We converted the marine microalga Nannochloropsis sp. into a crude bio-oil product and a gaseous product via hydrothermal processing from 200 to 500 °C and a batch holding time of 60 min. A moderate temperature of 350 °C led to the highest bio-oil yield of 43 wt %. We estimate the heating value of the bio-oil to be about 39 MJ kg−1, which is comparable to that of a petroleum crude oil. The H/C and O/C ratios for the bio-oil decreased from 1.73 and 0.12, respectively, for the 200 °C product to 1.04 and 0.05, respectively, for the 500 °C product. Major bio-oil constituents include phenol and its alkylated derivatives, heterocyclic N-containing compounds, long-chain fatty acids, alkanes and alkenes, and derivatives of phytol and cholesterol. CO2 was always the most abundant gas product. H2 was the second most abundant gas at all temperatures other than 500 °C, where its yield was surpassed by that of CH4. The activation energies for gas formation suggest the presence of gas-forming reactions other than steam reforming. Nearly 80% of the carbon and up to 90% of the chemical energy originally present in the microalga can be recovered as either bio-oil or gas products.

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We have investigated the catalytic treatment of a crude algal liquefaction bio-oil in supercritical water to discover how the properties of the treated oil depend on the experimental conditions. An L 9 (3 4 ) orthogonal array design (OAD) with four factors at three levels was employed. The four factors were temperature (varied from 430-530 C), time (varied from 2-6 h), catalyst type (Pt/C, Mo 2 C, HZSM-5), and catalyst loading (varied from 5-20 wt%). We used a direct analysis to determine the relationship between experimental conditions and properties of treated oils. The oil properties we examined were elemental composition, atomic ratios, chemical composition, and higher heating value. Of the four factors, the 100 C variation in temperature was always the most influential for each of the oil properties examined. Of the remaining three factors, catalyst type had the greatest influence on the fatty acid content of the treated oil and the fraction of N-and O-containing compounds in the oil. Catalyst loading had the greatest effect on the higher heating value and O/C ratio in the treated oil. Reaction time had the greatest effect on the H/C and N/C ratios. The results demonstrated that treatment in supercritical water at 430 C led to roughly a halving of the N and O content of the oil, a reduction in S to below detection limits, and about a 10% improvement in the higher heating value of the bio-oil. Within the parameter space investigated, the conditions leading to the highest content of saturated compounds in the treated oil are 430 C, 6 h, with a 10 wt% loading of Mo 2 C as the catalyst. Around 76 wt% of the carbon in the feedstock was retained in the treated oil at these conditions.

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