Synthesis of Biodiesel/Bio-Oil from Microalgae - Biotechnological Applications of Microalgae - page 103

rapidly and remains within specifications. Siegler et al. (2012) extracted oil from the
microalgae Auxenochlorella protothecoides and found it to be a potential source for
biodiesel production. The degree of unsaturation (DU) in the microalgal oil, which is
a measure of the unsaturated fatty acid content, was determined to be 137. Using the
DU, the cold filter plugging point value of the biodiesel was expected to be −12°C,
which can support the use of fuel even in cold climatic conditions.
Lardon et al. (2009) observed that despite biodiesel derived from microalgae
having immense potential to provide an alternative source of fuel, energy and
fertilizer consumption should be reduced for its economic viability. Using Chlorella
vulgaris as a model species, it has been found that a substantial portion of energy
consumption amounting to 70% and 90% of the total energy is used for lipid extrac-
tion when using wet and dry biomass, respectively. Hence, technologies must be
developed for economical extraction of oil from microalgal cells. Rosch et al. (2012)
advocate the reuse of residual algal biomass after oil extraction for the supply of
nutrients, which according to estimates may vary from 0.23 to 1.55 kg nitrogen and
29 to 145 g phosphorous (depending on the cultivation conditions of microalgae)
for the production of 1 L biodiesel.
8.3 THERMOCHEMICAL
The major cost attributed to the production of biodiesel is the dewatering and drying
step, which consumes 9 to 16 GJ of energy per ton of biodiesel produced (Chowdhury
et al., 2012). The dewatering and drying step can be negated if the microalgal oil is
subjected to pyrolysis for the synthesis of bio-oil as a biofuel. The thermochemical
method adopted for the preparation of fuel from microalgae is through pyrolysis,
where the organic compound is thermally decomposed at a high temperature in the
absence of oxygen. Zou et al. (2009) produced bio-oil by thermochemical catalytic
liquefaction of Dunaliella tertiolecta . A yield of 97.05% was obtained. The reaction
conditions were optimized and found to be H 2 SO 4 (2.4 wt%); reaction temperature,
170°C; and reaction time, 33 min. A high-quality bio-oil was produced that possessed
significant ester content. The bio-oil also possessed a low ash content of 0.4% to 0.7%.
However, the product had a low pH value (3.8 to 4.0) and thus necessitates storage in
acid-resistant bottles (e.g., polypropylene or stainless steel). Thermochemical treat-
ment resulted in a high calorific value of 28.42 MJ kg −1 . The bio-oil also had a low
nitrogen content compared to bio-oils produced by methods such as pyrolysis or direct
liquefaction. A high oxygen content was observed, thus providing the requirement
for deoxygenation of the bio-oil. The composition of microalgae bio-oil obtained
through thermochemical catalytic liquefaction consists of several methyl and ethyl
esters, which result from esterification between organic acids and the glycol solvent,
and is similar to that of biodiesel. Campanella et al. (2012) performed thermolysis of
microalgae (consisting of mixed wild culture with Scenedesmus sp . as the principal
constituent) and duckweed (primarily Wolffia and Spirodela species) in a fixed-bed
reactor using CO 2 as a sweep gas for the synthesis of bio-oil and called it “bioleum.”
The thermolysis of microalgae gave a higher bioleum yield in comparison to that from
the duckweed. This is attributed to the difference in composition of the two feed-
stocks. The fuel properties of the bioleum were found comparable to heavy petroleum