and Cheng 2002). The cellulose and hemicellulose hydrolysis are improved as both water and CO 2 molecules are
analogous in size. The penetration of CO 2 molecules within
the matrix of lignocelluloses is facilitated due to pressure
elevation caused by the explosion. High conversion yield,
zero inhibitor formation, nontoxic, cost-effective, noninflammable, and reduced decomposition of monosaccharides
are some the advantages of using CO 2 explosion method
(Kumar et al. 2009).
(d) Sulphur dioxide (SO 2 ) Explosion
The basic hypothesis behind the operation of SO 2 explosion
is comparable to CO 2 explosion. In this method, material is
subjected to SO 2 explosion coupled with acids, which leads
to solubilization of hemicelluloses even under low temperature and causes partial hydrolysis of cellulose. Generation
of high amount of degradation compounds and huge
requirement of equipment plagues the efficiency of this
process (Chen et al. 2017).
(e) Hot Water
Physicochemical pretreatment of biomass by using hot water
in the presence of high pressure eliminates an important part
of hemicellulose and hydrates the cellulose. No other
chemicals are used in this process which thereby eliminates
the use of an anti-corrosion product for the hydrolysis
reactor. Further, size reduction of raw material is not
required (Taherzadeh and Karimi 2008). Mostly, biomass is
kept in hot water at a temperature range of 200–230 °C for
about 15 min. During the process, hemicellulose fraction has
been removed completely by dissolving about 40–60% of
the total biomass. This pretreatment process is usually used
in the case of herbaceous crops as well as corn fibers (Mosier
et al. 2003).
3.3 Chemical Pretreatments
(a) Acid Pretreatment
Acid pretreatment of biomass using dilute acid hydrolysis is
solitary oldest and frequently used method. This process
solubilizes hemicelluloses at higher temperature or high acid
concentration, which helps to release pentose sugars (Alvira
et al. 2010) and make possible the release of substrate by
using enzymatic hydrolysis (cellulignin) (Taherzadeh and
Karimi 2008). Use of sulphuric acid (H 2 SO 4 ) helps the
breakdown of hemicelluloses into xylose and other sugars.
Though, few more acids viz. hydrochloric acid
(HCl) (Laopaiboon et al. 2010), phosphoric acid (Carvalho
et al. 2004), nitric acid (Rodrıguez-Chong et al. 2004), and
oxalic acid (Chandel et al. 2011) are also used for pretreatment process. Usually, the standard condition of performing
the process is in temperatures range of 120–180 °C and
residence time range of 15–60 min (Alvira et al. 2010). Use
of lower and intermediate temperatures and accordingly
decrease of energy costs is one of the significant advantages
of this process (Gírio et al. 2010). However, acid with
high concentration can create problems such as expensive
maintenance cost to prevent equipment from corrosion
(Alvira et al. 2010). Apart from this, the possibility for
generation of other byproducts which are considered as an
inhibitory compounds towards microbial fermentation, such
as phenolic compounds, carboxylic acids, furfural, furans,
acetic, levulinic, and formic acids. In this regard, a step of
detoxification is essential for eliminate these unwanted
compounds to raise the hydrolysate fermentation (Yamashita
et al. 2010).
(b) Alkaline Pretreatment
Alkali pretreatment of biomass is carried out by using bases
like potassium, sodium, ammonium hydroxides, calcium,
etc. In this regard, sodium hydroxide is known as one of the
commonly used base. Alkali pretreatment is usually done in
ambient conditions though it requires an excess time limit
for completion of the reaction. On alkali pretreatment, lignin
structure of biomass disturbed and results in acetyl elimination, cellulose decrystallization, and the different substitutions of uronic acid on hemicelluloses. As a result, there is
an increase in the accessibility of enzymes to hemicelluloses
and cellulose. Thus, a step to neutralize is required to
remove inhibitors (furfural, phenolic acids, aldehydes, and
salts,) and lignin before enzymatic hydrolysis. Alkaline
pretreatments help to recover more caustic salts and make
less sugar degradation as compared to acidic pretreatment
(Banerjee et al. 2011).
(c) Oxidative Delignification
In oxidative delignification process, in the presence of peroxidase enzyme, lignin degradation is catalyzed along with
hydrogen peroxide (H 2 O 2 ) (García-Cubero et al. 2009). The
process involves delignification as well as chemical breaking
of cellulose along with saccharification of enzymatic (Sun
and Cheng 2002). This pretreatment process can operate
even at low H 2 O 2 with high (approximately 40% solids)
loading of biomass yet it is very less explored. However, for
an enormous diversity of biomass, it becomes an appropriate
method such as bamboo, sugarcane bagasse wheat straw,
corn stover, barley straw, and rice straw (Ruzene et al.
