How CO2 Scrubbing Works
by Jennifer Horton
Browse the article How CO2 Scrubbing Works
Introduction to How CO2 Scrubbing Works
These are perilous times we live in. The Intergovernmental Panel on Climate Change recently declared that to avoid
disastrous temperature increases, global greenhouse gas emissions must not rise after the year 2015. Yet the International Energy Agency predicts our power usage will increase 50 percent by the year 2030, and if past performance is
any indication, it's probably right [source: The Guardian].
Having burned roughly 551 billion tons of carbon since the beginning of the industrial revolution, the world's
atmospheric concentration of the gas is now 100 parts per million higher than it was just a little more than 200 years
ago [source: The Guardian]. With data like that clogging up the air, it's pretty clear humans may have to make some
pretty weighty decisions regarding their energy consumption: Goodbye SUV; hello hybrid.
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That's not fog: It's smog -- evidence that the air is becoming ever more polluted with greenhouse gases.
But what if we could have our cake and eat it too? Or in the case of carbon sequestration and CO2 scrubbing in
particular -- our coke. As you may already know from What is Clean Coal Technology? and How Carbon Capture
Works, coke is a solidified form of carbon, and the term carbon sequestration refers to a wide range of processes that
capture carbon dioxide and send it away for permanent storage or productive use. CO2 scrubbing is a particular form of
carbon capture that takes place after fossil fuel has been combusted, but before the exhaust is released into the air.
Aside from its currently hefty price tag, many people see CO2 scrubbing as one of the easiest ways to reduce carbon
dioxide emissions since it doesn't require any lifestyle changes. No solar panels to set up or wind farms to connect to;
no guilt trips about accidentally leaving the lights on all day. Simply keep on burning that midnight oil and let the
scrubbers handle the rest. But just what does "the rest" entail? Keep reading to find out.
The CO2 Scrubbing Process
Regardless of what country you live in, unless you reside in a yurt, the electricity powering your home is most likely
coming from a power plant. The United States burns fossil fuels for more than 85 percent of its energy needs, and
power plants are sprouting up in China at the rate of two per week [source: Herzog/ CCS].
To produce energy, most power plants burn coal (or another fossil fuel) in air to create steam. The steam turns a
turbine, which generates electricity. Aside from steam, though, a hodgepodge of flue gases are also created and
released into the atmosphere. Many of those extraneous emissions are greenhouse gases that contribute to the
greenhouse effect.
But not all greenhouse gases are created equal. Even though carbon dioxide usually makes up no more than 15 percent
of a power plant's emissions by volume, it's responsible for 60 percent of the greenhouse gas effect [source: U.S.
Department Of Energy/Marion].
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CO2 scrubbing works not by physically scrubbing the coal, but by isolating CO2 from the other flue gases before
they're released into the air.
In order to prevent the CO2 from escaping into the atmosphere, post-combustion carbon capture (as its name implies)
works by isolating CO2 from the other flue gases after combustion. Once the flue gases have been removed, or
scrubbed, they're released into the air. Some scrubbing techniques also isolate other greenhouse gases like sulfur and
mercury, but since CO2 is the biggest problem, it gets the most attention.
Currently, using a liquid solvent to bind with the CO2 and separate it from the other gas components is the most
widespread method for isolation. Two solvents commonly used are aqueous ammonia and monoethanolamine (MEA).
Regardless of the particular solvent chosen, though, the process is essentially the same:
After the fossil fuel is combusted in air, the resulting gases are collected and chilled. The solvent is then added and
absorbs the CO2, forming a new compound in a reversible chemical reaction. The new compound separates out from
the other gases by entering a more solid state that gets pumped to a new chamber and reheated. The heat causes the
CO2 to come back out of solution so that it can be diverted to storage. The solvent is sent back to the beginning of the
cycle to be reused, and the cleaned flue gas is released into the atmosphere.
Aside from using solvents, other CO2 scrubbing methods include:
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Utilizing adsorbents that attract CO2 to their surface where it can be removed
Using selectively permeable membranes that prevent CO2 from passing through but allow the more benign
gases to escape
Cooling the flue gases to a temperature that forces CO2 to condense out of the solution for separation
Even though each of the listed techniques has proven effective, because of the challenges inherent in separating CO2
from flue gas, scientists are investigating better options as we speak. The race is on to find the best and cheapest way to
clean up the air. Learn about the challenges involved in CO2 scrubbing next.
