Quality Improvement of Biomass Pyrolysis Oil through Esterification

Simultaneous esterification and acetalization of bio-oil with ethanol using commercial Amberlyst catalysts at temperatures between 60 and 170 °C were carried out to obtain a less acidic bio-oil with improved stability for small residual boilers. The bio-oil used in the experiments was produced from bark-free pine sawdust, and it was partially dewatered using azeotropic distillation with n-heptane before the esterification experiments. A mediocre acidity (carboxylic acid number, CAN 30–50 mg KOH/g) bio-oil was obtained by dewatering and blending the bio-oil with an alcohol. A low-acidity bio-oil (CAN 10–20 mg KOH/g) was obtained with esterification using 50 wt % of ethanol at 80 °C. At higher temperatures (110–170 °C), bio-oil sugars were hydrolyzed and converted into acids, esters, and acetals. The reactions were very dependent on the amount of ethanol. With low concentrations of ethanol (15–20 wt %), dehydration was the main pathway, which also produced water-insoluble humins in larger quantities. Higher concentrations of ethanol reduced the amount of humins and improved the stability of the oil (carbonyl content 0.82 mmol/g) at a short residence time (6 h).