The role of allocation and coproducts in environmental evaluation of microalgal biofuels: How important?

• This work performs a life cycle assessment of microalgal biodiesel production.
• Multiple coproduct options and allocation schemes were studied.
• The environmental performance of microalgal biofuels is dependent on coproduct/allocation option.
• Microalgal biodiesel generates higher impacts relative to petroleum diesel.

This work evaluates the life cycle environmental impacts of producing microalgal biodiesel via baseline and improved technological routes under several coproduct options and allocation schemes, and compares the results to traditional petroleum diesel. Multiple environmental impact categories, as well as sustainability metrics are utilized to assess the environmental sustainability and viability of emerging microalgal biofuels. Analysis reveals that for baseline scenarios the fossil energy return on investment (EROIfossil) ranges from 0.16 to 0.45, energy return on investment (EROI) from 0.15 to 0.40, and life cycle greenhouse gas (GHG) emissions (gCO2 eq./MJ-fuel) from 142 to 352—depending on the choice of allocation methodology and coproduct option. For improved scenarios EROIfossil range from 0.50 to 1.87, EROI from 0.39 to 1.18, and life cycle GHG emissions from 35 to 141. Further analysis reveals that microalgal fuels provide benefits in only two out of the ten examined environmental impact categories–relative to petroleum diesel. The results are compelling as they suggest that the choice of production pathway and allocation scheme has a significant impact on algae’s energy and environmental performance, and in select cases can alter the net energy balance and GHG reduction potential of microalgal fuel production from negative to positive.