United States

Department

of Agriculture

New Technologies in

Ethanol Production

C. Matthew Rendleman and Hosein Shapouri

Agricultural

Economic

Report

Number 842 

Office of 

the Chief 

Economist

February 2007

Office of 

Energy Policy

and New Uses

Abstract

Fuel ethanol production has increased steadily in the United States since the 1980s,

when it was given impetus by the need to reduce energy dependence on foreign

supplies. The momentum has continued as production costs have fallen, and as the

U.S. Clean Air Act has specified a percentage of renewable fuels to be mixed with

gasoline. The fraction of annual U.S. corn production used to make ethanol rose

from around 1 percent in 1980 to around 20 percent in 2006, and ethanol output rose

from 175 million gallons to about 5.0 billion gallons over the same period. New

technologies that may further increase cost savings include coproduct development,

such as recovery of high-value food supplements, and cellulosic conversion. High oil

prices may spur the risk-taking needed to develop cellulose-to-ethanol production.

Developments such as dry fractionation technology, now commercially viable, may

alter the structure of the industry by giving the cheaper dry-grind method an edge

over wet milling. Dry milling requires smaller plants, and local farmer cooperatives

could flourish as a result. Though improvements in processing and technology are

important, however, the fluctuating price of inputs such as corn, the cost of energy

alternatives, and environmental developments play larger roles in the fortunes of the

industry.

Acknowledgments

The authors wish to thank a number of people who made valuable suggestions and

corrections to the paper. They include Don Erbach and Andrew McAloon of the

Agricultural Research Service, USDA, Jack Huggins of the Nature Conservancy,

and Vijay Singh of the Dept. of Engineering at the University of Illinois at

Urbana-Champaign.

About the Authors

C. Matthew Rendleman is with the Dept. of Agribusiness Economics, Southern Illinois University, and Hosein Shapouri is with the Office of Energy Policy and New

Uses, USDA.
Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Changes Since the 1993 ERS Analysis of Ethanol Production . . . . . . . . . . 3

Ethanol’s Energy Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Ethanol Production Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Input Improvements: Higher-Ethanol-Yielding Corn . . . . . . . . . . . . . . . . . 8

Process Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Advances in Separation Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . 10

New Ways of Fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

New Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Distillation Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Control Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Environmental Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Technologies Involving Coproducts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

The Growing Supply of Feed Coproducts . . . . . . . . . . . . . . . . . . . . . . . 16

Sequential Extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Corn Germ Recovery for the Dry-Mill Process . . . . . . . . . . . . . . . . . . 17

Centrifugal Corn Oil Separation from the Distiller’s Grain Stream . . . 17

CO2 Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Stillage Clarification and Other Uses of Membranes . . . . . . . . . . . . . . 18

Biorefinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Extraction of Compounds from DDGS . . . . . . . . . . . . . . . . . . . . . . . . . 19

Corn Fiber Oil Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Regional Impacts of Ethanol Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

National Benefits from Ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Biomass: Ethanol’s Future? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Cellulose to Ethanol: The Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Supplying Biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Biomass Byproducts: Problems with Acid and High Temperatures . . . 23

Other Biomass-to-Ethanol Improvements . . . . . . . . . . . . . . . . . . . . . . . 25

Conclusions: Ethanol’s Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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Introduction

The use of ethanol for fuel was widespread in Europe and the United States

until the early 1900s (Illinois Corn Growers’ Association/Illinois Corn

Marketing Board). Because it became more expensive to produce than

petroleum-based fuel, especially after World War II, ethanol’s potential was

largely ignored until the Arab oil embargo of the 1970s. One response to the

embargo was increased use of the fuel extender “gasohol ” (or E-10), a

mixture of one part ethanol made from corn mixed with nine parts gasoline.

Because gasohol was made from a renewable farm product, it was seen in

the United States as a way to reduce energy dependence on foreign

suppliers.

After the oil embargo ended, the use of ethanol increased, even though the

price of oil fell and for years stayed low. Ethanol became cheaper to make

as its production technology advanced. Agricultural technology also

improved, and the price of corn dropped. By 1992, over 1 billion gallons of

fuel ethanol were used annually in the United States, and by 2004 usage had

risen to over 3.4 billion gallons. Many farm groups began to see ethanol as a

way to maintain the price of corn and even to revitalize the rural economy.

This economic support for ethanol coincided with a further justification for

its use: to promote clean air. A 10-percent ethanol mixture burns cleaner

than gasoline alone (reducing the emission of particulate matter, carbon

monoxide, and other toxins), giving ethanol a place in the reformulated

gasoline (RFG) market. 

Ethanol use has also been boosted by the U.S. Clean Air Act and its various

progressions. Originally, the Clean Air Act required wintertime use of

oxygenated fuels in some urban areas to ensure more complete burning of

petroleum fuels. Since ethanol contains 35 percent oxygen, this requirement

of the act could be met by using an ethanol-containing blend. The current

Energy Act eliminates the need for oxygenates per se in RFG, but it specifies the minimum amount of renewable fuels to be added to gasoline.

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1980 82 84 86 88 90 92 94 96 98 2000 02 04 06

0

2,000

4,000

6,000

8,000

10,000

12,000

Ethanol

Other uses

Figure 1

Ethanol’s use of U.S. corn production

Millions of bushels

Source: Feed Situation and Outlook Yearbook, USDA, Economic Research Service,

various years. 
By 1980, fuel ethanol production had increased from a few million gallons

in the 1970s to 175 million gallons per year. During the 1990s, production

increased to 1.47 billion gallons, and total production for 2006 is expected

to be about 5.0 billion gallons. Annual U.S. plant capacity is now over 4.5

billion gallons, most of it currently in use. Demand is rising partly because a

number of States have banned (or soon will ban) methyl tertiary-butyl ether

(MTBE), and ethanol is taking over MTBE’s role (Dien et al., April 2002).

Ethanol provides a clean octane replacement for MTBE. The California

Energy Commission and the California Department of Food and Agriculture

now support ethanol development, and ethanol’s use in California alone is

expected to reach 1.25 billion gallons by 2012 (Ross).

The fraction of the Nation’s annual corn production used to make ethanol

rose from around 6.6 million bushels in the early 1980s (1 percent) to

approximately 2 billion bushels in 2006 (20 percent) (fig. 1).

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Changes Since the 1993 ERS

Analysis of Ethanol Production

In 1993, USDA’s Economic Research Service (ERS) published Emerging

Technologies in Ethanol Production, a report on the then-current state of

ethanol production technology and efficiency (Hohmann and Rendleman).

The report included a summary of production costs (table 1) and predictions

of “near-term” and “long-term” technological advances that many believed

would bring down ethanol costs.

The numbers were based on the costs of wet milling, which was then by far

the greatest source of output. (Milling types are explained in the next

section.) The estimate included a capital cost component, which distinguished this estimate from others done at the time. Other estimates ranged

from $1.08 to $1.95 per gallon.

The near-term technologies listed in the ERS report were as follows:

• Gaseous injection of sulfur dioxide and the use of special corn hybrids,

• Membrane filtration,

• Other advances, including improved yeast strains and immobilization of

yeast in gel substrates.

