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Coal-gas outburst is a violent phenomenon of energy release and strong dynamics is generated in underground coal mining. Coal-gas outbursts are accompanied by strong coal pulverization, and the gas desorbed by pulverized coal will in turn promote the development and spread of outbursts. Quantitative study on the role desorption gas played can improve the disaster-causing mechanism of coal-gas outburst and guide the work of outburst prevention and disaster reduction. Based on established gas emission model, the gas desorption is combined with the outburst process and its dynamic characteristics in this paper. The energy contribution and outburst dynamics with desorption gas are quantitatively studied under various desorption conditions, verified by outburst experiments. The results indicate that the energy contribution of desorption gas is 15.49–58.59% for JG71 coal, 4.49–57.26% for JG82 coal. The coal pulverization in the JG82 coal seam has a great influence on the energy contribution of desorption gas. After correction of gas desorption, the energy contribution increases by ∼20%. The maximum energy release rate (MERR) of the outburst with 0.075–0.15 mm JG71 coal sample reaches is 2.87 times that of the 1–2.36 mm coal sample at 3 MPa, and even 5.72 times that of the JG82 coal sample. For the outburst experiments, the energy contribution of desorption gas is 31.97–40.06% with 1–2.36 mm coal, 50.29–52.76% with < 0.25 mm coal. This paper presents a novel quantitative study defining the role of gas desorption in outburst energy and dynamics, applicable to disaster risk assessment and an accurate method to derive the energy contribution of desorption gas in experiments is proposed. [ABSTRACT FROM AUTHOR]

• A two-phase gas-solid flow model for outburst shock waves is established. • Three parameters of the release of gas pressure potential energy are defined according to the energy transformation during gas expansion process. • The gas dynamic characteristics of two-phase flow in the initiation stage are quantitatively studied under the four factors: initial gas pressure, particle volume fraction, particle size and outburst hole size, which is verified by experiments. The dynamic effect of coal and gas outburst seriously threatens the safety of underground equipment and workers in mines. In this paper, the models for outbursts are established, and according to the energy transformation during gas expansion process, three parameters of the release of gas pressure potential energy are defined and the gas dynamic characteristics of two-phase flow in the initiation stage are quantitatively studied under the four factors: initial gas pressure, particle volume fraction, particle size and outburst hole size. The gas dynamic characteristics in numerical simulations are verified by experiments. The results show that for the influence of the factors on the gas dynamic characteristics of two-phase flow, initial gas pressure >outburst hole size >particle volume fraction ≈particle size. The solid particles block the pressure potential energy release from chamber, so does the caliber. However, the maximum of pressure drop occurs when the caliber is 80% of the chamber, which explains the outbursts in mines are so violent due to the caliber smaller than the internal space. This paper provides a novel quantitative research method of gas dynamic characteristics of coal and gas outburst, which is of guiding significance to the design of outburst prevention facilities and emergency rescue measure. [ABSTRACT FROM AUTHOR]

Coal is an important energy resource internationally. However, accidents have severely restricted the clean and safe production of coal resources. Among such accidents, coal and gas outburst accidents are a kind of coalmine disaster with high destructive power. Previous research on coal and gas outburst accidents mainly focused on gas factors but ignored the role of human factors. This paper analyses the coal and gas outburst accidents in China from 2008 to 2018 and studies its macroscopic laws. To better understand the causes of coal and gas outbursts, this paper uses the 24modelel to analyse coal and gas accidents and suggest measures for accident prevention from the two aspects 'gas' (risk control) and 'humans' (behavioural safety). Macroscopic law research found the following: (1) March, May, July, and August are the predominant months for accidents. (2) The second to fourth hours of the working hours and the first hour before the end of work are the peak periods for accidents. (3) Guizhou, Hunan, Henan, Sichuan, Yunnan, and Chongqing are the provinces with the most coal and gas outburst accidents. (4) An overall 75.82 % of accidents occurred in the driving face, and 81.08 % of accidents occurred in coal and gas outburst mines. (5) Blasting, drilling, driving, and coal falling are the main inducing factors. Case analysis of accidents found the following: (1) Human error is the leading cause of accidents. Among the errors, the lack of strict enforcement of outburst prevention measures, illegal command, and the illegal operation of miners are the main unsafe acts. (2) Safety knowledge and awareness of miners is not generally high, and serious habitual violations and unsafe psychologies exist. (3) The gas comprehensive prevention system and supervision system in the coal mine safety management system (SMS) can be easily operated improperly, and the safety training system and emergency management system can be absent. (4) Coalmine enterprises seriously lack safety culture. [ABSTRACT FROM AUTHOR]

Analyzing the causes of accidents, excavating accident paths, and applying accident prevention are important tasks in safety management. Focusing on coal and gas outburst accidents, this study examined the primary accident path and conducted applied research on the reasoning of the accident case. First, combined with the obtained accident causes, a coupling analysis of the causes of coal and gas outburst accidents was conducted. Second, using the method of data mining coupled with Apriori algorithm, the coupling relationship between each cause module of the coal and gas outburst accident was obtained, and consequently, a path map of the coal and gas outburst accident was drawn. Third, a Bayesian network model for the causes of coal and gas outburst accidents was established based on the accident path map and the probability of occurrence of each cause. Finally, considering the safety concept element (SC1) as an example, the Bayesian network model was used to conduct a sensitivity analysis of accident causes. Thereafter, considering the coal and gas outburst accident of the Sanjia Coal Mine in Guizhou Province as an example, probabilistic reasoning research on the cause of the accident was conducted. The application results showed that (1) under normal conditions, there are approximately 797,280 accident paths for coal and gas outbursts. Following data mining, 188 main accident paths were found. (2) Sensitivity analysis determined 19 factors that were sensitive to safety concept elements (SC1), of which the three most sensitive factors were (i) resource management system procedures (SM7), (ii) safety policy (SM1), and (iii) safety training system procedure (SM8). 13 paths exhibited a sensitivity ≥0.5%, of which 7 exhibited strong sensitivity. (3) The absolute accuracy rate of accident cause reasoning in the Sanjia Coal Mine in Guizhou Province was 71.43%, while the relative accuracy rate was close to 100%. Thus, it was concluded that: (1) the accident path mining method proposed in this paper is feasible for main accident path mining. (2) The Bayesian network model for the causes of coal and gas outburst accidents established in this study can be practically applied for the sensitivity analysis of accident causes and exhibits high reliability in the probabilistic reasoning of accident causes. The results of this study is expected to aid in the prevention of coal and gas outburst accidents, and provide reference and help for the path mining of other accident causes and the probabilistic reasoning of accident causes. [ABSTRACT FROM AUTHOR]

The coalbed methane energy resources waste, environmental pollution and miner's health remain challenging problems in worldwide mining operations accompanied by coal-gas outbursts; thus, the safe and efficient yield of these resources plays a vital role in the world's energy market. The tectonic coal development is a significant geological feature of coal and gas outburst sites, and the study of coal and gas outburst mechanism should be based on more precise geological occurrence characteristics. In this paper, the geological characteristics of coal and gas outburst sites were analyzed. The failure law of coal body under the stratified occurrence of tectonic coal and raw coal seam with different thickness was studied. The mechanical equilibrium equation of non-synchronous deformation of tectonic coal and raw coal was constructed, the initiation criterion of coal and gas outburst was established based on the support body model, and a case study was carried out. The results show that with the thickness increase of the tectonic coal seam, the peak displacement of the driving face increases. The outburst start-up and development of coal and gas outburst, coal and gas extrusion outburst and coal and gas collapse outburst were deeply analyzed, as well as the internal reasons of the occurrence of coal-gas outburst portent are explained, based on the support body model. The development process from the start to the end of coal and gas outburst is revealed, and a new perspective is proposed for the prevention and control of coal and gas outburst: improving the strength of support body is also of great significance for the prevention and control of coal and gas outburst. • Asynchronous deformation induced coal-gas outburst mechanism. • The initiation criterion of coal and gas outburst was established. • The start and development process of the three types outburst were revealed. • New theoretical method and direction into dynamic disaster monitoring. [ABSTRACT FROM AUTHOR]

