Commercial production of ethanol in the San Luis Valley, Colorado. Final Report Page: 34 of 222
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188.8.131.52 Cellulosic Conversion
In the search for a supplement to conventional fuel sources, increased
attention has been focused on the ethanol production market. The major sources of
industrial ethanol in the United States are ethylene and the fermentation of sugars or
grains. Forest and crop residues present a disposal problem. Every year millions of
tons of crop and forest residues are burned in open fields. In addition to aesthetic
considerations, odors, reduced visibility and possible health effects make this method of
disposal unattractive. Lignocellulose is being seriously considered as an alternative to
traditional ethanol feedstocks.
The value of grains as a food source and the price and availability of
ethylene are negative factors for ethanol production. Because of the limited com-
mercial value of residual forest, pulp or paper wastes and municipal solid waste, the
feedstock costs for a cellulose plant would be extremely low. However, in order to use
lignocellulose, fermentable sugars must be first liberated and separated from the lignin
and hemicellulose fractions of the feedstock.
Most current processes for cellulosic conversion are energy intensive.
There are three primary approaches to the liberation of sugars in the lignocellulose;
weak acid, strong acid and enzymatic hydrolysis.
Weak acid hydrolysis such as the Madison-Scholler process (Yu, 1980) uses
0.5 percent sulfuric acid at relatively low temperatures of 140-1900C (284-3740F) to
degrade the cellulose. At higher temperatures and shorter retention times (5000C;
9320F), weak acid hydrolysis results in a higher glucose yield. The temperatures
required for either of these conversion techniques exceed the expected temperature of
the geothermal reservoir in the San Luis Valley.
Strong acid hydrolysis uses a sulfuric acid concentration between 70 and
80 percent to hydrolyze the cellulose bonds at temperatures between 10 and 450C (50-
1130F) (Oshima, 1965). Economic feasibility requires acid recovery. Generally the acid
recovery operations are highly energy intensive. Acid hydrolysis creates a corrosive
atmosphere for all equipment used in the processing steps. Special materials and con-
struction techniques may lessen the severity of the attack on holding vessels and other
process equipment. The additional cost for capital equipment and the safety hazards
associated with strong acid hydrolysis preclude the use of this process until cellulose
conversion and acid recovery become economically attractive.
Some of the problems of cellulose as an ethanol feedstock arise because
of the undesirable by-products (degradation) formed during conversion to glucose.
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Hewlett, E.M.; Erickson, M.V.; Ferguson, C.D.; Sherwood, P.B.; Boswell, B.S.; Walter, K.M. et al. Commercial production of ethanol in the San Luis Valley, Colorado. Final Report, report, July 1, 1983; United States. (digital.library.unt.edu/ark:/67531/metadc874948/m1/34/: accessed August 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.