In 2006, more than 5 billion gallons of fuel ethanol (90% from corn) and 250 million gallons of biodiesel (mostly from soybean oil) were produced in the U.S.
Dry-grind is the most prevalent process for ethanol production and much of the current expansion of the industry uses this technology. However, fuel ethanol is also produced using wet milling processes. 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. The wet mill process is 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 (e.g., specialty starches, high-fructose corn syrup, corn oil, citric acid, and thickeners).
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 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, which produced 4.2 billion gallons of fuel ethanol in 2004. 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.
Biodiesel is composed of mono-alkyl esters of long chain fatty acids. Through a process called transesterification, biodiesel is made by reacting vegetable oils or fats with alcohol (methanol) in the presence of a catalyst to produce either fatty acid methyl esters (FAME) or fatty acid ethyl esters (FAEE) and glycerol. While soybean oil is the primary resource used to produce biodiesel in the U.S., other virgin vegetable oils (e.g., Canola, castor, peanut, palm, rapeseed, sunflower) and greases and recycled oils (e.g., tallow, yellow grease, trap grease) can also be potentially used depending on their availability and price.