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CARBONDIOXIDE CAPTURING AND STORAGE
Approximately one third of all CO2 emissions due to human activity come from fossil fuels used for generating electricity, with each power plant capable of emitting several million tones of CO2 annually. A variety of other industrial processes also emit large amounts of CO2 from each plant, for example oil refineries, cement works, and iron and steel production. These emissions could be reduced substantially, without major changes to the basic process, by capturing and storing the CO2. Other sources of emissions, such as transport and domestic buildings, cannot be tackled in the same way because of the large number of small sources of CO2.
Carbon capture and storage (CCS) is an approach to minimize global warming by capturing carbon dioxide (CO2) from large point sources such as fossil fuel power plants and storing it instead of releasing it into the atmosphere CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without CCS.
Carbon dioxide (CO2) is a greenhouse gas that occurs naturally in the atmosphere. Human activities are increasing the concentration of CO2 in the atmosphere thus contributing to Earthâ„¢s global warming. CO2 is emitted when fuel is burnt â€œ be it in large power plants, in car engines, or in heating systems. It can also be emitted by some other industrial processes, for instance when resources are extracted and processed, or when forests are burnt.Currently, 30 Gt per year of CO2 is emitted due to human activities.The increase in concentration of carbon in the past two hundred years is shown in the Fig 1.1
Fig 1.1 Increase in concentration of CO2 in past two centuries
Fig 1.2 Increase in global temperature in past 200 years.
One possible option for reducing CO2 is to store it underground. This technique is called Carbon dioxide Capture and Storage (CCS).
In Carbon capture and storage (CCS), carbon dioxide (CO2) is capured from large point sources (A point source of pollution is a single identifiable localized source of air, water, thermal, noise or light pollution).such as fossil fuel power plants and storing it instead of releasing it into the atmosphere. Although CO2 has been injected into geological formations for various purposes, the long term storage of CO2 is a relatively untried.
CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without CCS.
Fig 1.3 Power plants with and with out CCS.
The section2 presents the general framework for the assessment together with a brief overview of CCS systems. Section 3 then describes the major sources of CO2, a step needed to assess the feasibility of CCS on a global scale. Technological options for CO2 capture are then discussed in Section 4, while Section 5 focuses on methods of CO2 transport. Following this, each of the storage options is addressed on section 6. Section 6.1 focuses on geological storage, Section 6.2 on ocean storage, and Section 6.3 on mineral carbonation of CO2 section 7 discus the risk of CO2 leakage, The overall costs and economic potential of CCS are then discussed in Section 8, followed by the conclusion in Section 9.
2. CARBON DIOXIDE CAPTURE AND STORAGE
One technique that could limit CO2 emissions from human activities into the atmosphere is Carbon dioxide Capture and Storage (CCS). It involves collecting, at its source, the CO2 that is produced by power plants or industrial facilities and storing it away for a long time in underground layers, in the oceans, or in other materials
The process involves three main steps:
1. capturing CO2, at its source, by separating it from other gases produced by an industrial process
2. transporting the captured CO2 to a suitable storage location (typically in compressed form)
3. storing the CO2 away from the atmosphere for a long period of time, for instance in underground geological formations, in the deep ocean, or within certain mineral compounds.
Fig 2.1 The three main components of the CCS process
Fig 2.2 The Esbjerg Power Station, a CO2 capture site in Denmark
3. SOURCES OF CO2 EMISSIONS SUITABLE FOR CAPTURE AND STORAGE
Several factors determine whether carbon dioxide capture is a viable option for a particular emission source:
Â¢ the size of the emission source,
Â¢ whether it is stationary or mobile,
Â¢ how near it is to potential storage sites, and
Â¢ how concentrated its CO2 emissions are.
Carbon dioxide could be captured from a large stationary emission sources such as a power plants or industrial facilities that produce large amounts of carbon dioxide. If such facilities are located near potential storage sites, for example suitable geological formations, they are possible candidates for the early implementation of CO2 capture
and storage (CCS).
Small or mobile emission sources in homes, businesses or transportation are not being considered at this stage because they are not suitable for capture and storage.
Fig 3.1 The Gibson coal power plant, a good example of a large stationary source.
