First Principles of Pretreatment and Cracking Biomass to Fundamental Building Blocks 203
mechanical communition and no recycling or environmental costs. The conventional
mechanical method requires 70% more energy than steam explosion to achieve the
same particle size reduction (wright, 1998). Steam-explosion pretreatment with the
addition of a catalyst is the technology that has been claimed to be the closest to
commercialization (wright, 1998). This pretreatment has been tested extensively for a
large number of different lignocellulosic feedstocks. The technology has been scaled-up
and operated at the pilot-plant scale at the Iogen demonstration plant in Canada.
The steam-explosion is recognized as one of the most cost-effective pretreatment
processes for hardwoods and agricultural residues. But it is less effective for softwoods.
Kobayashiet al. (2004) investigated methane production from bamboo wood using
a steam-explosion pretreatment. Methane could not be produced from raw bamboo,
but methane production is enhanced by steam-explosion pretreatment. The maximum
amount of methane production of about 215 ml was obtained from 1 g of exploded
bamboo wood at a steam pressure of 3.53 MPa and a steaming time of 5 min. Ballesteros
et al. (2002) evaluated the effect of particle size on steam-explosion pretreatment
of herbaceous lignocellulosic biomass. In this study, chipped biomass (5% moisture)
with particle size (2–5, 5–8, 8–12 mm), temperature 190 and 210∘C, and residence
time (4 and 8 min) was used. Larger size steam-exploded (8–12 mm) particles results
in higher cellulose and enzymatic digestibility. After pretreatment, the water-soluble
fiber was enzymatically hydrolyzed to determine the maximum accessible sugar yield.
Cellulase enzyme loading was 15 filter paper unit (FPU) g−^1 of substrate. Enzymatic
hydrolysis was performed at 50∘C on a shaker incubator at 150 rpm for 72 h and at 2%
(w/v) substrate concentration. Lower enzymatic hydrolysis yield (70%) was obtained at
190 ∘C for 4 and 8 min pretreatment. Higher enzymatic hydrolysis yields (about 99%)
were obtained at 210∘Ctemperature.Inarecentstudy,Violaet al. (2008) reported
steam-explosion pretreatment of wheat, barley, and oat straw. The steam-explosion
pretreatment was optimized at the batch scale on the basis of carbohydrate recovery.
Caraet al. (2008) investigated the production of ethanol fuel from olive tree. Olive
tree pruning was subjected to steam-explosion pretreatment in the temperature
range of 190–240∘C with or without previous impregnation with water or sulfuric
acid solutions. The influence of both pretreatment temperature and impregnation
conditions on sugar and ethanol yields was investigated by enzymatic hydrolysis and
SSF on the pretreatment solids. Results showed that the maximum ethanol yield (7.2 g
ethanol/100 g raw material) is obtained from water-impregnated, steam-pretreated
residue at 240∘C. Nevertheless, if all sugars solubilized during pretreatment are taken
into account, up to 15.99 g ethanol/100 g raw material may be obtained assuming the-
oretical conversion of these sugars to ethanol. Steam explosion has some limitations,
including destruction of a portion of the xylan fraction, in complete disruption of
the lignin–carbohydrate matrix, and generation of compounds that might have an
inhibitory effect on microorganisms used in the downstream stage of the fermentation
process. Due to the formation of the degradation products that may inhibit microbial
growth, enzymatic hydrolysis and fermentation-pretreated biomass needs to be
washed with water to remove the inhibitors along with water-soluble sugars obtained
from hemicellulose hydrolysis (McMillan, 1994). The washing decreases the overall
saccharification yield through the removal of soluble sugars, such as those generated
by hydrolysis of hemicelluloses. Pretreatment using hot water is also used occasionally.
In hot water pretreatment, pressure is used to maintain the water in the liquid state
