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Overburden separation grout filling (OSGF) technology compacts the fragmented rock mass in the goaf and changes the seepage characteristics and distribution of the “three zones” of spontaneous combustion in the goaf. In order to investigate the range of spontaneous combustion risks of residual coal in the goaf under the OSGF in the 22301 fully-mechanized caving face of the Tunlan Coal Mine, based on a combination of field measurement and numerical simulation, accurate demarcation of the spontaneous combustion “three zones” in the goaf was carried out. A three-dimensional mathematical model of borehole porosity was also proposed to evaluate the influence of different OSGF borehole locations on the goaf airflow field. The results show that the modified three-dimensional mathematical model of porosity can better reflect the gas flow field in the goaf after OSGF, and the residual accuracy of its simulation results is improved by 22% compared with the field measurement. Compared with the situation without OSGF, the danger of natural ignition in the goaf is effectively inhibited due to the compaction effect after OSGF. Meanwhile, the air leakage flow converges upward, resulting in an increase of oxygen concentration at higher positions in the caving zone of the goaf. The layout parameters of grouting holes will have an impact on the flow field of goaf, and the correlation degree is the spacing of holes, the number of holes and the influence radius of holes.

Abstract Saturation characteristics of water and capillary pressure are important for hydrofracturing or water flooding, which is widely adopted in dust suppression, methane displacement, and gas and coal outburst elimination. In this paper, we have studied the water saturation characteristics of coal samples with different coal ranks and their effect on capillary pressure. It was found that the isothermal adsorption of water vapor from coal samples can be divided into three processes: monolayer adsorption, multilayer adsorption, and capillary condensation. The dent isothermal adsorption model can well describe water vapor adsorption of coal samples, indicating water molecules were first adsorbed on the first‐order adsorption site of pore surfaces at low relative humidity and then on the second‐order adsorption site with increasing relative humidity. An obvious hysteresis occurred in the process of water imbibition and drainage. A capillary pressure of ~325 MPa or even larger was required to displace the water from the coal. Besides, micro‐ and mesopores play a more important role in capillary pressure. The capillary pressure of coal samples was positively correlated with wettability at lower water saturation, but it is not significant at higher capillary pressures. Therefore, the article's results provide a new direction for solving the water‐locking effect in coal seams.

The content of unfrozen water in the freezing process of coal body affects the microscopic pore structure and macroscopic mechanical properties of coal body and determines the permeability-enhancement effect of coal seam and the extraction efficiency of coal mine gas. To investigate the evolution mechanism of unfrozen water content in the melting process of lignite, this paper takes the melting process of lignite liquid nitrogen after freezing for 150 min as the research object and quantifies the spatial change process of unfrozen water distribution based on two-dimensional nuclear magnetic resonance technology. Through the accurate interpretation of the superimposed signals of different fluids, the 2D NMR technique can more easily obtain the spatial distribution of different fluids and even the specific content of fluids in different pores in coals. The results show that at −196 °C, the unfrozen water mainly existed in the small coal pore and the small ice pore in the large pore. As the temperature rose, the pores melted, and free water began to be produced. The mathematical model analysis shows that there was intermolecular potential energy between fluid molecules and the coal pore wall, and the pore wall exerted a part of pressure on its internal fluid, and the pressure affected the melting point of pore ice with pore diameter and melting temperature, resulting in the difference of unfrozen water content.

