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Nowadays, coalbed methane recovery faces challenges such as high gas content, micro-porosity and low permeability. An exploratory study on improving coal porosity by acetone treatment was carried out. Pore size distribution and fracture development before and after acetone treatment were evaluated using a suite of integrated diagnostic techniques including rock acoustical test, weight analysis, Scanning Electron Microscope (SEM), Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FT-IR) and mercury intrusion porosimetry (MIP). After acetone treatment, micropores are partially converted into mesopores and macropores. The porosity of coal increases by 2.66% after acetone treatment. Functional groups including hydroxy, methylene, oxygen functional groups and aromatic hydrocarbons reduce significantly and the removal of these hydrophilic groups significantly weakens the hydrophilicity of coal. New fractures appeared within 32.89% of the eroded zone, even large through-going fractures may appear after acetone treatment. The results indicate that acetone treatment is effective in improving gas productivity thus has the potential to become a new coalbed methane reservoir stimulation technology.

Gas outburst is a serious disaster in deep mining. In this paper, we first study the effect of water content on gas outburst, and found that increasing the water content of coal can significantly reduce the elastic modulus and uniaxial compressive strength, which is beneficial in reducing the peak stress concentration in the roadway and pushing the peak stress deeper. We also found that by increasing the water content of the coal seam, the firmness coefficient increases, while the initial speed of gas diffusion decreases simultaneously. Increasing the water content of the coal seam can also change the Langmuir adsorption constant of coal, reduce the a value of the limit adsorption capacity, increase the b value of the adsorption capacity, and reduce the amount of desorption gas in the coal. These studies indicate that increasing the water content of coal can significantly reduce the risk of gas outbursts. To increase the water injection volume, the technology of promoting coal seam infusion by blasting has been proposed. Blasting promotes equilibrium distribution in the stress field and generate new fractures, which promotes the water injection. In the 8th coal mine in the Pingdingshan coal field, the test results indicate that after loosening blasting, the water injection of a single borehole increased by an average of 54.6 times under the injection pressure of 3–7 MPa. The water content of coal is improved significantly, and the prediction index value of the gas outburst is reduced significantly. This technology is important for gas control in similar coal mines.

In recent years, high-voltage electric pulse (HVEP) technology has been suggested to improve the permeability of coal seams. However, the effect of moisture content on the structure evolution of coal subjected to HVEP is not clear, which restricts the wide spread application of this technology. In this study, the breakdown voltage of coal samples with different moisture content was tested, and an exponential function relationship was established between the average breakdown field strength and the moisture content of bituminous coal samples. We investigated the changes in pore structure by combing scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR) results, to better understand the pore structure evolution characteristics of coal with different moisture content. Furthermore, changes in the chemical structure of the bituminous coal samples with different moisture content subjected to HVEP were investigated by Fourier transform infrared spectroscopy (FTIR). The results show that many mesopores and macropores are formed in the coal body under the action of HVEP, and the connectivity between the mesopores and the macropores is very good. In general, the higher the moisture content of coal body, the more pores and cracks will be formed in the process of electric pulse breakdown. The total porosity of coal samples with moisture content of 1%, 1.5%, and 2% is significantly higher than that of coal samples with moisture content of 0% and 0.5% after electric pulse breakdown. Therefore, the increase of moisture content is beneficial to improve gas permeability in coal body. The FTIR spectral analysis indicates that oxidation occurs on the plasma channel surface in the breakdown process of coal samples with different moisture content. Furthermore, the increase of moisture content in coal favors the generation of more oxygen-containing functional groups, which will facilitate the desorption of methane.

In order to enhance the gas extraction rate of low-permeability coal seams, a study was made on variation characteristics of pore structure of the coal saturated by NaCl solution under the effect of high-voltage electrical pulses (HVEP) by adopting a self-designed experimental system of fracturing and permeability enhancing of the coal with HVEP. In addition, current waveforms in the process of electrical breakdown were also investigated. The results revealed the two types of electrical breakdown of the coal in air environment, namely, surface breakdown and internal breakdown with three forms of breakage, namely, complete comminution, breakage from one side and cracks on the surface. Furthermore, the coal was broken due to internal tension under the influence of HVEP. Based on energy dispersive spectroscopy (EDS) analysis, the coal underwent both physical and chemical changes where the current flowed. Results of scanning electron microscope (SEM) demonstrated that the higher the breakdown voltage, the more the fissures in coal, and the better the breakage effect. Fissures in the coal were observed via the 3D-XRM to extend radially from the center to boundary areas with the breakage effect weakening gradually from anode to cathode when needle electrode discharged. Besides, these fissures were interconnected with each other. The result of liquid nitrogen adsorption suggested that electrical breakdown could effectively boost gas desorption by improving fissures as well as pores and micropores in the coal. Characteristics of the current waveform showed that great thermal expansion stress was caused by huge energy injected into the coal in an instant. During the electrical breakdown of coal with the same voltage, the peak current was different which increased with the growing of breakdown voltage.

