The Influence of Energy Efficient Cultivation on the Chemical Composition of Algae Biomass for Biodiesel Production

The interest in finding renewable alternative to petroleum-based fuel has increased significantly during the last decades due to global warming and depletion of fossil oil sources. One of the most promising renewable alternatives to petroleum is microalgal biodiesel. Microalgae are advantageous because of significantly higher areal productivity compared to terrestrial plants. However, the main drawback that has hindered the microalgal biodiesel to establish a rigid market position is the energy consumption in the processing steps, which currently is larger than the potential output energy. Life cycle analyses show that the harvesting step may contribute up to 20-30 % of the biomass cost. The overall objective of this research was to investigate efficient ways of harvesting microalgae as well as growing them. The primary aim was to study how the chemical composition of microalgae changes in situation where they are partially degraded and have sedimented to the bottom of the cultivation pond compared with microalgae harvested in the water column. If the sedimented material marginally differs in energy content compared to fresh biomass, there is a great potential in energy savings by harvesting less frequently. This research project included the determination of the microalgal productivity as well as characterization of two batches of the microalgal strain Scenedesmus simris002 that were cultivated in circular, open ponds at SP Technical Research Institute of Sweden, Boras . The characteristics that were of main interest was to study how the lipid content as well as the composition of fatty acid methyl esters (FAME) differed between samples harvested at the bottom and in the water column. In addition to these tests, analyses of the elemental composition, concentration of chlorophyll and calorific heating values were also examined. As a complement to the experimental data, theoretical models were constructed to find ways of supplying carbon dioxide, ensure appropriate mixing and harvesting in order to increase the annual productivity at the same time as the energy consumption was minimised. Without any nutrient and pH control, the biomass productivity in cultures was at least 24 % of the theoretical maximum considering the environmental conditions during the cultivation. The lipid content ranged between 16-32 wt. %. From the characterization of fatty acid methyl ester, it was concluded that palmitic acid methyl esters (C16:0-Me) and oleic acid methyl ester (C18:1-Me) were the most abundant fatty acids in the algae investigated in the first and second batch, respectively. The calorific heating value was at most 24.5 MJ kg-1 for the sedimented algae, i.e. comparable with the energy content of fossil coal (17-27 MJ kg-1). The experimental study also confirmed that chlorophyll is the first component that is degraded, but did not have a significant effect on the energy content of the microalgae. The theoretical models emphasized the importance of injecting higher concentration of carbon dioxide in order to improve the productivity as well as decreasing the energy consumption. Despite somewhat lower amounts of lipids in the samples harvested at the bottom, it would be advantageous to harvest less frequent by only transferring the sediment to drying ponds.