Large quantities of biomass suitable for conversion into oil can be achieved by large scale production of fast growing microalgae. In production of this kind, the harvesting procedure is a crucial step. The most promising procedure is flocculaion followed by either settling or floating at the surface (due to gas bubbles). The chemical procedure has been tested under pilot plant conditions involving sedimentation tanks of 1000 to 1300 l. A local rise of the pH in the surface region of these tanks was sufficient to flocculate the algae in the whole volume. This technique is simple, reliable and economically acceptable. The expenditure corresponds to 5-10 United States cents per kg oil produced by the subsequent conversion process.
THE HARVESTING PROCEDURE IS CRUCIAL IN ANY LARGE SCALE PRODUCTION OF MICROALGAE. THE WELL KNOWN LABORATORY TECHNIQUES (MENBRANE FILTRATION OR CENTRIFUGATION) ARE NOT APPLICABLE AT LARGE SCALE. A PROMISING PROCEDURE IS INDUCED FLOCCULATION FOLLOWED BY EITHER SPONTANEOUS SETTLING OR FLOATING AT THE SURFACE (DUE TO GAS BUBBLES). THE ECONOMIC CONCEPT OF PRODUCING OIL FROM MICROALGAE BY LOW TEMPERATURE CATALYTIC CONVERSION INCLUDES RECYCLING THE RESIDUES OF THE PYROLYSIS WHICH MEANS ABOUT 60% OF THE NITROGEN, ALMOST ALL OF THE PHOSPHORUS AND ABOUT 30-40% OF THE NON-CONVERTED CARBON. BECAUSE OF THIS RECYCLING IT IS NOT POSSIBLE TO USE COMMONLY APPLIED FLOCCULATING AGENTS WHICH CONTAIN ALUMINIUM OR IRON. THESE METALS WOULD ACCUMULATE IN THE ALGAE PONDS. FOR THIS REASON, A PROCEDURE IS BEING APPLIED WHERE FLOCCULATION STARTS FROM SEAWATER-OWN COMPONENTS, A TECHNIQUE WHICH USES FINELY DISPERSED PRECIPITATES OF MAGNESIUM HYDROXIDE FORMED AT ABOUT PH 10 AS COAGULATION CENTRE FOR ALGAE CELLS. AFTER THE CULTURE HAS REACHED PH 9.7 DURING THE DAY, THE PH IS LOCALLY RAISED TO ABOUT 10 WHICH INDUCES THE WANTED FLOCCULATION. WE TESTED AN ELECTROCHEMICAL AND CHEMICAL WAY TO PERFORM THE LOCAL RAISE OF THE PH. IN BOTH METHODS, THE SEDIMENTATION EFFICIENCY AFTER FLOCCULATION TURNED OUT TO BE ABOUT 90%. THE CHEMICAL WAY COSTS APPROXIMATELY 2 US CENTS PER KG OF DRY ALGAE HARVESTED, I.E. ABOUT 8-10 US CENTS PER KG OF OIL PRODUCED. THE COMING EXPERIMENTS ARE AIMED AT FURTHER REDUCING THIS NUMBER.
Funding SchemeCSC - Cost-sharing contracts