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Adsorption characteristics of coalbed methane (CBM) are significant to investigate the absorption of coal, shale, and porous media. In particular, adsorption characteristics of CH4, CO2, and H2O play an important role in predicting CBM output and geologic sequestration potentials of CO2 in research fields of CO2-enhanced CBM recovery (CO2-ECBM) and sequestration of CO2. In this work, adsorption characteristics of CH4, CO2, and H2O in lignite molecules were simulated through the grand canonical Monte Carlo (GCMC) method and molecular dynamics (MD) method. Research results demonstrated that given the same temperature and pressure, the ultimate adsorption capacity of lignite per unit to H2O is the highest, followed by those of CO2 and CH4 successively. All isothermal adsorption curves conform to the “I-type” characteristics. In the saturated molecular configuration, gas molecules show different distribution patterns at two sides of the lignite molecule chain. Lignite has typical physical adsorption to CH4 and CO2, with adsorption energy provided by nonbonding energy. However, lignite has both physical adsorption and chemical adsorption to H2O, with adsorption energy provided by both nonbonding energy and hydrogen bond energy. High temperature is against adsorption of CH4, CO2, and H2O. Temperature might inhibit adsorption of gas molecules. Research conclusions lay foundations for the exploitation and development of CBM and relevant studies on sequestration of CO2.

Coal dust is a great threat to coal mine workers' health and safety in coal mine production. Wet dust removal is one of the effective dust removal methods. As a solid, coal has different rough surfaces, which have a certain effect on the wetting effect of coal. In this paper, three coal samples with different surface wettability are used as the research objects. Phase-field interface tracking method is used to simulate the wetting of droplets on rough surfaces. From the simulation results, it can be concluded that the influence of the rough interface on the contact angle of the droplets is in accordance with the change rule described in the Wenzel model. As the roughness increases, the contact angle of the hydrophilic lignite surface gradually decreases. As the roughness increases, the contact angle of hydrophobic coking coal gradually increases. The change trend of the contact on the surface of weakly hydrophilic anthracite coal is the same as that of lignite. Due to the local and global differences, the contact angles obtained from the numerical model are slightly different from the values calculated from the Wenzel model.

Coal structure could be roughly divided into four types. Among them, the two kinds of tectonic coal face a high risk of heat-induced gas outburst, which arises from the unfavorable temperature conditions in the coal structure. However, there is not yet an efficient way to identify the type of coal structure. The adjacent types of coal structures are often misjudged. The lack of an efficient identification method hinders the prevention of heat-induced gas outburst, making it difficult to realize energy-efficient and safe mining. To solve the problem, this paper first theoretically analyzes the ultrasonic properties of different types of coals, and applies backpropagation neural network (BPNN) to build up an intelligent identification model for the type of coal structure. Specifically, the characteristic parameters of ultrasonic signal were taken as the basis for judging the type of coal structure, the identification algorithm of BPNN was adopted to accurately identify the structure type of coal, and then the heat-induced gas outburst risk of the coal was evaluated preliminarily. Experimental results show that the proposed model could accurately identify the type of coal structure, and even differentiate between adjacent types of coals. The research results provide a reference for effective prevention of heat-induced gas outburst, and realization of energy-efficient and safe mining.

Protective layer mining (PLM) is one of the most effective regional measures against gas outburst, and the most preferred technique to prevent and control gas outburst. This paper attempts to disclose the effect of close-range PLM in Hezhuang Coal mine, Western Henan Coalfield. Firstly, coal seam I7 was selected as the lower protective layer, and coal seam II1 was taken as the overlying coal seam. Considering the situation of actual production, a simulation model for lower PLM was established on Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D). The established model was adopted to simulate the mining of the working face 11030# of the lower protective layer (I7) at the mining depth of 50, 100, 200 and 250 m, respectively. Through the simulation, the authors summarized the laws of stress evolution and deformation of the working face 11030# of the overlying coal seam (II1) in the upper part of the stope. The research results are as follows: After the lower PLM, the stress distribution of 11030# II1 in the strike direction can be divided into three parts from the open-off cut to the stopping line: original stress zone, pressure relief zone and stress concentration zone; the stress distribution of 11030# II1 in the dip direction can also be divided into the three parts from the top to the bottom. However, the stress distribution in the dip direction is less uniform than that in the strike direction. Meanwhile, the 11030# II1 had an unobvious deformation in the original stress zone, compressive deformation in the stress concentration zone, and expansive deformation in the pressure relief zone; the maximum expansive deformation was observed in the middle of the pressure relief zone. The research results lay the theoretical basis for engineering application of lower PLM.

