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Carbon dioxide air capture
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Carbon dioxide air capture is a form of carbon capture. It removes carbon dioxide from ambient air by carbon dioxide scrubbing. It is a different approach to removing CO2 from the stack emissions of large point sources, such as fossil fuel fired power stations. It is regarded as greenhouse gas remediation, which is a branch of geoengineering. Some commentators regard air capture as a form of carbon capture and storage, but CCS is usually used to describe capture at source rather than capture from ambient air.
Air capture is not generally seen as an attractive alternative to capture at large, point source emitters (such as power plants), as it is likely to be more efficient and cheaper to capture and store carbon dioxide from more concentrated streams. There are, however, some advantages of air capture as it removes the need for CO2 piping to transport the gas to underground storage sites, and allows the use of renewable energy and optimal storage sites.
It is particularly effective at dealing with small sources such as domestic heating systems and vehicle exhausts, where piping of exhaust gases is impractical.
This technique can give 1.43W/m2 of globally-averaged negative forcing, which is almost sufficient to reverse the warming effect of current levels of anthropogenic CO2 emissions.
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Encyclopedia
Carbon dioxide air capture is a form of carbon capture. It removes carbon dioxide from ambient air by carbon dioxide scrubbing. It is a different approach to removing CO2 from the stack emissions of large point sources, such as fossil fuel fired power stations. It is regarded as greenhouse gas remediation, which is a branch of geoengineering. Some commentators regard air capture as a form of carbon capture and storage, but CCS is usually used to describe capture at source rather than capture from ambient air.
Air capture is not generally seen as an attractive alternative to capture at large, point source emitters (such as power plants), as it is likely to be more efficient and cheaper to capture and store carbon dioxide from more concentrated streams. There are, however, some advantages of air capture as it removes the need for CO2 piping to transport the gas to underground storage sites, and allows the use of renewable energy and optimal storage sites.
It is particularly effective at dealing with small sources such as domestic heating systems and vehicle exhausts, where piping of exhaust gases is impractical.
This technique can give 1.43W/m2 of globally-averaged negative forcing, which is almost sufficient to reverse the warming effect of current levels of anthropogenic CO2 emissions. It is notable, however, that CO2 levels will have risen by the time this could be achieved.
Benefits
There are several benefits to using air capture:
- Capture of CO2 from dispersed sources (e.g. cars and domestic heating systems) is possible.
- Energy required for the process can be cheaply-generated renewables, as long distance transport of power is unnecessary.
- Geographic flexibility will allow the use of near optimal carbon storage geology.
- Administration and control will be easier with large, centralised facilities.
- Support industries could naturally 'cluster' around air capture plants, reducing costs.
- As a geoengineering approach, because it is a greenhouse gas remediation technique it is superior to solar radiation management techniques, as it removes CO2 from the atmosphere and thus prevents ocean acidification. It is, however, slower acting.
Proposed Methods
Artificial Trees (AKA Fake Plastic Trees)
A notable example of an atmospheric scrubbing process are the artificial trees (often referred to as 'fake plastic trees' after the Radiohead song) proposed by Klaus Lackner. This concept imagines huge numbers of fake plastic trees around the world to remove ambient CO2. The concept has been featured on TV documentaries.
The chemistry used is a variant of that described below, as it is based on sodium hydroxide. However, by using a polymer-based ion exchange resin, the process can be carried out at only 40C, and uses changes in humidity to prompt the release of captured CO2, instead of a kiln. This massively reduces the energy required to operate the process.
Scrubbing Towers
In 2008, the Discovery Channel covered the work of Professor David Keith, of University of Calgary, who built a tower, 4 feet wide and 20 feet tall, with a fan at the bottom that sucks air in, which comes out again at the top. In the process, about half the CO2 is removed from the air.
This device uses the chemical process described in detail below. Reagents are recycled within the tower. The main cost appears to be the electricity to run it, as supported by cost modeling done at the M.I.T.
To put this into perspective, people in the U.S. emit about 20 tonnes of CO2 per person annually. In other words, each person in the U.S. would require a tower like this to remove this amount of CO2 from the air, requiring an annual 2 Megawatt-hours of electricity to operate it. By comparison, a refrigerator consumes about 1.2 Megawatt-hours annually (2001 figures}.
Quicklime process
Quicklime will absorb CO2 from atmospheric air mixed with steam at 400C, (forming calcium carbonate) and release it at 1000C. This process, proposed by Steinfeld, can be performed using renewable energy from thermal concentrated solar power.
Example CO2 scrubbing chemistry
Various scrubbing processes have been proposed to remove CO2 from the air, or from flue gases. These usually involve using a variant of the Kraft process. Scrubbing processes may be based on sodium hydroxide. The CO2 is absorbed into solution, transferred to lime via a process called causticization and released in a kiln. With some modifications to the existing processes, mainly an oxygen-fired kiln, the end result is a concentrated stream of CO2 ready for storage or use in fuels. An alternative to this thermo-chemical process is an electrical one in which an electrical voltage is applied across the carbonate solution to release the CO2. While simpler, the electrical process consumes more energy as it splits water at the same time. It also depends on electricity and so unless the electricity is renewable, the CO2 produced during electricity production has to be taken into account. The early incarnations of air capture used electricity as the energy source and therefore depended on carbon-free sources. A thermal Air Capture system uses heat that can be generated on-site, reducing the inefficiencies associated with producing electricity, but of course it still needs a source of (carbon-free) heat. Concentrated solar power is an example of such a source.
Zeman and Lackner outlined a specific method of air capture.
First, CO2 is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate. The absorption reaction is a gas liquid reaction, strongly exothermic, (below)
2NaOH(aq) + CO2(g) -> Na2CO3(aq) + H2O(l)
? H° = -109.4 kJ/mol
The carbonate ion is removed from the solution by reaction with calcium hydroxide (Ca(OH)2), which results in the precipitation of calcite (CaCO3). The causticization reaction is a mildly exothermic, aqueous reaction that occurs in an emulsion of calcium hydroxide (below)
Na2CO3(aq) + Ca(OH)2(s) -> 2NaOH(aq) + CaCO3(s)
? H° = -5.3 kJ/mol
Causticization is performed ubiquitously in the pulp and paper industry and readily transfers 94% of the carbonate ions from the sodium to the calcium cation (10). Subsequently, the calcium carbonate precipitate is filtered from solution and thermally decomposed to produce gaseous CO2. The calcination reaction is the only endothermic reaction in the process and is shown (below).
CaCO3(s) -> CaO(s) + CO2(g)
? H° = + 179.2 kJ/mol
The thermal decomposition of calcite is performed in a lime kiln fired with oxygen in order to avoid an additional gas separation step. Hydration of the lime (CaO) completes the cycle. Lime hydration is an exothermic reaction that can be performed with water or steam. Using water, it is a liquid/solid reaction as shown (below).
CaO(s) + H2O(l) -> Ca(OH)2(s) ? H° = -64.5 kJ/mol
Economic factors
There is currently a market for concentrated CO2 at around $300/tonne, based on demand from horticulture in greenhouses, and oil extraction. This has the potential to allow proof of concept plants to be developed without relying on state funding for geoengineering uses.
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