Ciência já busca método para produzir etanol a partir de árvores (em inglês)
2007-03-12
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 high-tech
twist. The idea is to make ethanol, a biofuel that usually comes from maize
(corn) or sugar cane, from trees instead. Politicians and environmentalists
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 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, 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. The price
of a bushel of corn rose by 70% between September 2006 and January 2007 to
reach its highest level in a decade. Mexico's president, Felipe Calderón,
even capped the price of corn tortillas in January as America's fast-growing
ethanol industry caused prices to rocket. There are clear signs of a
backlash against ethanol made from food crops. Supply is struggling to keep
up, and as more governments introduce schemes to promote biofuels 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 such as America's Natural Resources Defence Council
argue that it is merely a short-term, first-generation approach to making
ethanol. Most energy experts reckon that using maize-based ethanol as a
substitute for petrol can reduce America's demand for petrol by 10-15% at
best. As for sugar, its growing value as a biofuel feedstock means that in
Brazil, which is now one of the world's largest producers and exporters of
ethanol, 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).
Trees to the rescue
Cellulosic ethanol would address many of these problems. Writing in the Wall
Street Journal recently, Vinod Khosla—a Silicon Valley venture capitalist
who has made a fortune by spotting opportunities in fields from
biotechnology to software—argued that America needs “cellulosic biofuels to
win the war on oil...we must encourage research on biomass feedstocks,
tomorrow's energy crops.”
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.
Ethanol is the result of the fermentation of sugars, which is why it can be
so simply and efficiently made from sugar cane. Making ethanol from maize is
a bit more complicated: the kernels are ground into flour and mixed with
water, and enzymes are added to break the carbohydrates from the maize down
into sugars, which can then be fermented into ethanol. Making ethanol from
cellulosic feedstocks is harder still, however, since it involves breaking
down the tough, winding chains of cellulose and hemicellulose from the walls
of plant cells to liberate the sugars. This can be done using a cocktail of
five or six enzymes, says Edward Shonsey, the boss of Diversa, a biotech
firm based in San Diego. The problem is that although such enzymes exist,
they are expensive. It is no use being able to produce ethanol from trees if
it costs $5 a gallon.
The lure of bioprospecting
So if cellulosic ethanol is to live up to its promise, researchers will have
to find cheaper and more efficient enzymes. 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 cellulose 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.
Treethanol has particular appeal in countries that have a lot of trees and
import a lot of fossil fuel. Top of the list is New Zealand: in 2005 the
country exported lumber worth NZ$411m ($290m) and imported fossil fuel
costing NZ$4.5 billion. In January two of New Zealand's Crown Research
Institutes, Scion and AgResearch, announced a research partnership with
Diversa. The aim is to investigate the feasibility of producing enough
ethanol from trees to fuel all the vehicles on New Zealand's roads without
fossil-fuel imports—in other words, to make the country self-sufficient in
energy.
BioJoule, a start-up based in Auckland, New Zealand, is planning to build a
pilot plant to produce ethanol from a type of willow. The idea, says James
Watson, BioJoule's co-founder, is that farmers would grow coppiced willow
trees which could be processed into wood chips and then transported to a
conversion plant to be turned into ethanol. The process would produce two
useful by-products: unsulphonated lignin, a commercially valuable polymer,
and xylose, a type of wood sugar used in dyeing and in foods for diabetics.
Selling these by-products, Mr Watson calculates, means his plant should be
able to produce ethanol for a direct cost of $1.13 per gallon, which
compares favourably with ethanol from American maize ($1.44) and is not much
more than Brazilian sugar-cane ($0.95). “Treethanol has particular appeal in
countries that have a lot of trees and import a lot of fossil fuel, such as
New Zealand and Sweden.”
Because willows are fast-growing and can thrive even on nutrient-poor soils,
BioJoule's technology could also be used in other parts of the world where
there is strong demand for energy, but the soil is not suitable for food
crops. Mr Watson thinks China and India look promising.
Another country keen on cellulosic ethanol is Sweden, which is relying
heavily upon wood-based solid and liquid biofuels as part of its plan to
wean itself off oil by 2020. But where New Zealanders favour willows, the
Swedes prefer poplars, since they are abundant and their biology is well
understood, says Mats Johnson of SweTree Technologies, based in Umea in
northern Sweden.
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.
And now for Frankentreethanol
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.
But Steven Strauss, a forest biologist at Oregon State University, says that
because of the great genetic variation in willows and poplars, genetic
modification may not be necessary. By screening existing varieties it ought
to be possible to identify those well suited to ethanol production.
Conventional breeding and cloning are very efficient when there is such a
variety of species and hybrids to choose from, he says, and the tight
regulation of genetically modified organisms makes using the technology
expensive and time consuming.
Hundreds of thousands of years ago, when man first gained mastery over fire,
wood was his primary fuel. In the past few centuries fossil fuels have risen
to prominence, with calamitous consequences for the world's climate. A
diversity of new fuels and energy sources seems the most likely future. It
would be fitting if humanity's portfolio of new energy technologies had a
place for wood, the oldest of them all.
By Derek Bacon
(The Economist, 08/03/2007)
http://www.economist.com/displaystory.cfm?story_id=8766061