Today's crop residues are tomorrow's fuel. The challenge is liberating cellulose's abundant sugars from its carbon-bond grip to ferment into ethanol.

Besides the promise of boosting U.S. energy security, cellulosic-based ethanol is more energy-efficient to produce, produces fewer greenhouse gases and has more plentiful feedstocks than starch-based ethanol. Plant cell walls are the most plentiful biomass resource in the world.

“It's the holy grail…if you can make it work,” says John Felmy, chief economist at the American Petroleum Institute.

Cellulosic biomass is much more difficult than starch to break down into sugars due to the complex structure of plant cell walls. What occurs naturally in ruminant digestion has confounded armies of scientists. They are trying to make cellulose processing more cost-effective. While promising technologies and genetic engineering advances hold promise, they have yet to be commercialized.

Corn-based ethanol is a mature technology not likely to see significant cost reductions, according to the Energy Information Administration. Major cost reductions are possible, however, if cellulose-based feedstocks replace corn. Those feedstocks include corn stover, straw, dried distillers' grains (DDG), perennial grasses, ag and municipal wastes and wood products. (Corn stover is the stalks, cobs and leaves remaining after corn harvest.)

These feedstocks can also someday replace petroleum-based chemicals with renewable biobased ones such as: adhesives, cleaning compounds, detergents, dyes, inks, hydraulic fluids, lubricants, solvents, sorbents, paper, plastic fillers, paints, packaging materials and motor fuels, says Robert Brown, Iowa State University (ISU) professor of mechanical, chemical and biosystems engineering.

Brown helped to start the country's only graduate program in biorenewable resources and ISU's Bioeconomy Initiative. The Initiative has accumulated more than $40 million in related funding and involves more than 60 Iowa State researchers. Its projects include producing hydrogen gas for energy and a bio-based plastic from switchgrass, fermenting sugars into commodity chemicals, producing building materials from natural fibers and turning peanut shells into cooking gas.

Streamlining the enzymatic breakdown of cellulose into fermentable sugars is one challenge to commercializing cellulosic ethanol production. “It takes 100 times more enzymes to digest cellulose compared to starch,” says Kevin Gray, director of alternative fuels for Diversa, which makes specialty enzymes.

“Some people predict that this may require a $1 billion investment in a single plant because of the multiple processes and their complexity,” Brown says. “Which specific technologies will be most feasible is still part of the great unknown.”

How might the transition to cellulosic-based ethanol evolve over the next 25 years?

One scenario envisioned would require 2,000 tons of corn stover delivered daily to a biorefinery. Studies have reported this would require on average a 40-mile trade radius, based on the assumptions that 30% of the land is corn acreage, with 50% farmer participation in biomass production.

At Iowa State University, researchers are evaluating different single-pass corn stover harvesting systems and the associated harvest transportation logistics to achieve this vision. This work is partially funded by a joint USDA/DOE project and also supported by John Deere and Company.

Conventional wisdom has corn stover as the next feedstock, after DDG, for cellulose-based fuels. “Stover is the largest single available biomass at the moment,” says Ken Moore, an ISU agronomist who evaluates various perennial grasses and grains as cellulosic feedstocks. “We could use the same machinery and add one extra harvesting step.”

Projections for this model assume 50-60% removal of corn stover for cellulosic ethanol production, and nearly universal adoption of no-till farming practices to increase stover supplies.

About 30% of corn stover by weight is lignin, the woody material that prevents corn from lodging. It's a hydrocarbon-like compound that can't be fermented. The energy released can help fuel the cellulose refinery, improving the energy equation of cellulosic conversion.

A third step toward a cellulosic-based ethanol industry might involve cultivating dedicated high-biomass yield energy crops, such as switchgrass, miscanthus, hybrid poplars and willows. Each geographic region will ultimately choose the biomass crops best suited to local conditions and economies.

A newly released University of Minnesota study estimates that mixed prairie grasses grown on marginal farmland would yield 51% more energy per acre than corn cultivated on fertile land. The study was led by David Tilman, regents professor of ecology there.

The U.S.'s goal is to replace 30% of U.S. gasoline consumption with biofuels by 2030. A DOE/USDA report finds that 1.3 billion dry tons of cellulosic biomass is feasible to produce annually by 2030 (see feedstockreview.ornl.gov/pdf.billion_ton_vision.pdf). This is considerably above what is required to meet the U.S. goal for biofuels. The year 2030 is when “large-scale bioenergy and biorefinery industries are likely to exist,” the report says. This annual potential represents more than a seven-fold increase in biomass production currently consumed for bioenergy.

Of the billion tons biomass required to fuel biorefineries mid-century, 73% is expected to come from ag lands, the report says. Another 27% will come from forestlands.

