Bio-oils comprise substantial amounts of carboxylic acids, such as acetic and formic acids, which leads to low pH values of 2–3. The bio-oil of pine had a pH of 2.6, while that of hardwood was 2.8 [10]. Acidity makes bio-oil very corrosive and extremely severe at elevated temperature, which imposes more requirements on construction materials of the vessels and the upgrading process before using bio-oil in transport fuels.