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As an important industrial raw material, syngas can be used for hydrogen production, and its main components include hydrogen and carbon monoxide. Experiments were conducted on syngas (ratio of CO and H2 is 1:1) to study the effects of the nozzle inclined angle and diameter as well as the heat release rate on the characteristics of syngas jet flame. Three nozzles with different diameters, 5 mm, 10 mm, and 15 mm, were used in this study. Four different inclined angles of the nozzle corresponding to 0°, 30°, 60°, and 90° were employed. It was found that the normalized horizontal extension length of jet flame (L/D) varies with nozzle diameter and fuel flow rate, and increases exponentially as the dimensionless heat release rate ( Q ∗ ) increases for the nozzle with inclined angles of 0°–60°. By modifying the dimensionless heat release rate and fitting the experimental data, a unified empirical model, which is the first of its kind to date, is established. The model can be used to estimate the horizontal extension length of the syngas jet flame for nozzles with different diameters and inclined angles. For jet flame at an inclined angle of 90°, the normalized jet flame width was exponentially proportional to the modified Froud number with a power of 0.39.

The preparation of suitable specimens is important for obtaining credible mechanical and methane migration parameters for tectonic coal, which help to guide methane extraction and disaster prevention. In this study, a dual-deformation mechanism for porous media was introduced along with two powder compression models, and the issues that should be considered in the preparation of coal specimens were analyzed. By compression tests, the relationship between bed relative density and the applied stress in the compression of coal particles was obtained. The method of coal specimen preparation was introduced in detail. The results indicated that the Kawakita model is suitable for describing the compressive process of tectonic coal powders and guiding the preparation of tectonic coal specimens. The key parameters a and b in the Kawakita model are 0.411 and 0.108, respectively. The bed relative density shows a slight increasing trend followed by an obvious rising tendency with an increase in the applied stress. A compressive stress of 150 MPa was determined to be suitable for preparation of the tested coal specimens.

The diffusion coefficient of methane in coal is a key parameter that influences the methane extraction effect. Former studies on the methane diffusion coefficient mainly paid attention to the intact coal. The diffusion coefficient of tectonic coal is different from that of intact coal, which may be caused by the effect of tectonism on the pore characteristics. In this study, the pore characteristics of the intact coal and tectonic coal were tested using mercury intrusion porosimetry (MIP) and the physisorption method, while the counterdiffusion experiments were adopted to measure the methane diffusion coefficients of both coal samples. The results indicate that the methane diffusion coefficient of the tectonic coal is higher than that of the intact coal, and the diffusion coefficients decrease with the increasing methane pressure. The tectonism increases the volumes of mesopores and macropores and the porosity of coal, which is the main reason for the higher methane diffusion coefficient of the tectonic coal. The pore structure of coal becomes more winding or complicated by tectonism, thus showing a higher tortuosity of the tectonic coal. The tectonism makes it easier to extract methane, and coal mining in the tectonic region should receive more attention.

This paper investigated the effect of separation distance on the flame appearance and radiation heat flux of medium-scale pool fires dominated by radiation. A series of experiments were conducted on two adjacent heptane pool fires with diameters of 20 cm, 40 cm and 60 cm at separation distances ranging from 0 to 60 cm. According to experimental results, flame height and flame inclination angle increase monotonously with the increase of pool diameter. As separation distance increases, flame height increases first and decreases gradually for 3 different sized pool fires. However, the variation law of flame inclination angle with separation distance varies with pool diameter. The normalized flame height ( H f / D ) is found to increase exponentially with the dimensionless heat release rate ( Q * ) with an exponent of 0.71. The radiative heat flux at the center of the pool surface peaks at the critical separation distance of flame merging, which results from increased flame emissivity and view factor. Based on thermal analysis, a model to predict the radiative heat flux on the fuel surface is developed. The predicted radiative heat fluxes quantitatively match the experimental results but with 20% overvalued, due to the neglection of radiation blockage effect of cool fuel vapors zone and soot. The blockage effect is more pronounced for the merging pool fires with a diameter of 60 cm.

