Page 158 - Lignocellulosic Biomass to Liquid Biofuels

132 Lignocellulosic Biomass to Liquid Biofuels

estimated that to produce a cost-competitive lignocellulosic biodiesel, the
price of the oil required is in the range of $70 $85/bl [20]. Some studies
estimated the value that can be potentially generated by the lignocellulosic
industry. Hertel et al. [21] gave a value to second-generation industry of
$64 billion under baseline conditions.
A number of novel technologies are being developed to refine ligno-
cellulosic biomass for the production of renewable oil and green mono-
mers [22]. In addition, the number of biorefinery-related pilot and
demonstration plants is increasing [23], though only few companies, such
as Lignol, Verenium, and Mascoma, have so far focused their attention on
the development of biorefining technologies for the production of
advanced biodiesel, biochemicals, and biomaterials from nonfood cellu-
losic biomass feedstocks [18].
Lignocellulosic biomasses consist of micro- and macrofibrils organized
in crystalline structures, that need to be pretreated in order to promote
access to their cellulose, hemicellulose, lignin and small amounts of pectin,
protein, and ash fractions in the subsequent hydrolysis step [24]. Cellulose,
hemicellulose, and lignin fractions depend by species, age, and growing
conditions of lignocellulosic biomasses [25,26].
The cellulose is the most abundant constituent of plant cells. It is a
polysaccharide characterized by a linear sequence of D-glucose molecules
linked by β-1,4-glycosidic bonds with a high degree of polymerization
equal to 10,000 or even higher. It consists of intra- and intermolecular
hydrogen bonds able to influence the specific crystallinity. It is chemically
stable and resistant to microbial degradation [27,28]. The cellulose fibrils
are responsible for the great tensile strength of the cell wall [29]. This
structural and inherent integrity of cellulose plays an important role in the
recalcitrance of lignocellulosic biomass [30]. The hydrophobic surface
involves the formation of water layer that prevents the diffusion of
enzymes and the degradation of the products on the surface [31].
Hemicellulose is the second most abundant polymer of plant cells.
Unlike cellulose, hemicellulose has a random and amorphous structure,
which is composed by several monomers including D-glucose, D-galactose,
D-mannose, D-xylose, L-arabinose, D-glucuronic acid, and 4-O-methyl-D-
glucuronic acid. It is characterized by a degree of polymerization lower
than 200. Hemicelluloses are embedded in the plant cell walls to form a
complex network of bonds that provides structural strength by linking
cellulose fibers into microfibrils and cross-linking with lignin.