Assessment of Conversion Technologies for Bioalcohol Fuel ...

Achieving such cost reductions would require substantial reductions in the current cost
of producing cellulase enzymes and increased yield in the conversion of non-glucose
sugars to ethanol.
A number of companies worldwide are developing improved enzyme systems for the
production of cellulosic ethanol. Besides applications to cellulosic ethanol production,
some of this development progress benefits conventional sugar- and starch-based
ethanol production as well. A major focus is on the conversion of corn stover and
other biomass feedstocks to not only alcohol fuels but in broader industrial
applications, possibly even the use of corn stover as an alternative feedstock for
products currently derived from petrochemicals.
Fermentation of Sugars
Ethanol is produced from the fermentation of the five major free sugars by enzymes
produced from specific varieties of yeast. These sugars are the five-carbon xylose
and arabinose and the six-carbon glucose, galactose, and mannose (M. McCoy,
“Biomass Ethanol Inches Forward,” Chemical and Engineering News, December 7,
1998). Traditional fermentation processes rely on yeasts that convert six-carbon
sugars to ethanol. However, other enzymes need to be added to convert the fivecarbon
sugars to ethanol.
It is estimated that as much as 40 percent of the sugars contained in typical forms of
celulosic biomass are of a type that normal yeast won’t metabolize. Therefore, the
biochemical cellulosic ethanol processes starts out at a 40 percent efficiency
disadvantage to corn- or sugarcane-based ethanol processes, which produce sugars
that are 100 percent convertible with normal yeast.
Once the hydrolysis of the cellulose is achieved, the resulting sugars must be
fermented to produce ethanol. In addition to glucose, hydrolysis produces other sixcarbon
sugars from cellulose and five-carbon sugars from hemicellulose that are not
readily fermented to ethanol by naturally occurring organisms. They can be converted
to ethanol by genetically engineered yeasts that are currently available, but the ethanol
yields are not sufficient to make the process economically attractive. It also remains to
be seen whether the yeasts can be made hardy enough for production of ethanol on a
commercial scale.
The resultant sugars are combined with the sugars from the first step and neutralized.
The sugars are fermented then purified to produce alcohol. A byproduct of the
neutralization is gypsum.
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