Saline Microalgae Cultivation for the Coproduction of Biofuel and Protein in the United States: An Integrated Assessment of Costs, Carbon, Water, and Land Impacts

Abstract

The development of microalgal biorefineries, utilizing high-value coproducts, offers a strategy to lower production costs, while the use of saline-tolerant microalgal species contributes to reducing freshwater consumption. This study evaluates the sustainability performance of saline microalgae cultivation and conversion at a national scale by analyzing economics, greenhouse gas (GHG) emissions, marginal GHG avoidance cost (MAC), water scarcity footprints, land-use change emissions, and resource availability. The Algal Biomass Assessment Tool (BAT) is applied for site selection, while algae farm and conversion models are used for techno-economic analysis (TEA). The Greenhouse Gases, Regulated Emissions, and Energy use in Technologies (GREET) model is employed for life-cycle assessment (LCA) by integrating the outputs from BAT and TEA. Our findings demonstrate that electricity and nutrient consumption are the primary drivers of base case GHG emissions, while biomass yield is the key factor determining both GHG emissions and economic performance. Saline microalgal biorefineries can achieve a carbon price limit of $80-200/tonne when high-value bio-coproducts, such as whey protein concentrate, are benchmarked, contingent on supply-demand conditions and other market drivers. However, this reduction is insufficient for the MAC to meet the current carbon price threshold. Further reductions in energy and nutrient usage, along with the careful selection of high-value protein coproduct targets with high GHG emissions during the design stage, are recommended. Additionally, saline microalgal biorefineries show great potential in addressing water stress, as the electricity requirements for desalinating brackish and saline water are relatively low compared to the overall system electricity demand.