CAPTURE AND STORAGE OF CO2
WHAT NEEDS TO BE DONE?
David Wallace
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INTRODUCTION
Increasing concentrations of carbon dioxide (CO2) and other gases in the earth’s atmosphere are aggravating the natural greenhouse gas effect and leading to unwanted climate change, with consequent risks of extreme weather, rising sea level and adverse effects on agriculture and biodiversity. The main source of CO2 emissions is the combustion of coal, oil and gas in power stations, for transportation and in homes, offices and industry. Atmospheric concentrations of CO2 have risen by about one-third since preindustrial times and are expected almost to double between now and 2100. Under the Kyoto Protocol of the United Nations Framework Convention on Climate Change (UNFCCC), developed countries have agreed to reduce their emissions by 5.2% below 1990 levels by 2008-2012. If they wish to stabilise CO2 concentrations at twice pre-industrial levels by the end of this century, the developed countries will have to reduce their emissions to around half of their 1990 levels, or even lower. Such deep cuts would require a combination of greatly reduced energy consumption, massive switching from high-carbon fuels (coal to natural gas), widespread use of renewable or nuclear energy, and enhancement of natural “sinks” for CO2, such as new forest growth. However, detailed studies of well-proven technologies indicate that capturing and permanently storing CO2 emissions from fossil fuel-fired power plants could be a low-cost option for achieving large reductions in CO2 emissions. Successful application of these technologies would allow countries to pursue a strategy providing:
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very large and relatively rapid reduction in CO2 emissions; continued use of fossil fuels or a less rapid change to non-fossil energy sources.
CAPTURING EMISSIONS OF CO2 CO2 capture in power generation
Capture and storage technologies are best suited to large-scale sources of CO2 such as power stations, which account for about one-third of global CO2 emissions. The two main technologies for power generation are natural gas combined cycles (NGCC), and pulverised coal-fired (PF) steam cycles. Other large fossil power plant configurations, such as integrated coal gasification combined cycles (IGCC), are also suitable for CO2 capture.
Post-combustion CO2 capture
Concentrations of CO2 in power station flue-gases range from around 4% by volume for NGCC plants to 9% for IGCC plants and 14% for PF plants. CO2 could be captured using amine solvents to scrub the flue-gases. Amine solvents have already been widely used in the chemical and oil industry for CO2 capture, and this technique can be adapted for application on flue-gas streams. The amine leaving the scrubber is heated to release high-purity CO2 and is then re-used.
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Although amine scrubbing is relatively straightforward in NGCC plants, additional measures are required in coal-fired plants to prevent contamination of the recovered CO2 by other flue-gas impurities. Post-combustion capture does have its disadvantages. The low concentration of CO2 in power- station flue-gases means that a very large volume of flue-gas has to be treated. Equipment is correspondingly large and capital costs are high. If the capture technology is based on a solvent, such as amine, large amounts of energy are required for solvent regeneration. Using concentrated oxygen instead of air for combustion will increase CO2 concentrations in the flue-gas to, typically, more than 90%. However, producing the oxygen requires expensive equipment and, again, high levels of energy consumption.
Pre-combustion CO2 capture
A pre-combustion capture technology, producing a CO2 concentration of 35% to 40%, can avoid many of these problems. Pre-combustion CO2 capture involves reacting the fuel with oxygen or air and, in some cases, steam, to produce a gas consisting mainly of carbon monoxide (CO) and hydrogen. A catalytic “shift” reaction with steam in a catalytic reactor (shift converter) gives CO2 and more hydrogen. The CO2 is removed and the hydrogen passes to a gas turbine, or possibly a fuel cell. This technique needs more gas-purification stages when applied to coal or oil, rather than natural gas. Most of the technology required is well proven in ammonia production and other industrial processes. However, the use of hydrogen as a turbine fuel is novel. At least two turbine manufacturers are seeking to establish criteria for the combustion of hydrogen-rich fuels.
Performance of known CO2 capture technologies
A study by the International Energy Agency Greenhouse Gases R&D Programme has estimated the following benefits and disadvantages for a new 500 MW gas- or coal-fired plant incorporating CO2 capture, with CO2 compression to 110 bar:
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an 80% reduction in CO2 emissions to the atmosphere; a reduction in electrical generation efficiency of between 8 and 13 percentage points; an increase in capital costs of between 50% and 100% ; an increase in the cost of electricity generation of about 50% in gas-fired plants and IGCC plants with pre-combustion capture, and about 70% in PF plants with post-combustion capture.
