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Coal seam gas content is frequently measured in quantity during underground coal mining operation and coalbed methane (CBM) exploration as a significant basic parameter. Due to the calculation error of lost gas and residual gas in the direct method, the efficiency and accuracy of the current methods are not inadequate to the large area multi-point measurement of coal seam gas content. This paper firstly deduces a simplified theoretical dynamic model for calculating lost gas based on gas dynamic diffusion theory. Secondly, the effects of various factors on gas dynamic diffusion from coal particle are experimentally studied. And sampling procedure of representative coal particle is improved. Thirdly, a new estimation method of residual gas content based on excess adsorption and competitive adsorption theory is proposed. The results showed that the maximum error of calculating the losing gas content by using the new simplified model is only 4%. Considering the influence of particle size on gas diffusion law, the particle size of the collected coal sample is below 0.25 mm, which improves the measurement speed and reflects the safety representativeness of the sample. The determination time of gas content reduced from 36 to 3 h/piece. Moreover, the absolute error is 0.15–0.50 m3/t, and the relative error is within 5%. A new engineering method for determining the coal seam gas content is developed according to the above research.

The aim of this study was to explore the evolution and release rule of internal energy storage in the process of coal and gas outburst and to further reveal the mechanism of coal and gas outburst from the perspective of energy. In this paper, the experiment of gas expansion energy release of coal samples under different adsorption pressures and with different moisture contents was carried out with the self-developed experimental device for release of gas-bearing coal expansion energy under load, and the energy of the whole outburst process was divided into three parts: the total expansion energy of gas, the energy consumed by destroying and throwing out coal body and the energy released inefficiently. On the basis of reasonable assumption, the energy evolution calculation model of each part was constructed with mathematical method. By analyzing the changes and distribution rules of three parts of energy under different experimental conditions, this paper explored the controlling effects of gas pressure, water content, and other variables on the energy evolution rules in the process of coal and gas outburst. Experimental and theoretical studies showed that in the gas-dominated coal and gas outburst process, the destruction of coal body was in the form of stratification; under each experimental condition, there existed a critical gas pressure value for the occurrence of coal and gas outburst, and there was a sudden change of energy evolution near this value; the existence of water made the critical pressure and the minimum energy consumption of coal and gas outburst increase obviously; under the experimental conditions, there was a linear relationship between the critical gas pressure and water content and a positive exponential relationship between the minimum energy consumption and water content.

In order to supply benefits for safe production of coal underground and efficient exploitation of coalbed methane, a self‐developed gas seepage experimental device considering gas adsorption and desorption was proposed to study the seepage properties of gas‐saturated coal in this paper. A series of gas seepage experiments under different loading conditions were carried out to investigate the change rules of permeability of gas‐saturated coal. The experimental results covered the significant effects of confining pressure, gas pressure, temperature change, creep stress, and complete stress‐strain process on the seepage laws of gas‐saturated coal. The experimental results showed that the permeability of gas‐saturated coal is strongly sensitive to the effective stress and decreases with the increase of the effective stress. Under the fixed confining pressure, the permeability of gas‐saturated coal showed an obvious Klinkenberg effect, but the Klinkenberg effect would no longer be evident once the gas pressure was larger than 1.0 MPa. And the change law of permeability of gas‐saturated coal with temperature was mainly reflected in the comprehensive effect of thermal stress and effective stress on the control of pore deformation. Under the complete stress‐strain loading conditions, the change rules of permeability were mainly dependent on the failure mode of gas‐saturated coal. And the change rules of permeability, for the condition of triaxial creep stress, were relied on the stage of creep deformation. The deformation at decay creep stage was responsible for a steady permeability after the failure of gas‐saturated coal, while the deformation at nondecay creep stage was responsible for a rapid increase of permeability after the failure of gas‐saturated coal. The results in this work can offer some helpful suggestions for efficiently exploring coalbed methane in the future.

