Environmental and economic assessment of microalgae-derived jet fuel

Abstract

Significant efforts must be undertaken to quantitatively assess various alternative jet fuel pathways when working towards achieving environmental and economic United States commercial and military alternative aviation fuel goals within the next decade. This thesis provides lifecycle assessments (LCAs) of the environmental and economic impacts of cultivating and harvesting phototrophic microalgae; extracting, transporting, and processing algal oils to hydrocarbon fuels; and distributing and combusting the processed renewable jet fuel for a pilot scale facility. Specifically, lifecycle greenhouse gas (GHG) emissions, production costs, freshwater consumption, and land use were quantified for four cultivation and two extraction technology sets. For each cultivation and extraction type, low, baseline, and high scenarios were used to assess the variability of each performance metric. Furthermore, sensitivity analyses were used to gain insights as to where efforts towards improving certain technologies could have the largest impact on improving the lifecycle metrics. The four cultivation technologies include open raceway ponds, horizontal serpentine tubular photobioreactors (PBRs), vertical serpentine tubular PBRs, and vertical flat panel PBRs. Open raceway ponds were modeled from previous literature, while the PBRs were modeled, validated and optimized for specific constraints and growth inputs. The algal oil extraction techniques include conventional dewatering, drying, and extraction using hexane in a similar process to seed oil extraction (termed dry extraction in this study) as well as algal cell lysing with steam and potassium hydroxide as well as fluid separation and washing processes (termed wet extraction). Overall, open raceway pond cultivation with wet extraction performed most favorably when compared with the other scenarios for GHG emissions, production costs, freshwater consumption, and areal productivity (including the entire cultivation and extraction facility), yielding 31.3 g-CO2e/MJHEFA-J, 0.078 $/MJHEFA-J (9.86 $/galHEFA-J), 0.38 Lfreshwater/MJHEFA-J and 17,600 LTAG/ha/yr for the baseline cases with brackish water makeup. The lifecycle GHG emissions and production cost metrics for the open raceway pond with wet extraction low scenario were both lower than that of conventional jet fuel baselines. For all cases, the inputs most sensitive to the lifecycle metrics were the cultivation system biomass areal productivity, algal extractable lipid weight fraction, and downstream harvesting system choices.