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Ethanol

Starch- and Sugar-Based Ethanol Production

Today, U.S. ethanol is primarily produced from corn crops by dry-mill or wet-mill processing. Although wet-mill facilities were common in the industry's early days, dry-mill facilities now account for more than 80% of industry capacity. Between 2000 and 2007, the number of ethanol plants more than doubled and production capacity tripled in the United States.

Most of the growth came from dry-mill plants because they are dedicated and optimized for the production of ethanol. Dry-mill plants are typically smaller than wet-mill plants and use less energy per gallon of ethanol produced. In both dry- and wet-mill plants, the production of co-products consumes a third or more of total process energy. These co-products provide an important revenue source to ethanol producers.

Corn Dry Mills

Dry-mill ethanol plants are optimized to produce ethanol with carbon dioxide (CO2) and animal feed as co-products. In these facilities, the corn is ground into coarse flour. Next, water and enzymes are added, and the mixture is "cooked." Yeast is added, and the mixture is fermented. This "mash" is sent to the distillation system and molecular sieves to remove the water to produce 200-proof ethanol. The ethanol is denatured (usually with gasoline) to make it unfit for human consumption and sent to ethanol storage tanks.

The solids and liquids remaining after distillation are generally recombined for sale as high-protein animal feed (known as wet distillers grains with solubles or WDGS). Some facilities also incorporate dryers to remove the moisture from the WDGS and to extend its shelf life. This dried co-product is called dried distillers grains with solubles (DDGS). The CO2 co-product is commonly captured and marketed to the food processing industry for use in carbonated beverages or the production of dry ice.

Most dry-mill plants generate thermal energy (steam and hot air) on site by burning fossil fuels such as natural gas or coal. Electricity is typically purchased from a utility. One way to improve the efficiency of dry-mill plants is to use combined heat and power (CHP) systems. In a CHP system, thermal and electrical energy are generated together on site. According to the U.S. Environmental Protection Agency (EPA), CHP can reduce the energy used during ethanol production by 10% to 25%. For more information on CHP, see the EPA's Combined Heat and Power Partnership and the fact sheet Combined Heat and Power: An Energy-Efficient Choice for the Ethanol Industry (PDF 187 KB). Download Adobe Reader.

A diagram showing the dry mill ethanol process. Corn is delivered to the plant, goes through a grinder, cookers, CO2 scrubber, fermenter, distillation columns, and molecular sieve, then gets placed in ethanol storage and is transported to market. At the distillation column phase, the material not turned into fuel goes into a centrifuge, which then can send the material through an evaporation system and rotary drum dryer - the output is distillers grain that can be sent to market. Material leaving the centrifuge also becomes wet stillage, thin stillage, or syrup for market use.

Source: Renewable Fuels Association

Corn Wet Mills

Wet-mill plants primarily produce corn sweeteners, along with ethanol and several other co-products (such as corn oil, animal feed, and starch). In these mills, the first step is to soak the corn grain in hot water to separate the protein and starch. The product is then coarsely ground, and the germ is separated to be processed into corn oil. Next, the remaining slurry, which contains gluten, starch, and fiber, is finely ground and separated so the fiber can be blended into animal feed and the starch/gluten mixture can be further processed. The starch is then dried to make corn starch or processed to produce sugars, corn syrup, and beverage sweeteners. The sugars are then fermented to produce ethanol.

Most wet-mill plants produce their own thermal energy and electricity using CHP systems. This practice enhances energy efficiency and reliability.

Ethanol also can be produced using cellulosic feedstocks. To learn more, see Cellulosic Ethanol Production.

A schematic of the wet mill ethanol process. Corn goes to steeping then to starch-gluten separation, which goes to either wet gluten or starch. Wet gluten gets dried and turns into 60% protein gluten meal. Starch can go into a drying process, fermentation process, or syrup refining. If dried, starch is the result. If fermented, ethanol chemicals is the result. If syrup refined, high fructose corn syrup is the result. The steeping process can also go directly to the starch process or go to a grinding screening process. After the grinding screening, corn is turned into either fiber or goes to germ separation. Fiber becomes feed product (wet feed). The germ separation process results in corn oil.

Source: Renewable Fuels Association