Thermochemical conversion of raw and defatted algal biomass via hydrothermal liquefaction and slow pyrolysis

Thermochemical conversion is a promising route for recovering energy from algal biomass. Two thermochemical processes, hydrothermal liquefaction (HTL: 300 °C and 10–12 MPa) and slow pyrolysis (heated to 450 °C at a rate of 50 °C/min), were used to produce bio-oils from Scenedesmus (raw and defatted) and Spirulina biomass that were compared against Illinois shale oil. Although both thermochemical conversion routes produced energy dense bio-oil (35–37 MJ/kg) that approached shale oil (41 MJ/kg), bio-oil yields (24–45%) and physico-chemical characteristics were highly influenced by conversion route and feedstock selection. Sharp differences were observed in the mean bio-oil molecular weight (pyrolysis 280–360 Da; HTL 700–1330 Da) and the percentage of low boiling compounds (bp < 400 °C) (pyrolysis 62–66%; HTL 45–54%). Analysis of the energy consumption ratio (ECR) also revealed that for wet algal biomass (80% moisture content), HTL is more favorable (ECR 0.44–0.63) than pyrolysis (ECR 0.92–1.24) due to required water volatilization in the latter technique.

► HTL and pyrolysis bio-oils display similar elemental composition and IR spectra.
► Bio-oil chemistry is influenced by biomass composition and conversion method.
► Pyrolysis bio-oils exhibit lower Mw and boiling points compared to HTL bio-oils.
► HTL is more energetically favorable than pyrolysis for wet biomass (80% moisture).
► Low biomass moisture increases energetic favorability of thermochemical conversions.