Spontaneous combustion of coal has long been a thermal dynamic hazard during coal mining, storage, and transport, posing a great threat to coal mine safety. Especially coal spontaneous combustion in longwall goaf poses a great threat to underground working crew. To investigate such a mine safety issue with more insights and “what if” scenarios, this study employs CFD technique to develop a proactive inertisation plan in a longwall goaf. Based on real on-site conditions of the longwall goaf, a three-dimensional transient non-equilibrium thermal CFD model was developed to study heating evolution and proactive inertisation plans of the longwall goaf. The theoretical model incorporated a set of governing equations including low temperature kinetics of coal oxidation, energy and mass conservation, momentum balance, and continuity equation. After the base model (1000 m) was validated and calibrated with field gas monitoring data, another model (500 m model) was studied to obtain an optimum inertisation plan. Both steady state and transient simulations were conducted to study the flow dynamics of air velocity, oxygen ingress, dispersion of gaseous products and heating evolution in the longwall goaf. Based on the flow dynamics field, proactive inertisation plans using nitrogen to suppress the onset and development of goaf heatings were then developed and studied.

The current study aimed to investigate the degree of pollution from drilling mud on different scales of fractures in coal reservoirs. Drilling mud for field utilization was prepared in the laboratory and different scales of fractures were generated in coal and rock samples before and after being polluted by the drilling fluid and observed using scanning electron (SEM) and an optical microscope. Using Monte Carlo modelling, the networks of the millimeter (mm), micron- and nano-meter sized fractures, before and after pollution were separately determined. After programming, seepage models of mm, micron- and nano-meter sized fractures were separately constructed using Matlab software. The degrees of pollution of the drilling fluid on the different scales of fractures were calculated according to the seepage principle and compared with the measured permeability of the polluted boreholes. The micron- and nano-sized fractures were more sensitive to drilling fluids compared to the mm-sized and larger fractures. When the original permeability of the coal reservoir was low, different scales of fractures were all greatly polluted by drilling fluids, macroscopically showing high damage to permeability of the coal reservoir. However, when the original permeability of coal reservoir was large, the pollution degrees of micron, and nano-sized fractures failed to be macroscopically observed, having insignificant influences on the permeability of the coal reservoir. The results showed that the combination of the Monte Carlo method and the Matlab numerical simulations can more distinctly reflect the pollution degrees of the drilling fluids on different scales of fracture compared to traditional methods that have been used to test the permeability of coal reservoirs before and after pollution.

Supercritical carbon dioxide (SC-CO2) jet is a promising technology for the extraction of coalbed methane. As a key component in the process of jet formation, the nozzle is an essential factor affecting the efficiency with which an SC-CO2 jet breaks coal and rock. The nozzle structure directly affects the energy conversion rate and flow field structure of the jet. Thus, the use of a suitable nozzle can maximize SC-CO2 energy utilization, reducing energy loss and engineering costs while achieving energy conservation and emission reduction. Based on the conical convergent nozzle and the Laval nozzles, this study conducted an SC-CO2 jet coal breakage experiment and an I-Scan pressure testing experiment using experimental research methods. The influence of nozzle structure on SC-CO2 jet coal breakage was studied and the results indicate that when the inlet pressure is constant, the Laval nozzle has a higher energy conversion rate and a stronger coal breakage effect than the conical convergent nozzle. Furthermore, considering the combined effects of the energy conversion rates of Laval nozzles and energy dissipation under ambient pressure, a key parameter for assessing Laval nozzles was discovered: the expansion ratio. The results indicate that the impact of jet temperature on the expansion ratio is small and that increasing the erosion effect of an SC-CO2 jet by increasing the jet temperature is an energy-consuming and inefficient method. Essentially, the optimum coal breakage effect of the SC-CO2 jet is obtained when the expansion ratio of the Laval nozzle is between 1.07 and 1.29.

