146 Organic waste recycling: technology and management
illustrated in Figure 4.1, to gain some additional heat value (from the biogas)
while the other benefits are still retained. Depending on the composition of the
raw materials, organic loading applied to the digestor and time and temperature
of anaerobic decomposition, some variations in the composition of biogas can
be noticed, but approximately it conforms to the following:
Methane (CH 4 ) 55-65 %
Carbon dioxide (CO 2 ) 35-45 %
Nitrogen (N 2 ) 0.3 %
Hydrogen (H 2 ) 0-1 %
Hydrogen sulphide (H 2 S) 0-1 %Of the different gases produced, CH 4 is the desirable gas, because it has a
high calorific value (≈ 9000 kcal/m^3 , equivalent to 13 kcal/g or 211 kcal/gmole
at standard temperature and pressure.); the approximate heat value of the biogas
is 4,500-6,300 kcal/m^3 , depending on the contents of other gases besides CH 4.
The three main types of biomass raw materials suitable for ethanol
production are: (a) sugar containing materials (such as sugarcanes, molasses and
sweet sorghum etc); (b) starch- containing materials (such as cassava, corn and
potato etc); and (c) cellulose materials (such as wood and agricultural residues
etc). Among the above, because the sugar content is already in the fermentable,
simple sugar form, the sugar containing materials can be readily fermented to
produce ethanol. For the other two raw materials (the starch and cellulosic
materials), the carbohydrates have to be biochemically converted into sugars
first before being fermented by yeasts into ethanol. Finally, the fermented
ethanol needs to be separated from water and other fermentation products by
distillation prior to application. The calorific value of ethanol is 7.13 kcal/g or
328 kcal/gmole. Description of the biogas and ethanol production technology is
given below.