Life-cycle assessment of five microalgae-to-biofuels processes of varying complexity

"Cradle-to-gate" life cycle analysis surrounding five algae-to-fuel/fuel precursor scenarios was studied. The different processes modeled were: an open pond producing either triacylglycerides (TAG) or free fatty acid methyl ester (FAME); a solar-lit photobioreactor producing either FAME or free fatty acids; and a light emitting diode irradiated (LED-lit) photobioreactor-producing TAG. These processes were chosen from amongst the simplest to most sophisticated approaches available in literature. The scenarios of production with open ponds are close to being sustainable environmentally. On the other hand, the production scenarios with solar-lit and LED-lit photobioreactors are both far from being sustainable. The reason for this is the higher embedded and operating life-cycle impacts associated with the materials in the growth reactor (and some other equipment) in these two types of production facilities, as well as the artificial photon source used in the latter. Many difficult-to-achieve improvements are required to make these processes less energy intensive. Algae strains with higher lipid productivity as well as changes in the number, the complexity, and energy expenditures in operation steps are always required to reduce overall life-cycle impacts when production of commodity fuels is the focus. An important perspective to keep in mind with algae-based processes is that there are currently no significant economies-of-scale with the environmental impacts for growth systems, since they are additive above a baseline production level.