To understand the effects of thermal shock on microcrack propagation and permeability in coal, thermal shock tests were conducted on coal specimens by using a constant temperature drying oven (105 °C) and a SLX program controlled cryogenic tank. The growth and propagation of microcracks were measured with computer tomography (CT) scanning and scanning electron microscope (SEM) tests. Results showed that thermal shocks improved the permeability of coal significantly. Notably, the permeability of coal after thermal shocks increased from 211.31% to 368.99% and was positively correlated with temperature difference. CT scanning images revealed that thermal shocks increased the crack number, crack volume and crack width as well as smoothened and widened the gas flow paths, thereby enhancing coal permeability. Moreover, SEM images showed that heating-cooling shocks created more new microcracks, forming more complex crack propagation paths and better connectivity among microcracks in coal compared to cooling shocks. We proposed a crack propagation criterion for coal to explain the mechanism of crack failure and propagation during thermal shocks. Our experiment results and theoretical analysis indicate that the heating-cooling shock is more effective in damaging and breaking coal than the cooling shock. Thus, it can be used as an alternative approach to enhance coal permeability in the production of coalbed methane (CBM).

Physicochemical structure is essential for understanding physiochemical properties of coal reservoir and coalbed methane (CBM) occurrence. In this work, ultimate analysis, 13C nuclear magnetic resonance spectra (NMR), Fourier transform infrared spectroscopy (FTIR) and gas chromatograph/mass spectrometer (GC/MS) tests were adopt to investigate the coal molecular structure information. Then, by means of molecular simulation, stable three-dimensional (3D) molecular structure and physicochemical structure models of Tunlan coal were established to reveal the physicochemical response at a molecular level. Results show that compared with the single molecular structure, angle and torsion energies in nanopore structure play a more important role in maintaining a stable and stereoscopic structure. While the electrostatic energy in non-bond energy can simulate to form nanopore space in coal. Then, with the dissolution of low molecular weight compounds (LMWCs), the pore volume (PV) of micropores increases gradually, while the corresponding specific surface area (SSA) decreases first and then increases, suggesting that pore expansion and pore increase effects are dominant in the early and the later dissolution stages, respectively. Furthermore, observing physicochemical response, total pore structure distribution depends on whether vertical or horizontal changes of basic structure units in coal make a great contribution in this evolution stage. Actually, when the aromatic layer is mainly the increase of the stacking height Lc, it is easy to result in the decrease of total SSA and PV. In contrast, for mainly the increase of the diameter La, all pores in coal present an increase characteristic to induce the increase of total SSA and PV.

To enhance desorption of coalbed methane and increase extraction efficiency, a technique using microwave irradiation is proposed. Methane desorption experiments without and with microwave irradiation were carried out in the laboratory using an experimental system developed to study methane desorption. The experimental results show that microwave irradiation causes the total quantity of methane desorbed to increase from 1.91 to 3.92 times larger than the quantity desorbed without MI. Two hour trials were performed, applying microwaves for 4, 8 or 16 min, equivalent to output energies of 192, 384 and 768 kJ. These results show that 1 kJ of microwave energy can cause methane desorption to increase by 0.0088 ml per gram of coal. Under successive microwave irradiations of 40 s, the desorption rate increases by a factor of 10.2. Kinetic analysis also found that microwave irradiation enhances the methane diffusion coefficient and decreases the attenuation coefficient. Regardless of which diffusion model is used to determine the diffusion coefficient, the quantity of gas desorbed increases as the microwave irradiation time increases. The pore and surface features of coal samples exposed to microwaves were determined by using mercury intrusion porosimetry and nitrogen adsorption and specimens were examined by scanning electron microscopy. Methane desorption evidently results from both the microwave thermal effect and structural damage to the coal. The results reported herein reveal the mechanisms that promote methane desorption by microwave irradiation and suggest a new field technique for extracting coalbed methane.

