Structure of tars derived from low-temperature pyrolysis of pure vitrinites:: influence of rank and composition of vitrinites

In this paper, a gas chromatography (GC)/mass spectrometry (MS) analysis of the volatile fraction of low temperature tars obtained at 550 degreesC by means of the Gray-King pyrolysis of four Stephanian (Carboniferous) monomaceralic coals (pure vitrinites) and a structural study of the whole material by means of nuclear magnetic resonance (NMR, H-1 and C-13) are undertaken. Furthermore, a precise assignment of the different spectral regions is achieved through C-13 editing of the protonated carbons, identification of quaternary carbon atoms and 2D proton detected H-1,C-13 correlation experiments. The study gives valuable information about chemical variability within the vitrinite maceral group. Apart from variation with rank, the results show the effect of factors such as the nature of the precursors, degree of gelification and preservation of the structure, and depositional environment. This effect is not very well understood and it is not always reflected in the conventional bulk characterisation parameters. However, there is no doubt that it affects the thermal behaviour of the material and, therefore, has an indirect effect on its conversion processes. The low-temperature tars are made up mainly of mono- and di-aromatic structures with a preponderance of phenolic compounds. All of the above mentioned factors (rank, nature of precursors, degree of gelification, preservation of the structure, depositional environment) affect the internal distribution of phenols in a complicated manner. Nevertheless, the higher proportion of telinite and initially structured macerals in the raw vitrinite and its precursors is clearly reflected in the amount of 2,4-dimethyl phenol and 4-ethyl phenol. Vitrinites made up of more gelified and homogeneous material give rise to tars with a higher amount of aromatic hydrogen and a higher quantity of condensed aromatic systems. Variation in the maceralcomposition within the vitrinite maceral group leads to differences in the amount of aliphatic carbon and in the distribution of the aliphatic moieties. Such variations seem to be closely related to the relative amounts of telinite and collotelinite and the proportion of collodetrinite present in the raw materials. (C) 2001 Elsevier Science B.V. All rights reserved.