Process Number of sources Emissions (MtCO2 yr-1)
Fossil fuels Power 4,942 10,539
Cement production 1,175 932
Refineries 638 798
Iron and steel industry 269 646
Petrochemical industry 470 379
Oil and gas processing N/A 50
Other sources 90 33
Bioethanol and bioenergy 303 91
Total 7,887 13,466
Table 3.1 Profile by process or industrial activity of worldwide large stationary CO2 sources with emissions of more than 0.1 MtCO2 per year.
In 2000, close to 60% of the CO2 emissions due to the use of fossil fuels were produced by large stationary emission sources, such as power plants and oil and gas extraction or processing industries (see Table 3.1).
Four major clusters of emissions from such stationary emission sources are: the Midwest and eastern USA, the northwestern part of Europe, the eastern coast of China and the Indian subcontinent (see Figure 3.2).
Fig 3.2 Global Distribution of large CO2 sources
Many stationary emission sources lie either directly above, or within reasonable distance (less than 300km) from areas with potential for geological storage (see Fig 3.2 & Fig 3.3)
Fig 3.3 Possible storage sites
4. CO2 CAPTURE
The purpose of CO2 capture is to produce a concentrated stream of CO2 at high pressure that can readily be transported to a storage site. Although, in principle, the entire gas stream containing low concentrations of CO2 could be transported and injected underground, energy costs and other associated costs generally make this approach impractical. It is therefore necessary to produce a nearly pure CO2 stream for transport and storage. Applications separating CO2 in large industrial plants, including natural gas treatment plants and ammonia production facilities, are already in operation today. Currently, CO2 is typically removed to purify other industrial gas streams. Removal has been used for storage purposes in only a few cases; in most cases, the CO2 is emitted to the atmosphere. Capture processes also have been used to obtain commercially useful amounts of CO2 from flue gas streams generated by the combustion of coal or natural gas. However, there have been no applications of CO2 capture at large (e.g., 500 MW) power plants.
Three systems are available for power plants: post-combustion, pre-combustion, and oxy fuel combustion systems. The captured CO2 must then be purified and compressed for transport and storage.
Fig 4.1 CO2 capture process.
4.1 Post-Combustion Systems
This system separate CO2 from the flue gases produced by the combustion of the primary fuel in air. These systems normally use a liquid solvent to capture the small fraction of CO2 (typically 3â€œ15% by volume) present in a flue gas stream in which the main constituent is nitrogen (from air). For a modern pulverized coal (PC) power plant or a natural gas combined cycle (NGCC) power plant, current post-combustion capture systems would typically employ an organic solvent such as monoethanolamine (MEA).
Fig 4.2 Gas turbine combine cycle with post-combustion
4.2 Pre-Combustion Systems
In this process the primary fuel in a reactor with steam and air or oxygen to produce a mixture consisting mainly of carbon monoxide and hydrogen (synthesis gas). Additional hydrogen, together with CO2, is produced by reacting the carbon monoxide with steam in a second reactor (a shift reactor). The resulting mixture of hydrogen and CO2 can then be separated into a CO2 gas stream, and a stream of hydrogen. If the CO2 is stored, the hydrogen is a carbon-free energy carrier that can be combusted to generate power and/or heat. Although it is costly than post-combustion systems, the high concentrations of CO2 produced by the shift reactor (typically 15 to 60% by volume on a dry basis) and the high pressures often encountered in these applications are more favorable for CO2 separation.
Fig 4.3 Pre-combustion capture of CO2
4.3 Oxyfuel Combustion Systems
This system use oxygen instead of air for combustion of the primary fuel to produce a flue gas that is mainly water vapour and CO2. This results in a flue gas with high CO2 concentrations (greater than 80% by volume). The water vapour is then removed by cooling and compressing the gas stream. Oxyfuel combustion requires the upstream separation of oxygen from air, with a purity of 95â€œ99% oxygen assumed in most current designs. Further treatment of the flue gas may be needed to remove air pollutants and non- condensed gases (such as nitrogen) from the flue gas before the CO2 is sent to storage. As a method of CO2 capture in boilers, oxyfuel combustion systems are in the demonstration phase. Oxyfuel systems are also being studied in gas turbine
Current post-combustion and pre-combustion systems for power plants could capture 85â€œ95% of the CO2 that is produced. Higher capture efficiencies are possible, although separation devices become considerably larger, more energy intensive and more costly. Capture and compression need roughly 10â€œ40% more energy than the equivalent plant without capture, depending on the type of system. Due to the associated CO2 emissions, the net amount of CO2 captured is approximately 80â€œ90%. Oxyfuel combustion systems are, in principle, able to capture nearly all of the CO2 produced. However, the need for additional gas treatment systems to remove pollutants such as sulphur and nitrogen oxides lowers the level of CO2 captured to slightly more than 90%.
