Detailed characterization of coal-derived liquids from direct coal liquefaction on supported catalysts

Coal liquids from Direct Coal Liquefaction (DCL) are one of the possible substitutes of conventional hydrocarbons resources. However the primary liquids obtained from DCL (i.e. outflow liquids of the first reactor in a two stages liquefaction process) do not meet the product specifications and need to be upgraded via downstream catalytic hydrotreatment processes. The in depth knowledge of the feed of this upgrading step is of key importance to design better catalysts. In this study, we applied new cutting-edge techniques to improve the characterization of a coal derived liquid obtained from a liquefaction step starting from Scottish bituminous coal. For this purpose, bidimensional chromatography (GC × GC), and 13C nuclear magnetic resonance (NMR) were used to analyze four cuts of the coal liquid with the following boiling points intervals: heavy naphtha (initial boiling point: IBP-215 °C), gas oil (215–343 °C), vacuum gas oil (343–524 °C) and vacuum residue (524+ °C). The analyses showed high aromaticity of all four cuts. The weight content of aromatics in the gas oil fraction, was 75%, with a majority of two rings aromatics and the remainder consisting mostly of normal paraffins. In the vacuum gas oil cut, which contains mostly four aromatic rings, we found 11 wt% of asphaltenes (compounds in n-heptane), an unexpected high amount comparing to petroleum VGO fractions. The amount of heptane-insoluble (asphaltenes, preasphaltenes and sediments) is as high as 80% in the vacuum residue fraction (524+ °C). Aromaticity given by 13C NMR in this cut is as high as the level observed in petroleum asphaltenes (Caro = 53 wt%). High content of heteroelements, which highly increase the polarity of the molecules, in the coal liquid and especially in the vacuum residue can explain the enhanced amount of n-heptane-insoluble compounds.