To quantitatively evaluate the effect of plasma on the permeability of coal, and to fill the gap in the research on the effect of plasma on the permeability of loaded coal by electrical breakdown, this study investigates the variations of the permeability of plasma-breakdown coal under different gas pressures, axial pressures and confining pressures with the aid of an established integrated experimental system for permeability enhancement by plasma. The following results are concluded: (1) The plasma breakdown based on the principle of electrical breakdown can raise the permeability of loaded coal. (2) When the gas pressure increases in the range of 1–2 MPa, the permeability of original coal corresponds to a V-shaped variation while that of the breakdown coal declines gradually. (3) The interconnected fracture network formed in breakdown coal can raise the coal permeability by tens to hundreds of times. (4) Due to the existence of the fracture structure, the permeability of the breakdown coal is more sensitive to the variation of external stress; (5) The flow state in coal changes from “Darcy” flow to “non-Darcy” flow, and the fracture structure formed after breakdown conduces to fluid migration. The above results suggest that the plasma technology based on the principle of electrical breakdown can effectively improve coal permeability and promote the application of plasma technology in coal mining.

In this work, an improved dual-channel porous combustor is designed. Extensive numerical investigations are conducted to compare the thermal performance between the single-channel and the improved dual-channel combustor under various inlet velocity, methane concentration and the pores density of foam ceramics. Moreover, the effective thermal conductivity model of the foam ceramic based on the Kelvin decahedron was introduced. The external and internal heat circulation and the heat circulation efficiency of the burner are calculated and analyzed quantitatively. Results show that, at low methane concentrations conditions, the pores density of foam ceramics have no significant effect on the blow-off limits for both single-channel and dual-channel porous burners. However, under the higher methane concentrations conditions, the blow-off limit would be increase with the pores density of foam ceramics. Compared with the single-channel burner with 20 PPI foam ceramic, the mean nonuniformity of the foam ceramics temperature for the improved dual-channel combustor would be reduced from 2.015% to 26.562% when the inlet velocity increases from 0.1 m/s to 0.16 m/s. However, for the 10PPI to 40PPI burners, the mean nonuniformity of the solid phase temperature of the improved dual-channel combustor would be reduced by 9.78% to 4.36%. Moreover, the preheat zone inside the porous medium would be greatly minimized by the addition of external heat recirculation to the porous media burners.

This paper is aimed at investigating the influence of plasma technology on desorption and adsorption features of coal in the fracture scale and proving that plasma technology boasts a wide prospect in the field of coalbed methane extraction. In this paper, CT was combined with low-temperature liquid nitrogen adsorption to analyze the pore and fracture variations of coal before and after plasma breakdown. More importantly, a self-designed desorption experimental system was used to explore variations of gas desorption with time in the fracture scale. According to the research results, the fragmentation effect of plasma is reflected by the generation of cracks in the macro-scale and the improvement of pores in the micro-scale; the fracture is generated from the section near the electrode, and the fracture growth rate after breakdown can reach 18–41 times; the action of plasma can crush coal into little pieces and promote the porosity by over hundreds of times. The appearance of semi-open pores in the liquid nitrogen adsorption test shows that part of closed pores in the coal body are opened by plasma. Moreover, the plasma channel can weaken the adsorption capacity of coal to gas, which is similar to the temperature effect. Meanwhile, the plasma channel increases the desorption amount of coal by 50%–70% and greatly improves the desorption rate of coal due to the influence of interconnected cracks.

