A group of researchers from 
Argonne National Laboratory’s Center for Transportation Research recently published a scientific paper that identifies key parameters for algal biofuel production using the GREET model, which models greenhouse gas emissions, regulated emissions and energy use for transportation fuels. The paper, titled “Methane and nitrous oxide emissions affect the life-cycle analysis of algal biofuels,” was published in a recent edition of Environmental Research Letters.
In the report, the researchers also described several goals they had with this analysis. First, they aimed to establish a framework that could be used to facilitate comparisons among algae scenarios and with other transportation fuels. Second, the research team wanted to identify which parameters produce the most significant impact to life-cycle analysis. As part of this, the paper noted that the team paid a great deal of attention to the amount of energy present within lipid-extracted algae. For example, this includes the energy potential associated with processing lipid-extracted algae in an anaerobic digestion system. The researchers also considered the impact of fugitive methane emissions, the fate of unrecovered nitrogen, as well as the potential for nitrous oxide (N2O).
According to information contained in the study, the baseline scenario considered by the researchers produced 55,400 grams of carbon dioxide equivalent (g CO2/e) per 1 million Btu of biodiesel, compared to 101,000 g CO2/e for low-sulfur diesel. The paper stated that the baseline scenario for algae biodiesel features the use of anaerobic digestion for energy and nutrient recovery from lipid-extracted algae. Alternatively, a reduced emissions scenario featured the use of catalytic hydrothermal gasification rather than anaerobic digestion.
On a full life cycle, or “well-to-wheels” basis, the modeling showed that algae biodiesel reduced the total amount of fossil energy use and petroleum used when compared the amount of these inputs required to make petroleum-based diesel. This was true even though the production of algae-based fuel consumed more energy during the production stage. The high energy use associated with the algae-biofuel production was attributed primarily to electricity use and fertilizer production. In addition, the total greenhouse gas emissions per million Btu of algae-biodiesel were less than those associated with petroleum diesel. The research paper largely attributed this to the substantial CO2 credit that results from the reuse of carbon contained within flue gas emissions exiting a power plant.
As a result of the analysis, the research team concluded that while algae biofuel production is energy intensive, substantial reductions in greenhouse gas emissions of 45 to 60 percent are still achieved when compared to conventional diesel fuel. According to the paper, most of the energy use was associated with circulating the algae culture and moving the culture to and from the first dewatering step.
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