Long-term technologies (potentially available in 5 to 10 years) were as

follows:

• Bacterial fermentation,

• Conversion of corn fiber to ethanol (cellulosic conversion),

• Coproduct development.

Though the savings from technological improvements are significant, they

tend to be small compared with fluctuations in the net cost of corn, the main

ethanol feedstock. This is illustrated in table 2, which presents data on corn

costs and profits from the coproduct DDGS (distiller’s dried grains with

solubles) in dry-mill ethanol production from 1981-2004. Since 1981, sales

of DDGS have recovered nearly half the cost of each bushel of corn used to

produce ethanol, peaking in 1986, when over 66 percent of the feedstock

cost was recovered this way. In recent years, the percentage of recovery has

fallen because increased demand for ethanol has led to an abundance of

DDGS, lowering its price on the feed market. 

The near- and long-term technologies listed in the 1993 ERS analysis were

predicted to save from 5 to 7 cents per gallon in the short term and from 9

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Table 1

Ethanol wet-mill cost estimates, 1993

Cost category Cost per gallon

Feedstock $0.44

Capital $0.43

Operating $0.37

Total $1.24 

Source: Hohmann and Rendleman (1993).
to 15 cents by 2001. The savings have been as anticipated, but they have not

come in the manner predicted.

Gaseous injection of sulfur dioxide was beginning in 1993 and is a part of

the quick-germ (QG) and quick-fiber (QF) techniques currently being developed. There is revived interest in the use of special corn hybrids high in

starch, though their use is still not widespread. Membrane filtration and

yeast immobilization were being used in some plants in 1993, but their use,

contrary to expectations, has not increased. Bacterial fermentation is still not

used commercially, nor is cellulosic conversion of corn fiber. There have

been no outstanding developments in coproducts, but the potential remains

for their future exploitation. Most of the cost savings have been through

plant automation and optimization of existing processes.

The industry is still improving technologically. It is far more mature than in

1993, and new developments appear poised to bring costs down further and

to reduce the environmental impact of producing ethanol. In this report, we

examine various production technologies, beginning with input improvements and then discussing process improvements, environmental technologies, and technologies involving coproducts. Finally, we look at niche

markets and briefly examine cellulosic conversion.

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Table 2

Net corn costs of dry milling,1981-20041

Year Corn DDGS* Byproduct Net corn Net corn

value value value cost cost

$/bu $/bu % corn cost $/bu $/gal. ethanol

1981 2.47 1.25 50.5 1.22 0.45

1982 2.55 1.21 47.5 1.34 0.50

1983 3.21 1.47 45.7 1.74 0.65

1984 2.63 0.83 31.6 1.80 0.67

1985 2.23 0.92 41.1 1.31 0.49

1986 1.50 1.00 66.4 0.50 0.19

1987 1.94 1.16 59.9 0.78 0.29

1988 2.54 1.20 47.2 1.34 0.50

1989 2.36 1.06 44.8 1.30 0.48

1990 2.28 1.08 47.3 1.20 0.45

1991 2.37 1.04 43.9 1.33 0.49

1992 2.07 1.04 50.4 1.03 0.38

1993 2.50 1.05 42.1 1.45 0.54

1994 2.26 0.91 40.1 1.35 0.50

1995 3.24 1.29 39.7 1.95 0.72

1996 2.71 1.21 44.8 1.50 0.55

1997 2.43 0.93 38.4 1.50 0.55

1998 1.94 0.72 37.2 1.22 0.45

1999 1.82 0.68 37.3 1.14 0.42

2000 1.85 0.69 37.0 1.16 0.43

2001 1.97 0.68 34.6 1.29 0.48

2002 2.32 0.75 32.3 1.57 0.58

2003 2.42 0.98 40.6 1.44 0.53

2004 2.06 0.65 31.3 1.41 0.52

Average 2.32 0.99 43.0 1.33 0.49

*Distiller’s dried grains with solubles. 12.7 gal. of ethanol and 17 lbs of DDGS per bushel of corn.

Source: ERS Feedgrains Database, http://www.ers.usda.gov/db/feedgrains.
Ethanol’s Energy Efficiency

Improvements in ethanol’s energy consumption have continued since largescale commercial production began in the 1970s. The process has become

more efficient at using the starch in the corn kernel, approaching the theoretical limit of about 2.85 gallons of ethanol per bushel. Energy for conversion has fallen from as high as 70,000 Btu’s per gallon in the late 1970s

(Wang, August 1999) to 40,000 Btu’s or less for modern dry mills and to

40,000-50,000 Btu’s for wet mills. Modern energy-saving technology and

process optimization account for the improvement.

In 2002, Shapouri et al. surveyed energy values and reported that fuel

ethanol from corn produced about 34 percent more energy than it took to

produce it. That figure was based on a weighted average of a 37-percent

increase in energy from ethanol produced in dry mills and a 30-percent

increase from wet mills. This value was revised in 2004 by updating energy

estimates for corn production and yield, improving estimates of energy

required to produce nitrogen fertilizer and energy estimates for seed corn,

and using better methodologies for allocating energy for producing coproducts. With these revisions, the energy gain is 57 percent for wet milling and

77 percent for dry milling, yielding a new weighted average of 67 percent. 

The energy content, however, may be less important than the energy

replaced. A gallon of ethanol can save 26,575 Btu’s of energy by replacing a

gallon of gasoline because of ethanol’s higher combustion efficiency

(Levelton Engineering Ltd. and (S&T)2 Consulting Inc.). A gallon of

ethanol containing 76,330 Btu’s is able to replace a gallon of gasoline

containing about 115,000 Btu’s because ethanol’s higher octane rating (113-

115, compared with 87) allows high-compression engines to perform as well

with fewer Btu’s.

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Ethanol Production Processes

Though new technology may eventually blur the distinction between them,

ethanol is produced by one of two processes: wet milling and dry milling.

Wet mills are more expensive to build, are more versatile in terms of the

products they can produce, yield slightly less ethanol per bushel, and have

more valuable coproducts. Wet milling initially accounted for most of the

ethanol fuel production in the United States, but new construction has

shifted to dry mills, partly because dry mills cost less to build. In 2004, 75

percent of ethanol production came from dry mills and only 25 percent from

wet mills (Renewable Fuels Association). As a result, most new technologies are being developed for dry-mill production. 

Dry-milling plants have higher yields of ethanol; a new plant can produce

2.8 gallons per bushel, compared with about 2.7 gallons for wet mills. The

wet mill is more versatile, though, because the starch stream, being nearly

pure, can be converted into other products (for instance, high-fructose corn

syrup (HFCS)). Coproduct output from the wet mill is also more valuable.

In each process, the corn is cleaned before it enters the mill. In the dry mill,

the milling step consists of grinding the corn and adding water to form the

mash. In the wet mill, milling and processing are more elaborate because

the grain must be separated into its components. First, the corn is steeped in

a solution of water and sulfur dioxide (SO2) to loosen the germ and hull

fiber. This 30- to 40-hour extra soaking step requires additional tanks that

contribute to the higher construction costs. Then the germ is removed from

the kernel, and corn oil is extracted from the germ. The remaining germ

meal is added to the hulls and fiber to form the corn gluten feed (CGF)

stream. Gluten, a high-protein portion of the kernel, is also separated and

becomes corn gluten meal (CGM), a high-value, high-protein (60 percent)

animal feed. The corn oil, CGF, CGM, and other products that result from

the production of ethanol are termed coproducts.

