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BioEnergyPurdue extensio

Hw F eha i Ma fm CNathan S. Mosier and Klein Ileleji

Department of Agricultural and Biological EngineeringPurdue University

ID-3

Introduction

Fuel ethanol has become a very importantagricultural product over the past two

decades. In 2005, more than 13% o U.S.corn production went toward making thisuel additive/uel extender, which lessensU.S. dependence on oreign oil imports,is cleaner or the environment, and hassubstantial impact on the rural economyand agriculture production.

Fuel Ethanol

Ethanol is an alcohol produced by yeastrom sugars. It is the same alcohol pro-duced by yeast in beer, wine, and spirits.

Fuel ethanol is ethanol that has beenhighly concentrated to remove water andblended with other compounds to renderthe alcohol undrinkable. Fuel ethanol canbe used alone as a uel, such as in IndyRacing League cars, or can be blendedwith gasoline and used as uel.

All cars and trucks on the road today canuse gasoline/ethanol blends o up to 10%ethanol (90% gasoline), also called E10.Blends o up to 85% ethanol, also known

as E85, can be used as transportationuel by cars and trucks with slight modi-cations (approximately $100 per vehicle).Tese exible uel vehicles can use eithergasoline or ethanol blends, including E85.

Yeasts Role in EthanolProduction

All ethanol production is based uponthe activity o yeast (Saccharomycescerevisiae), an important microorganism

to humans. Trough a process calledermentation, yeast eat simple sugarsand produce carbon dioxide (CO

2) and

ethanol as waste products. For each

pound o simple sugars, yeast can produceapproximately pound (0.15 gallons)o ethanol and an equivalent amount ocarbon dioxide.

Corn as Ethanol Feedstock

In 2005, approximately 11 billion bushelso corn were produced in the U.S. Indianacorn production in 2005 was approxi-mately 889 million bushels (USDA, 2006).Ethanol production in the U.S. topped 4

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billion gallons in 2005 and con-sumed 1.4 billion bushels o corn,valued at $2.9 billion (NCGA,

2005). Tis represents the thirdlargest demand or U.S. corn aeranimal eed and export markets.With additional construction oethanol plants and increasingethanol demand, uel ethanol pro-duction is expected to exceed 7.5billion gallons beore the year 2012target set orth in the Energy PolicyAct o 2005 (EPAC05).

Te value o corn as a eedstock

or ethanol production is due tothe large amount o carbohydrates,specically starch, present in corn(able 1). Starch can be rather easily processed tobreak it down into simple sugars, which can then beed to yeast to produce ethanol. Modern ethanol pro-duction can produce approximately 2.7 gallons o uelethanol per bushel o corn.

Industrial Ethanol Production

Commercial production o uel ethanol in the U.S.involves breaking down the starch present in corn into

simple sugars (glucose), eeding these sugars to yeast(ermentation), and then recovering the main prod-uct (ethanol) and byproducts (e.g., animal eed). womajor industrial methods or producing uel ethanolare used in the U.S.: wet milling and dry grind. Dry-grind ethanol production represents the majority oethanol processing in the U.S. (> 70% o production),and all newly constructed ethanol plants employ somevariation on the basic dry-grind process because suchplants can be built at a smaller scale or a smallerinvestment.

Wet MillingWet milling is used to produce many products besidesuel ethanol. Large-scale, capital-intensive, corn-processing wet mills produce such varied productsas high ructose corn syrup (HFCS), biodegradableplastics, ood additives such as citric acid and xanthangum, corn oil (cooking oil), and livestock eed.

Component

Percent (average)

Dry MatterCarbohydrates (total) 84.1%

Starch 72.0%

Fiber (NDF) 9.5%

Simple Sugars 2.6%

Protein 9.5%

Oil 4.3%

Minerals 1.4%

Other 0.7%

Table 1. Composition of Corn

(rom Corn: Chemistry and Technology, 1987)

Wet milling is called wet because the rst step inthe process involves soaking the grain in water (steep-ing) to soen the grain and make it easier to separate(ractionate) the various components o the cornkernel. Fractionation, which separates the starch,ber, and germ, allows these various components tobe processed separately to make a variety o products.Te major byproducts o wet-mill ethanol production

are two animal eed products, corn gluten meal (highprotein, 40%) and corn gluten eed (low protein, 28%),and corn germ, which may be urther processed intocorn oil.

Dry Grind

In the dry-grind ethanol process, the whole grain isprocessed, and the residual components are separatedat the end o the process. Tere are ve major steps inthe dry-grind method o ethanol production.

dry-Grin ethanol Procing stp

1. Milling2. Liqueaction3. Saccharication4. Fermentation5. Distillation and recovery

Milling

Milling involves processing corn through a hammermill (with screens between 3.2 to 4.0 mm) to produce

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a corn our (Rausch et al., 2005). Tis whole cornour is slurried with water, and heat-stable enzyme(a-amylase) is added.

Liqueaction

Tis slurry is cooked, also known as liqueaction.Liqueaction is accomplished using jet-cookers thatinject steam into the corn our slurry to cook it attemperatures above 100C (212F). Te heat and me-chanical shear o the cooking process break apart thestarch granules present in the kernel endosperm, andthe enzymes break down the starch polymer into smallragments. Te cooked corn mash is then allowedto cool to 80-90C (175-195F), additional enzyme(a-amylase) is added, and the slurry is allowed to con-tinue liqueying or at least 30 minutes.

