Page 53 - Lignocellulosic Biomass to Liquid Biofuels

32 Lignocellulosic Biomass to Liquid Biofuels

2.3.3 Alkali-catalyzed pretreatment
2.3.3.1 Alkaline delignification
Alkaline delignification is one of the most viable pretreatment approaches
because of its strong pretreatment efficiency and relatively simple process
scheme. It utilizes various alkalis, such as sodium hydroxide, sodium car-
bonate, calcium hydroxide (lime), and ammonia (aqueous, liquid, and gas-
eous), as active chemicals for delignification. The alkali digests the lignin
structure and breaks the linkages between lignin and carbohydrate, which
makes the carbohydrates in the biomass more accessible for enzymatic
degradation [75]. Alkali pretreatment can also make the biomass swelling,
hence increasing the internal surface area and porosity of the biomass and
decreasing both the DP and cellulose crystallinity [76]. Generally speaking,
alkali delignification is more effective for grass species with low lignin
content than woody species, especially softwood [77]. In addition, alkaline
delignification pretreatment is not as energy-intensive as some of the other
pretreatments since it can be carried out at lower temperatures and pres-
sures [78]. Alkaline delignification can be performed at ambient condi-
tions; however, longer pretreatment times, usually hours or days rather
than minutes or seconds, may be needed to achieve the same level of
digestibility [79]. Compared with acid pretreatment, alkaline delignifica-
tion has less influence on sugar degradation, but a disadvantage occurs
since some of the alkalis are converted to irrecoverable salts or incorpo-
rated as salts into the biomass during the pretreatment [76]. In alkaline sys-
tems, β-aryl ether bonds in both phenolic arylpropane units and
nonphenolic arylpropane units can be cleaved. Alkaline cleavage of β-aryl
ether bonds is shown in Fig. 2.3 [65,80].
Pretreatment of biomass with NaOH has been studied for a long time
and received much attention. NaOH pretreatment is usually performed
with concentration ranging from 0.5% to 10% for 1 24 h at room tem-
perature to 180°C [81]. Compared with 20% saccharification (g sugar/g
stover) of untreated corn stover, 52% saccharification of the pretreated
substrates during enzymatic hydrolysis was obtained by 2% NaOH pre-
treatment at 150°C for 5 min. Soto et al. found that more delignification
was achieved when using higher NaOH concentration; nevertheless, sac-
charification was negligibly affected by alkali concentration [82]. Using
lime instead of NaOH may reduce the cost of the alkali agent, and lime
seems to be especially suitable for agricultural residues, such as corn stover
or hardwood materials, for example, poplar [83]. Lime is an inexpensive
and safe alkali. It could be recovered as calcium carbonate through an