Post-outburst coal pulverization: Experimental insights with representative accident samples

Coal and gas outburst accidents pose a serious threat to safe coal mining. Coal pulverization and transportation are integral to outbursts, aiding in understanding outbursts process and holding great potential for elucidating outburst mechanisms. However, the lack of on-site coal sample makes it challenging to assess coal pulverization characteristics during outbursts. We collected coal samples from the forefront of five outburst sites in typical tectonic areas over the past decade. Three coal powders and coal blocks underwent particle size, low-pressure N2/CO2 adsorption and adsorption/desorption experiments to characterize their coal-gas properties. The impact of coal pulverization on coal transportation and outburst initiation was evaluated based on energy conservation principles. The results indicate that the particle size range of coal blocks is 0.27 μm to 2.6 mm, with over 50% consisting of particles smaller than 50 μm. The formation of micron-sized outburst coal powder is associated with tectonic movements. Pulverization increases micropore and some mesopore pore volume, and specific surface area initially increases and then decreases with decreasing particle size. The ultimate adsorption capacity of outburst coal at three particle sizes ranges from 16.8 to 36.0 m3/t. Pulverization affects gas adsorption capacity by influencing micropore volume. The methane desorption velocity within the first 60 s ranged from 0.0091 to 0.0998 ml/g/s, with the minimum particle size being 1.6 to 4.4 times the maximum particle size. Coal pulverization leads to an increase in pore quantity, a reduction in pore length, and the disappearance of high adsorption potential pores, resulting in rapid methane desorption. In the five outbursts, 246 to 25,580 m3 (30°C, 0.1 MPa) of desorbed gas is involved in coal transport. This requires gas desorption velocities ranging from 0.014 to 0.171 ml/g/s, with coal particle sizes reaching 5–875 μm. The abundant presence of micron-sized coal powder is essential for outburst initiation and sustained coal ejection, considerably increasing the risk of outbursts in tectonic coal seams.