Liquid fuel generation from algal biomass via a two-step process: effect of feedstocks.

Background: In this study, a two-step processing method (hydrothermal liquefaction followed by catalytic upgrading) was used to produce upgraded bio-oil. A comprehensive screening analysis of algal species, including four microalgae and four macroalgae, was conducted to bridge the gap between previous accounts of microalgae and macroalgae hydrothermal liquefaction and the upgrading process of the resulting crude bio-oils.

Results: Hydrothermal liquefaction using eight algal biomasses was performed at 350 °C for 1 h. The microalgae always produced a higher crude bio-oil yield than the macroalgae due to their high lipid content, among which Schizochytrium limacinum provided the maximum crude bio-oil yield of 54.42 wt%. For microalgae, higher amounts of N in the biomass resulted in higher amounts of N in the crude bio-oil; however, contrary results were observed for the macroalgae. The crude bio-oils generated from both the microalgae and macroalgae were characterized as having a high viscosity, total acid number, and heteroatom content, and they were influenced by the biochemical compositions of the feedstocks. Next, all eight-crude bio-oils were treated at 400 °C for 2 h with 10 wt% Ru/C using tetralin as the hydrogen donor. The hydrogen source was provided after tetralin was transformed to naphthalene. All the upgraded bio-oils had higher energy densities and significantly lower N, O, and S contents and viscosities than their corresponding crude bio-oils. However, the H/C molar ratio of the upgraded bio-oils decreased due to the absence of external hydrogen relative to the crude bio-oils. The S content of the upgraded bio-oil produced from upgrading the Schizochytrium limacinum crude bio-oil was even close to the 50 ppm requirement of China IV diesel.

Conclusions: Microalgae are better feedstocks than macroalgae for liquid fuel production. Biochemical components have a significant impact on the yield and composition of crude bio-oil. Tetralin does not perform as well as external hydrogen for controlling coke formation. The S content of the upgraded bio-oil can be reduced to 76 ppm for the crude bio-oil produced from Schizochytrium limacinum . Upgraded bio-oils have similar properties to those of naphtha and jet fuel.