Microstructure Study of High-Rank Coal in an Alkaline Solution at the Chengzhuang Mine.

To study the adsorption performance of coal bodies after alkaline solution erosion and the microscopic mechanism of alkali erosion on coal bodies, isothermal adsorption experiments at different pH values and with different numbers of soaking days were conducted on high-order coal bodies from the Chengzhuang mine. The results showed that the adsorption capacity of the coal bodies after alkali leaching was improved compared to that of the original coal, all of which was in accordance with the Langmuir equation. The unit adsorption capacity of coal samples increased gradually with an increase in the number of soaking days and solution pH, reaching the maximum at pH 13 and eight soaking days. The adsorption constant a of the coal sample was positively correlated with the pH, and the number of soaking days was a power exponential function; the adsorption constant b increased gradually with an increase in the pH of the solution and increased first and then decreased with an increase in the number of soaking days. The change in the adsorption of coal samples occurs because the alkaline solution reacts with the minerals in the coal as well as the mineral ions, and the resulting complex gels and precipitates block the pore channels of the coal body, which in turn inhibits the adsorption of gases. The presence of Na, Mg, Al, Si, Ca, Fe, and other elemental compounds detected in the generated sediments verified the mechanism of alkaline solution erosion. The changes in the microscopic pore structure of the coal body were quantified by low-temperature liquid nitrogen adsorption experiments. The small and medium pore volumes of the coal samples reached the maximum values at pH 13 and with eight soaking days, which is in agreement with the conclusion of optimal alkali modification.

Numerical Simulation of Integrated Mechanics of Drilling and Mechanical Cavitation in Coal Seam.

For coal and gas outburst and difficult extraction in soft, low-permeability, and high-gas seam, the integration technology of drilling and mechanical cavitation is put forward to relieve pressure and improve permeability of coal seam. Through the test of mechanical parameters of the coal body, the physical model of drilling and cave-making in coal seam is established, and the mechanical characteristics of drilling and mechanical cavitation are simulated numerically. The results show that the peak strength of coal increases linearly with confining pressure. When the drill bit just touches the coal, the maximum stress occurs at the center of the coal sample. With the increase in the drilling depth of the drill bit, the maximum stress point shifts to the depth. When the coal at the center of the sample is broken, the position of the maximum stress shifts to the surrounding. With the increase of drilling depth, the maximum contact number between the drill bit and coal body increases sharply. When the bit body basically enters the coal body, the maximum contact number between the drill bit and coal body remains unchanged. When the drill pipe rotates, the reaming tool collides with the hole wall at the reaming position on the axis. In general, the contact area increases with the opening of the reaming tool, but the contact between the reaming tool and the hole wall is random. As time increases, the contact force begins to increase and then basically stabilizes; at this time the reaming tool has been fully opened.

Study on the Critical Value of Residual Gas Content Based on the Difference of Adsorption Structure between Soft and Hard Coal.

In view of the difference of the adsorption structure between soft and hard coal, there is a big difference in the critical value of the inspection index for the regional outburst risk caused by the gas content. For the coal seams with soft and hard coal stratification, the model of gas content in the equilibrium state was established first, and the microscopic parameters of different rank coals were determined by the low-temperature liquid nitrogen adsorption test and mercury intrusion test. Then, the adsorption capacity of coal samples was determined by the adsorption test. Finally, the residual gas content of the coal seam in the equilibrium state was calculated based on the adsorbed gas content, and the critical value of prediction indexes of regional outburst based on the residual gas content was studied. The results show that for the same metamorphic degree, the specific surface area of soft coal is larger than that of hard coal. However, under the same gas pressure, the residual gas content of hard coal of anthracite and lean coal is greater than that of soft coal with the same metamorphic degree, while that of meager-lean coal and gas-fat coal is opposite. It is suggested to adopt the small value (rounded) of the measured gas content of soft and hard coal at 0.74 MPa as the critical value of the residual gas content in the regional effect test from the economic perspective. It is of great significance to determine the critical standard of the residual gas content in the regional effect test according to local conditions for reducing the cost of outburst prevention work.

Dynamic mechanical properties of structural anisotropic coal under low and medium strain rates.

