Exxon Mobil and genome expert Craig Venter hope
to strike it green with oilgae, but a few obstacles
remain on the path to commercialization of biofuel
Earlier this summer, Exxon Mobil announced that
it plans to produce algae-based biofuels in partnership
with Synthetic Genomics, a biotechnology company
founded by human genome pioneer Dr. J. Craig Venter.
Yet even with its US $600 million investment,
Exxon representatives have said that large-scale
production of algal biofuels is still "5-10 years
away." Indeed, there are many pros and cons to
using algae as a biofuel feedstock, as well as
hurdles to its commercialization.
|"Generally, only a portion
of the crude algal oil is suitable for making
biodiesel, but all of it can be used to make
gasoline and jet fuel."
--Dr. Yusuf Chisti, Professor of Biochemical
Engineering, Massey University
The Algal Advantage
Algae have indisputable advantages as a biofuel
feedstock. Of all the green fuel options, "only
algae appear to have the potential to provide
the huge quantities of renewable oil required
for substantially displacing petroleum-based transport
fuels," said Dr. Yusuf Chisti, Professor of Biochemical
Engineering at Massey University in New Zealand,
whose laboratory researches the cultivation and
processing of algae for biofuel production.
Microscopic algae yield up to 100 times more
oil per acre than soybeans and other common biodiesel
feedstocks, according to Mary Rosenthal, Executive
Director of the Algal Biomass Organization. Microalgae
can be up to 80% oil by dry weight, although that
number is for wild strains that are slow growers,
according to Dr. Margaret McCormick of the technology
company Targeted Growth. Genetically engineered
microalgae, such as those created by Targeted
Growth, approach 35%-45% oil by dry weight, but
achieve dense cultures in one day. Through genetic
manipulation, scientists can also control the
oil composition, and generate strains specialized
for particular growth conditions, such as high
salinity or temperature extremes.
When grown photosynthetically, microalgae are
a two-for-one environmental benefit - CO2 mitigation
plus a renewable energy source. Microalgae can
capture sunlight 20-40 times more efficiently
than plants, and unlike corn- or soy-based feedstocks,
they do not create a "food or fuel" dilemma. Some
can be cultured using seawater. Finally, much
of the groundwork for algal biofuels was done
by the United States Department of Energy Aquatic
Species Program, which developed strains, techniques
and pilot programs from 1978-1996.
From Cells to Oil: Many Paths
The versatility of microalgae means it's hard
to predict the most promising avenue for harvest,
processing and finally commercialization. While
more than 40,000 wild algal species exist, algal
biofuel leaders like Solazyme and Sapphire Energy
use genetically selected or engineered strains
for oil production, according to company representatives.
In addition to growing photosynthetically, with
sunlight as an energy source and CO2 as a carbon
source, microalgae can be grown heterotrophically,
using sugar, glycerol or cellulosic biomass for
energy and carbon. Solazyme uses the latter technique,
which gives up the solar advantage in exchange
for faster growth, a higher culture density for
easier harvesting and a process that fits the
existing industrial fermentation infrastructure.
Solazyme's heterotrophic cultivation requires
growth in a closed tank system, or bioreactor.
Other companies like Sapphire Energy and Solix
Biofuels grow microalgae photosynthetically, Solix
in photobioreactors and Sapphire Energy in ponds
on non-arable land.
Once the microalgae are cultivated, biofuel manufacturers
are faced with two major technical hurdles: harvesting
and dewatering. Microalgae cultures can be 80%-90%
water, so cells must be collected by settling,
which is time-consuming, although this can be
hastened with flocculating agents that cause cells
to clump and precipitate. More high-tech methods
like centrifugation and filtering are faster,
but are more costly in both dollars and energy.
Once harvested, cells may be air- or sun-dried,
requiring a large surface area and significant
time, or they can be dried using heat or a vacuum,
again increasing the cost and reducing energy
Finally, extracting the oils is another challenge.
Options include extraction with solvents like
hexane, enzymatic digestion of cell walls, or
physical disruption with ultrasonic sound waves
The Exxon-Synthetic Genomics partnership genetically
engineers strains to continuously secrete oil.
Professor Chisti explains that in the future,
microalgae might be engineered to "rupture at
a certain age and release their oil content."
In either method, the complexities of collecting,
drying and breaking open the algal cells would
be bypassed since the oil could be harvested by
simply skimming the culture.
Powering Trucks and Jets
Oil obtained from microalgae can be used as a
straight vegetable oil fuel, but this requires
a modified engine. Dr. Eric Jarvis, a scientist
at the National Renewable Energy Laboratory (NREL),
said that while the home hobbyist might enjoy
modifying engines to use algae biofuel, "no one
wants to do it at the commercial level."
Biodiesel can be used in existing diesel engines
and is produced by straightforward and established
transesterification technology. This chemical
reaction starts with simple triglyceride lipids,
which are fats and oils from plants, waste foods
or algae. The triacylglycerols are chemically
reacted with alcohol, with the help of enzymatic
or chemical catalysts. The resulting biodiesel
has the characteristics of petroleum diesel and
can be used alone or in a blend.
The big pay-off in algae biofuels will be as
drop-in replacements for gasoline or jet fuel.
Successful test flights have already been run
on mixtures of petroleum and algal-based jet fuels.
Chisti says, "generally, only a portion of the
crude algal oil is suitable for making biodiesel,
but all of it can be used to make gasoline and
jet fuel." For this, the fatty acids in the algal
oils are refined by hydrogenation and hydrocracking.
NREL’s Jarvis believes the refinery pathway has
the most flexibility, in part because the techniques
are already established for petroleum. He says
that "oil chemists know how to do the cracking
and hydrogenation, so they can change the fatty
acids into what they need." Also, refining is
necessary "to get the energy-dense targets like
jet fuels. You can't use ethanol on airplanes."
In addition, less refined products have problems
with gelling, which Jarvis cautions, "you don't
want happening at 30,000 feet."
Even with the proven potential of algal biofuels,
cost-effectiveness is an issue. Biofuels currently
compete with petrochemical fuels, which have economy
of scale. A 2007 analysis of the economics of
algal biofuels by Chisti suggested that a five-fold
reduction in production costs was needed to compete
with plant- or petroleum-based diesel. Now, Chisti
says, "issues relating to climate change may leave
us with no choice but to replace petroleum fuels
with renewable, carbon-neutral algal fuels, despite
a somewhat higher cost."
Algal-based Biofuel Manufacturing Yields Valuable
Algal biofuel manufacturers have another ace
up their sleeves: coproducts. Algae excel at making
complex organic compounds like B and C vitamins
and beta-carotene that are used as fragrances,
flavorings, pigments and supplements. These can
sell for hundreds of dollars a kilogram, so harvesting
both the coproducts and feedstock oils can potentially
offer manufacturers another revenue stream and
make cultivating and processing microalgae more
Even after lipid and coproduct extraction, the
remaining proteins and carbohydrates in the biomass
can be used as animal feed, or fermented by anaerobic
bacteria to generate methane. The coproduct strategy
lets algae manufacturers achieve economic feasibility.
Plus, the Exxon-Synthetic Genomics partnership
gives algal biofuels a big publicity boost. Dr.
McCormick of Targeted Growth says it's "great
for the industry…this shows that companies are
looking to see how they can make algae work for
them, and we welcome that investment."
Chris Tachibana, Ph.D, is a science writer
based in Seattle and Copenhagen, Denmark. Visit
her website here.