**3. Manufacturing Processes**

3

**Figure 4.** U.S. corn production (bu) and consumption according to major categories of use (adapted from [16]).

Fig. 3. U.S. dry grind corn-to-ethanol manufacturing plants. A. 450 x 106 L/y plant. B. 80 x 106 L/y plant (Photos

L/y plant. B. 80 x 106 L/y plant (Photos

Fig. 2. U.S. fuel ethanol (gal) and DDGS (t) production over time; RFS denotes levels mandated by the Renewable

Fuel Standard. Inset shows number of U.S. ethanol plants over time [12, 13, 14, 15].

courtesy of Rosentrater).

courtesy of Rosentrater).

**Figure 3.** U.S. dry grind corn-to-ethanol manufacturing plants. A. 450 x 106

A

84 Biofuels - Status and Perspective

B

Corn ethanol is produced by two main commercial approaches. These include dry grind processing and wet milling. Over the last 20 years, many new fuel ethanol plants have been constructed in the U.S. (Figure 2). During this time, the industry has grown, evolved, and matured. Dry grind processing has become the most popular method for ethanol manufac‐ turing for a variety of reasons, including lower capital and operational costs, lower energy use, and lower water consumption. In fact, most new ethanol plants in recent years have been dry grind facilities, and comprise approximately 80% of ethanol facilities in the U.S. [13].

Dry grind processing (Figure 5) consists of multiple unit operations. Primary steps include grain receiving, cleaning, storage, grinding, cooking, liquefaction, saccharification and fermentation, solids separation, distillation, ethanol storage and loadout, whole stillage centrifugation, coproduct drying, coproduct storage, and loadout. Additionally, ethanol plants entail heat recovery, waste management, CO2 extraction, dust control, and facility instrumen‐ tation systems. Specific details will vary, though, as each ethanol plant is somewhat unique in design and operation. Oil extraction is a relatively new technology for creating additional coproduct streams, and will be discussed later.

Grinding, cooking, and liquefying increase surface area and allow the corn starch to be broken down into dextrins by enzymes (e.g., alpha-amylase) during saccharification, and subsequent‐ ly into glucose molecules (using gluco-amylase), which are then consumed during fermenta‐ tion by yeast (*Saccharomyces cerevisiae*). After fermentation, ethanol is removed from water and nonfermentable materials (e.g., proteins, fibres, oils, minerals) by separation and distillation. Dewatering results in wet cake (e.g., suspended solids) and soluble solid-laden water, evaporation of the soluble solids in the water stream, mixing the condensed solubles with the wet cake, and then drying this mixture can result in a variety of coproducts. These are known as "distillers grains", and can be wet or dry, and may or may not have condensed soluble materials added into the mix. Distillers Dried Grains with Solubles (DDGS) is the most common coproduct. It is typically dried to about 10% moisture content or less, to ensure proper shelf life and reduce flowability problems. Distillers grains are generally sold to local livestock producers. It is also transported by truck and rail to livestock producers throughout the nation. DDGS has increasingly been exported to overseas markets in recent years. Distillers Wet Grains (or DWG) is popular with beef and dairy producers near ethanol plants, due to better digest‐ ibility and lower price (because it is not dried). In fact, it has been estimated that, nationwide, approximately 25% to 30% of distillers grains sales are for DWG. But, because the moisture contents are generally greater than 50 to 60%, their shelf life is very limited (less than one week, generally), especially in summer months, and shipping large quantities of water is expensive. DDGS is still the most prevalent type of distillers coproduct in the marketplace.

Dry grind ethanol manufacturing generally results in three key products. These include fuel ethanol, the primary end product; residual nonfermentable corn kernel components, which are further processed and then sold as distillers grains (or coproducts); and carbon dioxide. A rule of thumb used in industry is that each 1 kg of corn processed will produce approximately 1/3 kg of each of the products. Another general rule is that each bushel of corn processed (about

**Figure 5.** Flow chart of typical corn dry grind fuel ethanol and coproducts processing operations.

56 lb; 25.4 kg) will yield almost 2.9 gal (11.0 L) of ethanol (in reality, it will range from about 2.7 to 2.8 gal/bu, depending on the efficiency of the plant), 18 lb (8.2 kg) of distillers grains, and 18 lb (8.2 kg) of carbon dioxide (which is a byproduct of the yeast metabolism). These will vary from plant to plant due to production practices, equipment used, residence times, process temperatures, concentrations, maintenance schedules, equipment cleanliness, environmental conditions, as well as the composition and quality of the raw corn, the location where the corn was grown, and the growing season that produced that corn.

During fermentation the yeast's metabolic processes which convert glucose into ethanol result in carbon dioxide as a byproduct. This can be captured, compressed, and sold to gas markets such as beverage or dry ice manufacturers. This is especially attractive to ethanol plants near metropolitan areas. Most of the time it is just released to the atmosphere because logistics make CO2 sales impractical. Additional detailed information on ethanol and DDGS manufacturing can be found in [17, 18].