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Other opportunities for CO2 capture
Certain industrial processes, as well as oil and gas production wells, already produce concentrated streams of CO2. These could be captured at little cost. Hydrogen might become established as a major fuel for cars, aeroplanes and heat and power generation. Centralised, large-scale production of hydrogen from fossil fuels would be well-suited to pre-combustion capture of CO2 emissions.
CO2 TRANSPORTATION
After capture, transportation of CO2 to a long-term storage site would be by high-pressure pipeline or by tanker. CO2 is largely inert and easily handled and is already transported in large quantities. In addition, there are likely to be opportunities for power production to take place at such long-term storage sites as coal beds and oil and gas reservoirs. Siting decisions will need to take account of the fact that it is cheaper to pipe CO2 than to transmit electricity.
CO2 STORAGE Potential storage options
Carbon dioxide storage will be an effective way of avoiding climate change only if the CO2 can be stored for several hundreds or thousands of years. The four most promising storage options are: oil and gas reservoirs, deep saline reservoirs, unminable coal beds, and the deep ocean (Figure 3). FIGURE 1. OPTIONS FOR CO2 STORAGE
Power Station with CO 2 Capture Unminable Coal Beds Pipeline Pipeline Ocean
Depleted Oil or Gas Reservoirs Deep Saline Aquifier
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The estimated global CO2 storage capacity of each of these options is shown in Table 1. TABLE 1. GEOLOGIC RESERVOIRS FOR CO2 STORAGE AND THEIR CAPACITY, IN GIGA TONNES OF CO2, Gt CO2
Reservoir type Below ground Ocean Storage option Depleted oil and gas fields Deep saline reservoirs Unminable coal reserves Deep ocean Global capacity % of emissions to 2050 920 45 400-10,000a 20-500 >15 >1 Uncertain Gt CO2
a Estimates from early 1990s. More recent estimates suggest much greater capacity. Further research required to establish more accurate potential. Source: IEA Greenhouse Gas R&D Programme.
By comparison, other options are unlikely to be economically competitive. These include storage in specially created underground caverns, in a thermally insulated repository as solid dry ice or in carbonate form as a result of reaction with naturally occurring minerals.
Oil and gas reservoirs
Thousands of oil and gas reservoirs have been depleted to the extent that, given existing extraction techniques and current fuel prices, they are no longer viable. The geology of these reservoirs is well understood. They are known to have stored liquid and gaseous hydrocarbons for millions of years and their existing infrastructure might be suitable for CO2 storage. The natural-gas industry has routinely used depleted natural gas fields for the underground storage of natural gas. It is also possible to use CO2 injection for enhanced oil recovery (EOR) in active, producing oil and gas reservoirs, instead of existing energy-intensive EOR techniques (Figure 2). In some cases, the benefits would more than offset the costs of CO2 capture and injection.
Deep saline reservoirs
Deep aquifers that contain only saline water and have a relatively impermeable cap rock could be used to store CO2. In some formations, the CO2 would react with minerals in the water to form carbonates, thereby becoming locked up permanently. Injection techniques would be similar to those used for depleted oil and gas fields. In the Norwegian Sleipner project, CO2 is being separated from a natural gas stream and injected into a deep saline reservoir below the North Sea (Figure 3). The project is being monitored and modelled as part of an international initiative established by Statoil, the Norwegian state oil company, with the IEA Greenhouse Gas R&D Programme. This should help to resolve many of the uncertainties associated with storage in deep saline reservoirs.
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FIGURE 2 CO2-ENHANCED OIL RECOVERY
CO2 Injection Well Production Well
CO2
Miscible Oil Zone Bank
Additional Oil Recovery
FIGURE 3 CO2 INJECTION INTO A DEEP SALINE RESERVOIR BELOW THE NORTH SEA (COURTESY OF STATOIL)
Sleipner A Sleipner T
Gas from Sleipner West CO2 Injection Well Utsira Formation Sleipner East Production and Injection Wells CO2
Sleipner East Field
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Unminable coal beds
When CO2 is injected into unminable (very deep) coal beds, the CO2 is adsorbed onto the surface of the coal and displaces methane. The CO2 is locked up permanently, provided the coal remains unmined. Because coal can adsorb, by volume, about twice as much CO2 as methane, the coal bed provides net CO2 storage, even if the displaced methane is burnt as a fuel. The IEA Greenhouse Gas R&D Programme is helping in a field test of such enhanced coal-bed methane production, using CO2 and nitrogen mixtures.
