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Fueling America Through Renewable Resources BioEnergy Purdue e x tension How Fuel e thanol is Made from Corn Nathan S. Mosier and Klein Ileleji Department of Agricultural and Biological Engineering Purdue University ID-328 Introduction Fuel ethanol has become a very important agricultural product over the past two decades. In 2005, more than 13% of U.S. corn production went toward making this fuel additive/fuel extender, which lessens U.S. dependence on foreign oil imports, is cleaner for the environment, and has substantial impact on the rural economy and agriculture production. Fuel Ethanol Ethanol is an alcohol produced by yeast from sugars. It is the same alcohol pro- duced by yeast in beer, wine, and spirits. Fuel ethanol is ethanol that has been highly concentrated to remove water and blended with other compounds to render the alcohol undrinkable. Fuel ethanol can be used alone as a fuel, such as in Indy Racing League cars, or can be blended with gasoline and used as fuel. All cars and trucks on the road today can use gasoline/ethanol blends of up to 10% ethanol (90% gasoline), also called “E10.” Blends of up to 85% ethanol, also known as “E85,” can be used as transportation fuel by cars and trucks with slight modifi- cations (approximately $100 per vehicle). These flexible fuel vehicles can use either gasoline or ethanol blends, including E85. Yeasts Role in Ethanol Production All ethanol production is based upon the activity of yeast (Saccharomyces cerevisiae), an important microorganism to humans. Through a process called “fermentation, ” yeast eat simple sugars and produce carbon dioxide (CO 2 ) and ethanol as waste products. For each pound of simple sugars, yeast can produce approximately pound (0.15 gallons) of ethanol and an equivalent amount of carbon dioxide. Corn as Ethanol Feedstock In 2005, approximately 11 billion bushels of corn were produced in the U.S. Indiana corn production in 2005 was approxi- mately 889 million bushels (USDA, 2006). Ethanol production in the U.S. topped 4 Fueling America Through Renewable Crops BioEnergy billion gallons in 2005 and con- sumed 1.4 billion bushels of corn, valued at $2.9 billion (NCGA, 2005). This represents the third largest demand for U.S. corn after animal feed and export markets. With additional construction of ethanol plants and increasing ethanol demand, fuel ethanol pro- duction is expected to exceed 7.5 billion gallons before the year 2012 target set forth in the Energy Policy Act of 2005 (EPACT05). The value of corn as a feedstock for ethanol production is due to the large amount of carbohydrates, specifically starch, present in corn (Table 1). Starch can be rather easily processed to break it down into simple sugars, which can then be fed to yeast to produce ethanol. Modern ethanol pro- duction can produce approximately 2.7 gallons of fuel ethanol per bushel of corn. Industrial Ethanol Production Commercial production of fuel ethanol in the U.S. involves breaking down the starch present in corn into simple sugars (glucose), feeding these sugars to yeast (fermentation), and then recovering the main prod- uct (ethanol) and byproducts (e.g., animal feed). Two major industrial methods for producing fuel ethanol are used in the U.S.: wet milling and dry grind. Dry- grind ethanol production represents the majority of ethanol processing in the U.S. ( 70% of production), and all newly constructed ethanol plants employ some variation on the basic dry-grind process because such plants can be built at a smaller scale for a smaller investment. Wet Milling Wet milling is used to produce many products besides fuel ethanol. Large-scale, capital-intensive, corn- processing wet mills produce such varied products as high fructose corn syrup (HFCS), biodegradable plastics, food additives such as citric acid and xanthan gum, corn oil (cooking oil), and livestock feed. Component Percent (average) Dry Matter Carbohydrates (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 (from Corn: Chemistry and Technology, 1987) Wet milling is called “wet” because the first step in the process involves soaking the grain in water (steep- ing) to soften the grain and make it easier to separate (fractionate) the various components of the corn kernel. Fractionation, which separates the starch, fiber, and germ, allows these various components to be processed separately to make a variety of products. The major byproducts of wet-mill ethanol production are two animal feed products, corn gluten meal (high protein, 40%) and corn gluten feed (low protein, 28%), and corn germ, which may be further processed into corn oil. Dry Grind In the dry-grind ethanol process, the whole grain is processed, and the residual components are separated at the end of the process. There are five major steps in the dry-grind method of ethanol production. dry-Grind e thanol Processing s teps 1. Milling 2. Liquefaction 3. Saccharification 4. Fermentation 5. Distillation and recovery Milling Milling involves processing corn through a hammer mill (with screens between 3.2 to 4.0 mm) to produce Purdue e xtension How Ethanol Is Made from Corn ID-328 a corn flour (Rausch et al., 2005). This whole corn flour is slurried with water, and heat-stable enzyme ( a-amylase) is added. Liquefaction This slurry is cooked, also known as “l(fā)iquefaction. ” Liquefaction is accomplished using jet-cookers that inject steam into the corn flour slurry to cook it at temperatures above 100C (212F). The heat and me- chanical shear of the cooking process break apart the starch granules present in the kernel endosperm, and the enzymes break down the starch polymer into small fragments. The cooked corn mash is then allowed to cool to 80-90C (175-195F), additional enzyme ( a-amylase) is added, and the slurry is allowed to con- tinue liquefying for at least 30 minutes. Saccharification After liquefaction, the slurry, now called “corn mash, ” is cooled to approximately 30C (86F), and a second enzyme (glucoamylase) is added. Glucoamylase com- pletes the breakdown of the starch into simple sugar (glucose). This step, called “saccharification, ” often oc - curs while the mash is filling the fermentor in prepa- ration for the next step (fermentation) and continues throughout the next step. Fermentation In the fermentation step, yeast grown in seed tanks are added to the corn mash to begin the process of con- verting the simple sugars to ethanol. The other com- ponents of the corn kernel (protein, oil, etc.) remain largely unchanged during the fermentation process. In most dry-grind ethanol plants, the fermentation pro- cess occurs in batches. A fermentation tank is filled, and the batch ferments completely before the tank is drained and refilled with a new batch. The up-stream processes (grinding, liquefaction, and saccharification) and downstream processes (distil- lation and recovery) occur continuously (grain is continuously processed through the equipment). Thus, dry-grind facilities of this design usually have three fermentors (tanks for fermentation) where, at any given time, one is filling, one is fermenting (usually for 48 hours), and one is emptying and resetting for the next batch. Carbon dioxide is also produced during fermenta- tion. Usually, the carbon dioxide is not recovered and is released from the fermenters to the atmosphere. If recovered, this carbon dioxide can be compressed and sold for carbonation of soft drinks or frozen into dry ice for cold product storage and transportation. After the fermentation is complete, the fermented corn mash (now called “beer”) is emptied from the fermentor into a beer well. The beer well stores the fer- mented beer between batches and supplies a continu- ous stream of material to the ethanol recovery steps, including distillation. Distillation and Recovery After fermentation, the liquid portion of the slurry has 8-12% ethanol by weight. Because ethanol boils at a lower temperature than water does, the ethanol can be separated by a process called “distillation. ” Conventional distillation/rectification systems can produce ethanol at 92-95% purity. The residual water is then removed using molecular sieves that selectively adsorb the water from an ethanol/water vapor mix- ture, resulting in nearly pure ethanol (99%). The residual water and corn solids that remain after