Page 162 - Lignocellulosic Biomass to Liquid Biofuels

Lignocellulosic biomass to biodiesel 135

• Lipid extraction
Lipid production is mainly related to several technical challenges
including (1) the control of C:N ratio, exceeding 65 and near 100
[33,34]; (2) sugar concentrations (almost 100 g/L); and (3) the presence of
inhibitors, as side-products formed during the pretreatments, which can
include furfural, hydroxymethylfurfural, acetic acid, phenolic acids, and
other compounds [32].

4.4 Preprocessing of lignocellulosic biomass (mechanical,
chemical, and biological)
The lignocellulosic biomass is characterized by a complex structure that
requires pretreatment processes in order to make the structure accessible
to hydrolytic enzymes [24] and, in the same time, to prevent carbohydrate
degradation since the carbohydrates represent the most essential substrate
for the microorganisms. The pretreatment of the lignocellulosic biomass is
also required to prevent the generation of inhibitors directly related to lig-
nin fraction. Thus pretreatment processes are of crucial importance, due
to their large impact on all subsequent steps of lipid production.
However, these processes are usually energy-expensive and significantly
affect process costs [35,36]. For this reason, one of the objectives is to
optimize and reduce the energy demand and to improve the processing of
lignocellulosic biomasses. The pretreatment methodologies can be classi-
fied in physical, chemical physical, chemical, and biological. A list of the
most promising and commonly employed pretreatment processes is
reported in Table 4.2. A single method cannot represent the universal
choice due to the variability of lignocellulosic biomass composition.
Usually, extra costs are necessary to solve the negative effects of pretreat-
ments on subsequent steps.

4.5 Hydrolysis of lignocellulosic biomass
4.5.1 Enzymatic hydrolysis

The pretreatment of lignocellulosic biomasses is important to facilitate the
access of hydrolytic enzymes to the crystalline structure Enzymatic hydro-
lysis of cellulose and hemicellulose is carried out under mild conditions
(pH 4.5 5.0 and temperature 40°C 50°C), which ensure reducing cor-
rosion problems, low energy consumption, and reduced generation of
toxic by-products [80].