Mining protective coal seams is an effective approach for coal and gas outburst control, but a large amount of gangue is usually produced when mining a thin protective seam and dealing with the gangue can be a challenging task. In the paper, a novel integrated mining system by extracting two panels simultaneously was demonstrated in the 12th coal mine of the Pingdingshan coalfield: one panel was mined in the thin protective coal seam, and the other one was mined in the protected coal seam. The thickness of the protective seam was only 0.8 m, but the mining height reached 1.8 m, indicating that there was about 1 m thick roof stone being mined. An underground coal preparation plant was built, and the mixture of coal and gangue from the protective panel was sorted underground. The sorted coal was delivered to the surface stockpile, while the waste rock was directly backfilled into the gob of the panel in the protected coal seam. Thus, waste rock did not need to be lifted up to the surface, which can reduce material handing cost and minimise mining footprint on the surface. In order to prevent the migration of pressure relief gas into the working areas of the protective coal seam, the main reasons causing gas exceedance in the upper corner of U-shaped ventilation system were investigated, and then the Y-shaped ventilation system and various gas extraction boreholes were implemented to effectively control the pressure relief gas. Numerical models were developed to assess the effect of applying gob backfilling during the extraction of two panels in parallel. The results show that the gob backfilling can reduce the surface subsidence and horizontal movement, which is very helpful for the protection of ground water and surface infrastructures. [ABSTRACT FROM AUTHOR]

For the characteristics of high frequency and strong suddenness of dynamic disasters in deep coal mines, the traditional detection and evaluation techniques applied to shallow coal mine failed to accurately judge the risk degree of disasters. Therefore, it is of great significance to use advanced detection technologies and appropriate evaluation methods to improve the accuracy and efficiency of risk assessment in the process of coal mining. The present paper applies the rapid detection and multi-source dynamic detection technologies used in the field of mining with the purpose of improving the reliability of detection technologies for typical dynamic disaster. In this study, the data fusion technology was used to analyze data obtained from laboratory experiments, engineering survey, detection and historical data, so as to form the final dynamic and static indicators. Then, the new combined evaluation models with time series of coal and gas outburst as well as rock burst were established respectively to carry out the comprehensive risk evaluation using the least-squares method and the time-varying weight method. After the comprehensive analysis on the results of the above two evaluation models, the risk areas of the typical dynamic disasters were judged and classified. Finally, the evaluation models were coded to build an early-warning software platform that could achieve automatic evaluation and the actual 3D visualization of coal mining areas. The early-warning software platform was applied to risk assessment of dynamic disasters in Gengcun Coal Mine in Yima City, Henan Province, China. The results of the 6-month experiment showed that the risk assessment accuracy and reliability of the proposed evaluation models was 100% and 90% respectively, which indicates that the newly developed approach is reliable and can be recommended for applying in more coal mines to improve the process safety risk control. [ABSTRACT FROM AUTHOR]

• 1 Stress distribution was recalculated by stress-softening and prominent dilatancy. • 2 Gas-bearing coal under impact loading was a compaction followed by ruin process. • 3 Measures principles were mainly aimed at fracture and stress-concentration areas. • 4 Preliminary goal-reaching parameter based on principles was proposed and applied. As the increasing demand for coal and the depletion of shallow coal resources, the tendency of coal mining in China is progressively shifting to deeper areas, which brings about a circumstance where the frequency and intensity of coal and gas outburst accidents have further deteriorated. Nowadays, there are many outburst prevention measures being applied on site, while this disaster still occurs from time to time. Therefore, to better improve the preventing outcome of them, this paper came forward with some principles, based on the mechanical theory of deep mining and damage evolution procedure concerning coal containing gas underneath the impact load. The primary conclusions were as follows: the stresses, strains, and associated widths in the three areas, namely fracture area, plastic area, and elastic area, in front of the working face were recalculated by considering stress-softening and dilatancy of coal mass. The damage evolution of gas-bearing coal mass under impact loading was displayed in the LS-DYNA numerical simulation as a process of compaction followed by destruction. The proposed principles could be split into two perspectives, one for the fracture area, specifically, to minimize the descending of the coal mass's strength in this area; another for the stress-concentration zone, namely, to abate the thrust due to extrusion and deformation, on the coal body which was located in the front area. According to those principles, a preliminary goal-reaching parameter was proposed and it was implemented for the evaluation of advanced drilling. The results will cooperate to rationalize the design and layout of local outburst prevention measures, thereby enhancing the efficiency of control and ameliorating the deteriorating scenario of outburst disasters. [ABSTRACT FROM AUTHOR]

The low permeability of coal seams is the primary difficulty for gas production. Hydraulic cutting is one of the most effective technologies for increasing permeability and stimulating gas production. This paper investigates the hydraulic cutting styles in boreholes and the optimal equivalent borehole diameter after discharging the pulverized coal particles. This paper introduces the hydraulic cutting equipment and cutting styles and determines that the cutting cylinder is the most suitable style for a soft coal seam that is prone to gas outburst. The numerical simulation suggests that the radius of the stress-relaxed range increases quickly when the equivalent borehole radius is small, and the rate slows when the borehole radius becomes large. Then the result was verified by a field experiment in Pingmei coalfield. The gas drainage flowrate and stress-relaxed range greatly increased after initially outputting 2 tons of coal from the borehole, whereas the second outputting 2 tons of coal did not appreciably increase the stress-relaxed range and the flowrate. Too much or too little coal discharge is not optimal, and the borehole radius is recommend to be between 0.3 m and 0.4 m after the hydraulic cutting considering the pros and cons in Pingmei coalfield. The results are helpful for the engineering application of the hydraulic cutting technology, for increasing coal seam permeability, and for stimulating gas production. The method for finding the optimal coal discharge in this paper is also helpful for other coal mines. [ABSTRACT FROM AUTHOR]

Protective layer mining (PLM) is one of the most effective regional measures against gas outburst, and the most preferred technique to prevent and control gas outburst. This paper attempts to disclose the effect of close-range PLM in Hezhuang Coal mine, Western Henan Coalfield. Firstly, coal seam I 7 was selected as the lower protective layer, and coal seam II 1 was taken as the overlying coal seam. Considering the situation of actual production, a simulation model for lower PLM was established on Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D). The established model was adopted to simulate the mining of the working face 11030# of the lower protective layer (I 7) at the mining depth of 50, 100, 200 and 250 m, respectively. Through the simulation, the authors summarized the laws of stress evolution and deformation of the working face 11030# of the overlying coal seam (II 1) in the upper part of the stope. The research results are as follows: After the lower PLM, the stress distribution of 11030# II 1 in the strike direction can be divided into three parts from the open-off cut to the stopping line: original stress zone, pressure relief zone and stress concentration zone; the stress distribution of 11030# II 1 in the dip direction can also be divided into the three parts from the top to the bottom. However, the stress distribution in the dip direction is less uniform than that in the strike direction. Meanwhile, the 11030# II 1 had an unobvious deformation in the original stress zone, compressive deformation in the stress concentration zone, and expansive deformation in the pressure relief zone; the maximum expansive deformation was observed in the middle of the pressure relief zone. The research results lay the theoretical basis for engineering application of lower PLM. [ABSTRACT FROM AUTHOR]

