Ethanol and carbon dioxide emissions
Ethanol, also known as ethyl alcohol, is a transportation fuel derived primarily from the fermentation of various materials, with the majority in the United States coming from cornstarch. As a gasoline substitute, ethanol offers certain environmental benefits; blending it with gasoline can reduce carbon dioxide (CO2) emissions by up to 20% compared to gasoline alone. However, the production process of corn-based ethanol raises concerns, as it requires significant energy, often from fossil fuels, leading to substantial CO2 emissions during its lifecycle. Conversely, lignocellulosic ethanol, produced from non-edible plant materials, could have nearly zero net emissions since it uses renewable resources that absorb CO2 during growth. While ethanol can contribute to reducing dependency on foreign oil, it also competes with food resources, impacting food prices. Advances in technology and biorefinery strategies hold promise for making ethanol production more efficient and climate-friendly, with research indicating a significant reduction in emissions associated with U.S. corn ethanol over recent years. The potential for biofuels to achieve net carbon reduction is being actively explored, marking an important area of ongoing research and development.
Subject Terms
Ethanol and carbon dioxide emissions
Definition
Ethanol (ethyl alcohol, or grain alcohol) is a transportation fuel that is used as a gasoline substitute. Henry Ford’s first car, the Model T Ford, was designed to run on pure ethanol. Ethanol is a colorless liquid with the chemical formula C2H5OH. It is produced through a biological process based on fungal or bacterial fermentation of a variety of materials. In the United States, most ethanol is produced by yeast (fungal) fermentation of sugar from cornstarch. Sugar is extracted using enzymes, then yeast cells convert the sugar into ethanol and carbon dioxide (CO2). The ethanol is separated from the fermentation broth by distillation.
Ethanol can also be produced chemically from petroleum. Ethanol produced by fermentation is commonly referred to as bioethanol to differentiate it from chemically produced ethanol. Brazil, the second largest bioethanol producer after the United States, generates bioethanol from sugarcane. Brazilian production of ethanol from sugarcane is more efficient than is the U.S. corn-based production method. Several factors contribute to that efficiency, including climate and cheap labor. In addition, sugarcane has much a higher sugar content than corn, and no enzymes are necessary to extract cane sugar. Therefore, sugarcane yields of ethanol are twice as great as corn yields.
When energy demands and oil prices increase, ethanol becomes a valuable option as an alternative transportation fuel. In 2005, the U.S. Congress passed an energy bill that required gasoline sold in the United States to be mixed with ethanol in order to decrease the nation’s dependence on oil. There is a limit to the amount of ethanol that can be mixed with gasoline in the American market, however. Nearly all modern automobiles can use E10, fuel that contains 10 percent ethanol. By contrast, E85, containing 85 percent ethanol and only 15 percent gasoline, requires specially equipped flexible-fuel engines. In the United States, only a fraction of motor vehicles use such engines. Most cars in Brazil have a flex engine, however, as ethanol use in vehicle fuels has been mandatory in that country since 1977. Brazil’s ethanol policy allowed the nation to achieve in 2006.
Blending ethanol with gasoline oxygenates the fuel mixture, which then burns more completely and emits less carbon monoxide. However, ethanol has about two-thirds the energy content of gasoline by volume, so vehicles can travel less far on a gasoline-ethanol mixture than they can on pure gasoline. Ethanol also tends to be more expensive than gasoline. In addition, carcinogenic aldehydes, such as formaldehyde, are produced when ethanol is burned in internal combustion engines.
Significance for Climate Change
Burning ethanol in produces carbon dioxide (CO2), a major greenhouse gas (GHG). However, vehicles that run on pure ethanol or ethanol/gasoline mixtures can produce 20 percent less CO2 than vehicles that burn gasoline alone. Emissions of nitrogen oxide (another GHG) are about equal for both ethanol and gasoline.
