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156 Lignocellulosic Biomass to Liquid Biofuels

Figure 4.6 Conventional process (A) and direct transesterification process (B) for
microbial biodiesel production. Adapted from J.Y. Park, M.S. Park, Y.C. Lee, J.W. Yang,
Advances in direct transesterification of algal oils from wet biomass, Bioresour. Technol.
184 (2015) 267 275 [164].

Biodiesel production through conventional process demands high
energy consumption during lipid extraction and biodiesel conversion pro-
cess. Recently, the combination of lipid extraction and biodiesel conver-
sion in one step, called direct or in situ transesterification, has been
investigated. Direct transesterification could be an alternative to reduce
the amount of necessary equipment. A comparison of the direct transes-
terification and the conventional process is shown in Fig. 4.6.

4.9 Catalysts for biodiesel synthesis

There are three categories of catalysts used for the transesterification:
alkali, acids, and enzymes. The limited use of enzyme catalysts is related
to their long reaction time and high cost, though they are attractive as
they avoid soap formation and promote simpler processes of purification.
Homogeneous alkaline catalysts are the more traditionally used for the
synthesis of biodiesel, as they offer higher reaction rates in comparison to
the acid catalysts. Hydroxides and methoxides from sodium and potassium
are the most commonly used alkaline catalysts.
Homogeneous acid catalysts offer a lower reaction rate in comparison
to their alkaline counterpart, though they are often used when the triglyc-
eride feedstock contains higher levels of free fatty acids, as in the case of