The static mechanical properties of coal rock show anisotropism, which makes the permeability have anisotropic characteristics partly. The dynamic impact mechanical characteristics of structural anisotropic coal under low and medium strain rates were studied by using self-made vertical Split Hopkinson Bar (SHPB) equipment. The peak stress, the strain rate, dynamic elastic modulus and failure characteristics of raw coal with three coring directions were analyzed under the influence of five impact loads and structural anisotropy. The peak stress increases linearly with impact load, and the maximum strain rate and the dynamic elastic modulus increase exponentially with impact load. The coal samples display anisotropic mechanical characteristics. The values of maximum strain rate, peak stress and dynamic elastic modulus are ranked with directions by the perpendicular to bedding direction (Z direction), the parallel to bedding direction (X direction), and the oblique 45° to bedding direction (Y direction). Dynamic mechanical properties of structural anisotropic coal provide a theoretical basis for gas seepage in far-blasting field.

Experiment and Modeling of Permeability under Different Impact Loads in a Structural Anisotropic Coal Body.

A mount of bedding and cleat in a coal body causes that the mechanical property and gas permeability are anisotropic in a coal seam, partly. To reveal the permeability change law of the impacted coal, a self-developed vertical split Hopkinson pressure bar (SHPB) device is used to carry out the dynamic impact mechanical property tests of coal samples in three different coring directions under five impact loads and then the permeability of the impacted coal samples is measured by a permeability measuring instrument under different gas pressures. Finally, a calculation model for the anisotropic coal permeability is established to analyze the permeability distribution law in any direction with different angles to the bedding plane. The results show that with an increase in impact height the dynamic peak stress of coal samples increases gradually, which shows a linear growth relationship. The permeability of the impacted coal samples is much larger than that of raw coal samples, and the bigger the impact load, the larger the permeability. Moreover, under the same impact load and gas pressure, the permeability is the largest in parallel to the bedding direction, followed by that in oblique 45° to the bedding direction, and the smallest in perpendicular to the bedding direction. The permeability calculated by the anisotropic model in oblique 45° to the bedding direction is in good agreement with the measured results, and the errors are no more than 10%, which will provide a theoretical basis for the permeability distribution law of the coal seam after deep-hole blasting.

Freezing method for rock cross-cut coal uncovering I: Mechanical properties of a frozen coal seam for preventing outburst.

A comprehensive technology is proposed to realize fast and safe rock cross-cut coal uncovering (RCCCU) based on artificial freezing engineering method. This comprehensive technology includes four steps, namely, drilling a borehole, wetting the coal body by water injection, gas drainage and freezing the coal seam by liquid nitrogen injection. In this paper, the compressive strength, tensile strength and shear strength of frozen coal specimens are tested to obtain the mechanical parameters of the specimen. Then, for RCCCU under freezing temperatures, the outburst prevention effects are calculated and quantitatively analysed with regard to three aspects, namely, the enhancement of coal the mechanical properties, the reduction in the coefficient of outburst hazard (COH) in the distressed zone and the reduction in the interfacial elastic energy ratio (IEER) between the coal seam and the roof/floor. The results show that a considerable improvement in the mechanical properties of frozen coal and that the coal mechanical parameters, such as the compressive strength and the tensile strength, increase linearly with decreasing temperature. The coefficient of outburst hazard in the distressed zone decreases rapidly and drops from above 0.8 to below 0.3. The interfacial elastic energy ratio is greatly reduced from dozens of times of that of the roof/floor before freezing to several times of that of the roof/floor after freezing, which effectively weakens the sudden change of the elastic energy at the coal-rock interface.

Optimal layout of blasting holes in structural anisotropic coal seam.

Compared with the idealized isotropic coal seam, the effectively cracking distances of deep-hole pre-splitting blasting in different directions have significant difference in the structural anisotropy of coal seam. In this paper, based on the test results of coal mechanical parameters in different coring directions, the blasting crack distances in these coring directions of coal seam are theoretically calculated and analyzed, and then a new method of blasthole layout in coal seam is designed and the blasting crack effect is investigated according to the test data of gas drainage. The research results show that both compressive strength and tensile strength of the direction of perpendicular to bedding are larger than that in other directions, especially parallel to bedding. The blasting crack distances of calculation results are in great agreement with that of underground in-situ measurement, so the blasting crack zone caused by the differences of mechanical properties in different directions can be approximately treated as an ellipse. Comparing the conventional method of blasthole layout, the blasting crack effect with new method has greatly enhanced the methane concentration and gas drainage quantity. Pure methane quantities with the new method and conventional method are about 2.8 times and 1.67 times as large as that before blasting, and pure methane quantity with the new method is about 1.46 times that with the conventional method. Therefore, considering the anisotropic characteristics of coal seams, the reasonable blasthole layout plays an important role for enhancing gas drainage in low permeability and outburst working face.