Page 55 - Lignocellulosic Biomass to Liquid Biofuels

34 Lignocellulosic Biomass to Liquid Biofuels

including very high solids loadings [89]. During AFEX pretreatment, cel-
lulose and hemicellulose are well preserved since only a small amount of
biomass is solubilized. AFEX results in decrystallization of the cellulose.
Furthermore, hemicellulose is partially hydrolyzed and deacetylated [90].
Some lignin carbohydrate bonds are disrupted, and the material structure
is rigorously altered, which results in the increased water-holding capacity
and higher digestibility [91]. It is not necessary for neutralization prior to
the enzymatic saccharification of AFEX pretreated biomass. In addition,
there is almost no inhibitor formed for microbial fermentation [92].As
reported, compared with dilute acid pretreatment, the formation of furans
was 36-fold lower for AFEX, while the yield of carboxylic acids (e.g., lac-
tic and succinic acids) was 100 1000-fold lower than sodium hydroxide
pretreatment [93]. The hydrolyzate can be directly used for fermentation
without any detoxification. Furthermore, the residual nitrogenous salt is
an important nitrogen source for microbial fermentation [94]. AFEX has
been reported to achieve high conversion rates (80% 97%) of different
kinds of biomass, such as corn stover [89], switchgrass [95], wheat straw
[96], rice straw [97], birch, and willow [98]. However, AFEX is less effec-
tive for the biomass with relatively high lignin content, such as aspen and
newspaper [99]. Another advantage of AFEX is that the ammonia used
in the pretreatment can be recovered and recycled. Nevertheless, the
associated complexity and costs of ammonia recovery are high. Therefore
the process still needs integration and optimization to achieve the industri-
alization of AFEX [100].

2.3.3.2 Alkali-catalyzed organosolv pretreatment
Alkalis are also used to facilitate the organosolv pretreatment process.
Using alkaline catalysts, such as alkalis and neutral alkali earth metal, can
assist organosolv pretreatment under mild conditions. With regards to the
issues derived from acid catalyzed organosolv pretreatment, such as being
corrosive, hazardous, and inhibitory characteristics, alkali-catalyzed orga-
nosolv pretreatment has recently attracted considerable attention [101].In
order to increase the enzymatic hydrolysis, the removal of lignin seems to
be more helpful than the removal of hemicellulose under a mild condi-
tion (,100°C) [102]. Alkaline catalysts, such as sodium hydroxide, potas-
sium hydroxide, ammonia, and lime, have been used for pretreatment
due to their lignin removal effectiveness [101].
Ethanosolv pretreatment catalyzed by NaOH was applied to improve
the enzymatic hydrolysis of moso bamboo [103]. The cellulose to glucose