Saccharifcation

Aer liqueaction, the slurry, now called corn mash,is cooled to approximately 30C (86F), and a secondenzyme (glucoamylase) is added. Glucoamylase com-pletes the breakdown o the starch into simple sugar(glucose). Tis step, called saccharication, oen oc-curs while the mash is lling the ermentor in prepa-ration or the next step (ermentation) and continuesthroughout the next step.

Fermentation

In the ermentation step, yeast grown in seed tanks are

added to the corn mash to begin the process o con-verting the simple sugars to ethanol. Te other com-ponents o the corn kernel (protein, oil, etc.) remainlargely unchanged during the ermentation process. Inmost dry-grind ethanol plants, the ermentation pro-cess occurs in batches. A ermentation tank is lled,and the batch erments completely beore the tank isdrained and relled with a new batch.

Te up-stream processes (grinding, liqueaction, andsaccharication) and downstream processes (distil-lation and recovery) occur continuously (grain is

continuously processed through the equipment). Tus,dry-grind acilities o this design usually have threeermentors (tanks or ermentation) where, at anygiven time, one is lling, one is ermenting (usually or48 hours), and one is emptying and resetting or thenext batch.

Carbon dioxide is also produced during ermenta-tion. Usually, the carbon dioxide is not recovered and

is released rom the ermenters to the atmosphere. Irecovered, this carbon dioxide can be compressed andsold or carbonation o so drinks or rozen into dryice or cold product storage and transportation.

Aer the ermentation is complete, the ermentedcorn mash (now called beer) is emptied rom theermentor into a beer well. Te beer well stores the er-mented beer between batches and supplies a continu-ous stream o material to the ethanol recovery steps,including distillation.

Distillation and Recovery

Aer ermentation, the liquid portion o the slurryhas 8-12% ethanol by weight. Because ethanol boilsat a lower temperature than water does, the ethanolcan be separated by a process called distillation.Conventional distillation/rectication systems canproduce ethanol at 92-95% purity. Te residual wateris then removed using molecular sieves that selectivelyadsorb the water rom an ethanol/water vapor mix-ture, resulting in nearly pure ethanol (>99%).

Te residual water and corn solids that remain aerthe distillation process are called stillage. Tis wholestillage is then centriuged to separate the liquid (thinstillage) rom the solid ragments o the kernel (wetcake or distillers grains). Some o the thin stillage(backset) is recycled to the beginning o the dry-grindprocess to conserve the water used by the acility.

Te remaining thin stillage passes through evaporatorsto remove a signicant portion o the water to producethickened syrup. Usually, the syrup is blended with thedistillers grains and dried to produce an animal eedcalled istillers ried grains with olubles (DDGS).When markets or the eed product are close to theplant, the byproduct may be sold without drying asdistillers grains or wet distillers grains.

Energy Use in Ethanol Production

It is true that the laws o physics dictate that energy

will be lost in converting one orm o energy to an-other. Tus, ethanol does have less energy than thecorn used to produce it. However, this is also true orconverting crude oil to gasoline and coal to electricity.Te important questions about ethanol production areis ethanol truly a renewable uel? and how muchossil uel is used?

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Purdue AGriCulture NEW 12/06

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Yes; ethanol is a renewable uel. Te energy used toproduce ethanol includes uel or tractors, combines,and transportation o the grain to the ethanol plant, as

well as the energy in processing the corn to ethanol.However, the largest portion o the total energy pres-ent in corn is solar energy captured by the corn plantand stored in the grain as starch. When these amountsare totaled, the energy in the ethanol exceeds the ossiluel energy used to grow and process the corn by 20 to40% (Farrell et al., 2006).

Most o the energy or processing corn to ethanol isspent on the distillation and DDGS drying steps o theprocess. When wet distillers grain can be ed to live-stock close to the ethanol plant, the savings in natural

gas or drying can be as high as 20% o the total energycost or processing corn to ethanol.

Conclusions

Modern dry-grind ethanol plants can convert corngrain into ethanol (2.7-2.8 gallons per bushel) andDDGS (17 pounds per bushel). Tis rather energy-efcient process produces a renewable liquid uel thathas signicant impacts on the agricultural economyand energy use in the U.S.

Increasing ethanol production presents many oppor-

tunities and challenges or U.S. agriculture as demandson corn production or eed, uel, and export marketsincrease. Additionally, advances in biotechnology andengineering are opening possibilities or new rawmaterials, such as switch grass and corn stover, to beused or even greater uel ethanol production intothe uture.

Reerences and Links toFurther Inormation

Farrell, A. E.; Plevin, R. J.; urner, B. .; Jones A.D.; OHare, M.; Kammen, D. M. Ethanol CanContribute to Energy and Environmental Goals,Science 311(5760): 506 508, (2006).

National Corn Growers Association (NCGA) AnnualReport (2005).

Purdue Laboratory o Renewable ResourcesEngineering .

Rausch, K. D.; Belyea, R. L.; Ellersieck, M. R.; Singh,

V.; Johnston, D. B.; umbleson, M. E. Particle SizeDistributions o Ground Corn and DDGS FromDry Grind Processing, Transactions of the ASAE,48(1):273277, (2005).

U.S. Department o Agriculture, National AgricultureStatistics Service .

Watson, S. A., Structure and Composition, Corn:Chemistry and Technology, Watson, S. A. andRamstad, P. E. (eds). American Association oCereal Chemists, Inc. pp 53-82, (1987).

Visit orree, downloadable copies o all o the publicationsin the Purdue Extension BioEnergy series.