To examine the characteristics of the anisotropic seepage of the coal containing gas, a study of the anisotropic seepage rule of the coal containing gas was carried out based on the tri-axial seepage experimental system with raw coal samples. The calculation method determined the principal value and the azimuth of the anisotropic permeability of the coal containing gas. Also, the permeability anisotropic ratio of the coal containing gas was defined. The permeability anisotropic dynamic variation rule and the phenomenon of the change of the dominant flow direction of the coal containing gas were thoroughly analyzed. The results showed that the flow of the methane in the coal had very obvious characteristics of anisotropy. The calculation method of anisotropy permeability of the coal containing gas proposed in this study was found to be simple and effective. The coal containing gas displayed strong stress-sensitivity. The variation rule between the permeability of the methane in the coal and the effective stress was determined to be in accordance with the negative exponential function. The change of the permeability anisotropy of the gases in the coal with effective stress showed an obvious dynamic change law of development, and changes were observed in the dominant flow direction of the gas in the coal.

Elastic energy of coal reservoir is the driving force of gas and water production. However, its evolution characteristic in the process of gas and water production is unclear. In view of this, the calculation model of gas elastic energy was improved. Based on the stress analysis of coal, a mathematical model for the mutual transformation of elastic strain energy of coal matrix was established. The concept and calculation model of swelling strain energy of coal matrix were proposed. Furthermore, the energy evolution characteristic and its relationship with coal reservoir parameters were analyzed. The relationship among coal reservoir energy, parameters, and drainage method was revealed. The research shows that gas elastic energy controls the variation of permeability, and the elastic energy of adsorbed gas has an absolute advantage. The swelling strain energy belongs to a kind of elastic strain energy of coal matrix, and their variation characteristics are opposite. The swelling strain energy of coal matrix is mainly controlled by the adsorption strain, and increases with increasing strain. The elastic strain energy of coal matrix is mainly affected by the fluid content. With the increase of fluid content, the reduction rate is gradually accelerated. Moreover, the energy and pore pressure of coal reservoir affect the migration of gas and water, and the swelling strain energy of coal matrix can inhibit fluid migration. Besides, the coal reservoir energy is the internal reason for the variation of coal reservoir parameters, and the variation of coal reservoir parameters is the external performance of the change of coal reservoir energy. The drainage method is the fuse that causes a series of variations. The outcomes have guiding significance for analyzing the variation of coal reservoir parameters during coalbed methane (CBM) production.

In order to study the response characteristics of infrasound in coal samples under the uniaxial loading process, coal samples were collected from GengCun mine. Coal rock stress loading device, acoustic emission tested system and infrasound tested system were used to test the infrasonic signal and acoustic emission signal under uniaxial loading process. The tested results were analyzed by the methods of wavelet filter, threshold denoise, time–frequency analysis and so on. The results showed that in the loading process, the change of the infrasonic wave displayed the characteristics of stage, and it could be divided into three stages: initial stage with a certain amount infrasound events, middle stage with few infrasound events, and late stage gradual decrease. It had a good consistency with changing characteristics of acoustic emission. At the same time, the frequency of infrasound was very low. It can propagate over a very long distance with little attenuation, and the characteristics of the infrasound before the destruction of the coal samples were obvious. A method of using the infrasound characteristics to predict the destruction of coal samples was proposed. This is of great significance to guide the prediction of geological hazards in coal mines.

To study the precursory failure infrasonic characteristics of coal samples, coal rock stress loading system and infrasonic wave acquisition system were adopted, and infrasonic tests in uniaxial loading process were made for the coal samples in the studied area. Wavelet filtering, fast Fourier transform, and relative infrasonic energy methods were used to analyze the characteristics of the infrasonic waves in the loading process, including time domain characteristics, and relative energy. The analysis results demonstrated that the frequencies of the infrasonic signals in the loading process mainly distribute within 5–10 Hz, which are significantly different from noise signals. The changes of the infrasonic signals show clear periodic characters in time domain. Meanwhile, the relative energy changes of the infrasonic wave also show periodic characters, which are divided into two stages by the yield limit of coal samples, and are clear and easy to be recognized, so that they can be used as the precursory characteristics for recognizing coal sample failures. Moreover, the infrasonic waves generated by coal samples have low frequency and low attenuation, which can be collected without coupling and transmitted in long distance. This study provides an important support for the further in-situ prediction of coal rock failures. Keywords: Infrasound, Relative energy, Time-frequency analysis, Failure prediction, Identification feature

