Study on the influence of the dynamic evolution law of the void ratio in the withdrawal stage of the working surface on the self-ignition hazard area in the gob

To study the changing dynamics of the self-ignition area during the withdrawal stage of the working surface, this paper compares different advancing stages (The normal advancing stage, the ending mining stage and the withdrawal stage of the working surface). Caving accumulation and top coal fracturing near the gob (By gob, we mean the void space left behind after coal has been removed during mining operations), along with fully mechanized mining support, are analyzed. Based on the actual geological conditions of a mine in Shandong, the void ratio is calculated using the Particle Flow Code (PFC) program. Simulations of the distribution range of the self-ignition area under different conditions were conducted using FLUENT software. The conclusions are as follows: (i) During the withdrawal stage, cracks form in the top coal body. (ii) Oxygen concentration is high on both sides and low in the middle, leading to a decrease in the void ratio of the gob, which ranges from 0.01 to 0.35. (iii) The self-ignition area is most extensive in the middle of the gob (34-64 m), followed by the windward side (38-67 m), and smallest on the inlet side (0-10 m). (iv)Field measurement results indicate that the self-ignition area on the inlet side of the gob ranges from 0 m to 11 m, on the return air side from 37 m to 71 m, and in the middle from 30 m to 73 m.These findings closely match field measurements. The study's conclusions are crucial for identifying self-ignition hazard areas and preventing coal seam fires during the withdrawal stage of the working surface.