Page 41 - Lignocellulosic Biomass to Liquid Biofuels

20 Lignocellulosic Biomass to Liquid Biofuels

(chipping, cutting, grinding, or milling) and irradiation [gamma rays, elec-
tron beam (EB), or microwave] pretreatments [1,7].

2.2.1 Operation model of physical pretreatment

Mechanical comminution can reduce the size of lignocellulosic biomass
from centimeters to smaller sizes usually ranging from micrometers to
millimeters [3]. Usually, the energy consumption for efficient physical
pretreatment is high and is closely related to the final particle size [1,8].It
has been reported that for the same size reduction, 70% more energy
input was needed for mechanical comminution than steam explosion (SE)
pretreatment [9]. Biomass characteristics, type of the pretreating machine,
and final particle size are the dominant factors for energy requirement [4].
For example, the comminution of hardwood needs more energy con-
sumption than agricultural residues [10]. For this reason, mechanical pre-
treatment is usually combined with chemical or other pretreatment
processes, before or after, to reduce the particle size [4].

2.2.1.1 Mechanical comminution
Comminution is used to reduce the particle size of biomass and increase
accessible surface area, reducing crystallinity and degree of polymerization
(DP), thus increasing the biomass biodegradability [4]. Typical mechanical
comminution includes chipping and milling as shown in Table 2.1.
Chipping is usually necessary to reduce the size of raw lignocellulosic bio-
mass for further processing, for example, to make log to wood chips.
Milling process that can be used for biomass pretreatment involves various
types, such as ball milling, hammer milling, knife milling, vibro milling,
tow-roll milling, colloid milling, wet-disk, and attrition millings [4].As
indicated by Agbor et al., harvesting and preconditioning can reduce lig-
nocellulosic biomass from logs to coarse sizes (about 10 50 mm), and
chipping can reduce size to 10 30 mm, while grinding and milling can
reduce the size to 0.2 2mm [11]. Grinding and milling are also effective
to alter the inherent ultrastructure of biomass, such as crystallinity and DP
of cellulose [7]. As shown in Table 2.1, vibratory ball milling has been
found to be more efficient in reducing cellulose crystallinity of spruce and
aspen chips than ordinary ball milling [12]. Disk milling has been reported
to achieve higher enzymatic hydrolysis yield than hammer milling [13].
The moisture content of biomass should be also considered in the selec-
tion of milling methods [4]. For dry biomass, hammer and knife millings