM AUTHOR]

The gas contained in coal plays a crucial role in triggering coal and gas outbursts. During an outburst, a large quantity of gas originally absorbed by coal is released from pulverized coal. The role this part of the gas plays in the process of coal and gas outbursts has not been clearly elucidated yet. Therefore, investigating the changes in gas desorption rate from coal particles of different sizes could provide some meaningful insights into the outburst process and improve our understanding of the outburst mechanism. First, combining the diffusivity of coal of different particle sizes and the distribution function of broken coal, we present a gas desorption model for fragmented gas-bearing coal that can quantify gas desorption from coal particles within a certain range of size. Second, the gas desorption rate ratio is defined as the ratio of the gas desorption rate from coal being crushed to that from coal before breaking. The desorption rate ratio is mainly determined by the desorption index (γ) and the granularity distribution index (α). Within the limit range of coal particle sizes, the ratio of effective diffusion coefficient for coal particles with different sizes is directly proportional to the reciprocal of the ratio of particle sizes. Under uniform particle size conditions before and after fragmentation, the gas desorption rate ratio is the square root of the reciprocal of the effective diffusion coefficient. The gas desorption model quantitatively elucidates the accelerated desorption of adsorbed gas in coal during the continuous fragmentation process of coal during an outburst. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a violent mechanical instability of gas-containing coal, and its mechanism is crucial for prevention of gas dynamic disasters. The macroscopic instability of gas-containing coal is the result of the expansion and connection of micro cracks driven by unloading induced tensile stress and gas pressure. This study is aimed at solving the problem of crack instability of gas-containing coal during mining unloading. To this end, first, the governing equations of unloading-induced tensile stress and gas pressure evolutions in cracks were built, and the dynamic evolutions and main controlling factors of tensile stress and gas pressure were revealed. Furthermore, the evaluation indexes of crack stability under tensile stress and shear stress were established, and the variation of crack stability during unloading was investigated. Finally, the phase diagram of crack failure mode was drawn, and the transformation of crack failure mode was analyzed. The results show that unloading-induced tensile stress is generated when θ > 54.73°, and it rises gradually with the increase of original stress, unloading rate, unloading time and crack size and dip angle. Overall, gas pressure in cracks increases first and then decreases during unloading. It increases gradually with the increase of initial gas pressure, gas desorption rate and original stress, while it decreases as the unloading rate gets higher (k from 10−4 to 10−2 s−1) and the dip angle gets larger (θ from 50 to 90°). With the increase of unloading ratio, the crack stability declines slowly first and then plunges rapidly until the crack finally becomes unstable. In addition, burial depth of coal seam, unloading rate, gas desorption rate and mechanical strength of coal have significant effects on this process. Different factors have important influences on the crack failure mode. With the increase of burial depth from 100 to 2000 m, the crack failure mode gradually transforms from tensile failure to shear failure, and the higher the unloading rate is, the more easily tensile failure occurs. With the increase of cohesion (0.1–2 MPa), the scope of the shear failure zone shrinks gradually, and the crack is less prone to shear failure. The desorption rate has a notable effect on the scope of the non-failure zone. Under a higher gas desorption rate, the scope of the non-failure zone becomes smaller, which is due to the greater gas pressure in crack for a given time. The research in this paper has certain reference value for further grasping of the instability failure of gas-containing coal and the mechanism of coal and gas outburst. [Display omitted] • Evolutions of mining-induced tensile stress perpendicular to crack surface and gas pressure in crack were modeled. • Stability of crack under both tensile and shear stresses were evaluated and failure criteria were put forward. • Phase diagram and its evolution with the influencing factors were drawn to present the transformation of crack failure mode. [ABSTRACT FROM AUTHOR]

When rockbursts and coal and gas outbursts simultaneously occur in a coal mine, changes in gas adsorption (concentration of ambient methane) and displacement of coal and rock must occur. The co-associated minerals in coal reservoirs can affect the mechanical properties and methane adsorption capacity, which are commonly disregarded. It is important to construct compound molecular structure models of coal and rock and conduct molecular dynamic simulations to gain a microscopic understanding of underground disasters. In this work, the molecular structure models of anthracite and coking coal–rock compound models containing different contents of calcite and kaolinite were constructed, and the methane adsorption amount and mechanical properties considering temperature, pressure, and mineral contents were simulated and analysed. The results showed that the methane adsorption amount of the compound models increased rapidly, then increased moderately, and stabilized eventually with increasing adsorption pressure, and the Langmuir fitting findings were good. The saturation adsorption amount of methane in the coal models linearly decreased with increasing temperature, while the methane adsorption heat increased. The presence of minerals adsorbed a certain amount of methane, and the methane adsorption amount increased with increasing mineral contents. The mechanical properties of coal molecules changed when mineral molecules such as calcite and kaolinite were present, which had opposite contribution effects. The addition of kaolinite minerals to the coal molecular model always increased the bulk modulus and shear modulus, while the addition of calcite decreased the bulk modulus of the anthracite, causing an increase in the brittleness of the models. The results of the study further explain the adsorption behaviour and mechanical properties of methane in coal and minerals. [ABSTRACT FROM AUTHOR]