Environmental Engineering Reference
In-Depth Information
the cost of the chemicals required to produce it,
the cost of biodiesel is relatively high compared
with corn ethanol, gasoline, and standard diesel
fuel (Bourne, 2007).
It takes less energy to convert canola oil into
biodiesel than it does to make corn into ethanol.
The process is a fairly straightforward, and does
not require distillation, so the ratio of energy
output to fossil fuel energy input is around 2.5,
nearly twice that of corn ethanol. Because it uses
less fossil fuel in its production, biodiesel also
reduces greenhouse gas emissions by 68% over
an equivalent quantity of standard diesel fuel
(Bourne, 2007).
that the increased demand for laborers will lead
to exploitation (Bourne, 2007).
Cellulosic Ethanol
By contrast, cellulosic ethanol can be made from
agricultural residues, forestry wastes, municipal
solid waste, and prairie grasses such as switch
grass, grown on marginal land. Although reduc-
ing the current cost of enzymes for converting
cellulose to sugar is a key to this technology's
success (Allen, 2006), there is a great deal of
research and money being invested in ways to
cut those costs, and they are projected to decrease
by an order of magnitude (Lynd, 1996). The
sources of cellulose are numerous, and many are
relatively inexpensive. In addition to agricultural
residues, forestry waste, and energy crops like
poplar and switchgrass, municipal waste (grass
clippings, prunings, and the like) can actually be
an income-producing feedstock, in that tipping
fees are generally charged for their disposal (Graf
and Koehler, 2000).
The energy in cellulosic ethanol can be any-
where from 2 to 36 times the fossil fuel required
to produce it—the ratio largely depends on the
source of the feedstock and the methods used to
convert it to ethanol. If the unused portions of the
biomass, such as lignin, are used to produce the
heat and power needed for the rest of the process,
the energy balance can be improved. The reduction
in greenhouse gases is simililarly dramatic for cel-
lulosic ethanol, potentially creating 91% less than
a comparable amount of gasoline (Bourne, 2007).
Given that it is “very likely” that human-caused
greenhouse gas emissions have been the primary
cause of recent observed global warming trends
(Solomon et al., 2007), there is an increasing sense
of urgency for developing alternatives to fossil
fuels. It should be noted that not all experts agree
with these optimistic numbers for cellulosic etha-
nol. David Pimental, for example, has argued that
when the complete fossil fuel usage is taken into
account (including fuel usage by the workforce,
Cane Ethanol
Cane ethanol made from cane sugar grown
primarily in Brazil is an example of a foodstuff
turned to fuel that appears to be working, at least
in Brazil. Even though it has its drawbacks, 85%
of the cars sold in Brazil are flex-fuel vehicles,
and the pump price of alcohol is lower than the
price of gasoline (Bourne, 2007).
Sugar cane stalks are about 20% sugar, which
can be directly fermented to produce alcohol, un-
like corn which requires enzymes to convert the
corn starch into sugar before it can be fermented.
The leftover waste material from the cane (called
“bagasse”) can be burned to produce the energy
needed for distillation, reducing the need for
natural gas or other fossil fuels. By efficient use
of the entire sugar cane plant, the energy of the
ethanol produced is nearly eight times the fossil-
fuel energy required to make it. The reduction in
green house gases compared with an equivalent
amount of gasoline is about 56% (Bourne, 2007).
The story of cane ethanol is not all positive,
however. Most cane in Brazil is cut by hand, work
that is “hot, dirty, and backbreaking” according
to a recent National Geographic article (Bourne,
2007). Other concerns with cane ethanol produc-
tion are that expansion of the cane sugar planta-
tions will contribute to deforestation, the burning
