Biofuels in Perspective

180 Biofuels
Table 10.3 Demand of macronutrients for cane production required to produce of 1 m 3 of alcohol and
availability in the liquid and solid residues
kg/m3 of alcohol % of the demand
N P K N P K
Demand for production 30 5 30 – – –
In vinasse 4 3 20 13 60 67
In bagasse 16 0.4 1.2 53 8 4
Total in subproducts 20 3.4 21.2 67 68 71
significantly, thus reducing the potential for methane and power production. In principle,
the vinasse should be processed shortly after it has been produced. However the duration
of the sugar cane harvest is not more than about 180 days per year, so that during one
semester per year there would be no vinasse digestion nor associated methane production.
If year around energy is to be produced, one solution is to store part of the bagasse and use
this material between harvests.
A retention time of 10–12 days in the reactor is required for the conversion of 50 % of
the solids into methane. Consider a continuous solid anaerobic digester, for example the
DRANCO type (De Baere, 2001). Assuming that the digested bagasse has a 25 % solids,
this results in a loading rate of 250/12 ≈ 20 kg DM/m 3 .d. Because the production of 1 m 3
of alcohol leads to the release of 2 t DM of biogases, the required volume is 2000/20 =
100 m 3 of solids digester per m 3 /d of alcohol. If power production is spread out over the
whole year, the volume can be decreased to 50 m 3 , provided bagasse is stored: by the end of
the season half of the yearly bagasse production or 0,5*4*180 = 360 t per m 3 /d of alcohol
(50 % humidity) must have been accumulated. It is important to note that anaerobic bagasse
digestion or elutriation have not yet been implemented in continuous reactors at pilot or
full scale. Therefore, at this stage the costs of bagasse digestion cannot yet be evaluated
with a high degree of precision and reliability.
In Figures 10.7 and 10.8 it can be noted that the productive use of residues not only
increases very significantly the output of useful energy, it also enables the recovery of a
large fractions of the nutrients required for cane production. Table 10.3 shows the demand
of macronutrients as well as the masses or percentages present in stillage and bagasse.
Anaerobic bagasse digestion mineralizes practically all the nitrogen, so that after digestion
some 67 % of the required nutrients for production are in the liquid phase and can be
recovered just by recycling the effluents on the sugar cane field. Thus only about 30 % of
the nutrients in a production cycle are lost and must be replenished.
Table 10.4 illustrates that the conventional alcohol production in Brazil can be increased
in terms of output of products from 2775 US $ per ha per year to 3775 US $ per ha per year
(factor 1.36) by incorporating the anaerobic digestion of the stillage and biogases. However,
direct total digestion of the sugar cane biomass and conversion to green electricity is not
yet competitive to the production alcohol for motor vehicles.
Nationwide the electric power production potential of vinasse digestion and bagasse
combustion (1.5 MWh per m 3 of alcohol) at the current alcohol production rate of 15*10 6
m 3 /ano is 22.5 TWh/year or 2.6 GW if year around production is applied. This represents