Page 70 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 49

72%, while the SC-CO 2 combined with alkali pretreatment could achieve
20% higher glucose yield than alkali pretreatment only [183].

2.5.2.1 Sulfite pretreatment to overcome recalcitrance
of lignocellulose
Sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL)
pretreatment developed by Zhu et al. [184] is a combined pretreatment of
using sulfite to soften wood chips followed by size reduction with a disk
milling. SPORL pretreatment showed efficient improvement of cellulose
digestibility for woody biomass. In this process, sulfuric acid and aqueous
sulfite solution are used as the active chemical agents to cook woody chips
at relatively high temperatures (usually between 130°C and 170°C) for
different duration time. The cooked chips are then mechanically milled
for size reduction [39]. SPORL pretreatment partially removes hemicellu-
loses and lignin with great modification of lignin structure that dramati-
cally deconstructs cell wall structure and reduces the nonproductive
adsorption of cellulase on lignin. However, it has been found that hemi-
celluloses removal seems to be more crucial than delignification for the
improvement of cellulose digestibility. The amounts of inhibitor formed
in this process are low, with HMF and furfural of only 5 and 1 mg/g
untreated wood, respectively [184]. When spruce chips were pretreated
by SPORL process with 8% 10% bisulfite and 1.8% 3.7% sulfuric acid
on spruce wood (oven dried) at 180°C for 30 min, more than 90% of cel-
lulose conversion could be obtained. SPORL pretreatment also worked
well on Douglas-fir, poplar, and red pine [185,186]. To integrate the
influence of various key factors on SPORL pretreatment, a combined
hydrolysis factor has been defined and applied to predict xylan hydrolysis,
formation of inhibitors, and enzymatic digestibility of pretreated substrates
for woody and herbaceous biomass [187]. However, the disadvantage of
this pretreatment is the high cost of chemical recovery and effluent of
sulfur-containing wastewater if the formed lignosulfonate is not well
recovered [64].

2.5.2.2 Subsequent acid and alkaline pretreatments
Acid pretreatment may hydrolyze a considerable part of hemicelluloses,
while alkaline pretreatment is efficient to remove lignin. Therefore com-
bination of acid and alkaline pretreatments may achieve a fractionation of
lignocellulosic biomass with great exposure of cellulose. Maekawa [188]
pretreated a wide range of biomass materials including rice straw,