(384d) A Review of Algae-Based Biofuel Production: Sustainability and Life Cycle Assessments

The National Alliance for Advanced Biofuels and Bioproducts (NAABB) is a consortium of universities, bioenergy companies, and national laboratories whose goal is to overcome technical, economic, and environmental barriers to large scale algae-based biofuels production.  One of the important areas in the NAABB research portfolio is environmental sustainability, and life cycle assessment of the entire algae-based biofuels value chain is one of the key integrative projects in this area.  

Due to lack of quality land and competition with food production, current terrestrial sources of carbohydrates for conversion to bio-ethanol or oils for conversion to bio-diesel are inadequate for fully meeting the United States' transportation fuel needs.  Algal sources of biomass offer many novel sustainable opportunities to partially or fully replace the use of fossil fuels for motor vehicle use, due to large yields per unit of land area and low reliance on other inputs relative to other alternative feedstocks.

This work evaluates past peer reviewed life cycle assessments (LCAs) of biofuels production using algae feedstocks to identify areas where prior analyses have missed opportunities for putting sustainability into a whole life cycle context, and also to conduct an analysis of the range of potential data being used to describe this emerging industry.  System boundaries of prior studies are described to highlight limitations of past assessments and to aid in building more complete analyses with the use of additional experimental data.  Regional and technological limitations are also evaluated, so that future work can consider these issues in their evaluations. Data quality of previous LCAs is discussed by distinguishing input in all studies as being assumed, modeled, or derived from experimental data, and by using our meta-analysis to provide appropriate uncertainties on several key system inputs.

Our review of the LCA literature shows that there is a wide range in the quality of inventory data, and a prevalence of assumed (estimated) or modeled data as opposed to measured data.  In addition, many previous studies omitted critical processing steps altogether due to lack of data and lack of knowledge.  For example, previous assessments have not considered cell disruption methods and the energy requirements added to the total material and energy needs.  Water evaporation from raceways has been either ignored or simplified without considering local weather through annual cycles to identify the water burden on the local region.  When considering biodiesel production in warm sunny areas such as the southwestern United States, this may play a large role in the overall sustainability of the process.

Models of the missing elements discussed above are introduced to show how the life cycle inventories (LCIs) and the resulting LCAs are impacted by the addition of new data.  In many cases, it is found that neglecting some material and energy flows was unimportant to overall sustainability, but others are critically important, depending on the location of biofuel generation and processing.  Impacts of different nutrient input sources with various levels of material recycling are also illustrated in an effort to indicate how system co-location or integration with other utilities and industries may be a critical factor in the economic and environmental sustainability of this technology.