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.
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 K2CO3 catalyst. The corrosion rate of the alloy noticeably increased with temperature and the presence of inorganic corrodants (S2− 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 K2CO3 catalyst. From the corrosion perspective, the alloy is a suitable candidate for construction in the next phase of pilot-scale validation assessment.
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