With so many advantages, why are there still no
production facilities using lignocellulosic materials?
Ethanol is currently produced from sugar cane and
starch-containing materials, where the conversion of
starch to ethanol includes a liquefaction step (to make
the starch soluble) and a hydrolysis step (to produce glucose).
The resulting glucose is then readily fermented.
Although there are similarities between the lignocellulosic
and the starch process, the techno-economic challenges
facing the former are large. There are several options for
a lignocellulose-to-ethanol process but, regardless of which
is chosen, the following features must be assessed in
comparison with established sugar- or starch-based
ethanol production.
(i) Efficient de-polymerization of cellulose and hemicellulose
to soluble sugars.
(ii) Efficient fermentation of a mixed-sugar hydrolysate
containing six-carbon (hexoses) and five-carbon (pentoses)
sugars as well as fermentation inhibitory
compounds.
(iii) Advanced process integration to minimize process
energy demand.
(iv) Cost-efficient use of lignin.
The first step in the conversion of biomass to ethanol is
size reduction and pretreatment. In this review, only the
enzymatic process (Figure 1) will be discussed because it is
considered to be the most promising technology [4–6]. The
hemicellulose and cellulose polymers are hydrolyzed with
enzymes or acids to release monomeric sugars. The sugars
from the pretreatment and enzymatic hydrolysis steps
are fermented by bacteria, yeast or filamentous fungi,