U.S. Department of Energy - Energy Efficiency and Renewable Energy
Alternative Fuels and Advanced Vehicles Data Center - Ethanol
Cellulosic Ethanol Feedstocks
Plants contain the cellulosic materials cellulose and hemicellulose. These complex polymers form the structure of plant stalks, leaves, trunks, branches, and husks. They are also in products made from plants, such as paper. Cellulosic feedstocks contain sugars within their cellulose and hemicellulose, but they are more difficult to biochemically convert into ethanol than starch- and sugar-based feedstocks. Cellulose resists being broken down into its component sugars. Hemicellulose is easier to break down, but the resulting sugars are difficult to ferment. The plant compound lignin also resists biochemical conversion.
Developing processes to break down these components of biomass economically has been the focus of research by the U.S. Department of Energy (DOE) and other government and industry groups. Significant progress has resulted in biochemical conversion processes to break down cellulose and hemicellulose and thermochemical conversion processes to break down lignin. Together, these processes could unlock the potential of cellulosic feedstocks for ethanol production. Visit the DOE Biomass Program's Deployment page to learn about DOE-supported cellulosic ethanol biorefinery projects and view a project map.
Cellulosic feedstocks suited to ethanol production include the following:
- Agricultural residue—crop residues such as wheat straw and corn stalks, leaves, and husks
- Forestry residue—logging and mill residues such as wood chips, sawdust, and pulping liquor
- Grasses—hardy, fast-growing grasses such as switchgrass grown specifically for ethanol production
- Municipal and other wastes—plant-derived wastes such as household garbage, paper products, paper pulp, and food-processing waste
- Trees—fast-growing trees such as poplar and willow grown specifically for ethanol production
These feedstocks have many advantages over starch- and sugar-based feedstocks. They are much more abundant and thus can be used to produce more substantial amounts of ethanol to meet U.S. fuel demand. They are waste products or, in the case of trees and grasses grown specifically for ethanol production, can be grown on marginal lands not suitable for other crops. Less fossil fuel energy is required to grow/collect them and convert them to ethanol (see Energy Balance of Ethanol), and they are not human food products.
However, limitations on cellulosic feedstock quantities do exist. For example, limits must be placed on the amount of crop residue removed to protect lands from erosion and to sustain soil organic carbon. The U.S. Department of Agriculture's Renewable Energy Assessment Project is determining the amount of residue needed to protect the soil resource, comparing economic implications of using stover as a bioenergy feedstock versus a source of carbon to build soil organic carbon, and providing harvest rate recommendations and guidelines.
To learn more, see the DOE Biomass Program's Bioethanol Feedstocks page.