Biofuels in Perspective

182 Biofuels
about 4 % of the total demand of electric power in Brazil. An additional advantage of
the power production from residues at alcohol distilleries is the diffuse generation, since
distilleries are operating in most parts of the country. Thus the need for transmission lines
is reduced.
Figures 10.7 and 10.8 show that the productive use of bagasse and stillage for energy
generation transforms the use of sugar cane for alcohol production in a sustainable industry
where solar energy is transformed into useful energy forms: automotive (alcohol) and
electric energy with minimal use of materials for plant growth and a small environmental
impact. The energy balance can be improved further by abandoning the actual practice of
burning the cane before cutting to remove the leaves and the (sugar poor) tops and thus
making harvesting easier. By harvesting also leaves and tops the bagasse production can
be doubled, which would lead to an increase of 1,25 MWh by digestion and 0,5 kWh by
combustion taking the total to 4 MWh per m 3 of alcohol, representing 28 % of the energy
in the processes sugar cane.
The feasibility of productive use of the subproducts depends essentially on a comparison
of investment and operational costs on the one hand and the value of the produced energy and
recovered nutrients on the other. In Brazil electric power is mainly generated at hydropower
plants and can be bought at prices as low as US$ 40 per MWh on the spot market, although
power from the regular grit costs about US$ 80, even for large consumers. The Brazilian
government is developing a special program to stimulate the production of ‘green’ energy
(PROINFA) to complement the production of electric hydro power. Presently most of the
hydroelectric power potential is already in use and additional power is being developed
using fossil fuels, mostly in the form of natural gas. The expectation is that green energy
will be bought by the government at US$ 80 to 100/MWh. Under those conditions there
would be room for a comfortable profit for the distilleries by using the subproducts for
electric power production.
In the above evaluation the benefit of CO2 emission reduction by using renewable energy
has not yet been considered. The effect can be estimated as follows: The production of
1.5 MWh of electric power per m 3 of alcohol is effected by using 2 t dry bagasse and the
methane production of 100 kg of methane from vinasse digestion. If natural gas were used
to produce energy (at 35 % efficiency), about 300 kg CH4 would be required to produce
1.5 MWh and in the process 825 kg CO2 would be released. If renewable resources are
used for power production the CO2 release is ‘pre-paid’ in the sense that no new CO2 is
emitted, because the organic material of the subproducts was produced by photosynthesis,
using CO2 as the carbon source. While the incentive for decreased CO2 emission has not
yet been fixed, its value is expected to be 4 to 5 US$ per ton of CO2, or US$ 16 to 20 per
ha of cane field per year in the case of autonomous distilleries and US$ 3 to 4 /ha.year
for sugar mills with attached distilleries. While the absolute value of these figures is not
considerable in relation to the value of the main products (sugar, alcohol or electric power),
it may be significant in terms of profitability of the companies.
It is interesting to analyze the perspective of energy production if electric power generation
rather than automotive fuel production is the prime objective. In that case it probably
would not be advantageous to apply fermentation and the cane juice would be digested
directly, whereas the bagasse would receive the same treatment as proposed above: combustion,
eventually preceded by anaerobic digestion. In that case the entire equipment of
the distillery would become redundant. If it is assumed that the organic material of the