Study on distribution law of “three zones” under CO₂-N₂ mixed inertization in goaf of gently inclined coal seam

To address the problem of spontaneous combustion of residual coal in goaf of gently inclined coal seams, a numerical model of gas migration in goaf was established based on the porous medium model and gas component transport equation. The influences of injection position and flow rate of pure CO₂ and CO₂-N₂ mixture on the distribution range of the “three zones” in goaf were investigated, and the gas migration law under CO₂-N₂ mixed inertization was analyzed. The results show that when CO₂ is injected from the intake air side to inertize the goaf, the area of the oxidation zone first decreases and then increases with the increase of the distance between the CO₂ injection position and the working face. With the increase of CO₂ injection rate, the oxidation zone area decreases, and the CO₂ migration pattern presents a “trumpet shape”, forming “inertization blind zones” near the working face on the intake air side and in the middle of the goaf. When CO₂ is injected on the intake air side and N₂ on the return air side for mixed inertization, the oxidation zone area first decreases and then increases with the increase of the distance between the N₂ injection position and the working face. With the increase of N₂ injection rate, the oxidation zone area decreases, and the N₂ migration pattern shows a “wave shape”, which reduces the area of the “inertization blind zone” in the goaf. The optimal inertization effect is achieved when the CO₂ injection position is 40 m from the working face with an injection rate of 750 m³/h, and the N₂ injection position is 50 m from the working face with an injection rate of 750 m³/h. Under these conditions, the CO₂ volume fraction in the return airway does not exceed 1.5%, the oxidation zone area decreases from 4 100.445 m² to 2 463.205 m², which is 14.84% lower than that under single CO₂ inertization. An “isolation zone” is formed in the middle of the goaf, which effectively restrains the migration of O₂ to the deep goaf.