The formation water intruding into the coal seam through cross-layer seriously reduces the gas extraction efficiency. In this work, a series of experiments were carried out to investigate the influence of long-time pressure water on the mineral content and pore structure of coal. Results show that, the water intrusion makes a decrease in the types and contents of mineral in coal. Furthermore, full-scale pore size was analyzed by using low-temperature N2 adsorption and CO2 adsorption experiment, indicating that the macropore shows a different change trend before and after water intrusion. For TY and YZ sample, the macropore volume decreased by 18.56% and 0.98%, respectively. For QC sample, the macropore volume increased by 3.20%, while the micropore volume, mesopore volume and total volume of three samples increase in different extent, which are 16.67%–30.77%, 11.56%–17.59%, and 16.10%–125.00%, respectively. Then, the fractal dimension of coal before and after water intrusion was calculated by using FHH model, showing that the fractal dimension D 1 decreases while the fractal dimension D 2 increases after water intrusion because the water intrusion is able to create and expand pore of coal but it can reduce the roughness of the pore surface. Finally, the influence mechanism of water intrusion on the pore structure of coal is analyzed based on clay mineral collapse, mineral dissolution and coal swelling. The clay minerals in coal is a key factor impacting the pore structure during water intrusion process due to its poor water stability. In addition, the mineral dissolution and coal swelling also play the significant roles in pore structure. This work is significant for the gas control and prevention of coal mine in rich-water coal strata.

The dynamic evolution characteristics of fractures in coal under different compressive loads are of great significance to the recovery of coalbed methane (CBM). In this paper, the X-ray computer tomography (CT) detections of coal samples under uniaxial and triaxial compressive loading conditions were performed by using a developed triaxial in situ rig and an industrial CT scanner. The image processing technique, three-dimensional reconstruction method and statistical principles were employed to describe two-dimensional and three-dimensional fracture evolution in coal under uniaxial and triaxial compressive loads. The results showed that the developed triaxial in situ rig has an enough scanning resolution and is suitable to carry out CT scanning inspections under uniaxial and triaxial compressive loads for porous materials of coal and rock. The two-dimensional cross-sectional CT images effectively reflect the dynamic evolution of two-dimensional fractures during the deformation of coal. Confining pressure and minerals have a significant effect on fracture propagation in coal. The evolution characteristics of three-dimensional fractures can be clearly revealed by the reconstructed three-dimensional digital fracture models of coal. Euler number effectively reflects the fracture connectivity in coal at different deformation stages. During the deformation of coal, the fracture volume, fracture rate, fracture density and fracture connectivity mainly undergo a slow decrease stage, a slow increase stage and a sharp increase stage, display a good linear correlation with fractal dimension, and quantitatively characterize the evolution process of the three-dimensional fractures in coal. The fractal dimension calculated by the box-counting method effectively mirrors fracture propagation and fracture geometric complexity and can be employed to uncover the fracture dynamic evolution in coal under different compressive loading conditions.

The diffusion of gas in water-saturated porous media plays a significant role in CO2 geological sequestration and mine gas prevention and control. However, it is lack of an accurate model to calculate the effective diffusion coefficient of gas in a water-saturated coal core. In this work, an improved model was constructed to calculate the effective diffusion coefficient of gas in a water-saturated coal core by considering both the gas adsorption in residual pore and the gas dissolution in pore water. By using a PVT (Pressure-Volume-Temperature) test method, the calculation results of improved model are in the range of 1.03 × 10−12 ~ 30.40 × 10−12m2/s, which are 5 ~ 6 orders of magnitude lower than those calculated by traditional model, because the traditional model only considers the gas dissolution in pore water. Therefore, the results of improved model are more in line with the actual situation. Furthermore, the saturated moisture, equilibrium moisture and NMR experiments were carried out. Results shows that, the saturated moisture and equilibrium moisture decreases with the rising coal rank. The pore size changes from micropore/mesopore-dominated to micropores-dominated as the coal rank rise. In addition, the proportion of open porosity gradually reduces with the rising coal rank, while the closed porosity increases, indicating that the pore connectivity decreases with the growing coal rank. Finally, the influences of adsorbability, pore structure, pressure and liquid properties on the effective diffusion coefficient were analyzed.

