CO2 Capture and Storage System

  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.

1.   INTRODUCTION

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 Earth’s global warming. CO₂ is emitted when fuel is burnt – 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  

                 

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.

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.

3.   THE CHARACTERISTICS OF CCS

Capture of CO2 can be applied to large point sources. The CO2 would then be compressed and transported for storage in geological formations, in the ocean, in mineral carbonates2, or for use in industrial processes. Large point sources of CO2 include large fossil fuel or biomass energy facilities, major CO2-emitting industries, natural gas production, synthetic fuel plants and fossil fuel-based hydrogen production plants (see Table 3.1).

Potential technical storage methods are: geological storage (in geological formations, such as oil and gas fields, unminable coal beds and deep saline formations3), ocean storage (direct release into the ocean water column or onto the deep seafloor) and industrial fixation of CO2 into inorganic carbonates. This report also discusses industrial uses of CO2, but this is not expected to contribute much to the reduction of CO2emissions.

4. 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:

• The size of the emission source,
• Whether it is stationary or mobile,
• How near it is to potential storage sites, and
• 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 4.1 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 Biomass Bioethanol and bioenergy 303 91 Total 7,887 13,466 Table 4.1 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 4.2).

Many stationary emission sources lie either directly above, or within reasonable distance (less than 300km) from areas with potential for geological storage (see Fig 4.2 & Fig 4.3)  

5. 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 5.1 CO2 capture process.
 

5.1  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 3–15% 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).

5.2 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.

 

5.3 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 95–99% 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 85–95% 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 10–40% 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 80–90%. 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%.

 

 

6. 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 Mt CO₂ per year from natural and anthropogenic sources, and it is mainly used for EOR. These pipelines operate in the ‘dense phase’ 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 -20ºC 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. 

 

7. 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.

7.1 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. Fig7.1.1 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.

.

Reservoir type	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 7.1.1 Storage capacity for several geological storage options.

8. METHODOLOGICAL FRAMEWORK FOR CO2 CAPTURE AND STORAGE SYSTEMS

8.1  Greenhouse Gas Inventories

The two main options for including CCS in national greenhouse gas inventories have been identified and analysed using the current methodological framework for total chain from capture to storage (geological and ocean storage). These options are: • Source reduction: To evaluate the CCS systems as mitigation options to reduce emissions to the atmosphere;

Sink enhancement: To evaluate the CCS systems using an analogy with the treatment made to CO2 removals by sinks in the sector Land Use, Land-Use Change and Forestry. A balance is made of the CO2 emissions and removals to obtain the net emission or removal. In this option, removals by sinks are related to CO2 storage. In both options, estimation methodologies could be developed to cover most of the emissions in the CCS system (see Figure 9.1), and reporting could use the current framework for preparation of national greenhouse gas inventories. In the first option, reduced emissions could be reported in the category where capture takes place. For instance, capture in power plants could be reported using lower emission factors than for plants without CCS. But this could reduce transparency of reporting and make review of the overall impact on emissions more difficult, especially if the capture process and emissions from transportation and storage are not linked. This would be emphasized where transportation and storage includes captured CO2 from many sources, or when these take place across national borders. An alternative would be to track CO2 flows through the entire capture and storage system making transparent how much CO2 was produced, how much was emitted to the atmosphere at each process stage, and how much CO2 was transferred to storage.

The second option is to report the impact of the CCS system as a sink. For instance, reporting of capture in power plants would not alter the emissions from the combustion process but the stored amount of CO2 would be reported as a removal in the inventory. Application of the second option would require adoption of new definitions not available in the UNFCCC or in the current methodological framework for the preparation of inventories. UNFCCC (1992) defines a sink as ‘any process, activity or mechanism which removes a greenhouse gas, an aerosol, or a precursor of a greenhouse gas from the atmosphere’. Although ‘removal’ was not included explicitly in the UNFCCC definitions, it appears associated with the ‘sink’ concept. CCS11 systems do not meet the UNFCCC definition for a sink, but given that the definition was agreed without having CCS systems in mind, it is likely that this obstacle could be solved (Torvanger et al., 200 ). General issues of relevance to CCS systems include system boundaries (sectoral, spatial and temporal) and these will vary in importance with the specific system and phases of the system. The basic methodological approaches for system components, together with the status of the methods and availability of data for these are discussed below. Mineral carbonation and industrial use of CO2 are addressed separately.

