**3. Lignocellulose pretreatment technologies**

Application of biorefining to bagasse requires lignocellulose fractionation into cellulose, hemicelluloses and lignin [17]. This step involves pretreatment where a considerable part of hemicelluloses is solubilized. In this regard, the cellulose portion is activated initially towards enzymatic hydrolysis and subsequently, for ethanol/biogas production. The use of pretreatment in the conversion of LB for bioenergy production is to enhance the release of cellulose and disrupt lignin and hemicellulose [13]. The sole aim is to remove lignin and hemicellulose, thereby enhancing the cellulose crystallinity and porosity for easier accessibility of microbes to breakdown lignocellulosic feedstock [18]. Various feedstocks that have been employed in literature as pretreatments methods are presented in **Table 2**. Lignin is an amorphous and water-insoluble heteropolymer, and as stated earlier, it is composed of phenylpropane units (coniferyl, p-coumaryl and sinapyl alcohol) held together by different linkages as discussed earlier [19]. A simplified diagram of biomass pretreatment techniques showing the major components of lignocellulose is presented in this chapter (**Figure 4**). The fermentation of LB is difficult due to


**433**

**3.1 Hydrothermolysis**

**Figure 4.**

approximately 15%.

**3.2 Ionic liquid pretreatment**

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

*(Accessed at: https://doi.org/10.1016/B978-0-12-802323-5.00001-3).*

the high recalcitrant lignin and inadequate accessibility to sites for enzyme activity. Studies have shown that irrespective of its solubilization, the lignin content change is related to the solidification and re-deposition which is due to cooling after severe pre-treatment. Therefore, only re-allocation of lignin takes place, instead of lignin removal during pre-treatment at high temperatures and pressures [21]. Lignin inhibits hydrolysis by forming physical barriers and non-productive adsorption of cellulase enzymes. Thus, lignin restricts the enzymes from reaching the cellulose, thereby reducing the active enzyme sites for cellulose hydrolysis. Brandt et al. [20] observed that 80–90% of lignin was recovered from a solid fraction of hardwood through hydrothermal pre-treatment at 180–220°C. Therefore, as the severity of hydrothermal pre-treatment increases, the lignin content in the pre-treated solids also increases due to the simultaneous de- and re-polymerization reactions of lignin.

*Simplified diagram of biomass pretreatment technique showing the major components of lignocellulose* 

Some pretreatment methods are summarized in the following sections.

During hydrothermolysis, the lignocellulosic changes that occur for bioenergy production were found to be an efficient method to disrupt lignin and hemicellulose and expose cellulose [20]. The authors [20] concluded that it is impossible for this pretreatment method to completely remove all the lignin present in a lignocellulosic feedstock. In the hydrothermolysis of sunflower oil cake for 1, 2, 4 and 6 h intervals at 25–200°C, it was observed that the cellulose solubilization rate was low (5%) while the hemicellulose content decreased from 13 to 6% at 200°C [29]. In the case of wheat straw at 200°C, cellulose crystallinity reduced as the cellulose hydrolysis rate was increased [30]. Lignin repolymerization occurred at 140°C -180°C for wood in 12–192 minutes with a removal of 75% of lignin [31]. Biogas production from sugarcane bagasse by hydrothermolysis was studied [25]. The authors [25] finding was that pretreatment by hydrothermolysis increased the biogas yield by

The search for a green solvent such as ionic liquids (IL) in the pretreatment of LB for biofuel production has gained increasing recognition for decades. ILs do solubilize complex biomass, thus providing industrial scale-up potential [23]. The unique abilities of ILs to selectively dissolve components of biomass or whole native biomass have been demonstrated [24]. Most ILs have been reported to be

#### **Table 2.** *Feedstocks and pretreatment methods.*

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

**Figure 4.**

*Biotechnological Applications of Biomass*

water and minerals to be conducted through the xylem under negative pressure without collapse of the tissue. In addition to mechanical support, lignin contributes to protective functions in plants by, for example, increasing resistance to biodegradation and environmental stresses, such as changes in humidity and water balance.

Application of biorefining to bagasse requires lignocellulose fractionation into cellulose, hemicelluloses and lignin [17]. This step involves pretreatment where a considerable part of hemicelluloses is solubilized. In this regard, the cellulose portion is activated initially towards enzymatic hydrolysis and subsequently, for ethanol/biogas production. The use of pretreatment in the conversion of LB for bioenergy production is to enhance the release of cellulose and disrupt lignin and hemicellulose [13]. The sole aim is to remove lignin and hemicellulose, thereby enhancing the cellulose crystallinity and porosity for easier accessibility of microbes to breakdown lignocellulosic feedstock [18]. Various feedstocks that have been employed in literature as pretreatments methods are presented in **Table 2**. Lignin is an amorphous and water-insoluble heteropolymer, and as stated earlier, it is composed of phenylpropane units (coniferyl, p-coumaryl and sinapyl alcohol) held together by different linkages as discussed earlier [19]. A simplified diagram of biomass pretreatment techniques showing the major components of lignocellulose is presented in this chapter (**Figure 4**). The fermentation of LB is difficult due to

