shows; it consumes water at high temperature, i.e., 170 °C–
230 °C and uses pressure up to 5 MPa as compared to steam
(Agbor et al. 2011). This causes hemicellulose hydrolysis and
lignin removal to produce more accessible cellulose, which
inhibits the formation of inhibitors at the highest temperature. It
can be based on the direction of water flow and biomass into the
reactor performed in three different ways. Cocurrent pretreatment: In this method, both water and biomass slurry heated at
the required temperature with controlled pretreatment conditions before cooling. Current counter pretreatment: In this
procedure, hot water is propelled in controlling conditions
against biomass. In the third procedure, biomass behaves like a
stationary phase and hot water acts like a mobile phase and
flows through it, and fractions undergo hydrolysis passed out
from the reactor (Kumar and Sharma 2017).
Ammonia Fiber Explosion
Ammonia fiber explosion pretreatment takes place at a
higher temperature. Aqueous ammonia is a type of ammonia
fiber explosion that treats biomass with ammonia in the
aqueous form at 30 °C–60 °C in a batch reactor. In an
ammonia fiber explosion, lignocellulosic biomass through
liquid ammonia is heated in a closed vessel at 60 °C–90 °C
temperature and 3 MPa pressure for 30–60 min. After
holding the vessel at the required temperature for 5 min then
the vessel valve is opened for release of pressure and
ammonia evaporation with a drop in temperature of the
system. This methodology shows similarity with the steam
explosion but utilizes ammonia in the place of steam. Other
methods that used ammonia for pretreatment are soaking
aqueous ammonia and ammonia fiber explosion (Ayeni
2020; Rabemanolontsoa and Saka 2016).
CO 2 Explosion
CO 2 explosion done lignocellulosic biomass pretreatment uses
CO 2 gas which behaves like a solvent. CO 2 gas is passed through
a vessel with high pressure enclosing the biomass. At mandatory
temperature, this vessel is heated and placed for a few minutes.
CO 2 gas with high pressure enters into biomass and generates
carbonic acid that hydrolyzes hemicellulose. This gas with high
pressure when released upsets the lignocelluloses structure that
increases the surface area. This method is not good for that
biomass with less moisture percentage. Greater the moisture
content greater hydrolytic yield (Ayeni 2020).
2.1.4 Combined Bioconversion Technologies
In the combined method strategy, the experimental procedure involved more than two pretreatment methods. The
combination involves chemical or mechanical crushing,
biological or physical treatment, microwave‐machine-driven
crushing‐chemical process, machine-driven‐chemical‐steam
explosion, mechanical‐electronic radiation‐alkaline action
methods. Only one pretreatment methodology does not show
more efficient results due to its restricted specific mode of
functioning and some disadvantages. But these methodologies face many technologic trials, generate pollution, consume a lot of energy, slow procedure, and are destructive for
apparatus. But they also have several benefits over single
methods like enhanced hydrolysis efficiency, decreasing
pretreatment harshness, increased methane (CH 4 ) production, and agreeing more complete biomass utilization.
However, these methods increased the pretreatment cost, so
economic analysis must be carried out for collective lignocellulosic biomass pretreatment methodology (Chen et al.
2017; Ummalyma 2019; Zheng et al. 2014).
2.1.5 Biological Bioconversion Technologies
Biological pretreatment is closely linked to the action of
microbes like white, brown, and soft tissue molds that are
able to produce degrading enzymes for hemicelluloses and
lignin (Sindhu et al. 2016). The breakage of lignin assemblies on cell walls using microbial enzymes as biocatalysts is
often referred to as the first hydrolysis step in the pretreatment process (Tanjore and Richard 2015). The usage of
cellulases to convert cellulose into oligo and monomers is
called enzymatic saccharification and happens in the second
hydrolysis stage. These natural processes keeping separate is
perfectly reasonable, but it should be considered that most of
the relevant microorganisms simultaneously cause hydrolysis of lignin and cellulose to gain carbon and biomass
energy. To remove physical barriers required for hydrolysis,
several biological pretreatments required that involved
chemical and enzyme intermediaries to deal with such as the
mixing of enzymes can work harmoniously by enlarging
small pores and increasing contact by the opening of the cell
wall matrix (Laca et al. 2019). But, biological pretreatment
of lignocellulosic biomass has to be environmentally
favorable so with the expansion of technology, knowledge,
and advancement in genetic manufacturing, microbes will
perform an imperative role in the above procedure that
brands the appliance of biological pretreatment approach
more remarkable (Chen et al. 2017).
