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

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 degrees C for up to 168 hours. Fouriertransform 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 biooil and bio-oil/methanol mixtures, at 50 and 80 degrees 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 degrees C, indicating that adding methanol was effective in stabilizing bio-oil. For corrosivity investigation, CS corroded severely in tested bio-oil mixtures. At 50 degrees C, it was found that CS immersed in bio-oil mixtures with higher methanol concentration corroded at a more significant rate; whereas at 80 degrees 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 degrees 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 degrees 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.