The establishment of the equivalent structural model (ESM) is the foundation to simplify the structures of the coal matrix and fractures for the study of methane migration. The ESM of the intact coal has been proposed and widely used by many scholars, but the ESM of the tectonic coal has not been found in the literature. The application of the ESM of the intact coal to the tectonic coal is not reasonable for the study of the methane migration properties, so the establishment of the ESM for the tectonic coal is necessary and meaningful. In this study, a tectonic coal specimen was remodeled from collected coal powders, and methane permeability tests were conducted. Then the ESM of the tectonic coal was established by analyzing the fracture structure. The results show that the tectonic coal can be idealized as a model containing a cubic matrix with square section fractures located at the twelve edges of the matrix. Because of the variation of the ESM, the permeability and the fracture porosity has a square relation for the tectonic coal, rather than the cubic relation for the intact coal. The matrix shape factor of the tectonic coal has also been proposed, which has a value of 480bf/7a3m and is much smaller than that of the intact coal.

To reduce the great losses caused by gas explosions in the gas transportation process and to improve the explosion venting efficiency in pipelines, a small pipeline with both a side vent and an end vent was developed, and premixed methane/air explosion characteristics were studied in the pipeline with different side venting areas. The experimental results show that in the same side venting position, with an increasing venting area, the propagation time for flames through the pipeline increases on a large scale, the peak overpressure of the methane explosion drops significantly, and the venting effect is greatly enhanced. Meanwhile, when the flame propagates upstream a side vent, the propagation can be slightly promoted by a side vent with a small area in the corresponding period of time, whereas a side vent with a large area can hinder the flame propagation all the time. After the flame passes the side vent, side venting plays a strong role in flame propagation, and the flame propagation velocity drops rapidly under a large side venting area. It is also found that overpressure in the vicinity of a side vent is less affected by the side venting area, while the effect of the venting area on overpressure is relatively large at a far distance from the side vent position.

Coal spallation is a visible product of outburst, and it represents a unique failure type for coal. A theoretical analysis of coal spallation was constructed to study its formation mechanism. Then, physical experiments and numerical simulations were performed to determine the physical features of coal spallation and evaluate the theoretical analysis. In addition, the effect of gas, stress and coal strength on coal spallation was discussed. The results show that coal spallation is a comprehensive product of multiple physical factors, including stress, gas and the coal strength. The formation process is related to stress transfer and mechanical behavior changes, gas migration and the evolution of the coal strength. Coal is eventually subjected to tensile failure, and the mechanical condition for coal spallation requires that the gas pressure difference ( p i − p a ) near the exposed surface is enough to overcome the tensile strength ( σ t ). The gases in coal provide the power necessary for coal spallation; thus, coal gas promotes plastic failure and is the leading factor for tensile failure. Stress is the leading factor for plastic failure and has a great influence on the extent of coal spallation. Meanwhile, with an increase in the coal strength, which represents the resistance to coal spallation, the acreage of the plastic area decreases, and thus, a greater pressure difference is required for tensile failure.

With increasing attention to energy efficiency and environmental protection, syngas has attracted great interest as a clean fuel with broad development prospects. In order to further study the effect of the initial gas flow on the explosion of syngas, an explosion experiment of stoichiometric syngas/air premixed gas was carried out in a 3 × 3 × 100 cm square explosion pipe. The composition of the syngas used in the experiment is 50CO:50H2, the flow rate of the experiment gas is 0L/min-250L/min, and the Reynolds number (Re) is 0-9598.41. The experimental results show that the maximum explosion pressure Pmax and the maximum explosion pressure rise rate (dp/dt)max gradually increase with the increase of the initial flow rate and Re. The gas flow increases the flame area, and increase the combustion rate, thereby making the explosion more violent.

To reduce the huge losses caused by natural gas explosions in transport pipelines, a small simulation pipeline with internal dimensions of 100 mm × 100 mm × 1000 mm was constructed, and the effects of the side venting position in the pipeline on methane explosion characteristics were studied. The experimental results showed that the venting effect of a side vent on flame propagation and explosion overpressure is not linearly increasing with shortening the distance between the side vent and the ignition point. The venting effect is significantly influenced by both the side venting position and the effect of an end vent. As the flame propagates upstream a side vent, the flame propagation is mainly affected by an end vent, and little affected by the side vent. Whereas when the flame propagates downstream the side venting position, the flame propagation velocity becomes to be gradually reduced by side venting effect. Besides, after a side vent is opened, the explosion overpressure is released by the side vent immediately, and the duration time of explosion overpressure is sensitive with the side venting position, but not in direct proportion with the peak overpressure. This illustrates the overpressure development affected by a side vent is influenced by both the side venting position and the interaction between the side vent and the end vent.

