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Jasper Wu
Professor Aubertin
WRIT – 340 (66813)
3 May 2013
Algal Carbon Sequestration: The Solution to Cutting Carbon Emissions?
Climate change is becoming more and more of an issue. As the human population continues to
increase, steps must be taken to ensure that our footprint on the Earth does not damage it
permanently. Research is underway regarding methods of reducing the amount of carbon
dioxide released into the atmosphere, as greenhouse gases are one of the main causes of climate
change. Algal carbon sequestration is one such process, appealing because algae grow through
photosynthesis, consuming carbon dioxide and organic compounds. This means that algal
growth can be nurtured via power plant emissions and wastewater, reducing both pollution and
the release of carbon dioxide. If carbon sequestration systems can be implemented, algae can
certainly decrease the carbon dioxide emissions while simultaneously generating additional
energy.
Introduction
Currently, 80 percent of the United States’ energy demand is met through the burning of
fossil fuels. In generating energy through the combustion of fossil fuels, huge quantities of
carbon dioxide are generated, a worldwide problem that has become a significant environmental
issue. As a result, carbon dioxide levels in the atmosphere have been significantly increasing
since the industrial revolution, to levels highest in the last 800,000 years [1]. While the topic of
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climate change is a controversial topic, it is generally agreed upon that increased carbon dioxide
levels lead to a more severe climate, as well as higher overall global temperatures.
There are two possible ways to approach this issue. One is to cut energy use, thereby
reducing the amount of fossil fuels combusted, and thus reducing our overall carbon footprint on
the world. However, this approach cannot accommodate for the world’s increasing population or
the industrialization of previously undeveloped nations. The other way to tackle the increasing
amounts of emitted carbon dioxide is to capture it after it is produced – in other words, producing
the same amount of energy, while allowing less of the carbon dioxide to escape into the
atmosphere. This process is known as carbon sequestration. Currently, there are three methods of
sequestering carbon – geological, chemical, and biological. While all three are viable ways to
reduce the amount of carbon dioxide in the air, only biological carbon sequestration through the
use of algae generates additional energy to be used. This makes the process one of the most
favorable solutions available today.
Current Power Plants
Most of American’s power comes from fossil fuel power plants, in large part because it is
one of the oldest methods of producing energy. Many of the original fossil fuel power plants in
the United States are from the early 20th century, and this is largely because no other power
plant is as developed or as easy to use [2]. Fossil fuels are a valuable resource, because they
contain lots of energy, while not consuming very much energy in generating the energy. Wind
and solar generators rely on environmental factors, and are expensive and cannot generate the
volume of power that fossil fuel power plants can. Nuclear is one of the few renewable energy
sources that can match the amount of power produced by conventional power plants, but pose
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extreme danger if not properly maintained. Not only are conventional power plants more
powerful, they are also much more refined than renewable energy sources. Coal power plants
have been in existence since 1901, and have been continuously improved in the last century [2].
As such, the efficiency of fossil fuel power plants is plateauing, as new improvements become
more and more marginal.
Petroleum-based power plants have the advantage of refinement and already being in
place, but do have some major drawbacks. These include high carbon dioxide emissions, as well
as significant amounts of waste heat that must be dealt with. The presence of a conventional
power plant significantly impacts the surrounding environment, and the carbon dioxide produced
greatly adds to the amount in the atmosphere.
Combining Technology
The appeal of algal-based carbon sequestration is that the process can be integrated right
into existing coal or natural gas power plants. The carbon dioxide produced by burning fossil
fuels can be used to feed the algae, which in turn generate biomass and can be converted into
biodiesel. Additionally, nutrients for the algae can be provided by wastewater, another benefit of
using algae to sequester carbon. This is engineering at its finest – taking the problems and
unwanted byproducts of existing processes, and generating favorable products and other
additional benefit from it.
To allow algae to develop, rows of transparent tubes, or photobioreactors, are placed on
racks optimizing the amount of received sunlight. Exhaust gas from the power plant is scrubbed,
and then piped directly into the photobioreactors to provide carbon dioxide and heat for the algae.
Additionally, wastewater or other nearby water rich in organic compounds is also piped into the
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photobioreactors, to provide nutrients for the algae. Because the algae are so efficient at
absorbing sunlight, and thus reproduce quickly, the algae can be harvested often. The harvesting
of algae is actually the most energy-intensive step in generating biofuel, with almost ninety
percent of the total energy spent on harvesting and drying the algae [3]. Many techniques are
used to harvest algae, such as coagulation, flocculation, sedimentation, among other processes.
As carbon sequestration becomes more widely used, the techniques will surely become more
efficient and streamlined. The collected algae are then processed into biofuel, through the
fermentation or the metabolisms of the algae themselves. The production of biodiesel from algae
is both technically and economically plausible, though much research and development are
needed to increase overall efficiency [4].
Why Algae?
