hemicellulose and lignin cause lignocellulosic biomass
structure disruption. Both physical and chemical methods
are used separately but when they are used in combination, it
boosts the biomass digestibility and increases the desired end
product yields (Kumar and Sharma 2017; Pattanaik 2019).
Figure 1 represents the pretreatment methods for lignocellulosic biomass.
2.1.1 Physical Conversion Technologies
The physical process of pretreatment causes changes in a
particular surface area, size of particles, crystalline index, or
polymerization amount of biomass. This treatment eludes
chemical usage, causes a reduction in the waste generation,
and shortcuts for the following reactions. Pretreatment
methods: microwave, mechanical, and ultrasound are commonly designed to advance the efficacy of key processes in
biomass treatment. In these processes, the hard structure of
lignocellulosic material is dislocated and its cellulose portion is exposed. Lignocellulosic substances pretreatment is
carried to overcome the resistance that faces through the
combination of structural and chemical changes in the carbohydrates and lignin (Amin et al. 2017; Onumaegbu et al.
2018). Table 3 enlists the benefits and drawbacks of different conversion technologies.
Mechanical
Mechanical pretreatment methods can be performed using a
variety of procedures; however, the most abundant are
grinding, milling, extrusion, or chipping. Ball milling pretreatment yielded significantly lower particle size compared
to chipping or mashing but showed results in lower efficiency of hydrolysis (Yang et al. 2018). It was reported that
extrusion is an effective action to reduce particle size
including boiling, stirring, and trimming processes leading to
alterations in the behavioral and compositional properties of
biomass (Jedrzejczyk 2019). One of the benefits of using this
method is the lack of the chemicals needed during this
process, which lessens the quantity of post-processed garbage. The biggest drawback of mechanical pretreatment is
the utilization of high power, which has an impact on the
high cost of processing lignocellulosic materials. Therefore,
information about equipment is necessary for the correct
choice of material required for the processing of biomass,
which must ensure the right balance among the cost and
effectiveness of the process (Gu et al. 2018; Naimi and
Sokhansanj 2018).
Microwave
Microwave is non-ionizing electromagnetic radiation having
frequency among radio and ultraviolet waves. Microwave
radiation exposure on the matter has the proper capacity to
excite the vibration of molecules; however, its power is too
squat for breakage of chemical bonds. The microwave
electric field system transferences its energy to particles,
leading to the production of thermal energy. Some benefits
of microwave heating as compared to conventional process
for heating are low power utilization, short times for a
reaction, and avoidance of contact with feedstock. On the
other way, longer microwave exposure surges the degradation of the polysaccharides (Bhutto et al. 2017; Kostas et al.
2017).
Ultrasound
The ultrasound usage in green technology imparts
high-quality production of value-added compounds and
biofuels by active decay of lignocellulosic waste. The key
benefits of ultrasonic fabrication are the short duration
required for processing, low operating temperature, and the
endless volume of chemicals utilized during the additional
process of valorization. In addition, it has the power of
integration with further technologies (Chatel 2018; Subhedar
and Gogate 2016).
Table 2 Types and sources of
lignocellulosic biomass
Types
Forest
Agricultural
Industrial lignin
Wood
Industrial
residues
Sources
Treetops
Peat
moss
Bark
waste
Limbs
Wood
chips
Saw dust
Slashes
Pruning
residues
Cashew
nutshells
Bagasse
Corn bran
Lignosulfonate
Organosolvent
Lignin
Lignin obtained after steam
explosion of birch
Ground
softwood
Sawdust
woodchips
Softwood
bark
Mixed
hardwoods
Hardwoods
Pine
Paper waste
Birch wood
waste
Black pulping
liquor
Wood
industry
residues
Lignin from
newsprint
Creosote
treated waste
352
T. Mehmood et al.
structure disruption. Both physical and chemical methods
are used separately but when they are used in combination, it
boosts the biomass digestibility and increases the desired end
product yields (Kumar and Sharma 2017; Pattanaik 2019).
Figure 1 represents the pretreatment methods for lignocellulosic biomass.
2.1.1 Physical Conversion Technologies
The physical process of pretreatment causes changes in a
particular surface area, size of particles, crystalline index, or
polymerization amount of biomass. This treatment eludes
chemical usage, causes a reduction in the waste generation,
and shortcuts for the following reactions. Pretreatment
methods: microwave, mechanical, and ultrasound are commonly designed to advance the efficacy of key processes in
biomass treatment. In these processes, the hard structure of
lignocellulosic material is dislocated and its cellulose portion is exposed. Lignocellulosic substances pretreatment is
carried to overcome the resistance that faces through the
combination of structural and chemical changes in the carbohydrates and lignin (Amin et al. 2017; Onumaegbu et al.
2018). Table 3 enlists the benefits and drawbacks of different conversion technologies.
Mechanical
Mechanical pretreatment methods can be performed using a
variety of procedures; however, the most abundant are
grinding, milling, extrusion, or chipping. Ball milling pretreatment yielded significantly lower particle size compared
to chipping or mashing but showed results in lower efficiency of hydrolysis (Yang et al. 2018). It was reported that
extrusion is an effective action to reduce particle size
including boiling, stirring, and trimming processes leading to
alterations in the behavioral and compositional properties of
biomass (Jedrzejczyk 2019). One of the benefits of using this
method is the lack of the chemicals needed during this
process, which lessens the quantity of post-processed garbage. The biggest drawback of mechanical pretreatment is
the utilization of high power, which has an impact on the
high cost of processing lignocellulosic materials. Therefore,
information about equipment is necessary for the correct
choice of material required for the processing of biomass,
which must ensure the right balance among the cost and
effectiveness of the process (Gu et al. 2018; Naimi and
Sokhansanj 2018).
Microwave
Microwave is non-ionizing electromagnetic radiation having
frequency among radio and ultraviolet waves. Microwave
radiation exposure on the matter has the proper capacity to
excite the vibration of molecules; however, its power is too
squat for breakage of chemical bonds. The microwave
electric field system transferences its energy to particles,
leading to the production of thermal energy. Some benefits
of microwave heating as compared to conventional process
for heating are low power utilization, short times for a
reaction, and avoidance of contact with feedstock. On the
other way, longer microwave exposure surges the degradation of the polysaccharides (Bhutto et al. 2017; Kostas et al.
2017).
Ultrasound
The ultrasound usage in green technology imparts
high-quality production of value-added compounds and
biofuels by active decay of lignocellulosic waste. The key
benefits of ultrasonic fabrication are the short duration
required for processing, low operating temperature, and the
endless volume of chemicals utilized during the additional
process of valorization. In addition, it has the power of
integration with further technologies (Chatel 2018; Subhedar
and Gogate 2016).
Table 2 Types and sources of
lignocellulosic biomass
Types
Forest
Agricultural
Industrial lignin
Wood
Industrial
residues
Sources
Treetops
Peat
moss
Bark
waste
Limbs
Wood
chips
Saw dust
Slashes
Pruning
residues
Cashew
nutshells
Bagasse
Corn bran
Lignosulfonate
Organosolvent
Lignin
Lignin obtained after steam
explosion of birch
Ground
softwood
Sawdust
woodchips
Softwood
bark
Mixed
hardwoods
Hardwoods
Pine
Paper waste
Birch wood
waste
Black pulping
liquor
Wood
industry
residues
Lignin from
newsprint
Creosote
treated waste
352
T. Mehmood et al.