Simulation on Harmful Gas Control by Balanced Pressure Ventilation in the Fully Mechanized Caving Face under Sealed Fire Area of Small Coal Mine.

The harmful gas in the sealed fire area of a small coal mine rushes into the mining face of the lower coal seam, which restricts the efficient promotion of the working face. In this paper, based on the evolution law of caving coal rock dilatation coefficient, the characteristics of the heterogeneous distribution of permeability and voidage in goaf were obtained, and the mathematical model of gas migration in goaf is constructed. The numerical solution of gas migration in goaf under the sealed fire area of a small coal mine was realized by using the Free and Porous Media Flow module and the Transport of Dilute Matter in Porous Media module in COMSOL Multiphysics, and the corresponding measure was proposed. The results show that the fresh air flows into the goaf from both the inlet air roadway and the working face and then flows out from the upper corner. Driven by the air flow, the CO in the overlying sealed fire area of a small coal mine flows out from the upper corner of the working face, resulting in the CO overlimit. Due to the influence of air leakage and the CO overlimit in the working face, low oxygen occurs in the working face. According to the characteristics of gas emission, balanced pressure ventilation technology is proposed to control the low oxygen in the working face and the CO overlimit in the upper corner. It is found that the balanced pressure ventilation obviously increases the pressure of the working face, reduces the pressure difference between the two ends of the working face by 45.7-26.7%, and decreases the air leakage to the goaf in the upper corner of the inlet air roadway. The field application shows that the problems of low oxygen in the working face and a CO overlimit in the upper corner are effectively solved.

Scale Effect on Uniaxial Compressive Properties of the Outburst-Prone Coal.

Coal is a naturally discontinuous, heterogeneous, and anisotropic brittle material. The uniaxial compressive strength of coals is significantly affected by the sample size-dominated microstructure of minerals and fractures. The scale effect of the mechanical properties of coal is a bridge connecting the mechanical parameters of laboratory-scale coal samples and engineering-scale coal. The scale effect of coal strength is of great significance in explaining the fracturing law of the coal seam and reveal the mechanism of coal and gas outburst disaster. The uniaxial compressive strength of outburst-prone coal samples with different scale sizes was tested, the variation law of uniaxial compressive strength with increasing scale was analyzed, and the mathematical models of both were constructed. The results show that the average compressive strength and elastic modulus of outburst coal decrease exponentially with the increase in scale size, and the decrease rate is reduced. The average compressive strength of the tested coal samples decreased from 10.4 MPa for size 60 × 30 × 30 mm3 to 1.9 MPa for scale 200 × 100 × 100 mm3, which decreases by 81.4%.