A biofuel is a fuel that contains energy produced by relatively recent carbon photosynthesis. Commonly used fuel sources such as petroleum, coal, and natural gas all resulted from photosynthesis, but that photosynthesis occurred millions of years ago.

Plants produce biomass from photosynthesis and that biomass can be used by humans for fuel. For most fuels, at least some conversion has taken place. The conversion may be relatively minor, such as pellets made from switchgrass plants before use in electricity generation systems. Or, the conversion may be extensive, such as the thermal conversion of biomass to synthetic gases.

The Renewable Fuels Standards (RFS) resulted from an act of the US Congress. The standards were expanded in the Energy Independence and Security Act. Fuels impacted by Renewable Fuel Standards are liquid transportation fuels. Although plants exhibited in the MU Biofuels Garden have energy uses other than transportation, liquid fuels for transportation (blended into gasoline or diesel fuel) is the primary focus of the Garden.

I. Plants That Produce Feedstocks for Ethanol

Ethanol (ethyl alcohol) is a simple molecule with two carbon atoms. During combustion in an engine the bond between the two carbon atoms is broken and energy is released. Part of that energy is harnessed to move the vehicle and the rest dissipates as heat.

Fuel ethanol is produced during fermentation, similar to how beverage ethanol is made. The organisms responsible for fermentation, yeasts, (e.g. Saccharomyces cerevisiae) are members of the Fungi kingdom. Although sometimes called anaerobic respiration, the yeasts used to make fuel ethanol can, and do, produce ethanol in the presence of oxygen.

Ethanol is produced in a two phased process. In the first phase, 6-carbon sugars (e.g. glucose, fructose) are metabolized in a process called glycolysis to form pyruvate. During the second phase, pyruvate is converted to ethyl alcohol. Yeasts obtain energy for growth and reproduction through fermentation. During fermentation, one molecule of carbon dioxide is released as each molecule of ethanol is produced.

Most yeast species require free sugars such as glucose to produce ethanol. Sucrose (disaccharide of glucose and fructose) can be used, but the enzyme, invertase, first converts sucrose to two molecules of glucose. Many plants store carbohydrate in complex molecules such as starch or cellulose. Sucrose, starch, and cellulose are three common ethanol feedstocks made by plants.

Regardless of the feedstock, fermentation from glucose to ethanol takes about 40 to 50 hours. During fermentation, the components (mash) are agitated and kept cool to facilitate the activity of the yeast. When fermentation is terminated, the liquid portion of the slurry is 8 to 12% ethanol. In some facilities, carbon dioxide is captured as a valuable by-product with various uses (e.g. soft drink carbonation, dry ice).

Nearly all of the water must be removed before ethanol can be used for fuel. So, the resulting “beer” is transferred to distillation columns where the ethanol is separated from the “stillage.” Ethanol is concentrated to 190 proof (95% ethanol) during distillation. Distillation is possible because ethanol and water have different boiling point temperatures. Ethanol is then dehydrated to approximately 200 proof (100% ethanol) in a molecular sieve system. The anhydrous ethanol must be denatured (usually with 5% gasoline) to make it undrinkable and therefore, not subject to beverage alcohol tax.

A. Plants that produce sucrose feedstock for ethanol

Two plants, sugarcane and sugar beet, account for the nearly all of the world’s production of sugar. A third plant, sweet sorghum, can be used although acerage is small. The disaccharide, sucrose, is extracted, processed, and used as a sweetener. Although all plants produce sucrose, sugarcane and sugar beet store sucrose at concentrations large enough to be commercially viable sources for sugar. Unlike either starch or cellulose, fermentation yeasts can use sucrose directly without pretreatment. Because pretreatment is not required, ethanol production from sugarcane and sugar beet may be more efficient.

1. Sugarcane

Although several plant species are called sugarcane, Saccharum officinarum is the species cultivated for sugar production in the USA. It is a tropical, perennial grass plant that is more than 10 feet tall at harvest. Sugarcane plants, cannot tolerate freezing temperatures, so it is grown in parts of only four US states (Florida, Hawaii, Louisiana, and Texas) where frost is rare.