Correction for ‘Selective oxidation of alcohols on hydrogen titanate nanotubes under visible light irradiation: relationship between nanostructure and catalytic activity’ by Juan Yang et al., Catal. Sci. Technol., 2016, 6, 7604–7614, DOI: https://doi.org/10.1039/C6CY01345G

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.

In order to determine the gas pressure for water-intrusion borehole, a new method of gas pressure measurement in water-intrusion borehole based on gas solubility was proposed. A mathematical model of gas migration in water-intrusion borehole considering the effective diffusion coefficient was established. The influencing factors, such as water-intrusion radius, diffusion coefficient and solution convection, on the dissolution equilibrium of gas in borehole water were investigated. Finally, the feasibility of the new gas pressure measurement method was discussed. Results showed that the gas diffusion coefficient and water-intrusion radius of coal seam are the key factors influencing the equilibrium duration of gas dissolution. During the accumulation stage of formation water in borehole, the gas is able to quickly dissolve and diffuse in borehole water. The amount of dissolved gas can reach more than 90% of solubility in three days. After the formation water enters the coal seam, the equilibrium time grows sharply because the effective diffusion coefficient of gas in water-saturated coal is much lower than that in pure water. In addition, the water convection causes a higher amount of gas dissolved at the borehole bottom. Finally, the field applicability for the new method of pressure measurement were analyzed by combining the data from previous works. This work provided a new idea for the gas pressure measurement of water-intrusion borehole.

In order to analyze variable-mass permeation characteristics of broken rock mass under different cementation conditions and reveal the water inrush mechanism of geological structures containing broken rock masses like karst collapse pillars (KCPs) in the coal mine, the EDEM-FLUENT coupling simulation system was used to implement a numerical simulation study of variable-mass permeation of broken rock mass under different cementation conditions and time-dependent change laws of parameters like porosity, permeability, and mass loss rate of broken rock specimens under the erosion effect were obtained. Study results show that (1) permeability change of broken rock specimens under the particle migration effect can be divided into three phases, namely, the slow-changing seepage phase, sudden-changing seepage phase, and steady seepage phase. (2) Specimen fillings continuously migrate and run off under the water erosion effect, porosity and permeability rapidly increase and then tend to be stable, and the mass loss rate firstly rapidly increases and then gradually decreases. (3) Cementation degree has an important effect on permeability of broken rock mass. As cementing force of the specimen is enhanced, its maximum mass loss rate, mass loss, porosity, and permeability all continuously decrease. The study approach and results not only help enhance coal mining operations safety by better understanding KCP water inrush risks. It can also be extended to other engineering applications such as backfill paste piping and tailing dam erosion.

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.

Cyclones have been widely used as separation devices during pneumatic conveying. The pipelines and cyclones inlet are connected by diameter change joints. The inlet cross section area is always larger than the pipeline. This results in the velocity of gas becoming lower than particle velocity when the gas-particles flow into the cyclone. The gas-solid flow in cyclones are investigated using FLUENT-EDEM method. The results show that the existence of particles had major effects on the gas flow of the cyclone within the inlet and conical separation parts. The tangential, axial, and radial velocities are only slightly changed when solid gas ratio at the same gas velocity/particle velocity ratio, while the velocities increase with the increasing of gas velocity/particle velocity ratios under the same solid gas ratio. The cyclone pressure drop decreases at a low solid gas ratio, and is maintained at an almost constant level with the high solid gas ratio. However, with the lower solid gas ratio, the particle strands are not clear, and the particle strand is found to become clearer and remained constant when the solid gas ratio is 6.42 kg/kg at the same gas velocity/particle velocity ratio. The particle strands form early with the increasing of gas velocity/particle velocity ratios at the same solid gas ratio. At the same time, the particle-wall interaction force is determined to be the main reason for the particle breakage and wall attrition. The results could potentially provide support for the development of a pressure drop theory for cyclones. Refereed/Peer-reviewed

The gas permeability of the coal samples, subjected to cyclic temperature impact under different conditions, was considered in the article. Meanwhile, the change of the coal sample permeability and fractures were analyzed through scanning electron microscopy. The obtained results show that the cyclic temperature impact has largely positive effect on the increase of the coal sample permeability and that new fractures and permeability growth increase with the higher temperature difference.

