Life cycle assessment of the production chain of oil-rich biomass to generate BtL aviation fuel derived from micraoalgae

Considerable efforts are made to generate drop-in aviation fuels from microalgae to avoid competition with food production. Synthetic biofuel from oil-rich biomass is produced along four process lines: cultivation, harvest, extraction of raw material and conversion to fuel. This study deals with the life cycle assessment of fuel obtained from cultivation of the fresh water alga Auxenochlorella protothecoides and concentrates on the cultivation in open ponds as well as the harvesting steps preconcentration, electroporation and dewatering. Energy balance and environmental impact is analysed using GaBi software and data base. The main goal is to identify those factors or processes exerting the strongest impact, either environmentally or from the point of view of the energy balance. Production of one kilogram of dry oil-rich algal biomass (kg DM) consumes 118.56 MJ of primary energy. The primary energy demand is apportioned as follows: 71.7 % during proliferation in Erlenmeyer flags and bubble columns, 15.5 % by cultivation in raceway ponds and 12.8 % in preconcentration, electroporation and dewatering. This converts into a net energy ratio (NER) of 0.266 and a CO2-equivalent of 6.45 kg CO2 per kg DM. These values are disadvantageous when compared to kerosene (NER=0.867, 0.384 kg CO2 per kg kerosene). Production can be optimized using process energy from regenerative sources such as hydroelectric power (NER = 0.545, 1.27 CO2 per kg DM). In this case total primary energy input must be corrected for the portion of renewable sources resulting in a NERcorr of 3.04. CO2-equivalents per kg DM remain unfavourably high as compared to kerosene; the main driver responsible for this discrepancy is the usage of freshwater and fertilizer.