A gas migration controlling equation was formulated based on the characteristics of the dual pore–fracture media of coal mass and in consideration of the matrix exchange between pores and fractures. A model of permeability dynamic evolution was established by analyzing the variation in effective stress during gas drainage and the action mechanism of the effect of coal matrix desorption on porosity and fracture in the coal body. A coupling model can then be obtained to characterize gas compressibility and coal deformability under the gas–solid coupling of loading coal. In addition, a 3D model of boreholes was established and solved for gas drainage based on the relevant physical parameters of real mines. The comparison and analysis results for the law of gas migration and the evolution of coal body permeability around the boreholes before and after gas extraction between the dual media and the single-seepage field models can provide a theoretical basis for further research on the action mechanism of gas drainage. Keywords: Pore–fracture, Matrix exchange, Coupling model, Numerical simulation, Gas drainage

A gas–solid coupling model involving coal seam deformation, gas diffusion and seepage, gas adsorption and desorption was built to study the gas transport rule under the effect of protective coal seam mining. The research results indicate: (1) The depressurization effect changes the stress state of an overlying coal seam and causes its permeability to increase, thus gas in the protected coal seam will be desorbed and transported under the effect of a gas pressure gradient, which will cause a decrease in gas pressure. (2) Gas pressure can be further decreased by setting out gas extraction boreholes in the overlying coal seam, which can effectively reduce the coal and gas outburst risk. The research is of important engineering significance for studying the gas transport rule in protected coal seam and providing important reference for controlling coal and gas outbursts in deep mining in China. Keywords: Protective coal seam mining, Seepage characteristic, Coal and gas outburst, Numerical simulation

To understand the gas penetration properties during unloading confining pressures and permeability enhancement mechanism in coal, temperature shocks, seepage tests, NMR, CT and SEM scanning tests were carried out with trimmed coal samples. The results showed that the coal permeability increased with the decrease of confining pressures after temperature shocks performed, and that the coal permeability fully followed an exponential change law. To achieve a higher gas permeability, heating-cooling shocks should be executed in low stress regions and stress-releasing areas in coal. According to the NMR results, the increase in pore volume improved the gas diffusion coefficient of coal. The CT and SEM scanning results declared that the growth and extension of internal and surface cracks eventually enhanced the coal permeability. After temperature shocks performed, it can be concluded that coal anisotropy and excessive thermal stresses are the essential causes of the coal damage and gas permeability enhancement.

The characteristics of pore-fracture networks in coal seams and their evolution under compression are important for exploration and development of coalbed methane (CBM) and for the forecasting of coal dynamic disasters. Significant process has been made in fracture characteristics of rocks, however, the visualization and detailed quantitative characterization of real-time dynamic evolution of micro fractures in coal under triaxial compression are poorly understood. In this study, uniaxial and triaxial compressive tests and real-time CT scanning tests on anthracite were carried out using X-ray micro-Computed Tomography (X-ray μCT) equipped with the triaxial compression facility. The image processing technique, three-dimensional reconstruction and statistical principles were employed to extract geometric morphology, fracture growth and evolution. The results showed that confining pressure had certain inhibitory effect on the development of fractures in coal and also had effect on the forms and spatial distribution features of failure fractures in coal. The gray value, the fracture volume and also CT porosity could be the quantitative damage variable to evaluate the damage evolution of coal. The qualitative and quantitative analysis of fractures in anthracites indicated that their damage evolution process could be divided into three phases under uniaxial test and four phases under triaxial test, the precise degree of each phase was also given. The image and stereology analysis results reflected the microscopic fracture evolution process of anthracites under compression test agree well with the experimental results. The work of the paper is of great academic and realistic significance for further studies of coal structure and their dynamic evolutionary mechanism under loading using X-ray μCT.

