Water-gas masking effect of the primary active sites in coal and room temperature oxidation of coal after desorption

Abstract

The oxidation of coal at room temperature provides the initial heat source for spontaneous coal combustion (CSC). In the CSC theoretical study, exploring the active substances in coal that can be oxidized at room temperature is a complex problem. Previous thermal decomposition experiments have found that coal after pyrolysis contains active sites that can exist stably in inert gases and be oxidized at room temperature. Thus it is speculated that there may also be primary active sites that are forced to be stored under inert media in coal. In order to explore the primary active sites of coal, the vacuum drying technology is applied. Based on that, the water in the raw coal can reach the boiling point under the low-temperature environment of negative vacuum pressure to complete the removal of water and gas. Under different experimental conditions (coal, desorption temperature, oxidation temperature, particle size), the cyclic oxidation online monitoring technology is used to design and implement the room temperature oxidation experiment of the desorbed coal samples. At the same time, the reaction mechanism is analyzed by the corresponding low-temperature nitrogen adsorption, XPS, ESR experiments. After vacuum desorbed, the oxidation experiments under cyclic conditions show that the massive gaseous oxidation products such as CO and CO2 will be formed in the process of raw oxidation at room temperature, and the gases appear and accumulate soon after oxygen is introduced, which proves that coal oxidation can occur at room temperature. The room temperature oxidation experiment after the desorption of raw coal indicates that there are a large number of active sites affected by water-gas masking in the raw coal. They are unable to undergo the oxidation exothermic processes, while the coal after the desorption of water and gas under negative pressure exposes massive active sites and forms a channel conducive to oxygen transport and reaction, which rapidly occur oxidation exothermic phenomenon and lead to the rise of coal temperature. Therefore, the active structure leading to the spontaneous warming of the raw coal is found, and the experiments extend the view of room temperature oxidation of the active sites from the particular case state of the pyrolysis to the general state. From the comparison of gas product generation, it can be seen that CO, which is not easy to be adsorbed by pores, is produced quickly at the moment of contact between the primary active site and oxygen. Therefore, it can be concluded that CO is more suitable as an intuitive gas evaluation index of the concentration of the active sites compared with CO2. The room temperature oxidation of the primary active sites of coal helps reveal the mechanism of coal spontaneous combustion and provides a solution to the problem of spontaneous combustion during gas extraction in high-gas mines and the problem of CO over-limit in low-rank coal mines.

Keywords

• coal spontaneous combustion
• room temperature oxidation
• water-gas masking effect
• negative pressure desorption
• primary active sites