"Woodchips, Stalks, and Switchgrass" - Cellulosic Biomass:
A Discussion of Future Energy Crop Supply Potential

President Bush spoke in his January 31, 2006, State of the Union address of producing biofuels by 2012 using "woodchips, stalks and switchgrass" as the source of cellulosic biomass. These represent both existing and potential biomass resources.

The "woodchips and stalks" represent resources that are currently available from forestry and agriculture, though very underutilized. One of the largest unexploited categories is wood that needs to be removed from forests to reduce the risk of forest fires. Well over 8 billion dry tons of biomass has been identified by the U.S. Forest Service as needing fuel treatment removal. The amount of this biomass potentially available for bioenergy uses is estimated to be about 60 million dry tons annually. This estimate takes into consideration factors affecting forest access, residue recovery and the desirability of using some of the recoverable biomass for conventional wood products. The fraction that could be available for bioenergy and bioproducts is less than 1% of the total size of the fuel treatment biomass resource. Factors affecting the rate at which this source of material will become available include public opinion toward this type of removal, as well as delivered costs and the extent to which technology is developed for utilizing small diameter wood for products other than bioenergy. The other large underutilized forest sources of woodchips are logging residues and urban wood residues. In both cases, the relatively high costs of removal, handling, and transportation has not compared favorably to their relatively low value as an energy resource. Also, the compost market could compete for urban wood resources. From the agricultural sector, the major cellulosic resources are corn "stalks" and wheat straw. Both are left in the field after the grain is harvested in much of the U.S. While a portion of this residue does have a value in maintaining soil quality and crop productivity, in some higher yield areas of the U.S. there is an excess of residue produced that could be beneficially removed. It is important to use this resource in a way that is environmentally and economically sound and that supplies the needs of biorefineries in terms of cost, quality and consistency.

Switchgrass is a thin-stemmed, warm season, perennial grass that has shown high potential as a high-yielding crop that can be grown in most areas of the nation that are also suitable for crop production. There are, in fact, many perennial crops (grass and tree species) that show high potential for production of cost-competitive cellulosic biomass. The “best” crop for a given area can only be determined by local soil and climate conditions and the desired end-use. Thus “switchgrass” can be viewed as a surrogate for many “perennial energy crops” when doing biomass supply analysis. Other perennial energy crops that might be preferred in some situations include other thin-stemmed grasses, such as Reed Canary grass or Big Bluestem grass, or thick-stemmed grasses with rhizomes, such as Miscanthus, Energy cane, or Arundo (all may sometimes be marketed as E-Grass); trees grown as single stem row crops, such as poplars, eucalyptus, silver maple, sweetgum and sycamore, or trees grown as multiple stem row crops such as willow or poplar coppice. Some annual crops are also being evaluated as dedicated energy crops including corn, sorghum, and kenaf (a woody annual crop), because of very high yields. The perennial crops will normally show better environmental performance due to lower chemical requirements and better erosion control. Cost of production of energy crops is very sensitive to yield, thus development of better energy crops involves traditional genetic selection and/or molecular genetics. It is also extremely important to select appropriate sites and optimize agronomic or silvicultural management techniques to eliminate weed competition and assure that adequate nutrients and water are available (but without over-fertilizing or irrigating).

Crop residues and high yield dedicated energy crops will not become cellulosic biomass supplies unless efficient, integrated biomass supply systems are developed. This means first of all, fully integrating crop production, harvesting and collection, storage, preprocessing, and transportation for each crop type and end use scenario. A roadmap developed jointly by researchers, producers, and users in 2003 spells out the R&D needs for assuring that cellulosic biomass can be supplied in a way that meets the cost, quality and consistency requirements of biorefineries (or biopower or biofuels production facilities).

Source: U.S. Department of Energy, 2003. Roadmap for Agricultural Biomass Feedstock Supply in the United States. Available at: http://www1.eere.energy.gov/biomass/document_database.html (search for Roadmap).