A Techno-economic Analysis of Hydrogen, Biodiesel, Gasoline and Other Fuels Production from Microalgae Based on Energy Requirements

摘要

Carbon neutral renewable fuels have great advantages over the traditional fossil fuels for greenhouse gases reduction. Microalgae, which are photosynthetic microorganisms, can convert water, carbon dioxide and nutrients into oil rich algal biomass with the irradiation of light. Microalgae provide an alternative solution for future transportation fuels production. Production of different fuels from microalgae requires different microalgae diatom species. Biodiesel production from microalgae Oedogonium sp. and Spirogyra sp. by extraction yielded 0.22 gram/gram dry-alga biodiesel and 0.26 gram/gram dry-alga biodiesel, respectively. A different microalga Chlorella can produce biodiesel in 7.14 gram/L/day using fermentation method. For methanol production, 0.64 gram of methanol could be produced from 1 gram of dry microalga Spirulina by gasification. For hydrogen production, green alga Chlamydomonas reinhardtii produces hydrogen gas at a rate of 0.02 mmol/h/gram-algae for 70 hours by photosynthetic method. Through fermentation method, green alga Scenedesmys produces hydrogen at a rate of 0.431 mmol/h/L FVV (FVV = Free Volume Vessel). Hydrogen production from a blue alga Synechococcus leopoliensis reached a rate of 0.130 mmol/L/h. For ethanol production, a production rate of 1.73 mol/mol glucose consumed was obtained from Synechococcus leopoliensis using aerobic fermentation. Gasoline production rate from green alga Botryococcus braunii was 0.018 gram/L/day. Dry microalga biomass can also be used as solid fuel, and the heating value is about 4,000 kcal/kg, while the heating value for heavy oil is 10,000 kcal/kg (1 kWh = 0.245 kg of heavy oil). Energy balance, which is defined as the ratio of the energy of fuel produced to the total required energy, indicates that the process is plausible as an energy producing process if the energy balance is > 1.0. From our estimate, fuels production processes from microalgae are energy producing process with an energy balance ratio of 2.48. During the calculation, we assume that the microalgae production rate is 30 gram/m2/day. Since currently microalgal biomass production consumes ~60-70% of the total required energy, the energy efficiency of the microalgae harvest is the key issue for the economics of microalgae fuels production.