Multiple coal seams widely develop in the deep Chinese coal-bearing strata. Ground in situ stress and coal seam gas pressure increase continuously with the increase of the mining depth, and coal and gas outburst disasters become increasingly severe. When the coal is very deep, the gas content and pressure will elevate and thus coal seams tends to outburst-prone seams. The safety and economics of exploited first-mined coal seams are tremendously restricted. Meanwhile, the multiple seams occurrence conditions resulted in different methane pressure systems in the coal-bearing strata, which made the reservoir reconstruction of coal difficult. Given the characteristics of low saturation, low permeability, strong anisotropy and soft coal of Chinese coal seams, a single hydraulic fracturing surface well for reservoir reconstruction to pre-drain the coalbed methane (CBM) of multiple seams concurrently under the different gas pressure systems has not yet gained any breakthroughs. Based on analyses of the main features of deep CBM reservoirs in China, current gas control methods and the existing challenges in deep and multiple seams, we proposed a new technology for deep CBM reservoir reconstruction to realize simultaneous high-efficiency coal mining and gas extraction. In particular, we determined the first-mined seam according to the principles of effectiveness and economics, and used hydraulic fracturing surface well to reconstruct the first-mined seam which enlarges the selection range of the first-mined seam. During the process of mining first-mined seam, adjacent coal seams could be reconstructed under the mining effect which promoted high-efficiency pressure relief gas extraction by using spatial and comprehensive gas drainage methods (combination of underground and ground CBM extraction methods). A typical integrated reservoir reconstruction technology, “One well for triple use”, was detailed introduced and successfully applied in the Luling coal mine. The application showed that the proposed technology could effectively promote coal mining safety and simultaneously high-efficiency gas extraction. Keywords: Reservoir reconstruction, Coalbed methane, Multiple seam, Surface well, Gas drainage

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.

Chained explosions happened occasionally, leading to heavy casualties and huge property losses. However, the development regime of the chained explosions is unclear. Experiments of both single and two vertically distributed vapor clouds with different sizes, volume concentrations and ignition conditions were conducted. The experimental results show that the maximum overpressure generated by the primary methane/air mixture explosion increases with vapor diameter, but hardly changes with the ignition energy. The effect of ignition position on chained explosions is significant since the development trajectory of the primary explosion varies with the ignition position. The secondary explosion is more likely to be triggered in the case of ignition at the center of the primary vapor cloud. Because of the enhanced ignition and turbulence induced by the primary explosion, the maximum overpressure and rate of pressure rise of the secondary explosion are larger than that of the primary explosion and increase with a decrease in the separation distance between the two vapor clouds. In addition, the development of the explosion flame for the secondary explosion induced by a primary explosion is asymmetric due to the uncertain ignition point.

A coal and gas outburst can lead to a catastrophic failure in a coal mine. These outbursts usually occur where the distribution of coal seam gas is abnormal, commonly in tectonic belts. To study the effects of the abnormal distribution of this gas on outbursts, an experimental apparatus to collect data on simulated coal seam outbursts was constructed. Experiments on specimens containing discrete gas-enriched areas were run to induce artificial gas outbursts and further study of these outbursts using data from the experiment was conducted. The results suggest that more gas and outburst energy are contained in gas-enriched areas and this permits these areas to cause an outburst easily, even though the gas pressure in them is lower. During mining, the disappearance of the sealing effect of a coal pillar establishes the occurrence conditions for an outburst. When the enriched gas and outburst energy in the gas-enriched area is released suddenly, a reverse unloading wave and a high gas pressure gradient are formed, which have tension effects on the coal. Under these effects, the fragmentation degree of the coal intensifies and the intensity of the outburst increases. Because a high gas pressure gradient is maintained near the exposed surface and the enriched energy release reduces the coal strength, the existence of a gas-enriched area in coal leads to a faster outburst and the average thickness of the spall is smaller than where is no gas-enriched area.

