Page 57 - Lignocellulosic Biomass to Liquid Biofuels

36 Lignocellulosic Biomass to Liquid Biofuels

[106,107]. At a higher temperature, such as 172°C 227°C, the oxidation
is easier to occur, resulting in the formation of a large amount of fragmen-
tations, which are further decomposed to carbon dioxide, water, and
carboxylic acids [77]. During wet oxidation process, phenolic compounds
are further oxidized to carboxylic acids [6]. In addition, furfural and HMF
are not produced [86], which are potential inhibitors in the subsequent
fermentation process.
Wet oxidation has been successfully used for the pretreatment of
wheat straw, newspaper waste, maize silage, sugarcane bagasse, rice husk,
spruce, etc. [108]. Pedersen and Meyer [109] studied the effects of sub-
strate particle size and wet oxidation on physical surface structures and
enzymatic hydrolysis of wheat straw. The result showed that the released
glucose from the smallest particles achieved 90% of the theoretical maxi-
mum after 24 h enzymatic hydrolysis. Wet oxidation pretreatment could
tear up the surface structures of the particles, increasing the enzymatic
xylose and glucose yields by 5.4 and 1.8 times, respectively. After a wet
oxidation pretreatment at 200°C for 10 min with 12 bars of oxygen, the
sugar yield in a 72 h enzymatic hydrolysis could reach 79% of theoretical
for a bark-free Norway spruce, a softwood [110]. However, compared
with many other delignification pretreatments, the lignin obtained in
wet oxidation pretreatment cannot be used as a solid fuel, decreasing
the potential profit from by-products in large-scale production [91].
Moreover, the capital and maintenance cost for this process are high
primarily due to the high temperature and pressure used in the process.

2.3.4.2 Alkaline hydrogen peroxide
Alkaline peroxide pretreatment majorly refers to pretreating lignocellulosic
biomass with peroxide, typically hydrogen peroxide, under alkaline con-
dition. This pretreatment results in delignification of lignocellulosic mate-
rials with a cellulosic residue highly susceptible to enzymatic hydrolysis.
For alkaline hydrogen peroxide pretreatment the delignification reaction
is strongly pH-dependent with an optimum pH of 11.5 11.6, which is
the pKa for the dissociation of H 2 O 2 [111]. As a result of its decomposi-
2
tion, the highly reactive oxygen species superoxide (O ) and hydroxyl
2
radical (HO ) are produced, which are the primary species to oxidize lig-
nin [112]. The H 2 O 2 -derived radicals promote the depolymerization of
lignin by attacking the lignin side chains and fragmenting the lignin mac-
rostructure into low molecular weight compounds [113]. A number of
chemical changes occur in the alkaline peroxide pretreatment of biomass