Low-temperature oxidation of free radicals and functional groups in coal during the extraction of coalbed methane

Abstract

To analyze the low-temperature oxidation of coal in an air flow containing coalbed methane (CBM), the low-temperature oxidation of coal under different CH4 concentrations was analyzed based on the oxidation reaction of free radicals and functional groups. Based on the experimental platform for simulating the low-temperature oxidation of coal in different atmospheres, the experimental process was carried out by using the electron spin resonance (ESR) and Fourier transform infrared (FTIR) technologies. Furthermore, the influences of CH4 on the activity of coal during low-temperature oxidation and the oxidation reaction of functional groups on the surface of coal were explored. The results showed that, at different oxidization temperatures from 20 °C to 200 °C, the concentration of free radicals decreased with increasing CH4 concentration (0%–45%), and CH4 inhibited the oxidation activity of coal. Under the condition that the CH4 concentration was lower than 30%, CH4 showed a large inhibitory effect on the oxidation activity of coal. After the oxidization temperature was higher than 80 °C, the promotion effect of the oxidization temperature on the oxidation activity of coal was significant. Moreover, –CH2 and –CH3 exhibited high activities during low-temperature oxidation and were the major reactive groups. When the coal was oxidized in dry air, the contents of –CH2 and –CH3 constantly decreased at temperatures of 40 °C–110 °C until the CH4 concentration was higher than 25%. This finding implied that CH4 inhibited the consumptions of –CH2 and –CH3 in the oxidation process of coal. The –OH content generally decreased at first and then increased, showing high activity during low-temperature oxidation, and was considered the major reactive group. With increasing CH4 concentration in the oxidation atmosphere, the reduction amplitude of –OH decreased, indicating that CH4 can inhibit the consumption of –OH in the oxidation process. When the CH4 concentration was higher than 35%, CH4 influenced both the consumption and production processes of –OH, such that the consumption rate of –OH was gradually equal to its production rate. The content of –C–O increased overall, which implied that –C–O was the main product of low-temperature oxidation. As a relatively stable group, the –C–O group was not produced below a certain temperature in the oxidation process. Additionally, the unchanged stage disappeared when the CH4 concentration was higher than 25%, which indicated that CH4 promoted the production process of the –C–O functional group. –COOH showed high activity during low-temperature oxidation, and its content rapidly decreased at first and then increased quickly during coal oxidation in dry air, indicating that it was the main reactive group during low-temperature oxidation. With increasing CH4 concentration in the oxidation atmosphere, the reduction in the –COOH content gradually decreased, and then the –COOH content remained unchanged after the CH4 concentration was higher than 25%. This result indicated that CH4 inhibited the consumption of –COOH in the oxidation process.