2007).
86
N. Bordoloi et al.
analogous in size. The penetration of CO 2 molecules within
the matrix of lignocelluloses is facilitated due to pressure
elevation caused by the explosion. High conversion yield,
zero inhibitor formation, nontoxic, cost-effective, noninflammable, and reduced decomposition of monosaccharides
are some the advantages of using CO 2 explosion method
(Kumar et al. 2009).
(d) Sulphur dioxide (SO 2 ) Explosion
The basic hypothesis behind the operation of SO 2 explosion
is comparable to CO 2 explosion. In this method, material is
subjected to SO 2 explosion coupled with acids, which leads
to solubilization of hemicelluloses even under low temperature and causes partial hydrolysis of cellulose. Generation
of high amount of degradation compounds and huge
requirement of equipment plagues the efficiency of this
process (Chen et al. 2017).
(e) Hot Water
Physicochemical pretreatment of biomass by using hot water
in the presence of high pressure eliminates an important part
of hemicellulose and hydrates the cellulose. No other
chemicals are used in this process which thereby eliminates
the use of an anti-corrosion product for the hydrolysis
reactor. Further, size reduction of raw material is not
required (Taherzadeh and Karimi 2008). Mostly, biomass is
kept in hot water at a temperature range of 200–230 °C for
about 15 min. During the process, hemicellulose fraction has
been removed completely by dissolving about 40–60% of
the total biomass. This pretreatment process is usually used
in the case of herbaceous crops as well as corn fibers (Mosier
et al. 2003).
3.3 Chemical Pretreatments
(a) Acid Pretreatment
Acid pretreatment of biomass using dilute acid hydrolysis is
solitary oldest and frequently used method. This process
solubilizes hemicelluloses at higher temperature or high acid
concentration, which helps to release pentose sugars (Alvira
et al. 2010) and make possible the release of substrate by
using enzymatic hydrolysis (cellulignin) (Taherzadeh and
Karimi 2008). Use of sulphuric acid (H 2 SO 4 ) helps the
breakdown of hemicelluloses into xylose and other sugars.
Though, few more acids viz. hydrochloric acid
(HCl) (Laopaiboon et al. 2010), phosphoric acid (Carvalho
et al. 2004), nitric acid (Rodrıguez-Chong et al. 2004), and
oxalic acid (Chandel et al. 2011) are also used for pretreatment process. Usually, the standard condition of performing
the process is in temperatures range of 120–180 °C and
residence time range of 15–60 min (Alvira et al. 2010). Use
of lower and intermediate temperatures and accordingly
decrease of energy costs is one of the significant advantages
of this process (Gírio et al. 2010). However, acid with
high concentration can create problems such as expensive
maintenance cost to prevent equipment from corrosion
(Alvira et al. 2010). Apart from this, the possibility for
generation of other byproducts which are considered as an
inhibitory compounds towards microbial fermentation, such
as phenolic compounds, carboxylic acids, furfural, furans,
acetic, levulinic, and formic acids. In this regard, a step of
detoxification is essential for eliminate these unwanted
compounds to raise the hydrolysate fermentation (Yamashita
et al. 2010).
(b) Alkaline Pretreatment
Alkali pretreatment of biomass is carried out by using bases
like potassium, sodium, ammonium hydroxides, calcium,
etc. In this regard, sodium hydroxide is known as one of the
commonly used base. Alkali pretreatment is usually done in
ambient conditions though it requires an excess time limit
for completion of the reaction. On alkali pretreatment, lignin
structure of biomass disturbed and results in acetyl elimination, cellulose decrystallization, and the different substitutions of uronic acid on hemicelluloses. As a result, there is
an increase in the accessibility of enzymes to hemicelluloses
and cellulose. Thus, a step to neutralize is required to
remove inhibitors (furfural, phenolic acids, aldehydes, and
salts,) and lignin before enzymatic hydrolysis. Alkaline
pretreatments help to recover more caustic salts and make
less sugar degradation as compared to acidic pretreatment
(Banerjee et al. 2011).
(c) Oxidative Delignification
In oxidative delignification process, in the presence of peroxidase enzyme, lignin degradation is catalyzed along with
hydrogen peroxide (H 2 O 2 ) (García-Cubero et al. 2009). The
process involves delignification as well as chemical breaking
of cellulose along with saccharification of enzymatic (Sun
and Cheng 2002). This pretreatment process can operate
even at low H 2 O 2 with high (approximately 40% solids)
loading of biomass yet it is very less explored. However, for
an enormous diversity of biomass, it becomes an appropriate
method such as bamboo, sugarcane bagasse wheat straw,
corn stover, barley straw, and rice straw (Ruzene et al.
2007).
86
N. Bordoloi et al.