Page 59 - Materials Chemistry, Second Edition

The Application of Life Cycle Assessment on Agricultural 45

Fig. 4 Production of bioethanol by various feedstocks and processes (based on FAO 2008)

the two processes are completed in different units. A commonly used alternative is
simultaneous saccharification and fermentation, in which hydrolysis and fermen-
tation are completed in the same unit whereas a last option is represented by
consolidated bioprocessing (Di Nicola et al. 2011).
Pre-treatment is an important phase toward improvements of the production
rate and higher yield of monomeric sugars during hydrolysis in which unconverted
fractions (from the pre-treatment) of cellulose and hemicellulose are converted
into monomeric sugars. Hydrolysis is completed either chemically by acids or
enzymatically by addition of cellulases (Fig. 5). The monomeric sugars produced
at this phase include both pentoses and hexoses which can be fermented to ethanol.
The latter can be fermented quite easily, but the fermentation of pentoses is a
selective process which can only be done by a few organisms. Ethanol itself is an
inhibitor for the fermenting yeasts and bacteria along with the furans, phenolic,
carboxylic acids, and other soluble lignin compounds that are formed during
fermentation (Hendriks and Zeeman 2009). Ethanol is recovered from the fer-
mentation broth by distillation (Hendriks and Zeeman 2009) whereas the process
residuals (e.g., cellulose, hemicellulose, lignin, and other solid materials) can be
used to produce heat or to be converted into octane boosters or for the production
of chemicals (Wyman 1994).