the distillation process are called “stillage. ” This whole stillage is then centrifuged to separate the liquid (thin stillage) from the solid fragments of the kernel (wet cake or distillers grains). Some of the thin stillage (backset) is recycled to the beginning of the dry-grind process to conserve the water used by the facility. The remaining thin stillage passes through evaporators to remove a significant portion of the water to produce thickened syrup. Usually, the syrup is blended with the distillers grains and dried to produce an animal feed called “distillers dried grains with solubles” (DDGS). When markets for the feed product are close to the plant, the byproduct may be sold without drying as distillers grains or wet distillers grains. Energy Use in Ethanol Production It is true that the laws of physics dictate that energy will be lost in converting one form of energy to an- other. Thus, ethanol does have less energy than the corn used to produce it. However, this is also true for converting crude oil to gasoline and coal to electricity. The important questions about ethanol production are “is ethanol truly a renewable fuel?” and “how much fossil fuel is used?” Fueling America Through Renewable Crops BioEnergy Purdue AGri Culture NEW 12/06 It is the policy of the Purdue University Cooperative Extension Service, David C. Petritz, Director, that all persons shall have equal opportunity and access to the programs and facilities without regard to race, color, sex, religion, national origin, age, marital status, parental status, sexual orientation, or disability. Purdue University is an Affirmative Action institution. This material may be available in alternative formats. 1-888-EXT-INFO http:/www.ces.purdue.edu/new Y es; ethanol is a renewable fuel. The energy used to produce ethanol includes fuel for tractors, combines, and transportation of the grain to the ethanol plant, as well as the energy in processing the corn to ethanol. However, the largest portion of the total energy pres- ent in corn is solar energy captured by the corn plant and stored in the grain as starch. When these amounts are totaled, the energy in the ethanol exceeds the fossil fuel energy used to grow and process the corn by 20 to 40% (Farrell et al., 2006). Most of the energy for processing corn to ethanol is spent on the distillation and DDGS drying steps of the process. When wet distillers grain can be fed to live- stock close to the ethanol plant, the savings in natural gas for drying can be as high as 20% of the total energy cost for processing corn to ethanol. Conclusions Modern dry-grind ethanol plants can convert corn grain into ethanol (2.7-2.8 gallons per bushel) and DDGS (17 pounds per bushel). This rather energy- efficient process produces a renewable liquid fuel that has significant impacts on the agricultural economy and energy use in the U.S. Increasing ethanol production presents many oppor- tunities and challenges for U.S. agriculture as demands on corn production for feed, fuel, and export markets increase. Additionally, advances in biotechnology and engineering are opening possibilities for new raw materials, such as switch grass and corn stover, to be used for even greater fuel ethanol production into the future. References and Links to Further Information Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones A. D.; OHare, M.; Kammen, D. M. “Ethanol Can Contribute to Energy and Environmental Goals, ” Science 311(5760): 506 508, (2006). National Corn Growers Association (NCGA) Annual Report (2005). Purdue Laboratory of Renewable Resources Engineering . Rausch, K. D.; Belyea, R. L.; Ellersieck, M. R.; Singh, V .; Johnston, D. B.; Tumbleson, M. E. “Particle Size Distributions of Ground Corn and DDGS From Dry Grind Processing, ” Transactions of the ASAE, 48(1):273277, (2005). U.S. Department of Agriculture, National Agriculture Statistics Service . Watson, S. A., “Structure and Composition, ” Corn: Chemistry and Technology, Watson, S. A. and Ramstad, P . E. (eds). American Association of Cereal Chemists, Inc. pp 53-82, (1987). Visit for free, downloadable copies of all of the publications in the Purdue Extension BioEnergy series.
