BIOCHEMICAL CONVERSION: BIOFUELS BY INDUSTRIAL FERMENTATION - Biomass as a Sustainable Energy Source for the Future - page 406

A simplified elemental s yeast cell is

CH 1.748 N 0.148 O 0.596 S 0.0018 (Lange and Heijnen, 2001). Although the one-carbon mole
formula of carbohydrates is not very different (CH 2 O), modest consumption of
additional nutrients and modest formation of side products are required to close the
mass balance of the anabolic reactions.

formula per carbon mole of a baker '

13.1.2 Outline of This Chapter

Reduced products excreted by some microbial cells upon fermentation of carbohy-
drates can be isolated for use as biofuel. This approach has been very successful
for ethanol production, which therefore is the default biofuel produced by industrial
fermentation. Section 13.2 treats a traditional process for producing ethanol. Such a
so-called 1st-generation process consumes carbohydrate sources that might otherwise
be used as food. Therefore, 2nd-generation ethanol processes that consume nonfood
carbohydrate sources are being developed, and these are described in Section 13.3.
This includes the physical, chemical, and enzymatic process steps that are required
to liberate fermentable sugars.
Industrial production is also investigated for many other compounds resulting from
fermentative conversion of 1st- or 2nd-generation carbohydrate sources. Section 13.4
treats the production of 1-butanol, and Section 13.5 treats diesel-like compounds.
Which fermentation products should be produced remains a matter of debate, but in
Section 13.6, stoichiometric and thermodynamic criteria are used to facilitate such a
selection on the basis of fundamental knowledge. An outlook is given in Section 13.7.

13.2 FIRST-GENERATION BIOETHANOL PROCESSES

13.2.1 Ethanol

Ethanol (CH 3 CH 2 OH) is the best known alcohol. It is a clear liquid that mixes in all
proportions with water. It boils at 79 C and solidifies at 114 C.

Ethanol can be prepared from petrochemical resources or from renewable
resources such as sugar; in the last case, it is usually called

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“ bioethanol. ” Its ferment-

ative production from sugars has been known for thousands of years.
Distillation of ethanol water mixtures at atmospheric pressure does not lead to

pure ethanol (although it has a lower boiling point than water) but to a so-called
azeotropic mixture of 95.6 vol.% ethanol and 4.6 vol.% water, because their
molecules interact and do not show thermodynamically ideal behavior. Pure ethanol
was first obtained in 1796. Applications were developed as solvent, industrial
intermediate (e.g., for ethyl esters), and as fuel. The T-Ford was designed for
bioethanol use.
Between about 1950 and 2000, petrochemical resources were used for the
production of ethanol, but now, the production of bioethanol is again cheaper than
the production of this petrochemical ethanol. The most important reason for the return
of bioethanol is the need for an automotive fuel from renewable resources. Such

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