Sugar is stored in stems, so entire stems are harvested. After stems are cut and crushed, sugar is extracted into water for refining. It is not uncommon for commercial operations to produce both refined sugar for human consumption and ethanol for fuel.

The high fiber plant material that remains after sucrose extraction is called bagasse. Bagasse is primarily lignin and cellulose. After drying, it is often burned to produce heat and electricity, which meets some of the energy needs of the ethanol operation.

Sugarcane stems are 12 to 15% soluble sugars. Average sugar cane yield in the US is about 35 tons per acre. Although some ethanol yield estimates are much higher, about 600 gallons ethanol per acre is a reasonable estimate.

2. Sugar beet

Sugar beet, Beta vulgaris, is the same species as garden beets. Varieties grown for sugar produce large taproots (image1, image2) that store relatively large quantities of sugar. Sugar beet is a temperate crop adapted to northern latitudes. At least 10 US states (not Missouri) grow sugar beet with Minnesota producing about one third of the US crop. Before harvest, sugar beet tops are usually cut off the plants. The beet harvester lifts roots from soil. Roots are washed and sliced, and sugar is extracted into hot water.

Sucrose concentration in sugar beet roots may be as much as 20%. Average sugar beet yield in the US is about 30 tons per acre. Although some ethanol yield estimates are much higher, about 580 gallons of ethanol per acre is a reasonable estimate.

3. Sweet sorghum

Sucrose is stored in the stalks of sweet sorghum (image1, image2). Sap, containing the sugars, is squeezed from the stalks using mechanical pressure. Ethanol production of about 400 gallons/acre is possible from sweet sorghum.

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B. Plants that produce starch feedstock for ethanol

Starch is a complex carbohydrate consisting of long chains of glucose molecules. It is the primary storage compound in the seeds of cereal plants such as corn, grain sorghum, and various small grains. Two forms of starch exist. Amylose is composed of straight glucose chains, and amylopectin possesses branched chains of glucose. In either case, glucose is the only component of starch.

Yeasts used in ethanol facilities cannot use starch. So, enzymes or other treatments (e.g. heat) must be made to break starch into its component glucose. This pretreatment process is called liquefaction and saccharification. Although it adds expense, pretreatment is essential so that yeast has access to glucose.

Because all cereal grain seeds contain oil and protein, valuable by products are derived from ethanol production. A common by product is dry distillers grains with solubles (DDGS), which is used in livestock rations.

1. Corn

Corn, Zea mays, is the most important US crop in terms of acreage and cash receipts. More than 14 billion bushels of corn grain are produced from about 90 million acres each year. Corn is an annual, grass plant with a robust stem often 8 feet tall at maturity. Arrangement of flowers on plants is unusual in that nearly 1000 female flowers occur on an inflorescence called the ear (image1, image2), and hundreds of male flowers occur on an inflorescence called the tassel (image1, image2). Kernels and are actually single-seeded fruit in which the ovary wall (pericarp) is tightly fused to the seed coat. The technical name for grass plant fruit is caryopsis.

Corn is by far the most common feedstock for ethanol in the US. On a dry weight basis, corn kernels are more than 75% starch. Each bushel (56 pounds) produces about 2.8 gallons of ethanol and 17 pounds of DDGS. Average yield in Missouri is about 150 bushels; so an average acre of corn can produce about 420 gallons of ethanol.

2. Grain sorghum

Grain sorghum, Sorghum bicolor, is an annual, grass plant. Kernels are much smaller than corn kernels and are borne at the top of the plant in a panicle inflorescence. Starch composition and ethanol yield per bushel are similar to corn.

Grain sorghum plants can tolerate drier and hotter weather than corn plants, so production is centered in the Plain States. Production in the US is 450 million bushels from about 7 million acres. Average yield in Missouri is 95 bushels per acre, so an average acre of grain sorghum can produce about 266 gallons of ethanol.