如何由玉米生產(chǎn)乙醇
Nathan S. Mosier and Klein Ileleji
摘要: 燃料乙醇在過去20年來已成為一個非常重要的農(nóng)產(chǎn)品。 2005年，超過13％的美國玉米產(chǎn)量作出了對這種燃料添加劑/燃料擴展，從而減少美國對外國石油進(jìn)口的依賴，是對環(huán)境更清潔，并已對農(nóng)村經(jīng)濟和農(nóng)業(yè)生產(chǎn)產(chǎn)生重大影響的農(nóng)產(chǎn)品。
關(guān)鍵詞：玉米,乙醇,發(fā)酵法,發(fā)酵車間,蒸餾車間
1. 燃料乙醇
乙醇是糖由酵母發(fā)酵而成的。它是相同的發(fā)酵酒精生產(chǎn)與在啤酒，葡萄酒，和烈酒上。燃料乙醇是乙醇已高度集中，以消除與其他化合物混合，使水和酒精不能飲用。燃料乙醇可單獨使用作為燃料，如汽車在印地賽車聯(lián)盟，或者可以與汽油混合，作為燃料使用。
今天所有在道路上的汽車和卡車可以使用汽油高達(dá)10％/乙醇混合燃料乙醇（90％汽油），也被稱為“E10中。”高達(dá)85％的乙醇混合，也稱為為“E85燃料”，可以用作運輸?shù)钠嚭停s每車100美元）稍作修改卡車的燃料。這些靈活燃料汽車可以使用汽油或包括混合E85燃料乙醇。
2. 酵母在生產(chǎn)乙醇中的作用
所有的乙醇生產(chǎn)是根據(jù)酵母活性（酵母），一個重要的微生物到人類。通過這個過程稱為“發(fā)酵”，用簡單的糖和酵母產(chǎn)生的二氧化碳（CO2）和乙醇作為廢品。對于每一個簡單的糖磅，酵母可以生產(chǎn)大約一磅（0.15加侖）乙醇和二氧化碳等量。
3. 玉米作為乙醇原料
表1—2 玉米的組成
水分 蛋白質(zhì) 脂肪 碳水化合物 粗纖維 灰分
6—15 8.5 5—7 65—73 1.3 1.7
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2005年，約11億蒲式耳玉米生產(chǎn)了在美國印第安納州，2005年玉米產(chǎn)量約為8.89億蒲式耳（農(nóng)業(yè)部，2006）。美國乙醇生產(chǎn)在2005年突破4billion加侖，花費1.4億蒲式耳的玉米，價值在29億美元（NCGA，2005年）。這是后動物飼料和出口市場為美國玉米的第三大需求。隨著更多的乙醇工廠建設(shè)的需求和增加乙醇，燃料乙醇產(chǎn)量預(yù)計將超過之前設(shè)定的2005年能源政策法案（EPACT05提出了今年2012target 7.5billion加侖）。
????對玉米作為乙醇生產(chǎn)原料的價值，是因為其含有大量的碳水化合物，特別是淀粉，玉米目前的淀粉加工可以很輕松生成單糖，然后可以加入生產(chǎn)乙醇，現(xiàn)代乙醇生產(chǎn)技術(shù)可達(dá)到生產(chǎn)每加侖燃料乙醇用約2.7蒲式耳玉米。
4. 工業(yè)生產(chǎn)乙醇
在美國工業(yè)生產(chǎn)生產(chǎn)商業(yè)乙醇燃料乙醇是由玉米淀粉轉(zhuǎn)化為單糖(葡萄糖)，主要產(chǎn)品(乙醇)和副產(chǎn)品（例如，動物飼料）。兩大生產(chǎn)燃料乙醇工業(yè)的方法是用在美國：濕磨和干磨。干磨乙醇生產(chǎn)的乙醇處理代表在美國（> 70％的多數(shù)產(chǎn)品），所有新建乙醇工廠雇用了一些基本的干磨工藝變化，因為這種植物可以在一個較小的規(guī)模上建立了一個小投資。
4.1濕磨法
濕磨法是用于除生產(chǎn)燃料乙醇等的許多產(chǎn)品。大型的，資本密集，玉米深加工生產(chǎn)等不同的濕造紙廠的產(chǎn)品，高果糖玉米糖漿（高果糖漿），可生物降解塑料，食品添加劑，如檸檬酸，黃原膠，玉米油（烹調(diào)油），和牲畜飼料。
