Corrosion Assessment and Mechanisms of Materials in Pre-hydrolysis and Hydrothermal Liquefaction Biorefining Systems
Creators: Liu, Minkang
Publisher: name: University of Alberta Library
Year: 2022

<p>Advanced biomass conversion technologies, including hot dilute acidic pre-hydrolysis and hydrothermal liquefaction, have been developed for the generation of renewable bioenergy products and biochemicals from existing biomass resources and biowastes. However, the deployment of the technologies is hindered because of significant corrosion and materials technology gaps in terms of cost-effective construction and safe operation of biorefining reactors. This study intends to address the corrosion challenges and support the development of materials selection guidelines for the successful commercialization of the two technologies. <br>
Hot dilute acidic pre-hydrolysis biorefining technology is a pretreatment method used for direct conversion of raw biomass into fermentable and enzymatic intermediates in dilute acid (such as H2SO4 and H2SO3) at 100 - 200 °C. Little information on corrosion is available for determining materials performance in such environments. This study finds that Fe-based steels (SS316L and DSS 2205) suffer active corrosion, and their corrosion rates increase with increasing temperature, service time, and addition of corrodants (Cl-, S2-, and organic acid), while Alloy C-276 exhibits acceptable resistance. More importantly, it is found that increasing Cr content even up to 22.5 wt.% cannot provide satisfactory protection since the formed Cr-enriched oxides are thermodynamically unstable and experience chemical dissolution. Increasing Mo content in alloys, on the other hand, could significantly improve corrosion resistance due to the formation of stable Mo-enriched oxide layer which demonstrates the promising resistance to hot dilute acid and the corrodants released. This novel finding is completely different from previous studies about the effect of Mo on corrosion. <br>
Hydrothermal liquefaction is a thermochemical technology to convert wet biomass and biowaste to bio-oils and biochemicals in pressurized water at 250 - 370 °C and pressure up to 25 MPa. In this study, the effects of temperature, pressure and flow rate have been well investigated using high temperature autoclaves and environmental loops. It is found that increasing temperature up to 365 °C remarkably enhances the corrosion of P91 steel, and there is a transition temperature (around 310 °C) for SS310, above which its corrosion rate decreases with further increase in temperature. Increasing pressure and flow rate leads to a noticeable increase in the corrosion rate of P91 steel. Different from P91, the corrosion rate of SS310 only slightly increases with pressure, and decreases with an increase in flow rate due to the formation of an inner Cr-enriched oxide layer and suppressed nodular oxidation.<br>
Homogenous catalysts (such as K2CO3) are commonly used to achieve optimum carbon conversion efficiency. This study finds that the addition of 0.5 M K2CO3 results in a significant increase in the corrosion rates of P91 and SS310, and remarkably changes the microstructure of surface scales. At 310 °C, the presence of chloride and sulfide ions does not trigger pitting, but can cause an increase in corrosion rates due to the formation of less protective oxide layers. The presence of acetic acid further accelerates the corrosion. The corrosion mechanism in catalytic HTL environments has thus been proposed based on the testing results and thermodynamic simulations.<br>
In the non-catalytic and catalytic HTL environments, thermodynamic calculations indicate that major alloying elements (Fe, Cr, Ni and Mo) could experience general oxidation or active corrosion, depending on temperature and pH. Under the HTL conversion conditions, overall ranking in terms of increasing corrosion resistance is: P91 &lt; SS316L &lt; SS310 &lt; Alloy C-276 &lt; Alloy 625<br>
Increasing Cr and Mo contents in alloys should improve their resistance to the environmental attack. Increasing Ni content has a negligible effect in non-catalytic HTL environment, but can improve the corrosion performance in the catalytic environments. Post-mortem characterizations further reveal the roles of alloying elements in the corrosion layers and advance the mechanistic understanding of their impact on corrosion. Stress corrosion cracking is another concern when evaluating suitable alloys. Uniaxial constant strain and U-bending techniques were adopted to evaluate SCC susceptibility of SS316L and SS310 in a catalytic HTL environment at 365 °C. Preliminary results show that increasing strain deformation enhances the corrosion. However, the SCC risk of the tested steels, even under 3.3% deformation, is much lower than expected. More assessments are still needed to confirm the above findings and determine the SCC risk of welded construction steels and alloys.</p>