Energy and nutrient recovery efficiencies in biocrude oil produced via hydrothermal liquefaction of Chlorella pyrenoidosa

Biofuels derived from biomass conversion have the potential to mitigate the problems caused by over-exploitation of fossil fuels in recent decades. In this work, Chlorella pyrenoidosa, a fast-growing and low-lipid microalga species, was converted into bio-crude oils via a hydrothermal liquefaction (HTL) process. Response surface methodology (RSM) was applied to investigate the effects of operating conditions including reaction temperature, retention time and total solid ratio of feedstock on bio-crude oil yield and quality. A higher heating value (HHV), carbon recovery (CR) and nitrogen recovery (NR) were used as indicators of bio-crude quality. Reaction temperature was found to be the most influential factor affecting the yield and quality of bio-crude oils. Compared with the bio-crude oil sample obtained at boundary conditions (260 °C, 30 min, 35 wt%), the bio-crude oils at two optimized operating conditions (280°C, 60 min, 35 wt%; 300°C, 60 min, 25 wt%) were observed to have higher yields (43.26% and 39.55% versus 25.65%), higher HHV (34.21 MJ kg ^-1 and 36.51 MJ kg^-1versus 30.59 MJ kg^-1), higher carbon recovery (72.18% and 68.18% versus 61.22%) and slightly lower nitrogen recovery (33.15% and 33.28% versus 35.88%). TGA, FT-IR, GC-MS and NMR analysis indicated that the optimized bio-crude oils were observed to have higher boiling point distributions (250-500°C), a higher percentage of aliphatic functional groups (63-67%), a certain percentage of heteroatomic functionalities (21-26%) and a lower percentage of aromatic groups (1.5-3.3%).