Toward developing more sustainable marine biorefineries: A novel ‘sea-thermal’ process for biofuels production from microalgae

Abstract

This work explores the ‘sea-thermal’ treatment of the aquatic protein-rich microalga Chlorella Vulgaris to produce energy-dense biofuels and value-added, nitrogen-rich liquid products. The impact of the processing conditions (temperature, 200–300 °C; time, 20–180 min) and reaction medium (seawater/(deionized water + seawater) ratio, 0–100 wt%) on the yields and properties of these products has been addressed using a two-level, three-factor (23) Box-Wilson Central Composite, Face Centered (CCF, α: ±1) design. These processing parameters ruled the distribution of the overall reaction products, including gas (1–5 %), biocrude (17–57 %), aqueous product (32–47 %) and hydrochar (3–45 %), and the fuel and chemical properties of these fractions. The calorific values of the hydrochar and biocrude ranged from 2 to 25 MJ/kg and 23 to 32 MJ/kg, respectively. Using low temperatures and/or short reaction times favored biocrude formation, while higher temperatures and/or more prolonged processing times boosted the repolymerization of the biocrude to hydrochar. These transformations depended on the reaction medium, with seawater exerting different influences based on the reaction medium salinity. Diluted seawater promoted the dissolution and depolymerization of polysaccharides and proteins in the alga by disrupting the H-bonding networks within these macromolecules, while enriched seawater media favored the deoxygenation and repolymerization of the biocrude. These variations highlighted the bespoke nature of this ‘sea-thermal’ process to furnish liquid and solid biofuels and/or biochemicals. Process optimization based on the formulae developed from the ANOVA of the experimental data showed that 55–60 wt% of the alga could be converted either to energy-dense (30 MJ/kg) biofuels (36 wt% biocrude, with hydrocarbons and fatty acids/esters in high amount, and 20 wt% hydrochar) at 200 °C for 180 min using enriched seawater (64 wt% seawater) as a solvent or to value-added liquids (a nitrogen-rich (88 %) biocrude, including abundant fatty amides and N-heterocycles) at 237 °C for 169 min in seawater. These results lay the first stone toward developing more sustainable marine biorefineries using marine-based solvents to valorize marine feedstocks.