Thermal stability of dibenzothiophene in closed system pyrolysis: Experimental study and kinetic modelling

The aim of this study was to determine the thermal stability of a model compound, dibenzothiophene (DBT), that is representative of sulfur aromatic hydrocarbons of oils. Pyrolysis experiments were carried out in an anhydrous closed system (gold bags) over times ranging from 1 h to 2 months, under isothermal conditions (375-500 °C), at a constant pressure of 100 bar. Pyrolysis products were recovered quantitatively according to their solubility in different organic solvents. Gases were identified and quantified separately. Dibenzothiophene thermal cracking leads to the generation of sulfur-containing molecules such as hydrogen sulfide, DBT-dimers, benzothiophene, phenyl-dibenzothiophene or benzo-bis-benzothiophene, as well as sulfur-free compounds that include H2, methane, biphenyl and benzene. Pyrolyses corresponding to low conversions (<3%) were performed in order to identify and quantify primary cracking products which proved to be DBT-dimers, H2, hydrogen sulfide H2S, and biphenyl. A kinetic model for DBT degradation was proposed to describe qualitatively the generation of experimental primary products. Stoichiometries were deduced from this model and compared with experimental mass balances and good agreement was obtained for conversions below 3%. This kinetic model has to be quantitatively tested by additional ab initio modeling. For higher conversions, high-molecular weight sulfurised products are observed implying that the thermal cracking of dibenzothiophene leads mainly to aromatisation and, thus, is not a way to remove sulfur.