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Coalbed methane (CBM) is a clean energy source, but its utilization is inefficient due to the complexity and low accuracy of its emission prediction model. In this research, we constructed a mathematical model of gas emission from the excavation workface, and combined the experimental results to propose a new model for accurate and concise prediction. The new model was validated in the field workface and compared with the traditional prediction model. Moreover, the sensitivity of gas emission parameters and the participation ratio of gas emission sources were analyzed. The study results show that the new model has higher calculation accuracy than the old model prediction, with an average error reduction of 4.693%. In the excavation workface, the coal fall gas emission conforms to the negative power function equation, and the coal wall gas emission conforms to the negative exponential function equation. In the early stage of excavation, the proportion of coal fall gas emission is higher than that of coal wall gas emission, and the peak proportion reaches 58.5%. In the later stage, the proportion of coal fall gas emission gradually decreases to below 30%. The order of the sensitivity of gas emission parameters is coal wall gas initial velocity > coal fall gas decay coefficient > coal fall gas initial velocity > coal wall gas decay coefficient. The new model is successfully applied in engineering, which helps to improve the efficiency of coal mine gas disaster control and utilization. [ABSTRACT FROM AUTHOR]

Gas drainage technology is closely related to the flow of methane in coal. Desorption and diffusion of gas in coal is an important basic research area, and particle size is one of the influencing factors. In this study, the desorption and diffusion characteristics of coal samples mixed with different particle sizes over time were measured under constant temperature and pressure. The results show that in the initial stage, the desorption volume and desorption rate of coal samples are relatively large. The improved desorption model can well fit the data and the order of desorption volume is negatively correlated to the mean particle size of coal samples. The single small particle coal sample has the largest desorption volume which reached 3.77ml/g. Both the unipore model and the constructed unsteady dynamic diffusion model can well describe the diffusion of gas in coal, the diffusion coefficient of coal sample decreases with time, and the decay rate decreases gradually with time until it remains unchanged. The calculation results of the two models show that the diffusion coefficient ranges of gas in coal between 10−10m2/s and 10−12m2/s. For single-particle coal samples, the diffusion coefficient is positively correlated with particle size. The diffusion coefficient D is affected by the particle size and the degree of compaction between coal particles. These observations indicate that coal samples with different particle sizes will affect the desorption and diffusion of gas, and it has a positive impact on methane recovery. [ABSTRACT FROM AUTHOR]

Coal and gas outbursts are complex dynamic phenomena and are highly hazardous. This article established an experimental system to research the damaging effect of gas on particulate coal during coal and gas outbursts, which replicated the outburst scenario by instantaneously releasing different-pressure gas. The results show that the higher the gas pressure is, the larger the new surface area is, and the gas damages the pulverized coal more drastically. The mass ratio of new granular size coal is between 8–27%. The surface area increased significantly (73.0%-245.2%). The mean order of increasing surface area is SC (1.102 m2)> DC (0.9875 m2)> SN (0.8999 m2)> DN (0.8369 m2), which indicates that the extent to which gas destroys particulate coal is inversely proportional to the solidity coefficient and positively proportional to the adsorptivity. The ability of weakly adsorbable gases to break particulate coal is more affected by gas pressure. The degree of breakage of low-firmness coals is more responsive to gas pressure. The percentage of coal mass broken by gas is positively correlated with gas pressure and adsorptivity and negatively correlated with the firmness coefficient. Finally, the quick release of gas occurs in three stages: an acceleration section (0–0.05 s), a high-speed section (0.05–0.15 s), and a deceleration section (after 0.15 s). [ABSTRACT FROM AUTHOR]

Coal is a complex porous medium. When gas exists in the coal body, the fissure structure of coal and the occurrence of gas adsorption make the stress state of gas-bearing coal different from that of ordinary coal. Therefore, the stress state of gas-bearing coal cannot be accurately described by the Terzaghi effective stress, which ignores the effects of crack deformation and methane. To improve the accuracy of gas-bearing coal analysis, this study considered the corrosive effect of free gas and adsorbed gas and the crack closure effect to establish a novel statistical damage constitutive model with consideration to the combined effects of free gas, adsorbed gas, and coal cracking. The established model was used to elucidate the effects of gas-bearing coal on the coal's mechanical deformation and damage characteristics under different stress conditions. The model was validated through full stress–strain experiments on gas-bearing coal under different axial pressure and gas pressure. The results obtained by the proposed statistical damage constitutive model can better describe the stress state of the gas-bearing coal, particularly in the initial stage of void compaction, and are in good agreement with the experimental data. The proposed model provides the theoretical foundation for future studies on the characteristic mechanism of gas-bearing coal. [ABSTRACT FROM AUTHOR]

Understanding gas diffusion characteristics of coal particles is a significant theoretical basis for the exploitation of coalbed methane. Generally, diffusion experiments are performed under constant pressure conditions, and scholars seldom discuss gas desorption experiments with constant volume, but the recognition of this characteristic is also equally important. Based on this, the paper conducted coal particles desorption experiments under conditions of constant volume to explore gas diffusion characteristics under different adsorption equilibrium pressures and initial emission environmental pressures. The experimental data showed that the variation law of desorption amount with time under constant volume conditions conformed to the Langmuir-like equation. Meanwhile, the rising of initial emission environmental pressure reinforced the linear relationship between the limitation desorption amount and the adsorption equilibrium pressure. When the initial emission environmental pressure was 0.10 MPa, the correlation coefficient of linear fitting between them was as high as 0.9826. To better describe the diffusion process of this condition, a dynamic diffusion model of coal particles gas diffusion under constant volume conditions was established. The calculated values of this model had a preferable consistency with the experimental data, and the correlation coefficients were all over 0.99, which verified the reliability of the model. This research makes up for the theory of gas diffusion and provides a theoretical basis for coalbed methane extraction. [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]

