Biofuels from Lignocellulosic Biomass - Sustainable Biotechnology: Sources of Renewable Energy - page 37

An effective practical pretreatment process should meet the following standards
for use in future commercial facilities: (a) allow excellent cellulose digestibility by
commercial cellulases, (b) good recoveries of cellulose and pentoses from hemicel-
luloses, (c) minimal or no microbial inhibitory by-products, (d) good separation
of lignin, (e) be easily managed at large volumes, (f) be relatively inexpensive
(capex and opex), (g) not require large energy inputs, and (h) have environmentally
acceptable features.
Published economic analysis has suggested that the MESP (minimal ethanol
selling price) for cellulosic ethanol from corn stover, using different pretreatment
technologies, ranges from $1.41/gallon for the AFEX process to $1.7/gallon for hot
water treated corn stover [36]. More recently, Sendich et al. [37] indicated that the
MESP for AFEX treated corn stover could be as low as $0.81/gallon due to reduced
ammonia concentration and a simplified ammonia recycle process. However, we
believe the assumptions used are perhaps overly-optimistic. For example, a feed-
stock cost of $30/ton is very low, especially given the alternative nutrient and soil
texture improvement values for corn stover. More recently, the DOE reported a 2007
cellulosic MESP of $2.43/gallon [38]. In any case, and despite many years of R&D,
it is difficult to validate the assumptions since none of the conversion processes have
been evaluated at practical scale.

6.3 Cellulose Hydrolysis

Three methods are possible for hydrolyzing cellulose into glucose (C6 sugar for
fermentation): 1. dilute acid hydrolysis (<1% H 2 SO 4 , 215 ◦ C, 3 min with 50-70%
glucose yield) which is no longer a viable candidate; 2. concentrated acid (30-70%
H 2 SO 4 ,40 ◦ C, a few hours, >90% glucose yield), which has been used in Japan
and will be evaluated in a DOE-funded pilot facility (Table 1); and 3. enzymatic
hydrolysis (cellulase mixture, 50 ◦ C several days, 75-95% glucose yield).

The efficient enzymatic hydrolysis of cellulose by cellulases requires a coor-
dinated and synergistic action of three groups of cellulases: endoglucanase (EG,
E.C. 3.2.1.4), exoglucanases like cellodextrinase (E.C. 3.2.1.74) and cellobiohy-
drolase (CBH, E. C. 3.2.1.91), and

∼

β

-glucosidase (BG, E. C. 3.2.1.21). EGs and
CBHs act on insoluble cellulose molecules [39]. EGs randomly act internally on
the amorphous regions of a cellulose polymer chain and generate oligosaccharides
of various lengths and additional free ends (reducing and non-reducing ends) for
CBH action. CBHs usually hydrolyze both amorphous and crystalline cellulose and
cellooligosaccharide chains from the non-reducing ends in a sequential way with
cellobiose as the major product, but some CBHs can hydrolyze cellulose chains from
both reducing and non-reducing ends [40-42]. The hydrolysis products of these two
groups of enzymes include cellodextrins, cellotriose, cellobiose, and glucose. -

glucosidases hydrolyze soluble cellodextrins and cellobiose into glucose from the
non-reducing end and remove the product feedback inhibitory effect of cellobiose
on EG and CBH (Fig. 5 ).

β