Potentiale zur energetischen Nutzung von Biomasse in der ... - EPFL

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There are many reasons why ethanol (as opposed to methanol) is emerging as the best engine
alcohol. One of the serious drawbacks of methanol is that it is hazardous to both people and the
environment. Methanol indeed is highly corrosive, which leads to higher costs for the materials in
pipelines, filling equipment and so on. Methanol is also highly toxic and can cause blindness and
blood poisoning in the event of exposure to the skin. Also, methanol’s current cost advantages are
expected to disappear eventually as more efficient methods for producing ethanol are developed.
The process of bioethanol production
The method for producing ethanol is fermentation of biomass followed by distillation, for which a
variety of technical concepts have been developed. When used as a transportation fuel, ethanol can
be generated from a number of feedstocks (cf. Figure 1) which are usually categorised into sugar,
starch and lignocellulose-based feedstocks. For the production of bioethanol, the following raw
materials are the most interesting:
� Sugar-containing feedstocks (e.g. sugar cane, sugar beet, sweet sorghum, molasses)
The technique for fermentation of sugar is well known and the process design is simple. Sugar crops
produce simple sugars (glucose, fructose and sucrose) that can be directly fermented to ethanol. The
cultivation of sugar-containing agricultural products is mostly attractive for ethanol production in
countries with a sub-tropical climate, but is generally too expensive in countries at higher latitudes
(Mansson, 1998). In Brazil, the sugar in the sugar cane is used to produce ethanol for fuel purposes.
Waste products from the sugar cane (bagasse), however, are not at present used to manufacture
ethanol and are burned instead.
Figure 1 Fuel-ethanol production routes.
� Starch-containing substrates (e.g. corn, wheat, potatoes)
This process is slightly more complex as starch crops, including grains (corn, wheat, barley, grain
sorghum) and tubers (potatoes, sweet potatoes), require an extra processing step, called hydrolysis,
prior to fermentation. Hydrolysis converts the complex sugars or starches in the grains and tubers into
monomeric sugars suitable for fermentation. In the US, corn is extensively used for producing fuel-