Trends in Biotechnology
OpinionBiodiesel from microalgae beats bioethanol
Section snippets
Crop-derived biodiesel and bioethanol are unsustainable
Carbon neutral renewable liquid fuels are needed to eventually totally displace petroleum-derived transport fuels that contribute to global warming. Biodiesel from oil crops and bioethanol from sugarcane are being produced in increasing amounts as renewable biofuels, but their production in large quantities is not sustainable. An alternative is offered by microalgae.
Microalgae are photosynthetic microorganisms that convert sunlight, water and carbon dioxide to algal biomass. Many microalgae are
An integrated oil-production process
A conceptual process for producing microalgal oils for making biodiesel is shown in Figure 1. The process consists of a microalgal biomass production step that requires light, carbon dioxide, water and inorganic nutrients. The latter are mainly nitrates, phosphates, iron and some trace elements. Sea water supplemented with commercial nitrate and phosphate fertilizers, and a few other micronutrients, is commonly used for growing marine microalgae [5]. Fresh and brackish water from lakes, rivers
Production of microalgal biomass
Producing microalgal biodiesel requires large quantities of algal biomass. To minimize expense, the biomass must be produced using freely available sunlight and is thereby affected by fluctuations such as daily and seasonal variations in light levels. Microalgae can be grown on a large scale in photobioreactors 4, 5, 12, 15, 16, 17, 18, 19. Many different designs of photobioreactors have been developed, but a tubular photobioreactor seems to be most satisfactory for producing algal biomass on
Better than bioethanol
It is useful to compare the potential of microalgal biodiesel with bioethanol from sugarcane, because on an equal energy basis, sugarcane bioethanol can be produced at a price comparable to that of gasoline [35]. Bioethanol is well established for use as a transport fuel [3] and sugarcane is the most productive source of bioethanol [35]. For example, in Brazil, the best bioethanol yield from sugarcane is 7.5 m3 per hectare [35]. However, bioethanol has only ∼64% of the energy content of
Prospects of microalgal biodiesel
Impediments to large-scale culture of microalgae are mainly economic (Box 1). The economics of biodiesel production could be improved by advances in the production technology. Specific outstanding technological issues are efficient methods for recovering the algal biomass from the dilute broths produced in photobioreactors. Furthermore, extraction processes are needed that would enable the recovery of the algal oil from moist biomass pastes without the need for drying.
Algal biomass production
Concluding remarks
As discussed above, microalgal biodiesel is the only renewable biodiesel that has the potential to completely displace liquid transport fuels derived from petroleum. Existing demand for liquid transport fuels could be met sustainably with biodiesel from microalgae, but not with bioethanol from sugarcane. Algal biomass needed for production of large quantities of biodiesel could be grown in photobioreactors, but a rigorous assessment of the economics of production is necessary to establish
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