Ethanol Feedstocks
Almost any plant-based material can be an ethanol feedstock. All plants contain sugars, and these sugars can be fermented to make ethanol in a process called "biochemical conversion." Plant material also can be converted to ethanol using heat and chemicals in a process called "thermochemical conversion" (see Ethanol Production to learn more about these processes).
Selecting a feedstock depends on many factors, such as how difficult it is to grow a specific crop for ethanol, where crops can be cultivated (geographically), and whether the crops are being set aside for other uses, such as livestock feed or human nutrition. Crop residues and wood wastes can also be used as feedstock.
The U.S. Department of Energy's Bioenergy Technologies Office 2016 Billion-Ton Report: Advancing Domestic Resources for a Thriving Bioeconomy provides extensive information about existing and potential feedstocks and availability at various price points.
Starch- and Sugar-Based Ethanol Feedstocks
Today, nearly all ethanol produced in the world is derived from starch- and sugar-based feedstocks. The sugars in these feedstocks are easy to extract and ferment, making large-scale ethanol production affordable. Corn is the leading U.S. crop and serves as the feedstock for most domestic ethanol production. Corn ethanol meets the renewable fuel category of the Renewable Fuel Standard (RFS), which is limited to 15 billion gallons. This ensures there are enough feedstocks to meet demand in livestock feed, human food, and export markets.
Cellulosic Ethanol Feedstocks
Cellulosic feedstocks are non-food based and include crop residues, wood residues, dedicated energy crops, and industrial and other wastes. These feedstocks are composed of cellulose, hemicellulose, and lignin. Lignin is usually separated out and converted to heat and electricity for the conversion process. It's more challenging to release the sugars in these feedstocks for conversion to ethanol.
Cellulosic feedstocks offer several advantages over starch- and sugar-based feedstocks. They are abundant and can be used to produce cellulosic biofuels required by the RFS. They are either waste products or purposefully grown energy crops harvested from marginal lands not suitable for other crops. Less fossil fuel energy is required to grow, collect, and convert them to ethanol, and they are not used for human food. There are challenges with harvesting, collecting, and delivering cellulosic feedstocks. Researchers are studying these challenges to determine effective and affordable solutions to handling cellulosic feedstocks.
Detailed information on biomass feedstocks is available from Idaho National Laboratory's Biomass Feedstock National User Facility or in the Energy Research Centre of the Netherlands' Phyllis database.
