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With the increase of coal mining depth, the gas content in coal seams could also become larger and larger, which could suddenly cause an inrush of gas into the longwall mining face. It is very dangerous for miners' safety in the underground. The U-shaped ventilation pattern of longwall mining face that underground coal mines currently use is not enough to deliver sufficient air quantities to dilute gases in mining faces, which could result in the gas concentration over the required celling limit by government laws. Thus, the mine must stop production. In this paper, the high level roadway (HLR) is designed and the U + HLR new ventilation pattern is proposed to control gas emission in a longwall mining face. using computational fluid dynamics simulation (CFD) software, the flow field and gas transportation in the mine gob are studied. The optimized ventilation parameters are summarized. It is found that the best vertical distance of the HLR is 35 m over the coal seam and the horizontal distance is 25 m from the air return roadway. It is recommended that the negative suction pressure design of the high level roadway should be ranged from 9000 pa to 10000 pa. Based on the study outcomes, the gas emission could be well controlled in mining faces and avoid any gas disaster accidents. [ABSTRACT FROM AUTHOR]

The complex disaster of gas and coal spontaneous combustion is a major challenge in coal gob, especially under conditions of three-dimensional extraction. Taking the 15,101 mining face in Shigang Mine as the field background, COMSOL Multiphysics software was employed to construct models of airflow, methane migration, and stress distribution in gob under the stereo extraction method of "U+high-low level roadway." The range of oxidation zone was divided by combining the oxygen concentration and wind velocity field. The results showed that the methane can be discharged through the low-level airway and the return airway, and most of the methane at the top area in gob can also be discharged from the high-level roadway, indicating that the stereo ventilation was beneficial for methane dilution at both the upper and lower region. However, the oxidation zone in the middle of gob was the widest and closed to the mining face, it on the return side was narrower than intake side, and its distribution range was closer to the mining face. And finally, the range of the oxidation zone expanded with the increase in height in the vertical direction. These indicate the dangerous area for preventing gas and coal spontaneous combustion disasters in gob. [ABSTRACT FROM AUTHOR]

With the expansion of the coal mining depth and scales, the content of coal mine methane (CMM) is increasing, bringing the great challenge to the safe coal mining and greenhouse gas control. Researching CMM emission laws and its drainage technologies in underground coal mining is essential. This paper analyzed the sources of CMM emission in the workface and then put forward the grid method to measure the CMM emission laws in the single coal seam, which error was verified by the integral method. The CMM emission laws of the S2107 workface of the Yuwu mine, northeastern Qinshui basin, China were researched as a case study and it is found that the upper corner was the key position of CMM control in the workface. The sub-source CMM drainage methods by the strike high-level roadway and long vertical buried pipes were applied against the different CMM sources in the S2107 workface. The gas concentration in the return air was less than 0.6% during the CMM drainage, verifying the effectiveness of the sub-source CMM drainage methods. [ABSTRACT FROM AUTHOR]

The urban expressway system serves as a key role in the roadway transportation system. It provides an efficient and comfortable approach for long-distance travel within the city. However, the safety status of the urban expressways is becoming a critical issue as the high-frequent traffic crashes have severely influenced the traffic operations. Among the safety influencing factors, including traffic operational parameters (such as traffic speed and volume), geometric features and traffic participants' characteristics (such as vehicle roadway usage patterns), the traffic operational parameters and geometric features have been widely investigated. However, the impacts of traffic participants' characteristics on traffic safety have never been examined. This unprecedented study aims to link vehicles' roadway usage patterns with traffic safety through crash frequency analyses. First, the roadway usage patterns were identified using Latent Class Cluster Analysis (LCCA) based on their traveling rates. Then, the hourly-based crash frequency analysis data were formulated with traffic operational parameters, geometric features and crash data. Finally, crash frequency analysis models were developed to unveil the relationships between the crash occurrence and their influencing factors. The modeling results showed that the Random Effects Hurdle Negative Binomial Model (REHNBM) provided better goodness-of-fit. And it concluded that higher proportions of vehicles with low-level roadway usage pattern would substantially enhance the possibility of crash occurrence; while the proportions of vehicles with the medium-high-level roadway usage pattern had negative impacts on crash occurrence probability. Finally, safety improvement recommendations and strategies based on the modeling results were put forward. [ABSTRACT FROM AUTHOR]

