Page 383 - Refining Biomass Residues for Sustainable Energy and Bioproducts

346 Refining Biomass Residues for Sustainable Energy and Bioproducts

oxygen is removed from the biomass including water. Advantages of torrefied bio-
mass include better grindability, high-energy density, lower hygroscopic nature, and
better feeding in the reactor. However, torrefaction improved the quality of syngas
produced by reducing the carbon dioxide and increasing hydrogen and methane
content (Ren et al., 2013). In case of biological conversion processes, treatment of

biomass in the temperature range of 50 C 250 C enhances the digestibility of bio-
mass and also removes the pathogens. Thermal treatment includes steam explosion,
hydrothermal treatment, liquid hot water, microwave heating, and ultrasound irradi-
ation for biomass degradability. In steam explosion, the biomass is exposed to a hot
pressed fluid at high pressure and in the temperature range of 150 C 240 C for a

few minutes and then it is depressurized that explode the biomass leading to break-
age of carbohydrate linkages which ultimately enhances biomass property (Biswas
et al., 2011). Liquid hot water pretreatment is generally operated in the temperature
range of around 180 C 190 C with low drying matter content (1% 8%). Hot

water cleaves the hemiacetal linkages that liberate acids during biomass hydrolysis
and leads to ether linkages breakage in the biomass (Wyman et al., 2005).
Microwave heating and ultrasound irradiation are the other alternative pretreatment
methods at present. Ultrasound improves the AD process by enhancing the biogas
yield whereas microwave heating creates hot spots in the biomass (Bundhoo et al.,
2013). Unlike various advantages, the prolonged treatment of biomass at higher
temperature can cause unexpected reactions (Maillard reactions) that can form
inhibitory substances and decrease the efficiency.
Biological pretreatment involves the utilization of different types of enzyme and
fungi. When compared, other pretreatment methods are less energy consuming
because they are performed at milder conditions and economical (Yu et al., 2013)
but are slower as it requires several days. This pretreatment is performed by inocu-
lating the substrate with fungal spores (e.g., white rot basidiomycetes and actinomy-
cetes) or by enzymes (e.g., ferulic acid esterases and hemicellulases) (Lloyd and
Wyman, 2005). Enzymes are used for the hydrolysis of lignin. White-rot fungi were
used for the degradation of lignin while minimizing the polysaccharide consump-
tion (Sun and Cheng, 2002). Biological pretreatment of agricultural wastes using
rot-fungi or rots is a green technique and economical that does not involve any
energy input for lignin degradation. Biological pretreatment basically involves the
use of various types of fungi for lignocellulosic biomass.
Chemical pretreatment includes the involvement of different chemicals, such as
acids, alkalis, or ionic liquids to break down the organic components present in the
biomass. Basic principle of this pretreatment is to break the lignin carbohydrate
bond and crystalline cellulose structure. Different acids for the pretreatment are
H 2 SO 4 , HCl, HNO 3 , and H 3 PO 4 . Dilute acid pretreatment can be performed in
either batch or continuous mode (Lloyd and Wyman, 2005). The presence of certain
elements provides catalytic role and enhances the degradation process but its exces-
sive use sometimes degrades the process by the loss of fermentable sugar or
increase in the pH which requires neutralization. Dilute acid based treatment of
lignocellulosic biomass was carried out for the production of furfural (Zeitsch,
2000). In biological conversion processes, alkali pretreatment is more preferred.