Biofuels in Perspective

178 Biofuels
80 bar) can produce the energy with half the bagasse mass used in traditional units, so that
their conversion efficiency increases to 13 %. In part these figures are so low because of
the 50 % of water that makes part of the bagasse and evaporates during combustion. If the
bagasses were heat dried the efficiency could increase considerably. This is particularly
important at sugar mills where the energy demand is so high that most of the bagasse must
be burnt to produce the required heat and power to run the mill.
In explosion motors the efficiency of biogas generators is in the range of 30 to 37 %.
Thus, if anaerobic digestion of bagasse could be applied, the energy production could be
increased by a factor (30 to 37)/13 ≈ 2,5 even compared to application of modern, heavy
duty steam generators. If gas turbines instead of explosion motors are used for electric
power generation, the efficiency is even higher (40–45 %) corresponding to an increase of
energy production by a factor (45 to 45)/13 ≈ 3to3,5.
Anaerobic digestion can be applied to both the liquid and the solid residues of alcohol
and sugar production. It has been established that there is a release of about 500 kg of COD
in stillage per m 3 of alcohol, mostly as soluble organic material which can be converted
into biogas with an efficiency of more than 80 %. This means digestion of 400 kg COD
and hence generation of 100 kg methane per m 3 of produced alcohol (stoichiometrically
1kgCH4 is produced by the digestion of 4 kg of COD). In terms of energy the methane
represents about 23 % of the energy in the alcohol and 10 % of the energy in the processed
cane. In conventional generators with explosion motors the electric energy that can be
produced with 100 kg of methane is about 0,5 MWh.
Figure 10.7 shows the flow sheet of alcohol production with productive use of bagasse
by means of combustion and of the vinasse by digestion. It is assumed that state-of-theart
equipment is used for combustion (13 % efficiency) and that efficiency of methane
utilization is 35 %. The required volume of the digester can be estimated by considering a
COD release of 500 kg per m 3 of produced alcohol and a loading rate of 20 kg COD/m 3 .d,
so that a volume of 500/20 = 25 m 3 per m 3 .d of alcohol is needed for digestion during the
harvest season.
It is important to stress that the digestion of the stillage increases its applicability for
fertilization and irrigation: the digestion process preserves the nutrients, but the removal
of the biodegradable organic matter avoids ‘burning’ of the sugar cane leaves. Therefore
digested stillage can be applied at the most convenient moment to supply nutritional or
water demand, whereas raw vinasse has to be applied just after the cane has been cut, when
the cane fields are bare and there are no leaves.
Preliminary experiments show that even at environmental temperatures the digestion
efficiency of bagasse is more than 50 % at a retention time of 10–12 days. The remaining
50 % is composed mainly of fibres, which could still be used for power production by
combustion. Since power production is much more efficient if biogas instead of solid
bagasse is used as energy source, the power output is higher if bagasse is digested. The
electric power potential per m 3 of alcohol from the subproducts increases from 1,5 MWh
per m 3 of alcohol if stillage digestion and bagasse combustion is used (Figure 10.7) to 2,25
MWh if bagasse is digested at 50 % efficiency (Figure 10.8).
The anaerobic digestion of bagasse in association with vinasse digestion has an important
secondary advantage. Electric power can be sold at a higher price if stable production can
be guaranteed throughout the year, but raw vinasse cannot be kept for a long period: after
exposure of more than two months the concentration of organic material tends to decrease