5. CO2 TRANSPORTATION
After capture, the CO2 must be transported to suitable storage sites. Today Pipelines operate as a mature market technology and are the most common method for transporting CO2. Gaseous CO2 is typically compressed to a pressure above 8 MPa in order to avoid two-phase flow regimes and increase the density of the CO2, thereby making it easier and less costly to transport. CO2 also can be transported as a liquid in ships, road or rail tankers that carry CO2 in insulated tanks at a temperature well below ambient, and at much lower pressures.
The first long-distance CO2 pipeline came into operation in the early 1970s. In the United States, over 2,500 km of pipeline transports more than 40 MtCO2 per year from natural and anthropogenic sources, and it is mainly used for EOR. These pipelines operate in the Ëœdense phaseâ„¢ mode (in which there is a continuous progression from gas to liquid, without a distinct phase change), and at ambient temperature and high pressure. In most of these pipelines, the flow is driven by compressors at the upstream end, although some pipelines have intermediate (booster) compressor stations.
In some situations or locations, transport of CO2 by ship may be economically more attractive, particularly when the CO2 has to be moved over large distances or overseas. Liquefied petroleum gases (LPG, principally propane and butane) are transported on a large commercial scale by marine tankers. CO2 can be transported by ship in much the same way (typically at 0.7 MPa pressure), but this currently takes place on a small scale because of limited demand. The properties of liquefied CO2 are similar to those of LPG, and the technology could be scaled up to large CO2 carriers if a demand for such systems were to materialize.
Road and rail tankers also are technically feasible options. These systems transport CO2 at a temperature of -20Ã‚ÂºC and at 2 MPa pressure. However, they are uneconomical compared to pipelines and ships, except on a very small scale, and are unlikely to be relevant to large-scale CCS.
Fig 5.1 An LPG tanker-CO2 can be transported in the similar way.
6. CO2 STORAGE (SEQUESTRATION)
Various forms have been conceived for permanent storage of CO2. These forms include gaseous storage in various deep geological formations (including saline formations and exhausted gas fields), liquid storage in the ocean, and solid storage by reaction of CO2 with metal oxides to produce stable carbonates.
6.1 Geological Storage.
Also known as geo-sequestration, this method involves injecting carbon dioxide, directly into underground geological formations. Geological formations are currently considered the most promising sequestration sites, and these are estimated to have a storage capacity of at least 2000 Gt CO2 (currently, 30 Gt per year of CO2 is emitted due to human activities). Oil fields, gas fields, saline formations, unminable coal seams, and saline-filled basalt formations have been suggested as storage sites. Various physical (e.g., highly impermeable caprock) and geochemical trapping mechanisms would prevent the CO2 from escaping to the surface. CO2 is sometimes injected into declining oil fields to increase oil recovery (enhanced oil recovery).CO2 storage in hydrocarbon reservoirs or deep saline formations is generally expected to take place at depths below 800 m, where the ambient pressures and temperatures will usually result in CO2 being in a liquid or supercritical state. Under these conditions, the density of CO2 will range from 50 to 80% of the density of water. This is close to the density of some crude oils, resulting in buoyant forces that tend to drive CO2 upwards. Fig6.1.1 shows some of the methods used in geological storage.
This option is attractive because the storage costs may be partly offset by the sale of additional oil that is recovered
Unminable coal seams can be used to store CO2 because CO2 adsorbs to the surface of coal. However, the technical feasibility depends on the permeability of the coal bed. In the process of absorption the coal releases previously absorbed methane, and the methane can be recovered (enhanced coal bed methane recovery). The sale of the methane can be used to offset a portion of the cost of the CO2 storage.
Saline formations contain highly mineralized brines, and have so far been considered of no benefit to humans. Saline aquifers have 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 common occurrence. This will reduce the distances over which CO2 has to be transported. The major disadvantage of saline aquifers is that relatively little is known about them, compared to oil fields.
For well-selected, designed and managed geological storage sites, IPCC estimates that CO2 could be trapped for millions of years, and the sites are likely to retain over 99% of the injected CO2 over 1,000 years.
