As fossil fuel prices and consumption
both continue rising, the search is on for alternative
fuels. Fuel for vehicles is taking center stage, now
that 67% of U.S. petroleum consumption goes toward fueling
vehicles, according to the U.S. Department of Energy
(DOE). Could biomass energy derived from plant matter
supply a significant percentage of future transportation
fuel? The answer is yes, according to Biomass as Feedstock
for a Bioenergy and Bioproducts Industry: the Technical
Feasibility of a Billion-Ton Annual Supply, a report
funded by the DOE and the U.S. Department of Agriculture
(USDA) and issued by Oak Ridge National Laboratory in
April 2005.
The report defines biomass as all plant-derived
molecules, including grain, starch, sugar,
oil, and waste products, as well as the plant
structural components cellulose,
hemicellulose, and lignin. Fossil fuels and
animal matter are excluded. While the diversity
of resources is a strength
of biomass, it also raises a problem: different
facilities are needed to convert the array
of molecules from biomass
into the hydrocarbons needed for transportation
fuel. Furthermore, large changes in infrastructure
would be needed to harvest the various potential sources.
The report gives an overview of the biomass situation
today in the United States. About 190 million dry tons
of biomass feedstock are consumed annually. Biomass accounts
for about 3% of total U.S. energy supplies, and has recently
surpassed hydropower as the largest renewable energy source.
A great deal of biomass is waste material that is both
produced and consumed by industry. For example, forest-products
firms (including paper companies) use 96 million dry tons
of biomass, largely to power their factories. Currently,
3.4 billion gallons of ethanol are blended into gasoline
each year; that amount could soar to 80 billion gallons
by 2030.
The report states that by 2030, American acreage could
produce enough biomass to displace at least 30% of the
country’s current consumption of petroleum fuels
with some changes in land use and agricultural and forestry
practices, and up to 50% with advanced conversion technologies.
This calculates to up to 1.366 billion tons of biomass
produced annually.
Lynn Wright, who formerly worked in Oak Ridge National
Laboratory’s biomass program and now consults to
it, says the report was a response to a common question
from the energy industry. “We were hearing from people
with connections to oil companies that they hardly considered
it worth their while to think about biomass unless we could
show that it could supply as much as a billion dry tons
per year,” she says.
The Billion-Ton report does not attempt to assess
the economic viability of large-scale biomass energy, and
Wright explains that it’s difficult at best to predict
the relative price of fossil fuels and the various sources
of biomass in 25 years. Today, she says, when prices are
measured by energy content (British thermal units), energy
from switchgrass or corn stover, the residue that remains
in the field after a grain crop is harvested, is cheaper
than energy from oil but more expensive than energy from
coal.
Boosting Production
According to the report, wood could grow to supply 368
million dry tons of biomass by 2030. The supply could expand
with enhanced collection of urban tree trimmings and construction
waste, and greater efforts to prevent forest fires by clearing
deadwood from forests.
But transportation and processing costs may keep wood
expensive, cautions report coauthor Bryce Stokes, program
leader of vegetation management and protection research
at the USDA Forest Service. He says, “We still have
to overcome some economic and conversion efficiency barriers
. . . to make wood competitive” in transportation
fuels. Woody biomass will seem more competitive, he adds,
if the benefits of improving forest heath, reducing fire
risk, and recycling carbon from the atmosphere are held
in view.
Farms could potentially contribute a far larger quantity
of biomass (998 million dry tons), and much of that may
come from corn stover and perennial crops managed with
no-till production techniques and collected with advanced
harvesting equipment. However, Wally Wilhelm, a plant physiologist
with the USDA Agricultural Research Service, says it is
unlikely that all land will ever be switched to no tillage. “Use
of no-tillage methods and producing crops without tillage
is far more complex than simply not passing over the field
with a plow or disk,” he explains. “It takes
time and skill, and trial and error, to become proficient
at no-till farming. Not all farmers are willing, nor have
the flexibilitythe money in the bankto pursue
the knowledge and skill. ”
Corn grain is currently the source of most of the ethanol
used as motor fuel in the United States today. Corn production
has been growing by 1.7 bushels per acre per year for 30
to 40 years, says Achim Dobermann, a professor of soil
science and nutrient management at the University of Nebraska.
Dobermann says irrigated cornfields in Nebraska could produce
250 to 350 bushels per acre.
These yields require intensive inputs, especially in
the form of nitrogen fertilizer, which is usually derived
from natural gas. Farmers already manage nitrogen closely,
due to its price and potential for polluting groundwater,
but ever-higher yields will force them to work even harder
to carefully manage nitrogen. “It requires a more
fine-tuned type of management,” says Dobermann. “You
can’t just go in and apply anhydrous ammonia [a common
nitrogen fertilizer] in the fall and take off for vacation.” Instead,
he suggests multiple nitrogen applications, timed and placed
when and where the crop needs it.
More Mass, Sustainably
If biomass harvesting is to be sustainable, it must not
diminish soil’s fertility (its ability to supply
nutrients for plant growth) or other properties influencing
productivity. A market for stover creates an incentive
for farmers to remove more after the harvest. But crop
residue left in the fields reduces soil erosion; it also
improves soil fertility and structure through the addition
of organic carbon, which fuels microbial activity that
drives the cycling of nutrients and structures in productive
lands.
Estimates of how much stover must remain on the fields
if erosion is to be controlled rely on the concept of a
tolerable amount of soil loss, as defined for particular
soils by the USDA Natural Resource Conservation Service.
