Effect of the increase in temperature on the evolution of the physical and chemical structure of vitrinite

The effect of the progressive increase in temperature (off-line pyrolysis in an open-medium system) on a monomaceralic coal (low rank and pure vitrinite) has been studied on the basis of changes in its chemical and physical structure. The properties of the solid residues and liquid effluents obtained were studied using a wide range of petrographic and geochemical techniques. 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 structure 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 changes in the chemical structure of this maceral during heating rather than on changes in elemental composition. However, any relationship between reflectance and chemical properties needs to be established empirically. The mechanism during heating consists of a complex mixture of depolymerization and condensation reactions with the progressive homogenisation of the botanical structure of the vitrinite. Moreover, pyrolysis conditions lead to a rapid and massive expulsion of the thermally generated products, causing a significant decrease in the H/C atomic ratio and higher thermostability of the solid residues. The enhancement of cross-linking with the formation of C-O-C bridges, which have high activation energy also contributes to an increase in 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 similar rank. This is of special interest for coals with significant petroligenous potential or for coals that are gas-prone, depending on their composition and rank. (C) 1999 Elsevier Science B.V. All rights reserved.