CO2 Capture and Storage
Background
Maintaining the benefits of access to low cost energy derived from
fossil fuels, whilst at the same time significantly decreasing CO2
emissions to the atmosphere, is a major challenge in the world today.
The most promising technologies for significantly decreasing emissions
from large scale stationary sources of CO2 involve separating and
capturing the CO2, compressing and then storing it in geological
or other locations where it will not leak back into the atmosphere.
CO2 Capture
Technologies for capturing CO2 from emission streams have been
used for many years to produce a pure stream of CO2 from natural
or industrial CO2 emissions for use in the food processing and chemical
industries. The gas industry routinely separates CO2 from natural
gas before it is then transported to market by pipeline. Methods
currently used for CO2 separation include:
- Physical and chemical solvents, particularly monoethanolamine
(MEA)
- Various types of membranes
- Adsorption onto zeolites and other solids
- Cryogenic separation
These methods can be applied to a range of industrial processes.
However, their use for separating out CO2 from high volume-low CO2
concentration flue gases, such as those generated by conventional
pulverized coal-fired power stations, is much more problematic.
The very high capital costs of installing the huge post combustion
separation systems needed to process massive volumes of flue gases
is a major impediment to post combustion capture of CO2. The second
problem is the large amount of additional energy (25-35%) used to
release the CO2 from solvents or from solid adsorbents after separation.
Some major technical and cost challenges therefore need to be addressed
before retrofit (or new build) of post-combustion capture systems
becomes an effective mitigation option. The present cost of post-combustion
capture of CO2 is commonly quoted in the range of US$30-60 a tonne
of CO2. This would probably more than double the wholesale price
of electricity. However, capture technologies will undoubtedly improve
in the coming years, thereby improving economics.
A key to achieving lower capture costs lies in the production of a
more concentrated, pressurised stream of CO2 (the average PF generator
has only 10-14% CO2 in the flue gas stream). This can be achieved
through the pre-combustion capture of CO2 or oxyfuel combustion.
Pre-combustion capture is only possible together with integrated
gasification combined cycle (IGCC). This involves first the partial
combustion of coal or gas in air (or oxygen) to produce a CO plus
H2 gas stream, which is reacted with hot steam to produce CO2 plus
more H2. The H2 is then combusted in a gas turbine and the CO2 is
available for storage or use. Although IGCC plants are relatively
common, few are used for base load power or incorporate CO2 capture.
Figures currently available indicate that - even allowing for the
initial capital costs - new build IGCC producing a pure stream of
CO2 is a cheaper option than the retrofit of a power station coupled
with post-combustion capture.
An alternative approach is oxyfuel combustion, which relies on
the relatively simple principle of burning coal in an oxygen-rich
atmosphere to produce a pure stream of CO2. Whilst the principle
is simple, there are major issues to overcome, including the very
high combustion temperatures and the cost of producing the oxygen.
Oxyfuel combustion for power generation maybe a future option though
it has yet to confirm its operational and commercial viability.
Much the same oxycombustion technique is used in steel making and
consequently there may be no insurmountable technical barriers to
CO2 storage linked to oxyfuel power generation in the future. As
long as the main pressure on power companies is to decrease electricity
costs, it is unrealistic to expect them to voluntarily retrofit
postcombustion CO2 capture or move to IGCC or oxyfuels. However,
if in the future the main pressure is to reduce CO2 emissions, all
technical options, including perhaps post-combustion retrofit, will
receive increased attention given that many countries have made
massive investments in conventional thermal power stations.
Storing CO2
A number of CO2 storage options are being considered at the present
time - ocean storage, mineral storage and geological storage.
Ocean Storage
Ocean storage of CO2 involves two main options: one is the dispersal
of CO2 as droplets at intermediate water depths of around 500-1000m;
the other is disposal at abyssal depths (5000m or more) as liquid
CO2. Given the ocean is an enormous sink for CO2 and the system
is strongly buffered, the injected CO2 would probably have a negligible
effect on the chemistry of the ocean as a whole. Yet it would result
in a measurable drop in the pH of seawater in the immediate vicinity
of the injection site and impact on marine organisms. The ocean
is an open system and it would be difficult, if not impossible,
to monitor the distribution of the stored carbon to confirm residence
times of CO2. Also, the impact of elevated levels of CO2 on marine
ecosystems is poorly known and difficult to monitor. The potential
application of the London Dumping Convention to ocean storage of
CO2 also raises legal uncertainties. For all these reasons, there
is widespread opposition to ocean storage and it is most unlikely
to be a CO2 storage option in the foreseeable future.
Mineral Storage
Mineral storage has been suggested as a storage option. In some
cases it might also be possible to use mineral storage to counter
localised environmental problems such as alkaline groundwaters.
