Environmental Engineering Reference
In-Depth Information
etc.), the actual amount of energy derived from
cellulosic ethanol is less than the fossil fuels re-
quired to produce it (Pimental, 2003).
On the other hand, studies performed by Ar-
gonne National Laboratory suggest the following
benefits from cellulosic ethanol production: use
of a 10% mix with gasoline (E10) achieves a 6%
reduction in petroleum use, 6-9% reduction in
greenhouse gas emissions, and 6-7% reduction
in fossil energy use. Use of an 85% mix with
gasoline (E85) achieves a 70-71% reduction in
petroleum use, 68-102% reduction in greenhouse
gas emissions, and 70-79% reduction in fossil
energy use (Wang et al., 1999).
bodies, and so cannot live on wood or grass, but
cows and other ruminants have a large supply
of symbiotic bacteria in their multiple stomachs
capable of secreting enzymes that break down the
cellulose in grass, thereby providing nutritional
value (Lee, 1992).
The technological challenge for the cellulosic
ethanol industry is to develop an economically
feasible method for converting the cellulose in
biomass into fermentable sugars. Once the cel-
lulose in a piece of wood, grass, or paper has been
broken apart, the resultant sugars can then be
fermented into ethanol. The rest of the processes
for producing fuel alcohol from the sugar solu-
tion (fermentation, distillation, etc.) are relatively
well understood, although work to improve their
efficiency is ongoing. Yeast converts the sugars
to produce a solution that is 5% to 14% alcohol.
This solution can be distilled to produce a 95.6%
ethanol solution. Treatment with solvents or
adsorbants is required to make 100% (absolute)
alcohol (Lynd, 1996).
There are essentially three steps involved
in the process of converting cellulose to sugar:
pretreatment (which breaks down the cell walls
of the wood or grass feedstock and helps expose
the cellulose); hydrolysis of the cellulose into
fermentable sugar molecules; and separation of
the sugar solution from the residual materials, such
as lignin (Probstein and Hicks, 2006).
Description of the Science
behind Cellulosic Ethanol
The science and technology behind making etha-
nol from starchy plants like corn and potatos, and
sugary plants like grapes is at least as old as the
production of wine from grapes or moonshine
from corn. Naturally occurring starches and sugars,
found in corn, cane sugar, beets, grapes and many
other fruits and vegetables are readily converted
to alcohol for use as fuel.
Cellulose is another naturally occurring source
of carbohydrate which, along with lignin, com-
prises the bulk of the structural material found in
plants. Cellulose is a polysaccharide—it is made
up of sugar molecules linked together to form long
chains or polymers (see Figure 3), and cannot be
directly fermented with yeast to form alcohol.
The chemical bonds that join each of the sugar
molecules together must first be broken in order
for the enzymes in yeast to work on it (Probstein
and Hicks, 2006). That's good news if you are a
tree, since the resistance of cellulose to degradation
gives wood much of its strength and durability.
Fortunately for termites and those of us who
want to use cellulose to make fuel alcohol, certain
enzymes, called cellulase enzymes, are capable
of breaking apart the cellulose sugar chains. We
Pretreatment
Pre-treating cellulosic biomass makes it easier for
the enzymes to get to the cellulose and convert
it to fermentable sugars. Without pretreatment,
the yield of sugar is less than 20%, but with pre-
treatment it can be greater than 90%. Methods
include dilute-acid pretreatment, steam explosion,
ammonia fiber explosion, and treatment with
solvents (Lynd, 1996). Pretreatment combined
with hydrolysis also shows some promise (Ryu
and Lee, 1983). The choice of pretreatment is
important for at least two reasons: its costs typi-
