Organic Waste Recycling

Biofuels production 209

Storage and preparation of raw materials

Because most crops (as shown in Table 4.24) are normally harvested once a
year, to provide for year-round operation of ethanol fermentation plants, the
harvested raw materials have to be properly stored to avoid loss of
carbohydrates through spoilage, sprouting or (in temperate climate) freezing.
This is generally done by storing the raw materials in rooms equipped with
proper ventilation or drying to reduce the moisture content. For sugarcane, it is
advisable to extract the juice syrups from the crops by crushing or pressing and
concentrate them to about 20-24% sugar content to avoid microbial growth.
Except the sugarcane juice or molasses which contain readily fermentable
sugar compounds (C 2 H 12 O 6 ), the starch-containing materials such as cassava
and corns contain large amount of simple sugars bound up in complex
molecules of carbohydrates, not easily fermentable. To break down these
complex carbohydrates molecules, cassava or corns have to be milled or grinded
and water is added to form slurry containing about 65% water content. The
slurry is then heated to 65 – 93oC (150-200oF) and an enzyme alpha-amylase is
added to convert the starch to become long chain water soluble sugars called
dextrins (C 6 H 10 O 5 ). These dextrins are further converted to simple sugars
(C 6 H 12 O 6 ) by an enzyme gluco-amylase at temperature of 60oC (135-140oF).

Fermentation

Ethanol fermentation is a complex process accomplished biochemically by
yeast,Saccharomyces cerevisae, according to the following simplified equation:

C 6 H 12 O 6 ĺ 2C 2 H 5 OH + 2 CO 2 + heat (4.23)

Simple sugar ethanol

Theorectially, from Equation 4.23, 1 g of C 6 H 12 O 6 produces about 0.5 g of
C 2 H 5 OH, but actual C 2 H 5 OH yield at fermentation is less because about 5% of
sugar is used by the yeast to produce new cells and other by-products such as
glycerols, acetic acid and lactic acid etc.
For the ethanol fermentation or the yeast reaction to proceed at an optimum
rate, the following environmental conditions as summarized in Table 4.25
should be maintained in the fermenter:

• Sugar concentration should be controlled at 20 %. Too high sugar
  concentrations can inhibit the growth of yeast cells in the initial
  stage of fermentation, or they can result in too high ethanol
  concentration more than 10 % in the fermenter which are also toxic
  to the yeast cells.