Unlike in dry milling, where the entire mash is fermented, in wet milling

only the starch is fermented. The starch is then cooked, or liquefied, and an

enzyme added to hydrolyze, or segment, the long starch chains. In dry

milling, the mash, which still contains all the feed coproducts, is cooked and

an enzyme added. In both systems a second enzyme is added to turn the

starch into a simple sugar, glucose, in a process called saccharification.

Saccharification in a wet mill may take up to 48 hours, though it usually

requires less time, depending on the amount of enzyme used. In modern dry

mills, saccharification has been combined with the fermentation step in a

process called simultaneous saccharification and fermentation (SSF).

Glucose is then fermented into ethanol by yeast (the SSF step in most drymilling facilities). The mash must be cooled to at least 95o F before the

yeast is added. The yeast converts the glucose into ethanol, carbon dioxide

(CO2), and small quantities of other organic compounds during the fermentation process. The yeast, which produces almost as much CO2 as ethanol,

ceases fermenting when the concentration of alcohol is between 12 and 18

percent by volume, with the average being about 15 percent (Shapouri and

Gallagher). An energy-consuming process, the distillation step, is required

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to separate the ethanol from the alcohol-water solution. This two-part step

consists of primary distillation and dehydration. Primary distillation yields

ethanol that is up to 95-percent free of water. Dehydration brings the

concentration of ethanol up to 99 percent. Finally, gasoline is added to the

ethanol in a step called “denaturing,” making it unfit for human consumption when it leaves the plant.

The coproducts from wet milling are corn oil and the animal feeds corn

gluten feed (CGF) and corn gluten meal (CGM). Dry milling production

leaves, in addition to ethanol, distiller’s dried grains with solubles (DDGS).

The feed coproducts must be concentrated in large evaporators and then

dried. The CO2 may or may not be captured and sold.

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Input Improvements:

Higher-Ethanol-Yielding Corn

Efficient ethanol plants can convert 90-97 percent of the corn’s starch

content to ethanol. However, not all batches of corn leave the same amount

of starch residue. Studies of ethanol yields from different batches show

significant variability (Dien et al., March 2002). Even though it is the starch

that is turned into ethanol, researchers have been unable to find a correlation

between starch content (or even starch extractability) and the final yield of

ethanol (Singh and Graeber). Researchers believe some starches are in a

more available form (Dien et al., March 2002). They do not know, however,

what makes the starch break down easily to simple sugars and why this trait

varies from hybrid to hybrid (Bothast, quoted in Bryan, 2002). Some

research shows that although the ease with which the starch breaks down

varies among hybrids, most of the variability in the breakdown is due to

other factors (Haefele et al.).

Seed companies like Pioneer, Monsanto, and Syngenta are working to create

corn that will boost ethanol yield. “Current work centers on identifying

highly fermentable hybrids we already have,” says a Monsanto spokesman

(Krohn). Pioneer reports yield increases of up to 6 percent in batches using

what it calls HTF corn (High Total Fermentables), compared with the yields

from unselected varieties (Haefele et al.). Monsanto calls its selected varieties HFC for High Fermentable Corn.

Syngenta Seeds’ Gary Wietgrefe points out several of the impediments to

widespread adoption of HTF corn (Ed Zdrojewski in BioFuels Journal,

2003b). To begin with, starch and ethanol yield vary by geographic region

and from year to year, making an optimizing hybrid choice difficult.

Further, choosing a hybrid that maximizes ethanol qualities may mean a

tradeoff with yield and other potentially valuable qualities, such as protein

content and even test weight (because of moisture). Testing equipment presents its own challenges. Not all units are calibrated the same, creating

uncertainty. The technology may be less available to farmers than to ethanol

plants, and even if it were to become more readily available, sorting grain

by starch availability and making marketing decisions would be problematic

for the grower. Finally, with ethanol plants already able to convert between

90 and 97 percent of the corn’s starch, any new HTF or HFC genetics or

technology would have to overcome the problems and significantly improve

the profits of the ethanol plant and the farmer.

Higher ethanol yield leaves less DDGS for animal feed, possibly changing

the quality as well as the quantity of the feed coproduct. A lower quantity

might raise the protein percentage, but it could also concentrate some of the

undesirable contents of the DDGS. Any changes, however, are expected to

be minor. (See Haefele et al., p.14, on the selection of hybrids.)

Unlike many technologies that are adopted because they show an immediate

improvement in profit or a reduction in risk, corn with a higher ethanol

yield does not necessarily lead to additional profits for the farmer in today’s

marketing environment. Corn is not graded on the basis of fermentability,

nor is a premium offered in the wider marketplace for this trait. In order to

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overcome this market drawback, companies like Monsanto and Pioneer are

developing programs to encourage the adoption of their selected hybrids.

So far, hybrid-testing research has centered on dry-mill production, the

lower investment technique of choice for the new cooperatively owned

plants. The seed companies are targeting their incentive programs on dry

mills. Monsanto’s program, “Fuel Your Profits,” provides the participating

ethanol plant with high-tech equipment that profiles the genetics of

incoming corn and is calibrated to maximize ethanol yield (Rutherford). As

an incentive, Monsanto gives rebates on E85 vehicles (those designed to run

on 85%-ethanol fuel) and fueling stations. Pioneer has developed a wholegrain Near Infrared Test (NIT) to identify ethanol yield potential quickly.

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Process Improvements

New construction today is mostly of dry mills, and most new technology is

designed for them. Major technology changes are made more efficiently

while a plant is being built than when they are adopted later. 

Advances in Separation Technologies

New techniques that separate corn kernel components before processing will

blur the distinction between wet and dry milling (dry grind) by allowing the

dry mill to recover the coproducts from the germ. Process improvements are

also being made that will reduce the cost of wet milling, generally by shortening the soaking step. Some of the separation improvements we describe

here, though promising, are still experimental.

Germ and Fiber Separation

Modifications of the dry-grind facility have made the recovery of corn germ

possible in dry milling. Normally, neither corn germ nor any other corn

fraction is separated out before becoming part of the mash; all components

go through fermentation and become part of the feed coproduct, DDGS.

Various modifications of the process have made it possible to recover fiber

and corn germ—and thus corn oil—from both the endosperm and the pericarp (outer covering) of the kernel. 

A technique developed at the University of Illinois called Quick Germ (QG)

allows recovery of corn oil and corn germ meal from the germ, making the

dry mill a more profitable operation (Singh and Eckhoff, 1996, 1997; Taylor

et al., 2001; Eckhoff, 2001). Results published in 1995 and 1996 demonstrated that with a 3- to 6-hour soak step (as opposed to 24 to 48 hours for

the soak step in wet milling), the corn germ could be removed. Since then,

research has explored the parameters of the process and its savings potential

(Singh and Johnston). Another process, Quick Fiber (QF), can be used with

QG to recover fiber from the pericarp, a source of potentially valuable food

coproducts. Though these processes have not been used in commercial

applications, they hold promise for reducing the net cost of the input corn.