Artificial improvement of coal seam permeability is the key to solve the low gas drainage efficiency and prevent gas dynamic disaster. Plasma based on physical discharge is one of the effective means of coal seam antireflection. However, previous studies focus on the characterization of pore-fissure structure of single breakdown, ignore the polarization effect of plasma on coal, and lack of in-depth research on the electrical properties and the evolution characteristics of pore-fissure structure under the action of cyclical plasma of loaded coal. In this paper, the cyclical plasma breakdown experiments of coal samples were carried out. The voltage and current waveforms in the process of coal breakdown were monitored thorough combining with high-voltage attenuation rod and the Roche coil, and the variation of electrical parameters such as pre-breakdown period, plasma breakdown period, number of wave peaks, peak voltage, peak current and energy conversion efficiency were analyzed. The evolution of small pores, medium pores, large pores and micro cracks under the conditions of 1, 5, 10, 15 and 20 times of breakdown was tested by NMR. The fractal dimension of seepage pore was also discussed through geometric fractal theory. The results show that the pre-breakdown period is at the level of thousands of microseconds only for the first time, and then it will drop to tens of microseconds, while the plasma breakdown period shows a “ladder” growth with the increase of breakdown times. The energy conversion efficiency is maintained at 28.7%−55.9%, showing a trend of rapid increase at first and then stable, indicating that the polarization effect of plasma on the electrical properties of coal is limited. The growth rate of medium and large pores is the most obvious, and some micro fracture structures will achieve a “0” breakthrough, indicating that plasma will significantly improve the seepage of gas. The fractal dimension shows a decreasing trend after breakdown, which proves that the originally isolated pore structure will be connected by the fracture from the perspective of pore fracture space dimension. The similarity between the change trend of porosity and energy conversion efficiency shows that plasma has an impact on the electrical properties of coal, and the change of coal electrical properties will also have an impact on the distribution of plasma channels.

During the process of coal spontaneous combustion (CSC), a plethora of combustible gases alongside inert gases, such as CO2, are copiously generated. However, prior investigations have regrettably overlooked the pivotal influence of inert gas production on the propensity for methane explosions during CSC. To investigate the impact of the flue gas environment generated by CSC, containing both combustible and inert gases, on the risk of methane explosion, a high-temperature programmed heating test system for CSC was employed to analyze the generation pattern of flue gas. It was found that CO, CO2, and CH4 were continuously generated in large quantities during the process of CSC, which are the main components of CSC flue gas. The effect of the concentration and component ratio (CCO2/CCO) of the flue gas on the methane explosion limit was tested. It was found that the CSC flue gas led to a decrease in the methane explosion limit, and that the explosion limit range was facilitated at 0 < CCO2/CCO < 0.543 and suppressed at CCO2/CCO > 0.543. As the temperature of CSC increases, the risk of methane explosion is initially suppressed. When the coal temperature exceeds 330~410 °C, the explosion risk rapidly expands.

Abstract Protective seam mining is the preferred method for gas control and safe mining in high‐outburst coal seam groups. However, a hard‐thick sandstone roof (HTSR) triggers new gas problems when it covers directly above the first‐mined protective coal seam. To solve these problems, a typical case study in No. 12 Coal Mine was conducted. The simulation results indicate that the HTSR greatly prevents the plastic deformation of the upper roof formations without affecting the expansive deformation in the underlying protected seams. The pressure‐relief gas in protected seams will flow into the protective seam without continuing to migrate to roof strata, thereby turning the protective seam into a new high‐risk area. Therefore, roof presplitting (RPS) technology was proposed to effectively break the HTSR, create roof fracture passages, and divert the gob gas of the protective seam upward. Meanwhile, gob‐side entry construction (GSEC) was conducted to enhance gas discharge in the protective seam by adopting rapid filling, roof cutting, and active supporting technologies. Through the upper protective seam mining, RPS, and GSEC, gas migration passages were constructed step‐by‐step to divert the gas from high‐risk areas to low‐risk areas. As a key first step, gas diversion not only optimized the distribution of gas sources but also significantly reduced gas hazards. Source‐by‐source extraction was then performed to effectively control the high‐concentration gas in the newly formed gas source, further eliminating the mining risk. By applying the stepwise risk control method combined with gas diversion and extraction, the safe production was successfully achieved with excellent outburst prevention effects, and the gas extraction rate was significantly improved to more than 66%. This practice could be used for deep high‐gassy or high‐outburst coal seam group mining.