3. Small grains

Wheat (Triticum aestivum), oat (Avena sativa), barley (Hordeum vulgare) and rye (Secale cereal) are often classified as small grains. Both plant and kernel sizes are smaller than corn (sometimes called a coarse grain). Seeds from small grain crops are high in starch composition, so could be used to produce ethanol. However, high protein content and other kernel characteristics make it difficult for an ethanol facility that uses corn or grain sorghum to use wheat or other small grains. The MU Biofuels Garden does not display any of the small grain species, but they can be found in the MU Small Grain Crops Garden.

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C. Plants that produce cellulose feedstock for ethanol

Plants grown for cellulose feedstock are grown for their cell wall constituents instead of seeds or sugar-containing sap. Animals do not produce cell walls, but all cells of plants are surrounded with a tough, rigid layer. Plant cell walls contain several complex compounds that have important implications for ethanol production. These compounds are arranged in a complicated structure that requires extensive pretreatment before fermentation yeasts can use the constituents.

Cell walls of higher plants contain cellulose, hemicellulose, and lignin. Cellulose molecules are long chains of glucose molecules. Because cellulose contains glucose it is a potentially important feedstock for ethanol production.

The difference between cellulose and starch is the type of bond between the glucose monomers. Bonds in glucose are classified as α- 1,4. Humans and animals make enzymes that can break this bond, so starch is highly digestible. Enzymes used in pretreating starch, such as α- amylase, cleave these bonds to release glucose. Bonds in cellulose are classified as ß- 1,4. Humans and animals do not have an enzyme capable of breaking this bond (ruminant animals rely on microorganisms to digest cellulose), so cellulose is not digestible. Human dietary fiber is cellulose and is important because it cannot be digested. To release glucose for the fermentation yeasts, cellulosic materials must be treated with acids and/or enzymes.

Hemicellulose structure is more complex than the structure of cellulose. The most common constituent is the 5-carbon monomer, xylose. Other sugars in hemicellulose include mannose, galactose, and arabinose. Metabolism of hemicellulose requires yet another set of enzymes. And, few yeast species are capable of converting xylose and other 5-carbon sugars to ethanol.

Lignin possesses a structure that is far more complicated than either starch or cellulose and contains numerous phenolic subunits. It is not used as an ethanol feedstock, but lignin forms a crystalline structure that must be removed to uncover cellulose and hemicellulose. Lignin is a major component of bagasse and other residues from ethanol production. These residues can be dried and burned to produce heat and electricity at cellulosic ethanol facilities.

Because of the complicated chemistry and structure of cellulosic feedstock, considerable pretreatment must take place before fermentation is possible. Physical pretreatment involves grinding to reduce particle size. Chemical pretreatment may include both acids (e.g. sulfuric) and several enzymes (e.g. cellulase). After chemicals are removed, the free sugars enter into the fermentation process as described for sucrose and starch. Average ethanol yield from cellulosic material is about 100 gallons per dry ton of feedstock.

Widespread use of cellulosic feedstock for ethanol faces several technological challenges. Because of the amount of pretreatment required, the mixtures of sugars released, and the amount of residue produced, it is difficult to economically produce ethanol from cellulosic feedstock. In addition, because the material is less dense than grains, large amounts of material must be harvested, transported and stored to produce equivalent amounts of ethanol.

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1. Miscanthus

Miscanthus is a genus with at least 15 perennial grass species. Several species are known to gardeners and used for ornamental purposes. The species most often grown for ethanol, Miscanthus giganteus, is a cross between two species (M. sinensis and M. saccariflorus). It is a large plant (up to 12 feet tall) with potentially large biomass yields.

Miscanthus is sterile and does not produce seed. Plants are established by planting small pieces of rhizome (underground stem). Establishment is expensive, but several annual harvests can be taken over the lifetime of the field. Harvest will usually occur during very late autumn and winter. Yield may be less than if the plants were harvested green because leaves are lost, but ethanol yield per ton is greater.

