Roles of Co-solvents in Hydrothermal Liquefaction of Protein-Rich Algae

摘要

Galdieria sulphuraria (hereafter referred to as G. sulphuraria), a thermophilic and acidophilic red microalga, was effective in the urban wastewater treatment and energy recovery mainly due to its high nutrient removal ability and fast growth rate. Inthis study, hydrothermal liquefaction (HTL) was carried out to recover energy from G. sulphuraria with/without co-solvents under subcritical conditions. The optimal condition of HTL without co-solvents was 350 °C for 60 min with a bio-crude oil yield of 28 wt %. However, the quality of the bio-crude oil was hardly to meet the specification as an alternative to transportation fuel owing to high contents of nitrogen (>4%) and oxygen f>3%). To enhance the production of bio-crude oil and energy recovery from G. sulphuraria, three types of alcohol solvents (ethanol, isopropanol, and glycerol) were studied to improve HTL with an alcohol concentration of 40 wt % under a subcritical condition of310-350 °C and 5-30 min. Averagely, co-solvent HTL reactions yielded 14%-28% more bio-crude oil on a dry feedstock basis than that of HTL with pure water. It was found that HTL of G. sulphuraria with 40 wt % glycerol under 350 °C and 30 min gave the highest yield of bio-crude oil (73 wt %). The nitrogen content ofthe bio-crude oil increased by 8.1-18.6%, basically in agreement with the growth rate of bio-crude oil, and simultaneously the nitrogen levels in the aqueous phase decreased 19.9-31.7%, compared to HTL with pure water. The results suggest that the increase in bio-crude oil was primarily contributed by decomposition and conversion of G. sulphuraria, as the only nitrogen source, via promoting transferring of algae-derived protein fragments from aqueous to bio-crude oil phase with aid of alcohols. Lastly,an integrated bioreflnery of wastewater treatment algae and bio-diesel by-product was proposed and examined through a co-solvent HTL of wastewater treatment G. sulphuraria with crude glycerol under the optimal condition. Variety of characterization methods, including high-resolution Fourier transform mass spectroscopy (FT-MS), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR), offer an in-depth insight into the reaction pathways in HTL ofprotein-rich algae with the participation of co-solvents.