Influence of Major Operating Parameters (Temperature, Pressure, and Flow Rate) on the Corrosion of Candidate Alloys for the Construction of Hydrothermal Liquefaction Biorefining Reactors

Abstract

Hydrothermal liquefaction (HTL) is a promising thermochemical technology to convert wet biomass and biowastes into marketable bio-oils and biochemicals in an environmentally friendly manner. However, industrial deployment of this technology has been significantly hindered due to very limited materials and corrosion knowledge for the selection of appropriate alloys for the construction and long-term safe operation of HTL reactors. This study investigated the influence of operating temperature, pressure, and flow rate on the corrosion modes and extents of two candidate constructional steels (SS310 and P91) using high-temperature static autoclaves and environmental loop facilities under representative HTL conversion conditions, followed by post-mortem X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy/energy-dispersive spectroscopy, focused ion beam, and transmission electron microscopy characterizations of the formed corrosion products. The two steels experienced general and/or nodular oxidation in hot HTL water at 250–365 °C. Increasing temperature and flow rate resulted in a noticeable increase in the corrosion rate of P91. For SS310, there is critical temperature point (around 310 °C) above which its corrosion rate decreases with temperature. Increasing flow rate suppressed the nodular oxidation of SS310 and consequently led to a decrease in the corrosion rate. Increasing pressure from 9.8 to 25 MPa promoted the oxide formation on SS310 while caused an increased dissolution rate of the corrosion layer grown on P91 steel at 310 °C. In the simulated HTL conversion environments, the corrosion layer on P91 was mainly composed of magnetite (Fe3O4) and chromite (Fe3–x Cr x O4), while a compact and protective inner Cr-enriched layer was formed on SS310 along with the presence of other cations. Related corrosion mechanisms were also discussed and proposed.

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

“…High-temperature aqueous corrosion is a primary concern in HTL reactors, as they involve the use of high-temperature, high-pressure water as the conversion medium. Our recent findings indicate that operating pressure has only a marginal effect on corrosion [10]. However, the addition of homogeneous alkaline catalysts can significantly impact the corrosion modes and extent of construction alloys, as most Fe, Cr, and Ni oxides are thermodynamically unstable in aqueous environments with a pH higher than 11 [17].…”

“…Consequently, the impacts of these factors on HTL conversion efficiency are reviewed in the subsequent sections. Additionally, the corrosion of core component alloys is widely recognized as one of the key technical challenges in the commercialization of HTL technology [10,17]. The corrosion behaviors of candidate constructional materials are also thoroughly reviewed and discussed, specifically focusing on their performances in existing high-temperature, high-pressure systems.…”

“…However, temperature plays a more dominant role in the conversion process, affecting factors such as complete biomass conversion, overall yield of bioproducts, and the physicochemical properties of the final products [36]. It is worth noting that these two parameters also impact the cost of the conversion process and the long-term structural integrity of core components in HTL biorefinery plants, as highlighted in recent publications [10,37].…”

“…To address these corrosion challenges, it is crucial to select appropriate construction alloys, optimize process parameters, and develop cost-effective corrosion-resistant coatings. While commercial steels and alloys, such as austenitic, ferritic-martensitic, duplex stainless steels, nickel-based alloys, titanium alloys, and zirconium alloys, have been widely used in the high-temperature systems of boiler tubes, autoclave liners, and nuclear reactor cores, and similar, not all of them are suitable for constructing industrial-scale HTL plants, due to economic and practical considerations [10,17]. For instance, zirconium and titanium alloys exhibit excellent corrosion resistance in highly aggressive environments but are not practical for construction due to their high costs.…”

Reactivity and Stability of Natural Clay Minerals with Various Phyllosilicate Structures as Catalysts for Hydrothermal Liquefaction of Wet Biomass Waste

“…The addition of homogeneous alkali hydroxides, alkali carbonates, and organic acids into the HTL feedstock as in situ catalysts, in addition to the employment of optimal HTL operating conditions, has been reported to nearly meet these HTL-processing goals. , However, their utilization increases the overall HTL operational cost. − The post-HTL recovery of homogeneous alkali and organic acid catalysts requires energy-intensive separation processes. In addition, the whole unit process and operation require corrosion-resistant materials of construction.…”