Solvent Extraction of Superfine Pulverized Coal. Part 2. Free-Radical Characteristics

To better explore coal macromolecular models from the extraction aspects, the behaviors of free radicals during the solvent extraction of superfine pulverized coal were studied. The electron paramagnetic resonance (EPR) method was employed to characterize the extracts and extraction residues (ERs) from the pyridine (PY) and tetrahydrofuran (THF) extraction processes. The EPR parameters of different paramagnetic centers were analyzed through the peak deconvolution, and the detailed extraction mechanisms were discussed. The result suggests that the particle size and polarity of the reagent have the combined influences on the free-radical characteristics during the extraction process. Compared to the raw coals (rcs), the free-radical concentrations of the ER show a similar level, while these are 1 order of magnitude lower for the extracts (about 6 to 9% of rcs). In addition, PY with higher polarity is prone to attack the non-covalent interactions like hydrogen bonds, which can extract more abundant molecule components connected by charge-transfer forces, resulting in 35.42% higher spin concentrations compared to the THF extracts. On the other hand, THF with an affinity with oxygen-containing groups can loosen the coal structure, which extracts more stable oxygenated compounds. In addition, THF can effectively target the π–π interactions, and the paramagnetic centers on these aromatic clusters can be better preserved due to the steric hindrance effect. The study sheds light on better elucidation of coal macromolecular structures, which provides support on better understanding coal pyrolysis and liquefaction behaviors.