Page 44 - Advances in Eco-Fuels for a Sustainable Environment

Ecofuel feedstocks and their prospects 21

Algae (and microalgae) consist of a wide range of autotrophic organisms, growing
through photosynthesis, that can mitigate CO 2 emissions and provide a valuable sub-
strate for producing diverse fuels due to two specific characteristics. First, algae pro-
duce oil that can be easily refined into diesel or even gasoline components; second,
fermentative bacteria can degrade the biomass for butanol production and, more
importantly, can be genetically manipulated to directly produce a number of fuels,
including biodiesel, butanol, gasoline, methane, ethanol, vegetable oil, and jet fuel
[13, 14]. Table 2.4 reports the composition of microalgae cell walls and energy storage
products while Table 2.5 shows the carbohydrate content of different algal species.
Butanol (C 4 H 10 O) is a four-carbon alcohol primarily used as a solvent, an interme-
diate in chemical synthesis, and fuel. It is called biobutanol when produced biologi-
cally, and is of great industrial interest because it is exceptionally similar to gasoline,
with nearly identical energy density and an improved emissions profile. Until the
introduction of genetically modified algae, production of butanol was quite challeng-
ing, but today several commercial-scale facilities have been developed and are
expected to make butanol more popular than ethanol, not only due to its gasoline sim-
ilarity, but also because it does not cause engine damage or require modifications like
its predecessor.
Another major advantage of algae as feedstock is that they are capable of excep-
tional yields: they have been shown to produce up to 15000L biofuel per hectar,
10 times the best output ever generated by second-generation feedstocks.
Researchers have estimated that yields as high as 35000L/ha could be attainable
in the near future; according to the US Department of Energy (DoE), this means that
only 0.42% of the US land area would be needed to generate enough biofuel to meet
the country’s needs.
Algae can use a wide array of carbon sources: algae-growing facilities could, for
example, be tied to carbon-emitting point sources (power plants, industries, etc.) to
convert emissions into fuel directly without CO 2 release in the atmosphere, reducing
dramatically total emissions.

Table 2.4 Composition of microalgae cell walls and energy storage products [15, 16]

Storage
Cell wall products
Cyanophyta Lipopolysaccharides, peptidoglycan Cyanophycean
starch
Chlorophyta Cellulose, hemicellulose Starch/lipid
Dinophyta Absence or contain few cellulose Starch
Cryptophyta Periplast Starch
Euglenophyta Absent Paramylum/
lipid
Heterokontophyta Naked or covered by scales or with large Leucosin/lipid
quantities of silica
Rhodophyta Agar, carrageenan, cellulose, calcium Floridean starch
carbonate