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Carbon capture and storage CCS or carbon capture and sequestration [2] is the process of capturing carbon dioxide CO 2 before it enters the atmosphere, transporting it, and storing it carbon sequestration for centuries or millennia.
Usually the CO 2 is captured from large point sources , such as a chemical plant or biomass power plant , and then stored in an underground geological formation. The aim is to prevent the release of CO 2 from heavy industry with the intent of mitigating the effects of climate change.
This is because CCS is a relatively expensive process yielding a product with an intrinsic low value i. Hence, carbon capture makes economically more sense when being combined with a utilization process where the cheap CO 2 can be used to produce high-value chemicals to offset the high costs of capture operations.
CO 2 can be captured directly from an industrial source, such as a cement kiln , using a variety of technologies; including absorption , adsorption , chemical looping , membrane gas separation or gas hydration.
Most projects are industrial. Storage of the CO 2 is envisaged either in deep geological formations, or in the form of mineral carbonates. Pyrogenic carbon capture and storage PyCCS is also being researched.
Opponents point out that many CCS projects have failed to deliver on promised emissions reductions. Capturing CO 2 is most cost-effective at point sources, such as large carbon-based energy facilities, industries with major CO 2 emissions e. Extracting CO 2 from air is possible, [17] although the lower concentration of CO 2 in air compared to combustion sources complicates the engineering and makes the process therefore more expensive.
Impurities in CO 2 streams, like sulfurs and water, can have a significant effect on their phase behavior and could pose a significant threat of increased pipeline and well corrosion.
In instances where CO 2 impurities exist, especially with air capture, a scrubbing separation process is needed to initially clean the flue gas. Broadly, three different technologies exist: post-combustion, pre-combustion, and oxyfuel combustion:. The major technologies proposed for carbon capture are: [5] [28] [29]. Absorption, or carbon scrubbing with amines is the dominant capture technology. It is the only carbon capture technology so far that has been used industrially.
Adsorbents and absorbents require regeneration steps where the CO 2 is removed from the sorbent or solution that collected it from the flue gas in order for the sorbent or solution to be reused.
Thus, to optimize a MOF for carbon capture, low heat capacities and heats of adsorption are desired. Additionally, high working capacity and high selectivity are desirable in order to capture as much CO 2 as possible. However, an energy trade off complicates selectivity and energy expenditure. Metal reservoirs are another limiting factor. Optimizing capture would significantly increase CCS feasibility since the transport and storage steps of CCS are rather mature.
An alternate method is chemical looping combustion CLC. Looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of CO 2 and water vapor. The water vapor is condensed, leaving pure CO 2 , which can then be sequestered. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles for return to the combustor.
A variant of chemical looping is calcium looping , which uses the alternating carbonation and then calcination of a calcium oxide based carrier. A study found CCS plants to be less effective than renewable electricity.
The electrical energy returned on energy invested EROEI ratios of both production methods were estimated, accounting for their operational and infrastructural energy costs. Renewable electricity production included solar and wind with sufficient energy storage, plus dispatchable electricity production.
Thus, rapid expansion of scalable renewable electricity and storage would be preferable over fossil-fuel with CCS. The study did not consider whether both options could be pursued in parallel. In High Hopes proposed using high-altitude balloons to capture CO 2 cryogenically, using hydrogen to lower the already low-temperature atmosphere sufficiently to produce dry ice that is returned to earth for sequestration. In sorption enhanced water gas shift SEWGS technology a pre-combustion carbon capture process, based on solid adsorption, is combined with the water gas shift reaction WGS in order to produce a high pressure hydrogen stream.
After capture, the CO 2 must be transported to suitable storage sites. Pipelines are the cheapest form of transport.
Ships can be utilized where pipelines are infeasible, and for long enough distances ships may be cheaper than a pipeline. Rail and tanker truck cost about twice as much as pipelines or ships. For example, approximately 5, km of CO 2 pipelines operated in the US in , and a km pipeline in Norway, [44] used to transport CO 2 to oil production sites where it is injected into older fields to extract oil.
This injection is called enhanced oil recovery. Pilot programs are in development to test long-term storage in non-oil producing geologic formations. Various approaches have been conceived for permanent storage. These include gaseous storage in deep geological formations including saline formations and exhausted gas fields , and solid storage by reaction of CO 2 with metal oxides to produce stable carbonates.
