Geographically-resolved evaluation of the economic and environmental services from renewable diesel derived from attached algae flow-ways across the United States

Abstract

Harmful algal blooms (HABs) are becoming more invasive and ever more prevalent due to rises in nitrogen and phosphorus pollution in watersheds. Herein, we introduce a novel biorefinery that taps into the vast sources of nitrogen and phosphorus in watersheds. Contaminated water is diverted to flow over attached algae systems, feeding native, periphytic algal cultures and scrubbing excessive nutrients from the water with the produced biomass converted to fuels through hydrothermal liquefaction. The evaluation of the biorefinery concept is done with sustainability analysis that incorporates geographically-resolved growth modeling with nutrient resource availability based on all Hydrologic Unit Code-8 (HUC8) in the contiguous US. Life cycle analysis results demonstrate a global warming potential of 25 g CO2-eq MJ−1, a eutrophication potential of 1.3 ∗ 10−5 kg N eq. MJ−1, and a net energy ratio 0.33 of MJ MJ−1 in the Santa Monica Bay, CA subbasin. Techno-economic assessments found that renewable diesel can be produced for $1.20 per cubic decimeter (dm−3) or $4.56 per gallon of gasoline equivalent (GGE−1) under optimal conditions in the Santa Monica Bay, CA subbasin, with results dramatically varying across the US. Water quality trading was also incorporated into the analysis. Using modest nutrient credit values of $4.5 per kg of total nitrogen (kg-TN−1) and $4.5 per kg total phosphorus (kg-TP−1) removed enabled the renewable diesel to achieve parity with conventional diesel, $1.01 dm−3 ($3.84 GGE−1) in the Santa Monica Bay, CA subbasin. A more aggressive credit value of $45 kg-TN−1 and $45 kg-TP−1 made the price of the renewable diesel negative in Santa Monica Bay, CA, roughly $-4.45 dm−3 ($-16.8 GGE−1), and across the Midwest, the Gulf of Mexico, and major cities on the East and West Coast. These results highlight a path forward for mitigating eutrophication while also creating a sustainable fuel.