Page 60 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 39

Some reported works have shown that well-improved enzymatic
digestibility can be achieved by PAA pretreatment [130]. When bagasse
was pretreated with 50% PAA (based on raw biomass) and liquid-to-solid
ratio of 6:1 at 80°C for 2 h, more than 80% of cellulose to glucose con-
version was obtained [131]. Compared with acid and alkaline pretreat-
ment performed at the same conditions, PAA pretreatment was more
efficient for achieving higher sugar yield for Crofton weed stem [132].In
order to reduce the amount of expensive PAA used in a single-stage pre-
treatment, combined pretreatment with PAA and other chemicals, espe-
cially alkalis, was reported. By a first-step pre-pretreatment with NaOH
to swell the biomass and partially removing lignin, PAA loading used for
the second-step treatment can be significantly decreased to achieve similar
or even better cellulose digestibility [130].
PAA pretreatment can be carried out at a relatively low temperature
and atmospheric pressure, thereby avoiding the degradation of monosac-
charide to form inhibitors such as furfural and HMF [133]. In addition,
the PAA itself degrades into acetic acid, water, and oxygen. Therefore the
washed biomass after PAA pretreatment can be efficiently fermented to
make ethanol [134]. However, PAA is too expensive for commercial use
[135]. It is also corrosive to equipment and is explosive in a concentrated
form. This safety hazard results in the increased cost of storage and trans-
portation of PAA.

2.3.5 Cellulose solvent based pretreatment

Cellulose solvent based pretreatment refers to the process using cellulose-
dissolving solvents to pretreat biomass, thus increasing cellulose accessibil-
ity. A schematic process using cellulose solvents to produce bioethanol
from lignocellulosic materials is shown in Fig. 2.5. The most commonly
used cellulose solvents are N-methylmorpholine N-oxide (NMMO), con-
centrated acids, such as concentrated phosphoric acids (CPAs) and ILs.

2.3.5.1 N-Methylmorpholine N-oxide
NMMO contains N O bonds with high polarity, which makes it capable
of being used as a cellulose solvent. NMMO is industrially used in the
Lyocell process. NMMO can disrupt hydrogen bonds in cellulose and
form a new hydrogen bond with cellulose [136]. After dissolution in
NMMO, cellulose can be regenerated by rapid precipitation with water.
The regenerated cellulose is more accessible to cellulases because the crys-
talline structure of cellulose changes from cellulose I into II [137].