Environmental Engineering Reference
In-Depth Information
4 Cellulosic Ethanol
4.1 Background
Henry Ford test drove his first prototype automobile called the Ford Quadracycle in
July 1896 that ran on pure ethanol. He told the New York Times in 1925 that “The
fuel of the future is going to come from fruit like that sumach out by the road, or
from apples, weeds, sawdust -- almost anything” [41]. Ethyl alcohol, or ethanol, is
a two carbon, straight chain alcohol that is found in alcoholic beverages. Ethanol is
a renewable, biodegradable, clean burning, alternative fuel that is usually produced
by the fermentation of carbohydrates from sugar, corn, or fruits [13]. Ethanol has
replaced methyl tert-butyl ether (MTBE) as an emissions reducing additive in gaso-
line due to concerns of MTBE ground water contamination that arose in late 2005.
Ethanol can be used in current automobiles in blends up to 10% (E10) in gasoline
without any engine modifications. Higher percentages of ethanol blends (E85 and
E100) can be used in Flex Fuel Vehicles (FFVs).
Sugarcane-based ethanol edges out gasoline at an oil equivalent economic price
of $40 per barrel [42]. In contrast, US corn-based ethanol has an edge over gasoline
when oil price is $60 or higher. “Flex-fuel” vehicles are designed to run on ethanol,
gasoline, or a mixture of the two. Ethanol is made through the fermentation of sug-
ars, and sugar cane offers particular advantages. The energetic balance in ethanol
production shows that for each unit of energy invested, sugar cane based ethanol
yields eight times as much energy as corn [43]. Unlike corn-based fuels, sugarcane
requires no fossil fuels to process. Cellulosic ethanol, derived from a range of crops,
such as switchgrass and crop waste, is more economical than corn ethanol because
it requires far less energy to produce. However, the economics of corn or cellu-
losic ethanol has been discussed widely in many articles. A central argument is that
corn-based ethanol is literally a waste of energy. Detractors say that it takes more
energy to grow the corn, process it, and convert it to ethanol than would be saved
by using it. According to Pimentel and Pazek [44] “Ethanol production using corn
grain required 29% more fossil energy than the ethanol fuel produced.” Wang et al.
dispute this and state that it takes 0.74 BTU of fossil fuel to create 1 BTU of ethanol
fuel, compared with a ratio of 1.23 BTUs to 1 BTU for gasoline or 66% more than
ethanol [45]. The conclusions of Wang et al. have largely been corroborated by
Farell et al. [46]. According to them, “current corn ethanol technologies are much
less petroleum-intensive than gasoline but have greenhouse gas emissions similar to
those of gasoline.” The authors however opined that cellulosic ethanol would be key
to large-scale use of ethanol as a fuel. Hammerschlag compared data from ten dif-
ferent studies and used a parameter,
r
E
, defined as the total product energy divided
by nonrenewable energy input to its manufacture [47]. Thus,
r
E
> 1 indicates that
the ethanol has captured some renewable energy. The corn ethanol studies showed
r
E
in the range 0.84
≤
r
E
≤
1.65, and three of the cellulosic ethanol studies indicated
a range of 4.40
6.61.
Because ethanol is made from crops that absorb carbon dioxide, it generally
helps reduce greenhouse emissions. Although it is carbon neutral and renewable,
≤
r
E
≤
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