Fig 6.1.1 Geological storage options.
Lower estimate of storage capacity (GtCO2) Upper estimate of storage capacity (GtCO2)
Oil and gas fields 675a 900a
Unminable coal seams (ECBM) 3-15 200
Deep saline formations 1,000 Uncertain, but possibly 104
Table 6.1.1 Storage capacity for several geological storage options.
6.2 Ocean Storage
A potential CO2 storage option is to inject captured CO2 directly into the deep ocean (at depths greater than 1,000 m), where most of it would be isolated from the atmosphere for centuries. This can be achieved by transporting CO2 via pipelines or ships to an ocean storage site, where it is injected into the water column of the ocean or at the sea floor. The dissolved and dispersed CO2 would subsequently become part of the global carbon cycle. Fig 6.1.2 shows some of the main methods that could be employed. Ocean storage has not yet been deployed or demonstrated at a pilot scale, and is still in the research phase. However, there have been small- scale field experiments and 25 years of theoretical, laboratory and modeling studies of intentional ocean storage of CO2.
Fig 6.2.1 Ocean storage methods.
Fig 6.1.2 CO2can be injected into the deep ocean from oil platforms.
CO2 injection, however, can harm marine organisms near the injection point. It is furthermore expected that injecting large amounts would gradually affect the whole ocean. Because of its environmental implications, CO2 storage in oceans is generally no longer considered as an acceptable option
6.3 Mineral Storage
Through chemical reactions with some naturally occurring minerals, CO2 is converted into a solid form through a process called mineral carbonation and stored virtually permanently. This is a process which occurs naturally, although very slowly.
These chemical reactions can be accelerated and used industrially to artificially store CO2 in minerals. However, the large amounts of energy and mined minerals needed makes this option less cost effective.
Earthen Oxide Percent of Crust Carbonate Enthalpy change
CaO 4.90 CaCO3
MgO 4.36 MgCO3
Na2O 3.55 Na2CO3
FeO 3.52 FeCO3
K2O 2.80 K2CO3
Fe2O3 2.63 FeCO3
21.76 All Carbonates
Table 6.3.1 Principal metal oxides of Earth's Crust. Theoretically up to 22% of this mineral mass is able to form carbonates.
7. RISK OF LEAKAGE
The risks due to leakage from storage of CO2 in geological reservoirs fall into two broad categories: global risks and local risks. Global risks involve the release of CO2 that may contribute significantly to climate change if some fraction leaks from the storage formation to the atmosphere. In addition, if CO2 leaks out of a storage formation, local hazards may exist for humans, ecosystems and groundwater. These are the local risks.
Fig 8.1 Geological leakage routes
8. COST OF CO2 CAPTURE AND STOREGE OPERATIONS
CCS applied to a modern conventional power plant could reduce CO2 emissions to the atmosphere by approximately 80-90% compared to a plant without CCS. Capturing and compressing CO2 requires much energy and would increase the fuel needs of a coal-fired plant with CCS by about 25%. These and other system costs are estimated to increase the cost of energy from a new power plant with CCS by 21-91%.
Natural gas combined cycle Pulverized coal Integrated gasification combined cycle
Without capture (reference plant) 0.03 - 0.05 0.04 - 0.05 0.04 - 0.06
With capture and geological storage 0.04 - 0.08 0.06 - 0.10 0.06 - 0.09
With capture and Enhanced oil recovery
0.04 - 0.07 0.05 - 0.08 0.04 - 0.08
Table 8.1 Costs of energy with and without CCS (2002 US$ per kWh)
Large reductions in emissions of CO2 to the atmosphere are likely to be needed to avoid major climate change. Capture and storage ofCO2, in combination with other CO2 abatement techniques, could enable these large reductions to be achieved with least impact on the global energy infrastructure and the economy. Capture and storage is particularly well suited to use in central power generation and many energy-intensive industrial processes. CO2 capture and storage technology also provides a means of introducing hydrogen as an energy carrier for distributed and mobile energy users.
For power stations, the cost of capture and storage is about $50/t ofCO2 avoided. This compares favorably with the cost of many other options considered for achieving large reductions in emissions. Use of this technique would allow continued provision of large-scale energy supplies using the established energy infrastructure. There is considerable scope for new ideas to reduce energy consumption and costs of CO2 capture and storage which would accelerate the development and introduction of this technology
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