Hydrothermal Liquefaction of Algae Grown on Dairy Wastewaters

Algae-based biofuels have attracted significant research interest due to their advantages of not competing with land for food production, abilities to grow in low-quality water, higher growth rates, and strong CO2-mitigation abilities. In recent years, research has focused on hydrothermal liquefaction (HTL) of whole, wet algae biomass. HTL is a thermochemical process, considered one of the most promising techniques for valorization of wet algae biomass. HTL uses subcritical water (270-350°C and 8-18 MPa) both as a solvent and a reaction medium to convert organic biomass constituents into energy-rich bio-crude oil which can be refined into liquid fuels. HTL is most suitable for conversion of wet feedstocks including algae. In this study, we compare the oil yields and energy recoveries from the HTL of two algae polycultures used to remove excess nutrients from dairy manure effluents: a microalgae polyculture grown on brackish manure effluent in a photo bioreactor system at NMSU and a filamentous algae polyculture grown on freshwater dairy effluent as part of an algae Turfscrubber® system at ENMU.

Batch HTL conversion experiments were carried out using a 1.8 Liter autoclave reactor at reaction temperatures and retention times of 310°C and 350°C, and 30 and 60 minutes, respectively. Algae and reaction conditions were compared based on product yields and the higher heating values (HHV) of the bio-crude oils. For brackish water microalgae produced a yield of 10-24 wt. % oil, 19-38 wt. % char, and approximately 45MJ/kg for HHV. The yield for filamentous algae grown on freshwater effluent was 11-15 wt. % oil, 16-25 wt. % char, and approximately 30-44 MJ/kg. Both of these feedstocks presented high ash content, and presented a low moderate potential for energy recovery in liquid fuels. Future work will focus on bio-crude oil, and char characterization, as well as the evaluation of the dairy effluent waste water treatment and the renewable energy production combined process.