Adsorption Characteristics of Soaked Air-dried Coal and Reaction Characteristics of Free Radical Functional Groups in CH4-containing Oxidizing Atmosphere

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ABSTRACT This paper selects the unsoaked air-dried coal (USC), air-dried coal with 1 month’s water soaking (S1), air-dried coal with 3 months’ water soaking (S3) and air-dried coal with 5 months’ water soaking (S5) as the research objects to analyze the adsorption law of coal to CH4, oxygen, nitrogen, and mixed gas, and the changes of free radical concentrations, aliphatic hydrocarbons, and oxygen-containing functional groups under gas atmospheres with different CH4 concentrations during low-temperature oxidation process by using ESR and FTIR. The experimental results show that the effect of soaking time on CH4 adsorption amount is greater than that on O2 and N2 adsorption amounts. It can be seen by investigating the O2 concentration after the desorption process that there is no significant relationship between the O2 concentrations of three kinds of coals and different soaking time under the mixed gas with 0% CH4 concentration. However, there appears a certain law for O2 concentrations under the mixed gas with 25% and 45% CH4. The oxygen concentration of S1 is obviously higher than those of S3 and S5. Furthermore, there exists an upper limit of CH4 concentration to inhibit free radicals concentration of S1, about 35%. Obviously, the CH4 concentration points of S3 and S5, at which the decrease rate of free radicals concentration slows down, appear earlier than that of S1. The concentrations of -CH2 – and -CH3 of S1 gradually decrease in the initial stage and start to rise once the temperature exceeds 120°C. The change law of aliphatic hydrocarbon of S3 is similar to that of aliphatic hydrocarbon of S1. The concentration of aliphatic hydrocarbon of S5 starts to increase from 25% in the initial stage. The -OH concentration decreases sharply as starting oxidizing, and then the decrease gradually slows down. With the CH4 concentration increasing, the -OH concentration slowly increases, thus the decrease of -OH concentration reduces. Furthermore, the results show that the declining rate of -OH concentration in S1 is higher than those of -OH concentrations in S3 and S5. The -COOH of S1 rapidly decreases in the stage of 40–120°C. But with the increase of CH4 concentration, the decrease rate is obviously smaller. The decrease rate of S3 further slows down at the initial stage, and there is no increase in the -COOH content of S5.