Experimental study on the influence of coal oxidation on coal and gas outburst during invasion of magmatic rocks into coal seams

This research aims to explore the influence mechanism of coal oxidation on the outburst risk of coal seams after a magmatic rock invasion. Hence, the possibility of an outburst risk in coal seams caused by coal oxidation under different oxygen concentrations was evaluated by utilizing multiple indices to predict the outburst risk of coal and gas. By applying the BEL-MAX automatic analyzer for specific surface area (SSA), the change laws of the Brunauer–Emmett–Teller (BET) SSA and pore volume during coal oxidation were analyzed. Moreover, the change in multiple physical parameters during coal oxidation was measured by employing an industrial measurement instrument, gas chromatograph, initial velocity (△P) tester, and hardness tester. The results indicate that with the increasing oxidizing temperature of coal, the contents of moisture and volatiles in the coal declined constantly while the BET SSA and pore volume increased, thus strengthening the capability of coal mass for adsorbing gas. Moreover, the coal strength reduced with the constant development of pores, as shown by the Protodyakonov's coefficient (f value) of coal mass that decreased constantly and △P of gas diffusion for reflecting the diffusion capability of coal for the gas to increase gradually with increasing oxidizing temperature. With the increasing oxygen concentration in an oxidizing atmosphere, the required oxidizing temperature at which the comprehensive index reflecting the outburst risk of coal mass was larger than the critical value reduced gradually. Thus, the coupling effect of magmatic rock invasion into coal mass, thermal metamorphism, and oxidation reaction led to the increase in gas content and SSA of coal mass, thus improving the capability of coal for adsorbing and storing gas. Correspondingly, the mechanical strength of coal mass reduced and the initial velocity of gas diffusion from coal mass increased significantly. Additionally, owing to the sealing effect of magmatic rock inhibiting the migration of gas in coal mass in the area, the risk level of coal and gas dynamic disasters in the area increased significantly.