Synthetic Biofuels by Molten-Salt Catalytic Conversion: Corrosion of Structural Materials in Ternary Molten Chlorides

Abstract

The molten-salt catalytic conversion of biomass into synthetic fuels is one of the promising renewable energy topics of the 21st century. Mixtures of Zn-Na-K//Cl have gained attention, acting as catalytically active heat transfer fluids in the conversion process, due to their low costs and principally low melting points which effectively lower the required operating temperature in the catalytic reactor. Herein, the corrosion behavior of three candidate steels, SS316L, SS321, and Alloy 800, in the pure molten salt, in a salt-biomass mix, and in a salt-biomass mix containing a corrosion inhibitor (zinc oxide (ZnO)) is comprehensively investigated. The herein described study demonstrates that biomass additions increase the penetration depth of the salt, and the corrosion mechanism leads to partial spalling of the oxide scale. This is attributed to the presence of oxygen and moisture from the biomass leading to the formation of reactive HCl gas. The addition of ZnO reduced the corrosivity of the molten salt-biomass mix, and all alloys obtained a protective, adherent, and thin oxide scale leading to the conclusion that ZnO indeed is an effective corrosion inhibitor for the presented salt system. These findings constitute a major advance in the implementation of molten salt-catalytic reactors for renewable synthetic fuel production.