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Coal and gas outburst is a type of coal mine dynamic disaster with complex causes and serious consequences. In China, the number and scale of outburst disasters are among the highest in the world. It is an inevitable stage for outburst that the mechanical instability of the combination of intact coal and tectonic coal (CICTC) in strata. The dip angle of the interface has an important influence on the instability of CICTC-containing gas. To further understand the instability process of CICTC, the uncoordinated deformation of CICTC was defined. According to this, three strain conditions were delimited. The variations of stress under three strain conditions were analyzed, the expressions of additional interfacial stress were established. Then, based on the Mohr–Coulomb criterion, the strength characteristics and failure forms were analyzed. It was found that the dip angle deflects the principal stress in the interface, and the influences of additional stresses on the value of principal stress are opposite. These two reasons make the principal stress state and instability process of the combination complicated. Finally, combined with actual parameters, the instability processes of CICTC under different stress paths and buried depths were studied. It can be found that the failures of CICTC under different stress paths were concentrated appearing in the tectonic coal body, intact coal interface, and tectonic coal interface. With the increase of buried depth, the initial maximum principal stress increases and the reduction from additional stresses to the strength of intact coal interface enhances, which makes the combination damage easily and the failure form change. The research results in this paper can provide a theoretical basis to prevent and control the outburst of the combination containing gas considering the dip angle of the interface. Highlights: The uncoordinated deformation of combination considering the dip angle was first defined. The expressions of additional interfacial stress of combination considering the dip angle were established The influences of different stress paths and buried depths for the instability of combination were discussed quantificationally. [ABSTRACT FROM AUTHOR]

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

A large proportion of minable coal seams in China belong to low-permeability soft coal seams. Such coal seams suffer serious coal and gas outburst hazards and endure a high incidence of major disasters in coal mines. The adoption of the high-pressure water jet (HPWJ) hydraulic flushing cavity can effectively promote the gas drainage efficiency and volume and eliminate the hidden danger of gas disasters. Nevertheless, the shape and impact pressure of rotating HPWJ are rarely researched. In this study, on the basis of the numerical simulation, the axial and radial stress distributions of HPWJ and the energy-gathering effect of a conical-cylindrical combined nozzle were analyzed. It is concluded that the submerged condition will accelerate the attenuation of jet velocity and reduce the impact strength of the jet. The jet diffusion angle grows with the increases in the nozzle diameter and water pressure, and 24° is the optimal contraction angle. Finally, the influences of factors such as the rotation speed on the shape and impact pressure of HPWJ were explored, and the results show that the rotation speed should be controlled within 90 r/min. The research findings lay the foundation of the study on the mechanism of coal crushing by HPWJ and provide technical support for the research and development of drilling and flushing integrated equipment. [ABSTRACT FROM AUTHOR]

• The distribution of plastic failure and gas pressure of the coal in front of roadway during mining were analyzed. • The calculation model of gas outburst energy and the energy criterion of outburst were established. • The energy instability of gas outburst of coal combinations was assessed. The complexity of the coal and gas outburst process makes the gas outburst mechanism difficult to understand, leading to serious disasters of coal and gas outbursts in China. However, the presence of deformed coal may promote the occurrence of coal and gas outbursts, and several normal coal layers and deformed coal layers often occur at the same location. To better assess the energy instability of gas outbursts of coal combinations, the coal and rock were assumed to be homogeneous, isotropic mediums, and the deformation of coal and rock was assumed to be infinitesimal. Then, ignoring the influence of the change of gas pressure on the stress field during the process of gas emissions, the distribution of the horizontal stress, plastic failure and gas pressure in the coal in front of the roadway were analyzed in three different cases. The results show that there will be a large horizontal tensile stress between the normal coal and deformed coal, which may make plastic failure extend from the deformed coal to the normal coal through the interface. Due to plastic failure, the permeability of deformed coal in coal combinations will increase. Moreover, the deformed coal itself not only enables gas to migrate rapidly but also increases the gas emissions of the normal coal adjacent to the deformed coal. Then, a new model of gas outburst energy and an energy criterion of outburst were established. It was found that the total outburst energy of the normal coal in combinations is approximately 149 times that of a normal coal monomer, so the deformed coal increases the total outburst energy of the normal coal in coal combinations. In regard to the outburst energy of coal combinations, the desorbed gas expansion energy is the highest, which is 1–6 times the elastic energy and 5–17 times the gas expansion energy. In addition, measures should be taken to first reduce the expansion energy caused by desorbed gas, and second measures should be taken to reduce the elastic energy caused by ground stress. Finally, a series of control measures of gas outbursts for different coal seams were proposed. These research results, which were obtained under certain assumptions, can provide a theoretical basis for the selection of measures for the control of coal and gas outbursts of coal combinations. [ABSTRACT FROM AUTHOR]

• A general form of simplified diffusion model considering steady-state and unsteady-state diffusion was established. • The n in the model, as a gas diffusion state discriminant index, was determined. • The gas diffusion state was divided into three zones. As the prediction indexes of gas outbursts, the gas content and gas desorption index K 1 of drilling cuttings are affected by the gas diffusion in coal particles. According to the current research results, the steady-state and unsteady-state gas diffusion cannot be excellently defined, which leads to the measurement distortion of prediction indexes of gas outbursts in coal particles. Therefore, a general form of simplified diffusion model (GFSDM) considering steady-state and unsteady-state diffusion was established. And the experimental validation and reliability analysis were carried out. The results show that the fitting accuracy of GFSDM for gas diffusion data and the calculation accuracy of the lost gas amount are both greater than those of the simplified unipore diffusion model and simplified mathematical unsteady-state diffusion model. Then, the discriminant index of gas diffusion state and the boundaries of pure diffusion and pure steady diffusion were determined. Based on the boundaries of pure unsteady diffusion and pure steady diffusion, the gas diffusion state was divided into three zones, namely the ultra-negative unsteady diffusion zone, negative unsteady diffusion zone and positive unsteady diffusion zone. The influence mechanism of gas pressure, particle size and tectonism on the gas diffusion state was discussed. It is found that the main reasons for controlling the change of effective diffusion coefficients with time include the influence of diffusion length, influence of free gas pressure and influence of absorbed gas thickness. The research results can improve the gas diffusion theory and provide a theoretical basis for predicting coal and gas outbursts. [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]

Gas outburst disasters are becoming more serious as the underground coal mines become deeper in China, and a thick zone of deformed coal provides conditions favorable to coal and gas outbursts. The Daning coal mine's main mining seam is the No. 3 coal seam with coal and gas outburst hazard, which often contains two normal coal sub-layers and one deformed sub-layer. Considering both the geological conditions of the coal seam and applications of the in-seam directional longhole drilling technology, a new schematic diagram of in-seam directional longholes for gas drainage is developed. The two borehole layout models of longwall panel and main entries for gas outburst disasters control have been successfully applied. The gas drainage rates of both models are >70 %, and the residual gas contents are both <8 m/t, which can be considered that the gas outburst disasters were effectively controlled. To better guide gas drainage, gas drainage normal and failure modes have been obtained. Although in-seam directional longhole technology has been successfully applied for regional gas drainage with benefits to gas outburst control, there are also some problems that are detrimental to greenhouse gas reductions in gas drainage and gas utilization. The three main problems are air leakage failure in gas drainage, decreasing gas concentration and a low gas utilization ratio. To address the problems mentioned above, five improvements are suggested. [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]