Ethanol is commercially produced in one of two ways, using either the wet mill or dry mill process. Wet milling involves separating the grain kernel into its component parts (germ, fiber, protein, and starch) prior to fermentation. ICM-designed plants utilize the dry mill process, where the entire grain kernel is ground into flour. The starch in the flour is converted to ethanol during the fermentation process, creating carbon dioxide and distillers grain. Typical Ethanol Plant Grain is delivered by truck or rail to the ethanol plant where it s loaded in storage bins designed to hold enough grain to supply the plant for 7 10 days. Milling The grain is screened to remove debris and ground into course flour. Cooking (Hot Slurry, Primary Liquefaction, and Secondary Liquefaction) During the cook process, the starch in the flour is physically and chemically prepared for fermentation. Hot Slurry
The milled grain is mixed with process water, the ph is adjusted to about 5.8, and an alpha-amylase enzyme is added. The slurry is heated to 180 190 F for 30 45 minutes to reduce viscosity. Primary Liquefaction The slurry is then pumped through a pressurized jet cooker at 221 F and held for 5 minutes. The mixture is then cooled by an atmospheric or vacuum flash condenser. Secondary Liquefaction After the flash condensation cooling, the mixture is held for 1 2 hours at 180 190 F to give the alpha-amylase enzyme time to break down the starch into short chain dextrins. After ph and temperature adjustment, a second enzyme, glucoamylase, is added as the mixture is pumped into the fermentation tanks. Simultaneous Saccharification Fermentation Once inside the fermentation tanks, the mixture is referred to as mash. The glucoamylase enzyme breaks down the dextrins to form simple sugars. Yeast is added to convert the sugar to ethanol and carbon dioxide. The mash is then allowed to ferment for 50 60 hours, resulting in a mixture that contains about 15% ethanol as well as the solids from the grain and added yeast. Distillation The fermented mash is pumped into a multi-column distillation system where additional heat is added. The columns utilize the differences in the boiling points of ethanol and water to boil off and separate the ethanol. By the time the product stream is ready to leave the distillation columns, it contains about 95% ethanol by volume (190-proof). The residue from this process, called stillage, contains nonfermentable solids and water and is pumped out from the bottom of the columns into the centrifuges. Dehydration The 190-proof ethanol still contains about 5% water. It s passed through a molecular sieve to physically separate the remaining water from the ethanol based on the different sizes of the molecules. This step produces 200-proof anhydrous (waterless) ethanol.
Ethanol Storage Before the ethanol is sent to storage tanks, a small amount of denaturant is added, making it unfit for human consumption. Most ethanol plants storage tanks are sized to allow storage of 7 10 days production capacity. Co-Product Processing During the ethanol production process, two valuable co-products are created: carbon dioxide and distillersgrains.as yeast ferment the sugar, they release large amounts of carbon dioxide gas. It can be released into the atmosphere, but it s commonly captured and purified with a scrubber so it can be marketed to the food processing industry for use in carbonated beverages and flash-freezing applications. The stillage from the bottom of the distillation tanks contains solids from the grain and added yeast as well as liquid from the water added during the process. It s sent to centrifuges for separation into thin stillage (a liquid with 5 10% solids) and wet distillers grain. Some of the thin stillage is routed back to the cook/slurry tanks as makeup water, reducing the amount of fresh water required by the cook process. The rest is sent through a multiple-effect evaporation system where it is concentrated into syrup containing 25 50% solids. This syrup, which is high in protein and fat content, is then mixed back in with the wet distillers grain (WDG). With the added syrup, the WDG still contains most of the nutritive value of the original feedstock plus the added yeast, so it makes an excellent cattle ration for local feedlots and dairies. After the addition of the syrup, it s conveyed to a wet cake pad, where it is loaded for transport. Many ethanol facilities do not have enough nearby cattle to utilize all of the WDG. It must be used soon after it s produced because it spoils easily. So, it s often sent through a drying system to remove moisture and extend its shelf life. This dried distillers grain (DDG) is commonly used as a high-protein ingredient in cattle, swine, poultry, and fish diets. It s also being researched for human consumption.
