54
4 Effective Biomass Valorization Procedures Using Ultrasound …
also be converted into platform chemicals, such as ethanol, acetone, butanol, lactic
acid, itaconic acid and amino acids, using microbiological processes. Several technological challenges arise dealing with the conversion of lignocellulosic biomass, in
particular concerning the pretreatment step. The aim of pretreatment is to improve
the enzymatic access to cellulose for the hydrolysis, enhancing the sugars yields.
The hydrolysis of cellulose is limited by some factors, such as its degree of polymerization, crystallinity, available surface area, lignin and moisture contents. In the
past, different pretreatment options have been explored: wet oxidation, organosolv,
hydrothermal, acidic, alkaline, ammonia fibre explosion and, more recently, ionic
liquid pretreatment (Kumar et al. 2009; Hendriks and Zeeman 2009; Kumar and
Sharma 2017). Nevertheless, all of them appear as expensive, energy intensive and
utilize chemicals that require special disposal.
The widespread use of lignocellulosic resources that has occurred over recent
years has drawn considerable attention to the development of new but sustainable
processes based on non-conventional technologies. These technologies are certainly
capable of fractionating lignocellulosic biomass, depolymerising (hemi)cellulose and
lignin, and converting carbohydrates into more highly value-added chemicals. However, their energy consumption has to be assessed before any subsequent application
can be performed on an industrial level (Tabasso et al. 2015). Acoustic and hydrodynamic cavitation are able to provide severe physicochemical environment hardly
reachable using other engineering methods (Cintas et al. 2015), especially in terms
of energy efficiency, and to efficiently process recalcitrant lignocellulosic biomass
affording high-value chemicals. Cavitation can increase pore sizes and accessible
surface areas, while also decreasing the polymerization and crystallinity degrees of
cellulose, and improving the biodegradability and enzymatic hydrolysis of biomass
residues. Moreover, significant decreases in pretreatment time and temperature are
the most attractive features of US-assisted biomass pretreatment. In fact, US induces
faster structure damage through several mechanisms: fragmentation, erosion, sonocapillary effect, sonoporation and local shear stress.
The aim of this chapter is to highlight the impact that acoustic and hydrodynamic cavitation have on biomass valorization by investigating the role that the key
parameters play in the sustainability of the overall processes.
4.2 Biomass Pretreatment
4.2.1 Acoustic Cavitation as Suitable Biomass Pretreatment
The structure of lignocellulosic materials makes pretreatment essential in order to
obtain fermentable sugars in the hydrolysis step. One of the most important limiting factors to obstructing cellulose hydrolysis is the lignin content. Indeed, lignin,
being a structural polymer, imparts strength to plant cell walls by covalently linking to (hemi)cellulose, thus preventing carbohydrate exposure for enzymatic attack
4 Effective Biomass Valorization Procedures Using Ultrasound …
also be converted into platform chemicals, such as ethanol, acetone, butanol, lactic
acid, itaconic acid and amino acids, using microbiological processes. Several technological challenges arise dealing with the conversion of lignocellulosic biomass, in
particular concerning the pretreatment step. The aim of pretreatment is to improve
the enzymatic access to cellulose for the hydrolysis, enhancing the sugars yields.
The hydrolysis of cellulose is limited by some factors, such as its degree of polymerization, crystallinity, available surface area, lignin and moisture contents. In the
past, different pretreatment options have been explored: wet oxidation, organosolv,
hydrothermal, acidic, alkaline, ammonia fibre explosion and, more recently, ionic
liquid pretreatment (Kumar et al. 2009; Hendriks and Zeeman 2009; Kumar and
Sharma 2017). Nevertheless, all of them appear as expensive, energy intensive and
utilize chemicals that require special disposal.
The widespread use of lignocellulosic resources that has occurred over recent
years has drawn considerable attention to the development of new but sustainable
processes based on non-conventional technologies. These technologies are certainly
capable of fractionating lignocellulosic biomass, depolymerising (hemi)cellulose and
lignin, and converting carbohydrates into more highly value-added chemicals. However, their energy consumption has to be assessed before any subsequent application
can be performed on an industrial level (Tabasso et al. 2015). Acoustic and hydrodynamic cavitation are able to provide severe physicochemical environment hardly
reachable using other engineering methods (Cintas et al. 2015), especially in terms
of energy efficiency, and to efficiently process recalcitrant lignocellulosic biomass
affording high-value chemicals. Cavitation can increase pore sizes and accessible
surface areas, while also decreasing the polymerization and crystallinity degrees of
cellulose, and improving the biodegradability and enzymatic hydrolysis of biomass
residues. Moreover, significant decreases in pretreatment time and temperature are
the most attractive features of US-assisted biomass pretreatment. In fact, US induces
faster structure damage through several mechanisms: fragmentation, erosion, sonocapillary effect, sonoporation and local shear stress.
The aim of this chapter is to highlight the impact that acoustic and hydrodynamic cavitation have on biomass valorization by investigating the role that the key
parameters play in the sustainability of the overall processes.
4.2 Biomass Pretreatment
4.2.1 Acoustic Cavitation as Suitable Biomass Pretreatment
The structure of lignocellulosic materials makes pretreatment essential in order to
obtain fermentable sugars in the hydrolysis step. One of the most important limiting factors to obstructing cellulose hydrolysis is the lignin content. Indeed, lignin,
being a structural polymer, imparts strength to plant cell walls by covalently linking to (hemi)cellulose, thus preventing carbohydrate exposure for enzymatic attack