An innovative microwave irradiation device is developed to investigate effect of microwave irradiation on pore morphology of coal powder. This device with several distinguished highlights, such as real-time infrared temperature measurement, gas replacement, process parameters controls, and large size samples testing. Specifically, results of effect of temperature (25–300 °C) and microwave power (1–6 kW) on pore morphology was evaluated by the mercury intrusion porosimetry method. The pore connectivity decreases at first then increased with incremental processing temperature or microwave power. The percolation threshold increases with increased heating temperature or microwave power. The total pores volume increases with increased processing temperature or microwave power. The total specific surface area increases firstly then decreases with increased temperature or microwave power. These suggest that effect of microwave irradiation on pore morphology is obvious. Several possibilities for microwave energy application in the field were discussed, including drilling borehole from the ground to the payzone, drilling up-hole or down-hole from the roadway to the payzone, drilling borehole along the coalbed. The process economics were also discussed. It implies that as a new technology microwave energy may have the potential for the degassing coal seams.

The application of high-voltage electrical pulse (HVEP) technology to the improvement of pore structure of coal has achieved encouraging results. However, the very high breakdown voltage of some kinds of coal seriously affects the application of HVEP technology. In this study, the relationship between the concentration of NaCl solution and the breakdown voltage of coal sample was studied by improving the electrical breakdown effects of Linhua anthracite coal and Hongliu bituminous coal with different concentrations of NaCl solution, and a negative exponential relationship was established. The results indicate that the breakdown voltages of coal samples decrease with the increase of the concentration of NaCl solution, but there is a limit to the magnitude of the decrease. Additionally, nuclear magnetic resonance (NMR) and X-ray photoelectron spectrometer (XPS) were adopted in this study to investigate variations in pore structure and functional groups before and after the electrical breakdown. The NMR analysis demonstrates that the electrical breakdown mainly improves the number of mesopores and macropores and enhances the connectivity between them, thus providing a smoother channel for gas extraction. The XPS analysis reveals that a new oxidation reaction appears where the current flows through, which promotes gas desorption from surfaces of coal samples.

During the transport of low-concentration coal mine methane (CMM), water mist spraying into the pipeline is used to eliminate the risk of explosion, but it leaves abundant vapor in the methane. This study was aimed to explore the effects of water vapor addition on the low-concentration CMM combustion in porous media. Thereby, a 2D numerical model based on a two-section ceramic foam burner setup with high flame stability was established and multi-step kinetics mechanisms were imported to the model. In this paper, the effects of vapor concentrations on the temperature distribution, flame stability limit, and chemical reaction during low-concentration CMM combustion in ceramic foam were investigated. Results indicate that with the increase of vapor mole fraction in the inlet methane, the overall temperatures in the downstream section of the burner gradually decreased, while the vapor mole fractions were linearly and negatively correlated with the peak temperatures in the burner. A small amount of vapor was involved in the chemical reactions of combustion, and with the increase of vapor mole fraction, more vapor took part in the reactions when the vapor addition into the inlet methane was unchanged. As the vapor mole fraction in the low-concentration CMM increased, the velocity range of flame stability limit was gradually narrowed down. In addition, the lower limit of velocity changed very slightly and maintained at 0.13–0.20 m/s, while the upper limit dropped obviously. The key elementary reactions underlying the effect of vapor on combustion reactions were determined by defining the changing rate of peak reaction rate. Addition of vapor into methane affected the peak rate of each elementary reaction, and altered the area of axial region where elementary reactions occurred.

Research on electric pulse fracturing technology has led to significant advances in the field of coal mine fracturing and permeability enhancement. However, few studies have focused on nanoscale pores. In this study, we investigate the effect of high-voltage electric pulses on the nanoscale pore structure of coal using a custom-built experimental device. The results revealed that the high-voltage electric pulse breakdown process decreased the specific surface area, pore volume, and number of micropores (pore size 50 nm). In particular, the number of pores that were>100 nm increased significantly. Field emission scanning electron microscopy revealed that macropores were widely distributed on the surface of the post-breakdown coal samples and were connected to each other. The number of mesopores (pore size = 2–50 nm) were not significantly altered as they were supplemented by the formation of new pores while simultaneously expanding into larger pores. The electric pulse breakdown process increased the total pore volume, average pore size and pore connectivity, decreased structural complexity, and significantly improved the nanoscale pore structure of the coal samples. Raman spectroscopy further confirmed this observation. From a molecular point of view, post breakdown, the aromatic system in the coal sample dehydrogenated, the directly connected methyl groups broke, the carbon atoms were arranged in a more orderly manner, and the microcrystalline structure gradually improved, thereby providing suitable conditions for convenient gas desorption and diffusion.

