UNESCO resource kit - science and technology educa...

The ethanol debate
UNESCO Module 4: Problems with fossil fuels
Running cars on ethanol (mainly in Brazil) Producing ethanol
• In Brazil, about % of the fuel used for transport is ethanol.
• Fermenting sugar with yeast will produce a
solution that is about 12% ethanol. The yeast
• About 4 million cars use 'straight' ethanol
(actually a mixture of 95% ethanol and 5%
water) in modified petrol engines.
then dies in its own waste product.
• The weak ethanol solution produced by fermentation
is distilled to produce a mixture of 950/0ethanol
•
and 5% water. Energy is needed for the distillation
About a further 1 million cars use a mixture of petrol (80%) and ethanol (200/0) in unmodified
petrol engines.
•
process.
• Energy is also needed to grow the sugar cane
that is used to produce the ethanol (for example
In cooler countries, one problem with ethanol is starting the engine in cold weather. This problem
can be solved:
to make fertilisers and in the fuel for tractors).
• Unless all the processes are done very efficiently,
producing each litre of ethanol can use more
0 by starting the engine with petrol and then energy than you get from burning it.
switching over to ethanol;
0 by pre-heating the engine as we already do
with diesel engines.
• You could make ethanol production more
efficient and cheaper:
•
Both of these solutions make petrol engines more 0 by developing better strains of yeast to
expensive. produce a stronger solution of ethanol;
At present, ethanol is more expensive than
0 by reducing the pressure and distiling the
temperature.
petrol but this wasn't so in 1975 when the
Brazilian government
to ethanol.
started a big changeover
Environmental aspects I Social aspects
• Each ethanol molecule contains an oxygen atom
so it burns more cleanly than petrol:
o less carbon monoxide (a toxic gas) is
produced than with petrol;
o less unburnt hydrocarbon molecules (which
help to produce smog) are released into the
air than with petrol;
o slightly less nitrogen oxides are produced
than with petrol (but petrol-ethanol mixtures
actually produce more nitrogen oxides)
• Sao Paulo, a city in Brazil with 20 million
people, has much cleaner air than most smaller
cities in other countries.
• Burning a fuel made from plants has no overall
effect on carbon dioxide in the atmosphere. The
carbon dioxide taken from the air as plants grow
is put back into the air when the ethanol is
produced and then burned.
• In Brazil, ethanol is made from sugar cane.
Growing the sugar cane uses about 7.50/0of the
agricultural land.
• If all the cars in Brazil used 'straight' ethanol,
either 25% of the agricultural land would be
needed for sugar cane or large areas of the
remaining tropical rain forest would need to be
cleared.
• You could feed a small family quite well by growing
food crops on the land needed to produce the
ethanol used by an average small car.
• In Brazil, only the richer 200/0of families have
cars.
• In countries where sugar cane won't grow you
can make ethanol from sugar beet or corn
instead.
PageS