Microalgal biomass fractionation to enhance downstream biofuel production

Abstract

This work investigated microalgal/cyanobacterial protein fractionation to enhance biofuel production using wet biomass processing technologies (anaerobic digestion (AD) and hydrothermal liquefaction (HTL)). A method was developed for protein extraction from Spirulina platensis based on cell disruption, and a subsequent solubilisation and precipitation using alkali and acid respectively. At the optimized process conditions, extraction yield was 60.7 %. The obtained protein isolate had high protein content (80.6 %), and was enriched in essential amino acids and nutritional fatty acids, suggesting possible applications for human food or animal feed. The residual biomass had lower nitrogen and higher non-protein composition and was suitable for biofuel feedstock applications. AD of the protein extracted S. platensis residual biomass (PERB) resulted in 30.4 % higher methane yield than original (untreated) biomass. The rate of methane production was higher than that for original biomass (ORIB) and high pressure homogenizer disrupted biomass by 161 % and 38.9 % respectively. Biocrude oil produced from HTL of PERB was better in quality than that from ORIB owing to the presence of larger number of long chain hydrocarbons and fatty acids, and slightly lower nitrogen content (6.2 % versus 7.0 %). A comparison across AD and HTL suggested a better energy recovery for PERB in the former. Thereafter, the benefits of using protein extracted biomass residues generated from three different microalgal species (Chlorella pyrenoidosa, Tetraselmis chuii and Phaeodactylum tricornutum) by two different protein fractionation/ extraction methods (High pressure alkali-acid (HPAA) and low temperature hydrothermal treatment (LTHT)) were evaluated as feedstock for AD. HPAA method involved cell disruption and a subsequent protein extraction using alkali and acid. LTHT method involved low temperature hydrothermal treatment to extract proteins into the aqueous phase. Residues from the former method resulted in higher methane yields and methane production efficiencies than all other substrates of the respective species. The co-products (protein isolates) had a composition suitable for food/ feed applications. LTHT method was beneficial only for the species with the most recalcitrant cell wall (Chlorella pyrenoidosa). Based on this work, a microalgal biorefinery may be proposed with the integration of protein extraction and biofuel production processes.