How CO2 Scrubbing Works


That's not fog: It's smog -- evidence that the air is becoming ever more polluted with greenhouse gases.
That's not fog: It's smog -- evidence that the air is becoming ever more polluted with greenhouse gases.
Getty Images

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.

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

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.
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.
Getty Images

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].

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:

  • 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

This power plant w­ith scrubbing technology is not only a lot cleaner than regular plants, but also a lot more expensive.
This power plant w­ith scrubbing technology is not only a lot cleaner than regular plants, but also a lot more expensive.
Istock Photo

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].

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 CO2 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

Fossil fuel burning is the largest source of CO2 in the atmosphere.
Fossil fuel burning is the largest source of CO2 in the atmosphere.
Istock Photo

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].

Only the most stubborn person would dispute the fact that fossil fuels aren't going away soon. Because despite the two-pronged 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.

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.

For more on CO2 ­ scrubbing and other carbon capture and storage technologies, try the links on the next page.

­Related HowStuffWorks Articles

 

More Great Links

Sources

  • Allen,­ Paddy. "Carbon capture technologies." The Guardian. June 12, 2008. (July 16, 2008)
  • http://www.guardian.co.uk/environment/interactive/2008/jun/12/carbon.capture
  • CCS Education Initiative. "Carbon Capture and Sequestration: Frequently Asked Questions." (July 16, 2008) http://www.ccs-education.net/faqs.html
  • Green Car Congress. "BP and Powerspan Collaborate to Demonstrate and Commercialize CO2 Capture Technology for Coal-Fired Power Plants." Aug. 8, 2007. (July 16, 2008) http:­//www.greencarcongress.com/2007/08/bp-and-powerspa.html
  • GreenFacts. "Scientific Facts on CO2 Capture and Storage." Nov. 7, 2008. (July 16, 2008) http://www.greenfacts.org/en/CO2-capture-storage/l-3/3-capture-CO2.htm#1p0
  • Herzog, Howard J. "What Future for Carbon Capture and Sequestration?" Environmental Science and Technology. Vol. 35, Iss. 7. April 1, 2001. (July 16, 2008) http://sequestration.mit.edu/pdf/EST_web_article.pdf
  • Jha, Alok. "The cost of cleaning up fossil fuels- and the price of doing nothing." The Guardian. June 13, 2008. (July 16, 2008) http://www.guardian.co.uk/environment/2008/jun/13/carboncapturestorage.fossilfuels
  • Marion, John, et al. "Controlling Power Plant CO2 Emissions: A Long Range View." (July 16, 2008)http://www.netl.doe.gov/publications/proceedings/01/carbon_seq/1b2.pdf
  • Ronca, Debra. "How Carbon Capture Works." HowStuffWorks. 2008. (July 18, 2008)https://science.howstuffworks.com/carbon-capture.htm
  • RWE. "CO2 scrubbing process overview." (July 16, 2008) http://www.rwe.com/generator.aspx/konzern/fue/strom/CO2-minimiertes- kraftwerk/CO2-waesche/language=en/id=272122/page-CO2-waesche.html
  • Science Daily. "Lower Carbon Dioxide Emissions From Coal-fueled Power Plants Possible With Technology Development." Mar. 21, 2007. (July 16, 2008) http://www.sciencedaily.com/releases/2007/03/070319175954.htm
  • U.S. Department of Energy. "Carbon Capture Research." Sept. 6, 2007. (July 16, 2008)http://www.fossil.energy.gov/programs/sequestration/capture/index.html
  • U.S. Department of Energy: National Energy Technology Laboratory. "Carbon Sequestration." (July 16, 2008)http://www.netl.doe.gov/technologies/carbon_seq/core_rd/CO2capture.html
  • "Innovations for Existing Plants." (July 16, 2008)http://www.netl.doe.gov/technologies/coalpower/ewr/CO2/PostCombustion.html
  • Williams, Robert H. "CO2 Capture-Related Activities in US." 24 May, 2007. (July 16, 2008)http://belfercenter.ksg.harvard.edu/files/15-bob%20williams.pdf

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