Life-Cycle Assessment of Potential Algal Biodiesel Production in the United Kingdom: A Comparison of Raceways and Air-Lift Tubular Bioreactors

Life-cycle assessment has been used to investigate the global warming potential (GWP) and fossil-energy requirement of a hypothetical operation in which biodiesel is produced from the freshwater alga Chlorella vulgaris, grown using flue gas from a gas-fired power station as the carbon source. Cultivation using a two-stage method was considered, whereby the cells were initially grown to a high concentration of biomass under nitrogen-sufficient conditions, before the supply of nitrogen was discontinued, whereupon the cells accumulated triacylglycerides. Cultivation in typical raceways and air-lift tubular bioreactors was investigated, as well as different methods of downstream processing. Results from this analysis showed that, if the future target for the productivity of lipids from microalgae, such as C. vulgaris, of similar to 40 tons ha(-1) year(-1) could be achieved, cultivation in typical raceways would be significantly more environmentally sustainable than in closed air-lift tubular bioreactors. While biodiesel produced from microalgae cultivated in raceway ponds would have a GWP similar to 80% lower than fossil-derived diesel (on the basis of the net energy content), if air-lift tubular bioreactors were used, the GWP of the biodiesel would be significantly greater than the energetically equivalent amount of fossil-derived diesel. The GWP and fossil-energy requirement in this operation were found to be particularly sensitive to (i) the yield of oil achieved during cultivation, (ii) the velocity of circulation of the algae in the cultivation facility, (iii) whether the culture media could be recycled or not, and (iv) the concentration of carbon dioxide in the flue gas. These results highlight the crucial importance of using life-cycle assessment to guide the future development of biodiesel from microalgae.