7 Resource Utilization of Agricultural/Forestry Residues …
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monomers of five-carbon sugars xylose and arabinose as well as the six-carbon
sugars mannose and galactose), and lignin (cross-linked phenolic polymers). Fractionation of lignocellulosic biomass into its components (cellulose, hemicellulose
and lignin) has been widely adopted as a pre-treatment step for utilization of lignocellulosic biomass for bio-fuels from cellulose/hemicellulose (e.g., bio-ethanol) and
bio-based chemicals/materials from lignin [4]. The components fractionated from
agricultural/forestry residues can be further de-polymerized into monomers, e.g.,
glucose from cellulose, xylose from hemicelluloses and phenolic molecules from
lignin, which can be utilized as bio-feedstocks for the chemical industry [5]. In recent
years, the authors’ group had been successful in utilizing these components for the
production of various bio-based materials such as sodium carboxymethyl cellulose
(CMC) [6], phenol–formaldehyde adhesives [7], epoxy resins [8], and polyurethane
foams [9]. In this chapter, various fractionation methods for resource utilization of
lignocellulosic biomass are reviewed.
7.1 Lignocellulosic Biomass
Lignocellulosic biomass is the most abundantly available form of terrestrial biomass
on Earth, with an estimated 181.5 billion tonnes generated annually, among which
about 7 billion tonnes are produced from dedicated agricultural, grass and forest land
and another 1.2 billion tonnes stem from agricultural residues [10]. Thus, lignocellulosic biomass represents an immense, inexpensive and renewable resource for the
production of biofuels, bio-based chemicals and materials. It includes mainly crop
residues (cane bagasse, rice straw, rice hulls, corn stover, corn stalk, wheat straw,
barley straw and pulp), organic wastes (waste newsprint, recycled paper, pulp/paper
mill sludge), woody biomass (e.g., aspen and poplar, pine and spruce and forestry
residues), herbaceous biomass (coastal Bermuda grass, switchgrass, reed canary
grass and timothy grass) and organic fraction of municipal solid wastes [11].
Lignocellulosic biomass is mainly composed of cellulose, hemicelluloses and
lignin, as illustrated previously in Fig. 7.1. Cellulose is normally the largest fraction of
plant biomass (40–50 wt%) followed by hemicellulose (20–30 wt%) and lignin (10–
25 wt%), depending on types of biomass [12, 13]. Cellulose is a water-insoluble linear
polysaccharide with an average molecular weight of around 100,000 g/mol made of
disaccharide cellobiose chains composed of D-glucose units with a crystalline structure. Since about half of the organic carbon of lignocellulosic biomass is present in
the form of cellulose, conversion of cellulose into valuable chemicals and fuels is of
great importance. In contrast, hemicellulose is a non-crystalline, highly branched,
water-insoluble polymer with an average molecular weight of <30,000 g/mol. It is
composed of several heteropolymers such as xylan, galactomannan, glucuronoxylan,
arabinoxylan, glucomannan and xyloglucan. These heteropolymers are composed
of 5- and 6-carbon monosaccharides such as pentoses (xylose, arabinose), hexoses
(mannose, glucose, galactose) and acetylated sugars [12–15]. Lignin is an amorphous, three-dimensional polymer possessing a structure based on phenyl propane
181
monomers of five-carbon sugars xylose and arabinose as well as the six-carbon
sugars mannose and galactose), and lignin (cross-linked phenolic polymers). Fractionation of lignocellulosic biomass into its components (cellulose, hemicellulose
and lignin) has been widely adopted as a pre-treatment step for utilization of lignocellulosic biomass for bio-fuels from cellulose/hemicellulose (e.g., bio-ethanol) and
bio-based chemicals/materials from lignin [4]. The components fractionated from
agricultural/forestry residues can be further de-polymerized into monomers, e.g.,
glucose from cellulose, xylose from hemicelluloses and phenolic molecules from
lignin, which can be utilized as bio-feedstocks for the chemical industry [5]. In recent
years, the authors’ group had been successful in utilizing these components for the
production of various bio-based materials such as sodium carboxymethyl cellulose
(CMC) [6], phenol–formaldehyde adhesives [7], epoxy resins [8], and polyurethane
foams [9]. In this chapter, various fractionation methods for resource utilization of
lignocellulosic biomass are reviewed.
7.1 Lignocellulosic Biomass
Lignocellulosic biomass is the most abundantly available form of terrestrial biomass
on Earth, with an estimated 181.5 billion tonnes generated annually, among which
about 7 billion tonnes are produced from dedicated agricultural, grass and forest land
and another 1.2 billion tonnes stem from agricultural residues [10]. Thus, lignocellulosic biomass represents an immense, inexpensive and renewable resource for the
production of biofuels, bio-based chemicals and materials. It includes mainly crop
residues (cane bagasse, rice straw, rice hulls, corn stover, corn stalk, wheat straw,
barley straw and pulp), organic wastes (waste newsprint, recycled paper, pulp/paper
mill sludge), woody biomass (e.g., aspen and poplar, pine and spruce and forestry
residues), herbaceous biomass (coastal Bermuda grass, switchgrass, reed canary
grass and timothy grass) and organic fraction of municipal solid wastes [11].
Lignocellulosic biomass is mainly composed of cellulose, hemicelluloses and
lignin, as illustrated previously in Fig. 7.1. Cellulose is normally the largest fraction of
plant biomass (40–50 wt%) followed by hemicellulose (20–30 wt%) and lignin (10–
25 wt%), depending on types of biomass [12, 13]. Cellulose is a water-insoluble linear
polysaccharide with an average molecular weight of around 100,000 g/mol made of
disaccharide cellobiose chains composed of D-glucose units with a crystalline structure. Since about half of the organic carbon of lignocellulosic biomass is present in
the form of cellulose, conversion of cellulose into valuable chemicals and fuels is of
great importance. In contrast, hemicellulose is a non-crystalline, highly branched,
water-insoluble polymer with an average molecular weight of <30,000 g/mol. It is
composed of several heteropolymers such as xylan, galactomannan, glucuronoxylan,
arabinoxylan, glucomannan and xyloglucan. These heteropolymers are composed
of 5- and 6-carbon monosaccharides such as pentoses (xylose, arabinose), hexoses
(mannose, glucose, galactose) and acetylated sugars [12–15]. Lignin is an amorphous, three-dimensional polymer possessing a structure based on phenyl propane