• A new method for directly determining the coal seam permeability around roadway is proposed. • The coal seam permeability around roadway is divided into four stages. • The results are able to determine the sealing depth of in-seam borehole and guide the gas disaster preventions. Changes in coal seam permeability induced by roadway excavation may lead to many engineering problems, such as low gas extraction concentration in in-seam boreholes, coal and gas outburst disasters, and gas emissions in the process of coal roadway excavation. To solve these problems, this paper proposed a new method to directly determine the distribution of coal seam permeability around roadways. For this purpose, method principle was analyzed, equipment was developed, and field experiments were carried out in Baiyangling coal mine. Finally, the engineering applications of the method were analyzed, and a field application effect experiment was conducted to verify the new method. The results showed that the coal seam permeability around roadways was divided into four stages. In stages I and II, permeability increased, in stage III, permeability decreased, and stage IV maintained the initial permeability of the coal seam. These test results will aid in increasing the gas extraction concentration of in-seam boreholes and will guide the design of gas control areas. Using the verification experiment, it was concluded that the new method is able to accurately determine the redistribution of coal seam permeability caused by roadway excavation and provide theoretical guidance in solving engineering problems. [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]

Abstract: The effect of drainage and outburst elimination are obviously different between soft and hard coal sub-layers under per-drainage of coal seam, and hence, the location of per-drainage borehole related to the soft sub-layer is the key that determines whether the outburst can be eliminated in whole coal seam. This paper discusses the differences between soft and hard coal from fields of pore radius distribution, pore structure, surface area and permeability, and the per-drainage effect was numerical simulated under different distance between per-drainage hole and soft sub-layer. Moreover, the optimum hole arrangement of outburst elimination is given in the paper. The result of research shows that the macro-pore with aperture that is larger than 50,000nm, is dominant at about 50% in hard coal, correspondingly, the micro-pore with aperture less than 100nm accounts for about 41.23% in soft coal. The surface area of soft coal is 1.6 times as that of hard coal. The residual gas content in soft coal sub-layer is obviously more than it in hard coal sub-layer under the same pre-drainage condition. To 5.5m-thick coal seams which top is a soft coal sub-layer of 0.5m-thick, pre-drainage gas can eliminate danger of gas outburst in whole coal seam at the seam time after 455days drainage when the per-drainage holes are 1.0m distances from the soft coal sub-layer. [Copyright &y& Elsevier]

• Increasing water content in coal can reduce the elastic modulus and coal strength. • Water in coal reduces the initial gas diffusion speed and change adsorption constant. • Loosening blasting before water injection can promote cracks and water infusion. • Water injection after blasting reduces the gas outburst potential greatly. Gas outburst is a serious disaster in deep mining. In this paper, we first study the effect of water content on gas outburst, and found that increasing the water content of coal can significantly reduce the elastic modulus and uniaxial compressive strength, which is beneficial in reducing the peak stress concentration in the roadway and pushing the peak stress deeper. We also found that by increasing the water content of the coal seam, the firmness coefficient increases, while the initial speed of gas diffusion decreases simultaneously. Increasing the water content of the coal seam can also change the Langmuir adsorption constant of coal, reduce the a value of the limit adsorption capacity, increase the b value of the adsorption capacity, and reduce the amount of desorption gas in the coal. These studies indicate that increasing the water content of coal can significantly reduce the risk of gas outbursts. To increase the water injection volume, the technology of promoting coal seam infusion by blasting has been proposed. Blasting promotes equilibrium distribution in the stress field and generate new fractures, which promotes the water injection. In the 8th coal mine in the Pingdingshan coal field, the test results indicate that after loosening blasting, the water injection of a single borehole increased by an average of 54.6 times under the injection pressure of 3–7 MPa. The water content of coal is improved significantly, and the prediction index value of the gas outburst is reduced significantly. This technology is important for gas control in similar coal mines. [ABSTRACT FROM AUTHOR]

• Revelation of the main unsafe factors of working face crossing the fault. • The response characteristics of AE and EMR to the driving face crossing a fault. • The method of detecting and real-time monitoring the fault by AE and EMR. In this paper, the response characteristics of acoustic emission (AE) and electromagnetic radiation (EMR) to the driving face crossing a fault are analysed and tested. The method of real-time monitoring of the fault in front of the driving face by AE and EMR form coal is put forward. The results are as follows: (1) When passing through the fault, the large increase of stress in mining and fault superimposed stress, which makes it more likely to accumulate and release energy in the coal seam, is the main reason for the high risk of coal and gas outburst. (2) During crossing the fault, AE and EMR signals vary in stages, both the intensity and amplitude of the main frequency points of AE and EMR increase first and then decrease. (3) When the driving face is close to the fault, the main frequency of AE and EMR signals decreases and the frequency band concentrates. The main frequency ranges of AE and EMR signals are 3.66–22.5 kHz and 0.3–17.3 kHz, respectively. (4) The amplitudes of the main frequency points of the AE and EMR signals to the fault response. The change of the main frequency amplitude of AE and EMR signals can be used as the response index of the fault, and the response range is about 10 m. The research provides an effective monitoring method for structures in coal seam, which is of great significance to the safety production and rapid excavation of coal mines. [ABSTRACT FROM AUTHOR]

Abstract: An outburst of coal and gas is a major hazard in underground coal mining. It occurs when certain conditions of coal stress, strength and gassiness are met. The outburst is recognized as a two-step process, i.e. initiation and development. Each step has its own characteristics and requires different approaches in numerical modeling. As a first step, the initiation process of the outburst is numerically simulated with a coupled simulator. The simulator models the deformation and failure of a coal seam, adsorption and desorption of gas and flow of gas and water in coal. The simulator is developed by linking and sequentially executing two numerical codes: FLAC3D and COMET3. FLAC3D is a widely used commercial code designed for engineering mechanics computation, while COMET3 is a well-established code to model gas and water production from desorption-controlled reservoirs. In this paper, these two codes are linked and sequentially executed through coupling modules including a user-designed C++ module and several modules developed with the build-in FISH language in FLAC3D. These coupling modules are used to transfer relevant data between the codes. A simple example to simulate the initiation of the outburst is also presented in this paper to demonstrate the capability of the coupled simulator. [Copyright &y& Elsevier]

[Display omitted] • A non-coagulant long-time sealing material with excellent performance was developed. • The modified component was successfully inserted into the material group. • The sealing performance of the material is much better than the traditional material. Gas is produced during coal formation and is a major threat to the safety of coal mining operations. Gas-related accidents, including gas explosions and coal and gas outbursts, are particularly dangerous. However, gas also has great potential as a clean energy source. Therefore, effective gas extraction is essential for preventing accidents and utilizing its potential. The sealing material used for gas extraction boreholes directly affects the quality of the seal and, consequently, the efficiency of gas extraction. To address the limitations of cement-based and polyurethane sealing materials, this paper presents a novel non-coagulant long-term sealing material. Firstly, the optimal material ratio was determined through experiments involving constant temperature and weightlessness. The optimum proportion of modifier to be added is obtained by constant temperature weight loss experiments. Secondly, microscopic analysis revealed that the modified component was effectively inserted into the group. To simulate the underground gas extraction conditions in coal mines, drainage and plugging experiments were designed. The results showed that the new material had a 100% plugging performance during the observation time, while the plugging performance of cement-based and polyurethane sealing materials significantly decreased under the same experimental conditions. Finally, the non-coagulant long-term sealing material was applied in the gas drainage experiment of tunnel 53,081 at Changping Coal Mine. Compared to the original cement-based sealing material, the gas extraction concentration of the borehole increased by 16.5%. These results suggest that the newly developed sealing material is suitable for gas extraction and has significant potential for practical application. [ABSTRACT FROM AUTHOR]

