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The low-temperature nitrogen adsorption test was used to study anthracite from Jiulishan coal mine with different particle size ranges of 60–80 mesh, 150–200 mesh, and > 200 mesh. The adsorption isotherm, adsorption capacity, pore volume, pore specific surface area, and average pore diameter of coal samples were analyzed by BET and DFT models in order to study the influence of particle size on the pore structure of anthracite and determine the optimal range of particle size for low-temperature nitrogen adsorption test. The results indicate that the particle size plays a significant effect on the pore structure of anthracite and the adsorption capacity of soft coal is less affected by particle size, while hard coal is substantially affected by particle size. The adsorption capacity of hard coal with particle size of > 200 mesh is increased by 7 times when compared with the particle size of 60–80 mesh, indicating that the gas molecular mobility hindrance decline and pore connectivity improves with the decrease of particle size. The average pore diameter of hard coal decreases continuously from 3.1424 to 2.854 nm, while that of soft coal expands from 2.8947 to 3.2515 nm and then to 3.0362 nm with the decrease of particle size. The effects of particle size on the pore surface area of soft and hard coal are concentrated within the < 10 nm pore aperture. Effect of particle size on hard coal pore volume is mainly focused in the pore size < 10 nm, whereas that of soft coal is primarily concentrated in the pore with aperture ranges of 2–100 nm. When the particle sizes varies from 60–80 mesh to 150–200 mesh, the collapse of large pore of hard coal appears better than that of closed pore. When the particle size of hard coal reaches > 200 mesh, the collapse of closed pores and the damage to small pores are stronger than the collapse of large pores. The fractal dimensions with relative pressure of 0–0.20 and 0.20–0.995 are defined as D1 and D2, respectively, and when the fractal dimension D1 increases, the surface roughness and structural complexity of coal samples increase with the decrease of anthracite particle size, while the fractal dimension D2 shows the opposite trend, which indicates that anthracite of smaller particle size possess higher adsorption capacity. Therefore, 150–200 mesh is recommended as the preferred anthracite particle size in low-temperature nitrogen adsorption test. [ABSTRACT FROM AUTHOR]

In order to study the effects of different compression loads on the pore characteristics of coal, taking remolded coal as the research object, the mercury intrusion method was used to determine the pore structures of the briquettes under the compression loads of 50, 70, 90 and 110 MPa, and the Menger sponge model was used to conduct fractal research on the measured parameters. The results show that the compression load has a significant effect on the pore structure parameters of the briquettes. The hysteresis loop generated by the mercury-intrusion and mercury-extrusion curves of raw coal is small, and the pore connectivity is better. After different loads are applied for briquettes, the hysteresis loop becomes larger, and the pore connectivity becomes worse. From the process of the raw coal to the briquettes loaded at 50 and 70 MPa, the pore-specific surface area reduced from 5.069 m2/g to 1.259 m2/g, the total pore volume increased from 0.0553 cm3/g to 0.1877 cm3/g, and the average pore size increased from 43.6 nm to 596.3 nm. When the compression load reached 70 MPa, the specific surface area, total pore volume, and average pore diameter of briquettes remained basically stable with the change in the compression load. The minipores and visible pores and fissures of raw coal contribute 78% of the pore volume, and the micropores and minipores contribute 99% of the specific surface area. After being pressed into briquettes, the volume of mesopores and macropores increases, the volume of visible pores and fractures decreases and the volume of minipores changes little; additionally, the pore surface area contributed by mesopores and macropores increases, and the pore surface area contributed by micropores decreases, indicating that the effect of compression load on pores of 10–100 nm is not obvious, mainly concentrated in the 100–10,000 nm region. The fractal curve of briquettes is fitted into three sections, which are defined as low-pressure sections 1 and 2 and high-pressure section 3, and the fractal dimensions are D1, D2 and D3 respectively. The fractal dimension D1 of briquettes with different compression loads is close to 2, D2 is close to 3 and D3 is greater than 3. The pore structures of briquettes have obvious fractal characteristics in the low-pressure sections 1 and 2 but do not conform to the fractal law in the high-pressure section. Furthermore, in the micropore stage of briquettes, the measured surface area and volume are both negative, indicating that the mercury intrusion method used to test the pore structure of the loaded briquette is more likely to cause the collapse of and damage to the pores in the micropore (<10 nm) stage. [ABSTRACT FROM AUTHOR]

