Grass Makes Better Ethanol than Corn Does
Midwestern farms prove switchgrass could be the right crop for
producing ethanol to replace gasoline
By David Biello
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."
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