濕磨被稱為“濕”，因為在這個過程中的第一步是浸泡在水中的糧食（浸泡）軟化糧食，使其更易于分離（分餾）玉米粒的各個組成部分，使淀粉，纖維，細(xì)菌，等各組成部分分別進(jìn)行處理。
濕法磨乙醇生產(chǎn)的主要副產(chǎn)品是兩個動物飼料產(chǎn)品，玉米蛋白粉（高蛋白質(zhì)，40％）和玉米蛋白飼料（低蛋白，28％），玉米胚芽，可進(jìn)一步加工成玉米油。
4.2干磨
在干磨法生產(chǎn)乙醇的過程中，整個糧食處理主要有五個步驟：
·(1)干磨（粉碎）
(2)液化
(3)糖化
(4)發(fā)酵
(5)蒸餾和回收
4.2.1干磨
通過干磨法加工涉及錘磨把玉米粒粉碎到直徑為3.2之間（玉米4.0毫米），以產(chǎn)生一種玉米面粉，然后給玉米粉加水混合為水漿，加入熱穩(wěn)定酶（a-淀粉酶）。
4.2.1液化
這是煮漿，也稱為“液化”。液化是使用噴射爐，使玉米粉漿液在高于100℃（212 ° F）的溫度下。利用熱量和蒸煮過程的機械剪切斷使淀粉顆粒的內(nèi)核胚乳在酶的作用下分解成小片段的淀粉聚合物。煮熟的玉米泥，然后讓其冷卻至80-90℃（175-195 ° F），另外添加酶a-淀粉酶，并且繼續(xù)至少液化30分鐘玉米泥漿。
4.2.2糖化
液化后，漿料，現(xiàn)在被稱為“玉米醪”，被冷卻到大約30℃（86 ° F）時，添加第二個酶（糖化酶）被。糖化酶完成對成簡單的糖（葡萄糖淀粉崩潰）。這一步，被稱為“糖化”。經(jīng)常情況下，這一步的生產(chǎn)條件對整個下一步（在發(fā)酵罐發(fā)酵）的產(chǎn)品質(zhì)量有很大影響。
4.2.3發(fā)酵
在發(fā)酵過程中，酵母生長在發(fā)酵罐中，添加玉米醪開始簡單的糖轉(zhuǎn)化為乙醇的過程。在發(fā)酵過程，玉米粒（蛋白質(zhì)，油，等）的其他組成部分大致維持不變。在大多數(shù)干磨乙醇工廠，采用連續(xù)添加法，生產(chǎn)開始時，先將一定量的酒母打入發(fā)酵罐，然后根據(jù)生產(chǎn)，確定流加速度。一般從接種酵母后，應(yīng)于6—8小時內(nèi)將罐裝滿。
?上流過程（研磨，液化，糖化）和（蒸餾和回收）下游工序連續(xù)發(fā)生（糧食是不斷通過設(shè)備處理）。因此，這種設(shè)計的干磨設(shè)備通常有3個發(fā)酵罐（發(fā)酵罐）凡在任何時候，一個是填充，一個是發(fā)酵（通常為48小時），一個是清空，為下一批準(zhǔn)備。
??? 二氧化碳是發(fā)酵過程中產(chǎn)生的。通常情況下，二氧化碳不回收，并從發(fā)酵罐中釋放到大氣中。如果收回，這可以被壓縮二氧化碳和碳酸汽水出售或冷凍干燥成冰冷產(chǎn)品儲存和運輸。 4.2.4蒸餾和恢復(fù)
發(fā)酵后，發(fā)酵醪部分含8-12％（質(zhì)量分?jǐn)?shù)）的乙醇。因為乙醇的沸點為78.5℃,比水的沸點低，因此，乙醇可的提純開可以為“蒸餾”的過程。常規(guī)蒸餾可以生產(chǎn)出92-95％純度乙醇。為了進(jìn)一步提高乙醇的純度，可以使乙醇/水蒸汽的混合物經(jīng)過分子篩的吸附分離，從而可以得到濃度達(dá)到“99％（體積分?jǐn)?shù)）的乙醇產(chǎn)品。
蒸餾剩余的水和玉米固體，經(jīng)過蒸餾過程中仍然被稱為“酒糟。酒糟經(jīng)離心后，然后把薄酒糟（濕餅或酒糟固體碎片的液體）。回收到的干磨進(jìn)程的開始，用來把干磨后的玉米粉調(diào)為玉米粉漿，以節(jié)約設(shè)施使用的水。
余下的酒糟通過蒸發(fā)器除去水，很大一部分生產(chǎn)濃縮糖漿。通常，糖漿與酒糟混合，干燥，生產(chǎn)動物飼料的所謂“酒糟'干籽粒可溶物”（DDGS的）。當(dāng)飼料產(chǎn)品市場正好接近工廠，副產(chǎn)品直接可售，從而沒有酒糟或'濕酒糟干燥。