Electric pulse fracturing technology is a new type of coal seam fracturing and permeability increasing technology based on high-pressure shock wave technology. Compared with conventional technology, electric pulse fracturing has the advantages of a higher energy efficiency, less environmental pollution, and a shorter fracturing time, but having the disadvantages of rapid energy loss and a small fracture radius. To reduce the energy loss of electric pulses and increase the fracture radius, the experimental system of high-voltage electric pulse was employed in this study. The effects of NaCl, CaCl2, and AlCl3 conductive ions on the evolution of the pore structure induced by electric pulses were examined for medium- and high-rank coal bodies. The experimental results indicated the adsorption capacity of coal for ions was positively correlated with the valence of the cations and the soaking time, and the amount of ions adsorbed was larger when the solution was more alkaline. Amounts of ions adsorbed onto the coal surface decreased in the following order Al3+ > Ca2+ > Na+. After the treatment, the conductivity of the coal was significantly improved, making the plasma channel in the coal body more fully developed in the process of high-voltage electric pulse impaction, thus reducing significantly the breakdown voltage of the coal samples. Additionally, tthe total pore volume, total specific surface area, porosity, and average pore diameter of the treated coal increased and that the percentages of macropores and mesopores increased, indicating that the pore structure of coal samples is significantly improved.

Temperature is an important index to evaluate the microwave heating characteristics and affection of Formation microwave heating treatment. Experiments on microwave heating of coal powder were conducted to investigate the temperature rising characteristics of coal powder. The results indicated when the microwave heating power ranged from 1 kW to 5 kW, the temperature rising rate of the coal sample with grain size 0.18 mm–0.25 mm was larger than that with grain size 1.00 mm–2.00 mm. However, this relation was the opposite when the microwave power was 6 kW. The temperature rising rate increased with the increased moisture and pyrite content of the coal sample, while it decreased with the increased calcium carbonate, silicon dioxide, and potassium chloride of coal samples. Moreover, the correlation between oxygen concentration and the temperature rising was not obvious in this study. Finally, empirical equations between various factors and the heating temperature are given.

Coal and gas outburst is a typical dynamic disaster occurring during coal mining. To analyze its multi-field coupling mechanism, in this study, a multi-field (including coal stress field, damage field, gas diffusion field and seepage field) coupling model was established based on the heterogeneity of coal, and the mining process of horizontal and coal seam with fault were numerically simulated. Besides, the evolution of gas pressure, von mises stress and permeability under different parameters were analyzed. The results show that the stress relief zone in front of the working face is damaged by the mining disturbance, and the permeability increases sharply due to the full development of internal fractures. The coal in the stress concentration zone is affected by the stress concentration, which leads to the closure of fracture and the sharp decrease of permeability. As a result, the gas pressure gradient between stress relief zone and stress concentration zone increases, thus raising the risk of coal and gas outburst. The front geological structure of working face leads to the formation of multiple stress concentration zones in the coal seam, and the risk of coal and gas outburst increases with the increase of overburden stress. Based on the interaction between geological structure, gas pressure, coal mechanical properties and overburden stress, the mechanism of coal and gas outburst can be revealed. The research results can provide theoretical guidance for disaster prevention of coal and gas outburst.

The extensively applied plasma technology boasts potential application prospects in the field of coal seam permeability enhancement. However, the effect of electrode materials on coal breaking by plasma (CBP) has not been reported. For this reason, the effects of four kinds of electrode materials, namely aluminum, copper, brass and stainless steel, on CBP were investigated by using a self-built CBP experimental system. The experimentally obtained statistics show that the sizes of coal fragments differ under the action of different electrodes. Specifically, the coal sample tends to be broken into small particles under the action of aluminum electrode, whereas the broken fragments generally have a diameter of larger than 15 mm under the action of brass electrode. The scanning electron microscopy results suggest that the different electrode materials correspond to different modes of crack initiation on coal surface. The results of liquid N2 adsorption show that the aluminum electrode exerts the poorest effect on improving micro-pores in coal, while the copper electrode achieves the best effect. Furthermore, the effects of electrode materials on the current and voltage waveforms at different voltages were analyzed by using a Rogowski coil and a HV test rod, and the effects of electrode materials on deposited energy generated by plasma in the coal were also discussed. The results indicate that at low voltages, the voltage and current waveforms under the action of four electrode materials differ obviously; in contrast, at high voltages, the voltage and current waveforms become almost identical. At the same voltage, the deposited energies formed by different electrode materials also differ due to the difference in breakdown voltages and current waveforms. The deposited energies formed by the aluminum electrode, the brass electrode and the stainless steel electrode inside the coal fall first and then rise with the increase of breakdown voltage, whereas that formed by the copper electrode goes up gradually with the increase of breakdown voltage. Based on the photograph of plasma discharge process shoot by a high-speed camera, the law of plasma propagation inside the coal was analyzed, and the effects of electrode materials on plasma development were discussed. A plasma streamer which is accompanied by dazzling white light and extremely high temperature is formed inside the coal at the moment of discharge, and it continuously expands outward as time passes by. The brightness degrees of plasma streamers formed under the action of electrode materials are different. Among the four electrode materials, the brass electrode forms the brightest light and generates the greatest energy.

