How CO2 Scrubbing Works

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:

­­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.

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.

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.