Page 48 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 27

temperature and screw speed is important to reduce the energy consump-
tion. Some researchers have explored the applications of biomass extrusion
at low temperatures (less than 100°C) to get the promising results [43].
For mechanical energy consumed in extrusion pretreatment, torque is one
important factor to be considered. The torque is related to screw speed,
loading, and viscosity of the material inside the extruder. Lower torque is
required for thermo-softened biomass and operation at higher temperature
which reduces the viscosity. The relationship between temperature and
torque is negative. To compare the energy consumption under different
condition a parameter termed specific mechanical energy factor has been
defined by estimating the mechanical energy required to produce a unit
weight of pretreated biomass in a particular extrusion machine [44]. The
screw speed appears to be more important than temperature. When the
extrusion pretreatment was carried out at low temperature and high screw
speed, the lowest energy consumption of biomass was found to be
20.0 kJ/g [33]. Energy consumption can be reduced when the equipment
is running at low screw speed, but if the screw speed is too low, corre-
sponding feeding rate decreases with smaller processing capacity and lead-
ing to higher energy consumption per gram of biomass. For example, the
energy consumption for 100 rpm (20.0 kJ/g at 30°C and 27.8 kJ/g at
150°C) was lower than for 20 rpm (81.1 kJ/g at 30°C and 123 kJ/g at
150°C) [33]. Therefore to reduce energy consumption, extrusion pretreat-
ment is recommended to be working at low temperatures and low values
of torque to minimize the mechanical energy spent.

2.3 Chemical pretreatment
2.3.1 Liquid hot water pretreatment
Liquid hot water (LHW) pretreatment, also known as hydrothermal pre-
treatment, is a process using water as a heating medium at high tempera-
ture (usually from 130°C to 240°C) and high pressure to maintain water
in liquid phase without any chemicals added [4,45]. At high temperature,
water shows acid properties and can work as an acidic catalyst [46]. LHW
pretreatment as an effective process can be used as an alternative to dilute
acid pretreatment of herbaceous biomass, such as corn fiber [46].
Compared to dilute acid, the primary advantage of LHW pretreatment
refers to the avoided use of mineral acid with lower sugar degradation
[46]. LHW pretreatment has been found to partially hydrolyze hemicellu-
lose and disrupt the lignin and cellulose structures [47]. During the