**2. Lignocellulosic biomass**

Biomass is defined as a plant or animal-based organic matter of recent origin. Biomass includes agricultural crop residues (residues of agricultural crops that are not harvested for commercial use, including stalks, leaves, husks, etc.), forestry and wood residues (residues associated with the production of timber in the forest, as well as the processing of timber into their final products), dedicated energy crops (herbaceous and short rotation woody energy crops), aquatic biomass (algae, plants, and microbes found in water), sewage sludge, digestate (remains of anaerobic digestion), industrial crops, animal, industrial, municipal and food waste [1, 5]. A detailed discussion on biomass sources, classification, composition, and analysis is available in [1, 5]. If improperly managed, these biomass residues and waste could lead to environmental issues, including water contamination, greenhouse gas emissions, pests and insects breeding, and foul odor. Converting biomass residues and wastes into bioenergy (a) reduces the burden on waste management, (b) energy generated reduces the dependence on fossil fuels, and (c) reduces the amount of decomposing waste and associated environmental issues [5–7].

Biomass is a complex heterogeneous mixture mainly of organic matter (and a small amount of inorganic matter). Biomass is also classified based on its chemical composition as carbohydrates, lignin, essential oils, vegetable oils, animal fats, natural resins (gums), etc. The most abundant biomass on earth is lignocellulosic biomass (LCB), which includes agricultural crop residues, forestry and wood residues, dedicated energy crops, industrial crops, and food waste. LCB is a complex mixture of biopolymers consisting of carbon (C), oxygen (O), and hydrogen (H), with total content reaches typically above 95%. The elemental composition of C, O, and H in plant biomass on a dry basis is 42–47%, 40–44%, and 6%, respectively. Plant biomass has a substantial amount of oxygen with a carbon-to-oxygen (C/O) ratio of almost one. Because of the higher level of oxygen content, the energy density of biomass is relatively low compared to fossil fuels. Plant biomass also contains macronutrients, micronutrients, trace elements, and other heavy metals, which are present in varying amounts, depending on the plant species and environment, summing all together up to about 4% [1, 4, 8–10].

The three major constituents of LCB are cellulose, hemicellulose, and lignin. These constitute the cell wall of plants. The most abundant polymer of the lignocellulosic plant cell wall is cellulose, which provides structural support that gives trees and wood their strength. It is a linear polymer chain composed of D-glucose (pyranose) units linked by β-1,4 glycosidic bonds. Cellulose has a high (300–15,000)

#### *Hydrothermal Conversion of Lignocellulosic Biomass to Hydrochar: Production… DOI: http://dx.doi.org/10.5772/intechopen.112591*

degree of polymerization (DP) depending on the plant species. Cellobiose, glucose disaccharide, is the repeating unit of cellulose. Cellulose chains are grouped to form elementary fibrils, which are interlinked by hydrogen bonds and Van der Walls forces to form long microfibrils. Microfibrils represent the main component of the cell wall. Because of the high degree of hydrogen bonding, cellulose is highly stable and resistant to chemical attack. Interaction of hydroxyl (OH) groups present on the inner and outer surfaces of cellulose forms hydrogen bonds (intra- and intermolecular), which stiffens the chains and provides crystalline structure to cellulose. The interchain hydrogen bonds create crystalline (uniform and ordered) and amorphous (loose and disordered) regions of cellulose. The amorphous regions are more reactive than crystalline regions. The crystalline structure of cellulose leads to its mechanical strength and chemical stability and provides strength and toughness to leaves, roots, and stems. Cellulose is insoluble in almost any solvent [4, 5, 8, 11–13].

Hemicellulose is the second most abundant polymer in LCB. It is a branched heterogeneous structural polysaccharide composed of C5 and C6 sugars, including D-xylose, L-arabinose, D-mannose, D-galactose, and D-glucose units. Hemicellulose has a lower DP (80–200) than cellulose. Because of the branched structure, hemicellulose is amorphous and easy to hydrolyze by dilute acids, bases, and enzymes. Hemicellulose lateral chains form a tightly bound network with cellulose microfibrils through hydrogen bonds, which makes a highly rigid cellulose-hemicellulose-lignin matrix with the interaction of lignin *via* covalent bonds [4, 9, 11, 12].

Lignin is the third most abundant polymer in LCB. Lignin is an aromatic polymer composed of three phenylpropanoid monomers: guaiacyl, p-hydroxyphenyl, and syringyl. The phenylpropanoid monomers in lignin are linked in alkyl-aryl, alkyl-alkyl, and aryl-aryl ether bonds. Lignin, an amorphous and highly complex aromatic hydrophobic biopolymer, is the natural phenolic glue that tightly binds cellulose and hemicellulose. Lignin plays a cementing role for linkages between cellulose and hemicellulose to form a 3-D structure of lignin-polysaccharide complex in the cell wall, leading to a strong cell wall structure that makes it insoluble in water and provides mechanical strength to plants. Lignin provides sealing for a water-conducting system linking roots with leaves. The cross-linking between polysaccharides (hydrophilic and permeable) and lignin (hydrophobic and impermeable) creates vascular tissues for the efficient conduction of water in plants. Lignin 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 [5, 14, 15].

The key component in woody biomass is cellulose. Leaves and grasses are rich in hemicellulose, and lignin is the major component in shells. Hemicellulose is thermally least stable, and lignin is the most stable of all three. The cellulose, hemicellulose, and lignin content vary with the biomass falling in the range of 40–60%, 15–30%, and 10–25%, respectively. In addition to three major components, inorganic compounds and organic extractives, including fats, waxes, proteins, simple sugars, gums, resins, starches, phenolics, pectins, essential oils, and mineral compounds, are also present in biomass as nonstructural components, which are responsible for the smell, color, flavor, and natural resistance to decaying of some species. Inorganics present in LCB include nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), sulfur (S), calcium (Ca), chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), selenium (Se), silicon (Si), sodium (Na), aluminum (Al), titanium (Ti), cadmium (Cd), and chromium (Cr). Woody biomass contains a high (~90%) amount of cellulose, hemicellulose, and lignin, whereas agricultural and herbaceous biomass contain more extractives and ash [5, 8, 11, 12].

Biomass analysis, including proximate and ultimate analysis, as well as higher heating value (HHV), is essential in understanding the behavior of biomass in energy applications. The proximate analysis determines the volatile matter (VM), fixed carbon (FC), ash (noncombustible solid residue), and moisture (M) content. VM is the vapors/gases released during heating. FC is the nonvolatile solid carbon that remains after devolatilization. Inorganic matter, including silica, calcium, iron, aluminum, potassium, sodium, magnesium, and titanium, are the main constituents of ash. High-ash content biomasses include agricultural residues, grasses, and straws. Contamination of biomass with dirt, soil, rock, and other impurities during collection and handling partly contributes to ash content. Moisture content has a significant impact, and high moisture content is a major concern in biomass conversion. The moisture content of some biomasses, such as water hyacinth, can be very high (> 90%). Moisture can affect the storage and handling of biomass. High moisture content significantly increases transportation and energy costs. It lowers the calorific value as the energy released is used in the evaporation of moisture, which is not recovered. Moisture drains much of the deliverable energy during conversion. The ultimate analysis expresses biomass composition in terms of major elements (C, H, O, S, and N) on a mass percent, dry, and ash-free basis. These are useful for performing mass balances on biomass conversion processes. Typically, the sulfur and nitrogen content of biomass is very low. The characteristics of raw biomass include high moisture content, low bulk density, low energy density (calorific value), poor grindability, compositional non-homogeneity, hygroscopic nature, higher biological degradation, and lower storability [9, 16–18].