But this amounts to “an educated guess,” says
Wilhelm. “The assumption is that if we keep losses
below the tolerable level, we should not notice a significant
impact on productivity.” Erosion is affected by farming
practices, soil types, and weather, and Wilhelm says it’s “a
very good question” whether it’s possible to
predict what level of stover removal will hold soil organic
carbon loss below tolerable levels.
Because crop residue is converted into organic matter
that maintains soil structure, Wilhelm says levels of organic
matter may be a good metric of soil health and the amount
of stover that must be retained on or in the soil to sustain
productivity. Wilhelm, who is leading a project to develop
guidelines for sustainable removal of corn stover, says
extensive biomass extraction raises the danger that soil
organic carbon will be “mined” rather than
be treated as the irreplaceable resource that it is. He
adds that stakeholders must work together to develop systems
that enhance the use of renewable sources of energy and
produce renewable energy in a sustainable manner.
A Question of Impact
One of the key arguments over biomass energy concerns
the net energy contribution of biomasshow much energy
is gained from the crop. For example, David Pimentel, a
professor of ecology and agriculture at Cornell University,
and Tad Patzek, a professor of civil and environmental
engineering at the University of California, Berkeley,
published calculations in the March 2005 issue of Natural
Resources Research showing that ethanol derived from
corn contains only 71% of the energy used to grow, harvest,
and convert the grain into ethanol. At the other end of
the spectrum, calculations by federal researchers Hosein
Shapouri, James A. Duffield, and Michael Wang in the July
2002 Agricultural Economic Report Number 814 showed
a net energy gain of up to 130-140%.
Pimentel and Patzek based their calculations on average
U.S. corn production output for 2003140 bushels
per acre. As to the assertions put forth in the Billion-Ton report,
Pimentel contends that providing enough biomass to cover
30% of current U.S. gasoline and diesel use would require
a land area greater than that of the United States. He
believes the actual U.S. biomass capacity is about half
the 1.366 billion tons cited in the report.
But calculating net energy efficiency is difficult, says
Robert Anex, an associate professor of agricultural and
biosystems engineering at Iowa State University who studies
life-cycle assessments of biomass resources. “One
must account for all of the resources that are used, all
of the product created, and also those resources that are
saved via substitution of the biomass product for some
other probably petroleum-based product,” he says. “This
involves many assumptions about how crops are grown, harvested,
and converted, but also what resource use is avoided.” This,
he says, is why these sorts of measures are often contentious.
Biomass advocates such as Thomas Foust, biomass program
technology manager at the DOE National Renewable Energy
Laboratory, say more biomass should become available if
agricultural productivity continues its steady rise and
improvements in conversion technologies are madefor
example, ethanol production per bushel of corn has grown
by about 25% in the past 25 years. Furthermore, Foust says
that net energy balances for ethanol are not that useful,
and the real metric should be imported oil displacement,
which can be as high as 6 to 1 for ethanol.
The environmental health impact of gathering 1 billion
tons of biomass through whatever meansa plan that
could affect hundreds of millions of acresmust also
be investigated thoroughly. For example, the impact of
harvesting biomass from millions of acres of farmland now
set aside under the USDA Conservation Reserve Program remains
to be studied. One higher-production scenario in the Billion-Ton report
assumes that 60 million acres would be shifted from a combination
of Conservation Reserve Program land, pasture land, and
commodity crop production in order to produce woody and
grass crops as a source of biomass. The land used to produce
wood and grass crops would provide bird and mammal habitat
similar to the Conservation Reserve Program but would be
harvested more frequently.
Donald Waller, a professor of botany and environmental
studies at the University of Wisconsin-Madison, raises
other questions about the impact of boosted biomass energy
production on forest health. While noting that the report
does not call for building roads in roadless forests or
removing biomass from wilderness areas, he warns of broad
ecological consequences from removing massive amounts of
tree biomass and thus essential nutrients. “In most
forests, the old growth is dominated by decomposers in
terms of species number and complexity,” he says. “Deadwood
is there in far greater quantity than live wood. ”
Waller emphasizes the importance of leaving behind “biological
legacies”standing dead trees, live trees,
and tree material on the ground. “You can’t
take it all away without seriously diminishing the ecosystem
functions and the plant and animals that live there,” he
says.
Achieving Critical Mass
In the face of tight fossil fuel supplies, the federal
government is moving ahead with plans to expand biomass
output. The National Renewable Energy Laboratory, for example,
has a considerable effort working to improve biomass conversion
into liquid fuel. Wright suggests putting more effort into
pilot projects that use large amounts of biomass. “I
think it would help a great deal to get some demonstrations
in place on the part of farmers and power producers,” she
says.
Government subsidies similar to the tax credits already
offered to build wind power towers and install solar energy
panels may be another way to enhance the appeal of biomass.
In the 17 October 2005 issue of Newsweek, Frances
Beinecke, executive director and incoming president of
the Natural Resources Defense Council, says, “We
think subsidies or assistance from the federal government
should go to the new technologies that need to come to
the market. . . . Biofuels are definitely part of the renewables
portfolio. There’s growing interest in the agricultural
sector, because that way we could have home-grown fuels. ”
The energy business may be at a turning point. After
years of concern about funding levels for alternative energy
research, the prices of oil and natural gas have changed
the equation, says Wright: “If prices stay high,
I don’t think the government will have to do very
much [to jumpstart the biomass bandwagon].”
David J. Tenenbaum |