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) monomac eralic 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 ach ieved through C-13 editing of the protonated carbons, identification of qua ternary carbon atoms and 2D proton detected H-1,C-13 correlation experiment s. The study gives valuable information about chemical variability within t he vitrinite maceral group. Apart from variation with rank, the results sho w the effect of factors such as the nature of the precursors, degree of gel ification and preservation of the structure, and depositional environment. This effect is not very well understood and it is not always reflected in t he conventional bulk characterisation parameters. However, there is no doub t that it affects the thermal behaviour of the material and, therefore, has an indirect effect on its conversion processes. The low-temperature tars a re 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, depositi onal environment) affect the internal distribution of phenols in a complica ted manner. Nevertheless, the higher proportion of telinite and initially s tructured macerals in the raw vitrinite and its precursors is clearly refle cted in the amount of 2,4-dimethyl phenol and 4-ethyl phenol. Vitrinites ma de up of more gelified and homogeneous material give rise to tars with a hi gher amount of aromatic hydrogen and a higher quantity of condensed aromati c systems. Variation in the maceralcomposition within the vitrinite maceral group leads to differences in the amount of aliphatic carbon and in the di stribution of the aliphatic moieties. Such variations seem to be closely re lated to the relative amounts of telinite and collotelinite and the proport ion of collodetrinite present in the raw materials. (C) 2001 Elsevier Scien ce B.V. All rights reserved.