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Figure 1.3 Electricity generation from biomass [35].

Table 1.1 Estimated direct and indirect jobs in bioenergy, by country/region and technology, 2017-2018 [29].

      ^a Power and heat applications.

      ^b Traditional biomass is not included.

Table 1.2 Heat production from biomass in EJ [35].

Table 1.3 Bioenergy technologies [4–49].

1.2 Bioenergy Chemical Characterization

      If one clearly identifies the residue of biomass, one may find different components like lignin, hemicellulose and cellulose having different percentage. The chemical composition may vary depending on the structures of the bioenergy [50].

      1.2.1 Cellulose [C₆(H₂O)₅]_n

      The most important chemical component in the biomass is cellulose, which contains 90% of total cotton and 50% of total wood and possesses a very strong function in the plant cell wall [51]. The creation of an intermolecular hydrogen bond on different hydroxyl group makes it more firm and stable [52]. In addition to that, [53] confirmed that the group of hydroxyl have greater reactivity as compared to the secondary ones because of low impediment.

      1.2.2 Hemicellulose [C₅(H₂O)₄]_n

      Hemicellulose is different for different plants [54], and it is generally found to get decomposed in between 180-350 °C and produces aldehydes, ketones, non-condensable coal gas, furans and acids [55].

      1.2.3 Lignin [C₁₀H₁₂O₃]_n

      The content of lignin is also different for different species and ranges between 25-30% for soft plants and can be as high as 50% for ebony like hard species of plant. The major elemental components like carbon are up to 65%, for hydrogen, it is between 5-6%, and the rest are mostly oxygen [56]. The methoxylation of the compounds plays a very crucial role, and because of it, the composition varies [57]. At the temperature of around 128 °C, dry lignin begins to soften, but this also depends on the molecular weight and with the increase in molecular weight, the softening temperature is bound to increase [58]. The availability of lignin is quite high [59, 60]. It has got higher calorific value as compared to cellulose [61] and can be subsequently used for the production of bioplastics, additives etc. [62]. Several high valued components are produced from it [63–66].

      1.2.4 Starch

      Starch is found mainly in two forms in nature, i.e., hot water soluble amylose (25-27%) and water-insoluble amylopectin (73-75%) [67]. The residues of α-Dglucose are joined in starch to form long chains in order to create polymers [68].

      1.2.5 Other Minor Components of Organic Matter

      The important substrates of biomass that influences the treatment process are lipids, nucleic acid, proteins, acetyls and uronic acid [69–75].

1.2.6 Inorganic Matter

      Biomass often contains several inorganic substances and especially in the form of ash content [76]. The major elements that are constituents of biomass are potassium, sodium, silicon, calcium, iron and aluminium [77].

1.3 Technologies Available for Conversion of Bioenergy

      Three main routes are available for the conversion of biomass to usable forms. They are as follows: