Upgrading microalgae for petroleum alternatives: CO2 upcycling for photosynthesis-based society

Lipid-rich microalgae biomass, assimilating CO2 by photosynthesis, holds promise as a source of bio-crude oil. Such practice by hydrothermal liquefaction (HTL) offers a potential alternative to petroleum. However, the complexity of biomass thermal degradation presents challenges to industrialization. Thus, this review surveys recent advancements in non-conventional HTL technologies using microalgal biomass as feedstock, aiming to minimize energy input and enhance the yield of desired target products. Recent studies suggest that using methanol as a co-solvent in hydrothermal degradation enhances biocrude oil yield under mild temperature and pressure conditions. Moreover, zeolite-based in situ catalytic HTL shows a positive promise in refining biocrude oil, reducing its oxygen content, and increasing calorific value. Furthermore, microalgae-derived bio-crude can be processed into biofuel akin to petroleum via hydrotreating with Ni catalyst. Life Cycle Assessment/techno-economic assessment reveals that microalgae productivity critically impacts production costs. Reducing the microalgae biomass production cost to less than $300 per ton and increasing the biological oil conversion yield to more than 60 % are necessary for successfully replacing petroleum. Recent strategies have been suggested, including optimized photobioreactor operation, photosynthesis-enhancing additives, and genetic engineering. In particular, maintaining nitrogen concentration via N-fed batch operation in shallow photobioreactors and nanoparticle-mediated expansion of the photosynthesis-utilized wavelength range have proven effective in enhancing productivity. Lastly, prospects for increasing HTL yield and photosynthetic efficiency are proposed to speed up the transition towards a sustainable society.