Mechanism of Persistent CO Overlimit at the Upper Corner in Low-Rank Coal Mining Under Conventional Goaf Oxidation Prevention Measures: A Case Study of Nitrogen Injection

Low-rank coal reserves in China are vast. Residual coal in the goaf under fully mechanized top-coal caving mining can oxidize at ambient temperature, resulting in persistent over-limit carbon monoxide (CO) concentrations at the working face’s upper corner, threatening mine safety. Conventional inerting measures targeting the oxidation zone in the goaf have limited effectiveness in controlling over-limit CO concentrations, and the mechanism is unclear. Taking the 130X working face of Dananhu No.1 Coal Mine as an example, this paper establishes a mathematical model for low-rank coal oxidation at ambient temperature. It optimizes nitrogen injection parameters and analyzes oxygen consumption and CO generation/migration characteristics under the optimal scheme, revealing the CO production mechanism under ambient temperature. Results show that single-point nitrogen injection is ineffective, while dual-point injection at 10 m and 20 m in the intake gateway is optimal, reducing CO concentration. Further research indicates that oxygen concentration dominates CO generation. The dissipation zone, with high permeability and oxygen supply, is the main CO source and the reason for the failure of traditional measures. Thus, efficient oxidation inhibition technologies for the dissipation zone should be prioritized to control CO sources, providing a theoretical basis and technical direction for disaster control in similar mines.