CH391L/S13/Algal Biofuels

Algal biofuels represent various methods to produce highly reduced hydrocarbons from carbon dioxide using solar energy as the power source and algae, typically microalgae, as the machinery. The primary driving point for photosynthesis powered biofuels is that they do not increase the net carbon content in the air; every molecule of CO₂ released during combustion came from one molecule of CO₂ fixed during photosynthesis. Since their energy source is sunlight, algal biofuels are considered renewable. Algae are top choices in biofuel engineering due to their far greater photosynthetic efficiency and low growth requirements. In addition, countries that lack reserves of fossil fuels may desire economic independence by reducing imports through domestic fuel product.

Advantages over other Biofuels

Microalgae tend to have better photosynthetic efficiency than terrestrial plants, including corn and soybeans, the other major sources for biofuels[1]. While sugarcane, a major source of bioethanol, has a photosynthetic efficiency greater than most microalgae, it's land usage is still far greater than that of algae[2].

Cultivation

A Raceway Pond aquaculture. This method of aquaculture allows for good mixing and light exposure at low cost, although gas exchange, temperature control and sterility are poor[3].

One of the most distinct advantages of microalgae over other photosynthetic organisms is their tolerance and/or preference for marginal water sources[3]. This means that algal biofuels won't have to compete for resrouces like fresh water and arable land with food crops, unlike biofuels derived from corn and soybeans[4]. Most commercially relevant algal species grow well on either seawater or wastewater, both of which are inexpensive alternatives to freshwater[5].

For economical reasons, the majority of algal aquacultures today are open-air[3]. While this normally vastly opens up the risk for contamination, several of the most important algae, such as Chlorella, Spirulina and Dunaliella have traits that allow them to outgrow competitors under certain optimal conditions. That said, the future of algal biofuel reactors is likely to take place under closed conditions. Maximized efficiency in biofuel production is highly dependent on non-contamination and the absence of certain heavy metal pollutants that would be present in open aquacultures.

Summary of various Aquaculture methods[3].

iGEM Connection

The University of Washington's 2011 iGem Team attempted to use Fatty Acid intermediates into alkanes [6]. They used Acy-ACP Reductase (AAR) to convert long Acyl-ACPs into aldehydes, and then used Aldehyde Decarbonylase (ADC) to convert them into alkanes.

References

1. Biodiesel production—current state of the art and challenges

2. [Yusuf Chisti, Biodiesel from microalgae beats bioethanol, Trends in Biotechnology, Volume 26, Issue 3, March 2008, Pages 126-131, ISSN 0167-7799, 10.1016/j.tibtech.2007.12.002. Biodiesel from microalgae beats bioethanol]

4. DOE biofuel datasheet

5. Microalgal Reactors: A Review of Enclosed System Designs and Performances

6. UW Diesel Production

7. Commercial production of microalgae: ponds, tanks, tubes and fermenters