Yield is dependent on temperature, moisture, and fertility, but yields as large as 15 tons/acre have been recorded in research trials. A more modest yield of 7 to 10 tons per acre is likely in Missouri. So, an average Missouri acre of Miscanthus in Missouri could produce about 700 to 1000 gallons of ethanol.

2. Native perennial grasses

Several warm season grasses were components of the American tallgrass prairies. Examples of native perennial grasses are switchgrass (Panicum virgatum)(plant, inflorescence), big bluestem (Andropogon gerardii)(plant, inflorescence), eastern gamagrass (Tripsacum dactyloides)(plant, inflorescence), and Indiangrass (Sorghastrum nutans)(plant, inflorescence).These grasses are native to North America and are currently used as forages for livestock, to protect soil surfaces from erosion, and for wildlife cover. They are also frequently mentioned feedstocks for ethanol.

Native perennial grasses are established by planting seeds. Because these grasses are common in Missouri, Missouri farmers are experienced with their establishment and management. Like Miscanthus, harvest for ethanol would likely take place after frost.

Yields of native perennial grasses depend on temperature, moisture, and fertility, but yields in Missouri are likely to range from 3 to 6 dry tons per acre. So, ethanol yield would be about 300 to 600 gallons per acre.

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3. Forage sorghum

Forage sorghum is closely related to grain sorghum, but because it has no dwarfing genes, forage sorghum grows 2 to 4 times taller. Unlike native perennial grasses, forage sorghum is native to Africa and is an annual. It must be planted each year. Although most often used to produce silage for livestock feed, forage sorghum is also mentioned as a cellulosic feedstock. Forage sorghum is not commonly grown in Missouri, but yield would not likely exceed 6 dry tons/acre.

4. Plant residue

Grain crops produce fairly large quantities of plant residues. These residues may be called stover if produced by corn or grain sorghum and straw if produced by small grains. Plant residues can be used as cellulosic feedstock.

Plant residue should not be considered waste material. It functions as ground cover during winter, a provider of organic matter to improve soil structure, and a source of mineral nutrients for other crops. Careful removal of plant residues can be made so that the negative effects on soil characteristics are limited.

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II. Plants That Produce Feedstocks for Biodiesel

Biodiesel is produced from plant oils and animal fats. Plant oils are stored almost exclusively in the embryos of seeds. Plants that produce seeds with large embryos store large amounts of vegetable oil. Often these plants are called oilseeds. Vegetable oils have many uses, but more than 90% of the vegetable oil produced in the US is used for human consumption. Examples of food products made from vegetable oils include cooking oils, salad dressing, shortening, margarine, and mayonnaise. Industrial products made using vegetable oils include, paints, varnishes, soaps, inks, and plastics.

Vegetable oils are liquid at room temperature and composed primarily of triglycerides. A triglyceride is three fatty acids attached to a glycerol molecule. There are 5 to 7 common fatty acids in vegetable oil. Plant species and varieties within species differ for their fatty acid mix. Each fatty acid is connected to glycerol by an ester bond and contains 14 or more carbon atoms arranged in a straight chain.  

Petroleum-based diesel consists of a mixture of carbon chains with 8 to 21 carbon atoms. During combustion in a diesel engine bonds between carbon atoms are broken and energy is released. The chains of carbon atoms in fatty acids also release energy when combusted, so fatty acids can function like diesel oil after minimal processing.

Production of biodiesel starts with extraction and purification of vegetable oils in a process often identical to vegetable oil refining for human consumption. After extraction and purification, the triglycerides are treated with an alcohol (e.g. methanol or ethanol) along with a catalyst (usually sodium hydroxide). The three ester bonds of the triglyceride are cleaved and new ester bonds are made using a methyl group (if methanol is used) or an ethyl group (if ethanol is used). This reaction is called base-catalyzed transesterification. All fatty acids that did not react and glycol are separated from the biodiesel. Once a valuable product, oversupply of glycol because of biodiesel production has greatly decreased its worth.

