**2.2 Composition of biomass**

The chemical composition of biomass is different from fossil fuels. The most abundant biomass on the earth is LCB, including agricultural biomass, forestry and wood processing residues, dedicated energy crops, industrial crops and food waste, hence, the following sections primarily focus on LCB. Plant biomass is a complex mixture of polymers consisting of three key elements: 42–47% of carbon (C), 40–44% of oxygen (O) and 6% of hydrogen (H), all percentages in dry matter, whose total content generally reaches above 95% [24, 29]. Plant biomass also contains macronutrients such as nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulfur (S), calcium (Ca). These inorganics are required in relatively high amounts (> 0.1% of dry mass) and essential for plant life cycle and biomass production. In addition, plants need a small amount of micronutrients (essential elements required in relatively small amounts, 100 mg/kg of dry mass)

such as chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni). Trace elements like sodium (Na), silicon (Si), selenium (Se), titanium (Ti), vanadium (V), cobalt (Co), aluminum (Al) and other heavy metals may also be present in plant biomass at different levels depending upon the plant species and the environment [24, 30–32].

Plant biomass has a carbon-to-oxygen (C/O) ratio of almost one. Because of this high oxygen level, the energy density of biomass is relatively low relative to fossil fuels. The major component of plant biomass is cellular lignocellulosic material, which is the non-starch fibrous part of the plant materials. Cellulose, hemicellulose and lignin are the three major constituents of LCB constituting the cell wall of plants [5, 15, 21]. The main component of the plant cell wall is cellulose (a linear homogeneous structural polysaccharide composed of D-glucose units with molecular weight (MW) > 100,000), which provides structural support. The second most abundant polymer in LCB is hemicellulose, a ramified heterogeneous structural polysaccharide composed of D-xylose, L-arabinose, D-mannose, D-galactose and D-glucose units. The third most abundant polymer in LCB is lignin, a phenylpropanoid polymer composed of guaiacyl, p-hydroxyphenyl and syringyl units [33, 34]. The compositions (cellulose, hemicellulose and lignin) of common LCB are listed in **Table 1**. Cellulose macromolecules form tough microfibers that function as the skeleton material of the cell wall. The inner space is packed with branched amorphous hemicellulose and lignin linking material. Cellulose connects with hemicellulose or lignin mainly through hydrogen bonds, while hemicellulose connects with lignin via both hydrogen and covalent bonds. Lignins and carbohydrates link tightly together in lignin-carbohydrate complexes, which results in residual carbohydrate or lignin fragments in extracted lignin or hemicellulose samples. Wooden biomasses are usually rich in cellulose, leaves and grasses are rich in hemicellulose and the shells are mostly rich in lignin. Cellulose is thermally more stable than hemicellulose. Knowledge of cellulose, hemicellulose and lignin composition in LCB can be helpful in controlling the product chemistry [32, 37].

In addition to the three major components, some other compounds present in LCB include inorganic compounds and organic extractives. These exist as non-structural components that do not constitute the cell walls or cell layers. Organic extractives can be extracted by nonpolar solvents (such as toluene and hexane) or polar solvents (such as water and alcohol). These include fats, waxes, proteins, terpenes,


### **Table 1.**

*Composition of common LCB [35, 36].*

### *Recent Advances in Thermochemical Conversion of Biomass DOI: http://dx.doi.org/10.5772/intechopen.100060*

simple sugars, gums, resins, starches, alkaloids, phenolics, pectins, glycosides, mucilages, saponins and essential oils. Often these compounds are responsible for the smell, color, flavor and natural resistance to rotting of some species. A common classification divides them into aliphatic compounds (mainly fats and waxes), terpenes and terpenoids, and phenolic compounds. The inorganic compounds constitute less than 10% by weight of LCB, forms ash in the pyrolysis process [21, 32].

LCB contains varying amounts of inorganic materials (including alkali and heavy metals, chlorine, phosphorus and sulfur) collectively called ash. The ash contents in LCB depend on feedstock type, the environment in which it was grown, fertilizer use, and contamination with soil particles. Typically, softwood and hardwood have ash contents below 1 wt%, short-rotation woody crops have around 2 wt%. Herbaceous crops have high levels of potassium and silicon and ash contents up to 7 wt%. Ash content is also not uniformly distributed within biomass; bark has higher concentrations of inorganics [38]. Water in wet biomass contains in three phases: bound water (hygroscopic or adsorbed, in cell walls, believed to be hydrogen-bonded to the OH groups of primarily cellulose and hemicelluloses of the biomass); free or unbound water (liquid water in cell cavities or voids of the biomass if the moisture content is higher than the fiber saturation point); and water vapor which fills the cell cavities or voids of the biomass [23]. Depending on the type of LCB, the cellulose, hemicellulose and lignin content fall in the range 40–60%, 15–30% and 10–25%, respectively. Both cellulose and hemicellulose are carbohydrates (polymers of sugars) and can be hydrolyzed into fermentable sugars, which in turn can be converted into fuels and chemicals. Wood biomass contains much higher amounts of the three main components (~90%), while agricultural and herbaceous biomass contains more extractives and ash [21, 32].

