Optimization of key parameters for continuous and precise nitrogen injection in goaf based on response surface methodology

Research on the technology of uniformly injecting nitrogen into the porous long pipes in the gob of the gob-side entry retaining mining mode with roof cutting and pressure relief

The implementation of the Gob-Side Entry Retaining Mining Mode with Roof Cutting and Pressure Relief (GERRCPR) results in the gob connecting to the retaining roadway, creating an open space that causes significant air leakage and increases the risk of spontaneous combustion. A study was conducted during the implementation of the GERRCPR in the Xiaonan Coal Mine N1-1502 working face to investigate spontaneous combustion characteristics, along with fire prevention and extinguishing measures. To analyze gob airflow, Computational Fluid Dynamics (CFD) was employed to collect data on airflow conditions, O2 concentration, and temperature. Based on this, this study focuses on exploring the effects of nitrogen injection treatment under various rates and positions to optimize parameters for buried pipe nitrogen injection. Results indicated that within the GERRCPR, air leakage in the gob increased, leading to an increase in O2 concentration, expansion of the oxidation zone, and an elevated risk of spontaneous combustion. Air leakage primarily occurred from the retaining roadway and the working face near the intake-air roadway, peaking at a retaining roadway length of 500 m, with a flow rate of 226 m3/min. Following nitrogen injection treatment, the oxidation zone was significantly reduced, with optimal treatment achieved at a nitrogen injection depth of 70 m and a rate of 600 m3/h. Field monitoring data showed that the inertization measure of using porous long pipes, a nitrogen injection spacing of 30 m, and a nitrogen injection rate of 600 m3/h significantly decreased the O2 concentration within the gob. This reduction meets safety production requirements and outperforms the effectiveness of traditional buried-pipe nitrogen injection methods, thereby validating the simulation accuracy. Understanding the laws governing spontaneous coal combustion in the GERRCPR and enacting preventive measures for nitrogen injection can improve safety standards in mining operations. This proactive approach can effectively prevent spontaneous coal combustion accidents, resulting in substantial social benefits.

Study on Inerting Characteristics of Gas Coal by the Inerting Concentration and Ratio of an Inert Gas Mixture

To study the effect of the inert gas mixture concentration and ratio on the spontaneous combustion reaction of gas coal, a combination of experimental research and theoretical analysis was used to study the pyrolysis and combustion kinetics characteristics of gas coal and further explore the influence of inert gas on the inerting characteristics of gas coal. Research has shown that during the entire heating reaction process of gas coal, the concentration of inert gases has little effect on the drying and desorption stages, but there is a significant lag phenomenon in the characteristic temperature points of active decomposition and degassing stages. Under the same concentration of mixed inert gases, the higher the relative percentage content of CO2, the more significant the change and the better the inhibitory effect. The higher the volume fraction of the inert gas, the higher the cross-temperature point. In the late stage of rapid heating of coal samples, when the volume fraction of inert gas is 40%, the rate of temperature rise increases rapidly. In a pure air environment, CO begins to be released at 80 °C, and when the temperature rises to 130 ∼ 140 °C, the concentration of CO begins to rapidly increase. Under inert conditions, the higher the relative percentage content of inert gas is, the higher the temperature point at which CO is generated. When the experimental conditions are a mixture of 30% N2 and 10% CO2 as inert gas, the optimal inerting effect has been achieved. The research results provide a theoretical basis for determining the optimal ratio of inert gas inerting concentrations to achieve fire prevention and extinguishing.