Hydrothermal liquefaction of Nannochloropsis sp.: Systematic study of process variables and analysis of the product fractions

We investigated hydrothermal liquefaction of Nannochloropsis sp. at different temperatures (250–400 °C), times (10–90 min), water densities (0.3–0.5 g/mL), and biomass loadings (5–35 wt %). Liquefaction produced a biocrude with light and heavy fractions, along with gaseous, aqueous, and solid by-product fractions. The gravimetric yields of the product fractions from experiments at 250 °C summed to an average of 100 ± 4 wt %, showing mass balance closure at 250 °C. The gravimetric yields of the product fractions are independent of water density at 400 °C. Increasing the biomass loading increases the biocrude yield from 36 to 46 wt %. The yields of light and heavy biocrude depend on reaction time and temperature, but their combined yield depends primarily on temperature. Regardless of reaction time and temperature, the yield of products distributed to the aqueous phase is 51 ± 5 wt % and the light biocrude is 75 ± 1 wt % C. Two-thirds of the N in the alga is immediately distributed to the aqueous phase and up to 84% can be partitioned there. Up to 85% of the P is distributed to the aqueous phase in the form of free phosphate. Thus, N and P can be recovered in the aqueous phase for nutrient recycling. Up to 80% of the chemical energy in the alga is retained within the biocrude. The quantitative results reported herein provide the basis for a reaction network for algae liquefaction. [Display omitted] ► We systematically studied the processing effects on the liquefaction of a microalga. ► We are first to develop a reaction network for the liquefaction of algae. ► More severe reaction conditions produce more of the light biocrude fraction. ► Up to 80% of the chemical energy can be recovered in the biocrude product. ► Over 80% of N and P can be recovered in the aqueous phase for process reuse.