**4. Lignin types, isolation and pretreatment methods**

Base on the isolation and pulping process, lignin products is termed with five different names


As it known to us that the lignocellulosic biomass has three major components i.e. lignin, hemicellulose and cellulose that are linked together through different linkages and bonds between them. The details components and linkages are shown in the **Figures 3** and **4**. The isolation of lignin requires to disrupt the covalent, ether and hydrogen bonds between lignin and other fraction. The efficacy of the process arises from the yield, purity, and the degree of transformation in lignin structure. In each isolation process, either full or partial cleavage of bonds and functional groups are targeted. During the isolation method, the lignin linkages can be exposed to changes such as ether, carbon-carbon, oxygen-carbon bonds alteration and other than degradation to small fractions. Different kinds of pretreatment methods (**Figure 5**) are tested to effectively separate out the main components of plant biomasses. The methods tested includes physicals, biologicals and chemicals to degrade and hydrolyze

**Figure 5.** *Pretreatment methods and their effect on biomass.*

*Composition and Role of Lignin in Biochemicals DOI: http://dx.doi.org/10.5772/intechopen.106527*

the targeted biomasses into its monomers. The pretretments are necessory to remove lignin, one of the toughest structure that act as an obstacle for its utilization purposes. Thus lignin is considered to be remove from the biomasses and makes the cellulosic component expose for the transformation into usable materials [29].

The preeminent approach must have characteristics like (a) minimum chemicals uses (b) high carbohydrates reclamation (c) narrow quantities of by-products (d) economical far-reaching applications (e) appropriate for diverse biomasses and (f) decrease the concentration of enzymes used in hydrolysis of biomasses [30]. So far none of a single pretreatment method has all the good properties as supposed above, but in broad-spectrum, this section will concisely focus on the common methods reported for different biomass hydroylsis.

## **4.1 Mechanical treatment**

Mechanical method change the solid fragments of the biomass into granular shape, improve the precise surface area and degrade into lesser particles, remove water content, decrease the needs high amount of energy consumption in further pre-treatment reaction. Various mechanical methods like high-pressure homogenizer, piston press, bead mill, grinding and sonication are reported.

### **4.2 Thermal method**

A high temperature utilization is also applied at commercial scale that's disintegrate the plant biomass and direct conversion of biomasses into polysaccharides and carbon materials. The thermal treatment scale depends on the plant biomass materials and conversion into target molecules. Low thermal temperature ranges from 120 to 180°C are uses for proteins, carbohydrates and other sugars molecules solubilization. Wherase, high thermal treatment are applied for prouction of volatile organics, carbons and gases from biomass. Thermal heating in autoclave uses steam heat under high pressure on lignocellulosic biomass results in a considerable break in lignin linkages in biomass [31]. Electromagnetic radiation from microwaves with a frequency range of 250–300 MHz energy induced molecular vibration and is showed more efficient and less time consuming pretreatment method compared to conventional heating [32]. The microwave heating combine with acid, alkali and organic solvents has high impact than alone thermal treatment method [33]. So thermal treatment with any technology can be use to disintegrate the plant biomass into its components.

### **4.3 Lignosulfonate process**

The isolation of lignin using sulfite and bisulfite anions at industrial scale. In this process, sulfur dioxide reacts with water with control oxygen supply in sulfur combustion and produce sulfurous acid for making pulping liquor. This reaction use alkali base or carbonates for pH (1–6) adjustment, and temperature adjustment of 140–160°C. The final product lignosulfonate polymer is water soluble and comprises of 5% sulfur and carbohydrate impurities.

### **4.4 Kraft process**

The Kraft process is a universal delignification method used in paper and pulp industry for lignin removal. In this process sodium sulfide (NaS2) and sodium

hydroxide NaOH are used to disassociate lignin linkages in substrates to separate out cellulose. The addition of these alkali solutions at high temperature of 150–170°C for about 100–120 minutes, results a brown liquid and solid pulp. The lignin is precipitated through acidification from dark liquor and purified while cellulose and other carbohydrate residues and inorganic impurities are left aside. Purified Kraft lignin is then used to energy purpose through combustion in paper mills.
