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The Cellulose Quandary

By: Dennis Keeney

Cellulose-based ethanol may be a bust. Ethanol produced from switchgrass is proving very different. Much will depend on innovations in ethanol production from corn stover and corn cobs.

In the race to find new sources of energy, ethanol and biodiesel got the upper hand. These biofuels are easily produced—ethanol from corn grain, and biodiesel from soybean oil—and a combination of subsidies, import controls, and legislation further pushed ethanol to the front.*

But in the last few years, legislation has mandated which types of ethanol can be used to meet renewable fuels standards (RFS). While corn-based ethanol is easily produced, there are problems with using so much corn to produce fuel. Ethanol produced from cellulose seemed like a good alternative to corn ethanol. Cellulosic ethanol is produced from plant residues and woody materials, and as we realized that corn grain availability would be limited, and that plant residues and woody materials were in excess, we began to look toward cellulose-based ethanol as a solution.

As we’ve mentioned in this series already, in 2007 the RFS program was expanded to include mandates regarding diesel. Also, importantly, the legislation increased the mandate of renewable fuel (mostly ethanol) from 9 billion gallons in 2008 to 36 billion gallons by 2022. But for this renewable fuel mandate of 36 gallons by 2022, the corn ethanol contribution to this number was capped: by 2015, corn ethanol could contribute a maximum of 15 billion gallons. That cap on corn ethanol leaves a gap of 21 billion gallons that must come from cellulose. By 2012, 500 million gallons of ethanol were to be produced from cellulose. **

Alas, the renewable fuels requirement for cellulose-based ethanol is not to be realized in the foreseeable future.

“Cellulose” is the term for the plant material in grasses, crop residues, and woody plants. (Plant cells are structurally composed of cellulose, hemicellulose, and lignin.) Cellulose, as biologists and plant physiologists know, is built to last. The structure of the plant protects the starches buried inside the stems and leaves, and the lignin, which gives the plant strength, is hard to digest. Strong acids or special enzymes are needed to break these down so that the fermentable starch can be released. This adds a double whammy to the production of cellulose-based ethanol. And plant residues have special handling problems. They must be dried and stored, and are low in density, making them expensive to move from the field to the ethanol refinery.

The result of this cellulose quandary is that the production of cellulose-based ethanol has been very difficult and expensive. While research will certainly cut the production costs, the new plants that must be built to reach production scale economics are just being built. The down economy makes these plants risky, and private investors have not stepped up to the plate. The government has provided some support and investment, but the limited federal funds can go only so far. The U.S. EPA has lowered the requirement for this year to only 6.5 million gallons in recognition that production of cellulose biofuels is not proceeding according to the preset plans. This is a far cry from the original 500 million gallons.

The handling problems for low-density bulk materials are also proving difficult. Switchgrass, a native plant that grows well, especially in southern moist climates, was early on touted as the primary source of biomass for cellulose-based ethanol. But it has proven difficult to harvest and dry. Current cellulose biomass ethanol startup refineries are turning to corn stover. Iowa State, along with equipment manufacturers, are working on innovative ways to handle the stover.

Corn stover presents its own problems, particularly the concern that removing too much will deplete the soil of organic matter and nutrients, lower soil quality, and increase soil erosion and runoff. Studies show that around 20 percent of the stover could be removed without damage to the environment. Since corn stover is superior to other cellulose materials when it comes to collecting and handling, it appears to be the biomass of choice. This is unfortunate, because perennial plants, and especially grasses, offer many environmental benefits such as controlling soil erosion, adding organic matter to soils, capturing carbon, and increasing biological diversity.

Ethanol production takes considerable water, primarily lost as water vapor. About 150 million gallons of water are evaporated in the production of 55 million gallons of ethanol. We do not know if more water will be needed to produce ethanol from cellulose, but the extra steps will be water-intensive.

POET, a major ethanol company, is building a plant in northwest Iowa that would use corn cobs. And DuPont, the chemical and seed giant, will break ground next June for a plant just west of the current Lincolnway Energy plant near Ames, Iowa. This plant will cost several times more than the equivalent corn ethanol plant. The DuPont plant may well chart the future of cellulosic ethanol. It uses newly developed enzymes to break down the cellulose before fermentation to ethanol. This must be done carefully since products from the cellulose hydrolysis can interfere with fermentation to ethanol. The POET plant appears to be on hold until Federal funds and loan guarantees are available.

Cellulosic ethanol may not make the cut. Technical issues are major, and the public mood for ethanol subsidies has turned sour. A good idea pushed to the background by expediency to meet ethanol markets with corn has perhaps done cellulose ethanol in.

*The production of ethanol and biodiesel from corn and soy was further encouraged through government subsidies, controls on imports from other countries (especially Brazil), and the establishment of the renewable fuels standard (RFS) for the amount of renewable fuels that must used in the U. S. Further, state and federal subsidies to blenders of ethanol, and financial support for construction of facilities, ensured the rapid rise in ethanol production.

**By the time the 2015 requirement of 15 billion gallons of corn ethanol is reached, over 45 percent of the U.S. corn production will be assigned to ethanol. Even so, this is less than 5 percent of the liquid fuels used, on an energy basis, in the U.S.

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