Domino effects in process facilities can lead to catastrophic consequences. Many previous models devoted to assessing domino effects, but the influence of the combination of heat radiation, overpressure and fragment on the escalation is not considered. In addition, although the quantity of dangerous substances stored in the affected unit has a non-negligible effect on the propagation of domino effects, their inclusion in the modeling of domino effects has hardly been taken into account. To address these challenges, in this work, using the Fuzzy Analytic Hierarchy Process (FAHP), the escalation of domino effects under multiple physical factors is assessed, while considering other factors in varying degrees such as meteorological, mitigation measures and the affected units. Finally, the conditional probability table (CPT) in Bayesian networks (BNs) is updated by the FAHP. The application of the methodology has been demonstrated using an illustrative case study; the comparison of the results with that of previous study illustrates the efficacy and practicality of the development methodology. This work will be beneficial for land-use planning.

The method of explosion venting is widely used in industrial explosion-proof design due to its simple operation, economical and practical features. A dump vessel vented platform was built. By changing the vacuum level and the gas in the dump vessels and the structural size of linked vessels, the pressure in the explosion vessel and the dump vessel was compared, and the influencing factors of explosion venting investigated. The main conclusions are as follows: In the explosion venting process, the higher the vacuum in the dump vessel, the smaller the pressure peak of the explosion vessel and the dump vessel, and the faster the explosion pressure is lowered. When the dump vessel is under the same vacuum level and the gas in the dump vessel is CO2, the maximum pressure of the explosion vessel and the dump vessel is less than the maximum pressure when the containment medium is air. Under the same vacuum condition, the larger the volume ratio of the dump vessel and the explosion vessel, the smaller the pressure peak of the explosion vessel, the faster the explosion pressure drops, and the volume of the dump vessel reaches or exceeds the explosion vessel. Increasing the volume ratio of the containment vessel to the explosion vessel facilitates protection of the explosion vessel and the containment vessel. Under the same vacuum condition, when the gas explosion in 113 L vessel vents into 22 L vessel, the longer the length of the pipe, the greater the maximum pressure in the spherical vessel. When the gas explosion in 22 L vessel vents into 113 L dump vessel, as the pipeline grows, the maximum pressure in the two vessels decreases, but the reduction is not significant. In practical application, it is recommended to use a vacuum of 0.08Mpa or more for the dump vessel vented, and the containment medium is CO2.In terms of the structural size of the container, it is recommended that the ratio of the receiving container to the explosion container be as large as possible, and the pipe length be as long.

An experimental study of scale effects on hydrogen/methane deflagration of premixed flame in venting ducts is presented. The experiments focused on the effect of scale on flame structure, premixed flame propagation speed and overpressure during hydrogen/methane deflagration. Six ducts with different cross-sections and lengths were used in the experiments. Tests were performed for stoichiometric hydrogen/methane mixtures with five hydrogen fractions (hydrogen fraction from 0 to 100%) under ambient conditions. In the experiment, the “tulip” flame disappeared with increasing cross sectional area and hydrogen fraction. Meanwhile, the tulip flame gradually appears and a more pronounced “tulip” flame is observed with increasing duct length. The effect of cross sectional area coupled with duct length on flame propagation can be attributed to the influence of aspect ratio, while the “tulip” flame formation is only found when the aspect ratio > 6.7. In the experiments, the maximum flame speed has the similar trend with laminar flame speed regardless of whether there was “tulip” flame formation and the maximum flame propagation speed occurs with an aspect ratio of 10. The maximum overpressure decreases with cross sectional area and increases with the length of the duct. The time corresponding to maximum overpressure, t max decreases with hydrogen addition. Meanwhile, t max increases with cross sectional area and length of duct.

An experimental study has been conducted to investigate the effects of hydrogen addition on the fundamental propagation characteristics of methane/air premixed flames at different equivalence ratios in a venting duct. The hydrogen fraction in the methane–hydrogen mixture was varied from 0 to 1 at equivalence ratios of 0.8, 1.0 and 1.2. The results indicate that the tendency towards flame instability increased with the fraction of hydrogen, and the premixed hydrogen/methane flame underwent a complex shape change with the increasing hydrogen fraction. The tulip flame only formed when the fraction of hydrogen ranged from 0 to 50% at an equivalence ratio of 0.8. It was also found that the flame front speed and the overpressure increased significantly with the hydrogen fraction. For all equivalence ratios, the stoichiometric flame ( Φ = 1.0) has the shortest time of flame propagation and the maximum overpressure.

