Page 196 - Lignocellulosic Biomass to Liquid Biofuels

158 Lignocellulosic Biomass to Liquid Biofuels

significantly affected by the improvements offered by the metabolic and
genetic engineering.

4.11 Future prospects and conclusions
The biorefinery concept is becoming more attractive, taking into account
that the present economic model based on the nonrenewable fossil
resources for energy and industrial production is the reason of energy
instability, climate changes and cannot be considered sustainable. An
important output of biorefineries is the biodiesel, a biofuel increasingly
used as it reduces the net greenhouse generation. The cost of biodiesel is
primarily affected by the cost of the feedstock used, and secondarily by
the process adopted. Consequently, lignocellulosic biomasses are increas-
ingly studied as a feedstock for biodiesel production, since they are the
most abundant carbon-neutral renewable source and can be used as a sus-
tainable source of organic carbon with net zero carbon emission.
The hydrolysis of cellulose and hemicellulose (the main components
of lignocellulosic materials) is required to obtain fermentable sugars. In
this view a preliminary pretreatment of lignocellulosic biomasses is of cru-
cial importance to facilitate the access of hydrolytic enzymes to the crys-
talline structure. The correct research is devoted to solve the problems still
affecting this process, stemming from the loss and the inhibition of the
hydrolytic enzymes.
Different microorganisms can be used to obtain SCOs using the mix-
tures of fermentable sugars as feedstock. Microalgae are the most promis-
ing, due to the high flexibility of their metabolism, allowing higher
growth rates under operating conditions of industrial interest, as well as
higher cellular lipid fractions. Lignocellulosic hydrolysates can be used to
promote microalgal growth in a heterotrophic regime. In particular, mix-
otrophic cultures of microalgae are of great interest, as they sum the
advantages of heterotrophic and autotrophic cultures.
Significant advantages in biodiesel production can be achieved by
suitable choices of the reactor configuration. SSF offers very interesting
perspectives, as it prevents the inhibition phenomena and reduces the
investment costs.
Other factors affecting the economical balance of microalgal produc-
tion of biodiesel are the extraction of cellular lipids, and the choice of the
catalyst for esterification/transesterification reaction. These aspects require
a research effort to envisage satisfactory solutions.