Effects of solvent extraction on the storage stability of bio crude produced by hydrothermal liquefaction of Spirulina

Hydrothermal liquefaction (HTL) can convert wet biomass into biocrude, particularly with a comparable energy content with petroleum. The biocrude can be refined as fuel oils in farming machinery, cars, trains, ships, or planes. However, the biocrude from HTL is a complex mixture of oxygen and nitrogen, including phenols, alcohols, acids, esters, ketones, and amines. The biocrude is too tricky to be used, due to its high viscosity and low stability during storage. The extraction solvent is one of the most influencing factors to affect the biocrude yield and oil properties. Also, solvent extraction can serve as an efficient way to separate the biocrude. In this study, the multistep extraction was proposed to separate the biocrude into some components with different polarities, and then to analyze the effects of different components on the storage stability of biocrude. Spirulina was used as the feedstock for the HTL under the optimal reaction condition (reaction temperature: 260°C, retention time: 60 min, and total solid: 20%) from the previous study. Tetrahydrofuran, ethyl acetate, acetone, and n-hexane were used as the extraction solvents to separate the biocrude. Those solvents were chosen, depending on their polarity and ability to extract the biocrude yield. An accelerated aging test was carried out to investigate the effects of different extraction solvents on the storage stability of biocrude. The results showed that the multistep extraction performed the best to separate the biocrude into heavy, mild, and light oil. The BC12 extracted from ethyl acetate presented the lowest viscosity (316 mPa•s), the best fluidity, the lowest viscosity change rate (78.6%), and the best stability during storage. Once the n-hexane was used to separate the BC12 into M1 and L1, the viscosity of M1 was higher than that of BC12. There was a high viscosity change rate of M1, and then the fluidity was lost after four days of storage, even finally became the biochar was after seven days. The M2 also showed a higher viscosity and lower fluidity than the BC2 extracted by acetone. TG, GC-MS, and FT-IR analysis revealed that the solidification of M1 was closely related to the esterification, polymerization, and physical condensation. The better storage stability of biocrude was achieved in the absence of the high polar and heavy components. The biocrude aging was attributed to the severe polymerization in the components of heavy oil, including many macromolecular compounds, such as phenols and nitrogenous compounds. Element analysis indicated that the heavy oils were likely to contain more N, where the light oils contained more H. The higher heating value (HHV) changing rate of biocrude increased with the increase of N content, where the HHV of biocrude decreased by 0.4%-6.2% after storage. The solvent extraction removed the macromolecular compounds, and reduced the heteroatomic content in the biocrude, leading to improve storage stability of the biocrude. The presence of small non-polar compounds significantly reduced the viscosity of biocrude for better fluidity and storage stability. From the perspective of the oil quality, the low polar and light components of biocrude shared better storage stability than the high polar and heavy components, which can be used as transportation fuels. The heavy component can be used as asphalt or boiler fuel. The biocrude can be separated by distillation to produce different products in future industrial production.