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Bac 2009 : Series: C

Mankind has used trees as a source of fuel for thousands of years. But now the notion of exploiting trees for fuel is being updated with a new high-tech twist. The idea is to make ethanol, a bio-fuel that usually comes from maize (corn) or sugar cane, from trees instead. Politicians and environ mentalists are embracing ethanol for a number of reasons. Unlike oil, ethanol is renewable: to make more of it, you grow more crops. And blending ethanol into ordinary petrol, or burning it directly in special "flex-fuel" engines, reduces greenhouse- gas emissions.
Why 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 according to America's Energy Department, 1.3; in other words, the ethanol yields 30% more energy than was needed to produce it.
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, distil and refine wood quickly and cheaply.
Interest in cellulosic ethanol is growing as the drawbacks of making ethanol from maize and sugar become apparent. Both are important food crops, and as ethanol production is stepped up around the world, greater demand is driving up the prices of everything from animal feed to cola and biscuits. There are clear signs of reactions against ethanol made from food crops. Supply is struggling to keep up, and as more and more governments introduce schemes to promote bio-fuel and cut greenhouse- gas emissions, the tension between food and fuel will only intensify.
Growing maize requires a lot of land, water and agrichemicals, so environmental groups (... ) argue that it is merely a short-term, first-generation approach to making ethanol. As for sugar, its growing value as a bio-fuel feedstock means that in Brazil (... ) there is pressure to flatten rainforests to make more room for sugar production. One green objective (reducing dependency on fossil fuels) thus conflicts with another (preserving the environment).

Cellulosic ethanol would address many of these problems...Trees are particularly promising feedstock because they grow all year round, require vastly fess fertilizer and water and contain far more carbohydrates (the chemical precursors of ethanol) than food crops do.

The Economist Technology Quarterly, March 10t", 2007, pp. 16-17


To update something with a high-tech twist: mettre quelque chose à jour.
To blend: to mix with something
A feedstock: a source
A backlash: a strong reaction
To keep up: se maintenir.

Guided Commentary

1) Basing on the text, name three (3) different feedstocks from ethanol. (3 points)
2) Why is ethanol said to be the most advantageous source of energy according to the text? (3 points)
3) Relying on the text, underline the drawbacks of making ethanol from food crops. (4 points)
4) In your own words, explain how difficult it is to produce cellulosic ethanol. (4 points)
5) In your viewpoint, is bio-fuel a lasting solution to your country's energy issue? Give reasons. (6 points)

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