The tanks and equipment for the additional steps would increase the plant’s

capital cost, but could increase its capacity by reducing the amount of

nonfermentables in the mash.

Enzymatic Dry Milling

This process, which uses newly developed enzymes, is another method with

the potential for cost savings. In addition to recovering the germ and fiber

from the pericarp, it allows recovery of endosperm fiber. Savings come from

the recovered coproducts and from reduced energy consumption—the

process requires less heat for liquefaction and saccharification. Plant

capacity should be enhanced as well, since there is less nonfermentable

material in the substrate because it is removed earlier in the process. The

amount of DDGS is smaller, but of higher quality. Ethanol concentrations in

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the mash are also higher. Industrial- and food-grade products can be recovered from the fiber. Alternatively, the fiber can be fermented.

Dry Fractionation

This recent technology separates the corn kernel into its components

without the soaking step. Depending on the process—several companies

currently offer similar technologies—the feedstock may be misted with

water before being separated into bran, germ, and the high-starch

endosperm portion of the kernel (BioFuels Journal, 2005d,e).

The advantages of dry fractionation over processes that require a soak step

are threefold: lower costs because less energy is required for drying the feed

coproduct, lower emissions, and greater coproduct output because the mash

is more highly concentrated. The germ can be sold or pressed for corn oil,

and the bran also has potential for food or energy use. 

Dry fractionation is a process that has been tested and is in use in the food

industry (Madlinger). Both new and planned ethanol plant construction

employ the technology. Unlike some other new technologies, the dry fractionation equipment can be added to an existing dry mill.

With all the separation techniques, there appears to be less total ethanol recovered per bushel than with conventional dry-milling techniques, probably due

to removal of some starch with the coproducts (Singh and Johnston). Each

technique will change the nature of the resulting distiller’s grains, potentially

raising their value due to a higher protein content; the feed coproduct from the

separation processes is purported to be higher in protein and lower in fiber

than ordinary DDGS. However, research is needed to determine the feed value

of this altered coproduct. Preliminary feed trials with poultry and hogs, as yet

unpublished, are promising (Madlinger).

Ammoniation Process in the Wet Mill 

Researchers have also investigated a separation technique involving pretreatment with ammonia (Taylor et al., 2003). This process would facilitate

removal of the pericarp and reduce the soak time in wet milling or the QG

process. Anhydrous ammonia would take the place of the caustic soda solution usually used in debranning. In laboratory research, the pericarp was

more easily removed through ammoniation, but though the oil was not

degraded, its quantity was reduced compared with conventional techniques.

Continuous Membrane Reactor for Starch Hydrolysis 

This process, still experimental, uses enzymatic saccharification of liquefied corn in a membrane reactor. In a continuous membrane reactor, as

opposed to the traditional batch process, starch would be broken down and

glucose extracted continuously. Theoretically, the yield would increase, and

the automated continuous process would enable better control than the

batch process.

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Alkali Wet Milling

In an experimental modification of the wet-milling process, corn was soaked

briefly in sodium hydroxide (NaOH) and debranned (Eckhoff et al.). This

process cut the costly soaking time to 1 hour. The pericarp removed in alkali

wet milling becomes a potentially valuable part of the coproduct stream.

Additional work is needed to develop ways of disposing of or recycling the

NaOH before the technique can be commercialized.

New Ways of Fermentation

High-Gravity Fermentation

This technique, still experimental, would lower water use in ethanol production. Potential savings would come from the reduced cost of water and

wastewater cleanup, as well as from reduced energy use. This process would

involve less heating and cooling per gallon of ethanol. Very-high-gravity

fermentation accomplishes this saving in energy by using a highly concentrated mash with more than 30 percent solids. Experiments have resulted in

a 23 percent-alcohol fermentation, much higher than with the conventional

process. Commercial production at that level is not likely in the near future

because of difficulty in staying within the required tolerances. However,

incremental moves toward higher concentrations open the possibility of

lower production costs.

Improved Yeast 

For many years, researchers have been trying to improve yeast, which is a

highly effective converter of sugars to ethanol. The desired end product is a

yeast that would be more heat tolerant and better able to withstand high

alcohol concentrations, that would produce fewer undesirable byproducts,

and that might even be able to convert more types of sugar to ethanol.

Developers have already made progress in some of these areas. For

example, the ethanol tolerance of yeast is at least one-third higher today

than in the 1970s.

Some researchers believe a yeast tolerant of temperatures as high as 140o F

is the ideal. If such a yeast were to be developed—something increasingly

possible with recombinant DNA techniques—the ethanol conversion process

would look completely different than it does today (Novozymes and BBI

International). Another goal of industry researchers is to produce less glycerol, which is produced in response to stress and represents a loss of ethanol

during conversion.

Conversion of Pentose Sugars to Ethanol 

Sucrose from starch is not the only type of sugar in the corn kernel. Some of

the sugars are pentoses, or five-carbon sugars not normally utilized by

common yeast. Any organism that could ferment pentoses to ethanol would

be a valuable contribution to corn-ethanol conversion efficiency. This

conversion has been achieved in the laboratory using genetically modified

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yeasts (Moniruzzaman et al.) and in bacterial fermentation using E. coli

(Dien et al., 1997). These processes are not in commercial use, partly

because the engineered organisms are less hardy and less tolerant of environmental changes than conventional organisms. Researchers are also

concerned about how the nutritional content of the resulting feed coproduct

would differ from conventional DDGS and about whether the genetically

modified organisms remaining in the feed would be acceptable in the

commercial feed market.

New Enzymes

Enzymes for Liquefaction and Saccharification

Enzymes were first used in ethanol production in the 1950s, but they have

recently been improved and their cost brought down through the use of

special fermentations of microorganisms. Costs have fallen 70 percent over

the last 25 years (Novozymes and BBI International).

Enzymes enable chemical reactions to occur more easily, with less heat or a

more moderate pH, and therefore more cost effectively. Their use in ethanol

production improves liquefaction, saccharification, and fermentation.

Enzyme use also results in reduced soak time, higher starch and gluten

yield, better protein quality, and reduced water and energy use. USDA’s

Agricultural Research Service (ARS) is working with enzyme manufacturers to further reduce cost and improve effectiveness.

Enzymes To Reduce Sulfur Dioxide and Steep Time 

in Wet Milling

Part of the additional expense in wet milling as opposed to dry milling is the

necessity of soaking the corn before separation of the germ from the kernel.

