4.1
Introduction
Lignocellulosic biomass is a carbon-neutral or low-carbon, easy available, and
renewable feedstock available for the production of fuel and chemicals. Biomass is
currently the single, largest source of renewable energy worldwide, providing 10%
(50 EJ) of the global primary energy supply (IEA 2016). Lignocellulosic biomass is
composed of cellulose, hemicellulose, and lignin. For the production of secondgeneration biofuels and chemicals, these components need to be separated into the
individual component and must be fully utilized to make the lignocellulosic
biorefinery environmentally, socially, and economically feasible (Sims et al. 2010;
Singhvi et al. 2014). The lignocellulosic biomass-based research mainly focuses on
polysaccharides component of biomass, and lignin is discarded as waste with very
limited usage. Therefore, the challenges associated with the success of secondgeneration biomass-based biorefinery must be addressed (Singhvi et al. 2014). The
recent focus on the concept of lignocellulosic biorefinery, i.e., utilization of biomass
and byproducts leading to minimal or zero waste generation, can overcome the
problems associated with biofuel production at commercial scale. Lignin is very less
explored, and the advancement in approaches of lignin valorization into valuable
bioproducts is essential for overall economic viability and sustainability of lignocellulosic biorefinery (de Jong et al. 2012; Ragauskas et al. 2014).
Lignin is recalcitrant to degradation due to its complex and heterogeneous
structure. Its depolymerization is being done by chemical, thermochemical, and
biological processes for conversion into fuel and chemicals. Compared to the
chemical processes, the use of microorganism or enzymes for lignin depolymerization is less energy intense, cost effective, eco-friendly, and works at ambient
temperature with fewer inhibitors’ generation (Zhu et al. 2017). The discovery of
new microbial strains and understanding their enzyme system that is responsible for
lignin degradation will help in lignin depolymerization and its conversion into fuel
and chemicals (Kumar et al. 2017).
4.2
Lignocellulosic Biomass and Its Composition
Lignocellulose is the main constituent of plant cell wall and mainly refers to the dry
matter of plant biomass. Lignocellulosic biomass is a complex matrix composed of
cellulose, hemicellulose, lignin polymers, and a small amount of proteins,
extractives, and minerals (Moon et al. 2011; Menon and Rao 2012; Singh et al.
2017). The structure and composition of lignocellulosic biomass are represented in
Fig. 4.1. Two-thirds of lignocellulosic biomass are comprised of cellulose and
hemicellulose polysaccharides that are made of hexose (C6) and pentose (C5)
sugars. These polymers organize themselves into a complex three-dimensional
structure. Their organization is nonuniform with a varied composition of cellulose,
hemicellulose, and lignin depending on the types of lignocellulosic biomass (Gírio et
al. 2010; Scheller and Ulvskov 2010; Menon and Rao 2012). The presence of
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M. Kumar et al.
Introduction
Lignocellulosic biomass is a carbon-neutral or low-carbon, easy available, and
renewable feedstock available for the production of fuel and chemicals. Biomass is
currently the single, largest source of renewable energy worldwide, providing 10%
(50 EJ) of the global primary energy supply (IEA 2016). Lignocellulosic biomass is
composed of cellulose, hemicellulose, and lignin. For the production of secondgeneration biofuels and chemicals, these components need to be separated into the
individual component and must be fully utilized to make the lignocellulosic
biorefinery environmentally, socially, and economically feasible (Sims et al. 2010;
Singhvi et al. 2014). The lignocellulosic biomass-based research mainly focuses on
polysaccharides component of biomass, and lignin is discarded as waste with very
limited usage. Therefore, the challenges associated with the success of secondgeneration biomass-based biorefinery must be addressed (Singhvi et al. 2014). The
recent focus on the concept of lignocellulosic biorefinery, i.e., utilization of biomass
and byproducts leading to minimal or zero waste generation, can overcome the
problems associated with biofuel production at commercial scale. Lignin is very less
explored, and the advancement in approaches of lignin valorization into valuable
bioproducts is essential for overall economic viability and sustainability of lignocellulosic biorefinery (de Jong et al. 2012; Ragauskas et al. 2014).
Lignin is recalcitrant to degradation due to its complex and heterogeneous
structure. Its depolymerization is being done by chemical, thermochemical, and
biological processes for conversion into fuel and chemicals. Compared to the
chemical processes, the use of microorganism or enzymes for lignin depolymerization is less energy intense, cost effective, eco-friendly, and works at ambient
temperature with fewer inhibitors’ generation (Zhu et al. 2017). The discovery of
new microbial strains and understanding their enzyme system that is responsible for
lignin degradation will help in lignin depolymerization and its conversion into fuel
and chemicals (Kumar et al. 2017).
4.2
Lignocellulosic Biomass and Its Composition
Lignocellulose is the main constituent of plant cell wall and mainly refers to the dry
matter of plant biomass. Lignocellulosic biomass is a complex matrix composed of
cellulose, hemicellulose, lignin polymers, and a small amount of proteins,
extractives, and minerals (Moon et al. 2011; Menon and Rao 2012; Singh et al.
2017). The structure and composition of lignocellulosic biomass are represented in
Fig. 4.1. Two-thirds of lignocellulosic biomass are comprised of cellulose and
hemicellulose polysaccharides that are made of hexose (C6) and pentose (C5)
sugars. These polymers organize themselves into a complex three-dimensional
structure. Their organization is nonuniform with a varied composition of cellulose,
hemicellulose, and lignin depending on the types of lignocellulosic biomass (Gírio et
al. 2010; Scheller and Ulvskov 2010; Menon and Rao 2012). The presence of
84
M. Kumar et al.