The lignocellulosic materials that make up 50% of the total biomass in the world are not suitable for consumption as direct food and are made up of plant sources. Basically, there are three basic polymers: hemicellulose (C5H8O4)n, cellulose (C6H10O5)n and lignin [C9H10O3) (OCH3)0.9-1.7]n [4]. Typically, biomass contains 40-60% cellulose, 20-40% hemicellulose and 10–25% lignin. Extracts and minerals in lignocellulose are up to 10% of the weight of dry biomass. Other substances in the lignocellulose (extractives) are organic solutions or water-soluble substances, which make up a very small part (1-5%) of the lignocellulosic substance [6].
The cell walls of plants contain lignocellulose. If the lignin is removed, the polysaccharide derivative remains. Polysaccharides in the plant cell are also called halocellulose. Halocelluloses consist of celluloses and hemicelluloses. If halocellulose is hydrolyzed, C6 and C5 sugars, uronic acids and acetyl groups are obtained. C6 sugars are glucose, mannose and galactose. C5 sugars are mainly xylose and arabinose. The ratio of each compound varies depending on the plant source [7].
The main components of lignocellulosic natural sources are cellulose, hemicelluloses, lignin, extractives and inorganics [8]. Cellulose in nature is polysaccharides such as various starch, pectin, and hemicellulose. Hemicelluloses are galactose, mannose, xylose, arabinose and other sugars; they contain polymers and heteropolymers of uronic acids. In addition cellulose in nature exists as a mixture of cellulose-lignin [9, 10].
1.2.1 Cellulose
In the biosphere, which we call the world of living things, approximately 27x1010 tons of carbon is attached to living organisms and more than 99% of it is found in vegetable material. Approximately 40% of the carbon in plants comes from cellulose. In this regard, cellulose is the most abundant natural polymer on earth, and it has a wide spread from primitive plants (algae, moss, etc.) to highly organized plants (woods) and some bacteria [11, 12]. Cellulose is one of the most important structural polysaccharides that are found in the plant world and have the simplest structure and also located in the cell wall structure [13].
Cellulose is the most common polymer on earth which is a linear syndiotactic (alternative spatial arrangement) of the glucose polymer bonded by Β-(1 → 4)-glycosidic bonds. The size (degree of polymerization) of the cellulose molecule varies depending on the presence of less than 500 glucose units in each molecule in the secondary wall located on the wall of the plant cell [9]. Cellulose chains are formed by combining thousands of glucose units and these chains have hydrogen and Van der Waals bonds [14].
Cellulose is in the basic structure of all plants, and the most important task of cellulose is to provide strength, uprightness and support to plants. Cellulose does not exist in its pure form in nature. Cellulose, which is the main structure element of the cell wall, constitutes 40-50% of lignocellulosic biomass [15]. Cellulose is almost never found alone in nature. It is often found with other herbal ingredients. This affects the breakdown of cellulose in the natural environment [9].
There are several types of cellulose in nature. All of these are used for different purposes [16]. Cellulose types are distinguished from each other by the letters a, b, d. A-cellulose is the most important of all species. B-cellulose and d-cellulose, which take the name “hemicellulose”, are less resistant to acids and bases and have the ability to break easily [13].
1.2.2 Hemicellulose
Hemicelluloses are the most important component of lignocellulosic substances after cellulose. Hemicelluloses are formed by bounding simple sugars in different ways and found in plants at a rate of approximately 20-30%. Their polymeric structure is quite open (amorphous) and irregular (branching) compared to cellulose, and they are more sensitive to reactions than cellulose, which is arranged in the form of flat chains. Hemicellulose can be dissolved and swollen in water due to its amorphous structure [17].
Hemicellulose includes ether-bound pentoses, hexoses, sugar acids and ester-bound acetyl and feruloyl groups. It is a heterogeneous polymer consisting of 5C pentose sugars (such as xylose arabinose) and 6C hexose sugars (such as mannose, glucose and galactose) and sugar acids. Hemicellulose does not have a crystalline structure and therefore has a lower degree of polymerization than cellulose (mostly about 200) [18]. Unlike cellulose, hemicellulose is not chemically homogeneous. Hemicelluloses in hardwoods contain large amounts of xylan, hemicelulloses in softwoods contain more glucomannan [19]. In most plants, xylanes are a type of heteropolisaccharide. These include 1.4 linked homopolymeric chains of β-D-xylopyranose units. Unlike xylose, xylanes contain arabinose, glucronic acid or 4-O-methyl ether of glycronic acid and acetic, ferulic and p-coumaric acid. Branching contents and frequencies depend on the xylan source [20]. The xylan spine (skeleton) contains O-acetyl, α-L-arabinofranosyl, α-1.2 bound glucronic or 4-0-methyl glucronic acid. Hence, xylanes can be categorized as linear homoxylanes, arabinoxylanes, glucuronoxylanes and gluronoarabinoxylanes [21].
1.2.3 Lignin
Lignin is the most common natural polymer in the plant world after cellulose. Its main task in the cell wall is to hold cellulose fibers together due to its adhesive properties [15]. It is also known as the substance that forms the woody structure of the root and stem in the plant [16].
Lignin is a glycoside and can be easily decomposed into glucose and aromatic alcohol. This glycoside is called coniferin. Alcohol derived from this compound was also called coniferyl alcohol. Most of the polymeric structure of lignin contains three types of alcohols; synapyl, p-coumaryl and coniferyl alcohols [22].
It is understood from the fragmentation products that the main structure block of lignin consists of an aromatic core and a propane chain [23]. There are several functional groups in some parts of the molecule. The basic unit of lignin is called phenyl propane. Phenyl propane types produce lignin by connecting to each other in various styles [24]. The general structure of lignin can be explained by dehydrogenating polymerization of coniferyl, sinapil and coumaryl alcohols. The complex structure of the lignin and the formation of various bonds occur when the phenoxy radicals of these monomers are matched in different ways [25].
The structural duties of lignin in the plant cell are to give rigidity to the cell wall, to ensure the adhesion of different cells in the wood tissue, to make the cell wall hydrophobic, to protect the wood from microbial disintegration. The structure of the lignin is very suitable for performing these functions. Aromatic rings and hydroxyl groups provide non-covalent dipole aromatic interactions and hydrogen bonds between cellulose and hemicellulose. Bending of the structure is prevented due to the branches in the structure of the lignin [26].
Lignin can be used in many industrial areas: as water processors in heating-cooling systems, cement industry, road construction, oil well drilling mud, agglomeration of animal feed, ceramic production, paint production, pesticide drugs, pipelines, casting molding rods, plywood production [27].
1.3 Types and Category of the Biomass
Biomass is defined as any mass or residue of any natural or organic (decaying) substance obtained from existing plants or animals, of which biological origin is non-fossil and renewable. Biomass is the common name given to all organic materials including land and water-growing plants, forest and agricultural plants, animal waste, herbaceous and woody energy plants, organic wastes of cities, and industries and municipalities, which can be renewed in less than 100 years [28]. Biomasses which are carbohydrate compounds are transformed into solid, liquid and gaseous bioenergy. Bioenergy is passed through various physical, chemical and biological methods. It has commercial properties whose basic and specific