Life Cycle Design of an Algal Biorefinery Featuring Hydrothermal Liquefaction: Effect of Reaction Conditions and an Alternative Pathway Including Microbial Regrowth
Algae are an appealing source for bioenergy due to their high yields relative to terrestrial energy crops. The high cost of production, however, has prohibited commercialization. Hydrothermal liquefaction is a technology that converts more of the algae into oil than alternative technologies, thereby reducing the amount of expensive pond infrastructure and energy required for cultivation. We incorporate recent experimental results into an analysis that models the economic and life cycle performance of an algal biorefinery across a range of reaction conditions. Two strategies are explored: one pathway with gasification of the aqueous waste products for onsite energy recovery and another pathway featuring cultivation of Escherichia coli on the aqueous products and recycling of the biomass back through the reactor for boosted oil yields. We found that the maximum net energy ratio of 1.9 and minimum global warming potential of 1.0 kg CO2e L-oil–1 occurred with gasification, along with the minimum reaction temperature explored, 250 °C, and reaction times close to 1 h. The optimal economic and occupied land results occurred at a maximum temperature of 400 °C and with the shortest reaction time explored of 5 min. The cost of algal oil at these conditions was $1.64 L-oil–1 (or $263 bbl–1). For the regrowth pathway, the land footprint could be further reduced by 10%, and the optimal cost could be reduced to $1.59 L-oil–1. Forgoing gasification had a significantly detrimental effect on the other two metrics. Given the importance of economics to an algal biorefinery’s operations, this could be a viable option.