To achieve stabilization of atmospheric CO2 concentrations at an approximate doubling of pre-industrial levels, current carbon emissions of about 7 gigatons per year (GtC/year) will need to be reduced by about two thirds by the end of the century, followed by continued reductions in the following centuries. Technologies associated with CO2 capture and storage (CCS) are increasingly considered to be likely contributors to achieving these CO2 emission reductions. The term CCS incorporates many technologies associated with capturing CO2 and storing the associated carbon in a reservoir other than the atmosphere.
In one type of CCS scheme, the CO2 is captured from large point sources, mainly power plants. Several technologies exist to do this. CO2 can be absorbed from the flue gas of a conventional plant, typically with a regenerable liquid solvent like monoethanolamine (IPCC, 2005). It is recovered from the solvent during regeneration at a high enough concentration (> 90%) for compression. The process is termed "postcombustion capture". One can also remove CO2 from the fuel before combustion ("pre-combustion capture") by converting it to hydrogen and CO2 by means of a water-gas shift reaction. CO2 is collected after the shift then hydrogen is burned. The third option is to feed the combustor with pure oxygen instead of air, an "oxyfuel" system. This way, the flue gas has a high enough CO2 concentration to be compressed directly. Energy is saved by avoiding the solvent absorption and regenerating but expended for separation of oxygen from air. Once compressed to a liquid, the CO2 is ready for transportation to a sequestration site.
Several sequestration routes have been suggested.
In a typical geosequestration process, the captured and compressed CO2 is transported by pipeline to a suitable storage location. There, it is injected into a deep geological formation beneath the Earth's surface where it is intended that the CO2 will be stored in isolation from the atmosphere indefinitely. However geosequestration carries many unknown risks including potential leakage from unpredictable changes to the storage matrix or natural geophysical events.
Ocean sequestration involves the direct injection of a relatively pure CO2 stream to ocean depths greater than 1000 meters. The long term effectiveness and potential environmental consequences of this strategy however are also unknown.
But even if these schemes were employed present infrastructure does not lend itself to CO2 collection. Current fuel sources and power plants would require costly retrofits. In addition, more than half of all the CO2 emissions are virtually inaccessible to collection at the source. These inaccessible emissions encompass small distributed sources, like home heating and, most importantly, the entire transportation sector. While collecting CO2 on board of a vehicle has been discussed, it does not appear cost-effective. An obvious problem is the weight that would need to be stored on board. In the case of carbonates, this would be more than six times the weight of a full gasoline tank. Moreover this approach would require a massive infrastructure for recycling the sorbent and shipping the CO2. The mass handling alone would dwarf that of gasoline distribution. It may be possible to develop a carbon-free transportation sector, for example by moving to electric vehicles. However, the cost of abandoning the existing infrastructure and replacing it with an immature, and not yet practical one, would be prohibitively expensive.
Air extraction is an appealing concept, because it separates the source from disposal. One could collect CO2 after the fact and from any source. Air extraction could reduce atmospheric CO2 levels without making the existing energy or transportation infrastructure obsolete. There would be no need for a network of pipelines shipping CO2 from its source to its disposal site. The atmosphere would act as a temporary storage and transport system. Carbon dioxide capture from the atmosphere, in principle, can deal with any source, large or small. Indeed, the appeal of biomass for sequestration and of credits for growing trees is based on the very same premise. However, biomass affects climate not only through the carbon cycle; it also affects both the absorption of solar radiation and evapotranspiration. For example, it has been shown that large boreal forests have the potential to warm the planet, offsetting the benefits of carbon storage.
Aquatic biomass represents another large potential sink for sequestration of CO2. This second method is accomplished by the fertilization of phytoplankton with micro- or macro-nutrients in the sea, but again the long term ecological impact of large scale tampering with this complex and vital system has not yet been established.
The third method, soil carbon sequestration is the process of transferring carbon dioxide from the atmosphere into the soil through crop residues and other organic solids, and in a form that is not immediately re-emitted. Soil carbon sequestration can be accomplished by management systems that add high amounts of biomass to the soil, cause minimal soil disturbance, conserve soil and water, improve soil structure, and enhance soil fauna activity. Continuous no-till crop production is a prime example. However the worlds soils, are limited in their uptake capacity and it is unclear how policy could effectively mandate massive changes to agricultural practices on the scale necessary.
Current research is focused on designing a device that would rely on chemical reactions to capture CO2 directly from the air. This is a technical challenge due to the low concentrations of CO2 in the ambient atmosphere nevertheless several methods have been suggested.
These systems use either a Ca(OH)2 or NaOH solution to capture CO2 and then bury the resultant solid or regenerate the solution capturing the evolved gas. These methods differ from conventional mitigation in three key aspects. First, they remove emissions from any part of the economy with equal ease or difficulty, so its cost provides an absolute cap on the cost of mitigation. Second, they permit reduction in concentrations faster than the natural carbon cycle: the effects of reversibility are thus generally alleviated. Third, because they are weakly coupled to existing energy infrastructure, they may offer stronger economies of scale and smaller adjustment costs than the more conventional mitigation technologies.
However the effectiveness of such techniques for atmospheric capture have been hotly disputed. For solid sequestration a considerable amount of feed stock for the sorbent would have to be mined and milled with the associated energy overheads. While the energy requirements and utility of the chemical regeneration method are well established, as they are practiced on a large scale in the industrial kraft process used in pulp and paper production, the energy and land use requirements of the collectors for both routes are uncertain as this component is not currently implemented industrially.
One interesting possibility that has been suggested is that the captured gas from this method could be turned back into fuel via the Sabatier reaction in which carbon dioxide is combined with hydrogen in the presence of a ruthenium on alumina catalyst to produce methane. A second similar reaction produces ethane, propane, pentane, methanol, and ethylene. Discovered by the French chemist Paul Sabatier the reaction has been known since the 1800's and has been extensively researched by NASA as a method of refueling a future Mars mission. The potential synergy of using heat from a nuclear reactor to both generate hydrogen from water and free carbon dioxide from the sorbent to supply this process cannot be overlooked.
The carbon problem is an unprecedented challenge to humanity. It is global in scope, its time-scale is centuries, and the mitigation strategies required are often fraught with risks as large as the problem itself. It is unlikely that any of the methods discussed in this overview acting alone or in concert will provide a satisfactory solution to this issue without being combined with a sharp reduction in the use of fossil fuels and their replacement with an effective carbon neutral source such as nuclear energy, where practical.
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