### *2.2.1 Cellulose*

Cellulose is the most abundant organic polymer on the planet. It is one of the main structural constituents of the lignocellulose cell wall of green plants and is found in an organized fibrous structure. Cellulose is a polysaccharide consisting of d-glucose (pyranose) units linked by β-1,4 glycosidic bonds. The -linkages in cellulose form linear chains. The degree of polymerization (DP) is about 300–15,000, depending on the plant variety. Cellobiose is the repeating glucose disaccharide of cellulose. Because of its long and linear molecules, cellulose does not dissolve readily in water. The chemical formula of cellulose can be written as (C6H10O5)n, where n is the DP. It is the long flexible natural polymer in fibers that predominately gives trees and wood of their strength. Cellulose chains are grouped to form cellulose fibers, which are interlinked by hydrogen bonds and van der Waals forces, resulting in long microfibrils. These microfibrils are arranged as a mesh in the cell wall, giving it strength and shape. Hemicelluloses and lignin cover the cellulose microfibrils. Cellulose is highly stable and resistant to chemical attack because of the high degree of hydrogen bonding between cellulose chains [15, 32, 37, 39, 40].

The hydroxyl (OH) groups present on the inner and outer surfaces of cellulose forms intra- and intermolecular hydrogen bonds, which stiffens the chains and promotes aggregation into a crystalline structure. The crystallinity and stabilization of cellulose mainly originate from the presence of OH groups. As three OH groups are available in each glucose molecule, the inner and outer surfaces of cellulose are covered by OH groups. These OH groups make hydrogen bonds with other OH groups and other groups (such as O, N and S) available in lignocelluloses. The crystalline structure of cellulose leads to chemical stability and provides strength

and toughness to the roots and stems of a plant. Cellulose molecules have different orientations throughout the structure resulting in different levels of crystallinity. The energy of hydrogen bonds in water and cellulose is 15 and 28 kJ/mol, respectively and the energy of van der Waals in water is only 0.15 kJ/mol. The strength of cellulose mainly originates from the existence of hydrogen bonds rather than van der Waals forces. The interchain hydrogen bonds introduce order (crystalline) or disorder (amorphous) into cellulose structure, creating two forms of cellulose: crystalline and amorphous. Cellulose requires severe hydrolysis conditions for breaking it into simple glucose units due to its crystalline structure. As suggested by some authors, cellulose consists of three regions: true crystal, subcrystalline (disordered structure in true crystal regions) and or noncrystalline (subscrystalline) regions. The crystallinity index (CrI) usually characterizes the crystallinity of cellulose, increasing CrI leads to decreasing chemical and biological hydrolysis of cellulose [41, 42].

### *2.2.2 Hemicellulose*

Hemicellulose is the second most abundant natural organic polymer after cellulose on the planet. In contrast to the linear or one-dimensional structure of cellulose, hemicellulose is a two-dimensional polymer composed of short chain branched heteropolysaccharides side connections. It is one of the major constituents of plant cell walls and is strongly linked to the surface of cellulose microfibrils. Hemicellulose is a random heterogeneous polysaccharide of pentoses (xylose and arabinose), hexoses (galactose, glucose and galactose) and their acidified derivatives such as glucuronic and galacturonic acids. Because of the branched nature, hemicellulose is amorphous, which is relatively easy to hydrolyze (by dilute acids, bases and hemicellulose enzymes) to its constituent sugars compared to cellulose. The content and structure of hemicellulose differ among LCB. The general nature of the hemicellulose structure depends on the type of plant, with the result that certain types of lignocellulosic materials are easier to hydrolyze than others. The various sugar units are arranged with different substituents and in different proportions. Hemicellulose has a degree of polymerization of 80–200. It is much smaller than cellulose, with a relatively low MW (< 30,000). The general chemical formula of hemicellulose can be written as (C6H8O4)m, where m is the DP. Lateral chains of hemicellulose form the tightly bound network through hydrogen bonds with cellulose microfibrils. It makes a highly rigid matrix of the cellulose-hemicellulose-lignin with interaction of lignin via covalent bonds [15, 32, 37, 39, 43, 44].

