Opinion
Biodiesel from microalgae beats bioethanol

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Renewable biofuels are needed to displace petroleum-derived transport fuels, which contribute to global warming and are of limited availability. Biodiesel and bioethanol are the two potential renewable fuels that have attracted the most attention. As demonstrated here, biodiesel and bioethanol produced from agricultural crops using existing methods cannot sustainably replace fossil-based transport fuels, but there is an alternative. Biodiesel from microalgae seems to be the only renewable biofuel that has the potential to completely displace petroleum-derived transport fuels without adversely affecting supply of food and other crop products. Most productive oil crops, such as oil palm, do not come close to microalgae in being able to sustainably provide the necessary amounts of biodiesel. Similarly, bioethanol from sugarcane is no match for microalgal biodiesel.

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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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