**Pre-treatment methods Feedstocks References** Hydrothermal Sugarcane bagasse [23] Ultrasonic Sugarcane bagasse [24] Ionic liquids Water hyacinth [25, 26] Hydrothermal Sugarcane bagasse [14] Alkali Cattle dung [27] Thermochemical Water hyacinth [28]

**3. Lignocellulose pretreatment technologies**

*Chemical structure of the main monolignols of lignin.*

**432**

**Table 2.**

**Figure 3.**

*Feedstocks and pretreatment methods.*

*Simplified diagram of biomass pretreatment technique showing the major components of lignocellulose (Accessed at: https://doi.org/10.1016/B978-0-12-802323-5.00001-3).*

the high recalcitrant lignin and inadequate accessibility to sites for enzyme activity. Studies have shown that irrespective of its solubilization, the lignin content change is related to the solidification and re-deposition which is due to cooling after severe pre-treatment. Therefore, only re-allocation of lignin takes place, instead of lignin removal during pre-treatment at high temperatures and pressures [21]. Lignin inhibits hydrolysis by forming physical barriers and non-productive adsorption of cellulase enzymes. Thus, lignin restricts the enzymes from reaching the cellulose, thereby reducing the active enzyme sites for cellulose hydrolysis. Brandt et al. [20] observed that 80–90% of lignin was recovered from a solid fraction of hardwood through hydrothermal pre-treatment at 180–220°C. Therefore, as the severity of hydrothermal pre-treatment increases, the lignin content in the pre-treated solids also increases due to the simultaneous de- and re-polymerization reactions of lignin. Some pretreatment methods are summarized in the following sections.

## **3.1 Hydrothermolysis**

During hydrothermolysis, the lignocellulosic changes that occur for bioenergy production were found to be an efficient method to disrupt lignin and hemicellulose and expose cellulose [20]. The authors [20] concluded that it is impossible for this pretreatment method to completely remove all the lignin present in a lignocellulosic feedstock. In the hydrothermolysis of sunflower oil cake for 1, 2, 4 and 6 h intervals at 25–200°C, it was observed that the cellulose solubilization rate was low (5%) while the hemicellulose content decreased from 13 to 6% at 200°C [29]. In the case of wheat straw at 200°C, cellulose crystallinity reduced as the cellulose hydrolysis rate was increased [30]. Lignin repolymerization occurred at 140°C -180°C for wood in 12–192 minutes with a removal of 75% of lignin [31]. Biogas production from sugarcane bagasse by hydrothermolysis was studied [25]. The authors [25] finding was that pretreatment by hydrothermolysis increased the biogas yield by approximately 15%.

#### **3.2 Ionic liquid pretreatment**

The search for a green solvent such as ionic liquids (IL) in the pretreatment of LB for biofuel production has gained increasing recognition for decades. ILs do solubilize complex biomass, thus providing industrial scale-up potential [23]. The unique abilities of ILs to selectively dissolve components of biomass or whole native biomass have been demonstrated [24]. Most ILs have been reported to be

viscous in nature, requiring the use of co-solvents to enhance its fluidity and the recovery by a commonly employed aqueous biphasic system, or the use of acetone, sodium hydroxide or water [25]. Commonly used co-solvents are dimethyl sulfoxide (DMSO) and dimethylacetamide (DMAC). The application of ILs to LB in areas such as fractionation, cellulose composites preparation and its derivative and removal of pollutants is a new avenue for the efficient utilization of these solvents [26]. ILs have been found to be the most expensive research-grade solvents under investigation for the dissolution of biomass and provides further challenges with solvent recovery [20].

### **3.3 Acidic and alkaline pretreatment**

Lignocellulosic pretreatment with acids at ambient temperatures are carried out to enhance hemicellulose solubilization, thereby, making cellulose accessible for enzyme degradation with a dilute or a strong acid [14]. In this process, solubilized hemicelluloses are exposed by hydrolytic reactions to produce monomers, furfural, and other volatile products under acidic conditions [27]. In this regard, solubilized lignin quickly condenses and precipitates into acidic conditions. Hemicellulose solubilization and lignin precipitation are therefore noticeable during strong acid pretreatment. A disadvantage of this method is the risk of the formation of inhibiting compounds [14]. However, the use of dilute acid pretreatment has gained numerous research interests over the use of concentrated acids [28]. This is due to the fact that concentrated acids are toxic, corrosive, hazardous, and require reactors that need expensive construction materials which are resistant to corrosion.

### **3.4 Biological pretreatment**

The delignification of LB could also involve application of biological methods using enzymes or microorganisms. Wood degrading microbes including white, brown, soft rot fungi, and bacteria are used in biological applications [28]. Biodegradation releases the chemical components and opens up the structure of the LB which promotes enzyme action leading to further breakdown. The brown and soft rots have been reported to attack cellulose leading to lignin modifications, whilst the lignin components are degraded by the white rot fungi [28]. The biological pretreatment of wood chips with four different white-rot fungi for a period of 30 days was studied [3]. The glucose yield of the pretreated wood by *Trametes versicolor MrP 1* reached 45% by enzymatic hydrolysis while 35% solid was converted to glucose during fungi incubation. Some microbes that have been employed in the past decades include *Ceriporia lacerate, Sterum hirsutum, Polyporus brumalis* and *Phanerochaete chrysosporium.*
