Degradative solvent extraction of subbituminous coal with deep eutectic solvent and effect of reaction conditions on products

A degradative solvent extraction method for upgrading low-rank coal was performed at 200–350 °C for 90 min to obtain a substance dissolved in the solvent at room temperature (Soluble). Because the resulting mixture exhibited a high carbon content without ash, it could be readily used as a fuel. Furthermore, deep eutectic solvents (DESs) have attracted attention for improving the Soluble yield and decreasing the oxygen content in Soluble. DES is known to cleave oxygen-containing functional groups in biomass and is considered effective for deoxidizing low-rank coal. Herein, DES was prepared by mixing choline chloride, FeCl₃·6H₂O, and Adaro subbituminous coal (AD) and then added to 1-methylnaphthalene (1-MN) in a non-polar solvent, followed by degradative solvent extraction in the range of 200–350 °C. The effects of reaction temperature and added DES amount on the product yield and the composition were evaluated. As the reaction temperature and amount of DES added increased, the Soluble yield and carbon content increased. It was also found that the thermal decomposition temperature and oxygen content decreased with the increasing DES amounts. This decrease indicates that DES promotes the deoxygenation and decomposition of AD and increases the soluble yield of the fuel source.