8.2 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 8.2 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.

            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

8.3 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 (kJ/mol)
SiO2	59.71
Al2O3	15.41
CaO	4.90	CaCO3	-179
MgO	4.36	MgCO3	-117
Na2O	3.55	Na2CO3
FeO	3.52	FeCO3
K2O	2.80	K2CO3
Fe2O3	2.63	FeCO3
	21.76	All Carbonates

T

9. 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.

10 THE CURRENT STATUS OF CCS TECHNOLOGY

There are different types of CO2 capture systems: postcombustion, pre-combustion and oxyfuel combustion. The concentration of CO2 in the gas stream, the pressure of the gas stream and the fuel type (solid or gas) are important factors in selecting the capture system. Post-combustion capture of CO2 in power plants is economically feasible under specific conditions5. It is used to capture CO2 from part of the flue gases from a number of existing power plants. Separation of CO2 in the natural gas processing industry, which uses similar technology, operates in a mature market6.

The technology required for pre-combustion capture is widely applied in fertilizer manufacturing and in hydrogen production. Although the initial fuel conversion steps of pre-combustion are more elaborate and costly, the higher concentrations of CO2 in the gas stream and the higher pressure make the separation easier. Oxyfuel combustion is in the demonstration phase7 and uses high purity oxygen.

 This results in high CO2 concentrations in the gas stream and, hence, in easier separation of CO2 and in increased energy requirements in the separation of oxygen from air.

 Pipelines are preferred for transporting large amounts of CO2 for distances up to around 1,000 km. For amounts smaller than a few million tones of CO2 per year or for larger distances overseas, the use of ships, where applicable, could be economically more attractive. Pipeline transport of CO2 operates as a mature market technology (in the USA, over 2,500 km of pipelines transport more than 40 MtCO2 per year). In most gas pipelines, compressors at the upstream end drive the flow, but some pipelines need intermediate compressor stations.

Dry CO2 is not corrosive to pipelines, even if the CO2 contains contaminants. Where the CO2 contains moisture, it is removed from the CO2 stream to prevent corrosion and to avoid the costs of constructing pipelines of corrosion-

11. THE LOCAL HEALTH, SAFETY AND ENVIRONMENT RISKS OF CCS

The local risks24 associated with CO2 pipeline transport could be similar to or lower than those posed by hydrocarbon pipelines already in operation. For existing CO2 pipelines, mostly in areas of low population density, accident numbers reported per kilometre pipeline are very low and are comparable to those for hydrocarbon pipelines. A sudden and large release of CO2 would pose immediate dangers to human life and health, if there were exposure to concentrations of CO2 greater than 7–10% by volume in air. Pipeline transport of CO2 through populated areas requires attention to route selection, overpressure protection, leak detection and other design factors. No major obstacles to pipeline design for CCS are foreseen.

 With appropriate site selection based on available subsurface information, a monitoring programme to detect problems, a regulatory system and the appropriate use of remediation methods to stop or control CO2 releases if they arise, the local health, safety and environment risks of geological storage would be comparable to the risks of current activities such as natural gas storage, EOR and deep underground disposal of acid gas. Natural CO2 reservoirs contribute to the understanding of the behaviour of CO2 underground. Features of storage sites with a low probability of leakage include highly impermeable caprocks, geological stability, absence of leakage paths and effective trapping mechanisms. There are two different types of leakage scenarios: (1) abrupt leakage, through injection well failure or leakage up an abandoned well, and (2) gradual leakage, through undetected faults, fractures or wells. Impacts of elevated CO2 concentrations in the shallow subsurface could include lethal effects on plants and subsoil animals and the contamination of groundwater. High fluxes in conjunction with stable atmospheric conditions could lead to local high CO2 concentrations in the air that could harm animals or people. Pressure build-up caused by CO2 injection could trigger small seismic events.

