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).
Some plants are easier to process into ethanol than others. Some don't require many resources to grow, while others need many resources, as well as intensive care. Some plants are used for food as well as fuel, while others are cultivated exclusively for fuel production. Even residues left over from harvested crops can be made into ethanol. Climate and soil type determine the types and production potential of plants that can be grown in different geographic areas.
The U.S. Department of Energy's Bioenergy Technologies Office Billion-Ton Update provides extensive information about existing and potential feedstocks and availability at various price points. To view maps of feedstocks, see National Renewable Energy Laboratory's Biomass Maps or use BioFuels Atlas.
Starch- and Sugar-Based Ethanol Feedstocks
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. To meet requirements, the Renewable Fuel Standard (RFS) limits the amount of ethanol produced from starch-based feedstocks 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 feedstocks that include crop residues, wood residues, dedicated energy crops, and industrial and other wastes. These feedstocks are composed of cellulose, hemicellulose, and lignin (typically extracted to provide energy for production). It's more challenging to release the sugars in these feedstocks for conversion to ethanol. Commercialization of these processes is a funding priority of the U.S. Department of Energy's Bioenergy Technologies Office.
Cellulosic feedstocks offer many 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 collect and deliver cellulosic feedstocks.
Potential ethanol yields from feedstocks are determined by their properties, which can be found in the Biomass Feedstock Composition and Property Database, or in the Energy Research Centre of the Netherlands' Phyllis database. The table below shows the potential of some commonly considered feedstocks.
Example Theoretical Ethanol Yields of Selected Feedstocks
|Feedstock||Theoretical Ethanol Yield |
(gal/dry ton of feedstock)
|Cotton Gin Trash||56.8|
*74 Switchgrass Alamo Whole Plant
Source: US Department of Energy Biomass Program, Theoretical Ethanol Yield Calculator and Biomass Feedstock Composition and Property Database