Coal mine dust continues to be a health and safety issue in underground coal mines. Coal seam water infusion was developed and widely applied in European coal mines for dust control, and was also a common practice in most Chinese coal mines. This method typically involves the infusion of water into the coal seam to increase its moisture content, and therefore reduce dust generation during mining operations. With the availability of other dust control methods such as water spraying systems, the water infusion method has not been considered as a viable means for dust mitigation in modern mines. However, the increase in production output and the deployment of more powerful equipment for coal cutting and transport and intensive gas drainage practices mean that workers could be exposed to more dust contaminations. Whilst the mine operators are committed to suppress and dilute airborne dust particles using these passive measures, there is a need to critically examine and subsequently develop this proactive dust control technology for practical applications in Chinese coal mines. The paper provides a critical review of the water infusion technologies in view of its technological advances and practical application limitations. The methods of water infusion, mechanism of water flow in coal, the role of surfactants and the key parameters influencing the effect of water infusion on dust control are identified and discussed. Existing problems and prospects for water infusion are analysed.

Coal mine dust continues to be a health and safety issue in underground coal operations. Water infusion method was developed and widely applied in European coal mines for dust control, and is now a common practice in most Chinese coal mines. This method typically involves the injection of water into the coal seam usually ahead of mining to increase its moisture content and therefore reduce dust generation during mining. Field practices indicated that it was possible to inject 9–15 l of water into 1 m3 of coal and increase its moisture content by approximately 1% with a respirable dust reduction of 40–65% during mining. A limited scale of water infusion trials was conducted in Appin and West Cliff collieries of Australia in late 1980s; a 30–50% reduction of dust levels was reported. With the availability of other dust control methods such as water spraying systems, water infusion method has not been considered as a viable means for dust mitigation in modern mines and as such it has not been utilised in Australian underground mines. The increase in production output and the deployment of more powerful equipment for coal cutting and transport, intensive SIS and UIS gas drainage practices mean that workers could be exposed to more dust contaminations. Whilst the mine operators are committed to suppress and dilute airborne dust particles using these passive measures, there is a need to critically examine and subsequently develop this proactive dust control technology for practical applications in Australian coal mines. The paper provides a critical review of the water infusion technologies in view of its technological advances and practical application limitations. The key parameters influencing the effect of water infusion on dust control are identified and discussed, particularly coal moisture content. The use of gas drainage boreholes as secondary water infusion holes is discussed.

This study developed the equipment for thermo-fluid–solid coupling of methane-containing coal, and investigated the seepage character of loaded coal under different working conditions. Regarding the effective pressure as a variable, the variation characteristics of the gas permeability of loaded methane-containing coal has been studied under the conditions of different confining pressures and pore pressures. The qualitative and quantitative relationship between effective stress and permeability of loaded methane-containing coal has been established, considering the adsorption of deformation, amount of pore gas compression and temperature variation. The results show that the permeability of coal samples decreases along with the increasing effective stress. Based on the Darcy law, the correlation equation between the effective stress and permeability coefficient of coal seam has been established by combining the permeability coefficient of loaded coal and effective stress. On the basis of experimental data, this equation is used for calculation, and the results are in accordance with the measured gas permeability coefficient of coal seam. In conclusion, this method can be accurate and convenient to determine the gas permeability coefficient of coal seam, and provide evidence for forecasting that of the deep coal seam. Keywords: Permeability coefficient, Coal seam, Effective pressure, Loaded coal, Pore pressure