How Ethanol Production Works
Cellulosic Ethanol What Is Cellulosic Ethanol? All ethanol whether it is corn or cellulosic is the same chemical compound: C2H5OH. You might recall from elementary chemistry courses that the OH group at the end of the formula indicates that the compound is an alcohol. Alcohols can have varying numbers of carbon atoms. Alcohol with two carbon atoms is called ethanol. The other alcohols are generally too toxic to be ingested, and thus ethanol has been the libation of choice down through the ages. (Ethanol used as fuel is rendered nonpotable.) So corn ethanol and cellulosic ethanol don t signify different types of ethanol, but rather the different material (or feedstocks) used to produce them. Why Cellulosic Ethanol Can Be an Environmental Winner Corn ethanol is produced from kernels actually only a small part of the corn kernels the sugars and starches. Herein lies one of the limitations of corn ethanol. You see, sugars and starches comprise a tiny fraction of the corn plant s mass about 2-15%. Because only a small fraction of a plant is used to make corn ethanol, the amount you can produce is limited. Cellulosic ethanol is a different story. Most of the dry biomass as much as 80% is typically made up of cellulosic material the stuff that makes the plant sturdy. So you can make a lot of ethanol using a plant s cellulose instead of its sugars and starches. (By the way, even if the cellulosic material comes from corn, we still call it cellulosic ethanol. Corn ethanol is made solely from the sugars and starches of the corn kernel.) The Major Advantage of Cellulosic Ethanol Our guts are unable to digest cellulose, so we typically throw away that part of crops. A lot of it is left on the field or disposed of as agricultural waste. For corn, the cellulosic material includes the corn stover the leaves and stalk and the cob. Remember what made corn ethanol such an environmental negative? A main reason is that it requires that land being used to grow food (or left as forests or grassland) be converted to growing an energy crop. And that leads to lots of global warming pollution.
This is not a problem for cellulosic ethanol we can simply use the agricultural waste from food crops to make the ethanol and thereby avoid all those emissions. Why We Can t Fill Our Tanks With the Cellulosic Stuff Yet Unfortunately, right now, producing cellulosic ethanol on an industrial scale is too expensive. Unlike converting a plant s sugars and starches to corn ethanol, making cellulosic ethanol requires that we first break down the cellulosic material. But because this material is what makes a plant sturdy, the atoms in these compounds are strongly bonded together and that makes them hard to break apart. The processes we have available today to do this are too expensive to make cellulosic ethanol commercially competitive. But that will likely change. Scientists and engineers are working to make a commercially viable form of cellulosic ethanol. Some are developing new chemical processes; others are trying to genetically engineer new microbes that can ferment cellulose into ethanol like normal microbes that ferment sugars into ethanol. (The U.S.Department of Energy is helping fund six biorefineries.) Cellulosic Ethanol Could Help Cut U.S. Global Warming Pollution By some estimates, agricultural and forest wastes could supply as much as 35 billion gallons of ethanol per year, saving up to 76 megatons of global warming emissions per year. (These results are somewhat larger than but consistent with other recent estimates (e.g., see Smith et al. 2004).) Such savings would cut a little less than 5% of all our heat-trapping pollution and about 15% of the emissions from the transportation sector. By mid-century, cellulosic ethanol could supply as much as 86 billion gallons of ethanol, saving a little more than 180 megatons of global warming pollution per year or almost 12% of America s total global warming pollution and about 35% of the emissions from the transportation sector. These are significant numbers. But to reach such levels we would need to grow bioenergy crops such as switch grass. Such cultivation, in turn, would require converting lands for this purpose, and that could raise some of the problems discussed in last week s post. The Bottom Line of Biofuels: There Are Winners and Losers
The saying waste not, want not applies to biofuels. The best biofuels are made from agricultural or forests wastes or from plants cultivated on degraded or marginal lands. The product from such feedstocks cellulosic ethanol is where we should be directing our entrepreneurial energies.