Algae are one of the world’s most efficient organisms at absorbing sunlight through
photosynthesis, the process in which oxygen and glucose are produced from carbon dioxide,
water, and sunlight. In fact, even though algae only represent half a percent of the world’s
biomass, it produces almost seventy percent of the world’s oxygen [5]. In addition, algae
reproduce rapidly, resulting in rapidly increasing quantities of biomass. Both the process of
photosynthesis, and the process of reproduction absorb carbon dioxide, so the fact that algae
undergo both processes rapidly make it a perfect fit for sequestering carbon dioxide.
There are many kinds of algae, though they can be categorized into three groups –
emergents, macroalgae, and microalgae. While all three share the properties described earlier,
only microalgae has been successfully manipulated to produce usable oil and biomass.
Microalgae can be categorized even further, into four main groups separated in part by color –
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diatoms, golden algae, green algae, and blue-green algae. Each kind of microalgae has its own
advantages in terms of carbon sequestration and creation of biomass, but for the purposes of this
article they will all be referred to as microalgae.
Photobioreactors
Microalgae are cultivated in environments known as photobioreactors, in which nutrients
are added and the algae are allowed to reproduce. Photobioreactors for cultivating microalgae
can be either open or closed systems. In an open system, a pond or other body of water where the
algae grow is openly exposed to the environment and sunlight. In a closed system, the algae are
not directly exposed to the environment; instead, it is enclosed within transparent tubes or plates,
allowing the photosynthesis process to occur without disruptions from the outside environment.
This provides many benefits, as the open system could allow the algae to mutate, and the closed
system allows for more careful monitoring of conditions. In addition, closed systems have better
mass transfer, and can sustain larger amounts of algae in the same space [6]. For these reasons,
closed systems are preferable for algal carbon sequestration.
Many factors affect the effectiveness of photobioreactors, such as light intensity, amount
of carbon dioxide, levels of generated oxygen, temperature, and nutrient requirements of the
algae. Light intensity for photobioreactors can be increased through the use of thinner tubes,
which allow for more thorough sunlight penetration, or through internal illumination. However,
internal illumination adds to the energy used by the entire process, so the increased amount of
biomass produced must be weighed against the increased energy usage. Additionally, the
photobioreactors can be mounted on a rotating rack, so that more sunlight can be consistently
reaching the algae. Carbon dioxide is consumed and oxygen is produced in the photosynthesis
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process, and as carbon dioxide is depleted and oxygen is created the process will slow down due
to chemical equilibrium. By removing oxygen and supplying carbon dioxide (from the power
plant), maximum photosynthesis can occur. To accomplish this, the gas levels are monitored and
adjusted regularly. Because photosynthesis is a chemical reaction, increased temperature will
increase the rate at which the reaction occurs. The optimal temperature for algal growth is around
15 to 26 degrees Celsius, with maximum cell density achieved at 23 degrees Celsius. [7].
Nutrients are also necessary to assist in the growth of algae. Surprisingly, the organic
compounds present in wastewater are perfect for nurturing the growth of algae. In fact, another
environmental benefit can be provided by the algal carbon sequestration system is that the algae
can help synthesize organic compounds from wastewater in the process of sequestering carbon.
Essentially, algal photobioreactors work exceptionally well with the waste generated from
wastewater treatment and power plants, and even generate energy in the form of biofuel in the
process.
The Bottom Line
While not a comprehensive solution, carbon sequestration based on algae in
photobioreactors can provide a significant decrease in carbon emissions, while actually
increasing the amount of energy produced at fossil fuel-based power plants. The potential for a
significant reduction of carbon dioxide emissions from power plants is huge, as most of the
world’s pollution is from energy generation. While still in trial stages, algal carbon sequestration
should be invested in, as it provides a multiple benefits achievable simply by modifying the
existing energy infrastructure. However, stopping climate change is much more complex than
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simply reducing carbon emissions. More needs to be done, and carbon sequestration is but a
partial solution to a much larger problem.
Works Cited:
[1] W.K. Stevens. “Seas and Soils Emerge as Keys to Climate”. New York Times.
[2] S. Yeh and E.S.Rubin. "A centurial history of technological change and learning curves for
pulverized coal-fired utility boilers." Energy 32.10 (2007): 1996-2005.
[3] L. Lardon et al., (2009). “Life-cycle assessment of biodiesel production from microalgae”.
Environ. Sci. Technol. 43, 6475–6481.
[4] J. R. Benemann and P.M. Pedroni (2008). “Biofixation of Fossil CO2 by Microalgae for
Greenhouse Gas Abatement”. Treccani Encylopedia of Hydrocarbons, Volume III, pp 837
-861.
[5] J. Hall. “Earthworks & Systems: the most important organism?”. Ecology Global Network.
Internet: http://www.ecology.29. com/dr-jacksnatural-world/most-importantorganism/index.html.
[6] H. Znad et al., “CO2 Biomitigation and Biofuel Production Using Microalgae:
Photobioreactors Developments and Future Directions” in Advances in Chemical Engineering.
Rijeka, Croatia: InTech, 2012, ch. 9, pp. 229-241.
[7] H. Tamiya, H “Mass culture of algae”. Ann. Rev. Plant Physiol. 8, 1957 pp. 309–334.