In 2006, more than 5 billion gallons of fuel ethanol were produced in the U.S. About 90% of fuel ethanol produced in the U.S. uses corn grain as the feedstock. Corn (and similarly grain sorghum, the other major ethanol feedstock) is largely composed of starch which can be hydrolyzed (broken down) into sugars and fermented to ethanol using yeast.
Dry-grind is the most prevalent process for ethanol production in the U.S. and much of the current expansion of the industry uses this technology. The dry-grind process involves four major steps—grain preparation (grinding, liquefaction, and saccharifaction), fermentation of the sugars, recovery of the ethanol, and handling (centrifugation, evaporation, and drying) of the co-products. Using the dry-grind process, 2.7 – 2.8 gallons of ethanol and 17 pounds of distiller’s dried grains and solubles (DDGS—used as livestock feed) are typically obtained per bushel of corn.
Fuel ethanol is also produced in the U.S. using wet milling processes. Corn wet mills are generally larger in size than dry-grind facilities due to higher capital costs. And, in addition to the production of fuel ethanol, most corn wet mills are designed to fully fractionate the grain so that the major constituents (carbohydrates, lipids, and protein) can be efficiently recovered and purified for the production of value-added products such as specialty starches, high-fructose corn syrup (HFCS), corn oil, acidulants (citric acid) and thickeners (xanthan gum). Producing fuel ethanol from grains using the wet mill process involves four major steps—preparation of the grain, fermentation of the sugars, recovery of the ethanol, and handling of the co-products. The principal differences between the ethanol dry-grind process and the ethanol wet mill process are the grain preparation steps and the numbers and types of co-products recovered. Once the starch has been recovered the process of converting it to fuel ethanol and recovering the ethanol is similar in both processes.
Approximately 2.5 gallons of ethanol, 16.4 pounds of carbon dioxide, 2.1 pounds of oil, 2.6 pounds (dry mass) of corn gluten meal (used for livestock feed), and 11.2 pounds (dry mass) of corn gluten feed (used for livestock feed) are produced per bushel of corn using the wet milling process.
Analysts generally agree that the ability to significantly increase ethanol production using corn grain is limited and that large increases will require the use of lignocellulose resources such as agricultural residues, grasses, and wood.
The stability and chemical complexity of cellulose increase the difficulty of breaking it down into glucose for fermentation to ethanol. A number of approaches (involving chemical and/or physical methods) are being explored to overcome this problem. The fermentation process for producing fuel ethanol from cellulose is similar to that from corn grain. The major difference lies in the existence of relatively large amounts of pentose sugars (five carbon sugars such as xylose and arabinose) contained in the hemicellulose. These sugars also need to be fermented to make the overall process economically feasible. Efforts are underway to develop industrial microorganisms capable of efficiently converting xylose to ethanol.
Ethanol production from sucrose (table sugar), while not practiced in the U.S., predominates in tropical regions with sugarcane production such as Brazil. Under U.S. conditions, 19.6 gallons of ethanol can be produced per ton of harvested sugar cane if both the raw sugar and the molasses produced during processing are used.