Mechanism of deep eutectic solvent on coal spontaneous combustion

Abstract

Chemical inhibition is one of the important measures for the prevention and control of coal spontaneous combustion. This paper proposed a quasi-ionic liquid inhibition method based on deep eutectic solvents (DES). First, seven kinds of room temperature deep eutectic solvents were prepared and screened using a heating method. The changes in the functional groups and thermodynamic characteristics of different DES-treated coal samples were analyzed. On this basis, the density functional theory was utilized to analyze the differences in the modification of coal's physicochemical properties by the hydrogen bond strength in the DES, and the inhibition mechanism of deep eutectic solvents and their optimal hydrogen bond strength were deduced. The results showed that after the DES treatment, the hydrogen bond network in coal was disrupted and rearranged. The relative abundance of aliphatic and aromatic hydrocarbons increased by 10%−37%, the content of aliphatic side chains decreased by 9.38%−20.65%, the relative abundance of oxygen-containing functional groups (C=O and C—O) decreased by 22.88%−56.94%, and free low-molecular compound and minerals were leached out. After the DES treatment, the mass loss during the evaporation and desorption stage of coal and the oxygen uptake during the oxygen absorption stage decreased. The heat release during the low temperature oxidation stage and the thermal decomposition stage was reduced by 8.94%−77.51% and 5.40%−26.20%, respectively. The stronger the electronegativity of the hydrogen bond acceptor site in the HBA, the greater the hydrogen bond strength formed between HBA and HBD. The hydrogen bond strength in the DES was positively correlated with the degree of destruction of the hydrogen bond network in coal, and was locally correlated with the oxygen uptake during the oxygen absorption stage, the heat release during low temperature oxidation, and the mineral removal rate. The DES weakened the low-temperature oxidation reactivity of coal by dissolving its active components, and increased the bond dissociation enthalpy of coal by promoting the rearrangement of hydrogen bonds into more thermally stable [OH]4 and OH—N hydrogen bonds. However, the excessive strong hydrogen bond strength would inhibit the removal and dissolution of active side chains. Therefore, the hydrogen bond strength of deep eutectic solvents used to inhibit coal spontaneous combustion should be controlled between 69.45 kJ/mol and 160.00 kJ/mol.

Keywords

• deep eutectic solvent
• coal spontaneous combustion
• quantum chemical calculation
• hydrogen bond