Challenges of CO2 Scrubbing
As with many relatively new technologies, CO2 scrubbing faces its share of challenges. Obstacles depend on the
particular process used to remove carbon dioxide and may include degradation of the solvent by other flue gases,
corrosion of membranes, reduced adsorption by flue gas impurities, as well as increased energy costs and needs.
At present, the latter two issues tend to be the most problematic. Some analyses estimate that current capture
technologies cost around $150 per ton of carbon captured, adding between 2.5 cents/kWh and 4 cents/kWh to your
electric bill [source: U.S. Department of Energy]. Other estimates put the price closer to 9 cents/kWh -- an 84 percent
increase over electricity purchased from a plant without carbon capture technology [source: U.S. Department of
Energy].
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This power plant with scrubbing technology is not only a lot cleaner than regular plants, but also a lot more expensive.
Prices vary so widely because a number of factors affect costs, including the design of the scrubbing system, the
location of the plant, the efficiency of the plant and whether the carbon capture technology was original or added later.
For instance, while electricity generated at a new coal plant using CO2 scrubbing may cost 57 percent more than
electricity generated in a plant without carbon capture, electricity generated at an existing plant retrofitted with CO 2
scrubbing can cost up to 290 percent more [source: Williams].
In addition to being costly, CO2 scrubbing also requires a lot of energy. Not only does it require treating a high volume
of gas (remember that only a small percentage of flue gas is actually CO2), but it has to compress the captured CO2 into
a storable form -- a very energy-intensive process. So, ironically, capturing carbon actually ends up using its own share
of fuel. New coal plants fitted with carbon capture may use anywhere from 24 to 40 percent more fuel than those not
fitted with the technology, while retrofitted coal plants may use up to 77 percent more [source: Williams].
Even taking into account its own energy usage and emissions, though, CO2 scrubbing still removes a net amount of 80
to 90 percent of the carbon dioxide from flue gas [source: GreenFacts]. That's a good thing, since some scientists
estimate our emissions of CO2 may need to be reduced by 60 to 80 percent to avoid catastrophic climate changes
[source: Marion]. Next, find out about the applications for CO2 scrubbing.
Applications for CO2 Scrubbing
Perhaps one day you'll be able to scrub out CO2 just about anywhere. However at present, CO2 scrubbing is feasible
primarily at stationary carbon dioxide sources like fossil fuel-burning power plants. If you think that target area seems
limited, though, think again. Fossil fuel combustion is the single largest source of CO2 in the atmosphere: Power plants
alone emit more than one-third of total CO2 emissions worldwide [source: Herzog].
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Fossil fuel burning is the largest source of CO2 in
the atmosphere.
Only the most stubborn person would dispute the fact that fossil fuels aren't going away soon. Because despite the twopronged push to reduce energy consumption and switch to alternative sources of energy, people aren't that inclined to
change their ways. And although we now have the knowledge to build cleaner, more efficient plants, the newer plants
won't be widely available for several more decades due to the long life span of power plants (around 40 years) [source:
RWE]. Research indicates that by 2030, two-thirds of CO2 emissions will come from existing plants [source:
ScienceDaily].
Obviously, people need a way to clean up after themselves. As the only carbon capture method that can be applied to
existing plants, CO2 scrubbers are just the solution. They're essentially a way to buy time until we can make the full
transition to cleaner energy sources. U.S. Environmental Protection Agency (EPA) mandates and the Kyoto Protocol
(which sets limits for the amount of CO2 emissions each signee is allowed to emit) provide further incentive.
Giant Air Purifiers?
Some scientists are going a step or two beyond the present CO2 scrubbers designed for power plants and suggesting
devices that would literally pull CO2 out of thin air. The treelike units would stand out in the open and collect CO2 on
their surfaces. Similar to a tree taking in CO2 for photosynthesis, but many times stronger, the units would have a
special sorbent that would pull in CO2 from the surrounding air and store it.
Other countries have started emissions trading schemes that will set a price on carbon. The European Commission, for
instance, indicated that neglecting to use carbon capture could cost the region $80 billion more than installing it
[source: The Guardian]. All of which means that even though carbon scrubbing is still an expensive venture, it could be
equally expensive in the long run to do nothing.