Page 62 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 41

glycerol yield during fermentation, resulting in the decrease of ethanol
fermentation rate and final ethanol yield [146]. In order to reduce the
cost, efficient recycling of NMMO is necessary. However, some side reac-
tions, which are present in the Lyocell process, might also affect the pre-
treatment of lignocellulose and result in the acceleration of NMMO
degradation [147]. It was reported that the recycled NMMO was not as
effective as fresh NMMO for pretreating forest residues with a high lignin
content and bark after five cycles [138].

2.3.5.2 Concentrated acid for biomass hydrolysis and cellulose
decrystallization
Concentrated acid, usually .30% of acid concentration, can be used for
biomass hydrolysis at moderate temperatures (,100°C) under atmo-
spheric pressures to achieve a high sugar yield without subsequent enzy-
matic hydrolysis [54]. Concentrated acid pretreatment inevitably leads to
hydrolysis of polysaccharides by three sequential stages: (1) prehydrolysis
of hemicellulose fraction, (2) the main hydrolysis stage to dissolve cellulose
and convert it into soluble cello-oligosaccharides, and (3) posthydrolysis
to convert cello-oligosaccharides into monosaccharides [148]. In the initial
two stages, “solubilization” is responsible for the decrystallization of the
lignocellulose structure, while in the last stage, monosaccharides are liber-
ated from the fragments of cellulose and hemicelluloses [149]. The sugar
yields of this pretreatment are affected by operational variables, primarily
including acid concentration, solid loading, process temperature, and pre-
treatment time [59]. Compared with dilute acid saccharification, the sugar
yields of concentrated acid hydrolysis are much higher due to the
decreased degradation of sugars [150]. Furthermore, because of the low
temperature and atmospheric pressure used, inhibitors from sugar degrada-
tion are less than those from dilute acid pretreatment [148]. Sulfuric acid
is so far the most commonly used for concentrated acid hydrolysis of bio-
mass since it is inexpensive and highly effective, but other mineral acids,
such as hydrochloric, nitric, phosphoric, and trifluoroacetic acid, are also
studied. Concentrated sulfuric acid saccharification of wood chip, corn
stover, and bamboo has been reported [150], and the obtained sugars
show excellent fermentability. The formation of furfural and HMF is low,
and the main by-products in the hydrolyzate are organic acids [150].
Concentrated acid, such as CPA ( . 80 wt.%), is also an efficient cellu-
lose solvent for biomass pretreatment to increase cellulose hydrolyzability
[151]. It can be combined with an organic solvent (e.g., acetone or