3 High-Value Compounds
from Agro-Industrial Waste
Nowadays, agro-industrial wastes get much attention due to
the presence of high potential compounds to recover for the
development of value-added products to endorse the
bio-based economy. The worthy sources of these natural
products have diverse functions ranging from living systems
to industrial level. If we can’t reuse the large amount of these
agro-industrial wastes, resulting in a serious threat for the
356
T. Mehmood et al.
230 °C and uses pressure up to 5 MPa as compared to steam
(Agbor et al. 2011). This causes hemicellulose hydrolysis and
lignin removal to produce more accessible cellulose, which
inhibits the formation of inhibitors at the highest temperature. It
can be based on the direction of water flow and biomass into the
reactor performed in three different ways. Cocurrent pretreatment: In this method, both water and biomass slurry heated at
the required temperature with controlled pretreatment conditions before cooling. Current counter pretreatment: In this
procedure, hot water is propelled in controlling conditions
against biomass. In the third procedure, biomass behaves like a
stationary phase and hot water acts like a mobile phase and
flows through it, and fractions undergo hydrolysis passed out
from the reactor (Kumar and Sharma 2017).
Ammonia Fiber Explosion
Ammonia fiber explosion pretreatment takes place at a
higher temperature. Aqueous ammonia is a type of ammonia
fiber explosion that treats biomass with ammonia in the
aqueous form at 30 °C–60 °C in a batch reactor. In an
ammonia fiber explosion, lignocellulosic biomass through
liquid ammonia is heated in a closed vessel at 60 °C–90 °C
temperature and 3 MPa pressure for 30–60 min. After
holding the vessel at the required temperature for 5 min then
the vessel valve is opened for release of pressure and
ammonia evaporation with a drop in temperature of the
system. This methodology shows similarity with the steam
explosion but utilizes ammonia in the place of steam. Other
methods that used ammonia for pretreatment are soaking
aqueous ammonia and ammonia fiber explosion (Ayeni
2020; Rabemanolontsoa and Saka 2016).
CO 2 Explosion
CO 2 explosion done lignocellulosic biomass pretreatment uses
CO 2 gas which behaves like a solvent. CO 2 gas is passed through
a vessel with high pressure enclosing the biomass. At mandatory
temperature, this vessel is heated and placed for a few minutes.
CO 2 gas with high pressure enters into biomass and generates
carbonic acid that hydrolyzes hemicellulose. This gas with high
pressure when released upsets the lignocelluloses structure that
increases the surface area. This method is not good for that
biomass with less moisture percentage. Greater the moisture
content greater hydrolytic yield (Ayeni 2020).
2.1.4 Combined Bioconversion Technologies
In the combined method strategy, the experimental procedure involved more than two pretreatment methods. The
combination involves chemical or mechanical crushing,
biological or physical treatment, microwave‐machine-driven
crushing‐chemical process, machine-driven‐chemical‐steam
explosion, mechanical‐electronic radiation‐alkaline action
methods. Only one pretreatment methodology does not show
more efficient results due to its restricted specific mode of
functioning and some disadvantages. But these methodologies face many technologic trials, generate pollution, consume a lot of energy, slow procedure, and are destructive for
apparatus. But they also have several benefits over single
methods like enhanced hydrolysis efficiency, decreasing
pretreatment harshness, increased methane (CH 4 ) production, and agreeing more complete biomass utilization.
However, these methods increased the pretreatment cost, so
economic analysis must be carried out for collective lignocellulosic biomass pretreatment methodology (Chen et al.
2017; Ummalyma 2019; Zheng et al. 2014).
2.1.5 Biological Bioconversion Technologies
Biological pretreatment is closely linked to the action of
microbes like white, brown, and soft tissue molds that are
able to produce degrading enzymes for hemicelluloses and
lignin (Sindhu et al. 2016). The breakage of lignin assemblies on cell walls using microbial enzymes as biocatalysts is
often referred to as the first hydrolysis step in the pretreatment process (Tanjore and Richard 2015). The usage of
cellulases to convert cellulose into oligo and monomers is
called enzymatic saccharification and happens in the second
hydrolysis stage. These natural processes keeping separate is
perfectly reasonable, but it should be considered that most of
the relevant microorganisms simultaneously cause hydrolysis of lignin and cellulose to gain carbon and biomass
energy. To remove physical barriers required for hydrolysis,
several biological pretreatments required that involved
chemical and enzyme intermediaries to deal with such as the
mixing of enzymes can work harmoniously by enlarging
small pores and increasing contact by the opening of the cell
wall matrix (Laca et al. 2019). But, biological pretreatment
of lignocellulosic biomass has to be environmentally
favorable so with the expansion of technology, knowledge,
and advancement in genetic manufacturing, microbes will
perform an imperative role in the above procedure that
brands the appliance of biological pretreatment approach
more remarkable (Chen et al. 2017).
3 High-Value Compounds
from Agro-Industrial Waste
Nowadays, agro-industrial wastes get much attention due to
the presence of high potential compounds to recover for the
development of value-added products to endorse the
bio-based economy. The worthy sources of these natural
products have diverse functions ranging from living systems
to industrial level. If we can’t reuse the large amount of these
agro-industrial wastes, resulting in a serious threat for the
356
T. Mehmood et al.