Effect of acetic acid and CO2 on coal molecular structure and oxygen adsorption

Abstract

Adsorption oxygen inhibition is one of the main mechanisms in the CO₂ prevention of coal spontaneous combustion technology in goaf. Acidic solvent extraction technology can affect the adsorption properties of coal. However, there is a lack of research on the adsorption oxygen inhibition mechanism under the interaction of acetic acid and CO₂. Therefore, a series of coal extractions were conducted using acetic acid solvents with concentrations ranging from 0 % to 90 %. Elemental analysis, FT-IR, XPS, and ¹³C NMR experimental methods were employed to construct a molecular structure model of the coal. The reliability of the model was verified through isothermal adsorption simulation experiments and molecular simulation techniques. The effects of acetic acid on the microscopic structure of coal were discussed by comparing the raw coal and extracted coal. Furthermore, the adsorption behavior of coal towards O₂ under the effect of acetic acid and CO₂ at different temperatures and pressures was explored, leading to the identification of the optimal range of acetic acid concentrations. The research results indicate that acetic acid has a minimal impact on the aromatic macromolecular structure of coal. The coal molecules primarily consist of tri-substituted and tetra-substituted benzene rings, while aliphatic carbon structures are predominantly methylene and methyl groups. The nitrogen content mainly exists in the forms of pyridinic nitrogen and pyrrolic nitrogen, whereas oxygen is present in three forms: C–O, CO, and –COOH, with the highest content being CO. The molecular formulas of coal at acetic acid concentrations of 0–90 % are C₁₅₁H₁₀₆N₂O₂₅, C₁₅₁H₁₁₀N₂O₂₀, C₁₅₁H₁₀₈N₂O₂₁, and C₁₅₁H₁₁₂N₂O₁₉, denoted as KZ0, KZ30, KZ60, and KZ90, respectively. At different acetic acid concentrations, the adsorption of oxygen by coal is primarily dominated by alkyl ether and carboxyl groups, while the larger the free space volume of the coal sample, the more favorable it is for O₂ adsorption. The relationship between coal adsorption of oxygen under the action of acetic acid and CO₂ is as follows: KZ90 +CO₂ > KZ0 +CO₂ > KZ30 +CO₂ = KZ60 +CO₂, indicating that when the acetic acid concentration is 30 % and 60 %, the oxygen expulsion effect of coal on CO₂ is stronger. Based on the interaction energy and adsorption capacity, the optimal acetic acid concentration range is determined to be (30 %, 60 %]. For practical applications, a mobile downhole injection process was selected considering cost and safety factors. The findings contribute to enhancing the inerting effect in mined-out areas.