The different groups of polysaccharide molecules such as xylans, mannans, galactans and arabinogalactans make up hemicellulose. Xylan is the most common polysaccharide in hemicellulose consists of backbone chains that contain a varying number of D-xylopyranose linked by β −1,4 linkage (70–130 in softwood xylan and 150–200 in hardwood xylan). Mannans are made up of β−1, 4-linked D-mannose backbone mixed with D-glucose and D-galactose residues. These compounds include mannan (made up of mannose monomer), galactomannan (made up of mannose and galactose monomers), glucomannan (made up of mannose and glucose monomers), glucuronic acid (made up of mannose and glucuronic monomers). Galactan is composed of repeating galactose units as a polymer. Arabinogalactans consist of arabinose and galactose monosaccharides. The dominant hemicellulose component in hardwood, agricultural residues and herbaceous crops is xylan, with a small degree of acetylation and arabinose side groups. The main form of hemicellulose in softwood is glucomannan, highly acetylated and containing glucose and mannose [15, 32, 37, 44].

*Recent Advances in Thermochemical Conversion of Biomass DOI: http://dx.doi.org/10.5772/intechopen.100060*

### *2.2.3 Lignin*

After cellulose and hemicellulose, lignin is the third largest heteropolymer that occurs predominantly in the cell walls of woody plants. It is the main nonpolysaccharide constituent of plant biomass and the amount of lignin vary widely with plant species. Lignin primarily consists of macromolecules that contain highly branched phenolic compounds. Lignin is composed of three different phenyl propane (three-carbon chain attached to rings of six carbon atoms) monomers, including coniferyl alcohol (guaiacyl propanol), p-coumaryl alcohol (p-hydroxyphenyl propanol) and syringyl alcohol (sinapyl alcohol). The phenyl propane monomeric units in lignin are linked in different ways (alkyl-aryl, alkyl-alkyl and aryl-aryl ether bonds): through oxygen bridges between two propyl and phenyl groups, between a phenyl and a propyl group or through carbon–carbon bonds between the same groups. Lignin is generally considered as the natural phenolic glue that tightly binds cellulose and hemicellulose of LCB together; thus, leading to a strong cell wall structure and making it insoluble in water. The functions of lignin, an amorphous and highly complex aromatic hydrophobic biopolymer, are (a) to provide mechanical strength to the plants. It plays a cementing role for linkages (van der Waals, hydrogen bond and covalent bond) between cellulose and hemicellulose to form a 3-dimensional structure of lignin-polysaccharide complex in cell wall leading to a strong cell wall structure; (b) to provide sealing for waterconducting system linking roots with leaves. Polysaccharide components of the plant cell wall are hydrophilic and permeable, while lignin is hydrophobic and impermeable. The cross-linking between polysaccharides and lignin is a barrier for water absorption to the cell wall that create vascular tissues for efficient conduction of water in plants. Lignin exists in all vascular plants; and (c) to protect plants against biodegradation. It forms a natural protective shield protecting cellulose and hemicellulose in plants and makes plants resistant to pathogens, oxidative stresses and biodegradation by enzymes and microorganisms [42, 45–47].

The distinctive feature that differentiates lignin from cellulose and hemicelluloses is the presence of aromatic monomers. Lignin is less polar than cellulose or hemicellulose. Physically, cellulose microfibrils encrust hemicellulose whose empty spaces are filled up with lignin. Lignin is embedded within hemicellulose to provide additional rigidity to the plant. These lignin-hemicellulose fibers characterize woody plants, whereas the fibers in herbaceous plants are more loosely bound, indicating a lower amount of lignin [15, 35, 47]. The lignin content of plants varies with species and age. It is originated from not only content but also monomeric units and linkage types. The lignin content of softwood is in the range of 25–40%, which is higher than that of hardwood (18–25%), herbaceous crops (10–20%) and annual plants (10–12%). LCB with lignin percentage up to 40% has been reported. Softwood lignin is primarily made from coniferyl alcohol (>95%), with the rest consisting of coumaryl alcohol derived units and trace amounts of syringyl alcohol derived units. Hardwood lignin is composed of coniferyl alcohol and syringyl alcohol derived units in varying ratios. Lignin in grassy biomass has all three types of monomers. Annual plants lignin is composed of coumaryl alcohol. The elemental composition of lignin is approximately 61–65% carbon, 5–6% hydrogen and the remaining is oxygen. The carbon to oxygen (C/O) atom ratio for lignin is higher than 2:1, which is at least double that of hemicellulose and cellulose, where the C/O ratio is nearly 1:1. Therefore, lignin is a more energy-dense substance than polysaccharides. Lignin structure has many polar and hydroxyl groups allowing the establishment of strong intramolecular and intermolecular hydrogen bonds. These make lignin insoluble in any solvents except alkaline solutions [23, 37, 42, 44, 48].
