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:
- capturing CO2, at its source, by separating it from
other gases produced by an industrial process
- transporting the captured CO2 to a suitable storage
location (typically in compressed form)
- 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.
- 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)
- 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
REFERENCES
- Department of Trade and Industry (UK), Gasification of Solid
and Liquid Fuels for Power Generation, report TSR 008, Dec. 1998
- Department of Trade and Industry (UK), Supercritical Steam
Cycles for Power Generation Applications, report TSR 009, Jan. 1999
- Durie R, Paulson C, Smith A and Williams D, Proceedings of the
5thInternational Conference on Greenhouse Gas Control Technologies,
CSIRO(Australia) publications, 2000
- Eliasson B, Riemer P W F and Wokaun A (editors), Greenhouse Gas
Control Technologies, Proceedings of the 4th International Conference,
Elsevier Science Ltd., Oxford 1999
- Herzog H, Eliasson B and Kaarstad O, Capturing Greenhouse
Gases, Scientific American, Feb.
2000, 54-61
- Intergovernmental Panel on Climate Change (IPCC), Climate
Change 1995 -The Science of Climate Change, Cambridge University Press, 1996
- International Energy Agency, Key World Energy Statistics, 1999
edition.IEA Greenhouse Gas R&D Programme, Transport &Environmental
Aspects of Carbon Dioxide Sequestration, 1995, ISBN 1 898373 22 1
- IEA Greenhouse Gas R&D Programme, Abatement of Methane
Emissions, June1998, ISBN 1 898
373 16 7
- IEA Greenhouse Gas R&D Programme, Ocean Storage of CO2, Feb. 1999, ISBN 1 898 373 25 6
- IEA Greenhouse Gas R&D Programme, The Reduction of
Greenhouse Gas Emissions from the Cement Industry, report PH3/7, May 1999
- IEA Greenhouse Gas R&D Programme, The Reduction of
Greenhouse Gas Emissions from the Oil Refining and Petrochemical Industry,
report PH3/8, June 1999
- www.ipcc.ch
- www.Greenfacts.org
- www.ieagreen.org.uk
2 tier architecture