Mine disasters, as associated problems in the development of coal resources, always threaten the life safety of miners and the production safety of enterprises. In coal roadway excavation, the coal in front of the excavation face is in a state of concentrated stress and the deformation is serious. The instability of the coal seam under stress is an important cause of the occurrence of coal and gas outburst. Thus, knowing the mechanical characteristics of surrounding rock in front of the working face is of great significance for deep understanding and effective prevention and control of coal-rock dynamic disasters. By analyzing the characteristics of stress evolution and deformation and failure in front of the heading face, the analytical solution of plastic region distribution in front of the heading face is obtained. According to the established mechanical model, MATLAB numerical calculation software is used to analyze different examples. The influence of different mechanical parameters on the stress state of the working face is studied. The influence parameters of the morphological characteristics of the plastic region are determined, and the change law and characteristics of the plastic region in front of the working face are studied. At the same time, through the characteristics of stress field, the working face dynamic risk assessment of coal and rock dynamic disaster can be realized, which can provide a powerful guarantee for the prevention and control of gas dynamic disasters in deep mine. [ABSTRACT FROM AUTHOR]

Coal and gas outburst is a serious disaster in the mining, and crosscut coal is the most prone to cause the coal and gas outburst. In order to study the mechanism of outburst, this paper studied the stress distribution characteristics in front of the working face in the process of rock roadway driving before implementing crosscut coal firstly. On this basis, the hypothesis of stress distribution on the "strong and soft face" of coal-rock mass was proposed. And by using the hypothesis, the mechanical equation for the parameter changes of the plastic zone in front of the working face in crosscut coal was established, and the width and morphological characteristics of the plastic zone in crosscut coal were analyzed. The results show that during the process, as the working face moves forward, the shape distribution of the plastic zone in front of the working face changes from the "semielliptical" to "semibutterfly," And based on the distribution characteristics of plastic zone in working face in crosscut coal, modification proposals for outburst-prevention measures were proposed and provided technical support for the outburst-prevention measures and key parameters setting to prevent the coal outburst in crosscut coal. [ABSTRACT FROM AUTHOR]

The high‐pressure gas expansion‐induced deformation and dynamic fracture of coal are important parts of coal and gas outburst. To better understand the law of this process, laboratory experiments and numerical simulation are used to study the law of damage. A cavity with different pressures of CH4 or N2 was destroyed by a jack to achieve the rapid expansion of the gas and coal fracture inside. The particle size distribution of the coal particles before and after the experiment was measured, and the breakage ratio and the newly added surface area were calculated. The experimental results indicate that during the gas expansion process, the breakage ratio of coal and the newly added surface area clearly increase with the increase in gas pressure. Finally, a numerical model based on peridynamic theory was developed to simulate crack generation and the propagation of coal induced by the expansion of gases at different pressures. The numerical simulation results show that the higher the initial gas pressure is, the higher the number of failure units. Moreover, only when the gas pressure is large enough will the coal crack in various directions at the same time. [ABSTRACT FROM AUTHOR]

• The gas desorption process absorbs heat to reduce the temperature of the experimental system. • Coal sample particle size and adsorption equilibrium pressure have a significant effect on coal gas desorption. • As the coal sample particle size becomes smaller and the equilibrium pressure increases, the desorption amount and temperature changes increase, but the magnitude of the increase decreases. • The amount of desorption can be described linearly by the desorption temperature variation. Coal bed methane (CBM) is a clean energy source and its main component is methane, however, due to the complex conditions of its occurrence, its content detection is inaccurate and its utilization is not efficient. To further investigate the desorption and diffusion of methane in coal seams, this study uses an experimental system to investigate the desorption volume and temperature variations of gas desorption at different equilibrium pressures for three different particle sizes of coal samples. The results show that the desorption process is a heat absorption and cooling process, which is mainly influenced by the equilibrium pressure and the particle size of the coal samples. The amount of desorption and the absolute desorption temperature variation increases with increasing equilibrium pressure and increases with decreasing particle size of the coal sample. The desorption temperature variation at a desorption time of approximately 3000 s is close to the minimum. There is a linear relationship between the gas desorption volume and the absolute desorption temperature variation, which can be used to predict the desorption volume using the absolute desorption temperature variation. This will assist in the accurate measurement of coal bed methane content and the prevention and control of coal mine gas disasters. [ABSTRACT FROM AUTHOR]

The diffusion coefficient of gases in coal varies with time. This study aims to develop an unsteady dynamic diffusion (UDD) model based on the decay of diffusion coefficient with time and the change of integral. This study conducted a series of gas desorption and diffusion experiments with three different combinations of particle sizes and gas pressures and compared the diffusion coefficients of the three models. The UDD model exhibited good fitting results, and both the UDD and bidisperse models fitted the experimental data better than the unipore model. In addition, the dynamic diffusion coefficient (DDe) decreased rapidly in the initial stage but gradually decreased to a stable level in the later stage. All the effective diffusion coefficients of the three models negatively correlated with the particle size. In the unipore model, the diffusion coefficient of coal samples with three particle sizes increased with gas pressure. In the bidisperse and UDD models, the diffusion coefficients (Dae, Die, and DDe) of 0.25–0.5 mm and 0.5–1.0 mm coal samples increased with gas pressure. However, DDe and Dae of 1.0–1.25 mm coal samples increased first and then decreased. Furthermore, Die decreased first and then increased, with no sign of significant pressure dependence. Finally, the correlation and significance between the constant and diffusion coefficient in the UDD model was investigated. [ABSTRACT FROM AUTHOR]