A promising way to combat global warming is to capture CO₂ produced by industry and bury it in deep geologic formations. The processes are technically complex… and still expensive. A closer look.

To combat the accumulation of CO₂ in the atmosphere, capturing it and storing it underground offers a promising alternative until non-emitting energies become widespread. Large stationary sources are targeted first. Full-scale experiments are under way to capture CO₂ from these sources at an acceptable cost using available technologies.

Recovery technology still very expensive

Capturing CO₂ at the source requires separating it from the other components produced by industrial processes that burn oil, gas coal or biomass, such as water vapor, nitrogen and sulfur. For the moment, there appear to be three suitable technologies for doing that. They are used primarily in areas that are also conducting geologic disposal experiments, including the United States, Japan and Europe.

• Post-combustion capture consists in scrubbing flue gases, usually with amine solvents. It is easy to retrofit facilities with the technology, although it is really applicable only for large gas volumes at low pressure and low CO₂ concentrations. It is the most mature solution, but results in an electric kilowatt-hour that costs 50 to 70% more.
• Oxyfuel capture consists of replacing the use of air for combustion with pure oxygen. In this case, the concentration of CO₂ in the flue gases can be as high as 90%, facilitating capture by a cryogenic system at the outlet. This solution is also expensive and consumes a lot of energy. It is best suited to new facilities.

• “Pre-combustion” capture involves converting the fossil fuel before use into syngas, a mixture of carbon monoxide (CO) and hydrogen. The CO reacts with water in a step called “shift conversion”, forming CO₂ and hydrogen, which are then separated. The hydrogen can be used as a clean, non CO₂-emitting fuel to generate electricity or heat.
Once the CO₂ has been captured, it can be transported to storage sites either overland in high-pressure gas pipelines, or by sea in methane carriers, just like liquefied natural gas (LNG), i.e. in moderately pressurized liquid form, which reduces its volume. The United States has the largest land-based pipeline network, with some 50 million metric tons of CO₂ carried per year from enhanced oil recovery operations or from industrial plant emissions. Fewer than 3,000 kilometers of pipeline network are allocated to CO₂ transport worldwide, a sign of how inadequate the infrastructure is.

Three types of underground reservoirs tested so far

There are three main alternatives for deep geological storage of CO₂. In all three cases, the CO₂ is injected at depths of at least 800 meters. There, at temperatures of more than 31° C and pressures greater than 74 bar, it reaches a supercritical state in which it becomes denser and less voluminous.

• Deep saline aquifers – These brackish water-bearing layers represent the greatest volume potential for CO₂ storage, with up to 10,000 billion metric tons available. That is equivalent to several centuries’ worth of global CO₂ emissions. The aquifers are also very well distributed geographically. These two advantages make it easier to find an aquifer near an emission source. At the Sleipner natural gas production site in the North Sea, the 4 to 10% CO₂ contained in methane is extracted and re-injected more than 1,000 meters beneath the ocean floor. Each year, a million metric tons are buried under the ocean floor in Norwegian waters  rather than being released to the atmosphere. More study is needed to characterize the long-term behavior of these aquifers.

• Depleted or nearly depleted oil and gas reservoirs – We already know a lot about these onshore and offshore geological environments. As natural reservoirs, they have been proven to be leak-tight, as long as all the wells leading to them are sealed off. They are not ideally located, being far from CO₂-emitting industrial sites. Using them requires investing heavily in expensive infrastructure or other means to bring in the CO₂, such as pipelines or methane carriers. Demand also outstrips available volumes, estimated at about 930 billion metric tons. Injecting pressurized CO₂ for the enhanced recovery of oil or gas does provide some interesting possibilities, though. The Canadians are experimenting with it at the Weyburn oil field in Saskatchewan. CO₂ from a nearby U.S. coal gasification plant is injected under pressure into the oil layer, where it dissolves in the oil, lowering its viscosity and making it easier to recover. After treatment, the dissolved CO₂ is separated and reinjected into the underground reservoir.

• Unexploited coal seams – These can be used to take advantage of coal’s “affinity” for CO₂ which, when injected, replaces the methane naturally present in the coal bed. Gas companies can then extract the methane (the major component in natural gas), inject it into their pipeline networks, and market it for industrial and domestic use. Since 2001, the European RECOPOL research project in Poland has been testing the feasibility of injecting CO₂ into coal seams in the Upper Silesian Basin. The possibility of injecting large quantities of CO₂ into these low-permeability seams without boring a lot of injection wells, which would increase the risk of leaks, must be verified. At an estimated 40 billion metric tons, the global storage capacity of these seams is sorely inadequate.
There are two additional solutions. The first involves storing the CO₂ in carbon “lakes” in the ocean at a minimum depth of 1,500 meters, but this has been rejected due to concerns about the impacts on the marine ecosystem and how long the CO₂ would be contained. The second solution, carbon sequestration by mineral carbonation, is of more interest. Here, CO₂ reacts with naturally occurring subsurface calcium and magnesium to become a carbonated rock similar to limestone, which is insoluble and therefore perfectly stable over the long term.

Capture is 70% of the overall cost

The entire CO₂ capture, compression, transport and sequestration process can cost up to 42 euros (about 65 dollars) per metric ton of CO₂. This exceeds the floor price of 30 euros (about 47 dollars) per MT of CO₂ currently negotiated on the cap and trade market. Capture technologies represent 70% of the total in terms of cost, so they constitute a considerable economic challenge. They are in the most need of optimization from this viewpoint and are being tested in national and European projects.
The long-term safety of geological storage is a major criterion in deciding how much and how densely CO₂ can be stored underground. Using information from the many naturally occurring CO₂ deposits and lessons learned from pioneering storage sites, it is possible to establish rules for safe storage over more than 1,000 years. The European Commission published a draft directive in January 2008 that will be debated over the next few months in the European Council and Parliament. The directive sets the conditions for CO₂ storage licensing based on thorough site characterization and assessment and on an adequate monitoring program drawing on geophysical, geochemical and biological techniques.
Questions are already being raised about these underground storage reservoirs, destined to become permanent, so the final test will no doubt be whether or not the public accepts them.