Experimental determination of the outburst threshold value of energy strength in coal mines for mining safety

Abstract

An outburst of coal and gas is defined as a rapid release of a large quantity of gas in conjunction with an expulsion of coal into the mine workings in coal mines. It is a major dynamic hazard during coal mining and has to be controlled for mining safety. Despite significant advances made in outburst studies, the outburst incidence continues to occur. A variety of indices have been developed in assessing outburst risk such as gas content, gas desorption rate and coal strength. However these indices are empirical or based mainly on localized experience. Aiming to develop a more scientifically solid index in outburst risk assessment, energy approach was adopted in this study to explain the outburst process, i.e. the outburst is treated as a rapid energy release process in coal. Experimental investigations on outbursts with respective gas pressure of 0.4MPa, 0.6MPa, 0.8MPa, 0.9MPa and 1.0MPa were conducted. The mass and size distributions of ejected coal were analyzed. Based on testing results, the outburst effective energy in coal (including expansion energy of free gas in pore and the energy of desorption gas in coal structure), and the outburst energy of fragmenting and moving coal particles were both calculated. Finally, the outburst threshold value of energy strength was determined. Results show that: (1) the outburst effective energy and the energy for coal fragmentation and movement both positively correlate with gas pressure; (2) the minimum energy strength of about 1.1MJ/m3 in gas-bearing coal is required for an outburst to occur; (3) compared with free pore gas energy, the desorption gas energy from coal plays a decisive role in outburst initiation and propagation, which is in good agreement with the popular view that coal and gas outburst is gas-driven. Outcomes of this study indicate that the risk of outbursts could be effectively minimized through reduction in coal seam gas energy.