Of the biomass expected to come from ag lands, about 43% of the required biomass could come from crop residues (such as corn stover); another 38% from perennial dedicated crops such as grasses and dedicated fast-growing trees; 9% from grains and 11% from animal manures, the report says.

About 60% of the continental U.S. has some potential for growing biomass, the report says. “Currently, 75% of U.S. biomass consumption comes from forestlands (142 million dry tons). By 2030, seven times that amount of biomass will come from ag resources, the report says.

The single largest source of biomass from ag lands is obviously corn stover — about 75 million dry tons per year, the USDA/DOE report says. The current ethanol yields for ethanol derived from stover, straw and sugarcane bagasse (residue) is about 65 gal./dry ton. Thus, a moderately sized 65-75-million-gallon/year cellulosic biorefinery would need one million dry tons per year of feedstock.

Up to 60 million acres of cropland, cropland pasture and conservation acreage will ultimately be converted to perennial crop production, says the DOE/USDA report, once cellulosic biorefineries are commercialized.

Moore's job is figuring how to make this happen agronomically. One challenge is how to incorporate native grasses into a corn-soybean rotation while preserving soil tilth, reducing disease and maximizing biomass yield for ethanol. For example, there are short-term rotations with triticale, or longer-term rotations with switchgrass — perhaps three years corn, three years switchgrass. (Triticale is a winter annual similar to wheat.)

The perennial can offset the environmental impacts of corn, restore organic matter and maintain soil equilibrium. Another biomass rotation involves sweet sorghum, an annual with high levels of sucrose in the stalk which can be immediately converted to ethanol without other processing. “And it still produces a lot of biomass beyond those sugars,” Moore says.

Cellulosic biomass growth, harvest and processing could reconfigure present farming practices and economic incentives. Legions of university agronomists, engineers and economists aim to optimize the migration to cellulose-derived ethanol, chemicals and materials.

How this plays out locally hinges on individual farmers, soils and the policy incentives that evolve to reward stewardship and sustainability. Perennial biomass crops require longer rotations and longer-term commitments than does growing a year of corn or soybeans, says Anne Silvis, a University of Illinois Extension specialist in community and economic development who has studied factors that might lead to farmers' adoption of the biomass perennial crop miscanthus. “These are much more complex decisions,” she says.

From an agronomic perspective, “that will be a huge undertaking to develop a 1-billion-ton supply of biomass,” Moore says. “We're talking about wholesale changes in cropping systems in a relatively short term. As a frame of reference, today's agronomic systems took 70 years to develop.”

Is the vision of cellulosic ethanol a tall order? A pipedream?

“There are some big changes coming, specifically for places with a successful ag economy today,” says Brown. When asked about the feasibility of hauling vast quantities of biomass to refineries, and then transporting the resulting ethanol to distant population centers, he replies: “That's still a lot closer than Saudi Arabia.”

Using The Whole Plant

A handful of commercial and demonstration biorefineries are underway. A biorefinery converts biomass into fuels, power and chemicals currently derived from petroleum, while generating its own electricity and process heat.

Building a cellulosic biorefinery costs roughly two to four times per gallon of capacity what an ethanol refinery does.

In the marketplace, pilot biorefineries are operating to convert corn stover into ethanol. The first such biorefinery (800,000 gal./year) was built in 2004 near Ottawa, Canada by Iogen, which develops enzymes used in the process. Another two pilot cellulosic ethanol plants are under construction, in York, NE, and Salamanca, Spain. The York plant will produce ethanol and feed from corn, DDGs and corn stover blends.

“The commercial plants will probably be located with existing ethanol, forest products or food processing plants,” says John Ashworth, team leader, partnership development with the National Bioenergy Center and the National Renewable Energy Lab (NERL). “We think that the technology to make cellulosic ethanol for $1.07/gal. should be proven on the pilot scale by 2012 and scale up shortly thereafter.”

Presently, cellulosic ethanol costs about $2.26/gal. to produce, based on biomass costs of $53/ton, NERL says.

This figure could improve by releasing sugar streams from DDGs to yield an additional 90 gal. ethanol/ton of DDG, says Mark Emalfarb, CEO of the enzyme company Dyadic International. “If you unlocked 80% of the sugars in the DDG, $2 billion of additional ethanol revenues could be generated without additional corn.”

Broin, the largest U.S. dry mill ethanol producer, has partnered with DuPont and Novozymes to commercialize cellulosic ethanol. One proposed project adds cellulose capacity to an existing Iowa corn ethanol facility to process corn kernels' bran with corn stover. Ethanol yield there is expected to increase by 27% per acre.

A consortium of DuPont, Diversa, Pioneer Hi-Bred International, Deere & Co., Michigan State University and the NREL have collaborated to produce fuels and chemicals from an entire corn plant.