Environmental Engineering Reference
In-Depth Information
bioethanol has to be virtually free of water in order to prevent engine problems.
Since some decades, Brazilian car engines are suited for use of 100% ethanol and
for blends with gasoline, while cars and tank stations in other countries usually cannot
yet deal with 100% ethanol but only with blends. Ethanol, however, is by no means
an ideal automotive fuel. Combusting 1 kg liberates less energy than combusting 1 kg
gasoline does, because the oxygen atom in ethanol has no combustion value but makes
up a relatively large part of the mass of ethanol. Besides, ethanol cannot replace diesel.
13.2.2 Fermentation
As feedstock for ethanol fermentation, various carbohydrates are used:
Sucrose (from sugarcane in Brazil/India, sugar beet in Europe, etc.)
Starch (from corn in United States/China, wheat
in Europe, cassava in
Thailand, etc.)
Lignocellulosic sugars (from straw and other
“
wastes,
”
everywhere according to
many future scenarios as shown in Section 13.3)
Criteria for the choice of feedstock are:
Price
Availability
Convenience of handling
Stability
Effect on productivity of the process including product isolation
Societal issues (food vs. fuel discussion)
Enzymes and sometimes also acid catalysts are used to hydrolyze polysaccharides
and oligosaccharides to fermentable monosaccharides. Baker
s yeast (
Saccharomyces
cerevisiae
) is the default microorganism used for ethanol fermentation and has the
natural advantage of excreting the sucrose-hydrolyzing enzyme invertase. Other
advantages of baker
'
s yeast are its fast and easy growth, its potential to virtually
completely convert a range of C
6
monosaccharides into ethanol and CO
2
, and its high
microbial safety. Therefore, ethanol fermentations are traditionally carried out using
baker
'
s yeast. Still, alternatives are considered (other types of microorganisms or
recombinant strains of
S. cerevisiae
) because baker
'
s yeast does not naturally ferment
some important polysaccharides and C
5
monosaccharides and does only ferment up to
50
C, while a higher temperature could lead to energy savings in some process types.
'
13.2.3 The Brazilian Bioethanol Process
There are many varieties of bioethanol processes, mainly due to feedstock differences.
Here, we treat a typical sugarcane process such as performed in Brazil. A sugarcane
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