ABSTRACT
There is currently great interest in using microalgae for the production of biofuels, mainly due to the fact that microalgae can produce biofuels at a much higher productivity than conventional plants and that they can be cultivated using water, in particular seawater, and land not competing for resources with conventional agriculture. However, at present such microalgae-based technologies are not yet developed and the economics of such processes are uncertain. The production of microalgae biomass has been described in many publications, therefore we limit our review here on the various types of biofuels and bioenergy that can be generated from microalgal biomass, emphasizing the genus Dunaliella. We review power generation by direct combustion, production of hydrogen and other fuel gases and liquids by gasification and pyrolysis, methane generation by anaerobic digestion, ethanol fermentations, and hydrogen production by dark and light-driven metabolism. We in particular discuss the production of lipids, vegetable oils and hydrocarbons, which could be converted to biodiesel. Direct combustion for power generation has two major disadvantages in that the high N-content of algal biomass causes unacceptably high NOx emissions and losses of nitrogen fertilizer. Thus, the use of sun-dried microalgal biomass would not be cost-competitive with other solid fuels such as coal and wood. Thermochemical conversion processes such as gasification and pyrolysis have been successfully demonstrated in the laboratory but will be difficult to scale up commercially and suffer from similar, though sometimes not as stringent, limitations as combustion. Anaerobic digestion of microalgal cells yields only about 0.3 L methane per g volatile solids destroyed, about half of the maximum achievable, but yields can be increased by adding carbon rich substrates to circumvent ammonia toxicity caused by the N-rich algal biomass. Anaerobic digestion would be best suited for the treatment of algal biomass waste after value-added products have been separated. Algae can also be grown to accumulate starches or similar fermentable products, and ethanol or similar (e.g., butanol) fermentations could be applied to such biomass, but research is required on increasing solvent yields. Dark fermentation of algal biomass can also produce hydrogen, but, as with other fermentations, only at low yields. Hydrogen can also be generated by algae in the light; however; this process has not yet been demonstrated in any way that could be scaled up and, in any event, Dunaliella, is not known to produce hydrogen. In response to nutrient deficiency (nitrogen or silicon), some microalgae accumulate neutral lipids which, after physical extraction, could be converted, via transesterification with methanol, to biodiesel. Nitrogen-limitation does not appear to increase either cellular lipid content or lipid productivity in Dunaliella. Results from life cycle energy analyses indicate that cultivation of microalgal biomass in open raceway ponds has a positive energy output ratio (EOR), approaching up to 10 (i.e., the caloric energy output from the algae is 10 times greater than the fossil energy inputs), but EOR is less than 1 for biomass grown in engineered photobioreactors. Thus, from both an energetic as well as economic perspective, only open ponds systems can be considered. Significant long-term R&D will be required to make microalgal biofuels processes economically competitive. Specifically, future research should focus on (a) the improvement of biomass productivities (i.e., maximizing solar conversion efficiencies), (b) the selection and isolation of algal strains that can be mass cultured and maintained stably for long periods, (c) the production of algal biomass with a high content of lipids, carbohydrates, and co-products, at high productivity, (d) the low-cost harvesting of the biomass, and (e) the extraction and conversion processes to actually derive the biofuels. For Dunaliella specifically, the highest potential is in the co-production of biofuels with high-value animal feeds based on their carotenoid content.
