Energy grasses and woody fast- growing trees are being explored as dedicated energy crops to provide the raw material from which to make a new generation of liquid biofuel, cellulosic ethanol, for use in cars and some trucks.
These plants, unlike corn, sugarcane, or wheat, are non-food crops. One great promise is that they may thrive on marginal lands unsuitable for food crops.
They also have other beneficial aspects, including a better energy balance, a better carbon footprint, and often smaller land-use and water needs. But they are more difficult to process into liquid fuel than corn or sugarcane.
Advances in technology have occurred over the past decade and research and development into both the plants and the processing is rapidly continuing. Cellulosic biomass from grasses, trees, and from many kinds of agricultural, forest, and municipal wastes is already being used in many places of the world to generate electrical power.
A Fundamental Difference
Turning plant energy stored as starch into fuel is determined in large part by the source of the starch. Ethanol in the United States is made in large part from corn. The starch in corn grain (the kernels, but not the leaves or stalks) is relatively easy to break down into sugar to be fermented and refined into liquid fuel. It is a more difficult (and expensive) process to extract the sugars stored in the fiber of leaves, stems, grasses, and woody trees.
The fiber in these is composed of cellulose and hemicellulose. Inside the cell wall cellulose and hemicellulose are tangled with another tough material, lignin, which makes plants woody. A combination of chemicals, heat, and enzymes can be used to break down the fibers.
If researchers can learn to fully release and ferment the sugars in cellulosic and lignocellulosic biomass, it will make more ethanol per acre of land than using corn kernels.
Aspects of promising energy grass
-- It should be perennial and persistent
-- It should be noninvasive and exhibit positive
-- It should have minimal pest problems
-- It should have high biomass yield potential
-- It should be able to be established with seed or
relatively inexpensive vegetative means
-- It should have high nutrient and water use
-- It should have broad genetic variability
-- It should be harvested with available equipment
Adapted from Heaton EA, Clifton-Brown J, Voigt TB, Jones MB, Long SP, Miscanthus for renewable energy generation: European union experience and projections for Illinois, 2004.
Some Leading Candidates for Grasses
It is a warm-season perennial grass that grows up to 15 feet tall at maturity and produces the highest biomass yields among the grasses grown in temperate climates. It requires minimal fertilizer and has a wide growing area with a lifespan of 15 to 30 years. It is native to Asia and is grown in Europe and used for power generation.
It is native to North American tall grass prairies. It can grow to 10 feet and is a hardy herbaceous, adaptable perennial. It can be harvested either annually or semiannually, for 10 years or more before replanting is needed. Its value includes improving soil and water conservation.
It is another U.S. native grass that grows throughout the Northeast, Great Lakes, and Midwest states. It grows 4 to 8 feet tall and is a robust grass. It is being looked at for specialized situations because it is salt and moisture tolerant.
Also called elephant grass, it is native to the tropical grasslands of Africa and is used throughout the world as a forage crop. It is a tall perennial bunch-type grass that grows best in deep, well-drained soils. It has the ability to grow in poor soils and to draw out a number of pollutants, including heavy metals, from soils.
Hybrid Poplars and Willows:
These fast-growing trees can be planted in tree farms to sustainably produce woody biomass for cellulosic fuel. Hybrid poplar trees provide high yields per acre, short rotation, and the ability to regenerate after harvest. Willows and poplars can be cut back and the stumps regenerated through a process called coppicing. Trees have the advantage of being harvested any time of the year. They may also have a benefit as stable wildlife habitat because they are not disturbed by annual harvests.
OTHERS OF INTEREST
It is a large succulent native to Mexico and notable for tall, spiky flowers and long narrow leaves. It contains less lignin than other feedstocks so is more easily broken down to extract sugars. It is highly efficient in its use of water, allowing it to survive without irrigation between rainfalls. It requires minimal fertilizing.
It is the same species as sugarcane but is selected to produce large amounts of fiber rather than sugar. Both the fiber and the sugar can be used for ethanol. It grows in steamy and sandy soils, including the southern U.S, and may be a good crop for currently abandoned or minimally used pasture.
There are several types of sorghum, including sweet sorghum from which molasses is produced and energy grass varieties. Ethanol can be produced from both the sugar and the fiber. It is an annual grass that is drought and heat tolerant.
Reference Box for Numerical Symbols on Map:
1. Hybrid Popular/Switchgrass
3. Hybrid Poplars/Miscanthus/sorghum/Switchgrass
4. Hybrid Poplars/Switchgrass/Willows
5. Hybrid Poplars/Miscanthus/Sorghum/Switchgrass
6. Energy Cane
What is meant by cellulosic ethanol having a “better energy balance”?
Cellulosic energy crops require less energy to produce because most types use far less fertilizer than corn, and because the entire plant is used, the amount of energy produced per acre is increased.
Is the ethanol made from grasses and trees different from corn- based ethanol?
The ethanol made from corn is the same as cellulosic ethanol made from grasses and trees (and from the abundantly available biomass of agricultural and forestry wastes such as stalks, husks, leaves, straw and wood chips and from municipal waste that includes paper and garden clippings.) It is just that breaking down the cellulose and lignin in the plants requires more advanced technology.
What about greenhouse gas (GHG) emissions and other environmental issues?
Using cellulosic feedstocks for biofuels reduces GHG emissions significantly. According to the U.S. Environmental Protection Agency, GHG emissions from cellulosic ethanol are 50 to 80 percent lower than gasoline and 30 to 70 percent lower than corn ethanol. Another benefit is that the roots of perennial plants and grasses are effective at capturing carbon and improving soil quality. A question with using these crops, however, is how much fuel will be needed to collect and transport the bulky grasses to processing facilities.
Any plant-derived organic matter.
Biomass available for energy on a sustainable basis includes herbaceous and woody energy crops, agricultural food and feed crops, agricultural crop wastes and residues, wood wastes and residues, aquatic plants, and other waste materials including some municipal wastes.
The carbohydrate that is the principal constituent of wood and other biomass and forms the structural framework of the cells.
Dedicated energy crop:
A non-food crop grown specifically for its fuel value. These are crops such as poplar trees, Miscanthus, and switchgrass.
Any material used as a fuel directly or converted to another form of fuel or energy product.
Non-woody species of vegetation, usually of low lignin content such as grasses.
The major non-carbohydrate structural constituent of wood and other plant material that encrusts the cell walls and cements the cells together.
Refers to plant materials made up primarily of lignin, cellulose, and hemicellulose.
Source: National Renewable Energy Laboratory
The Briefing is a special section of Bioenergy Connection magazine, a publication of the Energy Biosciences Institute. We welcome your comments, questions, and ideas. Contact us at Bioenergyconnection@berkeley.edu
This map shows where in the U.S. dedicated energy grasses and fast- growing trees are best suited based on soil, rain, and temperature patterns.
Source: Ecological Society of America
Credits: This Briefing report was written by Marie Felde, editor of Bioenergy Connection, in collaboration with Tom Voigt , associate professor and extension specialist in the Department of Crop Sciences at the University of Illinois, Urbana-Champaign, where he also leads the Feedstock Production/Agronomy Program for the Energy Biosciences Institute.