27–32% whereas herbaceous plants consist around 14–25% [44].
Organosolv lignin and soda lignin are known as lignin without sulfur and processed on an industrial range, and another new resource of sulfur-free lignin with various uses is the second generation biorefinery method. Strong demand commodities on the market [45]. The presence of ash content Organosolv lignin is about 1.75% & lignin, owing to its hydrophobicity, is soluble in organic solution whereas impossible to solve in water [46]. It is derived by precipitation from the solvent. Organosolv’s most general techniques are focused on ethanol/water pulping and acetic acid pulping [47]. Soda lignin emerges from the pulping phase of soda. There is no sulfur content in soda lignin, but the presence of ash composition is about 0.7 to 2.3%, which is comparatively more than organosolv lignin [48]. In soda-based cooking methods [49], annual plants such as straw, flax and hardwood are used. The significant applications of soda lignin are employed in manufacture of phenolic resin, animal nutrition, as well as polymer manufacture.
Lignin is present in the most of vascular plants and second to cellulose in quantity between polymers in general. Because, like several other constituents of biomass, the photosynthesis reaction is formed. It is sustainable and the produced annually of lignin on earth has been estimated to be in the range of 5–36 * 108 tons. The composition of lignin is in the range of about 15–40% in the woody plants of gymnosperm and angiosperm phylum [5, 50]. In annual plants, low lignin content is also commonly found (Figures 2.3a, b, c). Many deciduous and coniferous species, along with certain types of animals, are given significant values for both the composition of lignin in different forms of plants which are having commercial importance; plants have a personal investment as a source of lignocellulosic materials to produce paper and board products.
Lignin is an essential component of cell walls in developing plants, along with chemical bonds to the monosaccharide components present. The key component of lignin has been found to be covalently bound to hemicellulose in spruce wood [51]. Some mechanical disintegration of the material must precede any efforts to separate lignin from wood or other forms of biomass. Typically, intense substance milling is used whereby structural integrity, cell layers and some homogeneity in macromolecule level are calculated as well as only the typical lignin structure can be derived from such materials.
Figure 2.3a Lignin content in various types of plants (Gymnosperms).
Figure 2.3b Composition of Lignin in different plants (Eudicotyledons).
Figure 2.3c Presence of Lignin in different plants (Monocotyledons).
2.3 Production of Bio-Aromatics (Bio-Aromatics as Lignin)
There are two general types of lignocellulosic biomass conversion technologies, namely treatment and processing for transformation into energy and non-energy goods (Figure 2.4). The aim of energy processing is to generate energy in various ways like using thermal energy and electricity, fuel oil, coal, methane, and biodiesel. Burning, pyrolysis and gasification are the key methods of conversion.
2.3.1 Pre-Treatment
Numerous preliminary treatment methods have been implemented in order to boost cellulose reactivity as well as to increase the yield of simple sugars and lignin content, including traditional pre-treatment goals.
1 Development of protein rich solids during enzyme hydrolysis, which increases sugar yields.
2 Trying to avoid the breakdown of sugar (mainly pentose), like hemicellulose enzymes.
3 Reducing inhibitor development for the following stage of fermentation.
4 Lignin treatment for conversion into useful aromatic products and products.
5 To be expense by performing in reasonable reactors and by decreasing demands for heat and fuel. Generally, there are four types of pre-treatment technologies are available and described in the following chapters.
Figure 2.4 Schematic diagram for processing of lignin.
2.3.1.1 Physical Pre-Treatment
Physical pre-treatment includes the dissolution by milling or grinding of biomass size and particle size. Enhanced outcomes of hydrolysis due to decreased crystallite size and improved characteristics of mass transfer from decreased particle dimension. The necessary energy necessary for physical pre-treatment are varies on the final size of particles as well as the decrease in lignocellulosic content crystallinity. In most situations, where physical pre-treatment is the only possible alternative, the energy needed is more than actual energy available in the biomass. It is a costly technique and probably won’t be employed in a continuous operation.
2.3.1.2 Chemical Pre-Treatment
2.3.1.2.1 Alkaline
Alkaline chemicals like Na, K, Ca, and NH4OH are much required for preliminary treatment of lignocellulosic biomass. Using an alkali allows the ester and the glycosidic side chain to degrade, resulting in structural lignin modification, swelling of cellulose, partial cellulose recrystallization [53–55] and partial hemicellulose solvation. NaOH has been widely researched for several years [56, 57], and the destruction of the biomass lignin structure has been shown to improve the user-friendliness of cellulose enzymes [51, 58, 59]. Lime is another alkali and has been used for biomass pre-treatment. Corn stover, turn grass, is a lignocellulosic feedstock, which has shown benefit from this pre-treatment process of straw bagasse, barley, and rice [60, 61–64].
The efficacy of multiple alkaline solutions by analyzing hemicellulose wheat straw delignification and dissolution. They observed that 1.5% NaOH for 144 h at 20 °C was the optimum state, resulting in 60% lignin release and 80% hemicellulose release. Alkaline pre-treatment conditions are generally less severe than most pre-treatments. It may be conducted at atmospheric temperature, but it needs longer pre-treatment periods compared to more temperatures. The alkaline method includes soaking biomass at a target temperature in alkaline solutions and mixing. The benefit of alkaline pre-treatment for specific period is that a given quantity of biomass lime cost needed is the less among alkaline treatment methods [65].
