Mutual replacement mechanism of inert gas with oxygen in coal and its effect on the low-temperature oxidation process of metamorphosed coal

Abstract

The ability of inert gas to inhibit the low-temperature oxidation of coal is crucial for preventing spontaneous combustion of coal. In this paper, the replacement process of inert gas and O2 in coal and the low-temperature oxidation process of inert metamorphosed coal samples were studied. The results show that when significant O2 is displaced by the inert gas, the displacement process is inhibited and some of the O2 is reabsorbed into the coal, resulting in a decrease in O2 displacement. This decrease in the amount of O2 displacement is called the amount of O2 reverse displacement (Vi). One-time and continuous replacement experiments were designed to elucidate the mutual replacement process between inert gas and O2 in coal. The Vi can be calculated as the difference between the continuous replacement amount (VC) and the one-time replacement amount (VO). In the continuous displacement process, when CO2 is used for replacement, O2 is replaced in large quantities over a short period, and the amount of O2 is replaced following the ExpDec2 function. When N2 is used for replacement, O2 is consistently replaced over a long period. This trend follows the Boltzmann function. In the one-time replacement process for 40–100 mesh coal samples, the replacement amount of O2 reaches its maximum at 90 min. When replaced by N2, the reverse replacement process of O2 is obvious, and the maximum Vi reaches 60.53 %, while 17.69 % by CO2. When replaced by mixed gas, VO increases with the increasing CO2 content in the mixed gas. After inserting, the amount of indicator gases such as CO, C2H4, and C2H6 produced during the low-temperature oxidation of coal decreases significantly, and the activation energy increases significantly. As the inerting time increases from 0 to 150 min, the inhibition first increases and then decreases, but is always present. At 90 min, the indicator gas production reached the lowest value, and the activation energy reached the peak (E1 of the metamorphic coal after N2 and CO2 insertion increased by 54.67 % and 218.78 %, respectively, in contrast to the raw coal). The activation energy of the CO2-inerted metamorphic coal sample was 202.23 % of that of the N2-inerted metamorphic coal sample.