Page 46 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 25

zone), while in counter-rotating equipment, the effect is radial (highest
velocity achieved at the screw tips) [11].
Screw configuration characteristic is another important factor for
improving the pretreatment efficiency. The addition of one reverse ele-
ment at the third mixing zone end can greatly improve the pretreatment
efficiency. Some researchers investigated the extrusion pretreatment com-
bined with chemical treatment (sodium hydroxide) on lignin removal.
For corncobs the glucose conversion of the lignin-removed substrates
could reach up to 56%, 64%, and 68% by conveying, kneading, and
reverse screw elements, respectively [11]. For pretreatment of sugarcane
bagasse, extrusion pretreatment did not change the CrI of the substrate,
being 54.1% and 57.3% for raw and pretreated bagasse, respectively [33].
Extrusion can be operated at high solid loading, moderate tempera-
ture, and pH, and the pretreated substrates usually show improved
enzyme hydrolysis [34]. However, extrusion is usually combined with
chemical pretreatment methods, such as SE or acid pretreatment [33].
Compared to batch pretreatment system, extrusion can be operated con-
tinuously with higher processing capacity [35]. Besides, like other physical
pretreatment processes, there are no or only a few inhibitors formed by
extrusion pretreatment itself [34]. However, some additives for changing
the flow properties of lignocellulose are needed to add into the system in
order to avoid clogging of the equipment, but these additives may also
cause the degradation of the materials [36]. It has been reported that the
substances with cellulose affinity are good options as extrusion additives
for biomass pretreatment, such as glycerol, ethylene glycol, and dimethyl
sulfoxide. They can effectively fibrillate biomass cell walls, opening struc-
ture, and lowering the equipment torque [36]. Moro et al. investigated
the extrusion pretreatment of sugarcane bagasse and straw using a corotat-
ing twin-screw extruder with water, glycerol, Tween 80, and ethylene
glycol as additives. The results showed that when the ratio between
biomass and glycerol was 1:0.75 for bagasse and 1:0.53 for straw,
the enzymatic hydrolysis of pretreated solid increased to 68.2% after a
multiple extrusion with the insertion of a reverse element in the screw
configuration [33].

2.2.1.4 Pulsed electric field pretreatment
Pulsed electrical field (PEF) is relatively new for biomass pretreatment. In
PEF pretreatment process a high voltage strikes the samples placed
between two electrodes with a very short burst at about 100 μm [4].