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

Abstract

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⁻¹ versus 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%).