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

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.

KEYWORDS:

• Soaked and air-dried coal
• different soaking time
• spontaneous oxidation
• free radicals
• functional groups

Article highlights

• The effect of soaked time on CH₄ amount is higher than that on O₂ and N₂.

• The oxygen concentration of S₁ is the highest during the desorption process.

• CH₄ concentration point of free radicals concentration slows down gradually earlier.

• The change law of functional group is related to the soaked time.

Additional information

Funding

This work was supported by the “Double First Rate” Independent Innovation Project of CUMT [2018ZZCX05]; the National Key R&D Program of China [2018YFC0807900].