The gas outburst potential is typically high when roadways uncover the gassy coal seams associated with outburst hazards. This paper describes the complex stress and displacement evolution that occurs in roadway tunnelling through coal seams under different original stress conditions, which is helpful for better understanding how the stress leads to an outburst accident, and proposing some relevant control measures. The stress concentration is primarily resulted from the strength difference between coal and rock, and the abutment stress increases gradually as the forward residual rock thickness decreases when the roadway excavation enters a coal seam from rock stratum. When the abutment stress is sufficiently high, the forward residual rock will suddenly be broken after the next cycle, and the stress will decrease rapidly to release elastic energy and generate new fractures. Meanwhile, the coal and rock in front of the heading face will move rapidly towards the free space. Therefore, an outburst accident may occur immediately when the gas pressure in front of the heading face is high enough. Once the outburst accident is triggered, the gas expansion energy and elastic energy distant from the heading face will contribute to developing the accident. The stress perpendicular to the roadway is the primary factor causing such outbursts. The abutment stress concentration will be higher when the maximum original stress is perpendicular to the roadway, which will increase the outburst potential. Therefore, it is suggested that arranging roadways parallel to the maximum original stress, if possible, can also reduce the outburst threat. Outbursts are triggered by the elastic energy near the heading face and developed by the gas expansion energy and elastic energy distant from the heading face. In this study, the method of “strong-weak” coupling circle-layers was proposed to reduce the outburst hazard when roadways excavated through gassy coal seams. This method was proven to be valid by the comparative experiment. The weak circle can be realized by boreholes and hydraulic cutting. It could not only reduce the elastic energy near the heading face but also prevent the distant energy from contributing to the outburst accident. In addition, the strong circle can be made by injecting water slurry and applying anchors. It could help to reduce the strength difference between coal and rock, thereby decreasing the local stress concentration, resisting the residual outburst energy near the heading face, and preventing the outburst from being triggered. The “strong-weak” coupling structure was successfully applied in Bailongshan coal mine when uncovering the #7,8 coal seam. This structure can effectively promote gas drainage, reduce the coal and gas outburst risk. As a result, the mining efficiency improves a lot, and the uncovering time greatly decreases from the expected 2 years to 5 months. [ABSTRACT FROM AUTHOR]

Closed pores, which have been confirmed to exist in coal, have a dual significance in illustrating the tremendous gas outburst content in coal mining and ascertaining the exact methane content in coal, although it is unknown if this methane can be exploited and utilized. In this paper, the principal objective is to explore the evolution and formation mechanisms of closed pores, regarding which there is a lack of knowledge and understanding. We investigated the varieties of closed pore contents in coalification, combining the small angle X-ray scattering (SAXS) and liquid nitrogen adsorption methods (LNA), and simultaneously analyzed the surface topography of closed pores by scanning electron microscopy (SEM). The closed pore volume represents a low-high-low evolution trend during coalification owing to the appearance of gas pores in the pyrogenic hydrocarbon generation stage. However, the specific surface area increases uniformly due to the increase in the number of micro-pores. The tectonic deformation effect impels the increase of closed pores, which reflected in the larger pore volume and specific surface area in strong deformation coal than that in weak deformation coal. The genesis of closed pores can be summarized as a result of heterogeneous matrix shrinkage, the tectonic compressive stress and the tectonic shear stress. Finally, this work provides a novel method to investigate the mechanism of geological CO 2 sequestration. [ABSTRACT FROM AUTHOR]

In underground coal mining operations, gas content in a coal seam is used to assess the risk of coal and gas outbursts. A broadly standard procedure has been adopted to estimate the gas content with coal core in the coal mining industry. In soft or friable or geologically complex sections of coal seams, it has proven to be extremely troublesome in taking coal cores as cores can be severely fragmented or sometimes it is simply impossible to core with conventional coring methods due to poor borehole stability, leaving coal cuttings from boreholes as the only practical alternative for sampling. When coal core is replaced with coal cuttings, concerns have been raised about the validity and accuracy of the procedure used for gas content determination with coal core, in particular with respect to the sampling method of coal cuttings and gas content calculation due to faster initial gas desorption rates of coal cuttings. The standard procedure for gas content determination with coal core has been modified to determine gas content of coal with coal cuttings from underground boreholes. This paper describes the modified method and presents and discusses the results when it was trialed in a soft coal seam at Huainan, China. [ABSTRACT FROM AUTHOR]

Researches on the failure process of coal during coal and gas outbursts are of great importance in underground mining. Based on the theories of coal spherical shell failure, coal spallation and coal powder pneumatic conveying, the flow state and transport mechanism of coal and gas outbursts were studied. This paper concludes that in the outbursts' development stage, with the front of the outburst being gradually exposed to air, the gas gradient behind the outburst acts as a reciprocating air knife of high pressure that cuts the ejected coal powders, thus forming a plug flow with a high solid-gas ratio and a low speed. By referring to the laboratory results from other researchers and data recorded from field outbursts, an appropriate explanation was obtained for the wave-like coal powder distribution and the sound explosion using the dense-phase plug flow pneumatic conveying theory, which conversely confirms the possible existence of the plug flow. [ABSTRACT FROM AUTHOR]

• Application problems of the Hodot's energy criterion were found. • The characterization methods of coal deformation energy and gas expansion energy were determined. • The influence mechanism of coal deformation energy on outburst was discussed. • A spatiotemporal evolution model of the release of outburst energy was proposed. Coal deformation energy and gas expansion energy are the main power sources for coal and gas outbursts (called outbursts for short). Although the combined effect hypothesis of the outbursts has been widely recognized, the contribution of the coal deformation energy to the outbursts is always despised compared with the gas expansion energy. In this paper, the famous energy criterion for outburst trigger proposed by Hodot was reviewed systematically. It was found that Hodot's energy criterion pays more attention to coal deformation energy and almost ignores gas expansion energy, different from the views in most literatures related to outburst energy. The characterization methods of coal deformation energy and gas expansion energy were summarized. Experimental results show that the deformation energy of tectonic coal with high outburst risk has a power function relationship with stress and the power exponent is between 1 and 2, indicating that the deformation properties of tectonic coal is between soil and elastomer. The releasing mode of the deformation energy of tectonic coal is different from that of intact coal, that is, the deformation energy of intact coal and tectonic coal mainly releases outward and dissipates inward respectively after they are damaged. The coal deformation energy was proved to have the same order of magnitude with the gas expansion energy, about 102 to 103 kJ/t. A new energy release model for whole process of outbursts was established to discuss the role of coal deformation energy in actual outbursts. It was finally determined that the pre-release of deformation energy of tectonic coal is the necessary condition for the accumulation of the free gas expansion energy and outburst trigger. [ABSTRACT FROM AUTHOR]