Abstract High‐voltage electrical pulse (HVEP) technology, a potential method for degassing coal seam in the future, has made great progress in field application. However, the previous studies mainly concentrated on the coal‐crushing effect of the action of single pulse, ignoring the influence of cyclic single pulse on the fracture structure of coal. In this paper, Huaibei anthracite coal and Inner Mongolia bituminous coal were taken as the experimental objects to study the effect of cyclic single pulse on the fracture structure of coal. The results show that the uneven distribution of electric field in the coal causes the crack to break along the interface between the mineral and the coal. Besides, the characteristics of cracks and functional groups of coal before and after electrical breakdown were investigated through scanning electron microscope and Fourier transform infrared spectroscopy. The results suggest that the relative contents of oxygen‐containing functional groups are reduced after electrical breakdown, which promotes the gas desorption from the surfaces of coal samples. Moreover, changes in CH4 adsorption capacity of HVEP‐treated coal were studied through a high‐pressure adsorption instrument, with the adsorption temperature set from 40 to 120°C. The results show that the adsorption amount of electrically broken coal is smaller than that of raw coal, which proves that temperature and current exert similar influences on the adsorption capacity of coal sample. Additionally, current waveforms indicate that the peak current increases with the number of cycle, whereas the breakdown time decreases with it. However, the peak current and breakdown time will eventually become stable as the number of cycles grows, demonstrating that electrical properties of coal are changed in the process of breakdown, which thus affects the next discharge.

The coal-rock projectiles induced by gas explosions in coal mines have a strong destructive effect on mine facilities and equipment, which are mostly made of steel. The LS-DYNA is used to simulate the process of coal projectile striking the steel plate. The results show that during the failure process, the morphological change of the projectile consists of three stages including plastic deformation, partial crushing, and complete crushing, and the steel plate pits at the impact point. The duration of the stress peak value of the steel plate increases as the impact velocity increases. The farther away from the center point, the smaller the stress peak value is, and the shorter the duration is. When the impact velocity is 100 m/s and 300 m/s, the axial velocity curve at the impact point of the projectile is pulsating. When the impact velocity reaches 500 m/s, the axial velocity of the projectile rapidly increases and then tends to be stable. As the impact velocity increases, the energy absorbed by the steel plate increases, and the rate of increase also increases. However, the proportion between the absorbed energy of the steel plate and the initial kinetic energy of the projectile decreases. The projectile conversion energy of the 1 mm steel plate is slightly larger than that of 1.5 mm and 2 mm steel plates, and the energy absorbed by the steel plate decreases with the increase in thickness of the steel plate. When the thickness of the steel plate is more than 2 mm, the thickness of the steel plate has little effect. The results of the study have a direct sense for the antiknock design of coal mine facilities and equipment.

Coalbed methane is a kind of high-quality clean energy. The underground gas drainage can achieve the efficient utilization of coalbed methane. Permeability is the key factor affecting gas drainage. At present, permeability research mainly considers the effect of gas adsorption-induced matrix swelling deformation as well as the effective stress. However, experiments indicate that the variation of pore pressure has an impact on the mechanical properties of coal, which further changes the coal permeability. Due to the differences between the theory and experimental results, firstly, the dynamic elastic modulus model was established in this paper. Then, an experiment was conducted to prove that the elastic modulus of coal-containing gas decreases with the increasing gas pressure. On this basis, a permeability model considering internal swelling deformation was constructed. Finally, the rationality of the model was validated through the laboratory experiment. Finally, through analyzing the influence of key parameters on the evolution law of permeability, we find that the parameters have three different impacts on permeability. The new permeability model is more accurate in predicting coalbed methane production capacity. The result can provide a theoretical basis for the research of the multifield coupling process during gas drainage.

Abstract Coal deformation caused by gas adsorption is a well‐known phenomenon and has played an important role in many engineering fields. For example, the bulk coal deformation induced by gas is a primary factor in coal and gas outbursts, and the changes in coalbed permeability are primarily determined by the evolution of the coal fracture volumetric strain during gas recovery. However, the current models of coal deformation induced by gas do not fully consider the coal seam field conditions or the deformation factors. To better fit coal seam field conditions and provide a basis for engineering applications, in this work, based on coal's physical structure, coal seam field conditions, the energy balance approach, poroelasticity, mathematical differentiation, and previous scholars' research results, the volumetric strain models of bulk coal, coal fracture, and coal matrix under coal seam field conditions are established. In addition, based on the analysis of the effect of gas pressure on coal deformation, the difference in gas pressure effects on the bulk coal deformation between experimental conditions and coal seam field conditions revealed that when the bulk coal deformation is tested in a laboratory, the gas pressure exerts a compressive effect on the bulk coal. However, under coal seam field conditions, the gas pressure has a swelling effect on the bulk coal. Additionally, gas pressure exerts an compression on the coal matrix and a swelling on the coal fracture under coal seam field conditions. At the end of the paper, the established models were analyzed and discussed from multiple perspectives. It was shown that the coal deformation models established in this paper are all accurate and reliable.

