Residues characteristics and structural evolution of Naomaohu coal during a mild direct liquefaction process

Abstract

Direct coal liquefaction (DCL) is an important way of coal conversion and is important to the global energy industry. In this paper, a typical liquefied coal–Naomaohu coal (NMHC)–was liquefied under mild conditions (400 °C, initial H2 pressure of 3 MPa), and a NMHC conversion of 90.6% was obtained with an oil+gas yield of 65.1%. In the DCL process, the solid particles of coal and its residues are not only reactants but also carriers of catalysts of DCL. To study the structural characteristics of these residues, the reaction mixture was extracted with n-hexane and tetrahydrofuran (THF), separately, yielding n-hexane insoluble residues (ReHEIS) and THF insoluble residues (ReTHFIS). Several patterns of the reaction progresses were identified. First, the particle sizes of ReHEIS and ReTHFIS have a decreasing trend, and the particle surface becomes loose and porous while raw coal has a relatively flat surface. Second, most of the aliphatic structure and some small aromatic compounds can gradually split off from coal skeleton structure, and the condensation degree of aromatic clusters in residues increases with the aromatic condensation degree of ReTHFIS being higher than that of ReHEIS. Third, the contents of carbonyl carbon and oxygen-attached carbon decrease while methylene chain and side chain of aromatic clusters are broken. Lastly, the aromatic structure of ReTHFIS becomes regular, and the graphitization degree increases, while the pyrolysis rate slows down. These results of element analysis show that the H/C of ReHEIS is larger than that of ReTHFIS, which implies that preasphaltene and asphaltene are relatively rich in hydrogen.