Molecular Characterization of Soluble Components in the Lignite by Sequential Solvent Extraction via Continuously Reducing Particle Size

A lignite was subjected to sequential solvent extraction via continuously reducing particle size from around 20 to more than 200 mesh. Five sets of n-hexane and methanol extracts from the particles were characterized by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) and gas chromatography–mass spectrometry. The total extract yield for lignite when using hexane and methanol as solvents could reach to 0.98 and 15.12%, respectively. The results showed that more molecules with a similar structure but different composition could be extracted by continuously reducing the particle size of the residues, indicating the nonuniform distribution of the low-solubility molecules trapped in the coal particles. The extracts were abundant in branched long-chain aliphatic moieties and oxygen-containing compounds. With the increasing of the extraction degree, the content of alkanes in the extracts decreased rapidly. On the contrary, the content of the compounds with higher condensation and more oxygen atoms increased. It should be noted that polycyclic aromatic hydrocarbons were almost steadily present in all the extraction steps. The molecular composition and distribution of organic molecules in the lignite provide clues to the understanding of coal structure, which is significant for the environmental emission and development of processing techniques for the clean and high value-added utilizations of such a low-rank and abundant coal resource.