Product characterization of multi-temperature steps of hydrothermal liquefaction of Chlorella microalgae

Hydrothermal liquefaction (HTL) is a promising technique for crude bio-oil (biocrude) production from microalgae. Instead of traditional direct HTL at one temperature with a residence time, the present work explored two temperature steps (TTS) and more temperature steps (MTS) of microalgae (Chlorella) HTLs for the first time in mini-batch reactors. Specifically, the reactions for the TTS of HTL were performed at a relatively low temperature (150-300 °C) for 10-40 min and then at a high temperature (350 °C) for 10-20 min. In the MTS of HTL, three or four temperature steps were adopted and each temperature stage (within 150-300 °C) was kept for 10 min. The results show that the low temperature pre-reaction stage significantly affected the yield, elemental composition, higher heating value, energy recovery and molecular component of biocrude. The biocrude derived from the TTS of HTL of 250*20-350 (i.e., a pre-reaction at 250 °C for 20 min followed by a HTL at 350 °C for 10 min) had the highest H content (9.00 wt%) and the lowest S content (0.55 wt%). Its yield, elemental composition, higher heating value, and energy recovery were comparable with those of the biocrude from the direct HTL at 350 °C for 30 min (with the highest yield and the best quality in tests). In comparison to direct HTL, a proper TTS of microalgae HTL was able to reduce reaction temperature on the premise of ensuring similar biocrude properties, so might be applicable in the algal bio-oil production.