Biofuels production 2074.7 Ethanol production
5.1 Objectives, benefits, and limitations
This section describes the principles of ethanol (C 2 H 5 OH) production by
fermentation of organic wastes and other biomass, such as molasses, sugarcane,
cassava and corn. Ethanol is a liquid fuel which, due to the increasingly high oil
price, is being produced in several countries for use as an alternative source of
energy. Since these organic materials contain high BOD concentrations,
fermenting them to produce liquid fuel would yield economic return and help
reducing pollution problems that could occur if they are improperly disposed off
to the water or land environment.
Table 4.24 compares ethanol yields from the four types of biomas raw
materials. Molasses and corn give relatively high ethanol yields of 280 and 310
L/ton, respectively; while, based on land area, sugarcane gives the highest
ethanol yield of 3,500 – 4,000 L/(ha-yr). Figure 4.20 describes the basic ethanol
production process which indicates that for starch-containing biomass (such as
cassava and corn), the carbohydrates need to be biochemically converted into
simple sugars (C 6 H 12 O 6 ) first by alpha-amylase and gluco-amylase enzymes.
Through the reactions of yeast Saccharomyces cerevisae, sugars are then
biochemically converted to ethanol. Theoretically, 1 g of C 6 H 12 O 6 can produce
about 0.5 g of C 2 H 5 OH (Figure 4.23).
It can be seen from Table 4.24 and Figure 4.23 that, based on economic and
technical aspects, sugarcane is among the most attractive biomass materials to
be used for ethanol production. Not only that sugar is easily converted by yeast
to become ethanol, the sugar manufacturing process also produces bagasse as a
by-product (section 2.3.3), which can be burnt to generate heat and steam
needed for the ethanol fermentation and distillation processes. Molasses
produced from the sugar manufacturing process (Figure 2.6) and contained
mostly C 6 H 12 O 6 can also be easily converted, through the yeast reaction, to
become C 2 H 5 OH.
Table 4.24 Ethanol yields from selected biomass raw materials (adapted from NRC 1983)
Biomass raw
materials
Ethanol Yield
(L/ton)Raw materials yield
tons/(ha-yr)Ethanol yield
L/(ha-yr)
Sugarcane 70-90 50-90 3,500-8,000a
Molasses 280 NA NA
Cassava 180 12-60a 2,000-5,000a
Corn 370 6 2,220
NA = Not applicable
a potential yields with improved cultivation technology