Upgrading Wood and Waste Fast Pyrolysis Oils by Esterification

The adoption of pyrolysis oils as renewable alternatives to conventional fossil fuels is limited by their high acidity, elevated water content, and inherent instability. In this study, esterification was used to upgrade pyrolysis oils derived from two feedstocks: beech wood as a representative biomass source and refused-derived fuel (RDF) as a waste-based material. The esterification was conducted using 1-hexanol as the alcohol and Amberlyst36 as a heterogeneous sulfonic acid-based catalyst. Esterification was studied using oil to 1-hexanol volume ratios of 3:1 (E31) and 1:1 (E11). In both cases, esterification improved the quality of the pyrolysis oils by reducing the water content by 98% for beech wood-based oils and at least 66% for RDF waste-based oils, reducing the total acid number (TAN) by 71% for beech wood-based oils and 78% for RDF waste-based oils, while simultaneously increasing their higher heating values (HHVs) from 15 to 34 MJ/kg for beech wood oils and from 37 to 40 MJ/kg for RDF-waste oils. Using the E31 pyrolysis oil to 1-hexanol ratio thus led to much improved oil properties, while using the E11 mixing ratio further reduced the water content, increased the H/C ratio, lowered the acidity, and improved the heating value. However, its benefits were mainly associated with dilution by unreacted 1-hexanol. The findings underscore the potential of 1-hexanol esterification to enhance the properties of pyrolysis oils for both biomass- and waste-based feedstocks, thereby expanding the applicability of these upgraded oils as sustainable fuels. Furthermore, this work contributes to providing knowledge on the feasibility of upgrading waste-derived pyrolysis oils by esterification.