But overall, reaction rates are slow and the quantities of CO2 stored
are likely to be very modest.
Geological Storage
The most comprehensively studied storage option is geological storage.
For the past 30 years, the oil industry has been injecting up to
30 million tonnes a year of CO2 (derived mainly fromnatural CO2
accumulations in Colorado) into the subsurface of West Texas, for
enhanced oil recovery (EOR). The Weyburn Project in Canada is a
recently initiated example of an EOR project However, most oils
are not suitable for Enhanced Oil Recovery (EOR) and in fact much
of the CO2 used for EOR is not stored but re-used. Therefore the
global opportunities for CO2 storage and EOR are limited.
- Enhanced Coal Bed Methane
Enhanced Coal Bed Methane (ECBM) recovery is seen as a potential
opportunity for sequestering CO2 in unmineable coal seams and
obtaining improved production of coal bed methane as a valuable
by-product. A demonstration CO2-ECBM project in the San Juan Basin
produced a positive, though modest, enhancement of the rate of
methane recovery. A new ECBM project (RECOPOL) has recently been
initiated by the IEA Greenhouse Gas Programme in Poland and the
results are awaited with interest. The true potential of CO2-ECBM
is difficult to gauge at this time. Coal bed methane is an important
energy source in the USA, but it has not to date been significant
in most other countries because of the lack of existing infrastructure
and expertise. An additional concern is that any increase in the
leakage of methane as a result of ECBM could counter the potential
benefits of CO2 storage.
- Injecting CO2 into the Subsurface
By far the greatest potential for geological storage of CO2 involves
injection of compressed CO2 into the subsurface, down to a depth
of 6-800 metres. The CO2 is compressed to a dense near critical
state; it is then very much denser than gaseous CO. This minimises
problems posed by the large amounts of CO2 that will need to be
stored if this method is to have a major impact on current emission
levels. Much of the CO2 will initially remain in a super-critical
state; some of it may react with the bedrock to form carbonate
minerals; some will go into solution. Over time, more will go
into solution, but provided the injection site is carefully chosen
the CO2 will remain stored for very long periods of time and can
be monitored. An obvious site for geological storage is depleted
oil and particularly gas reservoirs. In the USA, it is estimated
(by the US DOE) that the storage capacity of depleted gas reservoirs
is about 80-100 Gigatonnes of CO2 or enough to store US emissions
of CO2 from major stationary sources for 50 years or more.
Storing large amounts of CO2 in deep saline water-saturated reservoir
rocks, particularly sandstones, with the CO2 stored as a result
of hydrodynamic trapping, also offers great potential. One major
project to store CO2 in a deep saline aquifer is already being
conducted by the Norwegian company Statoil, in the sediments of
the North Sea Basin. A million tonnes a year of CO2 are being
injected into the Utsira Formation at a depth of around 1000m
below the sea floor.
A comprehensive regional analysis of the storage potential of
saline reservoirs has been undertaken in Australia as part of
the GEODISC project. This study has indicated a CO2 storage potential
for Australia, adequate to store CO2 emissions for many hundreds
of years at the current rates of emission. It is hoped that a
major demonstration project will be under way in Australia in
2005-2006.
Commonly quoted storage costs are US$10 a tonne of CO2 or less,
suggesting that excluding the costs of CO2 capture, the costs
of geological storage are likely to be cost competitive with other
sequestration options. Where the cost of capture also has to be
factored into project costs, such as in the case of post combustion
capture linked to a conventional coal-fired power station, then
obviously total costs rise considerably. New build IGCC is likely
to be a more effective technology for power generation combined
with CO2 storage.
Capture and storage of CO2 is not a "silver bullet"
that will overcome all the greenhouse gas problems. It is, however,
one of the most promising options that we now have for decreasing
CO2 emissions, whilst enabling us to continue to benefit from
access to widely available, low-cost fossil fuels.
 Geological
Storage Options for CO2 - Summary:
- Enhanced Oil Recovery
(EOR)
- Enhanced Coal Bed Methane
Recovery
- Depleted oil/gas fields
- Unmineable coal seams
- Voids and cavities
- Deep saline formations
Source:
Article 'A Pathway to Decreasing Carbon
Intensity' by Dr Peter Cook, Executive Director of the Co-operative
Research Centre for Greenhouse Gas Technologies (CO2CRC). Included
in the World Coal Institute's Ecoal Newsletter, Vol. 44, December
2002 and reproduced here with the kind permission
of the Author.
Other selected references:
CO2 Sequestration site - IEA Greehouse Gas R&D Programme
http://www.co2captureandstorage.info/
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