Fermentation is the biochemical process that converts sugars into ethanol (alcohol). In contrast to biogas production, fermentation takes place in the presence of air and is, therefore, a process of aerobic digestion. Ethanol producers use specific types of enzymes to convert starch crops such as corn, wheat and barley to fermentable sugars. Some crops, such as sugar-cane and sugar beets, naturally contain fermentable sugars.
Ethanol has a higher octane than gasoline, but its energy content is only about two-thirds the energy content of gasoline. Most new cars are designed to run on a blend of gasoline and ethanol. "Gasohol" is a mixture of 90-percent unleaded gasoline and 10-percent denatured ethanol. With modification, spark ignition engines can run on 100-percent ethanol. E-85 fuel consists of 85-percent ethanol and 15-percent gasoline. The major automobile manufacturers in the United States now produce flexible fuel vehicles that can use either E-85 fuel or gasoline.
Ethanol may also be used as a hydrogen source for fuel cells. A recent paper by the Renewable Fuels Association
concludes that there are no technical barriers to the use of ethanol in fuel cells. Because ethanol is easier to transport and store than hydrogen, fuel reforming (using a chemical process to extract hydrogen from fuel) may be a practical way to provide hydrogen to fuel cells in vehicles or for remote stationary applications. Ethanol is easier to reform than gasoline and most alternative fuels because of its relatively simple molecular structure.
Grain to Ethanol
Most of the ethanol produced in the United States today comes from grain (predominantly corn). In the wet mill process
, grain is steeped and separated into starch, germ and fiber components. In the dry mill process, grain is first ground into flour and then processed without separation of the starch.
Wet milling is more common. After the grain is cleaned, it is steeped and then ground to remove the germ. Further grinding, washing and filtering steps separate the fiber and gluten. The starch that remains after these separation steps is then broken down into fermentable sugars by the addition of enzymes in the liquefaction and saccharification stages.
To produce ethanol, yeast is added to a slurry (or "mash"), which is a solution of fermentable sugars and water. The yeast ferments the sugars, producing a solution called beer. The beer solution contains about 10-percent to 12-percent ethanol. The rate of the conversion process depends on the amount of water in the slurry and its acidity, temperature and oxygen content. Up to a third of the original dry weight of the feedstock leaves the fermentation process as carbon dioxide. The solids that remain after the mash has fermented still contain nutrients suitable for use as livestock feed. Distilling the beer produces a solution of 80-percent to 95-percent ethanol.
Producers can use several methods of dehydration to purify the ethanol solution further to 100-percent (200-proof) alcohol for use as a motor fuel.
Lignocellulosic Biomass to Ethanol
The use of lower-cost feedstock is of particular interest in Oregon and the Pacific Northwest region due to the abundance of potential feedstock. This regionally available feedstock (called lignocellulosic biomass) includes waste paper, wood waste, pulp sludge and grass straw. Mechanical preparation steps include cleaning, drying and reducing the size of biomass feedstock.
converts lignocellulosic feedstock (LCF) into component sugars, which are then fermented to ethanol. This technology is currently in an early stage of commercial development. However, as early as 1945, Oregon pioneered cellulose-to-ethanol technology. At that time, Dr. Raphael Katzen designed, built and operated a 17 million-gallon-per-year ethanol plant in Springfield, Oregon, that used wood feedstock
All LCF materials are made of cellulose, hemicellulose and lignin. Lignin acts like glue in plant material. It holds the other components together and gives trees and plant stalks their strength. The lignin removed in pretreatment is itself a biomass fuel. Burning the lignin produces heat, useful for other steps in the cellulose-to-ethanol conversion process.
Several methods are available to breaking down the chemical bonds of cellulose and hemicellulose and to remove the lignin. Methods include dilute and concentrated acid hydrolysis
and enzymatic hydrolysis. Hydrolysis releases fermentable sugars from cellulose and hemicellulose. This stage is sometimes called saccharification.
Fermentation, the next stage of the process, uses enzymes to convert the sugars into ethanol. As with the grain-to-ethanol process, the final stage is distillation of the fermented beer into ethanol that is about 95-percent pure.
Questions about Ethanol
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