Optimization of CO2/N2 injection ratios in goaf by saturation adsorption capacity

The inerting technology is a common means to prevent fires in goaf of coal mine, in which adsorption capacity is the key index to measure the inerting effect. Therefore, this paper obtained the coal spontaneous combustion (CSC) stage of Qianyingzi coal mine through programmed heating experiments, and determined the physical adsorption stage temperature (below 323.15 K) matching the actual environment. The oxidation characteristics of coal samples with different adsorption degrees were further studied by adsorption experiment and TG-DSC test. The results indicate that the combustion activation energy of coal samples follows the sequence of second-stage adsorption coal > dried coal > raw coal > first-stage adsorption coal. This implies that the CO2 saturated adsorption can exert a specific inhibitory impact on coal combustion. Therefore, the index of the saturation adsorption capacity (SAC) was proposed to evaluate the inerting effect. We further studied the adsorption capacity of coal at different ratios of CO2/N2 by molecular simulation under different pressures (0–5 MPa). The results show that CO2 dominates in both single-component adsorption and competitive saturated adsorption. The average of ratios (AOR) and average of differences (AOD) threshold are determined. The priority of SAC is obtained in the order of single-component (CS), CO2/N2 mixture ratio at 4:6 (C4:6), CO2/N2 mixture ratio at 2:8 (C2:8) from largest to smallest. Furthermore, a fuzzy optimization method for injection ratio of CO2/N2 under different pressures was established. During dynamic changes in environmental pressure, it is appropriate to inject CO2 separately at 0.2–2.5 MPa. At 3 MPa, a mixture of CO2/N2 at a ratio of 2:8 is the best for prevent fires in goaf. Similarly, an inert gas with a CO2/N2 ratio of 4:6 is injected at 3–5 MPa.