Study on the characteristics and role of the soluble fractions during direct liquefaction for two low rank coals. Part I: Structural comparison between the soluble fractions obtained from soxhlet and thermal extraction

The direct coal liquefaction (DCL) process is accompanied by the dissolution of the soluble fractions during the preheating stage. These soluble fractions remain in the system after dissolution and involve subsequent hightemperature hydro-liquefaction process, and their effect and role on the distributions and properties of products cannot be ignored. In this paper, soxhlet extraction (SE) with tetrahydrofuran (THF) and pyridine (PY) and in-situ thermal extraction (TE) in tetralin (THN) and 1-methyl naphthalene (MN) at 260-360 degrees C of Hami lignite (HM) and Bulianta sub-bituminous coal (BLT) were carried out to obtain SE soluble fractions (SESs) and TE soluble fractions (TESs), respectively. The effects of temperature and solvent on extraction behaviors of two coals were investigated and the structure and property characteristics of SESs and TESs were compared. The results indicate that the extraction yields of HM are higher than that of BLT, and TE significantly enhances the extraction yields, reaching 41.06 % and 36.40 % at 360 degrees C with THN and MN for HM, respectively. The TE yields (TEYs) increase with the raising of TE temperature (TET). The high TEYs received at 360 degrees C may be ascribed to the partial pyrolysis reactions of weak covalent bonds like side chain and bridge bond breaking in two coals. The main body of HM and BLT is a macromolecular network structure that is crosslinked by chemical bonds, and some low molecular weight compounds that are bound through non-covalent bond interactions constitute the guest. The SESs and TESs are mainly composed of aliphatics with high H/C, low condensation degree aromatics and oxygen-containing compounds containing C=O and C-O, with low heteroatom content and almost all of the mineral matters in raw coals are transformed to residues. There are significant differences in structures between SESs and TESs. As increasing the TET, the content of monocyclic aromatics in TESs decreases, and more polycyclic aromatics are dissolved, so H/C also declines. The weight-average molecular weights and molecular weight size distribution of TESs measured by the GPC method are generally larger than those estimated by MALDI-TOF-MS, which is correlated to the association structure formed between TESs and solvent or in TESs.