In order fully to evaluate ethanol’s carbon footprint, it is necessary to determine how much CO2 is emitting during production of the fuel. Such determinations depend on the method of ethanol manufacturing employed. For example, ethanol can be manufactured from various feedstocks, including starch (cornstarch), sugarcane, and lignocellulose. Lignocellulose is a combination of lignin, cellulose, and hemicellulose that strengthens plant cell walls. Cellulose and hemicellulose are made of sugars that can be converted into ethanol. Many steps are required to sustain this conversion. These steps include growing, transporting, and processing the feedstock.
Manufacturing ethanol from cornstarch requires considerable amounts of energy, usually obtained by burning fossil fuels. Burning these releases significant amounts of CO2. In addition, allocating corn crops to ethanol production leads to increases in food prices, because a great number of food items in the United States include corn-based ingredients, such as corn syrup and cornstarch. Corn-based ethanol is thus not a viable long-term biofuel, but it may help smooth the transition from a petroleum-based economy into a biofuel-based economy that utilizes ethanol from other sources.
Increasing corn ethanol use will help reduce U.S. reliance on foreign oil, but it will not do much to slow global warming and will lessen the availability of corn for food. By contrast, net emissions of CO2 during lignocellulose ethanol production can be nearly zero. Burning the lignin itself can provide enough energy to power ethanol production, alleviating the need for fossil fuels. Burning lignin does not add any net CO2 to the atmosphere, because the plants that are used to make the ethanol absorb CO2 during their growth. Most important, lignocellulose may be obtained from nonedible plants, such as switchgrass and poplar, or nonedible parts of other plants, such as corn stalks and wood chips. Thus, lignocellulose ethanol does not compete for food resources.
Lignocellulose is a very attractive ethanol fuel feedstock because it is in abundant supply. On a global scale, plants produce almost 90 billion metric tons of cellulose per year, making it the most abundant organic compound on Earth. In addition, cultivation of nonedible plants for ethanol production requires fewer nutrients, fertilizers, herbicides, acres of cultivated land, and energy resources.
Methods of processing the cellulosic parts of plants into simple sugars in order to ferment them into ethanol have so far been costly. The cost of ethanol generation from lignocellulose can be reduced substantially, however, by using a biorefinery-based production strategy. Similar in function to a petroleum refinery, a biorefinery utilizes every component of lignocellulose to produce useful products, thereby increasing revenues and cost-effectiveness. These products include ethanol fuel, electrical power, heat energy, animal feed, and chemicals such as succinic acid and 1,4-butanediol. The latter can be used to manufacture plastics, paints, and other products. Major research and development efforts are under way to improve the conversion cost of lignocellulose to ethanol.
Studies showed that ethanol-based fuels are becoming more climate-friendly over time. Carbon emissions from U.S. corn ethanol fell 20 percent between 2005 and 2019. This drop was due to numerous factors, including improved ethanol production processes and increased corn yields per acre of farmland. In the future, scientists believe that biofuels could surpass zero emissions, eventually resulting in net carbon reduction.
Bibliography
Demain, Arnold L., Michael Newcomb, and J. H. David Wu. “Cellulase, Clostridia, and Ethanol.” Microbiology and Molecular Biology Reviews 69, no. 1 (March 2005): 124-154. Review article about making ethanol from plant cellulose.
"Ethanol vs. Petroleum-Based Fuel Carbon Emissions." U.S. Department of Energy, 23 June 2022, www.energy.gov/eere/bioenergy/articles/ethanol-vs-petroleum-based-fuel-carbon-emissions/. Accessed 20 Dec. 2024.
Glazer, Alexander N., and Hiroshi Nikaido. Microbial Biotechnology: Fundamentals of Applied Microbiology. New York: Cambridge University Press, 2007. In-depth analysis of ethanol for fuel application.
Wald, Matthew L. “Is Ethanol for the Long Haul?” Scientific American, January 2007, 42-49.