Liquid and gas biofuels from the catalytic re-forming of pyrolysis bio-oil in supercritical water: Effects of operating conditions on the process

This work analyses the influence of temperature (310-450 °C), pressure (200-260 bar), catalyst/bio-oil mass ratio (0-0.25 g catalyst/g bio-oil) and reaction time (0-60 min) during the re-forming in sub- and supercritical water of a bio-oil obtained from the fast pyrolysis of pinewood. The original liquid has a 39 wt.% of water and the following elemental composition in dry basis: 54 wt.% C, 3.3 wt.% H, 41.3 wt.% O, 0.8 wt.% N and 0.6 wt.% S. The upgrading experiments were carried out in a batch microbomb reactor employing a co-precipitated Ni-Co/Al-Mg catalyst. Statistical analysis of the re-forming results indicates that the operating conditions and the water regime (sub-/supercritical) have a significant influence on the process. Specifically, the yields to upgraded bio-oil (liquid), gas and solid vary in ranges of 5-90 %, 7-91% and 3-31% respectively. The gas phase, having a medium-high lower heating value (2-17 MJ/STP m³), is made up of a mixture of H2 (9-31 vol.%), CO₂ (41-84 vol.%), CO (1-22 vol.%) and CH₄ (1-45 vol.%). Depending on the operating conditions, the amount of C, H and O (wt.%) in the upgraded bio-oil varies in ranges of 48-74, 4-9 and 13-48 respectively. This represents an increase of up to 42 and 152% in the proportions of C and H respectively, as well as a decrease of up to 69% in the proportion of O. The higher heating value (HHV) of the treated bio-oil varies from 20 to 32 MJ/kg, which corresponds to an increase of up to 68% with respect to the HHV of the original bio-oil.