5. 生產(chǎn)乙醇的能源利用
誠然，物理定律支配能量是在一個轉(zhuǎn)換到另一個形式的能源損失。因此，用玉米來制造能源乙醇也有不足。不過，這也是真正轉(zhuǎn)化為汽油和原油煤發(fā)電。
對乙醇生產(chǎn)的重要問題是“是一種真正可再生的燃料乙醇？”和“使用多少化石燃料？”
??? 是的，糧食能源用于生產(chǎn)乙醇，乙醇是一種可再生的燃料，包括拖拉機，聯(lián)合收割機燃料，糧食運到乙醇工廠，以及在處理玉米乙醇能源。然而，在目前玉米總能量最大的部分是玉米植株對太陽能的捕獲和儲存的糧食淀粉。用于種植和加工玉米生產(chǎn)乙醇的能源超過了化石燃料的能源，總計20％—40％（法瑞爾等人，2006）。
在玉米加工乙醇時，能量大部分是用于蒸餾和DDGS的烘干過程的步驟。'濕酒糟顆?？梢晕桂B(yǎng)牲畜且接近乙醇廠時即可節(jié)省烘干的能量，可為干燥玉米和加工乙醇節(jié)約高達(dá)20％的總能量消耗。
6.結(jié)論
現(xiàn)代干磨乙醇工廠可以把玉米粒轉(zhuǎn)換成乙醇（每蒲式耳2.7-2.8加侖）和DDGS的（每蒲式耳17磅）。這個相當(dāng)有效的過程產(chǎn)生的能源可再生液體燃料對 農(nóng)業(yè)經(jīng)濟和能源在美國使用的重大影響
美國作為農(nóng)業(yè)的要求和挑戰(zhàn)，增加乙醇生產(chǎn)帶來許多機會。玉米飼料，燃料的生產(chǎn)和出口市場的增長。此外，由于生物技術(shù)的進(jìn)步，人類利用這些新技術(shù)工程正在打開新的原料，如草，玉米秸稈等利用的可能性，相信將有更大的燃料乙醇在未來生產(chǎn)中使用。
參考資料
【1】Farrell, A. E.; Plevin, R. J.; Turner, B. T.; Jones A. D.; O’Hare, M.; Kammen, D. M. “Ethanol Can Contribute to Energy and Environmental Goals,” Science 311(5760): 506 – 508, (2006).
【2】National Corn Growers Association (NCGA) AnnualReport (2005).
【3】Purdue Laboratory of Renewable Resources Engineering
【4】 Rausch, K. D.; Belyea, R. L.; Ellersieck, M. R.; Singh, V.; Johnston, D. B.; Tumbleson, M. E. “Particle Size Distributions of Ground Corn and DDGS From Dry Grind Processing,” Transactions of the ASAE, 48(1):273?277, (2005).。
【5】U.S. Department of Agriculture, National Agriculture Statistics Service
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【6】Watson, S. A., “Structure and Composition,” Corn: Chemistry and Technology, Watson, S. A. And Ramstad, P. E. (eds). American Association of Cereal Chemists, Inc. pp 53-82, (1987).
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