Determining the width of the stress relief zone on roadway surrounding rocks is the premise to optimize drilling borehole effect and increase gas extraction efficiency. In this study, a new width measurement method of the stress relief zone on the roadway surrounding rocks was proposed, which determined the width according to gas pressure attenuation speeds in roadway boreholes at different depths. Then, the variation curve of the gas pressure in boreholes at different depths with the time was gained through a field test. On this basis, laws of the gas pressure attenuation and the gas transmission and loss in boreholes at different depths were explored through a numerical simulation based on COMSOL Multiphysics, thus concluding the stress on roadway surrounding rocks, the distribution of plastic zones, and the stress-permeability relation. The scientificity of the proposed method was illustrated theoretically. Finally, the proposed method was verified by the field test data and numerical simulation results of the gas extraction at different sealing depths. Research results demonstrate that the pressure in boreholes attenuates in the logarithmic function pattern. The attenuation speed decreases with the increase of the drilling depth. The width of the stress relief zone on roadway surrounding rocks in the studied area was determined to be about 11 m according to the proposed method. Both the numerical simulation and the field test of the gas extraction efficiency prove the feasibility and validity of the proposed method in determining the sealing depth of the borehole for the gas extraction. Research conclusions are of important significance to enrich width measurement methods of the stress relief zone on roadway surrounding rocks and to optimize sealing parameters of underground boreholes for gas extraction.

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

Nanostructured H-titanate materials with different morphologies, including nanotubes, nanosheets and nanofibers, are examined as active photocatalysts for the selective oxidation of alcohols to elucidate the relationship between the structure and the catalytic properties. H-titanate nanotubes (H-TNTs) exhibit superior photocatalytic performance under visible light irradiation to H-titanate nanosheets (H-TNS) and nanofibers (H-TNF) although they have similar crystal structures. N2 adsorption–desorption isotherms and low temperature electron spinning resonance (ESR) spectra indicate that H-TNTs possess a huge specific surface area and a large fraction of unsaturated surface Ti active sites. This facilitates the formation of a high concentration of surface complexes between unsaturated Ti atoms and adsorbed alcohols and consequently enhances the visible-light absorption, which play key roles in achieving high activity for ligand-to-metal charge transfer (LMCT) based photocatalytic systems. The unique structural features of 1D nanotubes promote the charge-transfer efficiency from the coordinated alcohol species to surface Ti atoms, which is evidenced by low temperature ESR under visible light and photoelectrochemical characterizations. Furthermore, pyridine-adsorbed Fourier transform infrared (FT-IR) spectra reveal that there are effective Bronsted acid sites on the surface of H-TNTs, which can accelerate the decomposition of Ti-peroxide species and the regeneration of Ti active sites. In brief, the features of 1D nanotubes, including large surface area, abundant unsaturated surface Ti sites, high concentration of surface complex, strong visible-light absorption, effective charge transfer and Bronsted acidity, contribute to the high photocatalytic activity of H-TNTs.

Although the impact of Karst Collapse Pillars (KCPs) on water inrush has been widely recognized and studied, few have investigated the fluid-solid interaction, the particles migration inside KCPs, and the evolution feature of water inrush channels. Moreover, an effective approach to reliably predict the water inrush time has yet to be developed. In this work, a suite of fully coupled governing equations considering the processes of water flow, fracture erosion, and the change of rock permeability due to erosion were presented. The inverse velocity theory was then introduced to predict the water inrush time under different geological and flow conditions. The impact of four different controlling factors on the fracture geometry change, water flow, and inrush time was discussed in detail. The results showed that the inverse velocity theory was capable of predicting the occurrences of water inrush under different conditions, and the time of water inrush had a power relationship with the rock heterogeneity, water pressure, and initial particle concentration and an exponential relationship with the initial fracture apertures. The general approach developed in this work can be extended to other engineering applications such as the tunneling and tailing dam erosion.

