Page 39 - Lignocellulosic Biomass to Liquid Biofuels

18 Lignocellulosic Biomass to Liquid Biofuels

to biodegradation of cell wall structure by enzymatic and microbial attack,
a suitable pretreatment process is required to facilitate the enzymes’ access
to the plant polysaccharides [1]. Pretreatment is a prerequisite step for bio-
conversion of lignocellulosic biomass and has a significant impact on the
cost of downstream processing including detoxification, enzymatic hydro-
lysis, waste treatment, and other variables [2]. It has been reported that
pretreatment constitutes more than 40% of the total conversion process
cost in some cases [2]. Many pretreatment methods have been developed
to remove the hemicelluloses and/or lignin, break down cell wall struc-
ture, increase specific surface area, create more pores, decrease cellulose
crystallinity, and so on, in order to promote the enzymatic hydrolysis or
any other microbial decomposition of cellulose [3]. Therefore pretreat-
ment has been considered as a key technical barrier and economic con-
sumption contributor to biomass bioconversion. A number of research
works have been published to improve the techno-economic efficiency of
pretreatment for cost-competitive production of biofuels and chemicals
from biorefining of biomass [4].
The different nature of biomass feedstocks makes the working
mechanisms of pretreatment techniques quite different. There is no gold
universal pretreatment for all biomass. Therefore a variety of pretreatment
methods have been tested during the last decades but each method still
has its own drawbacks and advantages. In general, pretreatment of biomass
can be classified into four categories according to their function mode and
the reagent used, namely, physical, chemical, biological, and combined
(hybrid) pretreatment, such as physicochemical pretreatment or subse-
quent use of physical, chemical, and/or biological pretreatments.
The ideal pretreatment should be effective and of low cost. There are
several important aspects that should be taken into consideration for pre-
treatment process as follows [5,6]:
(1) Wide adaptation for different raw materials, including crops species,
sites ages, and harvesting times.
(2) Highly digestible pretreated solid. Cellulose from pretreated sub-
strates should be easily, effectively, and highly hydrolyzed with yield
of more than 90%.
(3) No significant sugar degradation with the minimum formation of
toxic compounds. The yields of fermentable cellulosic and hemicel-
lulosic sugars achieved during the pretreatment steps should be close
to 100%. The liquid from the pretreatment process should be
fermentable with a low-cost and high yield.