(125c) Engineering Aspects of Open-Pond Algae Farming

Farming of micro-algae has the potential to provide a significant fraction of the transportation fuels for the entire country because of its high areal productivity (> 1,000 gallons per acre annually) and its ability to use land not suitable for row crops. Open ponds, particularly in the Southeast U.S., will likely be the only cost and energy effective means of production for algal biofuels feedstocks for the foreseeable future, since production costs will be constrained to around 20 cents per pound to compete with a $4 price at the pump, and since the energy budget will be limited to a small fraction of the ~ 6 Wh/g energy content of the algae. The engineering design of an algae farm must include a front end process which provides the nutrients, the high rate (20 grams per square meter per day) ponds, and a back end process which harvests the algae and produces a liquid transportation fuel. Small scale algae farming could employ animal litter as a source of nutrients via anaerobic digesters . Widespread algae farming would require concentrated carbon dioxide from carbon capture, since algae are 50% carbon and since air-to-pond transport of atmospheric carbon dioxide would be far too slow for high-rate ponds. In-ground bubble columns would be an efficient means of carbonating the pond water, and the pumps in them would provide the 10,000 gpm flowrates needed to circulate pond water in 5 acre ponds, thereby obviating the need for expensive paddlewheels. Harvesting micro-algae, at pond concentrations of a few hundred ppm, and concentrating them to 1-5 weight per cent for downstream processing, remains one of the key challenges. We are currently using ferrous ions in combination with cellulose or synthetic fibers to flocculate the algae for belt, or belt filter press, dewatering. Cost constraints require separation and recovery of the flocculation chemicals for re-use, and we have two options for this at present. Conversion of the algal biomass to fuels feedstocks is generally focussed on rendering the lipids for bio-diesel production. However, we believe that a better alternative would be anaerobic digestion of the entire algal biomass to methane, with the carbon dioxide co-product being returned to the ponds via bubble columns. The methane would be purified for injection in natural gas pipelines, or liquefied to LNG for truck transport. The overall scheme would therefore be: carbon dioxide in, methane out, via photosynthesis and anaerobic digestion. Development of algae farming, historically done by the life sciences, will require a highly interdisciplinary approach involving many areas of life sciences and engineering, especially chemical engineering, where we will be able to employ, well within our comfort zones, our training and experience in chemical process design.