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].
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.