Catalytic hydrotreatment of crude algal bio-oil in supercritical water

We report herein on the catalytic hydrotreatment of crude bio-oil, produced from the hydrothermal liquefaction of a microalga (Nannochloropsis sp.) over palladium on carbon (5% Pd/C) in supercritical water (SCW) at 400 °C and 3.4 MPa high-pressure H2. Influences of wide ranges of reaction time (varied from 1 to 8 h) and catalyst loading (varied from 5 to 80 wt%) on treated oil composition and yield, gas products composition and yield, and hydrogen consumption were explored. The C, H and energy recoveries were determined. The results demonstrated that longer reaction times and higher catalyst loadings did not favor the treated oil yield due to the increasing amount of gas and coke products formation but did lead to treated bio-oil with higher higher-heating-value (HHV) (41–44 MJ/kg) than that of the crude feed. Highest HHV of treated oil (∼44 MJ/kg) was obtained after 4 h using an 80 wt% intake of catalyst on crude bio-oil. The product oil produced at longer reaction times and higher catalyst loadings, which was a freely flowing liquid as opposed to being the viscous, sticky, tar-like crude bio-oil material, was higher in H and lower in O and N than the crude feed, and it was essentially free of sulfur (below detection limits). Typical H/C and O/C molar ratios ranges for the bio-oils treated at different reaction times and catalyst loadings were 1.65–1.79 and 0.028–0.067, respectively. The main gas-phase products were unreacted H2, CH4, CO2, C2H6, C3H8 and C4H10. Overall, many of the properties of the treated oil obtained from catalytic hydrotreatment in SCW in the presence of Pd/C are very similar to those of hydrocarbon fuels derived from fossil fuel resources.

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► Bio-oils derived from the hydrothermal liquefaction of microalgae show much promise.
► Processing the crude algal oil in SCW with Pd/C is an effective upgrading strategy.
► Higher H content in the treated oil benefited from the presence of high-pressure H2.
► Many of the properties of the treated oil are very similar to those of hydrocarbon fuels.