Algae for biofuels: solving the land-use problem

It’s becoming increasingly obvious that there isn't enough suitable land space to grow crops for food and feed as well as for biofuel, and to retain the forests and other land uses that sequester carbon in huge quantities. As the Nature blog ‘The Great Beyond’ points out, two articles published in Science in February argue that biofuels, especially those derived from agricultural crops (first generation biofuels), actually aren’t that great for the environment because of the required clearance of forests, grasslands, peatlands and savannas to make extra space for biofuel crops. Timothy Searchinger and colleagues argue that a wholesale switch to corn-based ethanol "nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years", whilst Joseph Fargione and colleagues say that converting land to produce food-based biofuels in Brazil, Southeast Asia, and the United States creates a "biofuel carbon debt".

So what’s the solution? We need a source of biofuel that doesn’t require a lot of land to produce it. How about that slimy green stuff we see floating on ponds? For several decades algae have been seen as having great potential as a source of fuel because of their high oil content which can amount to as much as 50% and their ability to grow quickly in sunlight. Now, due to increasing petrol prices government funds are again being channelled into new research on the use of algae as a biofuel and we’re starting to see developments that are making the production of biofuels from algae a commercially feasible proposition.

Compared to biofuels from agricultural crops, the amount of land required would be minimal. Trials in ideal conditions show that fast-growing micro algae can yield 1,800-2,000 gallons/acre/year of oil – compare this with 50 gallons for soyabeans, 130 gallons for rapeseed and ~650 gallons for palm oil. It can grow on fresh or brackish water on marginal land so that it doesn’t compete with areas for agricultural cultivation. As Sean Milmo points out in his article in Oils and Fats International, "oil from algae on 20-40M acres of marginal land would replace the entire US supply of imported oil, leaving 450M acres of fertile soil in the country entirely for food production." Biomass can also be harvested from marine algae blooms and algae can even be cultivated in sewage and water treatment plants.

However, many of the claims about yields are based on peak outputs during the summer period whilst during winter biomass productivity can be >10 times lower than in the summer due to low light, temperature and nutrient levels. The National Renewable Energy Laboratory (NREL) Aquatic Species Program (ASP) in the mid-1990s concluded that the average gross biomass productivity from algae was a maximum of 126 tonnes/acre/year – considerably below the average yields of ordinary topical energy crops. "The biggest challenge facing any algae project appears to be finding a cost-effective way to maintain the organism’s fast growth rates which, with some species, can amount to a doubling of their biomass every 3-4 days," says Milmo. In order to address this problem various photo bioreactors (PBRs) have been developed to enable organisms to grow in consistent light conditions, however these are expensive to build and operate.

But researchers are not giving up - recent advances from R&D projects are beginning to offer opportunities for making the production of biofuels from algae a commercially feasible proposition. Particularly, GreenFuel Technologies Corporation of Massachusetts are developing technologies that recycle CO₂ from the stack gases of power plants, so that it can be used as a nutrient for the growth of algae. The Arizona Public Service Company have trialled the GreenFuel technology with average yields recorded of 98g/m²/day with highs of more than 170g/m²/day. This average was 7 times higher than the maximum average reported by the NREL ASP and was 2 or 3 times higher than average current yields from PBRs.

Looking beyond the development of manufacturing systems, future developments could involve the genetic engineering of the algae itself to produce new, high-yielding strains. NREL has indicated that this could be a new target in its biofuels research programme.

So things are looking up. With further research and development it should soon be economically feasible to produce biofuels from this space-efficient green slime, then we can all get back to using our land for growing food and forests!

References

• Timothy Searchinger, Ralph Heimlich, R. A. Houghton, Fengxia Dong, Amani Elobeid, Jacinto Fabiosa, Simla Tokgoz, Dermot Hayes, and Tun-Hsiang Yu (2008) Use of U.S. Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land-Use Change. Science 319, 1238-1240.
• Joseph Fargione, Jason Hill, David Tilman, Stephen Polasky, and Peter Hawthorne (2008) Land Clearing and the Biofuel Carbon Debt. Science 319, 1235-1238.
• Sean Milmo (2008) From pond to fuel pump. Oils and Fats International 24 (2), 22-23.
• John Sheehan, Terri Dunahay, John Benemann, and Paul Roessler (1998) A Look Back at the U.S. Department of Energy’s Aquatic Species Program: Biodiesel from Algae – Close-Out Report.