7.3 Physical Pre-treatment Methods
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area and decrease the degree of polymerization and crystallinity [11, 18]. Since no
chemicals are used in physical pre-treatments, they have unique advantages of being
eco-friendly and reducing the generation of waste and inhibitors for the subsequent
processing [18, 20]. However, a major disadvantage of these methods is their high
energy consumption, which depends on the type of biomass. The most common
types of physical pre-treatments are mechanical methods, microwave-assisted pretreatment, extrusion and ultrasonication.
7.3.1 Mechanical Methods
Mechanical methods mostly consist of milling, grinding and chipping. They reduce
the biomass particle size and crystallinity, improve flow properties, and increase the
bulk density and porosity. The required power for the mechanical treatments depends
on multiple factors, e.g., the initial and final particle size, the biomass characteristics,
feed rate of the materials, moisture content, etc. [17, 21]. Although the high energy
requirements make them economically less attractive, they are still widely employed
for pre-treatment of lignocellulosic biomass due to their simplicity and no production
of toxic or inhibitory compounds during pre-treatment.
7.3.2 Microwave-Assisted Methods
Microwave heating causes molecular collisions by the dielectric polarization in an
applied electromagnetic field and generates thermal energy. The heat and extensive collisions could accelerate the biological, chemical and physical processes for
lignocellulosic biomass conversions. It is influenced by the dielectric properties of
the lignocellulosic biomass and can disrupt the biomass structure. This method can
be performed under either atmospheric or high-pressure environment. The highpressure microwave treatments are performed at 150–250 °C in a closed reactor. The
efficiency of microwave heating is determined by the loss tangent (the ratio of the
dielectric loss factor to the dielectric constant of a material). The dielectric constant
measures the ability of a material to store electromagnetic energy, while the dielectric loss factor is the ability of the material to convert the electromagnetic energy
into thermal energy. The microwave treatments can be used in combination with
other treatment methods for better efficiency. As an example, the microwave treatment combined with alkali pre-treatment proved to be more effective in removing
hemicelluloses and lignin from wheat straw compared to the traditional alkali pretreatment [18, 21]. It can also accelerate the cellulose dissolution in an ionic liquid
environment. The advantages of this method are easy operation, energy efficiency,
reduced inhibitor formation during the treatment and high heating capacity in a short
period of time. However, by far this method is applied on a lab-scale only, as it is
difficult to scale-up requiring more research and development work.
185
area and decrease the degree of polymerization and crystallinity [11, 18]. Since no
chemicals are used in physical pre-treatments, they have unique advantages of being
eco-friendly and reducing the generation of waste and inhibitors for the subsequent
processing [18, 20]. However, a major disadvantage of these methods is their high
energy consumption, which depends on the type of biomass. The most common
types of physical pre-treatments are mechanical methods, microwave-assisted pretreatment, extrusion and ultrasonication.
7.3.1 Mechanical Methods
Mechanical methods mostly consist of milling, grinding and chipping. They reduce
the biomass particle size and crystallinity, improve flow properties, and increase the
bulk density and porosity. The required power for the mechanical treatments depends
on multiple factors, e.g., the initial and final particle size, the biomass characteristics,
feed rate of the materials, moisture content, etc. [17, 21]. Although the high energy
requirements make them economically less attractive, they are still widely employed
for pre-treatment of lignocellulosic biomass due to their simplicity and no production
of toxic or inhibitory compounds during pre-treatment.
7.3.2 Microwave-Assisted Methods
Microwave heating causes molecular collisions by the dielectric polarization in an
applied electromagnetic field and generates thermal energy. The heat and extensive collisions could accelerate the biological, chemical and physical processes for
lignocellulosic biomass conversions. It is influenced by the dielectric properties of
the lignocellulosic biomass and can disrupt the biomass structure. This method can
be performed under either atmospheric or high-pressure environment. The highpressure microwave treatments are performed at 150–250 °C in a closed reactor. The
efficiency of microwave heating is determined by the loss tangent (the ratio of the
dielectric loss factor to the dielectric constant of a material). The dielectric constant
measures the ability of a material to store electromagnetic energy, while the dielectric loss factor is the ability of the material to convert the electromagnetic energy
into thermal energy. The microwave treatments can be used in combination with
other treatment methods for better efficiency. As an example, the microwave treatment combined with alkali pre-treatment proved to be more effective in removing
hemicelluloses and lignin from wheat straw compared to the traditional alkali pretreatment [18, 21]. It can also accelerate the cellulose dissolution in an ionic liquid
environment. The advantages of this method are easy operation, energy efficiency,
reduced inhibitor formation during the treatment and high heating capacity in a short
period of time. However, by far this method is applied on a lab-scale only, as it is
difficult to scale-up requiring more research and development work.