A deeper understanding of the mechanisms by which geological factors (depositional environment and tectonic evolution) control the occurrence of coalbed methane (CBM) is important for the utilization of CBM resources via surface-drilled wells and the elimination of coal-methane outbursts, the latter of which is a key issue for coal mine safety. Based on drill core data, high-pressure isothermal adsorption experiments, scanning electron microscopy experiments, mercury intrusion porosimetry, and X-ray diffraction experiments, the impact of the depositional environment and tectonic evolution on CBM occurrence of the II-1 coal seam of the Shanxi Formation in West Henan was analyzed. Results showed that the depositional environment led to the epigenetic erosion of tidal flat coal-accumulating structures by shallow-delta distributary channel strata. This resulted in the replacement of the original mudstone-sandy mudstone coal seam immediate roof with fine-to-medium grained sandstones, reducing methane storage capacity. Epigenetic erosion by the depositional environment also increased coal body ash content (from 6.9% to 21.4%) and mineral content, filling the cleat system and reducing porosity, reducing methane storage capacity. The maximum methane adsorption capacity of the coal body reduced from 35.7 cm3/g to 30.30 cm3/g, and Langmuir pressure decreased from 1.39 MPa to 0.909 MPa. Hence, the methane adsorption capacity of the coal body decreased while its capacity for methane desorption increased. Owing to the tectonic evolution of West Henan, tectonically deformed coal is common; as it evolves from primary cataclastic coal to granulitic coal, the angle of the diffraction peak increases, d002 decreases, and La, Lc, and Nc increase; these traits are generally consistent with dynamic metamorphism. This is accompanied by increases in the total pore volume and specific surface area of the coal body, further increasing the capacity for methane storage. Increases in micropore volume and specific surface area also increase the ability of the coal body to adsorb methane. Keywords: Coalbed methane, Depositional environment, Tectonic evolution, Epigenetic erosion, Tectonically-deformed coal

Hydraulic slotting in a gas drainage borehole is an effective method of enhancing gas drainage performance. However, it frequently occurs that a large amount of slotting products (mainly the coal slurry and gas) intensely spurt out of the borehole during the slotting, which adversely affects the slotting efficiency. Despite extensive previous investigations on the mechanism and prevention-device design of the spurt during ordinary borehole drilling, a very few studies has focused on the spurt in the slotting process. The slotting spurt is mainly caused by two reasons: the coal and gas outburst in the borehole and the borehole deslagging blockage. This paper focuses on the second reason, and investigates the hydraulic deslagging flow patterns in the annular space between the drill pipe and borehole wall. Results show that there are six deslagging flow patterns when the drill pipe is still: pure slurry flow, pure gas flow, bubble flow, intermittent flow, layering flow and annular flow. When the drill pipe rotates, each of those six flow patterns changes due to the Taylor vortex effect. Outcomes of this study could help to better understand the slotting-spurt mechanism and provide guidance on the anti-spurt strategies through eliminating the borehole deslagging blockage. Keywords: Coal hydraulic slotting, Cross-measure borehole, Borehole spurt, Deslagging flow pattern

In this article, we consider the p-Kirchhoff type problem $$ \Big(1+\lambda\int_{\mathbb{R}^N}|\nabla u|^p +\lambda b\int_{\mathbb{R}^N}|u|^p\Big)(-\Delta_p u+b|u|^{p-2}u) =f(u), x\in\mathbb{R}^N, $$ where $\lambda>0$, the nonlinearity f can reach critical growth. Without the Ambrosetti-Robinowitz condition or the monotonicity condition on f, we prove the existence of positive solutions for the p-Kirchhoff type problem. In addition, we also study the asymptotic behavior of the solutions with respect to the parameter $\lambda\to0$.