Coalbed methane (CBM) reservoir is generally believed to be under the uniaxial strain condition. Because of this stress/strain controlled boundary, the horizontal stress is passively changed with gas depletion. This dynamic stress evolution has great impact on permeability and stability of the CBM reservoir. Even though it is generally known that the stress depletion is both poromechanics and desorption controlled process, however, how the gas pressure and the gas type affect the horizontal stress profile and its induced coal failure is still unclear. In this work, we experimentally simulated the uniaxial strain condition in the lab. Both the horizontal stress and permeability were continuously monitored during of different gases (He, N2, CH4 and CO2) depletion. The results show that regardless of gas type, the horizontal stress decreases with the reduction of gas pressure. And gas with a higher adsorption capacity corresponds to a greater horizontal stress loss. During the depletion, the effective vertical stress rises, while effective horizontal stress show various trends for different gases. The effective horizontal stress increases during helium depletion, which leads to a reduction of the permeability. And it decreases during methane and carbon dioxide depletion, which corresponds to a significant increase of the permeability. While for nitrogen, the permeability decreases first and followed by a slight uptick. Both the lab test and modeling results show that a significant non-linearity exists in effective horizontal stress change during adsorptive gas depletion, which is expected to affect the reservoir stability. To evaluate the reservoir stability during depletion, a stability factor (SF) was put forward, and the influencing factors on this parameter were analyzed. With this parameter, the critical reservoir pressure indicating reservoir failure can be determined with appropriate geomechanical parameters input. This research will advance the understanding of the geomechanics change during CBM production.

To reveal the principles of fracture extension within coal under the impact of plasma, the fracture and mineral phase of Hongliu bituminous coal were experimentally extracted through 3D visual software Dragonfly. Besides, the principles of fracture extension within coal in different directions were further analyzed under the impact of plasma. The results show that fractures on axial and horizontal surfaces both extend along minerals, which indicates that the distortion of electrical field on the boundaries between minerals and coal leads to the concentration of stress. The distribution of minerals serves to lead fracture extension, and plasma forms a spatial network fracture structure within the coal. The results indicate that electrical field intensity can be distorted among dielectrics with various dielectric constants. The intensity of electric filed is low when passing through the dielectric with a large dielectric constant, whereas it is high when passing through the dielectric with a small dielectric constant. Furthermore, aiming at investigating selective fragmentation of coal sample by plasma path, an experiment was performed by drilling boreholes and infusing iron powder in coal samples before electric breakdown. The results show that fractures extend along the drilling boreholes and run through some drilling boreholes, suggesting that the iron powder that has infused in the boreholes disturbs the plasma path. Besides, plasma paths are also influenced by the location and amount of drilling boreholes, as well as their distances to the electrode center. The results of experiment and numerical simulation are mutually validated.

Increasing methane recovery rates from coal mines reduces greenhouse gas emissions and benefits mining safety. In recent years, raising the temperature of coal seams has been recognized as a potential method of improving methane recovery. However, the effect of elevated temperature (especially higher than 80 °C) on methane adsorption on coals is rarely studied. In this study, we investigated the effects of elevated temperature and pressure on methane adsorption on different types (ranking) of coal, and their thermal stability. We found that for all six coal samples measured, maximum methane adsorption capacities sharply decrease with increasing temperature, showing a linear relationship (r2 = 0.705–0.975), and that coals began to pyrolyze at 267–500 °C depending on the coal type. We defined the temperature-dependent decrease in methane adsorption of coal as its thermal desorption capacity, and found that coal types with higher fixed carbon content have a larger magnitude thermal desorption capacity. These results indicate that raising the coal seam temperature while capturing methane is a promising method for obtaining higher coalbed methane recovery rates, reducing greenhouse gas emissions, and improving mining safety.