• Fission gas burst release mechanisms under PWR fuel defective conditions are revealed. • The short-term fission gas release process has been simulated using the VOF method. • Sensitivity analysis of short-term release has been performed to find the critical factor. • Steam generation rate in the gap is the most relevant parameter for the short-term fission gas release rate. During the reactor operation, it is necessary to estimate the amount of fission gas release from the fuel-cladding gap to the coolant. The most common method for predicting the fission gas release process is based on the first-order kinetic model, which is used for analysis of the steady-state stage of the fission gas release. The purpose of this paper is to reveal the fission gas burst release mechanisms under PWR fuel defective conditions. The fission gas release process has been simulated using the CFD method. A validation calculation has been performed to validate the method. The simulation results are discussed in the paper including the two-phase flow evolution and the amount of fission gas release over time. For short stage after the equilibrium, the gas phase is released in the coolant by the form of jet. While for long stage after the equilibrium, the gas phase is entrained into the coolant by the vortex in the defect site. Sensitivity analysis of short-term fission gas release has been performed to evaluate which parameter critically influenced the release process. The results show that steam generation rate in the gap is the most relevant parameter for the short-term fission gas release rate. [ABSTRACT FROM AUTHOR]

Composite coal seam is a kind of special coal seam that is formed after complex geological structure action, including a tectonic coal sublayer. Gas can't be drained easily from this coal seam, which cause coal and gas outburst accidents (CGOA) occur frequently. A new hydraulic flushing technology was proposed in this paper to improve gas drainage. Accordingly, a new set of mechanical equipment, technology process, roadway layout system and gas extraction monitoring system are provided by drawing lessons from the Guhanshan coal mine. The stress variation and damage failure were investigated by numerical simulation on the basis of flushed-out coal and borehole geometry. The gas drainage effect, coal seam permeability and outburst risk-sensitive indexes of the special coal seam were obtained by field measurements. According to the results, hydraulic flushing can enlarge the stress unloading and plastic damaged zone around boreholes dramatically, and the coal seam permeability can be increased largely. • A new technology was proposed to enhance gas drainage in the special low-permeability composite coal seam with a tectonic coal sublayer. • The stress damage and permeability increase mechanism of hydraulic flushing were revealed by theoretical analysis and numerical modeling. • Abundant field experiments were carried to investigate and verify the gas drainage effect in the coal seam after hydraulic flushing. [ABSTRACT FROM AUTHOR]

Coal seam gas drainage is not only the effective measure for gas outbursts control, but also has the great significance for the use of gas energy and reduction of greenhouse-gas emissions. Coal permeability is the key parameter of gas drainage, and the permeability model of mining-damaged coal has the important theoretical guidance value for gas drainage in low permeability coal seams. In order to better understand the permeability model of damaged coal, the key points during the establishment of permeability model, which are the selection of coal structure, selection of boundary conditions and influencing factors, were first analyzed. Then, the expression and application of permeability models of mining-damaged coal in recent 20 years were reviewed. According to the contribution of new fractures, the permeability models of mining-damaged coal were divided into the empirical permeability models (EPM), volumetric strain permeability models (VSPM), plastic strain permeability models (PSPM) and damage variable permeability models (DVPM). After that, the quantitative characterization of key parameters, scientific issues of permeability models of mining-damaged coal were reviewed and discussed. Finally, the prioritization of 18 indexes affecting the permeability were determined, and the potential research directions of permeability models of mining-damaged coal were pointed out. The results of this paper can provide the theoretical guidance for the improvement of permeability model of mining-damage coal in the future. • The expression and application of permeability models of mining-damaged coal in recent 20 years were reviewed. • The quantitative characterization of key parameters, scientific issues of permeability models of mining-damaged coal were reviewed and discussed. • The prioritization of 18 indexes of factors affecting the permeability were determined. [ABSTRACT FROM AUTHOR]

This paper addresses airflow stabilization in airways induced by outburst gas flows after the disappearance of the outburst shockwave. Based on fluid dynamics theory, a mathematical model of an unsteady airflow in an airway was established. The formation conditions of additional force and gas ventilation pressure with their dynamic influences on airflow in an airway were investigated. The results of this study indicate that an additional force is created by the variation of the airflow rate after outbursts, and this force hinders changes to the airflow and maintains the airflow in its original state of motion. The gas ventilation pressure can be observed as an increase in the potential energy after coal and gas outbursts. For upward ventilation, the direction of gas ventilation pressure is positive and assists the ventilation. However, for downward ventilation, the direction of gas ventilation pressure is negative and hinders the ventilation. This hindrance may lead to an airflow reversal in the main airway. [ABSTRACT FROM AUTHOR]

• The effect of SiO 2 nanofluid with different concentrations on water adsorption capacity of coal is obtained. • The wettability-enhancing effect of SiO 2 nanofluid with different concentrations is investigated by contact angle tests. • The effect of nanofluid concentration on the wettability enhancement of coal is revealed. Water-based SiO 2 nanofluid, a wetting agent, has been proved to be feasible in improving the control effect of coal seam water injection against coal and gas outburst. However, the effect of nanofluid concentration on the control effect is not clear. Therefore, it is necessary to investigate the wettability-enhancing effect of nanofluid with different concentrations. In this paper, firstly, the effect of SiO 2 nanofluid with different concentrations on water adsorption capacity of coal was obtained. Then, the wettability-enhancing effect of SiO 2 nanofluid with different concentrations was investigated by contact angle tests. Finally, the effect of nanofluid concentration on the wettability enhancement of coal was revealed. The results showed that increasing the nanofluid concentration can enhance the water adsorption capacity and wettability of coal. In addition, the wettability-enhancing effect of nanofluid gradually got worse with time, while the stability of wettability-enhancing effect gradually got better. Furthermore, the 0.5 wt% nanofluid had the best sustainability and stability of the wettability-enhancing effect, while 1.0 wt% had the worst sustainability and stability. The research achievements could provide important basis for the selection of nanofluid concentrations in coal seam water injection. [ABSTRACT FROM AUTHOR]

Accurate determination of coalbed methane (CBM) content can not only prevent and control coal and gas outburst disasters in mines, but also facilitate the prediction of coal seam methane resource reserves, which is significant for injecting carbon dioxide to enhance CBM recovery. The freezing core sampling technique proposed in recent years for accurate determination of the CBM content has faced temperature-pressure coupling effects on gas desorption in coalin a low-variable temperature environment. In this paper, we utilized a self-developed high-low temperature alternating adsorption-desorption experimental device to study the gas desorption process in coal under temperature-pressure coupling conditions in a low-variable temperature environment. The experimental results show that the main promotion and inhibition effects of temperature on the gas adsorption and desorption in coal in a low-temperature environment were consistent with those in an ambient temperature environment, i.e., the decrease of temperature promoted coal gas adsorption, while the increase of the temperature promoted coal gas desorption. It is found that throughout the temperature change process (TCP), there existed a 'pseudo-desorption' phenomenon, and the cumulative desorption amount showed a consecutive rising-declining-rising trend and finally stabilized. The whole process can be regarded as consisting of a main desorption process and a retrograde desorption process. In the main desorption process with the same TCP, the cumulative desorption amount and initial desorption rate increased with the increase of initial equilibrium pressure. In the retrograde desorption process, there existed a peak retrograde desorption rate, and the staged retrograde desorption amount conformed to the linear decrease law. A comprehensive analysis of the temperature-pressure coupling effect on gas desorption throughout the TCP was conducted. Thus, we proposed some considerations and precautions for the configuration and supply monitoring of the cold source in the process of the measurement of CBM content using the freeze-coring method. • Temperature-pressure coupling effect is studied at low/ambient variable temperatures. • Gas desorption amount shows a rising-declining-rising trend. • Pseudo-desorption occurs in the temperature change process due to the coupling. • Linear-decreasing stepwise retrograde desorption process shows a peak desorption rate. [ABSTRACT FROM AUTHOR]

