Page 45 - Lignocellulosic Biomass to Liquid Biofuels

24 Lignocellulosic Biomass to Liquid Biofuels

found that the molecular weight and crystallinity of cellulose were
reduced, resulting in a well-improved cellulose accessibility. When the
biomass was pretreated with hot water extraction and 1000 kGy EB, the
hemicellulose content was reduced by 13.3% and the glucose yield by
enzymatic hydrolysis was enhanced by more than 4 times [26].

2.2.1.3 Extrusion pretreatment
Extrusion is a continuous and highly versatile pretreatment method and
has been considered as a promising thermo-mechanical process. Biomass
structure is altered due to the shearing, mixing, and heating actions, which
are mainly promoted in the mixing zone of the screw formed by succes-
sive kneading elements connected with a slight deviation angle between
them. Therefore extrusion pretreatment can result in the enhancement of
cellulose exposure ratio and increasing porosity and surface area [32,33].
There are two types of extrusion equipment, referring to single-screw and
twin-screw extruders [32]. Single-screw extruders only have one single
solid piece, while twin-screw extruders are composed of two small screw
elements and assembled in the shafts. The screw elements of twin-screw
extruders have three main types including kneading blocks, reverse screws,
and conveying screws. In addition, there are two classifications of twin-
screw extruders, such as counter-rotating and corotating [32]. The differ-
ent elements in the extruder have different functions including transport,
mixing, and shearing to disintegrate biomass.
The screw elements’ arrangements are also named as the screw profile,
which defines the position of each element, with the characteristics in
terms of pitch, stagger angle, length, and how the different elements are
spaced [29]. Screw profile is very important in extrusion processing, which
affects the transport of the material in twin-screw extruders significantly
[32]. Different types of extrusion machines have different mixing ways
and flow patterns depending on the inside of the barrel structures. Single-
screw extruders produce mainly a distributive mixing in the blending
agent with spreading through the lignocellulose matrix and obtain a good
spatial distribution. The force of single-screw extruder makes the material
exclusively frictional or viscous and the screw is one-piece element [32].
For twin-screw extruder it produces both distributive and dispersive mix-
ing, reducing particle size, and thus altering the physical properties of bio-
mass [4]. In corotating twin-screw extruders the shearing and plasticizing
effect is axial (the maximum velocity being achieved at the intermeshing