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Carbon Dioxide (CO₂) capture and storage has the potential for significantly reducing the amount of CO₂ released into the atmosphere. The technologies needed to separate CO₂ from other gases and to sequester CO₂ are known. Further development is needed to implement CO₂ capture and storage on a large scale.

By burning fossil fuels—coal, oil, and natural gas—we are adding to the concentration of CO₂, in the atmosphere. The CO₂ concentration is now 379 parts per million. This is significantly more than it has been at any time in the past 600,000 years. The consensus among climatologists is that the increased CO₂ concentration is the main cause of the increase in global temperature.

One way to reduce the amount of CO₂ being released into the atmosphere is to rely more on alternative energy sources that do not produce CO₂. These include hydroelectric, wind, solar, nuclear, geothermal, and tidal energy. Each of these has limitations, and it will be difficult to make a quick shift from fossil fuels to these other sources. But what if the CO₂ produced by burning fossil fuels did not reach the atmosphere? Instead of letting CO₂ go up the smokestack and into the air, can we capture it and put it somewhere? Is this possible?

Yes, it is. The process is called CO₂ capture and storage. It is being done on a small scale right now. It has the potential to make a significant difference in the amount of CO₂ we release into the atmosphere. As the name implies, there are two phases to the process. The first challenge is to capture the CO₂ instead of letting it go up the smokestack. Then it has to be stored or “sequestered” safely and for a long time. The idea of sequestering CO₂ to reduce the amount entering the atmosphere is fairly new. But the technology needed to do this has been developed for other reasons. We have a head start.

Capturing CO₂

The best place to capture CO₂ is at the major sources of emissions. Power stations that generate electricity produce about one-third of global CO₂ emissions. In addition, CO₂ is a by-product of iron and steel production, and cement production. CO₂ is also removed from natural gas before it can be used as a fuel. These industrial processes are good candidates for CO₂ capture and storage because they are large-scale sources in a fixed place. In contrast, it would be difficult to capture CO₂ emissions from automobiles. The chief fossil fuels for power stations are natural gas and coal. These fuels are burned in the presence of air. The resulting heat is used to create steam that drives turbines, which turn electrical generators. Or, gas may be burned to drive turbines directly. In either case, the oxygen in the air combines with carbon in the fuel to produce CO₂. The CO₂ is released into the air. When natural gas is burned, hydrogen from methane (CH₄) also combines with oxygen to form water.

But the air that was used to burn the fuel contained mostly nitrogen. This nitrogen does not participate in the combustion process. It is passed through and goes up the smokestack. The power-plant emissions, called flue gases, are typically only 10% to 15% CO₂ in a coal-burning plant and about 5% when natural gas is the fuel. In principle we could store all the flue gases, but this would fill storage capacity mostly with nitrogen that does not need to be sequestered. For the CO₂ to be stored efficiently, it first has to be separated from the other flue gases. How is this done?

There are three strategies:

1. Separate the CO₂ after combustion.
2. Take the carbon out of the fuel before combustion so that we burn only hydrogen and produce only water.
3. Burn fossil fuels in oxygen rather than air, resulting in concentrated CO₂.

Chemical solutions can be used to dissolve the CO₂ while passing the other gases to the atmosphere. The technique most widely used today employs a group of compounds called amines. They absorb CO₂ by forming chemical bonds, particularly when at high pressure and low temperature. This process is called “scrubbing.” The resulting chemical solution is later heated and the pressure reduced, releasing concentrated CO₂.

Other solvents are used to dissolve CO₂ without chemical bonding. In this physical absorption process the CO₂ dissolves under pressure and is later removed from the solvent by reducing the pressure. The solvent may then be reused. Another strategy for capturing CO₂ is to cool the flue gases to the point where the CO₂ becomes liquid. This process requires considerable energy for refrigeration. An advantage is that the liquid can be easily transported by truck or ship. It is also possible to separate gases by using thin films called membranes. Some gases will pass through a membrane faster than others. This allows the different gases to be separated from one another.

Getting the Carbon Out before Combustion

The fuel in natural gas is methane (CH₄). When burned it produces both CO₂ and water (H₂O). If we can take out the carbon before combustion we will be left with hydrogen, which produces only water when burned. Doing this involves reacting the fuel with oxygen and/or steam to produce carbon monoxide (CO) and hydrogen. The carbon monoxide is then reacted with more steam to produce CO₂ and more hydrogen.…Finally, the CO₂ is separated and the hydrogen is used as fuel in a gas turbine.

Combustion with Oxygen rather than Air

Nitrogen makes up 78% of air. It remains mostly unchanged in the fuel combustion process. It is the main gas that dilutes CO₂ in the flue gas mixture. If the fuel is burned in pure oxygen, rather than air, the CO₂ concentration in flue gas can be increased to over 80%. This process makes it unnecessary to do further concentrating of the flue gas before sequestration of the CO₂. The challenge is how to separate oxygen from the rest of the air, which is mainly nitrogen. The strategies are similar to those used to separate CO₂. The air can be chilled so that the oxygen liquefies. Membranes that pass oxygen and nitrogen at different rates can cause the separation. There are also materials that absorb nitrogen, thus separating it from the oxygen. They may then be made to release the nitrogen and be reused.

Sequestration Technologies

Once concentrated CO₂ has been captured, the next step is to store it somewhere. Here are some options.

Geological Formations

Storage in geological formations is currently the most promising solution for widespread, long-term sequestration of CO₂. Some projects are already under way. In order to reduce greenhouse gases and global warming, stored CO₂ must be kept out of the atmosphere for hundreds or thousands of years. Oil and natural-gas reservoirs, deep saltwater aquifers, and coal seams have existed for millions of years with only very gradual changes. There is strong evidence that if properly managed, these formations could provide for long-term storage of CO₂.

