Ethanol fuel
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 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 is possible only if the engines are designed or modified for that purpose. Anhydrous ethanol can be blended with gasoline 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. 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. By December 2011 Brazil had a fleet of 14.8 million flex-fuel automobiles and light trucks and 1.5 million flex-fuel motorcycles that regularly use neat ethanol fuel.
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.
Chemistry
During ethanol fermentation, sugars like glucose and others in the corn are converted into ethanol and carbon dioxide.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:
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.
Most ethanol is produced by fermentation.
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, as well as the energy and pollution balance of the whole cycle of ethanol production, especially from corn.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 fermentation of sugars, distillation, dehydration, and denaturing. Prior to fermentation, some crops require saccharification or hydrolysis of carbohydrates such as cellulose and starch into sugars. Saccharification of cellulose is called cellulolysis. 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 and starch 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 ethanol and 4.4% m/m. 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, 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.
With increasing attention being paid to saving energy, many methods have been proposed that avoid distillation altogether for dehydration. Of these methods, a third method has emerged and has been adopted by the majority of modern ethanol plants. This new process uses molecular sieves to remove water from fuel ethanol. In this process, ethanol vapor under pressure passes through a bed of molecular sieve beads. The bead's pores are sized to allow adsorption of water while excluding ethanol. After a period of time, the bed is regenerated under vacuum or in the flow of inert atmosphere to remove the adsorbed water. Two beds are often used so that one is available to adsorb water while the other is being regenerated. This dehydration technology can account for energy saving of 3,000 btus/gallon compared to earlier azeotropic distillation.
Recent research has demonstrated that complete dehydration prior to blending with gasoline is not always necessary. Instead, the azeotropic mixture can be blended directly with gasoline so that liquid-liquid phase equilibrium can assist in the elimination of water. A two-stage counter-current setup of mixer-settler tanks can achieve complete recovery of ethanol into the fuel phase, with minimal energy consumption.
Post-production water issues
Ethanol is hygroscopic, meaning it absorbs water vapor directly from the atmosphere. Because absorbed water dilutes the fuel value of the ethanol and may cause phase separation of ethanol-gasoline blends, containers of ethanol fuels must be kept tightly sealed. This high miscibility with water means that ethanol cannot be efficiently shipped through modern pipelines, like liquid hydrocarbons, over long distances.The fraction of water that an ethanol-gasoline fuel can contain without phase separation increases with the percentage of ethanol. For example, E30 can have up to about 2% water. If there is more than about 71% ethanol, the remainder can be any proportion of water or gasoline and phase separation does not occur. The fuel mileage declines with increased water content. The increased solubility of water with higher ethanol content permits E30 and hydrated ethanol to be put in the same tank since any combination of them always results in a single phase. Somewhat less water is tolerated at lower temperatures. For E10 it is about 0.5% v/v at 21 °C and decreases to about 0.23% v/v at −34 °C.