Ethanol Production and Distribution
Ethanol is a domestically produced alternative fuel most commonly made from corn. It can also be made from cellulosic feedstocks, such as crop residues and wood—though this is not yet common. U.S. ethanol plants are concentrated in the Midwest because of their proximity to corn production. Plants outside the Midwest typically receive corn by rail and are located near large population centers.
The production method of ethanol depends on the type of feedstock used. The process is shorter for starch or sugar-based feedstocks than with cellulosic feedstocks.
Starch- and Sugar-Based Ethanol Production
Most ethanol in the United States is produced from starch-based crops by dry- or wet-mill processing. Nearly 90% of ethanol plants are dry mills due to lower capital costs. Dry-milling is a process that grinds corn into flour and ferments it into ethanol with co-products of distillers grains and carbon dioxide. Wet-mill plants primarily produce corn sweeteners, along with ethanol and several other co-products (such as corn oil and starch). Wet mills separate starch, protein, and fiber in corn prior to processing these components into products, such as ethanol.
Making ethanol from cellulosic feedstocks—such as grass, wood, and crop residues—is more challenging than using starch-based crops. There are two primary pathways to produce cellulosic ethanol: biochemical and thermochemical. The biochemical process involves a pretreatment to release hemicellulose sugars followed by hydrolysis to break cellulose into sugars. Sugars are fermented into ethanol and lignin is recovered and used to produce energy to power the process. The thermochemical conversion process involves adding heat and chemicals to a biomass feedstock to produce syngas, which is a mixture of carbon monoxide and hydrogen. Syngas is mixed with a catalyst and reformed into ethanol and other liquid co-products.
To read updated studies about the conversion process, see Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn Stover and Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Thermochemical Pathway by Indirect Gasification and Mixed Alcohol Synthesis.
Schematic of Fuel Distribution System
Source: Dean Armstrong, National Renewable Energy Laboratory
Most U.S. ethanol plants are concentrated in the Midwest, but gasoline consumption is highest along the East and West Coasts (use TransAtlas to see the location of ethanol plants). This presents ethanol producers with unique distribution challenges.
According to the U.S. Department of Agriculture, 90% of ethanol is transported by train or truck. The remaining 10% is mainly transported by barge, with minimal amounts transported by pipeline. To put this into perspective, a tanker truck can carry 8,000 to 10,000 gallons of ethanol, and one rail car can carry approximately 30,000 gallons of ethanol.
Delivering ethanol by pipeline is the most desirable option, but ethanol's affinity for water and solvent properties require the use of a dedicated pipeline or significant cleanup of existing pipelines. Kinder Morgan ships batches of ethanol through its Central Florida Pipeline (see map). For details, see the Central Florida Pipeline Ethanol Project.