Generation of coproducts from nutrients is purported to improve the sustainability of algae-derived transportation fuels by minimizing life cycle impacts and improving economic sustainability. Although algae cultivation produces lipids that is upgraded to drop-in transportation fuel products, life cycle assessment and techno-economic analysis have shown that without coproducts, energy/economic returns are diminishing regardless of processing methods. This study utilizes a combined flash hydrolysis (FH), hydrothermal liquefaction (HTL), and coproduct conversion technology (atmospheric precipitation/AP; hydrothermal mineralization/HTM) to conserve the most recyclable nutrients for coproduct marketability. Six biofuel pathways were developed and compared in terms of “well-to-pump” energy, life cycle greenhouse gas (LC-GHG) emissions, and economic profitability: renewable diesel II (RDII), renewable gasoline (RG), and hydroprocessed renewable jet (HRJ) fuel, each were modeled for AP and HTM coproduct conversion. A functional unit of 1 MJ usable energy was employed. RG showed a promising energy-return-on-investment (EROI) due to multiple coproducts. All models demonstrated favorable EROI (EROI > 1). LC-GHG emissions tie in with EROI such that RG produced the least emissions. HRJ-HTM was determined to be the most profitable model with a profitability index (PI) of 0.75. Sensitivity analyses revealed that dewatering affects EROI and PI significantly. To achieve break-even, gasoline must sell at $4.10/gal, diesel at $5.64/gal, and jet fuel at $3.43/gal.
Microalgae as a feedstock source for biofuels have been researched extensively over the last decade as the renewable energy community strives to find an environmentally responsible and economically feasible source of transportation fuel. This study focuses on microalgae not as a source of transportation fuel but as a fundamental pillar of sustainable rural communities seeking to achieve a net-zero energy and waste status. Sources of sustenance, waste streams, energy demands, energy production, and recycle methods are evaluated in this life cycle assessment and techno-economic analysis to determine if a rural community can achieve a state of neutrality and self-sustainability. A functional unit of 20,000 MJ/year corresponding to the energy demand of a 902-person community is modeled after a town in Georgia. The model revealed that the total available land for algae production was 7.45 ha, but only 0.02 ha was required for producing 20,000 MJ/year, our functional unit in this study. At this magnitude of algae cultivation, the energy return on investment (EROI) was 0.54 (net negative energy community) and the profitability index was only 0.04 (community is at financial loss). When the algae cultivation area is increased to 1.8 ha, an EROI of 1 is achieved; however, PI is still <1 (0.07). In order to reach PI = 1 (the break-even community), the algae cultivation area must be increased to 1.55 ha, with a corresponding EROI of 3.87. The greenhouse gas offsets for all communities were determined to be substantially larger than their emissions.
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