The tanks increase capital cost, and the soak time slows the process. Soak

time can be reduced by adding sulfur dioxide to the steep water, but

research shows that the sulfur dioxide can be reduced or eliminated by using

enzymes. Recently, an experimental two-stage procedure reduced soak time

by up to 83 percent (Johnston and Singh; Singh and Johnston). In the

saccharification step, the protease enzyme hydrolyzed the protein matrix

around the starch granules and made it available for further breakdown. As

with most enzymes, cost is still an issue; however, small-scale experiments

seeking to optimize the process have so far reduced the enzyme requirement

severalfold. Research trials show that using a low level of sulfur dioxide

(more than 90 percent less than conventional levels) greatly reduces the

enzyme requirement. Small amounts of sulfur dioxide are still effective in

reducing bacterial contamination, a potential problem in continuous

processes. Though enzymes are an added expense, the procedure has the

potential to increase plant capacity (through the time savings), reduce

energy costs, and allow the use of otherwise unusable broken grains.

Replacing the conventional liquefaction and saccharification steps with a

single, low-temperature enzyme step has already been discussed in the

section “Advances in Separation Techniques.” 

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Distillation Technology

Standard distillation techniques leave about 4-percent water in the final

ethanol. In the early days of ethanol distillation, the basic production design

came from the beverage alcohol industry, where there is no need to remove

all the water (Swain). Fuel ethanol, however, must be almost pure or dry, so

ethanol producers began dehydrating their ethanol using a technique called

azeotropic distillation. This technique requires use of an ingredient, usually

benzene or cyclohexane, to break the azeotrope—the point after which

distillation becomes ineffective. The adoption of molecular sieves allows the

modern plant to use less power, reduce original capital outlay, and eliminate

potential exposure of workers to dangerous chemicals.

Molecular sieves use materials with microscopic pore sizes large enough to

allow a molecule of one size to get through while blocking another. For

example, in ethanol dehydration the molecular sieves have a pore diameter

that allows a water molecule to enter and be trapped but keeps out the larger

ethanol molecule. Since the late 1980s, vapor-phase molecular sieves have

been the industry standard, improving on the liquid-phase sieves by

reducing the required size (Novozymes and BBI International).

Control Systems

The 1993 prediction of cost reductions in ethanol production (Hohmann and

Rendleman) overlooked the incremental changes in efficiency that were

taking place due simply to increased control of fermentation and other

processes. Distributed control systems were already being used, but with the

evolution of technology, especially computing capabilities, these systems

have continued to reduce costs while optimizing the production process.

Distributed control systems are used in industrial and other engineering

applications to monitor and control a process remotely. Human operators

manage equipment distributed throughout the plant (or other application).

Examples, besides ethanol plants, include power distribution systems, traffic

signals, water management systems, and biorefineries. Instruments to

measure and control, usually digital, are wired ultimately to computers,

allowing a human-to-machine interface.

Merging the distributed control system with computer programs allows

timely monitoring of processes, and even allows prediction. Reports can be

compiled from stored data, and alarms can be set to alert operators if established parameters are breached.

Distributed control systems have cut costs in ethanol plants mainly by

reducing the labor required, but they have also improved production efficiency in other ways, letting operators fine-tune processes they could not

control as closely in the past. Better process control also reduces downtime

and maintenance.

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Environmental Technologies

In addition to reducing the amount of energy required for production,

modern ethanol plants give off fewer odors and emissions than ever before.

Technological advances hold promise for converting more of the feed

coproducts into ethanol, for reducing components of DDGS that might harm

the environment, and for utilizing waste streams from the process.

As noted, modern ethanol production uses less energy as techniques and

technologies improve. Both wet and dry millers are using less fuel and electricity per gallon produced, and farmers are producing corn more efficiently.

All these savings are an environmental plus for ethanol.

Ethanol plant emissions are a second area of improvement. In 2002,

Minnesota producers signed an agreement with the Environmental Protection Agency (EPA) to reduce emissions coming from their plants. That

agreement has become an industry standard. Since then, ethanol producers

in several States have agreed to install thermal oxidizers or other technologies that eliminate nearly all volatile organic compounds (VOCs) and other

pollutants, adding equipment that averages more than $2 million per plant.

The EPA estimates that the agreement will eliminate more than 63,000 tons

of pollution annually.

Though pollutants, like particulate matter and even VOCs, can originate

from fermentation and from other parts of the plant such as grain-handling

areas, most of the attention has been focused on dryer stacks. Thermal

oxidizers are now standard equipment in most new ethanol facilities. They

convert carbon- and hydrogen-bearing compounds into CO2 and water

through high-temperature oxidization. Besides eliminating odors and visible

emissions, thermal oxidizers can eliminate over 99 percent of oxides of

nitrogen and other hazardous air pollutants, as well as certain particulates.

Wastewater emission problems have been largely solved by the development

of anaerobic digester systems. Majumdar et al. report using membrane technology to recover VOCs such as hexane from process air emissions, giving

membranes a role in environmental remediation.

New techniques may make processing itself more environmentally friendly.

For example, a corn-steeping process being developed by scientists from ARS

and the University of Illinois uses enzymes and reduces the need for sulfites.

Another discovery—one that may hold the most promise for the future—is

the conversion of low-value and waste stream products into valuable

coproducts. (This will be discussed in the next section, “Technologies

Involving Coproducts.”)

Finally, overfeeding phosphorus to animals can be an environmental

concern because the phosphorus ends up on the land as manure. The phosphorus available in DDGS is more than is needed for proper animal nutrition. If the level excreted over time is excessive, phosphorus can move into

ground or surface water and create problems such as algae in the waterways.

Researchers are experimenting with membrane technology that would

remove phosphorus from the thin stillage (the liquid remaining after

removal of the wet distiller’s grains) before it becomes feed. The result is

likely to be more efficient feed rations and reduced environmental impact.

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Technologies Involving

Coproducts

In addition to animal feeds, other potential coproducts are produced along

with the ethanol. Both wet and dry milling create CO2 during fermentation.

Minor components, such as glycerol, may be collected from the processing

stream. Some research directions may alter the entire process, using

different fermentations to produce entirely different product lines.

Researchers are also hoping to turn much of the nonfuel product into

ethanol, or even something more valuable than the fuel that is currently the

primary product. Possibilities include protein and fiber that could be added

to human foods to increase nutritional value.

Enzymatic milling may also allow recovery of valuable coproducts. Conventional dry milling leaves as coproducts distiller’s dried grains with solubles

and, if recovered, CO2. ARS scientists from USDA’s Agricultural Research

Service are adapting the concept of enzymatic milling to the dry-mill

ethanol process, partly to recover additional high-value coproducts.

The Growing Supply of Feed Coproducts

Ethanol production does not exhaust the feed value of corn; it merely uses

up the starch portion, leaving protein, minerals, fat, and fiber to be dried and

sold for feed. A high percentage of ethanol producers’ revenue comes from

the feed coproduct. Dry milling turns a bushel of corn (56 lb) into 2.7 or

more gallons of ethanol and leaves 17 lb of distiller’s grains. Wet milling

produces only slightly less ethanol and coproduces around 16 lb of corn

gluten meal (CGM, 2.65 lb) and corn gluten feed (CGF, 13.5 lb), in addition

to corn oil. In 2004, 7.3 million metric tons of DDGS, 426,400 metric tons

of CGM, and 2.36 million metric tons of CGF were produced (Renewable

Fuels Association). Predictions for near-term increases are as high as 10

million tons by 2007-08, which would constitute a significant portion of all

the cattlefeed in the United States (BioFuels Journal, 2005c). 