The lithologies and permeabilities of caprocks have significant effects on coalbed methane (CBM) preservation. To study the controlling effects of redbeds deposits on CBM occurrence in the Xutuan and Zhaoji Coal Mines of the Huaibei Coalfield in China, the variations in mineralogical composition, microstructure, pore structure, permeability and diffusion among the redbeds caprock samples, Neogene clay rock and ordinary coal measure strata (sandstone, mudstone and siltstone) have been investigated. The results indicate that redbeds are porous rocks with weaker sealing capabilities than coal strata caprocks, allowing CBM to escape from the coal seam. The formation of the lithological properties of the redbeds is closely related to their mineralogical composition and diagenesis. Redbeds are mainly composed of illite (57–64%), calcite (25–33%), quartz (6–12%), muscovite (3–8%) and pyrite (

The influence of gas diffusion behavior on gas flow and permeability evolution in coal seams is evaluated in this paper. Coalbed methane (CBM) reservoirs differ from conventional porous media and fractured gas reservoirs due to certain unique features, which lead to two distinct gas pressures: one in fractures and the other in the coal matrix. The latter pressure, also known as the sorption pressure, will be used in calculating sorption-based volume changes. The effective stress laws for single-porosity media is not suitable for CBM reservoirs, and the effective stress laws for multi-porosity media need to be applied. The realization of the above two points is based on the study of the two-phase state of gas migration (involving Fickian diffusion and Darcy flow) in a coal seam. Then, a general porosity and permeability model based on the P-M model is proposed to fit this phenomenon. Moreover, the Klinkenberg effect has been taken into account and set as a reference object. Finally, a coupled gas flow and coal deformation model is proposed and solved by using a finite element method. The numerical results indicate that the effects of gas diffusion behavior and Klinkenberg behavior can have a critical influence on the gas pressure, residual gas content, and permeability evolution during the entire methane degasification period, and the impacts of the two effects are of the same order of magnitude. Without considering the gas diffusion effect, the gas pressure and residual gas content will be underestimated, and the permeability will be overestimated.

The mining of protective coal seams can cause changes in geostress, leading to changes in the permeability of coal rock and creating favorable conditions for gas extraction from coal seams. At the Haishiwan Coal Mine, field tests using remote protective coal seam mining were performed in the protected layer, which is rich in CO2 gas. In remote protective longwall mining, the permeability and composition of extracted stress-relief gas can vary. Under the conditions of remote protective longwall mining, the permeability of a protected coal seam can be generally described by the Liu model. During protective layer mining, the permeability of the protective layer increases rapidly with the release of stress, then decreases gradually with the recovery of the geostress. However, matrix shrinkage and decreased pore pressure caused by CO2 desorption from coal seams also cannot be ignored when considering the factors that affect the permeability. Thus, it is necessary to appropriately configure the cross-measure boreholes in advance to drain the stress-relief gas during remote protective layer mining. Stressrelief CO2 gas extraction presents multiple consecutive peaks. The No. 2 coal seam has different trap pressure systems as CO2 migrates into the coal seam. The protected seam experiences different effective stresses during protective layer mining, and the permeabilities appear to periodically increase due to differences in the original permeability. The various permeability and diffusion coefficients for CO2 and CH4 in coal induce CO2 and CH4 fractionation in the roof and floor of the No. 2 coal seam.

The fault has an important impact on coal bed methane (CBM) occurrence. The complexity of the fault exacerbates the variability of CBM occurrence, which increases the difficulty of mine gas prevention and thus threats to mining safety seriously. The coal-bearing strata in the Renlou coal mine located in the Linhuan mining area of the Huaibei coalfield have undergone three transformations caused by major tectonic movements since its formation. In addition, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. The normal faults F 3 and F 7 and reverse fault F 5 in southern Renlou coal mine are open structures. The coal seam between faults F 7 and F 5 is in the emission range of the two faults (partially in the superimposed range) in which a significant amount of gas escapes. The formation between faults F 5 and F 7 is horst, and it turns from NS to EW. The stress in the formation is large, which results in the presence of numerous fractures and cleats. Therefore, there is difference of methane occurrence between the two regions. Between faults F 7 and F 5 , the gas pressure gradient of coal seam No. 7 2 is 0.00356 MPa/m with a maximum value of 0.39 MPa (−633.5 m). However, the gas pressure gradient of coal seam No. 7 2 between faults F 3 and F 2 is 0.00726 MPa/m and the maximum measured gas pressure is 1.7 MPa (−692 m). The relative methane emission (RME) of working face 7257 located between faults F 7 and F 5 was 2.01 m 3 /t on average with a maximum value of 4.96 m 3 /t. The average RME of working face ІІ7211 located between faults F 3 and F 2 reached 12 m 3 /t in the area of 0–600 m away from F 3 fault and increased to 30 m 3 /t in the region of >600 m away from fault F 3 .

