Biocrude oil production and nutrient recovery from algae by two-step hydrothermal liquefaction using a semi-continuous reactor

We evaluated two-step hydrothermal liquefaction in a semi-continuous reactor for recovery of both nutrients and biocrude from the alga Coelastrum sp. in direct comparison with a one-step process. The influence of the operating temperature, pressure and water flow rate was investigated by means of a 2(k) factorial experimental design and response surface methodology. The two-step process gave a higher total biocrude yield (36 wt% (daf. basis)) and nutrient recovery level in terms of nitrogen containing compounds (60 wt% of the protein content in the original algae as ammonium and nitrate ions and protein/polypeptides) than the single-step process. The highest biocrude yield was achieved at first-step temperature of 473 K, second-step temperature of 593 K, pressure of 200 bar and water flow rate of 0.5 mL/min.