| Summary: | This study presents a life cycle greenhouse gas and energy assessment for two algal biofuel production pathways: biodiesel produced through lipid extraction (LE) and renewable diesel produced through hydrothermal liquefaction (HTL). The two production pathways generate different co-products, which are handled through allocation in life cycle assessment-based analyses. The method and assumptions used for co-product allocation affect the performance of the analyzed fuels, and are thus examined through scenario analysis; five co-product allocation strategies are tested for the LE pathway and six are tested for the HTL pathway. After allocation, the carbon intensity of renewable diesel varies from 36 to 54 gCO2e/MJ, and the primary energy consumption of renewable diesel varies from 0.7 to 1.2 MJ/MJ; while the carbon intensity of biodiesel ranges, remarkably, from −59 to 125 gCO2e/MJ, and the primary energy consumption of biodiesel ranges from 0.1 to 1.7 MJ/MJ. The optimal algal oil production pathway is determined by comparing open-loop and closed-loop systems, considering not only the estimated net environmental impacts, but also the confidence or uncertainty of those outcomes. © 2019 Elsevier Ltd
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