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 degrees 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 degrees 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 biofuel 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.