Before Sequestration
Regardless of the type of sequestration to be used, the same steps must be taken before the carbon dioxide may be stored:
I. Capture and separation of CO2 from contaminants.
II. Conversion of CO2 to the proper form if it was captured in a different form - ie liquid, solid, mineral
III. Transport to the disposal site. (In certain circumstances the CO2 may be transported to the disposal site in one form and converted to the form it will be in for disposal at the site.)
IV. Disposal of the carbon dioxide.
Capture and Separation
 |
| source
A CO2 Separation plant in Shady Point, Oklahoma |
One of the limitations of carbon sequestration technology is the problem of actually recovering the carbon dioxide. CO2 is never produced in pure form and must be separated from the other products of combustion. Unless the CO2 is removed directly from the air, which many believe to be impossible due to its dilute concentrations in air, separation must occur directly after combustion at the emissions site or before the "tail pipe" or "smokestack." It would be virtually impossible to have separation schemes for every mobile power plant, (ie car, truck, boat, train) due to cost and size restrictions. Therefore it is most economical and practical to collect CO2 from large point sources or power plants. The separation step is still usually the most expensive in the sequestration process. "Current equipment for capturing carbon dioxide from power plants would raise the cost of generating electricity by 50 to 100 percent. But because sequestration does not affect the cost of electricity transmission and distribution (a significant portion of consumers' electricity bills), delivered prices will rise by less, by about 30 to 50 percent. Research into better separation technologies should lead to lower costs." [1]
The capture and separation of carbon dioxide from exhaust gases is an entire area of research in and of itself. There are numerous technologies under development today to determine the "best" way to remove CO2 from the other products of combustion for disposal or use elsewhere.
The most common method for separating carbon dioxide involves mixing a solution of dilute monoethanolamine (MEA) with the flue gases inside the absorption tower of a plant designed to capture the greenhouse gas. The carbon dioxide in the exhaust reacts with the MEA solution at room temperature to form a new, loosely bound compound. This compound is then heated in a second column, the stripping tower, to approximately 120 degrees C to release the carbon dioxide. The gaseous carbon dioxide product is then compressed, dried, chilled, liquefied and purified (if necessary); the liquid MEA solution is recycled. Currently this technology works well, but it must become more energy efficient if it is to be applied to large-scale carbon sequestration. [2]
Regardless of the method employed, the technique must be efficient, cost effective, and, most importantly, safe. Some of the possibilities include separation as a solid/ice slurry, flue gas recirculation, sorbent energy transport system (fuel reduces a metal oxide), amine scrubbers, polymer membranes, or absorption directly from the air. Many of these separations are being developed to be retrofitted to modern fossil fuel burning power plants, while others (even some of the same techniques) will be built into new zero emission plants, where the fuel is not burned, but instead is converted to a clean source of energy such as hydrogen. Yegulalp et al. estimates an increase in the price of electricity by 30% to 40% for retrofitting a power plant with exhaust gas containing 10-15% CO2. [3]
Although, capture and separations technologies are beyond the scope of this website, we wanted to share with you one innovative technology for separation - direct removal of CO2 from the air. While this may seem a bit far fetched and unlikely due to the dilute nature of the carbon dioxide in air, recently Klaus Lackner and associates have put forth a very convincing argument for the possibility of removing the carbon dioxide directly from air:
Currently, only biomass generation extracts CO2 from air. The 1:3000 dilution has made this approach look too difficult. On the other hand the CO2 in a volume of air represents an amount of heat of combustion that exceeds the kinetic or wind energy of the same amount of air by two orders of magnitude (Lackner et al. 1999a). Since wind energy appears close to economically viable, CO2 extraction from air may prove to be attractive. Extraction from air would eliminate the need for a dedicated infrastructure for the transport of CO2 and no change would be needed in present combustion technology. Extraction from air would be particularly useful in counteracting the emissions of small, distributed and often mobile sources of CO2, which together amount to about half of all CO2 emissions. [4] For more information, click here.
Conversion
 |
| source
A Liquid Carbon Dioxide Plant |
Once the CO2 has been separated into a relatively pure form, conversion from one state to another is not very difficult, and this step is usually rolled over as part of capture, or it occurs at the disposal site (as in the case of storing liquid CO2 below ground where it reacts with the stone to form carbonate minerals). For example, in the case of separation from flue gas, the carbon is recovered in gaseous form, but for transportation and storage, it will most likely have to be converted to liquid form.
Transportation
Moving the CO2 to the site of disposal is a function of a number of factors: location of the site, form of the CO2, and distance between the origin of the carbon dioxide and the site. Most often the CO2 is transported as a liquid in trucks on land, in tankers on sea, or in pipelines on both land and sea. It can also be transported by container on trucks or tankers in solid dry ice form.
Once carbon sequestration is widely used, pipelines will be the most likely avenue for transportation. Because pipeline technology has been perfected over the years for transport of other liquids, the overall cost of sequestration should not be increased by too much. Many pipeline systems also already exist for areas involving oil and gas reservoirs. As cited in Yegulalp et al. from Aldus et al., estimated costs for implementing pipeline technology is merely $0.01/ton/km with some newer studies suggesting even lower costs. [5]
