Study of life cycle assessment: Transforming microalgae to biofuel through hydrothermal liquefaction and upgrading in organic or aqueous medium

Hydrothermal liquefaction (HTL) is a promising method for the utilization of low-lipid microalgae. For the upgrading of biocrude obtained from hydrothermal liquefaction of Dunaliella salina, two different catalysts for hydrodeoxygenation (HDO), which could function in organic solvents and aqueous phases respectively, were published in our previous work (Lin et al., 2023; Lin et al., 2022). Corresponding to quite different upgrading processes based on these catalysts, two systems for resource utilization of low-lipid microalgae were established, including microalgae growth & collection, HTL, HDO upgrading, and catalysts recovery, which were named case 1 and case 2, respectively. Through life cycle assessment (LCA), the product yield, energy consumption, and different environmental impact categories of the two systems were compared. It was found that both conversion processes of microalgae had significant environmental benefits for the reuse of wastewater in terms of global warming potential (GWP) and eutrophication (EP) by reducing up to a maximum value of 8.73 kg PO4-eq and 158.80 kg CO2-eq, respectively. In two systems, case 2 has more significant advantages in terms of low energy consumption, high product yield, and less hazardous substance emissions, which achieves 62 % total production yield and complete self-heating. The annual profits of case 2 increased by 41 % compared to case 1. Remarkably, the carbon-based catalysts contribute to the sustainability of the system due to the easy procedure of catalyst recovery. Through this work, the significance of technological innovation for environmental protection and economic development was emphasized.