Putting agricultural waste
to good use
Plants of all kinds are rich in sugar. This is obvious if you bite into
an apple, a sweet potato, or sugar cane. But many of these plants are
food sources. So, the best solution to our need for fuels derived from
crops, also called biofuels, would be to find some plant material that
is currently considered a waste product, and turn that into biofuel.
In the United States, millions of tons of discarded wood left over from
the lumber industry would be enough to generate more than 30 times
as much ethanol as the country currently uses. There are also huge
amounts of agricultural waste from leftover plant parts that remain after
harvest, such as the stalks and husks from corn plants. It would be ideal
if we could convert this waste into ethanol.
Corn was chosen to make ethanol in the first place
because it contains large amounts of relatively free and uncombined
glucose molecules. But what makes up the rest of the corn plant? It
turns out that the fibers in corn stalks, plant stems, shrubs, and trees
are made primarily from a compound known as cellulose (Fig. 3).
What is interesting about cellulose is that it is a natural polymer made up
of repeating subunits of, wait for it, glucose! The challenge is to free up the
glucose that makes up the cellulose, so it can be fermented into ethanol.
This is the challenge of cellulosic ethanol: Cellulose is made up of the
same molecules that are present in corn—glucose—but the nature of
the links holding the glucose molecules together is different in cellulose
than in corn, and yeast cannot break cellulose apart. Also, cellulose
molecules are wrapped up in lignin and hemicellulose, two groups of
dioxide in the atmosphere roughly constant. Petroleum, however, is
a product of vegetation that was buried and locked away millions of
So when we extract the petroleum and burn it, the carbon that had
remained locked away for so long is released into the atmosphere in the
form of carbon dioxide, increasing the amount of carbon dioxide already
present in the atmosphere.
According to the U.S. Environmental Protection Agency, the burning of gasoline and diesel in the United States accounts for 32% of
greenhouse gas emissions. The total amount of carbon dioxide released
worldwide into the air from burning gasoline and diesel accounts for the
rise of 1.4 ˚F (0.8 ˚C) in the average global temperature since 1980.
In the United States, ethanol is derived primarily from the glucose in
corn. Glucose (C6H12O6), a form of sugar, contains six carbon atoms
(Fig. 2) and can be easily converted into ethanol. Yeast is used to
metabolize the glucose, releasing ethanol as one of its waste products.
Hard, dried corn is shipped in trucks to an
ethanol plant and dumped into giant holding tanks. From there, the corn is soaked
in water, crushed, and sent to fermentation
tanks, where yeast digests the glucose. The
yeast converts each molecule of glucose to
two molecules of ethanol and two molecules
of carbon dioxide, a process called yeast
C6H12O6 ➔ 2 CH3CH2OH + 2 CO2
Some ethanol producers release the carbon dioxide into the atmo-
sphere, but most producers capture it and sell it for use in carbonating
soft drinks and the manufacturing of dry ice.
Ethanol has been part of our gasoline supply since 1995. But many
people are concerned that nearly one-third of the corn produced in the
United States is no longer used to make food but rather to produce
fuel. As the U.S. population continues to grow, many people consider
the demands for corn-based food supplies to be more important than
demands for corn ethanol.
Also, producing ethanol requires equipment that uses fossil fuels,
which raises questions about the impact of ethanol production on the
environment and the amount of greenhouse gases that are actually
released during ethanol production.
There is also concern that using corn to produce ethanol may be
pushing farmers in some parts of the world to convert rainforest to
farmland to meet the demand for ethanol fuel. This would encourage
deforestation and, as a result, the loss of trees that are known to
capture carbon dioxide in the air. With fewer trees, more carbon
dioxide builds up in the air, which defeats the original purpose of using ethanol instead of fossil fuels as a source
These problems may be alleviated with a different type
of ethanol, called cellulosic ethanol.
“Plant materials currently being left
to rot, burned, or sent to landfills
could replace half of our
petroleum needs in the
Figure 2. Chemical
structure of glucose
Figure 3. Cellulose is composed of a chain of glucose molecules.