Ethanol fuel is an alcohol-based fuel commonly produced by fermenting sugars from crops such as corn, sugarcane, and other biomass, although it can also be synthesized from petroleum derivatives. While it is the same type of alcohol as found in alcoholic beverages, it is most often used as an alternative to gasoline in transportation, either as a pure fuel or blended into gasoline mixtures as a biofuel additive, to reduce reliance on fossil fuels and lower greenhouse gas emissions. Ethanol fuel in Brazil and the United States dominate global ethanol production, together accounting for the majority of supply, and many countries mandate ethanol blending in automotive fuels.
Historically, ethanol has been used as a fuel since the early 20th century, with Brazil pioneering large-scale adoption during the 1970s energy crisis. Advances in flexible-fuel vehicle technology and government policies have expanded its use worldwide. Ethanol's chemical composition (C2H5OH) allows it to burn cleanly, producing carbon dioxide and water, and its high octane rating makes it suitable for high-compression engines. Most ethanol is produced through microbial fermentation of sugars, followed by distillation and dehydration, though synthetic ethanol from ethylene remains a small share of global output.
Ethanol is considered a renewable energy source, but its production raises environmental and economic concerns. While sugarcane-based ethanol in Brazil offers a favorable energy balance and lower carbon emissions compared to gasoline, corn-based ethanol in the United States provides more modest benefits. Large-scale cultivation for ethanol can affect food prices, water resources, and land use, and emissions from production and combustion vary by feedstock and process. Research continues into cellulosic ethanol and other advanced methods to improve sustainability and reduce environmental impact.
Several common ethanol fuel mixtures are in use around the world. The use of pure hydrous or anhydrous ethanol in internal combustion engines (ICEs) is possible only if the engines are designed or modified for that purpose. Anhydrous ethanol can be blended with gasoline (petrol) for use in gasoline engines, but only after engine modifications are made since pure ethanol contains only a fraction of the energy of an equivalent volume of pure gasoline. Despite its inefficiency compared to gasoline, ethanol is eco-friendlier and produces less greenhouse emissions upon combustion due to more complete combustion as compared to gasoline, leading to less toxic gases emitted, making it an eco-friendly alternative.
History and use Global ethanol production for transport fuel tripled between 2000 and 2007 and ethanol production in general has increased over time, although production fell in 2020 due to the COVID-19 pandemic. Ethanol-blended fuel is widely used in Brazil, the United States, Canada, and Europe (see also Ethanol fuel by country). Most cars on the road today in the U.S. can run on blends of up to 15% ethanol, and ethanol represented 10% of the U.S. gasoline fuel supply derived from domestic sources in 2011. Some flexible-fuel vehicles are able to use up to 100% ethanol.
The first production car running entirely on ethanol was the Fiat 147, introduced in 1978 in Brazil by Fiat. Since 1976 the Brazilian government has made it mandatory to blend ethanol with gasoline, and since 2007 the legal blend is around 25% ethanol and 75% gasoline (E25). By December 2011 Brazil had a fleet of 14.8 million flex-fuel automobiles and light trucks pp. 62–63. Carta de ANFAVEA 308 pp. 4. and 1.5 million flex-fuel motorcycles that regularly use neat ethanol fuel (known as E100).
World ethanol production for transport fuel tripled between 2000 and 2007 from to more than . From 2007 to 2008, the share of ethanol in global gasoline type fuel use increased from 3.7% to 5.4%. In 2011 worldwide ethanol fuel production reached with the United States of America and Brazil being the top producers, accounting for 62.2% and 25% of global production, respectively. US ethanol production reached in May 2017.AMIS Market Monitor No. 48 – May 2017, http://www.amis-outlook.org/fileadmin/user_upload/amis/docs/Market_monitor/AMIS_Market_Monitor_Issue_47.pdf
Chemistry
During ethanol fermentation, sugars like glucose and others in the corn (or sugarcane or other crops) are converted into ethanol and carbon dioxide. C6H12O6 → 2 C2H5OH+ 2 CO2 + heat
Ethanol fermentation is not 100% selective with side products such as acetic acid and glycols. They are mostly removed during ethanol purification. Fermentation takes place in an aqueous solution. The resulting solution has an ethanol content of around 15%. Ethanol is subsequently isolated and purified by a combination of adsorption and distillation.
During combustion, ethanol reacts with oxygen to produce carbon dioxide, water, and heat: C2H5OH + 3 O2 → 2 CO2 + 3 H2O + heat
Starch and cellulose molecules are strings of glucose molecules. It is also possible to generate ethanol out of cellulosic materials. That, however, requires a pretreatment that splits the cellulose into glucose molecules and other sugars that subsequently can be fermented. The resulting product is called cellulosic ethanol, indicating its source.
Ethanol is also produced industrially from ethylene by hydration of the double bond in the presence of a catalyst and high temperature. C2H4 + H2O → C2H5OH
Most ethanol is produced by fermentation.
