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Title: Development of microalgal biomass for biodiesel production
Keywords: Microalgae, Biodiesel, Screening, Mixing, Harvesting, Transesterification
Issue Date: 24-Feb-2010
Citation: PROBIR DAS (2010-02-24). Development of microalgal biomass for biodiesel production. ScholarBank@NUS Repository.
Abstract: Amid concern over climate change as a consequence of burning of fossil fuels, coupled with depleting fossil fuel reserves and increasing energy demand, the world is now on a quest for viable, alternative and sustainable fuel sources. Biodiesel from microalgae has the potential to significantly supplement global oil demand for liquid transportation fuels. Nannochloropsis sp., a local marine microalgae strain, was selected as a source of lipid feedstock for production of to fatty acid methyl ester (FAME)i.e biodiesel due to (i) its fast growth rate (specific growth rate = 0.64d1-), (ii) its ability to accumulate intracellular lipid, at up to 15% of its cellular mass, (iii) the ability to enhance lipid accumulation (up to 19% of cell mass) in the presence of a fixed organic carbon source i.e. glycerol; and (iv) and its ability to undergo cell division at elelvated salinity (i.e., 70ppt) levels.. The usual practice of microalgae culture in a photobioreactor (PBR) for biodiesel production is too energy intensive to produce the requisite biomass; but PBR cultures can be used for supplying the inoculum to the large capacity open systems, such as raceway ponds. Blue light emitting diode (LED) illumination at 470nm wavelength resulted in 75% and 40% higher biomass productivity for Nannochloropsis sp. in a PBR compared to red and green LED illumination respectively. Deploying an incremental light intensity (ILI) technique resulted in a 19% energy saving of the energy requirement for illumination of a photoautotrophic culture of Nannochloropsis sp. Using an incremental energy supply (IES,) for mixing the culture inside the PBR, together with the ILI technique, energy demand was reduced by 58.7%. An air sparged assisted coagulation-flocculation (ASACF) technique was developed to harvest both fresh and marine water microalgae. ASACF is at least 11.5 times less energy demanding, and much faster (i.e. entire process takes 10 minutes), than conventional harvesting techniques, and has excellent scalability. Harvested wet microalgae biomass (up to 95% water w/w) requires more heat energy to dry it than its actual calorific energy content. Therefore, a one-step transesterification (OST) process was developed to produce FAME directly from the wet biomass, thus avoiding biomass drying. The OST process also avoids the use of chloroform and yields higher FAME for an acid catalyzed reaction compared to a base catalyzed reaction, where H2SO4 catalyzed OST yielded 19% FAME at 1000C and in 30 minutes.
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