1. Introduction

The need for energy has continuously been a major issue in human society. Energy use per capita has been increasing at an average rate of 21.5 kg of oil equivalent annually since the year 2000 (value calculated from [1]). Increase in energy demands leads to the search for alternative sources for energy production. Biomass, as the fourth largest energy source after coal, oil, and natural gas, is a very promising resource for energy production due to its renewability and versatility [2]. Biomass is biologically originated materials or simply any materials that are not fossilized. Supplies of biomass could be from forestry, agriculture, and wastes. They could be used directly to produce energy by burning or could be refined to produce biofuels in the form of solid, liquid, or gas [3].

Lignocellulosic biomass is the biomass with the structure that is composed of lignin, hemicellulose, and cellulose. They can be divided into woody and nonwoody biomass. Woody biomass can be further categorized into hardwoods and softwoods, which differ in their reproduction; angiosperm for hardwoods and gymnosperm for softwoods. Examples of hardwoods include beech, mahogany, maple, and teak, while softwoods are cedar, pine, juniper, and spruce. Non-woody biomass are those of agricultural residues, grass family (Poaceae or Gramineae), and non-woody fibers such as cotton fiber [4, 5].

Apart from using biomass in co-firing with fossil fuels, gasification, and pyrolysis, its use in fermentation technology for liquid and gaseous biofuels production is

also applicable and widely studied. The hemicellulose and cellulose structures (so called holocellulose) in lignocellulosic biomass contain sugar monomers that could be utilized by microorganisms and converted to various biofuels via biological pathways.

Cellulose is considered a major composition of lignocellulosic biomass. It is a homopolymer containing glucose as the only monomer. Glucose molecules in cellulose are linked by β-1,4-glycosidic bonds (Figure 1). Cellulose chain is also known as β-1,4-glucan. The chains are packed into tiny and extremely long structure called microfibrils. These microfibrils are packed into lattices, which make most part of the cellulose fibers inaccessible by enzymes [6–8].

Hemicellulose is also the composition that consists of sugars that can be utilized by microorganisms. It is a heteropolymer of pentoses (xylose, arabinose), hexoses (mannose, glucose, galactose), and uronic acids (4-O-methylglucuronic acid, galacturonic acid). Due to its heterogeneity, different structures of hemicellulose are found in different types of biomass [9, 10].

In hardwoods, glucuronoxylan is the major hemicellulose. Its backbone consists of xylose connected by β-1,4-glycosidic linkage, with some acetylation at C2 and C3 of xylose molecules. In addition, side chains of 4-O-methylglucuronic acid are found attached to xylose with α-1,2-linkage. Main hemicellulose in softwoods is galactoglucomannan. As the name suggests, galactoglucomannan has mannose and glucose as the backbone, with galactose and acetyl group as the side chains. For grasses (including cereals), glucuronoarabinoxylan, wherein xylose is the backbone, is the major hemicellulose [9, 11]. Structures of main hemicellulose in lignocellulosic biomass are illustrated in Figure 2.

Some other hemicellulose, which can be found in multiple sources, include xyloglucan which could be found in all hardwoods, softwoods, and grasses, arabinoglucuronoxylan in grasses and softwood. In addition, glucomannan is found as a minor component in softwoods and hardwoods.

The last major component of lignocellulosic biomass is lignin. Lignin is the only non-sugar component of the biomass. It is the second most abundant biopolymer besides cellulose. It is an amorphous polymer with structures that vary among different types of biomass and environmental conditions. Primarily, lignin consisted of three phenylpropane units of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), which are originated from aromatic alcohols, p-coumaryl, coniferyl, and sinapyl alcohols [12].

Although the sugar monomers in holocellulose part of the biomass are of interest for use in biofuel production via biological pathways, breaking down the structure to obtain the monomers is not a simple task. All three components of lignocellulose are incorporated into complex structures and recalcitrant to hydrolysis. Not only the cellulose itself has a strong crystalline structure, its microfibrils are packed and interconnected with hemicellulose. In addition, lignin that fills the void of the structure adds additional strength, increases the hydrophobicity of the wall, and

Basic flow diagram for the use of lignocellulosic biomass in biofuels production through fermentation route.