Sources
About 5% of the ethanol produced in the world in 2003 was actually a petroleum product. It is made by the catalytic hydration of ethylene with sulfuric acid as the catalyst. It can also be obtained via ethylene or acetylene, from calcium carbide, coal, oil gas, and other sources. of petroleum-derived ethanol are produced annually. The principal suppliers are plants in the United States, Europe, and South Africa. Petroleum derived ethanol (synthetic ethanol) is chemically identical to bioethanol and can be differentiated only by radiocarbon dating.
Bioethanol is usually obtained from the conversion of carbon-based feedstock. Agricultural feedstocks are considered renewable because they get energy from the sun using photosynthesis, provided that all minerals required for growth (such as nitrogen and phosphorus) are returned to the land. Ethanol can be produced from a variety of feedstocks such as sugar cane, bagasse, miscanthus, sugar beet, sorghum, switchgrass, barley, hemp, kenaf, potatoes, sweet potatoes, cassava, sunflower, fruit, molasses, corn, stover, grain, wheat, straw, cotton, other biomass, as well as many types of cellulose waste and harvesting, whichever has the best well-to-wheel assessment.
In 2008 an alternative process to produce bioethanol from algae was announced by the company Algenol. Rather than grow algae and then harvest and ferment it, the algae grow in sunlight and produce ethanol directly, which is removed without killing the algae. It is claimed the process can produce per year compared with for corn production. In 2015 the project was abandoned.
The first generation processes for the production of ethanol from corn use only a small part of the corn plant: the corn kernels are taken from the corn plant and only the starch, which represents about 50% of the dry kernel mass, is transformed into ethanol. Two types of second generation processes are under development. The first type uses enzymes and yeast fermentation to convert the plant cellulose into ethanol while the second type uses pyrolysis to convert the whole plant to either a liquid bio-oil or a syngas. Second generation processes can also be used with plants such as grasses, wood or agricultural waste material such as straw.
Production
Bioethanol is a form of renewable energy that can be produced from agricultural feedstocks. It can be made from very common crops such as hemp, sugarcane, potato, cassava and corn. There has been considerable debate about how useful bioethanol is in replacing gasoline. Concerns about its production and use relate to increased food prices due to the large amount of arable land required for crops (see: Indirect land use change impacts of biofuels), as well as the energy and pollution balance of the whole cycle of ethanol production, especially from corn. Youngquist, W. Geodestinies, National Book Company, Portland, Oregon, p.499
Although there are various ways ethanol fuel can be produced, the most common way is via fermentation.
The basic steps for large-scale production of ethanol are: microbial (yeast) fermentation of sugars, distillation, dehydration (requirements vary, see Ethanol fuel mixtures, below), and denaturing (optional). Prior to fermentation, some crops require saccharification or hydrolysis of carbohydrates such as cellulose and starch into sugars. Saccharification of cellulose is called cellulolysis (see cellulosic ethanol). Enzymes are used to convert starch into sugar.
Fermentation
Ethanol is produced by microbial fermentation of the sugar. Microbial fermentation currently only works directly with sugars. Two major components of plants, starch and cellulose, are both made of sugars—and can, in principle, be converted to sugars for fermentation. Currently, only the sugar (e.g., sugar cane) and starch (e.g., corn) portions can be economically converted.
There is interest in cellulosic ethanol obtained from breaking down plant cellulose to sugars and converting the sugars to ethanol. However, cellulosic ethanol is currently uneconomical and not practiced commercially. According to a 2006 International Energy Agency report, cellulosic ethanol could be important in the future.
Distillation
For the ethanol to be usable as a fuel, the yeast solids and the majority of the water must be removed. After fermentation, the mash is heated so that the ethanol evaporates. This process, known as distillation, separates the ethanol, but its purity is limited to 95–96% due to the formation of a low-boiling water-ethanol azeotrope with maximum (95.6% m/m (96.5% v/v) ethanol and 4.4% m/m (3.5% v/v) water). This mixture is called hydrous ethanol and can be used as a fuel alone, but unlike anhydrous ethanol, hydrous ethanol is not miscible in all ratios with gasoline, so the water fraction is typically removed in further treatment to burn in combination with gasoline in gasoline engines.
Dehydration There are three dehydration processes to remove the water from an azeotropic ethanol/water mixture. The first process, used in many early fuel ethanol plants, is called azeotropic distillation and consists of adding benzene or cyclohexane to the mixture. When these components are added to the mixture, it forms a heterogeneous azeotropic mixture in vapor–liquid-liquid equilibrium, which when distilled produces anhydrous ethanol in the column bottom, and a vapor mixture of water, ethanol, and cyclohexane/benzene.
When condensed, this becomes a two-phase liquid mixture. The heavier phase, poor in the entrainer (benzene or cyclohexane), is stripped of the entrainer and recycled to the feed—while the lighter phase, with condensate from the stripping, is recycled to the second column. Another early method, called extractive distillation, consists of adding a ternary component that increases ethanol's relative volatility. When the ternary mixture is distilled, it produces anhydrous ethanol on the top stream of the column.