其他摘要 | As feedstock for biodiesel, algae have the advantages of fast growth, high lipid content and algae production located on non-arable land. As secondary bioenergy, the production stages of algae based biofuels are usually associated with certain amounts of fossil fuel use and pollutant emission. To assess the environmental sustainability of biodiesel derived from algae, method of life cycle assessment (LCA) is used to quantify the fossil energy efficiency ratio and potential environmental impacts during algal biodiesel production and use, and functional unit of 1 MJ biodiesel is used. The main contents and results are as following: (1) Life cycle initial fossil energy consumptions and pollutant emissions of process energy and materials needed during algal biodiesel production are calculated with GREET model. The results provide necessary supporting data for the calculation of life cycle fossil energy efficiency ratio and potential environmental impacts of algal biodiesel.(2) Effects of alternative CO2 capture technologies, nitrogen nutrient supply conditions, culture apparatus and algal oil extraction technologies on life cycle fossil energy efficiency ratio of algal biodiesel are assessed. The results show that compared to chemical absorption of CO2, the higher fossil energy efficiency ratio is observed for algal biodiesel with CO2 from membrane separation. Except for Phaeodactylum tricornutum, both the life cycle fossil energy efficiency ratios of biodiesel production from Tetraselmis suecica and Chlorella vulgaris are improved by nitrogen stress cultivation of algae. Compared to flat-plate PBR, the higher life cycle fossil energy efficiency ratio is observed for biodiesel produced from algae cultivated in raceway ponds. Compared to extraction of oil from dried algae, subcritical co-solvents extraction of lipid from wet algae achieves 43.83% reductions in life cycle fossil energy efficiency ratio of algael biodiesel when oil cake drying is not considered.(3) The potential environmental impacts generated in the life cycle of biodiesel produced from algae are quantified. The results show that the most prominent impact category in the life cycle of microalgae based biodiesel is photochemical ozone creation potential(POCP), and the second one is abiotic resources depletion potential(ADP). Except for POCP, algal biodiesel produced from Chlorella vulgaris uner low N condition in open ponds, with CO2 from membrane separation, and oil extracted from wet algae does not offer benefits in all the other environmental impact categories compared to biodiesel produced from dried algae. The higher POCP is mainly caused by the high hexane emission during oil extraction from wet algae. Processes of power, steam and methanol production and biodiesel combustion contribute significantly to the life cycle ADP, GWP (global warming potential), ODP (ozone depletion potential), AP (acidification potential), EP (eutrophication potential), ETP (eco-toxicity potential)and HTP (human toxicity potential) of algae based biodiesel. (4) Without considering transports of feedstock and products, the LCA results of algal biodiesel are compared to fossil diesel and biodiesel derived from soybean oil. Biodiesel produced from Chlorella vulgaris based on all technical routs have high life cycle fossil energy efficiency ratio than fossil diesel. Except for ADP, GWP and ODP, producing and driving with algal biodiesel does not offer benefits in all the other environmental impact categories and life cycle single score compared to fossil diesel. Except for algal biodiesel based on biomass grown in low N medium and open ponds, CO2 from membrane separation, and lipid extracted from wet Chlorella vulgaris, the life cycle fossil energy efficiency ratios of algal biodiesel based on all the other technical routes are lower than soybean based biodiesel. Without pesticide use and making better control of fertilizers fate effectively reduce the life cycle eutrophication and eco-toxicity potentials of algae based biodiesel compared to soybean derived biodiesel. (5) A sensitivity analysis is performed to determine key parameters affecting life cycle fossil energy efficiency ratio and environmental impacts of algae based biodiesel. Algal biodiesel produced from Chlorella vulgaris uner low N condition in open ponds, with CO2 from membrane separation, and oil extracted from dried algae has been taken as the baseline scenario. The algal oil conversion rate, energy content of algae and recovery rate of methanol are found to have the greatest effects on the LCA results of algal biodiesel production, followed by utilization ratio of algal residue, algae cultivation water recycling rate, energy demand for algae drying, capacity of mixing and productivity of algae. |
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