Ethanol Production Plants
Ethanol production plants convert biomass to ethanol for use as an additive to gasoline.
Ethanol is most often produced from corn or sugarcane by utilizing enzymes to convert starches to simple sugars and yeast to ferment the sugars into ethanol. Ethanol can also be produced from cellulose by a hydrolysis process, including a pretreatment step prior to the fermentation process to break down the hemicellulose and lignin that surrounds the cellulose in a protective sheath. Alternatively, in a gasification process, heat and chemicals can convert cellulosic biomass into syngas (carbon monoxide and hydrogen), which is then reassembled into products such as ethanol, methanol, and higher alcohols. No commercial cellulosic plants are in operation today; one is under construction in Georgia, however, that will use a gasification process.
Corn Ethanol Production Plant Components
A corn ethanol production plant includes the following major components:
A receiving building would be available for delivery of corn via truck or rail. Conveyors would lift the corn for discharge into a storage silo, and from there, it would be delivered to a hammermill for grinding.
The milled corn is mixed with hot water and enzymes and passed through a steam jet cooker to liquefy the starch. Saccarification is the process in which the hot mash and enzymes are cooled, and an additional enzyme is added to convert the liquefied starch to dextrose. The cooked mash is mixed with yeast and pumped through liquefaction tanks into the fermentation tanks. The enzyme converts the dextrose into glucose that is then converted by the yeast into ethanol and carbon dioxide. In the past, the carbon dioxide was vented to the atmosphere. The preferred method is to transfer the carbon dioxide to a plant where it is packaged for sale to a third party for use in the food industry. The fermented mash is pumped to a distillation feed tank and then to the top of a multicolumn distillation system. Steam is injected at the bottom of a stripper/rectifier column. Ethanol vapor travels up the column and the solids flow down and out the column. The 93% to 95% pure ethanol next enters a molecular sieve that removes the remaining water to produce pure (99.95%) ethanol. The ethanol is then denatured by adding 5% unleaded gasoline and stored in large tanks for eventual shipment.
The remaining solids are pumped into a surge tank and then fed to centrifuges. The wet product could then be sold as feed to farms and dairies. Alternatively, the wet product could be pumped into an evaporator followed by a dryer to produce a dry feed.
Additional processing equipment is necessary to produce the ethanol. A boiler provides steam for cooking, distillation, evaporation, and other plant uses. A stack is required for the exhaust gas stream, and cooling towers would cool plant process water.
Hydrolysis Cellulosic Ethanol Production Plant Components
A hydrolysis cellulosic ethanol production plant includes all of the components described for a corn ethanol plant. In addition, pretreatment of the cellulose requires a high-temperature, high-pressure dilute acid to break down the hemicellulose and dissolve the lignin. The addition of enzymes can increase the yield of sugars. To date, this production method is still under development.
Gasification Cellulosic Ethanol Production Plant Components
A gasification cellulosic ethanol production plant includes the following major components:
Forest residue and timber feedstock would be first processed in a chipper to produce a ¾-inch nominal chip size. A hammermill could also be utilized to produce finer biomass source material. The chips would be transferred to a storage pile until the time they would be fed into the biomass converter that converts cellulosic fuel into a gaseous mixture of carbon monoxide and hydrogen. The syngas is next cleaned to remove any carbon dioxide (which represents about 20% of the carbon introduced into the process by the feedstock) and volatile organic compounds and sent to catalytic syngas converters. The product is primarily ethanol, but does contain methanol and higher molecular weight alcohols. Alternatively, the syngas is sent to a fermenter that incorporates a bacterium that converts the carbon monoxide and hydrogen into ethanol. Distillation columns would then separate the various alcohols and water. The ethanol is transferred to a molecular sieve dryer to remove all of the remaining water and produce pure ethanol. Gasoline would be used to denature the ethanol.
Additional processing equipment is necessary to produce the ethanol. A boiler provides steam for cooking, distillation, evaporation, and other plant uses. A stack is required for the exhaust gas stream. Pollution control equipment (see Biomass Power Plant page) would be required, as well as cooling towers to cool plant process water.
Ethanol Production Plant Size
An ethanol production plant typically requires 15 to 80 acres for all the facilities. The total area of the site is usually larger, landscaped, and serves as an exclusion area to the public.