Effect of the increase in temperature on the evolution of the physical andchemical structure of vitrinite

The effect of the progressive increase in temperature (off-line pyrolysis i n an open-medium system) on a monomaceralic coal (low rank and pure vitrini te) has been studied on the basis of changes in its chemical and physical s tructure. The properties of the solid residues and liquid effluents obtaine d were studied using a wide range of petrographic and geochemical technique s. Results support the validity of this pyrolysis method for following the physicochemical processes that occur in vitrinite of bituminous coal rank. Of special interest is the information concerning the control of the struct ure on the vitrinite reflectance parameter in the bituminization stage. The evolution of properties and changes occurring at this stage show that the increase in vitrinite reflectance is strongly related and depends on change s in the chemical structure of this maceral during heating rather than on c hanges in elemental composition. However, any relationship between reflecta nce and chemical properties needs to be established empirically. The mechan ism during heating consists of a complex mixture of depolymerization and co ndensation reactions with the progressive homogenisation of the botanical s tructure of the vitrinite. Moreover, pyrolysis conditions lead to a rapid a nd massive expulsion of the thermally generated products, causing a signifi cant decrease in the H/C atomic ratio and higher thermostability of the sol id residues. The enhancement of cross-linking with the formation of C-O-C b ridges, which have high activation energy also contributes to an increase i n thermal stability. Results obtained from the production and expulsion of oil suggest that this method is suitable for providing an estimation of the amount of hydrocarbons that might be produced and migrate from coals of si milar rank. This is of special interest for coals with significant petrolig enous potential or for coals that are gas-prone, depending on their composi tion and rank. (C) 1999 Elsevier Science B.V. All rights reserved.