The equation used to model the unidirectional flow of methane gas in coal seams is usually formulated as a nonlinear partial differential equation, which needs to be solved numerically with a computer program. Nevertheless, for people without access to the computer program, the conventional numerical method may be inconvenient. Thus, the objective here is to seek some method simpler than the conventional one for solving the flow problem. A commonly used model of the unidirectional methane gas flow is considered, where the methane adsorption is described by the Langmuir isotherm and the free gas is treated as real gas. By introducing the similarity solution, a simple method for solving the flow model is proposed, which can be done on a hand calculator. It is shown by two examples that the gas pressure profile obtained by the proposed method agrees well with the direct numerical solution of the flow model. Keywords: Coalbed methane, Migration, Unidirectional flow, Similarity solution, Simplified solving, Numerical method

As one of the most popular research hotspot of lab-on-chip, digital microfluidic (DMF) technology based on the principle of electrowetting has unique advantages of high-precision, low cost and programmable control. However, due to the limitation of electrodes number, the throughput is hard to further upgrade. Therefore, active matrix electrowetting-on-dielectric (AM-EWOD) technology is a solution to acquire larger scale of driving electrodes. However, the process of manufacturing of AM-EWOD based on thin-film-transistor (TFT) is complex and expensive. Besides, the driving voltage of DMF chip is usually much higher than that of common display products.In this paper, a solution for mass production of AM-EWOD based on amorphous silicon (a-Si) is provided. Samples of 32 × 32 matrix AM-EWOD chips was designed and manufactured. A boost circuit was integrated into the pixel, which can raise the pixel voltage up by about 50%. Customized designed Printed Circuit Board (PCB) was used to supply the timing signals and driving voltage to make the motion of droplets programmable. The process of moving, mixing and generation of droplets was demonstrated.The minimum voltage in need was about 20 V and a velocity of up to 96 mm/s was achieved. Such an DMF device with large-scale matrix and low driving voltage will be very suitable for POCT applications.

Roadways excavated through a coal seam can exert an adverse effect on roadway stability. To investigate the effects of in-situ stress on roadway stability, numerical models were built and high horizontal stresses at varying orientations were applied. The results indicate that stress concentrations, roadway deformation and failure increase in magnitude and extent as the excavation angle with respect to the maximum horizontal stress increases. In addition, the stress adjacent to the coal-rock interface sharply varies in space and evolves with time; coal is much more vulnerable to deformation and failure than rock. The results provide insights into the layout of roadways excavated through a coal seam. Roadways should be designed parallel or at a narrow angle to the maximum horizontal stress. The concentrated stress at the top corner of the face-end should be reduced in advance, and the coal seam should be reinforced immediately after excavation. Keywords: Roadway stability, Numerical simulation, In-situ stress, Stress concentration, Failure, Deformation

In order to understand the kinetic characteristics of coal gas desorption based on the pulsating injection (PI), the research experimentally studied the kinetic process of methane desorption in terms of the PI and hydrostatic injection (HI). The results show that the kinetic curves of methane desorption based on PI and HI are consistent with each other, and the diffusion model can best describe the characteristics of methane desorption. Initial velocity, diffusion capacity and ultimate desorption amount of methane desorption after PI are greater than those after HI, and the ultimate desorption amount increases by 16.7–39.7%. Methane decay rate over the time is less than that of the HI. The PI influences the diffusion model parameters, and it makes the mass transfer Biot number Bi′ decrease and the mass transfer Fourier series F0′ increase. As a result, PI makes the methane diffusion resistance in the coal smaller, methane diffusion rate greater, mass transfer velocity faster and the disturbance range of methane concentration wider than HI. Therefore, the effect of methane desorption based on PI is better than that of HI. Keywords: Pulsating injection, Hydrostatic injection, Methane desorption, Kinetic characteristics