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

Abstract

Considerable efforts are made to generate drop-in aviation fuels from microalgae to avoid competitionwith 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 fuelobtained from cultivation of the fresh water alga Auxenochlorella protothecoides and concentrates on the cultivationin open ponds as well as the harvesting steps preconcentration, electroporation and dewatering. Energybalance and environmental impact is analysed using GaBi software and data base. The main goal is to identifythose factors or processes exerting the strongest impact, either environmentally or from the point of view of theenergy balance. Production of one kilogram of dry oil-rich algal biomass (kg DM) consumes 118.56 MJ of primaryenergy. The primary energy demand is apportioned as follows: 71.7 % during proliferation in Erlenmeyerflags and bubble columns, 15.5 % by cultivation in raceway ponds and 12.8 % in preconcentration, electroporationand dewatering. This converts into a net energy ratio (NER) of 0.266 and a CO2-equivalent of 6.45 kg CO2per kg DM. These values are disadvantageous when compared to kerosene (NER=0.867, 0.384 kg CO2 per kgkerosene). Production can be optimized using process energy from regenerative sources such as hydroelectricpower (NER = 0.545, 1.27 CO2 per kg DM). In this case total primary energy input must be corrected for theportion of renewable sources resulting in a NERcorr of 3.04. CO2-equivalents per kg DM remain unfavourablyhigh as compared to kerosene; the main driver responsible for this discrepancy is the usage of freshwater andfertilizer.