Depleted Oil and Natural-Gas Reservoirs

Many people believe that oil and natural gas are found in big underground caves. This is not the case. Rather, these hydrocarbons are in permeable and porous rocks such as sandstone. These rocks contain microscopic spaces, called pores, which fill with fluids. The fluids may be water, oil, or natural gas. An oil or natural-gas reservoir is more like a sponge than a bottle. Once an oil or natural-gas field has been productive for a period of time, a good portion of the hydrocarbons has been removed. There is space available to store CO₂. The porous and permeable rock layer that contains these fluids is covered by an impermeable cap rock—often salt or shale—that does not let them pass through. Normally, oil and natural gas will tend to migrate upward through permeable rock because they are lighter than the water that is also found in such rock formations. The cap rock traps them. Since oil and natural gas have been sequestered in such formations for millions of years, there is good reason to believe that CO₂ will remain there as well.

Enhanced Oil Recovery

Much of the technology needed to store CO₂ in oil fields is already being used for a process known as enhanced oil recovery (EOR). When a reservoir is newly tapped, the oil is typically under pressure and flows freely to the surface. As oil is removed, pressure drops and pumping is needed to recover more. At some point recovery becomes uneconomical and is stopped, or additional techniques are used to extract more oil. One approach is to pump CO₂ into the reservoir. This increases pressure so the oil flows more readily. Also, the CO₂ dissolves in the oil and causes it to become less viscous and flow more easily. It expands in volume as well, further increasing pressure. CO₂ is pumped into the reservoir through an "injection well." This forces the oil toward a "production well," where it rises to the surface.

In enhanced oil recovery, CO₂ is pumped into the reservoir through an injection well. It mixes with the remaining oil, forming a “miscible zone.” The pressure from the CO₂ and expanding oil pushes an oil bank toward the production well, where it rises to the surface. Then the CO₂ is separated from the oil and may be added to the stream of CO₂ going into the injection well. A side effect of enhanced oil recovery is that the CO₂ that was used to force oil out of the formation is now sequestered.

There are many EOR projects using CO₂ injection around the world. A good example is the Weyburn field in Canada.

A side effect of enhanced oil recovery is that CO₂ becomes sequestered in the rock formation. If the goal is CO₂ storage rather than oil recovery, then depleted or nearly depleted oil fields may be used for sequestration even if they are not good candidates for enhanced oil recovery.

What are potential problems with CO₂ sequestration? The big question is whether or not the CO₂ will leak out of the reservoir. Leakage would be a problem because it would return the sequestered CO₂ to the atmosphere. This would defeat the purpose of the project.

When an oil well is drilled, the final step in the process is to insert steel pipe in the borehole and to fill the space between the outside of the pipe and the borehole with cement. The pipe, called a casing, is later perforated to allow oil to flow into the pipe and up to the surface. When the well is no longer productive, it may be sealed at the top.

If the leakage was sudden, it could kill people. CO₂ is not poisonous, but if it substitutes for available oxygen, people can suffocate. Since CO₂ is heavier than air, it can accumulate in low places such as cellars and valleys. There have been natural releases of CO₂ that have caused death. One such disaster occurred at Lake Nyos in Cameroon. The lake is in a volcanically active area with carbon dioxide leaking into the waters of the lake from below. On August 21, 1986, there was a sudden release of CO₂, which flowed into valleys around the lake and killed 1,800 people in nearby villages. There is no reason to expect CO₂ sequestered in an old oil field to suddenly escape, but there are possibilities for slower leakage. Ironically, the wells themselves are a potential problem.

There may be hundreds of such wells in an oil field. If the field is old, the cement around the well casings may have deteriorated. This may provide a pathway for CO₂ to escape. There are ways of recementing wells so that they provide a reliable seal for sequestered CO₂.

Another possible pathway for CO₂ to leak would be cracks in the cap rock. CO₂ sequestration is proposed for regions that are geologically stable, where earthquakes are not likely to occur. However, the sequestering of CO₂ itself could result in increased pressure under the cap rock that could result in cracks. The solution is to monitor pressure and take care not to exceed the limits of the formation.

Aquifers

There are many underground sealed geological "traps" that have never contained oil or natural gas. Their pores are filled with water. These are called aquifers. The aquifers that are most suitable for CO₂ storage are deep underground. They are filled with salt water, so they are unsuitable for supplying or storing fresh water for human use. CO₂ would partially dissolve in the water in the aquifer. In some rock types, it might react with minerals to form stable carbonate deposits. This would permanently lock up the CO₂. Geological studies would need to be made, as is routinely done for oil and natural-gas reservoirs, to confirm that the aquifer would not leak CO₂.

The world’s first CO₂ injection program performed for climate-change considerations is offshore Norway in a deep saline aquifer in the North Sea known as the Sleipner field.

Coal Beds

Another potential storage medium is in coal deposits that are too deep to be mined. Coal is mostly carbon. It will absorb CO₂ and lock it up permanently. Coal deposits usually contain methane. When CO₂ is pumped into the coal, it is absorbed in preference to the methane, which is released. As with enhanced oil recovery, useful fuel is produced while sequestering CO₂.

A problem with this approach is that coal swells when it absorbs CO₂. This can result in reducing the pathways through which the gas flows. The result is more limited storage capacity.

Capture-and-Storage Case Studies

Two CO₂ capture-and-storage projects currently under way are in the Sleipner field in the North Sea and in the Weyburn field in Canada.