3 Pretreatment Process for Lignocellulosic
Biomass
This is the first and key step in processing of lignocellulosic
biomass that helps in their modification to make it accessible
for further processes or reactions in order to convert it into
biofuel. This involves altering of the structural and compositional complexity of lignocellulose biomass to enhance
hydrolysis and better yield of fermentable sugars (Pullammanappallil 2013). Figure 2 depicts the effect of pretreatment on the structural components of lignocellulosic
biomass. Lignocellulose materials are usually degraded
under certain pretreatment conditions. And several pretreatment techniques have been documented in literature and
they are grouped under major headings as physical, chemical, physicochemical and biological pretreatment. Figure 3
shows a detailed classification of the various pretreatment
techniques for lignocellulosic biomass. And the product
obtained from any of the above-mentioned techniques is
dependent on the operational conditions of the entire pretreatment process. The physical method tends to increase the
reactive surface area of the lignocellulose biomass by
reducing the size into smaller particles, thereby reducing the
degree of crystalline creating easy accessibility for enzymes.
This method is energy intensive and therefore involves lots
of cost (Sun and Cheng 2002). Chemical methods employ
chemical substances in the form of acid, alkaline, ionic liquid and organic solvent in altering the complex structure of
lignocellulose biomass to their constituent components like
cellulose, hemicelluloses and lignin or their reducing sugar
equivalent (Zhang et al. 2016; Elgharbawy et al. 2016;
Kumar and Sharma 2017). Physiochemical method employs
both techniques in physical and chemical methods in pretreatment of lignocellulosic biomass. This method is quite
effective because it improves lignin removal and increases
hydrolysis efficiency (Apilak et al. 2019). Biological methods employ the use of enzymes and microorganisms in
degrading the lignocellulosic biomass. This method is slow
and eco-friendly. The detailed benefits and limitations of the
four classes of pretreatment techniques are documented in
the literature (Kumar et al. 2009; Mosier et al. 2005; Hassan
et al. 2018; Seidl and Goulart 2016; Zhao et al. 2009;
Avellar 1998; Zhang et al. 2007; Teymouri et al. 2005;
Wang et al. 2009).
The choice of pretreatment method to be used must satisfy the following conditions:
i. Avoid size reduction biomass
ii. Preservation of hemicellulose fraction
iii. Less formation of degradation products
iv. Less energy use
v. Use of cheap catalyst and/or cheap catalyst recycle and
regeneration of high value lignin co-products (Kumar
and Sharma 2017; Wyman 1999).
There are several extraction methods that have been
explored in the literature for extracting hemicellulose from
its raw materials/sources such as alkaline peroxide extraction, liquid hot water extraction, steam treatment, microwave
treatment, ionic liquid extraction, alkaline extraction and
dilute acid treatment (Nguyen et al. 2000; Egües et al. 2012;
Hasegawa et al. 2004; Palm and Zacchi 2003; Froschauer
et al. 2013).
Fig. 1 Structure of
hemicellulose in comparison to
cellulose and lignin
Application of Hemicellulose in Biohydrogen Production
317
Biomass
This is the first and key step in processing of lignocellulosic
biomass that helps in their modification to make it accessible
for further processes or reactions in order to convert it into
biofuel. This involves altering of the structural and compositional complexity of lignocellulose biomass to enhance
hydrolysis and better yield of fermentable sugars (Pullammanappallil 2013). Figure 2 depicts the effect of pretreatment on the structural components of lignocellulosic
biomass. Lignocellulose materials are usually degraded
under certain pretreatment conditions. And several pretreatment techniques have been documented in literature and
they are grouped under major headings as physical, chemical, physicochemical and biological pretreatment. Figure 3
shows a detailed classification of the various pretreatment
techniques for lignocellulosic biomass. And the product
obtained from any of the above-mentioned techniques is
dependent on the operational conditions of the entire pretreatment process. The physical method tends to increase the
reactive surface area of the lignocellulose biomass by
reducing the size into smaller particles, thereby reducing the
degree of crystalline creating easy accessibility for enzymes.
This method is energy intensive and therefore involves lots
of cost (Sun and Cheng 2002). Chemical methods employ
chemical substances in the form of acid, alkaline, ionic liquid and organic solvent in altering the complex structure of
lignocellulose biomass to their constituent components like
cellulose, hemicelluloses and lignin or their reducing sugar
equivalent (Zhang et al. 2016; Elgharbawy et al. 2016;
Kumar and Sharma 2017). Physiochemical method employs
both techniques in physical and chemical methods in pretreatment of lignocellulosic biomass. This method is quite
effective because it improves lignin removal and increases
hydrolysis efficiency (Apilak et al. 2019). Biological methods employ the use of enzymes and microorganisms in
degrading the lignocellulosic biomass. This method is slow
and eco-friendly. The detailed benefits and limitations of the
four classes of pretreatment techniques are documented in
the literature (Kumar et al. 2009; Mosier et al. 2005; Hassan
et al. 2018; Seidl and Goulart 2016; Zhao et al. 2009;
Avellar 1998; Zhang et al. 2007; Teymouri et al. 2005;
Wang et al. 2009).
The choice of pretreatment method to be used must satisfy the following conditions:
i. Avoid size reduction biomass
ii. Preservation of hemicellulose fraction
iii. Less formation of degradation products
iv. Less energy use
v. Use of cheap catalyst and/or cheap catalyst recycle and
regeneration of high value lignin co-products (Kumar
and Sharma 2017; Wyman 1999).
There are several extraction methods that have been
explored in the literature for extracting hemicellulose from
its raw materials/sources such as alkaline peroxide extraction, liquid hot water extraction, steam treatment, microwave
treatment, ionic liquid extraction, alkaline extraction and
dilute acid treatment (Nguyen et al. 2000; Egües et al. 2012;
Hasegawa et al. 2004; Palm and Zacchi 2003; Froschauer
et al. 2013).
Fig. 1 Structure of
hemicellulose in comparison to
cellulose and lignin
Application of Hemicellulose in Biohydrogen Production
317