It was once suggested that CO 2 could be stored in the oceans, but this would exacerbate ocean acidification and was banned under the London and OSPAR conventions. Geo-sequestration, involves injecting CO 2 , generally in supercritical form, into underground geological formations. Oil fields , gas fields , saline formations, unmineable coal seams , and saline-filled basalt formations have been suggested as alternatives.
Physical e. Unmineable coal seams can be used because CO 2 molecules attach to the coal surface. Technical feasibility depends on the coal bed's permeability. In the process of absorption the coal releases previously absorbed methane , and the methane can be recovered enhanced coal bed methane recovery.
Methane revenues can offset a portion of the cost, although burning the resultant methane, however, produces another stream of CO 2 to be sequestered. Saline formations contain mineralized brines and have yet to produce benefit to humans. Saline aquifers have occasionally been used for storage of chemical waste in a few cases. The main advantage of saline aquifers is their large potential storage volume and their ubiquity.
The major disadvantage of saline aquifers is that relatively little is known about them. To keep the cost of storage acceptable, geophysical exploration may be limited, resulting in larger uncertainty about the aquifer structure.
Unlike storage in oil fields or coal beds, no side product offsets the storage cost. Trapping mechanisms such as structural trapping, residual trapping, solubility trapping and mineral trapping may immobilize the CO 2 underground and reduce leakage risks. CO 2 is occasionally injected into an oil field as an enhanced oil recovery technique, [50] but because CO 2 is released when the oil is burned, [51] it is not carbon neutral. CO 2 can be physically supplied to algae or bacteria that could degrade the CO 2.
It would ultimately be ideal to exploit CO 2 metabolizing bacterium Clostridium thermocellum. CO 2 can exothermically react with metal oxides, which in turn produce stable carbonates e. This process CO 2 -to-stone occurs naturally over periods of years and is responsible for much surface limestone.
Olivine is one such MOX. Ultramafic mine tailings are a readily available source of fine-grained metal oxides that can serve this purpose. Cost is a significant factor affecting CCS. The cost of CCS, plus any subsidies, must be less than the expected cost of emitting CO 2 for a project to be considered economically favorable. CCS technology is expected to use between 10 and 40 percent of the energy produced by a power station. Constructing CCS units is capital intensive.
Other applications are possible. CCS trials for coal-fired plants in the early 21st century were economically unviable in most countries, [64] including China, [65] in part because revenue from enhanced oil recovery collapsed with the oil price collapse.
Possible business models for industrial carbon capture include: [7]. Governments have provided various types of funding for CCS demonstration projects, including tax credits, allocations and grants. CO 2 can be captured with alkaline solvents at low temperatures in the absorber and released CO 2 at higher temperatures in a desorber.
Chilled ammonia CCS plants emit ammonia. Alternative amines with little to no vapor pressure can avoid these emissions. Plants equipped with selective catalytic reduction systems for nitrogen oxides produced during combustion [77] require proportionally greater amounts of ammonia.
A study concluded that half as much CCS might be installed in coal-fired plants as in gas-fired: these would be mainly in China and India. Plants equipped with flue-gas desulfurization FGD systems for sulfur dioxide control require proportionally greater amounts of limestone , and systems equipped with selective catalytic reduction systems for nitrogen oxides produced during combustion require proportionally greater amounts of ammonia. IPCC estimates that leakage risks at properly managed sites are comparable to those associated with current hydrocarbon activity.
It recommends that limits be set to the amount of leakage that can take place. Mineral storage is not regarded as presenting any leakage risks. Norway's Sleipner gas field is the oldest industrial scale retention project. An environmental assessment conducted after ten years of operation concluded that geosequestration was the most definite form of permanent geological storage method:.
Available geological information shows absence of major tectonic events after the deposition of the Utsira formation [saline reservoir]. This implies that the geological environment is tectonically stable and a site suitable for CO 2 storage.
The solubility trapping [is] the most permanent and secure form of geological storage. In March StatoilHydro issued a study documenting the slow spread of CO 2 in the formation after more than 10 years operation. Gas leakage into the atmosphere may be detected via atmospheric gas monitoring, and can be quantified directly via eddy covariance flux measurements. Transmission pipelines may leak or rupture. Pipelines can be fitted with remotely controlled valves that can limit the release quantity to one pipe section.
For example, a severed 19" pipeline section 8 km long could release its 1, tonnes in about 3—4 min. Large-scale releases present asphyxiation risk. In the Menzengraben mining accident , several thousand tonnes were released and asphyxiated a person meters away. Monitoring allows leak detection with enough warning to minimize the amount lost, and to quantify the leak size. Monitoring can be done at both the surface and subsurface levels.
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