12. THE LEGAL AND REGULATORY ISSUES FOR IMPLEMENTING CO STORAGE

1.                  Some regulations for operations in the subsurface do exist  that may be relevant or, in some cases, directly applicable to geological storage, but few countries have specifically developed legal or regulatory frameworks for long-term CO2 storage. Existing laws and regulations regarding inter alia mining, oil and gas operations, pollution control, waste disposal, drinking water, treatment of high-pressure gases and subsurface property rights may be relevant to geological CO2 storage. Long-term liability issues associated with the leakage of CO2 to the atmosphere and local environmental impacts are generally unresolved. Some States take on longterm responsibility in situations comparable to CO2 storage, such as underground mining operations.

2. No formal interpretations so far have been agreed upon with respect to whether or under what conditions CO2 injection into the geological sub-seabed or the ocean is compatible. There are currently several treaties (notably the London26 and OSPAR27 Conventions) that potentially apply to the injection of CO2 into the geological sub-seabed or the ocean. All of these treaties have been drafted without specific consideration

of CO2 storage.

13. THE IMPLICATIONS OF CCS FOR EMISSION INVENTORIES AND ACCOUNTING

The current IPCC Guidelines2 do not include methods specific to estimating emissions associated with CCS. The general guidance provided by the IPCC can be applied to CCS. A few countries currently do so, in combination with their national methods for estimating emissions. The IPCC guidelines themselves do not yet provide specific methods for estimating emissions associated with CCS. These are expected to be provided in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories. Specific methods may be required for the net capture and storage of CO2, physical leakage, fugitive emissions and negative emissions associated with biomass applications of CCS systems.

The few current CCS projects all involve geological storage, and there is therefore limited experience with the monitoring, verification and reporting of actual physical leakage rates and associated uncertainties. Several techniques are available or under development for monitoring and verification of CO2 emissions from CCS, but these vary in applicability, site specificity, detection limits and uncertainties.

CO2 might be captured in one country and stored in another with different commitments. Issues associated with accounting for cross-border storage are not unique to CCS. Rules and methods for accounting may have to be adjusted accordingly. Possible physical leakage from a storage site in the future would have to be accounted.

14 THE GAPS IN KNOWLEDGE

There are gaps in currently available knowledge regarding some aspects of CCS. Increasing knowledge and experience would reduce uncertainties and thus facilitate decision-making with respect to the deployment of CCS for climate change mitigation.

15 APPROACHES AND TECHNOLOGIES FOR MONITORING    ENVIRONMENTAL EFFECTS

Techniques now being used for field experiments could be used to monitor some near field consequences of direct CO2 injection. For example, researchers (Barry et al., 2004, 2005; Carman et al., 2004; Thistle et al., 2005) have been developing experimental means for observing the consequences of elevated CO2 on organisms in the deep ocean. However, such experiments and studies typically look for evidence of acute toxicity in a narrow range of species (Sato, 2004; Caulfield et al., 1997; Adams et al., 1997; Tamburri et al., 2000). Sub-lethal effects have been studied by Kurihara et al. (2004). Process studies, surveys of biogeochemical tracers, and ocean bottom studies could be used to evaluate changes in ecosystem structure and dynamics both before and after an injection. It is less clear how best to monitor the health of broad reaches of the ocean interior  Ongoing long-term surveys of biogeochemical tracers and deep-sea biota could help to detect long-term changes in deep-sea ecology.

