Thermal energy and electricity are the main types of energy used in both types of milling plants. Dry milling corn ethanol plants have traditionally used natural gas as their process fuel for production. The choice of natural gas as a process fuel may turn out to be advantageous costwise due to the sharp decrease in natural gas price in recent years as well as the shale gas boom in the United States. Natural gas is used to generate steam for mash cooking, distillation, and evaporation as well as also being used directly in DGS dryers and thermal oxidizers that destroy the volatile organic com­pounds (VOCs) present in the dryer exhaust [15]. DGS stands for "distillers grain with solubles." Due to increased production efficiencies and expanded fuel capabilities, combined heat and power (CHP) has become increasingly popular as an efficient energy option for many new ethanol plants. CHP is an efficient, clean, and reliable energy services alternative, based on cogen­eration of electricity and thermal energy on site. Therefore, CHP achieves avoiding line losses, increases reliability, and captures much of the thermal energy otherwise normally wasted in power generation to supply steam and other thermal energy needs at the plant site.

A CHP system typically achieves a total system efficiency of 60-80% com­pared to only about 50% for conventional separate generation of electricity and thermal energy [15]. By efficiently providing electricity and thermal energy from the same fuel source at the point of use, CHP significantly reduces the total fuel usage for a commercial ethanol plant, along with reductions in corresponding emissions of carbon dioxide (CO2) and other pollutants. Generally speaking, electrical energy is used mostly for grinding and drying corn, whereas thermal energy is used for fermentation, ethanol recovery, and dehydration. On the other hand, flue gas is used for drying and stillage processing as part of waste heat recovery and energy integra­tion efforts. The carbon dioxide generated from the fermentation process is also recovered and utilized to make carbonated beverages as well as to aid in the manufacture of dry ice as a by-product of the ethanol process. As mentioned earlier, based on the 2008 survey of 150 dry milling corn etha­nol plants in the United States [14], ethanol plants in 2008 used an average of 25,859 BTU of thermal energy and 0.74 kWh of electricity per gallon of etha­nol produced, which was 28.2 and 32.1% lower than the 2001 values of 36,000 BTU and 1.09 kWh, respectively. Ethanol productivity per bushel of corn also increased by 5.3% from 2.64 gallons in 2001 to 2.78 gallons per bushel in 2008 [12, 14]. It was also found that on average 5.3 pounds of dried distillers grains and 2.15 pounds of wet distillers grains (WDGs) as well as 0.06 gallons of corn oil per every gallon of ethanol are also produced as process coproducts. One U. S. bushel as a volume unit is equivalent to 35.23907 liters. Even though the U. S. corn ethanol industry has been considered a mature industry, the recent enhancements made on their process and energy efficiencies as well as the overall profitability are quite remarkable.