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 degrees C and 10-12 MPa) and slow pyrolysis (heated to 450 degrees C at a rate of 50 degrees C/min), were used to produce bio-oils from Scenedesmus (raw and defatted) and Spinilina 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 degrees 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. (C) 2012 Elsevier Ltd. All rights reserved.