Key Processing Factors in Hydrothermal Liquefaction and Their Impacts on Corrosion of Reactor Alloys

Despite intensive efforts to develop hydrothermal liquefaction for the conversion of wet biomass and biowaste feedstocks into valuable bio-oils, severe corrosion of conversion reactor alloys and other core components, induced by the pressurized hot water medium, catalysts, and inorganic and organic corrodants generated during the conversion process, has significantly hindered the industrial deployment of this attractive technology. In this paper, a general review of major operating parameters, including biomass feedstock types, temperature, pressure, and catalysts, was conducted to advance the understanding of their roles in conversion efficiency and the yield and properties of produced oils. Additionally, the corrosion performance of a representative constructional alloy (Alloy 33) was investigated in both non-catalytic and catalytic HTL environments at temperatures of 310 °C and 365 °C, respectively. The alloy experienced general oxidation in the non-catalytic HTL environment but suffered accelerated corrosion (up to 4.2 µm/year) with the addition of 0.5 M K 2 CO 3 catalyst. The corrosion rate of the alloy noticeably increased with temperature and the presence of inorganic corrodants (S 2− and Cl − ) released from biowastes. SEM/XRD characterization showed that a thin and compact Cr-rich oxide layer grew on the alloy in the non-catalytic HTL environment, while the surface scale became a double-layer structure, composed of outer porous Fe/Cr/Ni oxides and inner Cr-rich oxide, with the introduction of the K 2 CO 3 catalyst. From the corrosion perspective, the alloy is a suitable candidate for construction in the next phase of pilot-scale validation assessment.