Page 66 - Lignocellulosic Biomass to Liquid Biofuels

Pretreatment of lignocellulosic biomass for efficient enzymatic saccharification of cellulose 45

pretreat wheat straw pellets using solid-state fermentation. The results
demonstrated that the longest time (35 days) was needed for P. chrysospor-
ium, while shortest for TV53J (21 days) to achieve a good pretreatment
efficiency. After biological pretreatment the combination and connection
between single fibers became loose, which were advantageous to improve
the porosity of the substrates [173]. Saha et al. compared 26 white-rot
fungal strains to evaluate the fermentable sugar production from corn sto-
ver under solid-state cultivation at 74% moisture and 28°C for 30 days.
Pretreatment with Cyathus stercoreus NRRL-6573 obtained the highest
sugar yield of 394 6 13 mg/g of pretreated stover, followed by treatment
with Pycnoporus sanguineus FP-10356-Sp (393 6 17 mg/g) and Phlebia bre-
vispora NRRL-13108 (383 6 13 mg/g) [173]. Biological pretreatment is
also combined with other pretreatment processes, such as chemical pre-
treatment and physicochemical pretreatment. Yan et al. studied biological
pretreatment combined with dilute acid pretreatment and found that bac-
teria could enhance dilute acid pretreatment. This was because the used
fungi Cupriavidus basilensis B-8 could work on the lignin droplets formed
in dilute acid pretreatment thus recovering cracks and holes on rice straw
surface and leaving an open and porous structure for the easy access of
enzymes to the inner cellulose. The enzymatic digestibility of rice straw
was increased by 35% 70% and 173% 244% in combined pretreatment
process compared to dilute acid pretreated and untreated rice straw,
respectively [174]. Biological pretreatment is also applied in improving
biogas production and other bioconversion processes [173]. However,
process optimization and intensification are still needed to make the bio-
logical pretreatment more efficient [4].

2.5 Combined pretreatments

As aforementioned, each pretreatment has its own merits and drawbacks.
In addition, the recalcitrance of different raw biomass is different; and
thus, no single pretreatment process is universally effective for all kinds of
biomass. Therefore to achieve a high pretreatment efficiency, different
pretreatments are usually combinedly used. Generally, single mechanical
milling pretreatment is energy intensive to achieve a high cellulose digest-
ibility; biological pretreatment is limited by its low efficiency and long
pretreatment time; dilute acid pretreatment is effective for herbaceous and
hardwood, but poor for softwood with remaining lignin; alkaline pretreat-
ment process works well with hardwood, agricultural residues, and also