Page 152 - Materials Chemistry, Second Edition

Environmental Sustainability Assessment of Ethanol 139

Fig. 5 Life cycle diagram of
Agrochemicals, stem cuttings, diesel, water cultivation trash Fuel stock
molasses ethanol (cradle-to-
gate) Sugarcane Cane
Transport of
cane
Chemicals, water, enzymes, diesel, electricity Sugar mill Bagasse Fuel stock

Sugar Molasses
Transportation of molasses
Fuel and
Ethanol Distillery Stillage
feed stock
after the juice is extracted, called bagasse, is used as an energy source to produce
steam and electricity both for the sugar milling process and for export. The sug-
arcane juice is concentrated by boiling, and sugar is extracted in several steps by
crystallization. After the final economic extraction of sugar, the remaining
molasses (which still contains a substantial amount of sugar) is used for producing
ethanol. The sugar content of the molasses is the key quality criterion as it is
converted to ethanol. Energy-based allocation is used to share the environmental
burdens of sugarcane cultivation and sugar milling between sugar and molasses.
As for the bagasse, one part is used within the sugar mill itself and is treated as
internal recycling. The excess electricity produced from the bagasse is exported to
the grid, and credits are provided to the sugar milling system from the avoided
conventional electricity production. Molasses is transported to the ethanol plants
via trucks or through pipelines. The ethanol production process is similar to that
for cassava except that the liquefaction step is not required as molasses can be
directly fermented, followed by distillation and dehydration to produce 99.5 %
ethanol.

5.2.1 NER, NEB, and Renewability of Molasses Ethanol
The proportions of the energy used in sugarcane cultivation, sugarcane transpor-
tation, molasses transportation, and ethanol production are shown in Fig. 6a. The
sugar milling step is not included in the figure as this step actually yields energy in
excess of that being used by the process from the conversion of one of the co-
products, bagasse, which is used for steam and electricity production. As in the
case of cassava ethanol, the ethanol production step contributes the major share,
almost two-thirds of the total. In the case of molasses ethanol, the energy outputs
are not only from the ethanol but also from the exported bagasse electricity from
the sugar milling step. The net energy ratio is 1.12; though higher than one it is