Deep ocean
Injecting CO2 into the deep ocean is a longer-term option that would take advantage of the very slow natural interchange between the deep ocean and its surface layers. Computer models suggest that about 80% of the CO2 injected at a depth of 3,000 meters would still be retained in the ocean after 500 years. CO2 injected deeper than 3,000 meters at the seabed, would form a lake of liquid CO2 or CO2 hydrate. This might further extend retention periods. Studies are under way to address the substantial scientific uncertainty about the storage integrity and environmental impact of ocean storage. Among these are the Climate Technology Initiative (CTI) Ocean Sequestration Project.
Environmental implications and uncertainties
There are a number of environmental impacts and uncertainities that need further study:
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the length of time the CO2 must remain stored in order to mitigate climate change risks; the effect of slow or sudden release of CO2 on atmospheric CO2 concentrations; the effect of drilling on the integrity of depleted oil and gas field caps; likely reactions between CO2 and underground minerals, and their possible impact on CO2 sequestration periods and on the integrity of oil and gas field caps; the nature of deep saline reservoirs and their impact on CO2 storage over time; the possible impact of seismic activity; the impact on marine life of deep ocean storage of CO2 and of natural CO2 absorption from the atmosphere.
Verification
Accurate verification of the quantities stored is essential if CO2 storage is to be used as a basis for emissions trading or to meet national commitments to CO2 reduction. Accurate, low-cost measurement techniques already exist for storage of CO2 in depleted oil and gas fields and deep saline reservoirs.
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Validation of ocean storage is likely to be more difficult and costly, and appropriate techniques have yet to be developed.
CO2 CAPTURE AND STORAGE COSTS
The IEA Greenhouse Gases R&D Programme has estimated the costs of CO2 capture and storage for a range of coal- and gas- fired power plants, using pre- and post-combustion capture techniques. The overall cost of CO2 capture and storage is about $40 to $60 per tonne of CO2 emissions avoided. This compares favourably with other options, such as the widespread use of renewable energy sources. The cost has three main components:
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CO2 capture and compression to 110 bar: $30 to $50 per tonne of CO2 for a 500 MW gas- or coal-fired plant at current fuel prices; transportation by pipeline: $1 to $3 per tonne of CO2 per 100 km; storage: $1 to $3 per tonne of CO2.
These costs are expected to fall as the technology matures and the scale of application increase. The cost of CO2 capture and storage corresponds approximately to an increase in the price of electricity of 1.5 – 3 US cents per kilowatt hour. For comparison, in 1998, domestic electricity users in the OECD paid between 7 and 14 cents per kilowatt hour. Industrial users paid 4 – 9 cents.
KEY TECHNOLOGY NEEDS
Technology research and development, demonstration projects and assessments of the potential for CO2 capture and storage are taking place in many countries. Key technology needs are:
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accurate assessment of geologic storage potential; field tests to determine the fate of CO2 injected into geologic formations (oil and gas reservoirs, unminable coal beds and saline aquifers), and the deep ocean, and its environmental impact; cost reduction of existing CO2 separation techniques; R&D on novel capture and storage technologies; development of technologies for the production, transportation and use of hydrogen derived from fossil fuels.
RECOMMENDATIONS FOR POLICY MAKERS
To ensure that the CO2 capture and storage technology option is available in the coming decades, a major effort is justified:
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existing efforts need to be linked together; new technological ideas and approaches to CO2 capture and storage should be vigorously pursued;
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issues of storage integrity and environmental impacts should be resolved rapidly, through open, transparent research programmes; the studies, R&D and technology demonstrations outlined above need to be comprehensively addressed; this can be achieved most rapidly, and effectively, through the fullest possible international and public-private collaboration; given its strategic importance as a potentially large-scale and affordable mitigation technology, the attention given to CO2 capture and storage should at least be equal to that given to other major mitigation options, such as biomass, solar, nuclear technologies.
Successful resolution of these issues should lead to recognition within the UNFCCC process of CO2 capture and storage as an effective option for mitigating emissions of CO2.
Background information for this report can be obtained from “Technology Status Report: CO 2 Capture and Storage” (DTI URN 00/1081), prepared by the IEA Greenhouse Gases R&D Programme (mail@ieagreen.demon.co.uk), with the support of the UK Department of Trade and Industry, for the IEA Working Party on Fossil Fuels.
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