Abstract
As the world’s population and energy demand continue to increase, causing a shortage of fossil fuels, the need for a domestically-sourced, climate-neutral fuel is of growing concern. Across the world, biofuels have been gaining traction as an energy source and are predicted to continue to be a significant component of the world’s energy. Algae-derived biofuels are anticipated to be especially promising due to their purported sustainability benefits, which position them as one of the most promising alternative energies under development.
The environmental impacts, specifically greenhouse gas (GHG) emissions, of algae-derived biofuels have been evaluated by life cycle assessment (LCA). We hypothesize that previous LCAs have overlooked a possible key contributor to the overall global warming potential (GWP) of algae-derived biofuels; namely, nitrous oxide (N2O) emissions during cultivation. Scientific studies have observed significant N2O emissions from lakes exhibiting eutrophication, causing algal blooms. These lakes can be compared to the shallow, open cultivation ponds used in algae-derived biofuel production, and, as such, failing to account for N2O emissions during cultivation could lead to an underestimation of GWP. Under the EPA’s Renewable Fuel Standard (RFS2), life-cycle GHG emissions must be 50% less than the average life-cycle GHG emissions for gasoline or diesel used as transportation fuel in 2005. Because the GWP of N2O is 298 times greater than that of CO2 on a per mass basis, even small quantities of N2O emissions could pose a severe threat to the environment and could possibly prevent approval of an algae-derived biofuel process under RFS2.
The primary objective of this research was to determine if accounting for N2O production during algae cultivation could substantively change previous LCA-based estimates of the life-cycle GWP. This research was conducted in three parts: (1) thorough examination of existing literature on N2O emissions from eutrophic lakes similar to algae cultivation ponds, (2) bench-scale experiments for measurement of N2O emissions during simulated algae cultivation, and (3) integrating literature and laboratory results into an existing LCA framework for algae-derived gasoline.
Eutrophic lakes similar to algae cultivation ponds are estimated to produce fluxes in the range of 7-99 μg N2O/m2/hr from portions of the lakes containing dense algae cultures, making eutrophic lakes a significant contributor of N2O into the atmosphere. Bench-scale experiments simulating algae cultivation under different growth conditions (i.e., air- or CO2-sparging) were conducted. Higher N2O emissions were produced when cultivated under a headspace of CO2, which is anticipated to be the preferred method for commercial algae cultivation. N2O emissions produced under these conditions were up to 32 μg N2O/g algae biomass, suggesting that the previously undocumented contribution of N2O could substantially increase the life-cycle GWP of algae-derived biofuels. Literature and laboratory results were integrated into an existing LCA-based framework which reveals that accounting for N2O could dramatically increase the life-cycle GWP of algae-derived gasoline by as much as a factor of 10, depending on system boundaries and parameter selection. These results will provide a better understanding of how algae-derived biofuels can contribute to achievement of national energy and climate change goals, as laid out in RFS2.