Coal seam water injection is proved an effective method for preventing coal and gas outburst. However, the poor wettability of coal seam has resulted in poor effect of coal seam water injection in preventing coal and gas outburst. In addition, the coal seam is in a state of injected-uninjected cycle due to multiple water injection. Therefore, it is important to study the wettability of coal seam with adding wetting agent under multiple water injection. In this paper, the effect of cyclic water injection on the wettability of coal with different SiO 2 nanofluid treatment times was investigated through contact angle tests. The results showed that increasing the treatment time of SiO 2 nanofluid would enhance the wettability of coal. In addition, the wettability of nanofluid-treated coal samples gradually got worse with the number of cycles increasing. In general, the effect of SiO 2 nanofluid on enhancing the wettability was approximately 2 to 4.5 times better than that of water. However, the difference of wetting enhancement between water and SiO 2 nanofluid became less obvious as the number of cycles increased. Even though, the nanofluid-treated coal samples showed the better wettability than water-treated coal samples after 13th cycle. Furthermore, the potential application of SiO 2 nanofluid in coal seam water injection was proposed, which had important significance for improving the effectiveness of coal seam water injection. [ABSTRACT FROM AUTHOR]

Coal seam water injection is one of the important technical means to prevent mine disasters. It can effectively change the mechanical properties of coal, prevent dust, coal and gas outburst, and reduce gas concentration. In order to expand the range of coal seam water injection, make coal seam water injection more effective in wetting coal mass, and determine the effective type of ionic surfactant required for coal seam water injection, triaxial seepage experiments and spontaneous imbibition experiments are conducted on the long flame coal samples from Houwenjialiang Coal Mine in Ordos City, Inner Mongolia Autonomous Region, China in this paper. Moreover, the effects and mechanisms of different ionic surfactants on the seepage and wetting stages of coal seam water injection are studied, which provides a theoretical basis for on-site coal seam water injection. After analysis and discussion, it is found that the influence of cationic surfactants on the seepage stage of coal seam water injection first decreases and then increases while the influence of anionic surfactants on the seepage stage of coal seam water injection shows a gradually increasing trend. Furthermore, both cationic and anionic surfactants show more significant wetting effects as the concentration of the surfactants increases. In addition, the adsorption of cations to coal molecules is stronger than that of anions, and the water solubility of cations is better than that of anions. [ABSTRACT FROM AUTHOR]

• Release pressure and diameter determined the distribution range of smash district. • Filling level of LCO 2 directly control the distribution range of crack district. • LCO 2 filling parameters can be used to guide LCO 2 -PB field applications. Liquid carbon dioxide phase-transition blasting (LCO 2 -PB) is a promising alternative method for enhanced coalbed methane (ECBM) extraction and prevention of coal and gas outbursts. During the process of the LCO 2 -PB fracturing the coal seam, the improper application of fracturing model will reduce the efficiency of gas drainage and cause gas disasters. To clearly understand the ECBM modes of LCO 2 -PB fracturing coal seam process, this paper employed experiments to investigate the effect of leakage characteristics of the LCO 2 -PB on fracture models of coal cubes, including release pressure (P r), filling level (V), release diameter (D r) and blasting hole diameter (D b). We propose the combination I and II of the LCO 2 -PB leakage parameters and applied two methods to the in-situ tests. The results revealed that the gas flow of the boreholes of the LCO 2 -PB with the combination I is 0.368 m3/min, which is 2.07 times that of the combination II (0.178) in the soft coal seam. Besides, the average methane flow of the boreholes with combination II is 0.796 m3/min, which is 1.78 times the combination I (0.448) in the hard coal seam. The adjustment of leakage parameters of the LCO 2 -PB technology can be helpful for ECBM and reduce the risk of coal and gas outbursts in the soft and hard coal bed. [ABSTRACT FROM AUTHOR]

• The whole safety production accidents of coal mines in Guizhou Province were analyzed. • The time–space dynamic evolution of coal and gas outburst accidents were proposed. • Coal and gas outburst accidents, influence factors and suggestions in Guizhou Province were studied. The safety situation of coal mines in Guizhou has been improving for many years, but coal and gas outburst accidents continue to occur. This paper analyzes the safety situation of coal mines in Guizhou by collecting data on accidents and disasters in the last decade: 2011–2020. The distribution characteristics, time–space dynamic evolution, and factors influencing coal and gas outburst disasters were studied. Countermeasures and safety management suggestions for disaster prevention and control were proposed. Our analysis shows that the primary disasters occurring in coal mines in Guizhou province are gas, roof, and water disasters, which are mainly distributed in the western and northern regions, as well as closely related to the complex geological conditions in the region. Coal and gas outburst is the most serious disaster threatening the safety of coal mines in Guizhou province in the last decade. The occurrence and development of gas outburst accidents presents three stages. The coal and gas outburst disasters mostly occurred in the driving face in the last decade, while their distribution showed obvious regional characteristics. In particular, Liupanshui, Bijie, and southwest Guizhou have a greater risk of coal and gas outbursts. Thus, it is necessary to focus on and formulate strict and effective outburst prevention measures when performing mining operations. Finally, according to hazard analysis, objective factors and management factors underlying coal mine disasters in Guizhou, specific countermeasures, and safety management policy suggestions of government and enterprises are provided. This research has certain significance for the safety supervision by the government and the staff engaged in the prevention and control of coal and gas outbursts. [ABSTRACT FROM AUTHOR]

Abstract: An analysis of the stress distribution characteristics of shaft working faces in deep underground coal mines was performed using theoretical analyses and field tests. The analysis of the outburst risk within the studied shaft demonstrated that a funnel-shaped pressure relief zone existed below the working face, and a zone of concentrated stress surrounding the shaft was a critical area for a potential outburst disaster. We then divided the process of coal excavation within the shaft into four stages to manage and propose improved technical, drainage and protective measures as well as other requirements. When using the processes and requirements provided, the Haizi and Taoyuan coal mines, which are located in the Huaibei mining region and have depths of up to 1000m, realized safe and rapid excavation through outburst-prone coal seams. It was determined that the technical systems developed in this paper could provide for safe coal excavation in deep shafts, which could be established as new regulations for rapid and safe coal extraction in outburst-prone coal seams. [Copyright &y& Elsevier]

Abstract: In this paper, numerical simulation models built using FLAC3D were simulated 16 times with different in situ stresses and different driving cycle footages to investigate outburst risk of coal mine roadways. The results suggest that an outburst is most likely to happen when the maximum in situ stress is perpendicular to the roadway axis, and the risk increases to the maximum when the driving cycle footage is at the stress concentration peak. However, conditions are much safer when the maximum in situ stress is parallel to the roadway. The stress distribution was validated with field experiments. The results suggest that most of the roadways in coal mines should be arranged parallel to the maximum stress direction if practical, and that the driving cycle footage should not be at the stress concentration peak. The results will help improve coal mine safe design and provide a driving procedure to prevent gas outbursts. [Copyright &y& Elsevier]

Abstract: Dynamic collapses of deeply mined coal rocks are severe threats to miners. In order to predict the collapses more accurately using electromagnetic radiation (EMR), this paper researched the energy conversion mechanism in the damage process of coal rock mass, analysed its EMR and dissipated energy, and a relationship between which was established using the voltage amplitude of EMR signal and hysteresis loop generated in the loading and unloading cycles as a bridge; then a series of cyclic loading experiments using coal samples of Junde and Xinlu coal mines from Heilongjiang province were carried out, and EMR signal released during these cycles were collected. Results show that during a whole damage process of a sample, the cumulative values of EMR energy and corresponding dissipated energy (hysteresis loop area) well subject to the form of , whose correlation coefficients are above 0.90, and EMR signals received by different frequency antenna seldom impacted on this relationship; the total EMR energy released from the whole failure process of the samples obtained from adjacent sampling point in same mining area are different to some extent, so are the dissipated energy; compared with cyclic loading for coal rock mass orderly with the peak loads of 5kN and 10kN previously, the dissipated energy of direct using that of 15kN increase 17.8%, and EMR signal is more abundant; EMR energy received in each cycle increase steadily with the improvement of load level, and in a single loading and unloading cycle, sometimes it is very rich in the unloading phase, even more than the loading one. [Copyright &y& Elsevier]

