Bio-oil contains 30-40% oxygen (shown in the Table 5), which is similar to oxygen content in the original biomass that bio-oil is derived from. In comparison, petroleum crude oil and heavy fuel oil contain less than 1% oxygen. The oxygen in bio-oil is distributed in the form of many functional groups. High oxygen contents results in many of its undesirable properties such as low energy content and high acidity. Removal of oxygen from the bio-oil is one of the biggest barriers of using bio-oil especially for the production of drop-in hydrocarbon fuels.

High Acidity

The pH of bio-oil is 2.5 (shown in the Table 5), which makes it very acidic and highly corrosive. The acidity of bio-oil is due to presence of carboxylic acids such as acidic acid and formic acid (Zhang et al. 2007).