The occurrence of a steeply inclined coal seam is extraordinary, and the coal body is seriously damaged by extrusion. The most steeply inclined coal seam is a high-gas or -outburst coal seam, and protective layer mining is the safest and most effective measure for regional prevention of coal and gas outburst. Based on considering the coefficient of lateral pressure and vertical height of the section, the deflection of the basic roof of the steeply inclined protective layer in a mine in western Henan, China, was calculated using the deflection calculation method of the thin-plate theory of elasticity. Using MATLAB to understand the deflection, the deflection curve was obtained. The law of rock movement and deformation in the mining process of the protective layer was studied by a similarity simulation experiment. The results show that, after mining, the roof mainly sinks slowly without large-scale collapse, and the largest rock strata movement is located in the upper part of the slope. Rock strata movement and fracture development can relieve the pressure of the protected layer and provide a channel for gas migration and drainage. The mining conditions of the protected layer will not be destroyed, and mining this type of protected layer in this mine has better safety and feasibility. The conclusions of this study have a guiding and scientific significance for the control of surrounding rock and the layout of gas drainage boreholes of under-protective steeply inclined coal seam mining. [ABSTRACT FROM AUTHOR]

The accuracy of the drainage radius plays a vital role in the gas drainage effect, and the establishment of the drainage model and numerical calculation of the model has an essential value for the accurate determination of the drainage radius. Based on the elastoplastic constitutive model of mining coal and rock mass, effective stress equation, gas content equation, gas flow continuity, and control equation established by predecessors, the gas-solid coupling model of coal-rock deformation and pressure-relief gas flow in protective layer mining was established in this paper. The numerical simulation prototype was based on the mining engineering practice of a short-distance and underprotective steep seam mining in a mine in western Henan of China. COMSOL Multiphysics coupling software was used to numerically calculate the coupling model, and the influence radius of different mining pressure-relief area and different drainage time was obtained. The results show that under the same geological conditions and mining conditions, the impact range of drilling in the pressure-relief area of the protected layer is larger than that of the nonrelief area. As the working face of the protection layer advances, the pressure-relief area of the protected layer gradually increases, the influence range of the drainage borehole increases, the drainage borehole has undergone a process of initial stress-stress concentration-stress reduction-stress recovery successively, and the strike drainage radius is larger than the inclined. [ABSTRACT FROM AUTHOR]

As shallow coal resources deplete in industrialized nations, mining operations expand to the deeper resources to meet the requirements of the society. Deep coal seams in China have the characteristics of high gas pressure and high gas content. There is a persistent track record of gas-related accidents in Chinese coal mines. In an effort to eliminate gas explosions and gas outbursts, a new permeability improvement technology of "hydraulic punching plus air blasting" is developed for soft and low-permeability coal seams. To intensively enhance the permeability and achieve efficient extraction of gas from low-permeability coal seams, hydraulic punching is performed first, followed by air blasting. The LS-DYNA software was used to simulate the blasting process by the air cannons. Results were obtained from the Guhanshan mine. Comparisons were done for the stress field, deformation field of the coal body using the law of coal body deformation. Deformation at different blasting times was compared in combination with the field air blasting tests. With 40 times air blasting, the gas drainage increased by about two times compared to drainage without air blasting. The pure gas extraction volume was also observed to be about 2.2 times higher than that without air blasting. The results proved improvement of the coal permeability and gas drainage by this new technology. [ABSTRACT FROM AUTHOR]