Valorisation of Chlorella vulgaris biomass for multi-products synthesis via hydrothermal processing

Microalgal-based biofuel such as hydrochar has gained popularity in recent years owing to the high lipid content and fast growth rate of microalgal cells. However, commercial scale production of microalgal-based biofuel is not feasible due to the high processing cost of microalgal itself. One possible solution to overcome this limitation is to obtain other value-added byproducts to enhance the economic feasibility of the entire process in which such nature of research work is still very limited in the literature. Thus, in the present study, a two-step sequential hydrothermal carbonization (SEQHTC) was used to produce polysaccharides and hydrochar (potential solid fuel) from Chlorella vulgaris biomass. In the first step, whole algal (WA) biomass was subjected to hydrothermal subcritical extraction of polysaccharides whereas in the second step, the solid residue from the first step or identified as treated algal residue (TAR) was subjected to hydrothermal carbonization to produce hydrochar. The yield of hydrochar using WA biomass through SEQHTC process was 39.3 wt with 12.5 wt of polysaccharides. Apart from using WA, lipid-extracted algal (LEA) biomass was also used as feedstock for SEQHTC. The total extracted lipid, hydrochar and polysaccharides were 15.2 wt, 41.5 wt and 13.4 wt, respectively. For comparison purposes, the conventional direct hydrothermal carbonization (DHTC) method was also carried out on both biomass. This study successfully demonstrated the realization of microalgal biorefinery concept, in which value-added products (polysaccharides) could be extracted along with the production biofuel such as biodiesel and hydrochar as sustainable renewable energy sources from microalgal biomass. © 2023 The Authors