ABSTRACT

Catalytic hydrodeoxygenation (HDO) is a promising approach to upgrade crude pyrolysis oil to achieve the ambitious target of partial or complete replacement of fossil fuel with bio-oil. Our recent study indicates that formic acid is an alternative in-situ hydrogen source to effectively improve oil properties for final application and significantly reduce cost and safety concerns compared to using high pressure H2 gas. In this work, corrosion of UNS S30400 (a candidate reactor constructional steel) was investigated under the catalytic HDO of crude pyrolysis oil by supercritical ethanol and formic acid in the temperature range of 80 to 350 °C to simulate the commercialization of the developed HDO technology. After 10 cycles exposure, the corrosion rate of the steel was evaluated using direct weight change and weight loss measurements. The formed corrosion products were examined with modern characterization techniques (SEM, EDS, XRD) to advance the corrosion mechanism.

INTRODUCTION

Bio-oils are renewable and clean energy sources, which can be used to partial or completely replace fossil fuel.1 Fast pyrolysis is a promising and by far the only industrially realized approach to convert dry biomass into biofuels, particularly the liquid bio-oils. However, the poor quality of fast pyrolysis oil including thermal instability, high viscosity and acidity, high oxygen and water content, and low heating value makes it hard to be directly used as transportation fuels.2-4 Besides, the high acidity and water content in crude pyrolysis oil will raise a corrosion concern when handling it.5 Therefore, upgrading processes need to be carried out to improve oil properties.

Catalytic hydrodeoxygenation (HDO) using high pressure H2 is one of the most promising ways to upgrading crude pyrolysis oil.6 It can efficiently reduce the oxygen content while maintaining a high oil yield.7 However, safety and the costly high pressure H2 are the main concerns to further advance this process.8 Alternative hydrogen sources have been sought to substitute the high-pressure hydrogen gas commonly employed for the HDO of bio-oils. Formic acid, a hydrogen source that can be derived from bioresources, has drawn extensive attention.9 In addition, supercritical ethanol (Tc: 241°C, Pc: 63 bar) is an effective hydrogen-donating solvent that can remove the oxygen in crude pyrolysis oil via water formation.10 In our previous study,11 crude pyrolysis oil was upgraded through catalytic HDO with the presence of formic acid and supercritical ethanol. The results indicated that formic acid presented satisfactory performance as an in-situ hydrogen source especially at low upgrading temperatures (i.e., <300 °C). However, formic acid may cause corrosion issues to reactors during upgrading processes, and only limited studies have been conducted to address these challenges. Our previous work revealed that compared to high pressure H2 gas, formic acid created a less corrosive environment for austenitic stainless steels UNS S30400 and UNS S31603 during catalytic HDO of crude pyrolysis oil at 325 °C.12

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