If conveniently available, agricultural waste is an ideal fuel or fossil fuel supplement because it provides a use for waste material. Dry manure (less than 20 percent moisture) can also be burned directly. Use of these materials is most economical for ethanol plants located near farms or ranches since transportation costs can outweigh the energy value.

Conventional gasification or reforming uses high temperature and steam with limited or no oxygen to break down organic substances into synthesis gas (primarily carbon monoxide and hydrogen). These gaseous products are valuable for use as a fuel by itself or for building larger organic molecules (including ethanol) via a catalytic Fischer-Tropsch synthesis reaction. Gasification isn't a new technology, but its consideration for biomass feeds such as corn stover has only occurred over the past 30 years. Although there has been considerable recent interest in gasification for generating alternative liquid fuels from biomass, use of this technology in an existing ethanol plant at the present time is unknown.

Anaerobic digestion is a process where organic matter is decomposed by bacteria in the absence of oxygen to generate a gaseous mixture (referred to as biogas) consisting primarily of methane. The process is commonly used for wastewater treatment and has been utilized by several ethanol plants for processing relatively small quantities of off-spec fermentation batches. Digestion of the much larger quantities of stillage that are produced in a typical plant would generate a volume of gas that could justify its recovery and use as a fuel within the plant. Several designs where an anaerobic digestion system replaces the centrifuge, evaporator and dryer within an ethanol plant have recently been proposed.

Anaerobic digestion can easily accommodate other biomass sources such as agricultural waste and manure. The main hurdle that has prevented widespread use of anaerobic digestion by the ethanol industry has been cost (up to 30 percent greater in capital cost). However, elimination of two critical economic factors in plant operation—fuel and distillers dried grains with solubles (DDGS) prices—provides a powerful incentive that may make the initial high cost worthwhile.

Improved Efficiency, Energy Extraction
This is a key reason why energy use in ethanol plants has dropped considerably over the past few decades. The most efficient plants are getting at least two uses out of most of
their energy inputs. One way to increase energy production efficiency is through cogeneration. Plants have also been reusing waste heat and utilizing regenerative heating.

Examples include using hot gaseous effluent from the DDGS drum dryers to heat the thin stillage evaporators and using evaporator waste heat to augment what's needed for distillation. The improved energy costs must be weighed against the higher capital expense and more complex operation.

Another strategy to consider is cooperative energy. Excess heat or power can be captured and transferred from a nearby industrial facility to the ethanol plant or vice versa. This synergistic relationship fosters good neighbors and reduces overall energy costs. The downside to this strategy is the dependence on this synergy. For example, if there is a plant shutdown for maintenance, there needs to be a backup plan so that there is no effect on the collaborating plant. This backup system could result in additional capital costs and maintenance to ensure no disruption to the dependent processes.

In addition, pinch technology is a methodology that seeks to maximize heat recovery in a process facility by systematically identifying all heat gradients and then determining the optimum heat recovery scenarios. Many facilities that have applied this methodology have seen energy savings of 20 percent.

Elimination, Minimization of Drying
Drying distillers grains and solubles is one of the most energy-intensive unit operations in a dry-grind ethanol plant, consuming as much as one-third of a plant's entire energy requirements. Thus, any method of incrementally reducing the energy demand of the drying step can yield substantial savings. Of course, elimination of the dryers and evaporators altogether is the most straightforward solution if there is either a nearby market for distillers wet grains or if the stillage is used for its heating value within the plant.

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