Page 151 - Materials Chemistry, Second Edition

138 S. H. Gheewala

5.1.2 LCA of Cassava Ethanol
The impact assessment results from cassava ethanol production are presented in
Fig. 4. As anticipated from the energy balance evaluation, the ethanol conversion
step is the major contributor to all the impact categories considered. During this
step, steam is produced in a boiler fired by sub-bituminous coal and is responsible
for emissions of CO 2 ,SO 2 , NOx, CO, and particulates; it thus contributes 52, 51,
and 43 % to global warming, acidification, and human toxicity impacts, respec-
tively. The other big contributor is of course the electricity, which is largely
produced from natural gas and coal. Eutrophication is contributed largely by
fertilizer use in the cultivation stage and the wastewater discharged from the
upflow anaerobic sludge blanket (UASB) reactor after biogas recovery.

5.2 Ethanol from Sugarcane Molasses

The life cycle diagram of ethanol production from sugarcane molasses is shown in
Fig. 5. The first step in the life cycle is sugarcane cultivation, which consists of
planting stem cuttings in the initial year, followed by three ratoons. Manual labor
is used for land preparation, planting, farming, and harvesting. The sugarcane is
transported to the sugar mill in trucks and trailers. The next step is at the sugar mill
where the sugarcane is crushed to extract sugarcane juice; the residue remaining

Fig. 4 Potential environmental impacts of 1,000 L cassava ethanol production. a Global
3-
warming (1,922 kg CO 2 eq). b Acidification (16 kg SO 2 eq). c Eutrophication (2.79 kg PO 4 eq).
d Human toxicity (18.53 kg 1,4 DCBeq)