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Advancing process development in fractionation-based, fuel-oriented, microalgal biorefineries
Hull, Tobias C.
Hull, Tobias C.
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2024
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2026-04-09
Abstract
Microalgae have significant potential for decarbonizing fuel and chemical production. However, high cultivation costs limit their adoption, particularly for low-value products like fuels. Fractionation-based processing separates microalgae into distinct chemical fractions, enabling design of targeted conversion pathways to maximize the value of each component of the biomass. This dissertation develops processes to support renewable fuel and chemical production from fractionated microalgal biomass.
In the first study, a wet oxidation process is developed to recover nutrients from lipid-extracted algal residues. Under certain oxidation conditions, nitrogen in the residues is converted to ammonium and phosphorus is converted to phosphate, which can be recovered by sequential cation and anion exchange. At the same time, a portion of the carbon is converted to carboxylates, which remains unaffected by ion exchange and is suitable for further upgrading. This process offers an alternative to anaerobic digestion for nutrient recovery, adding flexibility and reducing risks in microalgal biorefinery design. The second study builds on the first by conditioning recalcitrant high-protein algal hydrolysates to facilitate the bioconversion of residual carboxylates formed during wet oxidation. Oxidative pretreatment removes nitrogenous inhibitors while preserving carbon, allowing oleaginous yeast to grow and accumulate lipids in treated hydrolysates. These lipid-rich organisms could be coprocessed alongside microalgae to supplement lipid content, improving overall oil yields and addressing the issue of low lipid availability in high-protein algal strains. This approach shows the feasibility of incorporating higher-protein algae into fractionation-based microalgal biorefinery designs. In the final study, a fully integrated sustainable aviation fuel (SAF) production pipeline is developed, incorporating acid pretreatment, cell separation, lipid recovery, esterification, short-path distillation, and hydroprocessing. Esterification followed by short-path distillation effectively purifies low-quality algal oils, removing impurities and outperforming alternative cleanup technologies. This purification step enables stable hydroprocessing with SAF yields (C9-C14) of 25%, comparable to model compound performance. These findings validate esterification and short-path distillation as a technically viable cleanup approach, highlight the importance of cleaner algal oils for efficient catalytic processing, and guide future research on catalyst designs and purification techniques.
This dissertation contributes to the literature by filling in knowledge gaps about processing fractionated microalgal biomass and then using these insights to design new technologies and validate their integration within fractionation-based frameworks. The processes developed here add to the growing portfolio of technologies available to process microalgae and are an important step forward in realizing the potential of microalgal biomass for the production of sustainable fuels and chemicals.
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