16 ENVIRONMENTAL IMPACTS, RISKS, AND RISK MANAGEMENT

Overall, there is limited knowledge of deep-sea population and community structure and of deep-sea ecological interactions. Thus the sensitivities of deep ocean ecosystems to intentional carbon storage and the effects on possibly unidentified goods and services that they may provide remain largely unknown. Most ocean storage proposals seek to minimize the volume of water with high CO2 concentrations either by diluting the CO2 in a large volume of water or by isolating the CO2 in a small volume (e.g., in CO2 lakes). Nevertheless, if deployed widely, CO2 injection strategies ultimately will produce large volumes of water with somewhat elevated CO2 concentrations (Figure 6.15). Because large amounts of relatively pure CO2 have never been introduced to the deep ocean in a controlled experiment, conclusions about environmental risk must be based primarily on laboratory and small-scale in-situ experiments and extrapolation from these experiments using conceptual and mathematical models. Natural analogues (Box 6.5) can be relevant, but differ significantly from proposed ocean engineering projects. Compared to the surface, most of the deep sea is stable and varies little in its physiochemical factors over time. The process of evolutionary selection has probably eliminated individuals apt to endure environmental perturbation. As a result, deep-sea organisms may be more sensitive to environmental disturbance than their shallow water cousins (Shirayama, 1997). Ocean storage would occur deep in the ocean where there is virtually no light and photosynthesizing organisms are lacking, thus the following discussion primarily addresses CO2 effects on heterotrophic organisms, mostly animals. The diverse fauna that lives in the waters and sediments of the deep ocean can be affected by ocean CO2 storage, leading to change in ecosystem composition and functioning. Thus, the effects of CO2 need to be identified at the level of both the individual (physiological) and the ecosystem.

Introduction of CO2 into the ocean either directly into sea water or as a lake on the sea floor would result in changes in dissolved CO2 near to and down current from a discharge point. Dissolving CO2 in sea water increases the partial pressure of CO2 (pCO2, expressed as a ppm fraction of atmospheric pressure, equivalent to µatm), causes decreased pH (more acidic) and decreased CO3 2– concentrations (less saturated). This can lead to dissolution of CaCO3 in sediments or in shells of organisms. Bicarbonate (HCO3 –) is then produced from carbonate (CO3 2–). The spatial extent of the waters with increased CO2 content and decreased pH will depend on the amount of CO2 released and the technology and approach used to introduce that CO2 into the ocean. Table shows the amount of sea water needed to dilute each tonne of CO2 to a specified .pH reduction. Further dilution would reduce the fraction of ocean at one .pH Photosynthesis produces organic matter in the ocean almost exclusively in the upper 200 m where there is both light and nutrients (e.g., PO4, NO3, NH4 +, Fe). Photosynthesis forms the base of a marine food chain that recycles much of the carbon and nutrients in the upper ocean. Some of this organic matter ultimately sinks to the deep ocean as particles and some of it is mixed into the deep ocean as dissolved organic matter. The flux of organic matter from the surface ocean provides most of the energy and nutrients to support the heterotrophic ecosystems of the deep ocean (Gage and Tyler, 1991). With the exception of the oxygen minimum zone and near volcanic CO2 vents, most organisms living in the deep ocean live in low and more or less constant CO2 levels.

table : Relationships between .pH, changes in pCO2, and dissolved inorganic carbon concentration calculated for mean deep-sea conditions.

17. 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 17.1 Costs of energy with and without CCS (2002 US$ per kWh)

18 THE FUTURE OF CO₂ CAPTURE AND STORAGE

• CO2 capture and storage is technologically feasible and could play a significant role in reducing greenhouse gas emissions over the course of this century. But many issues still need to be resolved before it can be deployed on a large scale.
• Full-scale projects in the electricity sector are needed to build knowledge and experience. More studies are required to analyse and reduce the costs and to evaluate the suitability of potential geological storage sites. Also, pilot scale experiments on mineral carbonation are needed.
• An adequate legal and regulatory environment also needs to be created, and barriers to deployment in developing countries need to be addressed.
• If knowledge gaps are filled and various conditions are met, CO2 capture and storage systems could be deployed on a large scale within a few decades, as long as policies substantially limiting greenhouse gas emissions are put into place.
• The scientific consensus views carbon capture and storage as one of the important options for reducing CO2 emissions. If it were deployed, the cost of stabilizing the concentration of greenhouse gases in the atmosphere would be reduced by 30% or more.

19 CONCLUSION

            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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