Harvesting natural algal blooms for concurrent biofuel production and hypoxia mitigation

Hypoxia;Eutrophication;Algae;Biofuel

Kuo, Chih-Ting ; Schideman ; Lance C.

关键词: Hypoxia;    Eutrophication;    Algae;    Biofuel;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/18541/Kuo_Chih-Ting.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

This study assesses the net energy balance and economic benefits of harvesting detrimental environmental algal blooms and conversion of the harvested biomass into biofuels. An engineering model was developed to compare the energy efficiency of different harvesting methods and biofuel conversion techniques. The modeling data was largely compiled from a variety of literature sources, but was also supplemented by some original experimental data as necessary. The recurring algal blooms that lead to the hypoxic zone in the Northern Gulf of Mexico was used as a case study situation. Three different harvesting techniques: plankton net trawling, traveling screen, and screw pump filtration, were compared in terms of energy consumption and harvesting efficiency. Among the various conditions modeled, the most favorable harvesting condition was produced by a 750 kW fishing boat with a plankton trawling net for harvesting algae biomass at 0.5 m/s harvest speed and harvesting from the surface to 0.5 m depth in the ocean. When harvesting a highly eutrophic area (40 mg-chlorophyll/m3) under these conditions, we estimate a plankton net trawling operation can collect 100 kg of dry algal biomass with 1 GJ of harvesting energetic consumption.Four different biofuel conversion processes, hydrothermal liquefaction (HTL), anaerobic digestion (AD), transesterification and fermentation, were compared in the model. Hydrothermal liquefaction and anaerobic digestion are generally more favorable in terms of energy production because they process a larger portion of the algal cell and require less dewatering. The dewatering energy required for transesterification and fermentation leads to a negative energy balance for these processes. Integrated conversion processes of transesterification combined with AD or HTL were also considered, but these combinations still produced negative energy balances. Overall, the energetic analysis revealed that the entire harvesting and conversion process can achieve an energy “break-even point” if the chlorophyll concentration is above 55 mg/m3. To evaluate potential improvements in energy efficiency, basic surface harvesting was compared with vertical focusing of algal biomass. Our estimates indicate that basic surface harvesting technology can harvest 1596 metric tons in 3 months and reduce the net cost of $93,357,016. Vertical focusing potentially could increase the harvest to 23,313 metric tons over 3 months and reduce the net cost to for $81,380,937. This analysis showed that vertical focusing technology can offset 98% of harvesting energy in April. Environmental analysis showed that harvesting natural occurring algae in Atchafalaya and Mississippi delta once a month reduces nitrogen flows into the Gulf of Mexico by 0.5%.