Coal and gas outburst is a common coal-rock dynamic disaster. Such accidents frequently occur, and the mechanism underlying the occurrence of these outbursts is complex. As a typical failure mode of a gas-filled and pressure-relieved coal body, the spallation mechanism should be investigated to reveal the mechanism of coal and gas outburst and guide outburst-prevention strategies. In this paper, a fluid-solid coupling model for coal seam gas flow is established. This model considers the adsorption characteristics of coal. Numerical calculations are used to simulate the stress field distribution and evolution of gas-filled coal bodies under different boundary conditions. The mechanical mechanism of the spallation occurrence after the pressure relief of coal is explained from the perspective of seepage breaking coal. The control of the flow and stress state of the gas to the spallation failure is analyzed. The mechanical-quantitative conditions for the initial failure of the coal body under seepage and the mechanical-qualitative conditions for the continuous advancement and termination of spallation are studied based on numerical solution results. The numerical calculation results show that the formation of a flow field after pressure relief will apply a drag force (tensile stress) on the porous media of coal. The presence of this force plays a crucial role in promoting the spallation and cracking of coal and, thus, the promotion of spallation. The tensile strength, initial adsorption pressure, and pressure relief rate of the coal body jointly control whether the initial failure can occur and the thickness of the fracture layer cracks. Spallation propulsion is mainly determined by the pressure relief conditions of the undestroyed coal body and pressure changes in the spallation space; the former can be quantitatively obtained by numerical calculations, whereas the latter is related to the thickness of the spalled layer and the degree of the layer-crack structure.

Based on the basic theory of gas seepage and coal seam deformation, using the numerical simulation method, this paper established the gas-solid coupling model of gas drainage from borehole. Using multi-physical coupling analysis software, the authors studied the stress change conditions around the drainage borehole, the influence of the gas drainage effect caused by the drilling gap, and the gas drainage effect under the conditions of different borehole radius and different permeabilities. The results show that the effective drainage radius is 1.03 m during 30 days of drainage. The effect of the diameter change of the drainage borehole is limited, but the influence of coal seam permeability is much bigger. After the same drainage period, the greater the permeability of coal seam is, the bigger the drainage radius is. For a low permeability coal seam, coal miners should take pressure-relief measures and increase the permeability to improve the drainage effects before draining gas through drilling.

Dynamic collapses of deeply mined coal rocks are severe threats to miners. In order to predict the collapses more accurately using electromagnetic emission (EME), we established a loaded coal rock EME electromechanical coupling model based on statistical damage mechanics. By using it, we numerically simulated both the accumulative pulse and strain ratios. We further improved the model with the Weibull pattern parameter, which has important effects on simulated results and can be applied to judge coal's homogeneity, and determined the pattern parameter and its value domain. Based on the revised model and the characteristics of coal rock deformation and fracture, we setup EME graded warning criteria against coal rock dynamic collapses by determining static critical coefficient and dynamic trend coefficient. We have applied this model to predict and deal with coal and gas outburst and rock burst occurring at Xie I and Taoshan Mines, respectively. All these verifications show that the model has many advantages and provides more sensitive and accurate warning for dynamic collapses.

Experimental system for investigating gas-desorption characteristics of deformed coal was established. The gas desorption law of deformed coal with different damage extent was investigated. The desorption characteristics of deformed coal having different size under different equilibrium pressures were determined. This provides a theoretical basis for the prediction of coal and gas outburst, the measurement of pressure and content of coal seam gas, and the estimation of gas emission of coal mining fell.

Based on laboratory results of time-dependent mechanical behavior tests, the creep property of coal containing gas is investigated under different load conditions, using servo-controlled triaxial rheology testing machine at room temperature. The effect of effective axial stress, effective confining pressure and gas pressure on the creep behavior of coal containing gas is discussed respectively in detail. At primary creep stage, creep strain rate of coal samples, which can be described by a power function, decreases gradually with testing time. At steady-state creep stage, effective axial stress, effective confining pressure and gas pressure are important factors which affect the creep strain rate of coal samples. An exponential function is proposed to model the effect of effective axial tress on creep strain rate, at the same time, a linear equation can be used to describe the effects of effective confining pressure on creep strain rate. While, gas pressure influences indirectly the creep behavior of coal containing gas through making a change of the value of effective stresses. Accelerating creep stage is the last stage of non-attenuating creep and the beginning of coal samples failure, as well. Macro-cracks will be produced and lead to the failure of coal samples finally as far as accelerating creep stage appears.