The urban expressway system serves as a key role in the roadway transportation system. It provides an efficient and comfortable approach for long-distance travel within the city. However, the safety status of the urban expressways is becoming a critical issue as the high-frequent traffic crashes have severely influenced the traffic operations. Among the safety influencing factors, including traffic operational parameters (such as traffic speed and volume), geometric features and traffic participants' characteristics (such as vehicle roadway usage patterns), the traffic operational parameters and geometric features have been widely investigated. However, the impacts of traffic participants' characteristics on traffic safety have never been examined. This unprecedented study aims to link vehicles' roadway usage patterns with traffic safety through crash frequency analyses. First, the roadway usage patterns were identified using Latent Class Cluster Analysis (LCCA) based on their traveling rates. Then, the hourly-based crash frequency analysis data were formulated with traffic operational parameters, geometric features and crash data. Finally, crash frequency analysis models were developed to unveil the relationships between the crash occurrence and their influencing factors. The modeling results showed that the Random Effects Hurdle Negative Binomial Model (REHNBM) provided better goodness-of-fit. And it concluded that higher proportions of vehicles with low-level roadway usage pattern would substantially enhance the possibility of crash occurrence; while the proportions of vehicles with the medium-high-level roadway usage pattern had negative impacts on crash occurrence probability. Finally, safety improvement recommendations and strategies based on the modeling results were put forward. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is one of the main gas hazards in coal mines, and pulse hydraulic fracturing is a new technique for preventing and controlling gas hazards in coal mines and is presented based on conventional hydraulic fracturing. S2107 is a coal seam with high gas content and coal and gas outburst hazard but with low gas concentrations when extracted. To safely drive air tunnels into S2107, the pulse hydraulic fracturing technique is used on cross-measure boreholes placed from a high-level roadway to the driving roadway to improve the effects of gas drainage and ensure safety during production. The mechanism of coal fracturing via pulsed pressure is analyzed. A variable frequency method is used to strengthen the fatigue damage effect, and the initial pressure is estimated based on previous studies. The layouts of fracturing borehole and guide borehole are designed according to the layer relation, and a new hole sealing method is developed. The results indicate the pressure changes during pulse hydraulic fracturing reflect crack extension, and it is conducive to extending and connecting cracks under pulse pressures. The initial pressure during pulse hydraulic fracturing is negatively related to the water volume and fracturing time but cannot directly influence the fracturing radius. Moreover, this pressure is 30-43 % less than the calculated value. After this application, the gas desorption index, K, at the driving workface decreases to below the critical value. The gas drainage volume of fracturing boreholes and their guide boreholes increases relative to normal boreholes by 3.32-fold and 3.07-fold, respectively. The new technique is promising for preventing and controlling gas hazards in the future. [ABSTRACT FROM AUTHOR]

High (low)-level roadways are effective gas control measures for coal seams with high outburst potential and low permeability. The spatial location of the low-level roadway directly determines the gas drainage efficiency and can affect the success of the whole engineering project. Currently, determining the location of a low-level roadway is based mostly on empirical engineering analogies and lacks relevant theoretical support. In this work, based on the Ji-15-24080 engineering project at the No. 10 Coal Mine of Pingdingshan Tian’an Coal Industry Co., Ltd., a numerical model for the location of a low-level roadway was established using ANSYS numerical software to analyze the stress distribution in the rock surrounding the conveyor roadway for the mining face and assess the permeability-enhancing effect of the low-level roadway when it is placed at different locations. A location directly beneath the conveyor roadway was found to provide a good location for a low-level shield roadway. In addition, a quantitative distance between the conveyor roadway and the low-level roadway can also be determined. These optimization results were then directly applied in the field. Relevant industrial field tests were conducted to determine fracture development, gas pressure and stability of the surrounding rock. The field test results confirm that the proposed location for a low-level shield roadway is reasonable. We recommend that when the low-level shield roadway is placed directly beneath a conveyor roadway, it should be within 50 m of the working face, and an effective support structure should be employed to support the rock surrounding the conveyor roadway to ensure the stability of the rock. We further recommend that, in the area in front of the working face, particularly within 25 m of the working face, densely distributed gas drainage boreholes should be drilled to drain gas from the coal seam to reduce the gas pressure and lower the outburst risk. The results of this work will provide guidance for similar engineering projects to determine the location of a low-level roadway and to ensure gas control. [ABSTRACT FROM AUTHOR]

