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.
Highlights
Biofuels currently are mostly produced from terrestrial plants containing oil, starch or sugar such as soy beans, raps, corn and sunflower seeds or palm oil
Modelling the production of the reference mass (1 kilogram of dry oil-rich algal biomass (kg DM)) with GaBi (Fig. 3) resulted in a total primary energy demand of 118.56 MJ distributed between process energy (39.27 MJ) and operational material (10.4 MJ related to 567.957 kg). 102.8 MJ out of the sum of 118.56 MJ are from nonrenewable sources and 15.76 MJ are from regenerative input
6 CONCLUSION This study quantitatively demonstrates that production of one kilogram of algal biomass from the fresh water microalga Auxenochlorella protothecoides consumes 118.56 MJ of primary energy
Summary
Biofuels currently are mostly produced from terrestrial plants containing oil, starch or sugar such as soy beans, raps, corn and sunflower seeds or palm oil. Considerable efforts are made to generate biofuels (including biodiesel) from other sources to avoid competition with food production. This is especially true for aviation fuels. Microalgae turned out to be an almost ideal alternative as they contain significantly higher concentrations of oil and carbohydrates, possess high to very high photosynthetic activity and require comparatively less land use than terrestrial plants 22. Possible processes to produce drop-in fuels focus on FischerTropsch synthesis applied to coal, gas or biomass (CtL, GtL, BtL) and hydration of vegetable oil (HVO). BtL and HVO from microalgae are considered an environmentally sensible alternative with high potential to replace fossil resources
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