Simultaneous production of gaseous and liquid biofuels from the synergetic co-valorisation of bio-oil and crude glycerol in supercritical water

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This work addresses the co-valorisation in supercritical water of bio-oil obtained from the fast pyrolysis of wood and crude glycerol yielded as a by-product during biodiesel production. The experiments were conducted at 380 °C and 230 bar for 30 min with a Ni-Co/Al-Mg catalyst, analysing the effects on the process of the catalyst loading (0–0.25 g catalyst/g organics) and feed composition (each material alone and all possible binary mixtures). The yields to gas, upgraded bio-oil (liquid) and solid varied as follows: 4–87%, 0–46% and 0–18%, respectively. A synergistic interaction between crude glycerol and bio-oil took place during the upgrading process, which allowed the complete and simultaneous transformation of both materials into gas and liquid bio-fuels with a negligible solid formation. The compositions of the gas and the upgraded liquid can be easy tailored by adjusting the catalyst amount and the composition of the feed. The gas phase was made up of H2 (7–49 vol.%), CO2 (31–56 vol.), CO (0–7 vol.%) and CH4 (6–57 vol.%) and had a Lower Heating Value (LHV) ranging from 8 to 22 MJ/m3 STP. The upgraded bio-oil consisted of a mixture of carboxylic acids (0–73%), furans (0–7%), phenols (0–85%), ketones (0–22%) and cyclic compounds (0–53%). The proportions of C, H and O in the liquid shifted between 66–77 wt.%, 7–11 wt.% and 15–25 wt.%, respectively, while its Higher Heating Value (HHV) ranged from 29 to 34 MJ/kg. An optimum for the simultaneous production of gas and liquid bio-fuels was achieved with a solution having equal amounts of each material and employing a catalyst amount of 0.25 g catalyst/g organics. Under such conditions, 37% of the bio-oil was transformed into an upgraded liquid having a HHV (32 MJ/kg) two times higher than the original material (16 MJ/kg) with a negligible solid formation; the rest of the bio-oil and all the crude glycerol being converted into a rich CH4 (55 vol.%) biogas with a high LHV (21 MJ/m3 STP). This represents a step-change in future energy production and can help to establish the basis for a more efficient and sustainable biomass valorisation.