(772c) Effects of Microalgal Polycultures on Quality of Biomass for Biocrude Oil Production Via Hydrothermal Liquefaction

Authors: 
Hietala, D. C., University of Michigan
Savage, P. E., The Pennsylvania State University
Cardinale, B. J., University of Michigan - Ann Arbor
Rising atmospheric carbon dioxide levels have created a continuously increasing demand for sustainable energy. Algal biomass as a feedstock for sustainable energy production is a compelling option, as algae cultivation does not necessitate arable land and algae have higher energy density than terrestrial plants [1]. Moreover, one biomass-conversion technology called hydrothermal liquefaction (HTL) can convert wet microalgae into an energy-dense biocrude oil without incurring energy penalties associated with drying the biomass [2, 3].

Raceway ponds are a primary method of growing microalgae, although the open nature of these ponds subjects algal cultures to external perturbations like light and temperature variability, predators, diseases, and invasive species, leading to crop instabilities [4, 5]. Multi-species communities, or polycultures, could potentially be more resistant to these environmental factors and thus more stable over time [6, 7]. In a recent study, we used HTL to assess the biocrude productivity and stability of 2-, 4-, and 6-species polycultures of Ankistrodesmus falcatus, Chorella sorokiniana, Pediastrum duplex, Scenedesmus acuminatus, Scenedesmus ecornis, and Selenasatrum capricornutum, compared to the monocultures of these species [8]. This study found that polycultures generally produced biocrude more stably than monocultures but not in higher amounts. However, the quality of the biomass feedstock and the resulting biocrude have yet to be considered, but are also important bases of comparison. Most importantly, these factors affect the extent of catalytic upgrading required before the biocrude can be inserted into existing refining infrastructure [9].

In this presentation, we expand upon earlier work [8] and compare the performance of polycultures and monocultures on several quality-based metrics, including biomass fatty acid content and biocrude yield, atomic ratios, and higher heating value (HHV). Microalgae richer in fatty acid content and biocrude with lower nitrogen and oxygen content and higher carbon and hydrogen content are more desirable. We found that some polycultures met or exceeded the best monocultures for each metric we examined; moreover, the best monocultures and polycultures were generally different for each metric, suggesting that optimization of polyculture composition based on the desired biocrude specifications is possible. These metrics of quality provide additional bases of comparison for monocultures and polycultures beyond those of biocrude productivity and stability reported in earlier work by [8], allowing for a more comprehensive evaluation of the benefits of biodiversity on biocrude oil production via microalgal HTL.

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