Cellulose is the fiber contained
in leaves, stems, and stalks of plants and
trees – the most abundant organic
compound on earth.
Unlike corn and sugar – the plants
now used to make most ethanol – cellulose
is not used for food, and it can be grown
in all parts of the world. Cellulosic ethanol
is expected to be less expensive and more
energy-efficient than today’s ethanol
because it can be made from low-cost feedstocks,
including sawdust, forest thinnings, waste
paper, grasses, and farm residues (e.g.,
corn stalks, wheat straw, and rice straw).
Switchgrass and other perennial grasses,
in particular, are considered to be promising
sources of cellulosic ethanol. Perennial
grasses are less expensive to produce because
they don’t have to be replanted each
year. Fast-growing woody crops, such as
poplar and willow, are also attractive options
because of harvesting and storage advantages.
Ethanol can be made from cellulose much
as it is today from corn – once the
very tightly bound sugars in the plant fiber
are broken down by enzymes. Accomplishing
this task at low cost has been the principal
obstacle to commercial development.
The enzymes needed to break cellulose down
into fermentable sugars are genetically
improved natural organisms. One such fungus,
Trichoderma reesei, plagued U.S. troops
with jungle rot that “ate” their
clothing in the South Pacific during World
War II.
Another promising source of enzymes is termite
guts. Termites, after all, sustain themselves
by converting woody biomass to sugars.
Thanks to biotechnology, the cost of such
enzymes is dropping rapidly, down 30-fold
in the last five years – to 10-18
cents per gallon of ethanol produced.
• Termites are one of the
planet’s most abundant creatures.
There are more than 2,500 species.
The termites’ digestion process
is fast and efficient – typically
achieving 95% conversion in 24 hours
or less.
• U.S. Patent No. 5,000,000
was awarded to University of Florida
Professor Lonnie Ingram in 1991
for using an E. coli bacterium to
metabolize multiple sugars and make
ethanol.
Acid
can also be used to break down cellulose,
or, alternatively, cellulose can be heated
and turned into a gas that can be converted
into biofuels – even bio-gasoline.
The success of cellulosic ethanol
will depend on how fast the technology is
commercialized and how much it costs compared
to the alternatives. These are some of the
promising recent developments:
• U.S. Secretary of Energy Samuel
Bodman has set a goal of making ethanol
a practical and cost-competitive alternative
by 2012 (at $1.07 a gallon) and displacing
30% of gasoline (60 billion gallons) by
2030.
• A Canadian company, Iogen, which
specializes in industrial enzyme production,
is operating a small pilot plant in Ottawa
and is planning to build a commercial plant
in Idaho, both operating on wheat straw.
• U.S. ethanol producer Broin intends
to convert one of the six corn-to-ethanol
plants that it currently operates in Iowa
into a biorefinery that will use both corn
grain and stover – the stalk, leaves,
and cobs that come with the grain. DuPont
and Novozymes are partners in the project.
• A Spanish company, Abengoa, is building
a cellulosic ethanol plant in Spain, using
wheat straw,
and is planning another unit for York, Neb.,
using corn stover and distillers grains.
• An independent analysis prepared
for the National Commission on Energy Policy
predicts, “Advanced biofuels production
facilities could produce gasoline
alternatives at costs equal to between $0.59
and $0.91 per gallon of gasoline by around
2015.”
The chief advantage of cellulose
is its abundance. Indeed, cellulose is estimated
to make up half of all the organic carbon
on the planet.
In the U.S., ethanol production from corn
is expected to hit a limit of 15 to 20 billion
gallons per year.
Additional feedstocks will be needed to
replace a larger share of gasoline demand,
now running at 140 billion gallons per year.
A
second advantage to processing cellulosic
biomass is that it also contains lignin,
a natural fiber. Lignin can not be converted
to ethanol but can serve as an energy-rich
boiler fuel. There is enough lignin in plants
to provide all the energy needs of an ethanol
production facility, with electricity left
over for sale to the power grid.
Switchgrass
(Panicum virgatum), shown at right,
is a prairie grass native to the United
States known for its hardiness and rapid
growth. It was once part of the tall-grass
prairie that covered most of the Great Plains
and also grew in Alabama and Mississippi.
Because it is native, switchgrass is resistant
to many pests and plant diseases and is
capable of producing high yields with relatively
low applications of fertilizer and other
agricultural chemicals. It is also tolerant
of poor soils, flooding, and drought. Half
the plant’s carbon is stored in its
root system, improving soil quality and
preventing erosion. It is an approved cover
crop for land protected under the federal
Conservation Reserve Program.
President Bush praised switchgrass in his
2006 State of the Union address
for its potential as a biofuel source. It
can yield 6 to 8 tons per acre, compared
to 4 tons per acre for corn, and progressive
breeding could double that yield over time.
The predicted distribution
of switchgrass cultivation at a market
price of $40 per ton.
Other
perennial grasses may be even more productive
than switchgrass, depending on the climate.
Miscanthus, shown at right,
is one such example. Tests in Illinois suggest
that it could yield 11 to 17 tons per acre.
Recent research also suggests that mixed
prairie grasses may be more productive than
monocultures.
In addition to grasses, some fast-growing
trees are prodigious producers of biomass,
including poplar, willow, sweetgum, and
cottonwood.
Switchgrass
(Panicum virgatum), shown at right,