Because feed coproducts and ethanol are produced in fixed proportions,

increased demand for ethanol will result in greater output of DDGS, putting

downward pressure on its price. Partly for this reason, research is continuing

on alternative coproducts.

Proving the nutritional value of DDGS for new uses will expand the market.

Most of the DDGS produced is fed to dairy and beef cows, but it is increasingly being tested and used in swine and poultry rations. Research is

planned on DDGS in equine diets.

The nonuniform character of DDGS makes it difficult to establish feeding

parameters because the product varies in consistency and nutritional value.

Ongoing research is aimed at establishing feed values for various forms of

DDGS, and some producers are developing proprietary DDGS brands with

guaranteed nutritional properties.

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Sequential Extraction

Lawrence Johnson of Iowa State University has initiated several projects

exploring the parameters of the sequential extraction process, or SEP (see,

for example, Hojilla-Evangelista and Johnson). SEP uses alcohol rather than

water to separate kernel components in an otherwise wet-mill process. The

researchers claim increased ethanol production (10 percent), higher quality

protein extracts (with no SO2 needed for extraction, and therefore less

degradation of the protein portion), and the production of corn fiber gum, a

gum arabic substitute, as a coproduct. Gum arabic is used in producing soft

drinks, candy, and pharmaceuticals. Cost remains a problem with the SEP

process: The initial capital outlay for an SEP plant would be much higher

than for a conventional wet mill. Extraction of proteins from the separated

germ remains a problem as well.

Corn Germ Recovery for the 

Dry-Mill Process

As explained in the “Process Improvements” section, both dry fractionation

and the quick-germ technique modify the dry-mill process and make the

recovery of corn germ possible. Recovery of the germ, and thus of the oil,

can make dry milling more profitable. The soak step in the quick-germ

process takes less time than in wet milling, 3-6 hours as opposed to 24-48

hours. The process has not yet been used commercially. Plants using dry

fractionation, however, are underway. Profitability will depend on the relative

cost of corn oil and the capital costs of the additional equipment required.

Centrifugal Corn Oil Separation 

from the Distiller’s Grain Stream

An additional technology for separating corn oil from distiller’s grain has

recently become available to dry millers. SunSource, a coalition of ethanol

producers and a technology company, is licensing a system that can be

installed in new plants or retrofitted to existing ones (Walker). The technology makes an additional coproduct available to ethanol producers, most

likely to be used in biodiesel production. The process uses centrifuge technology to extract the oil from the distiller’s grains in the evaporation step.

The developers of the process claim that removing the oil from the distiller’s

grains does not lower the value of the feed coproduct and makes it easier to

handle. The process also reduces volatile organic compounds emitted from

the dryers, an environmental bonus.

CO2 Recovery

Ethanol’s most abundant coproduct is CO2, produced by yeast in about the

same proportion as ethanol itself. Only about 25 U.S. plants find uses for

the gas (Lynn Grooms in BioFuels Journal, 2005b); the other plants,

because of the low commercial value of CO2, simply vent it into the air.

Most CO2 sold commercially is used in soft drinks and food processing,

while other uses, such as water treatment, welding, chemical processing,

refrigerants, and hydroponics, consume some of the remainder. 

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Another approach would be to find new uses for the gas, raising its value

and expanding the market. One experiment uses CO2 to enhance the

recovery of oil from depleted oilfields. The gas is pumped into the oil

production zones, forcing residual oil to the surface. If successful, the technique could greatly expand the market for CO2.

A number of experiments with CO2 are taking place at the basic science level

(Bothast). One idea, successful at that level, is to turn the gas into ethanol or

other fuel. However, the techniques are not yet commercially viable.

Bioconversion offers hope for increased CO2 exploitation. Through biological processes, it turns organic materials into usable products or energy. In

corn ethanol production, the possibility of bacterial bioconversion of CO2

into fuel (e.g., ethanol or methanol) is under study.

Stillage Clarification and Other Uses 

of Membranes

Membrane separation is now used in dry mills to treat incoming boiler

water and in wet mills to clarify dextrose. Based on the molecular size of

the particles permitted to pass through, membranes are classified as reverse

osmosis, nanofiltration, ultrafiltration, or microfiltration. They are made of

various materials, including organic polymer, ceramics, and stainless steel.

Membranes were once thought promising for removing the last of the water

from ethanol—the portion left by ordinary distillation—as processors

looked for substitutes for hazardous materials like benzene that were then

used for that purpose. In plants built today, however, the drying part of

ethanol production is done with molecular sieves.

Membrane systems are used in industry to separate, clarify, and concentrate

various feed streams and are most efficient when used on dilute broths. In

the future, they may be used to reduce the cost of production through recovering and purifying minor components of the ethanol product stream. The

recovery of lactic acid and glycerol from thin stillage is an application of

this research that may soon be commercially viable. Corn oil and zein, a

protein, are also potentially recoverable (Kwiatkowski and Cheryan).

Researchers think membranes can be useful in continuous (though not in

batch) reactors (Escobar et al.).

Biorefinery

One definition for a biorefinery is “a facility that integrates biomass conversion processes and equipment to produce fuels, power, and value-added

chemicals from biomass.” The concept is a bit like wet milling, but with

more coproduct possibilities. Corn or another feedstock might be decomposed and recombined into a number of products other than, or in addition

to, ethanol. Some of the products might be high-value, low-volume ones,

outside the traditional ethanol market, that could supplement a plant’s

primary, lower value product line.

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Extraction of Compounds from DDGS

The biorefinery concept meshes with a new area of research known as functional foods or nutraceuticals. Two categories of functional foods, dietary

fibers and oligosaccharides, can be derived from grains.

Several Canadian companies and some universities are experimenting with

extracting new coproducts from the distiller’s grains generated from alternative feedstocks. Examples include cosmetics from oat derivatives, phenolic

avenanthramides (useful in low-density liproprotein resistance to oxidation)

from oats and wheat, and beta-glucan, a fiber-type complex sugar derived

from yeast, oats, and barley fiber and useful in reducing cholesterol.

A biorefinery may also yield purified proteins used as human food or in

industrial processes. A potentially recoverable coproduct is zein, a corn

protein that can be used in adhesives and in coatings for pharmaceuticals

and packaging materials because of its good water-vapor barrier properties.

ARS scientists are working to develop a cost-effective way to recover zein

from the milling stream, potentially making its use more common.

Corn Fiber Oil Recovery 

Corn fiber is a byproduct of corn wet milling and may be a future product

of dry milling. ARS has been developing methods to obtain enriched

protein, starch, and fat from corn fiber. Corn fiber is also a source of corn

fiber oil, a valuable dietary supplement that contains high levels of cholesterol-lowering and antioxidant phytosterols. Corn fiber can also be recovered by other means, such as the quick-fiber technique. Its economical

recovery could give ethanol production another valuable coproduct line.

