Page 286 - Advances in Eco-Fuels for a Sustainable Environment

Ecofuel conversion technology of inedible lipid feedstocks to renewable fuel 249

from microalgae Nannochloropsis sp. by multiple enzymes assisted with pretreatment
using an alkaline solution. Under extraction temperature of 50°C and 30min extrac-
tion time, 90% lipids can be extracted. However, the cost of the enzyme is quite high
and the reaction time is longer than other methods; therefore, applying enzymatic dis-
ruption is still challenging [18]. In order to obtain high extraction efficiency, a fast
process, and low energy consumption, combining mechanical and nonmechanical
methods may be the best option [15, 18]. Ultrasound treatment followed by solvent
extraction using hexane/methanol can extract 41–42wt% lipid from Chlorella proto-
thecoides [42]. Those sequential methods can reduce the energy required, reduce the
solvent amount, and obtain a high lipid yield. Other factors necessary to consider in
order to increase high lipid efficiency are the algae strain, the algae condition and
composition, and the energy consumption [18].
Microalgae oil is reacted with excessive alcohol to obtain biodiesel. The alcohol used
can be methanol, ethanol, butanol, propanol, or amyl alcohol, but the common alcohol
used is methanol due to its reactiveness and cheapness [18]. Acid, base, and enzyme can
be used as catalysts in transesterification. Those catalysts can be in either the homoge-
neous or heterogeneous form. Enzymatic catalyst requires milder operating conditions,
results in higher yield, and needs simpler purification. However, the reaction time
needed is longer and the production cost is more expensive than other catalysts. Base
catalyst provides a faster reaction rate in mild operating conditions. However, a base
catalyst is not suitable for microalgae oil due to its high FFA content. Chen et al.
[16] reported that the FFA content of microalgae oil can reach up to 70.3% while Krohn
et al. [75] found that the FFA content in oil extracted from microalgae D. tertiolecta was
84% while the FFA content in microalgae oil of N. oculata was 41.7%. For such oil with
high FFA content, the acid catalyst is the only choice for biodiesel production, even
though the reaction time is longer, it needs a higher temperature, and product purifica-
tion is quite difficult [15, 76]. Using a homogeneous catalyst causes difficulty in product
separation [76]. On the other hand, a heterogeneous catalyst is reusable, easily recov-
ered, and is environmentally friendly [77].Nonetheless, Chenet al. [78] found that solid
catalyst Sr 2 SiO 4 was less effective than the homogeneous catalyst because the dosage of
the solid catalyst can be 12 times that of the homogeneous catalyst and cannot be reused
due to chlorophyll adsorption on the solid.
In order to reduce energy consumption, especially for obtaining dry biomass, wet
in situ trans(esterification) has been introduced to convert lipids into biodiesel. In situ
trans(esterification) extraction and trans(esterification) occur simultaneously and a
higher yield of biodiesel can be obtained due to elimination of oil loss during solvent
extraction. However, in situ trans(esterification) needs to use more alcohol because
alcohol plays the role of reactant and solvent. Due to excessive alcohol used, recovery
of alcohol adds to the additional cost. In addition, high water content in wet micro-
algae can lead to hydrolysis of biodiesel into alcohol and FFA. Water in wet micro-
algae forms a thin layer [76] and is miscible with reactant [20], hence preventing the
extraction of lipids and the transesterification reaction. Adding a cosolvent such as
chloroform or hexane can be one way to overcome the drawback of using large
amounts of alcohol and the presence of water. Cosolvent improved lipid extraction
and facilitated the mass transfer of the reactant [20].Imet al. [79] reported that