In order to utilize lignocellulosic biomass in production of biofuels via biological pathways, its tough structures have to be loosen, and hydrolysis of holocellulose needs to be achieved to release sugars for microbial usage. Block diagram in Figure 3 shows generalized scheme for handling and processing of lignocellulosic

biomass when applied in biofuel production by microorganisms through

Structures of main hemicellulose in hardwoods, soft woods and grasses. (a) Glucorunoxylan,

Bio-hydrogen and Methane Production from Lignocellulosic Materials

DOI: http://dx.doi.org/10.5772/intechopen.85138

(b) galactoglucomannan, (c) glucuronoarabinoxylan.

hence prevents the action of hydrolytic enzymes [13, 14].

fermentation process.

105

Figure 2.

Figure 3.

Figure 1. Structure of cellulose.

Bio-hydrogen and Methane Production from Lignocellulosic Materials DOI: http://dx.doi.org/10.5772/intechopen.85138

#### Figure 2.

also applicable and widely studied. The hemicellulose and cellulose structures (so called holocellulose) in lignocellulosic biomass contain sugar monomers that could be utilized by microorganisms and converted to various biofuels via biological

the cellulose fibers inaccessible by enzymes [6–8].

Biomass for Bioenergy - Recent Trends and Future Challenges

are found in different types of biomass [9, 10].

as a minor component in softwoods and hardwoods.

biomass are illustrated in Figure 2.

and sinapyl alcohols [12].

Figure 1.

104

Structure of cellulose.

Cellulose is considered a major composition of lignocellulosic biomass. It is a homopolymer containing glucose as the only monomer. Glucose molecules in cellulose are linked by β-1,4-glycosidic bonds (Figure 1). Cellulose chain is also known as β-1,4-glucan. The chains are packed into tiny and extremely long structure called microfibrils. These microfibrils are packed into lattices, which make most part of

Hemicellulose is also the composition that consists of sugars that can be utilized by microorganisms. It is a heteropolymer of pentoses (xylose, arabinose), hexoses (mannose, glucose, galactose), and uronic acids (4-O-methylglucuronic acid, galacturonic acid). Due to its heterogeneity, different structures of hemicellulose

In hardwoods, glucuronoxylan is the major hemicellulose. Its backbone consists of xylose connected by β-1,4-glycosidic linkage, with some acetylation at C2 and C3 of xylose molecules. In addition, side chains of 4-O-methylglucuronic acid are found attached to xylose with α-1,2-linkage. Main hemicellulose in softwoods is galactoglucomannan. As the name suggests, galactoglucomannan has mannose and glucose as the backbone, with galactose and acetyl group as the side chains. For grasses (including cereals), glucuronoarabinoxylan, wherein xylose is the backbone, is the major hemicellulose [9, 11]. Structures of main hemicellulose in lignocellulosic

Some other hemicellulose, which can be found in multiple sources, include xyloglucan which could be found in all hardwoods, softwoods, and grasses,

arabinoglucuronoxylan in grasses and softwood. In addition, glucomannan is found

consisted of three phenylpropane units of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), which are originated from aromatic alcohols, p-coumaryl, coniferyl,

The last major component of lignocellulosic biomass is lignin. Lignin is the only non-sugar component of the biomass. It is the second most abundant biopolymer besides cellulose. It is an amorphous polymer with structures that vary among different types of biomass and environmental conditions. Primarily, lignin

pathways.

Structures of main hemicellulose in hardwoods, soft woods and grasses. (a) Glucorunoxylan, (b) galactoglucomannan, (c) glucuronoarabinoxylan.

#### Figure 3.

Basic flow diagram for the use of lignocellulosic biomass in biofuels production through fermentation route.

Although the sugar monomers in holocellulose part of the biomass are of interest for use in biofuel production via biological pathways, breaking down the structure to obtain the monomers is not a simple task. All three components of lignocellulose are incorporated into complex structures and recalcitrant to hydrolysis. Not only the cellulose itself has a strong crystalline structure, its microfibrils are packed and interconnected with hemicellulose. In addition, lignin that fills the void of the structure adds additional strength, increases the hydrophobicity of the wall, and hence prevents the action of hydrolytic enzymes [13, 14].

In order to utilize lignocellulosic biomass in production of biofuels via biological pathways, its tough structures have to be loosen, and hydrolysis of holocellulose needs to be achieved to release sugars for microbial usage. Block diagram in Figure 3 shows generalized scheme for handling and processing of lignocellulosic biomass when applied in biofuel production by microorganisms through fermentation process.
