Abstract Three brown coals were extracted by a hydrothermal method at 350 °C and 20 MPa for 90 min, and the molecular compositions of the deposit extracts were analyzed in detail by high-resolution mass spectrometry measurements. The extract yields were as high as ∼30% for all brown coals, indicating that hydrothermal extraction is applicable to brown coals with differing ash and sulfur contents. The high-resolution mass measurements with Kendrick mass analyses identified the chemical compositions and clarified the abundance of common molecular species in all the extracts prepared from the three brown coals. The sum of mass intensities of overlapped CnHm, CnHmO, CnHmO2, CnHmO3, CnHmO4, and CnHmN species in the mass spectra of LY, NM, and YN extracts were 37.0%, 35.9%, and 50.8%, respectively, indicating the presence of abundant compounds with similar molecular compositions in all of the extracts, regardless of the parent coal.
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