Outburst simulation experiments facilitate understanding coal and gas outburst in underground mining. With the help of the mid-scale simulation system, a model based on similitude principle, coal seam sandwiched by roof and floor, was constructed to conduct an outburst experiment. It had a three-dimensional size of 1500 mm × 600 mm × 1000 mm with 0.5 MPa gas pressure. The experimental procedures include specimen preparation, moulding, sealing, gas charging and adsorption, and completion. The outburst process was investigated by analyzing the gas pressure variation, temperature variation, outburst propagation velocity, particle size of outburst coal and energy transformation. During the experiment, each gas charging was accompanied with gas pressure or temperature fluctuation because of coal behavior of gas adsorption-desorption. The outburst propagation velocity was 17.2 m/s, obtained by a mass-weighted calculation of velocities of outburst coal. The small-size coal particles have a higher desorption rate and tend to participate in outburst process. According to energy conservation law, the energy forms of the outburst included elastic strain energy (E_e), gas expansion energy (E_p), internal energy of coal (ΔU), breakage work (W₁), throwing out work (W₂) and gas-flow loss energy (ΔE), and each was calculated respectively. Gas potential energy, including gas expansion energy and internal energy of coal, registered a larger percent and was far greater than the strain energy. And it can be the main factor influencing the occurrence of low-threshold outburst. The experimental system provides a feasible way to study the initiation and evolution of coal and gas outbursts.

Multiscale fractal analysis of the pore system for coal is necessary to obtain more inner information. The techniques of Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) are combined to characterize the pore structure of natural coal. A total of eight coal samples, of a different rank and coalification degree, are prepared for experiments. Methods of SEM image processing, piecewise curve-fitting and correction of intrusion data are adopted to obtain more useful results. According to the pore size range of the MIP probe, pores in coal are classified as seepage pore (pore size ≥ 1000 nm), transition pore (pore size ≥ 50 nm and <1000 nm) and mesopore (pore size < 50 nm). Variations of multi-scale fractal dimensions are studied from the perspective of coalification degree or coal rank. Fractal dimension from SEM data (D₁) and fractal dimensions of seepage pore, transition pore and mesopore (D₂, D′₂ and D″₂) from MIP data are calculated by fitting curves, and consequently correlations of those with volatile matter (V_daf), pore volume and pore size are analyzed and discussed. The U-shape relationships between fractal dimensions (D₁, D₂ and D′₂) and V_daf are observed. Macropores are presented as the isolated clusters embedding in the network of smaller pores, and the difference of the order of magnitude of the pores’ size affects the connectivity between pores. Both the pore size and volume have a direct influence on multiscale fractal dimensions. Overall, multiscale fractal analysis is beneficial to explore the structure of natural coal.

The gas pressure in front area of heading face is essential to dynamically evaluate coal and gas outburst during coal mining. In this work, a novel inversion model of gas pressure in front area of the heading face was established on premise of the hypothesis that a time-dependent zone of steady flow exists within newly exposed face. The key parameters in the inversion model were obtained based on the gas emission models and field data of gas emission rate in different times, which were used to calculate the volumes of gas emission from different sources. The results show that the percentage of gas emission from the heading face, coal wall and collapsed coal ranges from 7% to 47%, 47% to 82% and 2% to 11%, respectively. Based on the calculated volumes of gas emission and gas pressure inversion model, the gas pressure was obtained and transformed to the gas content. The absolute errors between the gas content tested and transformed in every hour is 0.4%–33%, which proved the rationality of gas pressure inversion model. Furthermore, the daily drifting footage, the radius of gas pressure boundary and the gas permeability coefficient of coal seam were confirmed to have a great effect on the result of gas pressure inversion. The inversion results verify that the speedy excavation can increase the risk of coal and gas outburst. This work produces a useful method for gas disaster prevention and control that converts the gas emission rate to an index of gas pressure within coal seam.

Mine gas disasters are a major safety concern in underground coal mining. Protective layer mining is widely used in gas disaster control, but there are limited theoretical and experimental results that can provide guidance for site-specific mining circumstances. Taking the Xinji No. 1 mine as an example, gas disaster treatments were conducted in a new panel with overlying goaf located 85 m above the coal mine and adjacent goaf located at 30 m intervals. This study involved a comprehensive investigation, which included four steps: the selection of the first mining face, gas control and prevention, tracking and investigation, and effect analysis and assessment. The safety strategy focused on gas control planning in new mining areas or panels. The distribution and evolution characteristics of the stress, the gas permeability coefficient and the deformation volume within the protected layer were determined by numerical simulation. The coal deformation, gas emission and extraction effect were analyzed by field observation. The deformation and gas permeability of the coal seam were consistent with the stress evolution, for which the maximum compressional and expansional deformation of 6-1 coal were 18‰ and 28‰, respectively. Gas disaster control and prevention treatment of the mining face produced a significant protective effect on the underlying No. 6-1 coal seam. This work is beneficial for the planning of gas control in successive panels.