Abstract: Based on the second Fick’s law, the theoretical equation of gas desorption of particle coal under the non-uniform pressure condition was developed in this paper. The analytical solution of the theoretical equation and the method of gas desorption quantity of particle coal under non-uniform pressure condition were obtained. [Copyright &y& Elsevier]

Abstract: When coal and gas outburst occurs, high-speed gas flow and air shock wave with high kinetic energy could be created. In this paper, the formation process of outburst shock waves and gas flow has been analyzed firstly. Afterwards, the numerical simulation models of the roadways with right-angled intersection have been established, by which real-time simulation of the propagation of outburst gas flow and the process of gas transport has been conducted. Gas pressure, gas velocity and gas concentration can be simulated and shown. From analyzing the simulation results, qualitative and quantitative conclusions that the characteristics and patterns of the propagation and attenuation of outburst shock waves and gas flow can be arrived at. Finally, experimental models have been carried out to investigate the outburst shock waves and gas flow at the roadways with the similar shapes as the simulated ones. The results indicate that when shock wave and gas flow passes the intersection, most of the shock wave and gas flow will flow into the roadway of section opposite the intersection, and a little of it would flow into the roadway below the intersection. And turbulence will appear, shock wave reflects and diffracts at branches with more influence on the roadway below the intersection. [Copyright &y& Elsevier]

Abstract: Gas pressure is an important index for evaluating the outburst risk and determining the gas content in coal seams. It is recommended to predict coal-seam gas pressure of the workface at deep levels before extending mining activities to deeper levels. According to the prediction results, measurements are taken for gas outburst prevention and control and for workload estimation. At present, regression methods are always used to process the numerous gas pressure data for prediction. Because there are many factors that influence the gas pressure which could lead to a deviation from actual values, the measured data do not possess basic conditions for regression methods; this can cause unexpected dangers if the methods are adopted. Based on a statistical analysis of actual measured results of coal-seam gas pressure in a same geological section in certain coal mine, two symbol measured points are selected to make a line for prediction, i.e. safety line, and the other measured points should be below the line except the abnormal points due to the confined water. It has been successfully applied in numerous coal mines in China. Particularly, this method is analyzed in this paper for the case of the No. 82 coal seam in the Taoyuan coal mine in Huaibei coalfield, China. By comparatively analyzing the relationship between gas pressure and depth from surface using regression methods, it is found that the safety line method could lead to a better prediction for deep coal-seam gas pressure, and therefore promote early warning ability and mining safety. [Copyright &y& Elsevier]

Abstract: Accurate prediction of burst pressure plays a central role in engineering design and integrity assessment of oil and gas pipelines. Theoretical and empirical solutions for such prediction are evaluated in this paper relative to a burst pressure database comprising more than 100 tests covering a variety of pipeline steel grades and pipe sizes. Solutions considered include three based on plasticity theory for the end-capped, thin-walled, defect-free line pipe subjected to internal pressure in terms of the Tresca, von Mises, and ZL (or Zhu-Leis) criteria, one based on a cylindrical instability stress (CIS) concept, and a large group of analytical and empirical models previously evaluated by Law and Bowie (International Journal of Pressure Vessels and Piping, 84, 2007: 487–492). It is found that these models can be categorized into either a Tresca-family or a von Mises-family of solutions, except for those due to Margetson and Zhu-Leis models. The viability of predictions is measured via statistical analyses in terms of a mean error and its standard deviation. Consistent with an independent parallel evaluation using another large database, the Zhu-Leis solution is found best for predicting burst pressure, including consideration of strain hardening effects, while the Tresca strength solutions including Barlow, Maximum shear stress, Turner, and the ASME boiler code provide reasonably good predictions for the class of line-pipe steels with intermediate strain hardening response. [Copyright &y& Elsevier]

Abstract: Autofrettaging a pressure vessel improves its pressure capacity. This is reliable if there isn’t any crack or other type of flaws. In this paper, the effects of external surface cracks on bursting pressure of autofrettaged cylinders are studied. It is observed that bursting pressure decreases considerably (up to 30%) due to external cracks in the cylinders without autofrettage. This reduction increases for high levels of the applied autofrettage. External axial cracks have more effects than inclined cracks. Comparing experimental and numerical results show that the numerical methods can acceptably predict the bursting pressure of the autofrettaged cracked cylinders. These predictions are valid when the fracture parameter (J-Integral) is calculated from the modified equation that takes into account the effects of residual stresses. [Copyright &y& Elsevier]

Abstract: Most of the coal seams currently mined in the Southwest part of China have high gas content, are soft and prone to outbursts, which significantly influences the rate of gate road development and mining safety. It is believed that an outburst is mainly due to the internal energy stored in the coal seam, so it is necessary to release the energy to prevent the occurrences of coal and gas outbursts, which in turn improves the rate of gate road development. This paper presented herein discusses the usage of the waterjet slotting technique to prevent the occurrence of outbursts during gate road development, on the basis of the laboratory tests, numerical modeling and field work. The numerical modeling indicated that the internal effective stress levels can be reduced up to 70% and 45% for the vertical and horizontal stresses, respectively, when waterjet slotting is applied, and the field monitoring results showed that the gas concentration released from the face of excavation can be increased up to five times compared with the traditional borehole drainage method. As a result, the values S mix and K 1, which are determined by drill cuttings to describe the possibility of outburst occurrences, have been kept below the critical limits 70% of the excavation time and the rate of gate road development increases from 30m to 70m per month. [Copyright &y& Elsevier]

Abstract: An accident leading to bursting of a compressed natural gas (CNG) cylinder fitted to a passenger bus in India resulted one person died and four persons injured. This paper presents the incident, the human factor involved, safety issues and lesson learned. [ABSTRACT FROM AUTHOR]

Abstract: This paper proposes the use of a coupled fault tree analysis (FTA) and artificial neural network (ANN) model to improve the prediction of the potential risk of coal and gas outburst events during the underground mining of thick and deep Chinese coal seams. The model developed has been used to investigate the gas emission characteristics and the geological conditions that exist within the Huaibei coal mining region, Anhui province, China. The coal seams in this region exhibit a high incidence of coal and gas outbursts. An analysis of the results obtained from an initial application of an FTA model, identified eight dominant model parameters related to the gas content or geological conditions of the coal seams, which characterize the potential risk of in situ coal and gas outbursts. The eight dominant model parameters identified by the FTA method were subsequently used as input variables to an ANN model. The results produced by the ANN model were used to develop a qualitative risk index to characterize the potential risk level of occurrence of coal and gas outburst events. Four different potential risk alarm levels were defined: SAFE, POTENTIAL, HIGH and STRONG. Solutions to the prediction model were obtained using a combination of quantitative and qualitative data including the gas content or gas pressure and the geological and geotechnical conditions of coal seams. The application of this combined solution method identified more explicit and accurate model relationships between the in situ geological conditions and the potential risk of coal and gas outbursts. An analysis of the model solutions concluded that the coupled FTA and ANN model may offer a reliable alternative method to forecast the potential risk of coal and gas outbursts. [ABSTRACT FROM AUTHOR]