The permeability of coal is an important parameter in mine methane control and coal bed methane exploitation because it determines the practicability of methane extraction. We developed a new coal permeability model under tri-axial stress conditions. In our model, the coal matrix is compressible and Biot’s coefficient, which is considered to be 1 in existing models, varies between 0 and 1. Only a portion of the matrix deformation, which is represented by the effective coal matrix deformation factor \(f_\mathrm{m}\), contributes to fracture deformation. The factor \(f_\mathrm{m}\) is a parameter of the coal structure and is a constant between 0 and 1 for a specific coal. Laboratory tests indicate that the Sulcis coal sample has an \(f_\mathrm{m}\) value of 0.1794 for \(\hbox {N}_{2}\) and \(\hbox {CO}_{2}\). The proposed permeability model was evaluated using published data for the Sulcis coal sample and is compared to three popular permeability models. The proposed model agrees well with the observed permeability changes and can predict the permeability of coal better than the other models. The sensitivity of the new model to changes in the physical, mechanical and adsorption deformation parameters of the coal was investigated. Biot’s coefficient and the bulk modulus mainly affect the effective stress term in the proposed model. The sorption deformation parameters and the factor \(f_\mathrm{m}\) affect the coal matrix deformation term.

Thermography analysis of a roadway excavation experiment in 601 inclined stratified rocks W.L. Gong , J. Wang , Y.X. Gong , P.Y. Guo a,b a State Key Laboratory for Geomechanics and Deep Underground Engineering, Beijing 100083, China b School of Mechanics and Civil Engineering, China University of Mining and Technology, Beijing 100083, China c School of Mathematics and Statistics, South Central University for Nationalities, Wuhan 430074, China

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. Language: en

Coal and gas outbursts are among the most destructive dynamic disasters. It is very important to study the failure process and mechanism of coal and gas outbursts in coal mines. Based on outburst cases and outburst experiments, the outburst process and failure types of coal are discussed, and the flow process of outburst coal is studied in depth. This paper concludes that the whole process of outburst generally involves four stages (preparation, trigger, development and termination), which include both time and spatial dimensions. There are two types of coal failure: shear failure and tensile failure. Under the effect of the gas pressure gradient, the coal exhibits tensile failure, and the coal ultimately fails in the form of multiple similar layers. After the layer is separated, the pressure difference between the two sides of the layer is found to be stable for a period of time, and a pressure difference coefficient is introduced to characterize the change of this pressure difference before and after the separation of the layer. Furthermore, the flow process of outburst coal is divided into a quasi-static phase, a constant acceleration phase, a freefall phase and a deceleration phase. Accordingly, a simplified model of the coal flow is established, and the model is calculated using the data of outburst experiments. Based on the layered failure of outburst coal, the flow velocity of outburst coal is obtained. The distribution features of outburst coal in both the experiments and the outburst cases are explained as well.

The coal permeability is an important parameter in mine methane control and coal bed methane (CBM) exploitation, which determines the practicability of methane extraction. Permeability prediction in deep coal seam plays a significant role in evaluating the practicability of CBM exploitation. The coal permeability depends on the coal fractures controlled by strata stress, gas pressure, and strata temperature which change with depth. The effect of the strata stress, gas pressure, and strata temperature on the coal (the coal matrix and fracture) under triaxial stress and strain conditions was studied. Then we got the change of coal porosity with strata stress, gas pressure, and strata temperature and established a coal permeability model under tri-axial stress and strain conditions. The permeability of the No. 3 coal seam of the Southern Qinshui Basin in China was predicted, which is consistent with that tested in the field. The effect of the sorption swelling on porosity (permeability) firstly increases rapidly and then slowly with the increase of depth. However, the effect of thermal expansion and effective stress compression on porosity (permeability) increases linearly with the increase of depth. The most effective way to improve the permeability in exploiting CBM or extracting methane is to reduce the effective stress.