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.