Hydrothermal liquefaction (HTL) is seen as a promising thermochemical approach to convert wet and waste biomass feedstocks into biocrude oils and other valuable chemicals. One of the critical technical barriers that must be addressed for the industrial deployment of HTL technology is the corrosion of process core equipment, especially the refining reactors, due to the presence of the hot-compressed water medium, applied alkali catalyst, and aggressive intermediate and final products (such as aggressive sulfur and/or chlorinated compounds, organic acids) generated during the conversion. In this study, the corrosion performance of two candidate alloys (UNS N06625 and UNS R20033) was investigated in a batch reactor containing hot-compressed water, 5 wt.% K2CO3 catalyst and cellulose (a typical model compound of lignocellulosic biomass). Certain amounts of organic acids and phenolic compounds were present in the produced oil, implying the change of environmental pH (from mild basic to near neutral) during the conversion. The two tested alloys experienced general oxidation associated with localized oxide peel-off or nodular oxidation. Due to its higher Cr content, UNS R20033 had a lower corrosion rate compared to UNS N06625 under the HTL of cellulose.
The conversion of forestry/agricultural residues (lignocellulosic biomass) to biofuels or bioproducts has received increasing interest over the past years because of the demand for green products and the fact of abundant residual/waste streams in forestry and agricultural sectors. Forestry/agricultural residues/waste streams are advantageous bioresources to produce biofuels or bio-based chemicals since they do not compete with food resources.1 Typical conversion technologies involve biochemical and thermochemical processes. Biochemical conversion processes, mainly referring to fermentation of wet carbohydrate materials into bioethanol and anaerobic digestion to generate biogas at ambient operation conditions,2 is quite slow and sensitive to operating conditions (pH, temperature, etc.).3 In contrast, thermochemical conversion processes, such as combustion, gasification, and pyrolysis, are much faster due to the nature of high-temperature refining conditions. But these methods are only applicable for the conversion of relatively dry biomass with moisture content less than 20–30%.4 Different from them, hydrothermal liquefaction (HTL), typically operated at the temperature range of 200–400 °C and pressure up to 25 MPa, can be used for directly converting wet biomass or waste feedstocks with a high water content into biocrude oils in hot compressed water or mixed solvents of water and alcohol.5,6