Production of bio-fuels and chemicals by microwave-assisted, catalytic, hydrothermal liquefaction (MAC-HTL) of a mixture of pine and spruce biomass

This work firstly addresses the microwave-assisted, catalytic, hydrothermal liquefaction (MAC-HTL) of a mixture of pine and spruce biomass, examining the effects of the temperature (150–250 °C), pressure (50–120 bar), time (0–2 h) and catalyst amount (Ni–Co/Al–Mg; 0–0.25 g catalyst/g biomass). This hydrothermal process turned out to be a very promising route for the production of bio-oil with suitable physicochemical properties to be used as a precursor for liquid bio-fuels and/or renewable platform chemicals. The statistical analysis of the results revealed that the operating conditions exerted a significant influence on the process; the overall biomass conversion and the yields of gas and bio-oil varied by 13–77%, 7–67% and 1–29%, respectively. This liquid consisted of a complex mixture of esters (0–30%), aldehydes (4–69%), ketones (0–35%), alcohols (0–14%), phenols (0–83%), acids (0–28%), cyclic compounds (0–38%), acetates (0–11%), ethers (0–27%) and furans (0–12%). The proportions of C, H and O in the liquid shifted by 2–70 wt%, 4–11 wt% and 27–87 wt%, which varied the Higher Heating Value (HHV) between 4 and 28 MJ kg⁻¹. The optimisation of the process revealed that it is possible to transform up to 27% of the original biomass into a phenol-rich (47%) bio-oil with a relatively high HHV (20 MJ kg⁻¹) using a temperature as low as 250 °C, at 80 bar, and employing 0.25 g catalyst/g biomass for 1.9 h. The properties of the liquid produced under optimum conditions make it suitable to be used as a renewable bio-fuel precursor, a bio-based source of aromatics and/or a sustainable phenolic-rich antioxidant additive. Therefore, this process might represent a promising improvement in biomass pre-processing technologies, helping the development of novel routes for biomass valorisation.