Biodiesel is often blended with petrodiesel. The US and most other countries use a system that includes the letter “B” followed by a number to describe the blend. B20 means the mixture is 20% biodiesel and 80% petrodiesel. Unblended biodiesel is labeled B100.

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A. Soybean

Soybean, Glycine max, is the second most important US crop in terms of cash receipts. More than 4 billion bushels of soybean grain are produced from about 85 million acres each year. Soybean is an annual plant in the legume family. Because it is a legume, soybean plants form a synergistic relationship with a species of bacteria. That relationship allows for nitrogen fixation (changes nitrogen from a plant-unavailable form to an available form) to occur in structures called nodules located on roots. Unlike corn, nitrogen fertilizer is seldom applied to soybean fields in the US.

Soybean flowers possess a form typical of all legume plants with flower parts (other than pistil) in multiples of 5. The technical name for a legume plant fruit is pod. Soybean pods contain 1 to 4 seeds. More than 95% of a soybean seed volume is embryo.

Soybean is by far the most common feedstock for biodiesel in the US. On a dry weight basis, soybean seeds are about 20% oil and 40% protein. Each bushel (60 pounds) produces about 1.5 gallons of B100 biodiesel and 47 pounds of high protein meal. Average yield in Missouri is about 45 bushels per acre, so an average acre of soybean can produce about 73 gallons of biodiesel.

B. Rapeseed/Canola and Camelina

Rapeseed, Brassica napus, is a cool-season, annual plant that is a member of the mustard family. Rapeseed flowers are bright yellow and contain four petals in the form of a cross. The technical name for fruit of the mustard family is silique. Each fruit contains 15 to 30 brownish-black, round seeds. Seeds are quite small. There are about 160,000 seeds in a pound, which is nearly 45 times the number of soybean seeds in a pound.

Rapeseed seeds contain two toxins. Erucic acid is a fatty acid in the oil, and glucosinolate is in the protein meal. Canadian plant breeders have developed varieties that contain low amounts of both toxins. Collectively, they are known as Canola (Canada plus “ola” (oil)). Varieties classified as canola account for nearly all of the rapeseed grown in North America.

Canola is grown in 9 US states, but not in Missouri. North Dakota accounts for nearly 85% of the total US production. On a dry weight basis, canola seeds are about 44% oil. Average canola yield in the US is about 1700 pounds per acre, so an average acre of canola can produce about 100 gallons of biodiesel.

Camelina is in the same plant family as canola and has been suggested as an alternative oilseed. Oil content of seeds is about 35%. It has a short growing cycle of less than 100 days. Grain and oil yield estimates are few, but biodiesel production may be about 55 gallons/acre.

C. Sunflower

Sunflower, Helianthus annuus, is a annual plant that is a member of the composite family (Asteraceae). Unlike most grain crops, sunflower is native to North America. The structure we call sunflower seeds are actually fruits. The technical name for fruit of the composite family is achene.

Two types of sunflower are grown in the US. The black-seeded type (actually black fruit) is higher in oil and is the type grown for vegetable oil production. Confectionary type produces larger seeds with less oil. These varieties usually possess striped fruit to distinguish them in the market.

Sunflower is grown in 9 US states, but not in Missouri. North Dakota accounts for nearly 50% of the total US production. On a dry weight basis, sunflower seeds are about 40% oil. Average sunflower yield in the US is about 1650 pounds per acre, so an average acre of sunflower can produce about 89 gallons of biodiesel.

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  1. Plants Used to Produce Ethanol
    1. Plants that produce sucrose feedstock for ethanol
      1. Sugarcane
      2. Sugar beet
    2. Plants that produce starch feedstock for ethanol
      1. Corn
      2. Grain sorghum
      3. Small grains
    3. Plants that produce cellulose feedstock for ethanol
      1. Miscanthus
      2. Native perennial grasses
      3. Forage sorghum
      4. Plant residue
  2. Plants used to produce biodiesel
    1. Soybean
    2. Rapeseed/Canola
    3. Sunflower