Proposal: 4 What is Switch Grass? Switchgrass (Panicumvirgatum) is a summer perennial grass that is native to North america. It is a natural component of the tall-grass prairie which covered must of the great Plains, but which also was also found on the prairie soils in the Black Belt of Alabama and Mississippi. Many people do not realize that the natural vegetation of the Black belt was grassland, and not forst like most other parts of the southeastern USA. Because it is native, switchgrass is resistant to many pests and plant diseases, and it is capable of producing high yields with very low applications of fertilizer. This means that the need for agricultural chemicals to grow switchgrass is relatively low. Switchgrass is also very tolerant of poor soils, flooding and drought, which are widespread agricultural problems in the southeast. There are two main types of switchgrass: upland types, which usually grow 5 to 6 feet tall and are adapted to well drained soils, and low land types, that grow up to 12 feet tall and which are typically found on heavy soils in bottomland sites. Although switchgrass is native, plant breeders have developed a fairly large number of improved varieties for use as forage. 'Alamo' switchgrass is a robust lowland variety of switchgrass most suited to the southern US. In Auburn University test plots, it has frequently produced over 10 tons per acre per year, but on a commercial scale, it is more reasonable to expect 6 to 8 tons per acre. This is because test plots usually have perfect establishment, but commercial plantings almost always have weak spots in the field. However, for comparison, the average annual hay yield for Alabama is about 2.5 tons per acre, and the productivity of forests is only about half that of switchgrass. Despite poorer soils than in the Midwest, switchgrass yields are higher in the Southeast because of the adaptation of more productive switchgrass varieties in our region, and because we have a longer growing season. The seed of switchgrass is very small, and much of it is dormant (will not germinate) right after it is harvested. However, aging, treating it with water and chilling temperatures (stratification) or storing it in warm conditions will break dormancy. Partly because of the small size of the seed, switchgrass seedlings tend to be slow to develop, and are susceptible to weed competition. Unfortunately, there are no
herbicides approved by government for weed control during establishment of switchgrass. However, it can still be successfully established by no-till planting and other strategic approaches. Switchgrass reaches full yield only in the third year after planting; it produces a quarter to a third of full yield in the first year, and about two thirds of full yield in the second year. When managed for energy production it can be cut once or twice a year with regular hay or silage equipment. At maturity, widely spaced switchgrass plants can measure 20 inches in diameter at ground level. Switchgrass has a huge, permanent root system that penetrates over 10 feet into the soil, and weighs as much (6-8 tons/acres) as the above-ground growth from one year. It also has many fine, temporary roots. All these roots improve the soil by adding organic matter, and by increasing soil water infiltration and nutrientholding capacity. Switchgrass fields provide habitat and a home for many species of wildlife, including cover for deer and rabbits, and a nesting place for wild turkey and especially quail. Switchgrass has several other environmental benefits. If it is used to produce energy, it will reduce the risk of global warming by replacing fossil fuels (coal, natural gas and oil). When fossil fuels are burnt, carbon is removed from below ground (gas and oil wells and coal mines) and release into the atmosphere as carbon dioxide (CO2). This is a greenhouse gas that increases the risk of global warming. In contrast, switchgrass (like all other plants) removes CO2 from the atmosphere and incorporates it into plant tissue, both above and below the ground Together with its energy benefits, swicthgrass offers great opportunity for farmers. Because of its perenniality, compared to annual crops switchgrass is a true conservation crop which will substantially reduce soil erosion and release of soil carbon which are related to annual tillage, and it will reduce the use of toxic chemicals. It could also produce much needed farm income in many regions that are in desperate need of rural development, and it could substantially reduce the need for farm programs and disaster aid which are currently paid from tax dollars. What is needed to ensure that we fully realize all these potential benefits? Simple! We must demonstrate that the process of using switchgrass for energy can be profitable for energy producers, farmers, and consumers of energy. Can this be done? Probably especially in certain specific situations, and co-firing switchgrass with coal to produce electricity in existing plants offers on of the best near-term prospects. Perhaps most important, we must recognize that fossil fuels will be our main energy base for many years, and bioenergy from switchgrass is not intended to compete with these valuable resources, but rather, to complement them by softening their environmental impact.