Page 287 - Advances in Eco-Fuels for a Sustainable Environment

250 Advances in Eco-Fuels for a Sustainable Environment

chloroform gave the best performance as cosolvent in in situ trans(esterification) of wet
microalgae. Adding chloroform as the cosolvent resulted in 90.6% biodiesel yield in wet
microalgae with high moisture content (65%). Another research by Im et al. [80]
reported that the yield of biodiesel can increase 13.44% with chloroform as the
cosolvent. However, Kim et al. [81] stated that pure methanol was more effective in
wet in situ trans(esterification) because it resulted in a biodiesel yield up to 99.7%.
The ratio of chloroform to alcohol affected the biodiesel yield due to the different func-
tions of those solvents in in situ trans(esterification). The more alcohol in the solvent
mixture, the more oil can be converted into biodiesel because the cosolvent improved
lipid extraction and the mass transfer between oil and alcohol [76, 82]. Although the
cosolvent has a beneficial effect in wet in situ trans(esterification), separation and envi-
ronmental issues should be taken into account on cosolvent utilization.
Factors affecting biodiesel conversion in wet in situ trans(esterification) are micro-
algae species, temperature, moisture content, reaction time, ratio of biomass to alcohol
and to catalyst, and the agitation rate [83]. In general, severe operating conditions
result in higher biodiesel yield because mass transfer, solubility, and reaction rate
increase with temperature [82]. Sulfuric acid is the main choice of acid catalyst
due to its cost effectiveness. However, based on research conducted by Kim et al.
[84], HCl as the catalyst can give 15wt% higher FAME than using H 2 SO 4 as the cat-
alyst. Even with high moisture content (80%), the FAME yield obtained was 90%.
Using HCl as the catalyst, the biodiesel yield was not affected by moisture content,
cosolvent addition, and temperature change. Lower reaction temperature and easier
separation make the enzyme worth considering as the catalyst. Lopez et al. [85] car-
ried out in situ transesterification using high-pressure homogenization as the pre-
treatment process; 99.5% FAME can be obtained using enzyme as the catalyst.
However, the high cost of the enzyme, the low stability, and the low reaction rate make
the enzyme rarely used as the catalyst [85]. Table 9.6 shows the results of several cat-
alytic processes both in in situ trans(esterification) and two-step transesterification of
producing biodiesel from microalgae.
The high cost and energy intensiveness of several processes in microalgae-based
biodiesel production make commercial production unfeasible at this moment. The
drying cost has been known as the limiting factor in the economic feasibility of
microalgae-based biodiesel production because it consumes a huge amount of energy
[19–21]. To reduce energy consumption, elimination of the drying step by using wet
microalgae directly as the feedstock is one alternative. Catalytic biodiesel conversion
technologies face some hurdles in converting of wet microalgae into biodiesel due to
high water content will reducing catalyst concentration and it decreasing yield of bio-
diesel. Therefore, noncatalytic conversion technologies have been developed to over-
come those obstacles.
The noncatalytic process is conducted under either subcritical or supercritical con-
dition. Both supercritical and subcritical processes need higher temperature and pres-
sure than the common catalytic processes. Even though they are energy intensive,
subcritical/supercritical processes have several advantages: (1) under subcritical/
supercritical conditions, biodiesel conversion does not depend on the initial feedstock
condition such as water and free fatty acid (FFA) content. Thus microalgae with