Abstract: A new outburst prevention method based on injecting and freezing, considering of the important role of gas pressure and the mechanical characteristics of coal in the sprout and occurrence in the coal and gas outburst, was put forward in this paper. The curves of stress and strain with different freezing temperatures were drawn, and the effects of the water content, compaction force as well as freezing temperature on mechanical properties was discussed, through the experiment on the outburst-prone coal seam injected frozen water. It was found that the uniaxial compression strengths of coal obviously rise, its Young''s modulus show a rise tendency,, and the effective stresses of gas-filled coal obviously rise and its strengths is further improved after the gas pressure falls with the freezing temperature decreasing. [Copyright &y& Elsevier]

Abstract: In this paper, we describe the principle, technique, and steps of fast predicting the outburst risk before coal un-covering in shaft and cross-cut, including the selection of indexes for the prediction, fast test of gas pressure, coring technique for obtaining an whole coal core, and the determination of outburst parameters. Using this technique, we can finish testing the original gas pressure of coal within 20 h, making it possible to accurately predict the outburst risk before coal un-covering in shaft and cross-cut in 3~5 days. In addition, we can, according to the prediction result, give out the residual pressure and the smallest pre-drainage rate of gas. The comparison of prediction data among more than 10 shafts and cross-cuts shows that the index of initial gas relief expansion energy can more accurately reflect the outburst risk of coal seams. [Copyright &y& Elsevier]

Abstract: This paper, starting with the existence of general introduction of associated coal bed methane (CBM) with coal, expatiates in five parts on the transfer rules of CBM changing with time in three-dimensional rock space of the coal mine under the effect of mining activity and ventilation; On the basis above, the principle having been validated in practice is also pointed out for us to follow in methane extraction from coal mines. Finally, it draw conclusion that mining areas with high gas concentration(including coal and gas outburst), low permeability, and composite coal seams can realize the coal mine methane (CMM) extraction and ground development in a high efficient way as long as we follow the miming activities and the gas migration rules under the effect of negative pressure ventilation. [Copyright &y& Elsevier]

• A new gas pressure inversion model is established. • The application result of the new model proves the uneven distribution of gas pressure. • The gas pressure value shows an approximate normal distribution law in statistics. • The evolution law of outburst elimination in the coal seam presents the characteristics of non-uniformity. China is one of the countries most seriously affected by coal and gas outburst disasters. Gas pressure is an important parameter to evaluate the risk of coal and gas outburst. In order to solve the problem existing in the traditional gas pressure measurement method, a dynamic inversion model of gas pressure while drilling is established based on the gas emission rule of coal particle and the gas radial migration equation of borehole wall in this paper. Combined with the directional drilling system and the special gas emission measuring device, the dynamic inversion model is applied to measure the gas pressure in the coal seam. The results show that: (1) The theoretical value of gas pressure obtained from the new model has a small error range by comparing with the actual value of gas pressure, which indicates that the new model has good reliability and applicability; (2) The distribution range of gas pressure value is wide, which shows the non-uniformity of gas occurrence in coal seam. The heterogeneity of gas pressure value shows an approximate normal distribution law in statistics; (3) Influenced by the occurrence of original gas in coal seam, the evolution law of outburst elimination in each area of coal seam also presents the characteristics of non-uniformity. The research results have important practical significance to assist the early warning of coal and gas outburst and to achieve coal and gas co-mining with safety mining. [ABSTRACT FROM AUTHOR]

[Display omitted] • Rock outbursts and gas are the most serious natural hazards in underground coal mine. • The outburst sensor detects methane concentration, pressure and acoustic effects. • The dynamic properties of the sensor tested on the measuring bench in laboratory. • Field tests of the outburst sensor were carried out in the experimental mine. • The microphone and the pressure signals appear before of methane concentration. Outbursts of methane and rocks are, similarly to rock bursts, the biggest hazards in deep mines and are equally difficult to predict. The violent process of the outburst itself, along with the scale and range of hazards following the rapid discharge of gas and rocks, require solutions which enable quick and unambiguous detection of the hazard. The paper presents the functions of an integrated outburst detector which was equipped with three measuring and detection elements: a pressure sensor, a microphone and a chamber for constant measurement of methane concentration. A measuring stand for testing the dynamic properties of the integrated methane discharge sensor is described. It has been shown that both the pressure transducer and the microphone react immediately after a change in thermodynamic conditions, and the time constant of the pellistor methane concentration sensor was estimated at a few seconds. Field tests of the integrated sensor were also carried out, the main purpose of which was to verify the dynamic parameters of the sensor in medium scale experimental conditions during methane and coal dust explosions in the testing gallery at an experimental mine. [ABSTRACT FROM AUTHOR]

• A new mathematical time-dependent diffusivity model and its simplified model of deformed coal were established and validated. • The change law of gas diffusion coefficient of deformed coal with time was obtained. • The engineering application of SNMTDM was analyzed. Deformed coal provides favorable conditions for the occurrence of gas outburst disasters. The existing gas diffusion model cannot accurately calculate the lost gas amount during the sampling period of deformed coal. In order to solve the above problems, a new mathematical time-dependent diffusivity model (NMTDM) and its simplified model (SNMTDM) of deformed coal have been established and validated. At the same time, the performance and engineering application of models were also discussed. The results show that the fitting accuracy of NMTDM and SNMTDM to gas diffusion data is greater than that of unipore diffusion model (UDM) and simplified unipore diffusion model (SUDM), respectively, which indicates that it is reasonable to regard the diffusion coefficient of deformed coal as a time-dependent variable. The diffusion coefficient of adsorbed gas, diffusion coefficient of free gas and total diffusion coefficient of gas decrease with the increase of time, and SNMTDM can realize the accurate calculation of lost gas amount. However, when the application of SNMTDM extends beyond its applicable range, the calculating results of the model may deviate seriously from the actual values. The gas diffusion models established in this paper can improve the calculation accuracy of lost gas amount and gas content of deformed coal. [ABSTRACT FROM AUTHOR]

Acoustic emission (AE) and electromagnetic radiation (EMR) monitoring technology have been widely used in coal and rock dynamic disaster monitoring. However, the prediction of coal and gas outburst using original signal directly may cause the false and missing alarms due to the failure to identify valuable potential information, which may lead to the decrease of prediction accuracy. This paper presents a new method to identify the precursor features of coal and gas outburst using two-dimensional Convolutional Neural Network (CNN). Empirical mode decomposition (EMD) was performed for AE or EMR signals, and the Intrinsic Mode Functions (IMFs) with Variance Contribution Rate (VCR) greater than 90% were merged into modified signal. Since the frequency of AE or EMR signal can change with different coal and rock fracture stages, one-dimensional modified signal was transformed into a two-dimensional time-frequency graph using the short-time Fourier transform (STFT). The time-frequency graph was input into CNN that recognizes the time-frequency characteristics of AE or EMR using SoftMax regression at the end of the network. Compared with the original signal, the precursor characteristics of modified AE or EMR signals can be better learned by CNN. The proposed method was applied to the Jinjia Coal Mine database which was identified as having outburst risk, and it was found that the recognition accuracy was better than other approaches. The proposed method achieves maximum recognition accuracy of 98.00% (AE), and 97.20% (EMR) in Jinjia Coal Mine database, respectively. • A precursor recognition method using CNN for coal and gas outburst is proposed. • The modified signal and its time-frequency characteristics have been analyzed. • CNN is sensitive to the modified precursor features of coal and gas outburst. • The method has been verified in a coal mine with coal and gas outburst risk. • The method improved the precursor recognition accuracy of coal and gas outburst. [ABSTRACT FROM AUTHOR]