To study the movement law of methane in the goaf drained by a large-diameter borehole drilling machine under "U"-shaped ventilation, a simulation on a coal mine was conducted on Fluent to find the optimal spacing between large-diameter boreholes and the most appropriate distance between the borehole and the upper corner. The variation of borehole drilling and the methane concentration in the upper corner were obtained through a field test. Results show that the method of drilling large-diameter boreholes greatly reduces the methane concentration in the goaf and the upper corner, with the optimal borehole spacing being 30 m and the most appropriate distance between the borehole and the upper corner being 15 m. When the large-diameter borehole is drilled 25 m deep down into the goaf, it penetrates into the stress impact area, and the methane concentration increases rapidly, with the maximum being 3.7%. When the borehole is drilled 35 m down into the goaf, the methane concentration slightly decreases as a result of the drainage superposition effect. The methane concentration in the upper corner increases as the borehole is drilled deeper and is farther away from the upper corner. As a result of the drainage superposition effect, the methane concentration in the upper corner varies from 0.32% to 0.51% in a cyclical way. [ABSTRACT FROM AUTHOR]

With the increase of mine mining intensity, the scope of goaf continues to expand, a large amount of coal remains in goaf, and the spontaneous combustion fire of coal is particularly serious. In addition, the spontaneous combustion of coal under the condition of gas mining is not clear. At once, spontaneous combustion can induce gas explosion, resulting in the waste of manpower and material resources, bringing hidden dangers to the mine safety production. In order to prevent and control the fire caused by spontaneous combustion of left coal under the condition of gas mining, in this paper by means of laboratory test, field measurement, and numerical simulation, the law of spontaneous combustion of residual coal under gas mining is studied. The index gas of coal spontaneous combustion was determined by temperature-programmed experimental device. Secondly, the distribution of the stope working face, goaf, and O2 is discussed. It is of great significance to study the spontaneous combustion rule of three belts in goaf under acid leaching condition and then to discuss the best position of injection idling opening and put forward targeted precontrol measures, which is of great significance to guarantee the safety of mine production. The results show that CO begins to appear when the temperature rises to 40 °C, and its concentration increases exponentially with the increase of coal temperature, which can reflect the degree of coal oxidation in goaf. Therefore, CO should be selected as the indicator gas in the working face. Under the influence of negative drainage pressure, the dangerous area will be enlarged. The optimal drainage volume of high drainage roadway is 90 m3/min, and the optimal air intake volume of working face is 1400 m3/min. The area 0–34 m away from the working face is scattered zone, 34–135 m away is spontaneous combustion zone, and 135 m away is asphyxiation zone. When the position of the idler is 50 m behind the working face, the reduction of the oxidation zone is the largest. [ABSTRACT FROM AUTHOR]

Coalbed methane (CBM), which is extremely rich in deep coal reserves in China, is not only the material cause of gas explosion and coal‐gas outburst disasters but also a clean and environmentally friendly energy source. Gas drainage must be carried out during deep coal mining to avoid gas accidents while obtaining clean energy. The low‐level roadway is an effective gas drainage measure for coal seams with high coal‐gas outburst risk and low‐permeability, and it can simultaneously realize the functions of gas drainage before and during mining. Roadway tunneling has a strong influence on the stress and permeability properties of the adjusted coal seams, and further determines the gas drainage efficiency and the layout of the air‐return roadway. Taking the Pingshu Coal Mine as an example, this paper analyzes the influence of the vertical distance between the low‐level roadway and the underlying coal seam, and the influence of the width and height of the low‐level roadway on the stress relief and deformation characteristics. The results indicate that the low‐level roadway tunneling results in a nonuniform stress disturbance of coal seam #15. And a reasonable layout and size parameters of the air‐return roadway in coal seam #15 were proposed based on which gas drainage practices were carried out with good effects in panel #15211. The research can effectively guide the elimination of coal and gas outburst in return air roadway, and achieve the purpose of safe and rapid tunneling coal roadway under the environment of highly‐gassy coal seam. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In the process of improving coalbed permeability through pulse hydraulic fracturing, the cyclic loading effect influences the characteristics of micropores in coal matrix, thus affecting the process of gas migration. Therefore, it is essential to investigate the effect of cyclic loading on the pore structure of coal. Seven groups of loading tests at different frequencies and amplitudes were conducted on anthracite coal obtained from Shanxi Province, China, using a fatigue-testing machine. Subsequently, using a PoreMaster GT-60 Mercury-intrusion apparatus, the influence of the frequency and amplitude on the structural characteristics (including mercury-injection and mercury-ejection curves, pore size distribution, porosity, and specific surface area) of pores in coal samples was analyzed. Finally, the law and mechanism of action of the loading frequency and amplitude on pores in coal samples were comprehensively analyzed. The test results showed that, in the case of maintaining the sine-wave amplitude unchanged during loading while altering the loading frequency, the overall porosity and pore volume rise at different degrees. The growth of the loading frequency presents a more significant promotive effect on the initiation and development of pores and fractures. Moreover, it drives the transformation of micropores and transition pores into mesopores and macropores, thus increasing the proportion of seepage pores. Under the condition of large sine-wave amplitude during loading, macropores and mesopores are subjected to the repeated action of the external force, thereby reducing the overall porosity. In addition, the volume of the seepage pores declines, and the number of the coalesced pores decreases. Finally, in light of these results, the implications of frequency and amplitude selection in the process of pulse hydraulic fracturing are discussed. Therefore, the results of this research will provide an important theoretical basis for the field application of pulse hydraulic fracturing technology in coal mines. [ABSTRACT FROM AUTHOR]

