composition of algae is proteins, carbohydrates, and lipids, their percentage might
vary even in a single alga depending upon the conditions of growth and age of the
culture, and there are some algal species like Coleochaete thallus which even
contains lignin as well in smaller percentages [7].
Cellulose is a polymer of D-glucose subunits linked by α-(1,4)-glycosidic bonds
throughout its length that forms the organized fibrous linear structure. These long
fibrous polymeric chains are packed into bundles of microfibrils by hydrogen bonds
and weak van der Waals bonds. These microfibrils are concealed by hemicelluloses
and lignin, thereby adding additional strength to the cell wall. The hemicellulose is a
heteropolymer made of short lateral chains and branches of sugars and polysaccharides linked by α-(1,4)-glycosidic bonds like cellulose and occasionally by α-(1,3)glycosidic bonds. Xylose is the major sugar component, and it also includes arabinose, rhamnose, glucose, galactose, mannose, and other uronic acids like 4-Omethylglucuronic and D-glucuronic, and D-galacturonic acids [8]. Lignin has a
complex cross-linked polymeric structure with large molecular weight which mainly
constitutes coniferyl alcohol, p-coumaryl alcohol, and sinapyl phenyl propionic
alcohols as monomers which are linked together by alkyl-alkyl, alkyl-aryl, and
aryl-aryl ether bonds [9]. Extractives include some organic compounds like alkaloids, phenolics, glycosides, saponins, terpenes, pectins, waxes, gums, mucilages,
resins, and oils. They act as plant’s energy storage units and also as a defense against
insects and microbes that take plants as food. They also contribute to the wood
properties such as color, odor, and decay resistance. Ash is the inorganic final
residue that remains after complete combustion of the biomass at a high temperature
[10]. Proteins are made of one or more peptide chains with various percentages of
amino acids, namely, lysine, cysteine, methionine, and tyrosine folded together into
a solid or fibrous form. They act as the means of nitrogen storage for the algal cells
which decreases as it grows with age [11]. Carbohydrates consist of starches and
sugars that are formed as a result of energy absorbed by the chloroplasts and
reduction of carbon dioxide into 3-phosphoglycerate in the presence of ribulose1,5-bisphosphate carboxylase/oxygenase. They act as the structural part of the algal
cell wall and provide energy needed for metabolism under dark conditions
[12]. Lipids are long carbon chain molecules with carbon numbers 16–24 that
consist of polar and neutral constituents of fatty esters, fatty acids, hydrocarbons,
and triglycerides. They are formed in the inter-thylakoidal space of the chloroplast
and stored inside the cytoplasm which serves as a structural component of the cell
membrane. The increased lipid content in algae decreases the specific gravity,
making the cells buoyant [13].
The chemical composition of the biomass varies from one plant species to
another. The lignocellulosic and algal biomass compositions with their constituents
of some of the feedstock are given in Tables 2 and 3, respectively.
The chemical energy stored in the biomass can be converted into energy and
useful products by numerous ways. The two frequently and majorly used pathways
are (a) thermochemical processes and (b) biochemical processes. Thermal conversion processes include combustion, gasification, pyrolysis, carbonization, and
hydrothermal conversion techniques, while biochemical processes include anaerobic
digestion, fermentation, transesterification, and biohydrogen production. Though
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C. D. Venkatachalam et al.
vary even in a single alga depending upon the conditions of growth and age of the
culture, and there are some algal species like Coleochaete thallus which even
contains lignin as well in smaller percentages [7].
Cellulose is a polymer of D-glucose subunits linked by α-(1,4)-glycosidic bonds
throughout its length that forms the organized fibrous linear structure. These long
fibrous polymeric chains are packed into bundles of microfibrils by hydrogen bonds
and weak van der Waals bonds. These microfibrils are concealed by hemicelluloses
and lignin, thereby adding additional strength to the cell wall. The hemicellulose is a
heteropolymer made of short lateral chains and branches of sugars and polysaccharides linked by α-(1,4)-glycosidic bonds like cellulose and occasionally by α-(1,3)glycosidic bonds. Xylose is the major sugar component, and it also includes arabinose, rhamnose, glucose, galactose, mannose, and other uronic acids like 4-Omethylglucuronic and D-glucuronic, and D-galacturonic acids [8]. Lignin has a
complex cross-linked polymeric structure with large molecular weight which mainly
constitutes coniferyl alcohol, p-coumaryl alcohol, and sinapyl phenyl propionic
alcohols as monomers which are linked together by alkyl-alkyl, alkyl-aryl, and
aryl-aryl ether bonds [9]. Extractives include some organic compounds like alkaloids, phenolics, glycosides, saponins, terpenes, pectins, waxes, gums, mucilages,
resins, and oils. They act as plant’s energy storage units and also as a defense against
insects and microbes that take plants as food. They also contribute to the wood
properties such as color, odor, and decay resistance. Ash is the inorganic final
residue that remains after complete combustion of the biomass at a high temperature
[10]. Proteins are made of one or more peptide chains with various percentages of
amino acids, namely, lysine, cysteine, methionine, and tyrosine folded together into
a solid or fibrous form. They act as the means of nitrogen storage for the algal cells
which decreases as it grows with age [11]. Carbohydrates consist of starches and
sugars that are formed as a result of energy absorbed by the chloroplasts and
reduction of carbon dioxide into 3-phosphoglycerate in the presence of ribulose1,5-bisphosphate carboxylase/oxygenase. They act as the structural part of the algal
cell wall and provide energy needed for metabolism under dark conditions
[12]. Lipids are long carbon chain molecules with carbon numbers 16–24 that
consist of polar and neutral constituents of fatty esters, fatty acids, hydrocarbons,
and triglycerides. They are formed in the inter-thylakoidal space of the chloroplast
and stored inside the cytoplasm which serves as a structural component of the cell
membrane. The increased lipid content in algae decreases the specific gravity,
making the cells buoyant [13].
The chemical composition of the biomass varies from one plant species to
another. The lignocellulosic and algal biomass compositions with their constituents
of some of the feedstock are given in Tables 2 and 3, respectively.
The chemical energy stored in the biomass can be converted into energy and
useful products by numerous ways. The two frequently and majorly used pathways
are (a) thermochemical processes and (b) biochemical processes. Thermal conversion processes include combustion, gasification, pyrolysis, carbonization, and
hydrothermal conversion techniques, while biochemical processes include anaerobic
digestion, fermentation, transesterification, and biohydrogen production. Though
204
C. D. Venkatachalam et al.