The BTL (Biomass to Liquids) process aims to convert lignocellulosic biomass (wood, agricultural residues, etc.) into second-generation fuels (biodiesel and biokerosene). Research works conducted at IFPEN in partnership with the SPIN Centre of the École des Mines in Saint-Etienne have led to characterization of the physicochemical phenomena involved in the ZnO sulfidation reaction.
The process involves an initial gasification step to turn the biomass into a syngas, consisting of a mixture of CO and H2, followed by a Fischer-Tropsch reaction to produce liquid hydrocarbons. In so far as the catalysts used for this reaction are extremely sensitive to the presence of sulfur, desulfurization of the syngas is a key step.
ZnO is one of the principal oxides that can be used in final gas desulfurization processes. It combines with sulfur in the presence of H2S, leading to the formation of ZnS, which remains trapped in the desulfurization reactor. At present, numerous industrial gas desulfurization processes exist, but their performance could be improved by gaining a better understanding of the mechanisms involved in the conversion of ZnO into ZnS.
Research works conducted at IFPEN in partnership with the SPIN Centre of the École des Mines in Saint-Etienne have led to characterization of the physicochemical phenomena involved in the ZnO sulfidation reaction. In particular, an identified mechanism is based on external growth of the ZnS phase on the initial ZnO particles. This mechanism leads, in particular, to the formation of ZnS particles containing cavities, phenomena which limit the reaction kinetics.
Understanding the reactional mechanisms has led to the construction of a kinetic model taking into account the key parameters affecting the ZnO sulfidation rate and hence the performance of an industrial solid.
Ultimately, the knowledge acquired made it possible to define rules for the design of materials presenting optimal properties, thereby improving the efficiency of desulfurization processes.
Scanning Electron Microscope images of ZnO particles before (a) and after sulfidation (b) .