Although ground multi-branch horizontal wells can maximally connect the fractures in a coal reservoir and greatly increase the single well yield, this type of well is not appropriate in many mining areas in China, since conventional drainage and recovery must be carried out on the ground. In this work, a new coalbed methane (CBM) recovery technology is proposed, which is characterized by docking ground multi-branch horizontal wells with underground boreholes. In addition, three specific docking schemes are also given. In order to improve the efficiency of CBM recovery and avoid the huge investment of purchasing ground drainage and recovery equipment in LiuLin Block, an engineering test for connecting ground multi-branch horizontal wells with underground boreholes was carried out at the 24307 working face of the Shaqu coal mine. The engineering practice has proven that the CBM recovery technology characterized by docking ground multi-branch horizontal wells with underground boreholes captures CBM more efficiently than conventional underground CBM recovery, and it enables underground drainage and recovery when compared to conventional ground CBM recovery, thus avoiding the huge investment in purchasing ground drainage and recovery equipment, and increasing economic efficiency as well as improving the applicability of horizontal multi-branch horizontal wells. [ABSTRACT FROM AUTHOR]

Closely-spaced outburst coal seams (COCS) is the main condition of coal seams in southwest China, and gas disasters are one of the major problems affecting coal mine safety. Mining a protective seam and pre gas extraction are the most safety way to improve the efficiency of mining under these conditions. However, low pre-mining gas extraction efficiency coupled with the close proximity of adjacent working faces is a problem. When mining at an old working face has been completed but the new working face is not yet ready to be mined, coal-bed gasses can flow into the new working face from adjacent seams and this commonly causes methane monitoring instruments to sound an alarm. These gas extraction difficulties lead to a conflict between mine safety and profit. To solve these problems, a sequential approach for integrated coal and gas mining of closely-spaced outburst coal seams is introduced in this paper. Two fundamental principles are proposed: (1) Fully coordinating the spatiotemporal relationships between gas extraction, roadway development, and coal mining to maximize both mine safety and coal and gas production; (2) Defining a mining sequence for outburst coal seams and choosing the coal seam with the weakest outburst risk as the protective seam. A system for comprehensive gas extraction in underground coal mines is divided into four stages for gas extraction: gas extraction before coal roadway tunneling, gas extraction before coal mining, gas extraction during coal mining, and gas extraction from the goaf after coal mining. The Songzao mining area, China, is used as a case study to demonstrate the effectiveness of this model, and it brings three major benefits: it improves underground coal mine safety with physical gas accidents decreased by 66.8%, it makes underground coal-bed methane (CBM) extraction more efficient with the average gas extraction rate were respectively 45.13 m3/t and 62.4%, the highest in China, and it reduces greenhouse gas emissions equivalent to 3.5 million tonnes of carbon dioxide. This study could be used as a valuable example for other coal deposits being mined under similar geological conditions. [ABSTRACT FROM AUTHOR]