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Regional Impacts of 

Ethanol Plants

Ethanol production holds promise for rural communities that hope to add

value to locally produced corn. A new ethanol plant is seen as a way to

create jobs and revitalize the local rural economy. Such scenarios seem

more likely with widely dispersed, smaller scale plants—the kind that could

spring up to service small specialty markets with an unusual coproduct or to

take advantage of a local feedstock or other regional characteristic.

In the future, corn-processing facilities may be able to recover special

healthful elements. One example might be proteins recovered from the

process stream by ultrafiltration and made available as food.

Minor producers may take advantage of discard items like soft drinks or

candy past their expiration date or other unusual sources of feedstock. Two

cheese plants in California produce ethanol and at the same time solve a

disposal problem by using the whey—generally regarded as waste—as a

feedstock. Cheese whey contains lactose that, if not fermented, may require

special processing at the cheese plant to avoid extra charges for municipal

water-disposal treatment.

Some ethanol producers take advantage of their location to reduce production

costs by selling coproducts otherwise too bulky to transport, such as steepwater. Or they save by avoiding drying, for instance, by feeding their wet

distiller’s grains to cattle or even fish. Others may sell CO2, or may at least

capture some value from it by using it in greenhouses to boost plant growth.

With the development of commercial cellulosic techniques, ethanol

producers may actually be paid to take away material that they can use for

feedstock—for instance, waste from other production systems or even

municipal garbage.

Other potential niche markets may use animal waste. One current USDA

research project seeks to exploit the advantages of co-location (of an ethanol

plant and poultry farms) by generating electric power and steam for ethanol

production from chicken litter. Coproduction of power and steam from a

waste stream is being developed in another USDA project.

Many ethanol facilities recently opened or under construction are farmer

owned and dispersed across rural areas. Farmers expect the completion of

an ethanol plant in the area to increase the local corn price. McNew and

Griffith examined the impact on local corn prices of opening 12 ethanol

plants and found that, on average, price per bushel rose 12.5 cents at the

plant and that some price response was detected 68 miles from the plant.1

New rural ethanol plants also provide employment. Though the plants need

fewer employees than they would have just a few years ago when the

industry was using less labor-saving technology, the U.S. Department of

Energy (DOE) estimates that ethanol production is responsible for 40,000

jobs and $1.3 billion in increased annual household income.

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1McNew and Griffith have also

established a Web site at Montana

State University that shows how corn

prices are impacted by the opening of

an ethanol plant (BioFuels Journal,

2003a). The site allows the user to

investigate a small, medium, or large

plant and displays the results graphically in map format. The impacts are

based on econometric work by the

authors. The tool, called the “Ethanol

Plant Analyzer: A GIS-Driven Tool for

Assessing Ethanol Feasibility,” can be

found at http://extensionecon.

msu.montana.edu/eplantanalyzer/
National Benefits from Ethanol

Ethanol produced in the United States displaces imported foreign oil and

creates domestic economic activity. Gallagher et al. (2000) estimate that the

current program results in a $400-million net gain in overall social welfare.

More domestic production, which is likely with an MTBE phaseout, would

result in additional gain.

With ethanol production of 3.41 billion gallons in 2004, 143.3 million fewer

barrels of oil were needed, about 4.5 percent of annual U.S. use

(Urbanchuk). A 10-percent mixture of E-10 reduces petroleum use by 6

percent, greenhouse gas emissions by 1 percent, and fossil energy use by 3

percent (Wang et al.). 

Increased biomass production would also change the picture. Gallagher and

Johnson focus primarily on the benefits of a developed biomass-to-ethanol

industry. Assuming an industry based on corn stover, the largest single

source of biomass, they conclude that U.S. welfare would increase (a)

because of the expanded fuel supply and (b) because the oligopoly effects of

pricing by the Organization of Petroleum Exporting Countries (OPEC)

would be mitigated. A biomass-to-ethanol industry, based on something like

switchgrass, could add to farmers’ product line and “could significantly

increase profits for the agricultural sector” (De La Torre Ugarte et al.). 

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Biomass: Ethanol’s Future?

Though corn has been the feedstock of choice in the United States, ethanol

potentially can be made from any starch, sugar, or cellulosic feedstock. In

fact, ethanol has been created from a variety of grains and from grass and

straw, wood fibers, and sugarcane. Though ethanol production from corn

has become more efficient, some experts see it as a technology that has

already matured, with any significant reduction in production costs unlikely

(DiPardo). Substantial cost reductions may be possible, experts believe, if

cellulose-based feedstocks are used instead of corn. One industry publication editorializes: “Ultimately, if renewable automotive fuel becomes

economical in the United States it will have to be made from lignocellulosic

biomass” (Industrial Bioprocessing).

In the end, biomass-to-ethanol production may be attractive because

biomass would cost less than corn. In addition, selling the feed coproducts

from corn ethanol may become burdensome. As corn ethanol production

increases, an inevitable result of dry-mill expansions, more feed coproducts

will find their way into the market and drive down prices for DDGS.

The vision of cellulosic conversion is not yet commercial reality, however,

due to difficulties inherent in turning biomass into ethanol. Because the

cellulose and fermentable portions of woody biomass are tightly bound

together, researchers have had to focus on the problem of pretreatment and

hydrolysis. The necessary chemical conversion can take place using acids or

enzymes, but, to be commercially viable, costs must be brought down.

USDA and the U.S. Department of Energy are funding projects that have

this aim. Also, many of the sugars making up cellulosic feedstocks

(composed of cellulose and hemicellulose) are not readily convertible to

ethanol by ordinary yeast.

Researchers have not settled on the cost of producing ethanol from biomass

because ethanol is not yet being produced this way. A joint project by the

USDA and DOE estimated the unit cost at a 25-million-gallon per year

(MMgy) plant to be $1.50 per gallon (1999 dollars) (McAloon et al.).

Though some researchers put this figure lower, at $1.16-$1.44 per gallon

(Wooley et al.), recent estimates of the cost of commercial cellulose collection ($40-$50 per ton) and capital outlays (over $6 per gallon of annual

capacity) make the likely cost higher. A consideration for new facilities will

be the one-of-a-kind expense associated with setting up the first working

biomass-to-ethanol plants (e.g., scaleup and development costs). In its

$1.50-per-gallon estimate, the USDA/DOE study assumed these costs had

already been incurred.

Cellulose to Ethanol: The Process

In the same way that starch from corn must be hydrolyzed and saccharified

(decomposed further into simple sugars) before it can be fermented, cellulose must first be converted to sugars before it is fermented and turned into

ethanol. Cellulosic feedstocks are more difficult than corn to convert to

sugar. Cellulose can be converted by dilute acid hydrolysis or concentrated

acid hydrolysis, both of which use sulfuric acid. Hydrolysis can also be

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achieved with enzymes or by other new techniques, including countercurrent hydrolysis or gassification fermentation.

Cellulosic hydrolysis produces glucose and other six-carbon sugars

(hexoses) from the cellulose and five-carbon sugars (pentoses) from hemicellulose. The non-glucose (carbon) sugars must be fermented to produce

ethanol, but are not readily fermentable by Saccharomyces cerevisia, a

naturally occurring yeast. However, they can be converted to ethanol by

genetically engineered yeasts, though the process is not yet economically

viable (DiPardo).

