GRASS GAS: Turning fields of switchgrass like this one in northeastern Nebraska into ethanol produces 540 percent more energy than the amount consumed growing the native perennial.
COURTESY OF USDA-ARS
Farmers in Nebraska and the Dakotas brought the U.S. closer to becoming a biofuel economy, planting huge tracts of land for the first time with switchgrass—a native North American perennial grass (Panicum virgatum) that often grows on the borders of cropland naturally—and proving that it can deliver more than five times more energy than it takes to grow it.
Working with the U.S. Department of Agriculture (USDA), the farmers tracked the seed used to establish the plant, fertilizer used to boost its growth, fuel used to farm it, overall rainfall and the amount of grass ultimately harvested for five years on fields ranging from seven to 23 acres in size (three to nine hectares).
Once established, the fields yielded from 5.2 to 11.1 metric tons of grass bales per hectare, depending on rainfall, says USDA plant scientist Ken Vogel. "It fluctuates with the timing of the precipitation,'' he says. "Switchgrass needs most of its moisture in spring and midsummer. If you get fall rains, it's not going to do that year's crops much good."
But yields from a grass that only needs to be planted once would deliver an average of 13.1 megajoules of energy as ethanol for every megajoule of petroleum consumed—in the form of nitrogen fertilizers or diesel for tractors—growing them. "It's a prediction because right now there are no biorefineries built that handle cellulosic material" like that which switchgrass provides, Vogel notes. "We're pretty confident the ethanol yield is pretty close." This means that switchgrass ethanol delivers 540 percent of the energy used to produce it, compared with just roughly 25 percent more energy returned by corn-based ethanol according to the most optimistic studies.
The U.S. Department of Energy (DOE) is partially funding the construction of six such cellulosic biorefineries, estimated to cost a total of $1.2 billion. The first to be built will be the Range Fuels Biorefinery in Soperton, Ga., which will process wood waste from the timber industry into biofuels and chemicals. The DOE is providing an initial $50 million to start construction.
"Cost competitive, energy responsible cellulosic ethanol made from switchgrass or from forestry waste like sawdust and wood chips requires a more complex refining process but it's worth the investment," Energy Secretary Samuel Bodman said at the Range Fuels facility groundbreaking in November. "Cellulosic ethanol contains more net energy and emits significantly fewer greenhouse gases than ethanol made from corn."
In fact, Vogel and his team report this week in Proceedings of the National Academy of Sciences USA that switchgrass will store enough carbon in its relatively permanent root system to offset 94 percent of the greenhouse gases emitted both to cultivate it and from the derived ethanol burned by vehicles. Of course, this estimate also relies on using the leftover parts of the grass itself as fuel for the biorefinery. "The lignin in the plant cell walls can be burned," Vogel says.
The use of native prairie grasses is meant to avoid some of the other risks associated with biofuels such as reduced diversity of local animal life and displacing food crops with fuel crops. "This is an energy crop that can be grown on marginal land," Vogel argues, such as the more than 35 million acres (14.2 million hectares) of marginal land that farmers are currently paid not to plant under the terms of USDA's Conservation Reserve Program.
But even a native prairie grass needs a helping hand from scientists and farmers to deliver the yields necessary to help ethanol become a viable alternative to petroleum-derived gasoline, Vogel argues. "To really maximize their yield potential, you need to provide nitrogen fertilization," he says, as well as improved breeding techniques and genetic strains. "Low input systems are just not going to be able to get the energy per acre needed to provide feed, fuel and fiber."
