Influence of methanol addition on bio-oil thermal stability and corrosivity

Abstract

Bio-oil has been considered to be upgraded via co-processing with petroleum intermediates in existing fluid catalytic cracking (FCC) units to produce drop-in fuels. However, the low stability and high corrosivity of bio-oil are two major obstacles preventing further advances in bio-oil upgrading processes. Previously, efforts have been made to improve bio-oil stability through the use of additives, with methanol as a promising candidate. Yet, the effect of these additives on bio-oil corrosivity has not been fully understood. In this study, both the stability and corrosivity of bio-oil with methanol addition are analyzed. Bio-oil was blended with methanol at concentrations of 5–20 wt%. These mixtures were subject to accelerated aging at 50 and 80 °C for up to 168 hours. Fourier-transform infrared spectroscopy, gas chromatography–mass spectrometry, and thermogravimetric analysis were conducted to identify functional groups, chemical compounds, and thermal behaviors of bio-oil, respectively. Viscosity, density, water content, and pH were also measured to track the physical and chemical property changes of bio-oil and bio-oil/methanol mixtures during aging. Alongside, immersion experiments with common FCC structural materials such as carbon steel (CS) and stainless steels (SS) 304L and 316L were conducted in bio-oil and bio-oil/methanol mixtures, at 50 and 80 °C for 168 hours. The result of aging experiments showed that the viscosity increasing rate of bio-oil was dramatically lowered by adding methanol, especially at 80 °C, indicating that adding methanol was effective in stabilizing bio-oil. For corrosivity investigation, CS corroded severely in tested bio-oil mixtures. At 50 °C, it was found that CS immersed in bio-oil mixtures with higher methanol concentration corroded at a more significant rate; whereas at 80 °C, the corrosion rate of CS initially increased with methanol concentration in bio-oil and then declined. 304L SS exhibited moderate corrosion rates at 80 °C, while 316L SS showed minimal corrosion at the tested conditions. It has also been observed that CS accelerated the viscosity increasing rate of bio-oil, especially after being aged at 80 °C for 168 hours. After immersion experiments, abnormally high carbon, oxygen, and nitrogen contents were identified on rigorously cleaned metal coupon surfaces. A combination of viscosity measurements and surface characterization suggested that chelation between organic compounds and metal atoms/ions played a significant role in the corrosion of steels in bio-oil. A mechanism was proposed to justify the corrosion behavior of steels in bio-oil/methanol mixtures.