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Glossary of alternative-fuel terms

A primer on 'cellulosic biomass'

Ethanol is the fastest-growing energy source in the world, yet it provides only about three percent of America's transportation fuels. In the United States, corn is the feedstock for almost all of the ethanol produced today, but its growth potential is limited. The big hope to increase ethanol production rests on tapping cellulose, which makes up the vast bulk of all plant materials, and finding better ways to transform it into liquid fuels.

Cellulosic biomass – cellulose and hemicellulose – comprises upward of 75 percent of all plant material. This material can be used as a low-grade fuel that can be burned, but currently it is difficult and costly to turn it into a liquid fuel like ethanol. The potential, however, is tantalizingly near. Cellulose and hemicellulose are polymers of sugar, but they are complex compounds not easily broken down into their simpler component sugars. Several processes are being used with some success now, but researchers are seeking faster, cheaper, more efficient ways to break down cellulosic biomass into sugars. Yeast can then ferment these sugars into ethanol.

What would be the source of cellulosic biomass? Candidates include agricultural plant wastes, plant wastes from industrial processes (sawdust, paper pulp), and crops grown specifically for fuel production, such as switchgrass and poplar trees.

Says Steve Chu, Nobel laureate and director of Lawrence Berkeley National Laboratory, "We should develop rapidly growing, self-fertilizing plants that convert carbon dioxide, sunlight, water and modest amounts of nutrients into biomass, such as cellulose, and more efficient means to convert the biomass and biowaste into usable forms of energy. Nature has found ways to convert cellulose within the stomach of a termite and at the bottom of a swamp. A promising avenue of research is to improve these microorganism communities or develop biology-inspired enzymes that can replace existing, less efficient processes."

-- Jeffery Kahn   (2/1/07)
A fuel made from biological materials, usually plants. Ethanol and biodiesel are two different types of biofuels. Fossil fuels (which are formed underground) are distinct from biofuels.
A fuel derived from sources such as vegetable oils that is the equivalent of diesel refined from petroleum; diesel has a higher energy density than gasoline. A variety of oils serve as a source of biodiesel including rapeseed, soybean, and even waste vegetable oil. Other crops that show promise include mustard, flax, sunflower, canola, and even algae.
Living or recently living biological matter that can be used as a fuel. Biomass usually refers to plant matter but can also refer to animal or waste materials.
Carbon neutral
Over its life cycle, a product or process that does not add more carbon dioxide to the atmosphere. For instance, a plant consumes carbon dioxide while it grows, then when transformed into and used as fuel such as ethanol it releases carbon dioxide back into the atmosphere. Plant-derived fuels have the potential to be carbon neutral.
Carbon sequestration
The capture and long-term storage of carbon dioxide before it is emitted into the atmosphere. One example: a system for filtering CO2 out of the emissions of a coal-fired power plant and pumping the CO2 deep underground.
Cellulosic biomass
The fibrous, woody, and generally inedible portions of plants that make up 75 percent or more of all plant material. (See sidebar at right for more details)
Also known as grain alcohol, a liquid produced by fermentation in which yeast metabolizes sugar, producing carbon dioxide and ethanol.
A compound that accounts for roughly 25 percent of all plant material that provides rigidity and, together with cellulose, forms the woody cell walls of plants and the glue that binds these cells. Lignin is an excellent fuel and can be burned to provide heat, steam, and electricity.
Net energy
The energy provided by a fuel minus the energy required to produce or obtain it. For instance, the net energy of gasoline is reduced by the energy lost in extracting oil from deep in the earth, refining, and transporting it to consumers. Similarly, the net energy of corn ethanol is reduced by the energy required to make fertilizers; irrigate, grow, harvest and transport crops; and build refineries.
Synthetic biology
The design and construction of new biological entities such as enzymes, genetic circuits, and cells, or the redesign of existing biological systems. The field builds upon advances in molecular, cell, and systems biology and seeks to transform biology in the same way that synthesis transformed chemistry and that integrated circuit design transformed computing.