March 11, 2007

Why ethanol from wood makes sense

Taking a page from the urban waste management handbook, it is time to start thinking about a wood biostock hierarchy for the production of bioenergy. What wood, that isn't already being recycled, would we seek to use in biorefineries designed to convert wood into bioenergy? In what profile order would we seek woody biostock?

At the bottom of the pyramid would be wood industry residues and black liquor. They are part of the status quo - the waste of manufacturers that have long used them for cogenerating electricity.

Next would be wood waste that never makes it into the factory - including construction and demolition wood waste from municipal resorting facilities. Above that would be cleared unbrush and perhaps yard trimmings that isn't being composted. Next might be dead or diseased trees that represent a forestry health or fire hazard unless extracted and converted to bioenergy. Follow that with tree thinnings to prevent overgrown forests. Next would come fast growing "energy trees" developed, planted, and harvested for the specific purpose of producing bioenergy efficiently.

There are plenty of sources of wood that would be suitable short of logging live trees for the sake of producing renewable energy. Below are excerpts from an article found at the online version of The Economist on why wood looks attractive as a biostock for producing biofuels...

Woodstock Revisited
Could new techniques for producing ethanol make old-fashioned trees the biofuel of the future?
from the online version of The Economist

Why use trees, rather than maize or sugar cane, as a feedstock for ethanol? Because “treethanol” has the potential to be much more energy efficient. The ratio of the energy yielded by a given amount of ethanol to the energy needed to produce it is called the “energy balance”. The energy balance for ethanol made from maize is the subject of much controversy, but America's energy department puts it at 1.3; in other words, the ethanol yields 30% more energy than was needed to produce it. For ethanol made from sugar cane in Brazil, the energy balance is 8.3, according to the International Energy Agency.

But for ethanol made from trees, grasses and other types of biomass which contain a lot of cellulose, the energy balance can be as high as 16, at least in theory. In practice the problem is that producing such “cellulosic” ethanol is much more difficult and expensive than producing it from other crops. But the science, technology and economics of treethanol are changing fast. Researchers are racing to develop ways to chip, ferment, distill and refine wood quickly and cheaply.

Trees are a particularly promising feedstock because they grow all year round, require vastly less fertiliser and water and contain far more carbohydrates (the chemical precursors of ethanol) than food crops do.

Grass, trees and other biomass feedstocks consist of a mixture of cellulose, hemicellulose and lignin, a tough material that helps plants keep their shape. Two large producers of industrial enzymes—Genencor, an American firm, and Novozymes, from Denmark—are working to reduce the cost of cellulase enzymes, which can break down cellulose, to below $0.10 per gallon of ethanol. For its part, Diversa is developing enzymes capable of breaking down hemicellulose. One approach, says Mr Shonsey, is to tweak the structure of existing enzymes to try to make them work better. Another approach is “bio-prospecting”—looking for natural enzymes in unusual places, such as in the stomachs of wood-eating termites.

Even if the right cocktails of enzymes can be found, sceptics say treethanol will still have several problems to overcome. In particular, trees take much longer to grow than grass or food crops—so it might make more sense to make cellulosic ethanol from fast-growing grasses, or the leftover biomass from food crops. Some environmentalists worry that having struggled for years to protect forests from overexploitation, demand for biofuels could undermine their efforts.

One idea is to create new, fast-growing trees to address this problem, either through careful breeding or genetic modification. A team led by Vincent Chiang, a biologist at North Carolina State University, is investigating the production of ethanol from genetically modified trees, with funding from America's Department of Agriculture. “Our preliminary results clearly point out that transgenic wood can drastically improve ethanol-production economics,” says Dr Chiang.

A tree's rate of growth is limited by its lignin structure, which is what determines the tree's strength and form. Trees containing less lignin and more cellulose would both grow faster and also produce more ethanol. Some transgenic trees of this kind are being tested in America. Dr Chiang and his colleagues are also looking at ways to modulate the genes that determine the structure of a tree's sugar-containing hemicelluloses in order to make the breakdown and fermentation processes more efficient.

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