Supplying Biomass

Some research has centered on converting the cellulosic portion of the corn

kernel. Many of the researchers in this effort initially believed that cellulosic

conversion could begin with the cellulose in the current feedstock stream

(corn) and proceed to corn husks, and then to corn stover, before they finally

extended their research to include other sources of cellulose. A second line

of research has centered on converting cellulosic biomass directly into

ethanol from noncorn sources such as small-diameter trees and switchgrass.

Further research has focused on converting cellulose after it has entered the

waste or coproduct stream, through steam explosion, for example.

If converting biomass to ethanol can be made economically attractive, the

potential feedstocks are myriad. They include agricultural waste, municipal

solid waste, food processing waste, and woody biomass from small-diameter trees. Agronomic research is underway on improving dedicated-energy

crops such as hybrid willow, hybrid poplar, and switchgrass (DiPardo). One

project looks at genetic improvement of switchgrass to optimize its conversion. Grasses grow quickly, of course, while tree crops such as willow

require a 22-year rotation, with the first harvest in year 4 and subsequent

harvests every 3 years thereafter. Hybrid poplar trees require 6-10 years to

reach their first harvest.

Biomass from crop residue can be a source of farm profits, and with appropriate steps, its use in ethanol conversion can be environmentally friendly. A

USDA study by Gallagher et al. (2003) concluded that crop residues are the

cheapest prospective source of fuel for the U.S. market, with the energy

potential to displace 12.5 percent of petroleum imports or 5 percent of electricity consumption.

Biomass Byproducts: Problems with Acid

and High Temperatures

The dilute acid and concentrated acid hydrolysis used in biomass-to-ethanol

conversion produce byproducts that either must be disposed of or that

require recycling of sulfuric acid. In addition, the high temperatures

required take a toll on the sugars and thus on the ethanol yield (DiPardo).

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Potential Solutions to Acid and Heat Problems

Countercurrent Hydrolysis—Advances in biotechnology could reduce

conversion costs substantially. The National Renewable Energy Laboratory

(NREL) presents countercurrent hydrolysis as a new pretreatment. The

process uses steam to hydrolyze most of the hemicellulose. In a second,

hotter, stage, dilute sulfuric acid hydrolyzes the rest of the hemicellulose

and most of the cellulose. NREL researchers believe the countercurrent

hydrolysis process offers more potential for reducing costs than the dilute

sulfuric acid process. They estimate that it will increase glucose yields and

permit a higher fermentation temperature, resulting in an increased yield of

ethanol. They have achieved glucose yields of over 90 percent in experiments with hardwoods (NREL, 2004).

A New Approach—While acknowledging that a cellulose-to-ethanol

industry is in its infancy, Tembo et al. attempt to determine parameters that

would define a regional (Oklahoma) biorefinery industry. The authors assess

alternatives for future production, positing a gassification-fermentation technique that uses neither traditional acid nor enzymatic hydrolysis. Lignocellulosic biomass can be gassified, they say, in fluidized beds to produce

“synthesis gas.” The gas can then be converted by anaerobic bacteria to

ethanol. Advantages to such a lignocellulosic system include a theoretically

lower cost than for corn ethanol and the potential to use multiple perennial

feedstocks, with supposedly less environmental impact than corn use. The

researchers caution that the process is at the bench level and is not yet

commercially available.

Enzymatic Hydrolysis—NREL believes that the greatest potential for

ethanol production from biomass lies in enzymatic hydrolysis of cellulose.

Advances in biotechnology may eventually make the technique possible

through the use of genetically engineered bacteria and may also permit the

fermentation of the pentoses.

Reducing Enzyme Costs

The use of enzymes in biomass-to-ethanol conversion will require reductions in the cost of producing cellulase enzymes, along with an increased

yield in the conversion of nonglucose sugars to ethanol. The enzyme cellulase, already used in industry, replaces sulfuric acid in the hydrolysis step.

Higher sugar yields are possible because the cellulase can be used at lower

temperatures (Cooper). NREL reports that recent process improvements

allow simultaneous saccharification and fermentation, with cellulase and

fermenting yeasts working together so that sugars are fermented as they are

produced. NREL estimates that cost reductions could be four times greater

for the enzyme process than for the concentrated acid process and three

times greater than for the dilute acid process.

In speaking of both corn stover and forest product waste, Industrial Bioprocessing writes, “The big stumbling block in manufacturing ethanol from

biomass is the cost of hydrolyzing cellulose into fermentable sugar.” To

speed the quest for cheaper ways of using enzymes to convert biomass to

ethanol, DOE funded research by two commercial enzyme companies,

24

New Technologies in Ethanol Production, AER-842

Office of Energy Policy and New Uses, USDA
Novozymes and Genencor International Inc. Cellulase enzymes were

recently reported to cost 45 cents per gallon of finished ethanol, making

them too expensive for commercial use (NREL, 1998). NREL estimated that

the cost could be reduced to less than 10 cents with scaled-up production. In

fact, Novozyme recently announced that its researchers have successfully

completed this project and have reduced the enzyme costs to 10-18 cents

per gallon of ethanol produced (Susan Reidy in BioFuels Journal, 2005a).

Other Biomass-to-Ethanol Improvements

NREL estimates that, in addition to reduced costs of enzyme conversion,

improvements in acid recovery and sugar yield for the concentrated acid

process could save 4 cents per gallon and that process improvements for the

dilute acid technology could save about 19 cents per gallon.

Considerable success in cellulosic conversion has already been achieved at

the experimental level. Depending on the prices of alternatives and the

success of current scaleup efforts, commercial viability may be possible in

the near term. A Canadian firm, Iogen Corporation, in partnership with the

Canadian Government, has demonstrated a process that turns wheat straw

into fermentable sugar.

25

New Technologies in Ethanol Production, AER-842

Office of Energy Policy and New Uses, USDA
Conclusions: Ethanol’s Potential

In keeping with USDA’s early estimates of the savings to come, the cost of

ethanol production has indeed fallen. Though improvements in process optimization and technology have been important, the fluctuating prices of

inputs such as corn, the price of energy alternatives, and even environmental

developments such as a drop in MTBE use, play larger roles in the fortunes

of the industry.

Ethanol production is becoming a mature industry, with savings in the next

10 years likely to be smaller than those of the last 10-15 years.

Some developments, such as dry fractionation technology—soon to be

commercially employed—may alter the structure of the industry by giving

an edge to the less capital-intensive dry-mill method. This advantage for dry

milling may make it easier to build smaller plants that are cost competitive,

and local farmer cooperatives could flourish as a result.

Promising areas to be exploited by new technology include coproduct

development, cellulosic conversion, and niche markets. The recovery of

high-value food supplements may reduce financial risk by giving the

industry an outlet outside the capricious energy market. Continued high oil

prices may spur the risk-taking necessary to overcome the initial scaleup

and development costs of cellulose-to-ethanol production. Niche markets

that take advantage of locally available feedstocks, that have local outlets

for coproducts, or that produce unique coproducts may also contribute to

the industry’s growth.

26

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Office of Energy Policy and New Uses, USDA
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