Most ethanol is currently produced from corn using a dry-milling process. Conventional dry — mill ethanol production is a relatively simple procedure. Corn is cleaned, tempered with steam, ground, and wetted to a free flowing “mash.” The mash is superheated in a cooker where acid and/or enzymes are added to solubilize starch. Water is added to adjust solids concentration and temperature along with saccharifying enzymes. The mash is placed in a fermenter, and yeast is added to convert sugars to alcohol. When the fermentation is complete,

alcohol is removed by distillation and the residual “still bottoms” are recovered for animal feed. In such a plant, the only two products are ethanol and DDGS. Some plants now separate the germ before “mashing.” Food-grade corn oil can then be extracted from the germ, thus creating another salable product. Figure 8.1 provides a basic process flow diagram that describes the dry — milling process.

The rising demand and cost of corn has challenged the economic feasibility of conventional dry-mill processing in recent years. Conversion and retrofitting existing dry-mill ethanol plants such that they can process cellulosic biomass materials is becoming a topic of consid­erable interest. Most ethanol plants have been located in relatively rural settings. Therefore, access to relatively low-cost land is frequently available to allow for expansion of processing equipment as well as additional options for storing feedstocks and products. The DDGS produced in the dry-mill process has considerable value as livestock feed. However, practical methods for improving the value of this material and extracting additional marketable prod­ucts from the DDGS could greatly improve the economic viability of dry-mill ethanol plants.

This chapter discusses relatively nondisruptive methods for incorporating cellulosic feedstocks into existing dry-mill ethanol plants using existing off-the-shelf technology. Additionally, practical techniques for extracting marketable oil suitable for food, feed, or conversion to biodiesel will be discussed. By expanding both the feedstock options and the marketable products that can be produced at these facilities, dry-mill ethanol plants can improve their long-term competitiveness and viability.