Page 382 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Bioenergy generation from agricultural wastes and enrichment of end products 345

biomass is an essential step to break complex molecular structures to simple mono-
mers for a better output and is based on the pathway followed for the conversion
process (Kan et al., 2016). The main objective of the pretreatment is to enhance the
surface area, provide ease of accessibility to the enzymes, modify and solubilize the
lignin in case of biological processes, and also to minimize the total cost of opera-
tion. The various technologies for biomass pretreatment include physical (washing,
grinding, extrusion, etc.), thermal (steam explosion, torrefaction, and ultrasound/
microwave irradiation), biological (fungal, enzymatic, etc.), chemical (acid, alkali,
ionic liquids), and thermochemical. Effect of pretreatment on biomass is shown in
Fig. 15.4.
Physical pretreatment of lignocellulosic biomass includes milling, grinding,
chopping, extrusion, sonication, and high-pressure homogenization. The primary
objective of this process is to disintegrate the biomass into smaller particle size,
provide uniformity in its size, and enhance the surface area. The enhanced surface
area provides an easy access to bacteria and enzymes in biological processes,
whereas in thermochemical conversion, it promotes heat and mass transfer that
facilitate uniform temperature within the particles (Kan et al., 2016). Sonication is
another physical technique that uses sound energy to disrupt the particles. In a study
by Carrere et al. (2010), sonication of biomass before anaerobic digestion (AD)
enhanced the biogas yield in both batch and continuous processes. Use of gamma
rays is another method; however, it doesn’t change the particle size of the biomass
but it cleaves the glycosidic bonds in the biomass that decrease the cellulose crys-
tallinity and enhance the surface area. Extrusion of biomass involves high-pressure
treatment that converts biomass into pellet form which decreases the moisture con-
tent and enhances the volumetric energy density (Erlinch et al., 2006). The process
produces no odor, less energy consumption, but it clogs the equipment.
Thermal pretreatment is performed both at lab and industrial scale for easy
dewatering, viscosity reduction, and for pathogens removal (Edelmann et al., 2005).
For thermochemical conversion, drying and torrefaction are the two important pre-
treatment methods. Drying involves the removal of moisture from the biomass and
increases the efficiency of the process. In torrefaction, biomass is treated thermally
in inert atmosphere in the temperature range of 200 C 300 C, where sufficient

Cellulose

Physical
Lignin

Chemical Pretreatment Thermal

Biological

Hemicellulose
Figure 15.4 Effect of pretreatment on biomass components.