Published online 7 January 2008 | Nature | doi:10.1038/news.2008.415

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Prairie grass energy boost studied in the field

Using switchgrass as a biofuel yields five times more energy than is used to grow it.

Do prairie grasses hold the future of biofuels?SPL

Switchgrass, a prairie grass that sways around the borders of many US fields, offers 540% more energy than the energy sown into it, research has shown. The renewable fuel should be seriously considered as a low-greenhouse-gas, high-energy biofuel source, the researchers say.

Kenneth Vogel at the US Department of Agriculture and the University of Nebraska, Lincoln, and his colleagues did the first large-scale field test of the grass by studying its growth on the borders of 10 farms in Dakota. The farmers noted how much seed, fertilizer and fuel they used, how often it rained and how much grass they harvested over five years. Farms in different areas produced different amounts of switchgrass, mostly as a consequence of different amounts of rainfall.

Vogel used this information to estimate how much bioethanol could be produced from the grass, using information from current corn-ethanol plant technologies and small-scale studies on switchgrass conversion. He estimated that, on average, 60 gigajoules of energy per hectare could be produced per year if the switchgrass were turned into bioethanol. The net energy gain from the switchgrass was 540% — in the middle of previous predictions of 300-700%.

Soya bean biodiesel, in contrast, returns 93% more energy than is used to produce it, whereas corn grain ethanol currently provides only 25% more energy.

Greenhouse-gas emissions from the switchgrass would be 94% lower than emissions from petrol, they calculate — that's nearly, but not quite, carbon neutral. Vogel's findings are published in the Proceedings of the National Academy of Sciences1.

Field of dreams

The benefit of using prairie grasses and other crops as renewable fuels is widely debated — especially because of the competition for land that could otherwise be used to grow food. But Vogel is not intending to replace corn or other crops with switchgrass. “We are developing switchgrass for use on marginal cropland in the US. We do not expect it to replace corn or other grain or oilseed crops on the best land,” he says.

Switchgrass only needs to be planted once, after which it returns year on year. And its extensive underground root system locks carbon in the soil. Others have previously estimated that this might produce a net carbon sink after use as a fuel, although Vogel finds it to be a slight carbon source.

Rainer Zah, head of the Life Cycle Assessment & Modelling group of the Swiss Materials Science and Technology research institution, EMPA, in Saint Gallen, is more concerned about another greenhouse gas, namely dinitrogen oxide, which is much more potent than carbon dioxide. If this gas is not properly modelled, the greenhouse benefits of switchgrass could be wildly overestimated, he says.

Zah agrees that switchgrass holds promise as a fuel, but notes that there is some way to go before the energy yield estimated by Vogel can be realized.

Switchgrass locks its carbon in as woody lignin and cellulose, which needs to be broken down to sugar and starch before it can be used as a fuel. The facilities needed to do this currently exist only on small scales for pilot projects. “The technology to make full use of all the carbon in switchgrass is not yet established,” says Zah.

Scaling up

Until now, only lab-scale trials have been used to provide estimates of the amounts of energy that could be squeezed from prairie grasses. “Large-scale field trials are difficult to set up, establish and manage, particularly for an extended period of time in a large geographical area,” says Vogel. This is mainly because of the difficulty of using arable land to grow a crop for which there is, as yet, no real market.

The 540% net energy figure could be increased further in the future, with land-management and breeding improvements, says Vogel, and with advances in cellulosic-ethanol production techniques. Zah agrees that more progress on the technology that gets energy from grass will be made in the coming years, but adds that switchgrass is only one of a range of viable options for biofuel feedstocks, others including algae and jatropha.

Vogel has also looked at the financial aspects of using switchgrass as a fuel feedstock, the results of which will be published in a separate paper. “The fiscal economics of growing switchgrass as a biomass energy crop are also positive,” Vogel says. 

  • References

    1. Schmer, M. R., Vogel, K. P., Mitchell, R. B. & Perrin, R. K. Proc. Natl Acad. Sci. USA 105, 464-469 (2008).
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