Water-inrush in mine is one of the mine disasters caused by mining. In order to assess the risk of roof water-inrush in Wuyang Coal Mine based on the geological material of the coal mine, we built numerical models for the roof fracture and seepage development rule by using RFPA2D and COMSOL respectively, to analyze the changes in fracture zone, stress, water pressure and seepage vector with the advancement of working face, and compared the results with the field investigated data. The numerical simulation results indicate that: (1) with the advancement of the working faces, the stress relief range and fracture zone in the overlying strata increased rapidly up to about 90 m, and then tended to remain constant, reaching a final height of about 95 m which agrees with the field investigation; (2) the seepage flow constantly increased with a larger flow volume both in the front and rear area, where the stress concentration are the most serious. Keywords: Fracture zone, Numerical simulation, Water inrush, Wuyang coal mine

The effect of the fracture distribution on CO2 injection into coal seams was studied with a heterogeneous model having dual porosity to represent both the primary medium (the coal matrix) and the secondary medium (the fractures) under variable stress conditions. A numerical generation method and a digital image processing method were used to model the heterogeneous fracture distribution in the coal. The model solutions demonstrate that: (1) the fractures are the main channel for gas flow and their distribution has an important impact on the gas injection rate; (2) the fractures only affect the injection rate of CO2 into the coal but not the final storage amount; (3) when gas is injected into coal the fractures will first expand and then close due to the changing effective stresses and the adsorption induced swelling of coal grains. This fully coupled dual-porosity model with a heterogeneous fracture distribution provides a way to predict the CO2 injection into a coal seam. Keywords: CO2 sequestration, Dual poroelasticity, Fracture distribution, Digital image processing

Axial compression stress, produced by the pre-tightening force of a bolt, is a necessary condition for surrounding rock to form a whole structure. For this study, we built a mechanical model for an end-anchorage bolt, which represented the effect of a bolt on the surrounding rock in roadways in order to obtain its elastic solution. Simultaneously, we analyzed factors affecting the axial compression of the bolt on the surrounding rock and obtained the axial stress contours of the anchorage area through this elastic solution. The results indicate that 1) the axial compression stress in the anchorage area is proportional to the pre-tightening force and confirms the rule that stress declines sharply with the increase in axial distance from the bolt, with an effective stress radius of 1 m; 2) the maximum axial compression stress declines first and then rises with the increase in depth from the surface of the anchorage surrounding rock and 3) the size of the axial compression area is mainly determined by the length of the bolt.

Harmonic wavelets not only possess the traditional advantages of a wavelet function, they also have other merits such as clear expressions, more flexible time-frequency divisions, a simple transformation algorithm, a finer box-like frequency spectrum and others. Given the frequency distribution characteristics of the nondestructive testing signals from a rockbolt support system and based on the discrete harmonic wavelet transformation theory, we have effectively abstracted signals from frequency ranges concerned by removing useless high and low frequency signals from the testing signals of the rockbolt support system and obtained filtered signals with a reconstruction algorithm of harmonic wavelets. Finally, we applied the harmonic wavelet transformation in filtering analog signals and measured response signals of rockbolts. The results indicate that harmonic wavelets also have excellent filtering characteristics.

In order to study the movement characteristics of groundwater in a deep mining area and solve the dispute of the distribution rule of hydro-chemical zoning which is contradicted by lixiviation water zoning in a horizontal direction, we directed our attention to the source of deep groundwater, its seepage and hydro-chemical characteristics in a typical mining area. We used a neotectonic water-control theory, chemical and isotope methods, as well as a method for analyzing dynamic groundwater conditions. The results indicate that 1) Karst water in the deep and medium parts of this mining area is recharged by vertical leakage through neotectonic fractures rather than seepage along strata from subcrop parts or surrounding flows; 2) from surface to deep leakage paths, the variation in the types of chemical groundwater agrees with the normal lixiviation water distribution rule and the age of mixed groundwater increases; 3) the water-rich zones along neotectonic fractures correspond with water-diluted zones in a horizontal direction; 4) the leakage coefficient and water capacity of aquifers increases during the flow process of Karst water along the antidip direction (from west to east) and 5) Karst water in shallow mining areas forms a strong runoff belt along strikes and quickly dilutes the water from deep and medium mining areas. Overall, chemical and dynamic water characteristics actually agree with in terms of the entire consideration for differences in vertical leakage and abnormalities in the zone of water chemical distribution, along a horizontal runoff direction.