Page 71 - Lignocellulosic Biomass to Liquid Biofuels

50 Lignocellulosic Biomass to Liquid Biofuels

hardwoods, and softwoods using SE and alkaline hydrogen peroxide treat-
ment in succession and found that the enzymatic digestibility was
improved by 2 2.5 times compared with the single SE pretreatment. Pan
et al. [189] performed a two-stage strategy to pretreat Douglas-fir by
steam pretreatment and extraction with cold 1% NaOH. They found that
NaOH extraction reduced the lignin content by only approximately 7%,
but cellulose to glucose conversion was enhanced by about 30%, reaching
85% at a cellulase loading of 40 FPU/g substrate. Guo et al. developed a
combined acid and alkaline hydrothermal pretreatment using dilute sulfu-
ric acid and lime sequentially for pretreating Miscanthus in order to obtain
high recovery of both hexose and pentose. Under the optimal condition,
this two-stage process achieved .80% glucose yield and .70% xylose
recovery, and ethanol yield of 0.145 g ethanol/g biomass in a subsequent
fermentation. These works have demonstrated that two-stage methods
combining acid and alkaline treatments can achieve higher sugar recovery
than single stage processes and require less enzyme loading for efficient
hydrolysis of cellulose. However, the process complexity is also increased
so that the economic feasibility of this combined process still needs
evaluation.

2.6 Concluding remarks and prospective

Due to the biomass recalcitrance of lignocellulose, pretreatment to disrupt
the cell wall structure to increase cellulose accessibility is a necessary step
for biological conversion of the structural polysaccharides to biofuels and
biochemicals. An ideal pretreatment process should be of high lignin
removal, high hemicellulose removal, minimal carbohydrate loss,
suitable for various lignocellulose raw materials with little formation of
inhibitors, without using expensive chemicals and a good potential for
large-scale application. Various pretreatment methods have been devel-
oped in the past decades. Most of them are efficient in laboratory scale to
enhance cellulose digestibility; however, only a few of the pretreatment
processes seem to be economically feasible in commercial scale. The
mode of action, mechanisms, advantages, and disadvantages of different
pretreatments are summarized and compared in Table 2.2. Each pretreat-
ment has its own merits and drawbacks. Currently, versatile pretreatment
which can well improve the digestibility of both woody and grass biomass
with low energy consumption and cost has not been available yet.
Combined use of different pretreatments may provide new insights to