A national-scale comparison of resource and nutrient demands for algae-based biofuel production by lipid extraction and hydrothermal liquefaction.

Algae's high productivity provides potential resource advantages over other fuel crops. However, demand for land, water, and nutrients must be minimized to avoid impacts on food production. We apply our national-scale open-pond, growth, and resource models to assess several biomass to fuel technological pathways based on Chlorella sp. We compare resource demands between hydrothermal liquefaction (HTL) and lipid extraction (LE) to meet 1.89E+10 and 7.95E+10 L yr−1 renewable diesel targets. We estimate nutrient demands where post-fuel biomass is consumed as co-products and recycling by anaerobic digestion (AD) or catalytic hydrothermal gasification (CHG). Sites are prioritized based on fuel value relative to a set of site-specific resource costs. The highest priority sites are located along the Gulf of Mexico coast, but potential sites exist nationwide. Compared to LE, HTL reduces land requirements at least 50%, freshwater consumption at least 33%, and saline groundwater by 85%. Without recycling, nitrogen (N) and phosphorous (P) demand is reduced 44%, but remains significant relative to current U.S. agricultural consumption. The most nutrient-efficient pathways are LE + CHG for N and HTL + CHG for P (by 52%). Resource gains for HTL + CHG are offset by a 284% increase in N consumption relative to LE + CHG (with potential for further recycling). Nutrient recycling is essential to effective use of alternative nutrient sources. While modeling of availability and costs remains, for HTL + CHG at the 7.95E+10 L yr−1 production target, municipal sources can offset up to 20% of N and 49% of P demand and animal manure could potentially satisfy demands.