Thermal Treatment under High-Vacuum of Tars Relevant in Combustion and Material Science

The composition of tars, typically derived from coal and heavy fuel processing or formed in fuel-rich combustion, determines their transformation into carbons relevant in combustion and environmental fields as well as for material production. The speciation of the huge number of aromatic components of tars, usually found in form of viscous black liquid or solid, is not straightforward because of the tar complexity and high molecular weight, spanning from few hundreds up to thousands of Da. To this regard, the pre-separation of tar in lighter and heavier fractions simplifies the further characterization of its composition. The present work reports a fractionation method of a typical sample of combustion-formed tar based on moderate heating in high-vacuum conditions (10-6 mbar). It was preliminarily tested on a single polycyclic aromatic hydrocarbon, coronene, and on synthetic mixtures of polycyclic aromatic hydrocarbons, presumed to be the basic aromatic moieties of tar components. Lighter components obtained by condensation/deposition as thin films and/or crystals, as well as the heavier residue, were analysed by optical microscopy and spectroscopy. The separation procedure allowed to get more information on the components distribution also inferring the self-organization in cluster assembly and/or crystal forms.