A cotton gin is basically a very sophisticated solid-solid separation technology. It not only removes the fiber from the seed, but also cleans the plant material from the fiber. The fiber is then baled into 220-kg bales that are enclosed in a plastic sleeve for protection. The bales are then stored in a warehouse at the gin and are shipped in tractor-trailer trucks to meet the demand schedule of the cotton buyer, typically a textile mill.

A cotton gin produces three main products:

1. Cotton fiber (bales)

2. Mote cotton (short fiber removed from seed and used for a variety of industrial applications)

3. Cotton seed (an important ingredient in dairy cattle feed)

The remaining material is referred to as cotton gin waste (CGW). Some of this material is fed to beef cattle, some is composted, and some is land applied. There are continuing efforts to find higher value uses for CGW, and some of these efforts may lead to its use to produce a biofuel.

The cotton gin not only stores the bales for continuous delivery to the textile industry, but it also stores seed for year-round delivery to the feed market. In this sense it functions as a preprocessing plant in the total logistic system—field to factory. The function of a preprocess­ing plant is to receive raw material and produce a more homogeneous product, or products, which meet customer quality standards and can be shipped more cost competitively to meet a year-round delivery schedule. This is the job of the cotton gin.

The cotton gin is an interesting model for the type of preprocessing plant that might be included in a biomass system. The raw biomass is brought in from the field and processed into products sold to three different markets. After ginning the products have at least twice the value ($/kg) as the raw biomass. This increase provides an opportunity to ship longer distances (because a truckload has a higher value), a benefit that could potentially be realized with a preprocessing plant in a biomass system. Raw biomass can be bulky, and thus has a low energy density per unit volume. If it can be transformed into a material with higher energy density, a point made earlier in this chapter, then this material can be hauled further to support a larger processing plant.

There is another lesson to be learned from the cotton industry. There is a compromise between the level of separation done with the mobile machine (cotton harvester) and the stationary machine (gin). In general, the annual hours of operation for a mobile machine is limited by field conditions (condition of crop, weather, and daylight) and these machines require a liquid fuel (diesel). Stationary installations can operate 24/7 and use electric power, thus avoiding, at least until electric cars become popular, a direct competition with the trans­portation sector for energy. System cost (harvest plus logistics) is often minimized when the mobile machine is simplified and more processing steps are allocated to the stationary machine. This is certainly the case with the cotton system. No one has advocated that the cotton harvester be modified to accomplish the ginning function and thus become a “mobile gin.”