Carbon sequestration
Carbon dioxide sequestration is cited as a possible long term method of mitigating the effects of fossil fuel consumption. Before carbon can be sequestered, it will have to be collected in the form of gaseous CO 2 .
-
Pre-combustion capture
, in which the carbon content of the fuel is captured before the final combustion (i.e. not in the flue gas). This approach is used in coal gasification plant, which generates a syngas made mostly of CO and H
2 . The CO and H
2 are then separated and reacted in a controlled environment releasing almost pure CO
2 .
-
Post-combustion capture
, in which the CO
2 is captured from the flue gas. With this approach various amine-based processes already exist, and retrofit to existing plant is possible. These approaches are expensive at the moment.
-
Oxyfuel technologies
are an alternative approach, involving the use of enriched oxygen as feedstock for the combustion process. This, together with recycling of combustion products, results in CO
2 and H
2 O products, which are readily separated.
Once CO 2 has been gathered, there are eight broad approaches to CO 2 sequestration:
- Industrial use of CO 2 in plastics and other chemical industries
- Inorganic sequestration as carbonates
- Biological conversion to fuel
- Geological sequestration, in salt domes, or coal beds
- Injection into active oil wells
- Injection into exhausted gas or oil wells
- Injection into aquifers
- Ocean disposal
The capacity, cost and technical feasibility of these approaches is shown in Table 2. Global capacity for sequestering CO 2 in such reservoirs is sufficient for many years' combustion of fossil fuels. More detail of each approach is covered in the following paragraphs.
Table 2 comparison of capacity, cost, integrity and technical feasibility of sequestration methods
| Sequestration method | Relative capacity | Relative Cost | Sequestration integrity | Technical feasibility |
|---|---|---|---|---|
| Industrial use | small | low | high | established |
| Inorganic conversion | very large | high | excellent | not developed |
| Biological | small | moderate | good | needs scale-up |
| Geological sequestration in salt domes | large | very high | good | high |
| Storage in coal beds | not known | low | not known | not developed |
| Injection into active oil wells | small | v. low | good | in use |
| Injection into exhausted gas or oil wells | 650-1800G tonnes | low | good | in use on small scale |
| Injection into aquifers | 320-10,000G tonnes | not known | good | in use on small scale |
| Ocean sequestration | 5000-100 000 G tonnes | moderate-v. high | not known | not developed |
The many potential methods of sequestration are subject to significant limitations in terms of the logistics and economics of CO 2 capture and transport to the sequestration site. In some cases, such as the removal of CO 2 from natural gas, there is a possibility of re-injection of the gas into aquifers at or near the site of the gas well. In other cases, there will be a need for substantial transportation. In all cases there will be a demand for improved methods of concentrating the CO 2 , by means of membrane technologies, refrigeration or other means. Each of these supporting technologies will generate manufacturing opportunities in its own right.
Industrial use of CO 2
Although CO 2 production is a well-established industrial process, the volumes currently used in food and chemical processes are insignificant compared with the volumes that would be required to have any impact on global warming. Even if the entire US plastics industry were to convert to CO 2 as feedstock, it would only consume some 5% of current emission from US power plants. It is however true that industrial use of CO 2 could be favoured as part of an overall sequestration strategy.
Inorganic conversion
CO 2 sequestration occurs naturally when silicate rocks weather to form carbonate minerals. Similar chemical processes could be used industrially to convert power plant CO 2 and pulverised olivine and wollastonite sands into magnesium and calcium carbonates. The transport and conversion costs involved would be large (perhaps approaching the scale of the existing US coal mining industry), but the basic technologies involved in this approach are extensions of existing chemical processes. If the annual global output of CO 2 were sequestered by this approach, it would generate around 15km 3 of solid carbonate material.
Biological conversion to fuel
In the absence of suitable catalysts (as yet undeveloped), the direct chemical conversion of CO 2 to fuel gas is not attractive. The amount of energy consumed by available methods is almost equivalent to the energy available from the fuel generated.
The use of biological methods is more attractive, as essentially 'free' power from the sun or other biological sources can be brought to bear. There is scope for biomass conversion using trees, grasses or algae. Of these, forestry is the most established, but microalgae may give conversion rates an order of magnitude higher. While the manufacturing processes that underpin forestation are well understood, the biochemical methods required for significant algae culture and harvesting and processing are less developed. An algae-based sequestration system would require a large land area to sustain it: a 500MW power plant would require processing ponds of from 50-100 km 2 .
Geological sequestration in salt domes and brine aquifers
Salt domes are widely distributed geologically. (There are over 500 in the Gulf coast region from Mexico to Florida alone). They formed when thick layers of salt, buried over 4 miles deep extruded through overlying sediment to nearly reach the surface. Typically, a salt dome is roughly circular and 1-2 miles in diameter. They may be close to the surface, and extend 5-8000m deep. Brine aquifers associated with salt deposits are also widespread, both under land and offshore. Because both salt domes and brine aquifers are geologically sealed from fresh water resources, they are attractive sites for sequestration of CO 2 . In some cases, the association of salt formations with oil deposits allows the use of CO 2 as a means of increasing oil recovery rates.
Technologies for accessing and storing CO 2 in salt domes and aquifers are established on a small scale. Such installations could also make use of technologies developed for storage of natural gas in salt domes, an approach that has been used in the USA and Europe.
Difficulties associated with salt dome and aquifer sequestration are mainly associated with escape of CO 2 through geological faults, or displacement of brine into fresh water aquifers. There is a number of ongoing research projects associated with such sequestration, involving both oil bearing and oil-free salt formations.
Storage in coal beds
Coal deposits, (which typically contain large volumes of water) have significant CO 2 absorption capability. There is therefore potential for injecting CO 2 into deep, low grade coal beds that are being used as a source of methane. The injection process increases the methane recovery rate, but raises issues regarding the geological integrity of the coal seams. The technologies required for handling the injection process are similar to those used for oil reservoir sequestration operations, providing another outlet for sequestration equipment.
Injection into active oil wells
Direct injection into oil wells to increase recovery rates is an established technology, practised both on and offshore. The USA leads the use of CO 2 for enhancing oil recovery, and uses over 32 million tonnes of CO 2 for this purpose annually.
This approach is able to sequester carbon at low net cost, due to the revenues from recovered oil/gas. While the technologies involved in oil well enhancement are established, their more general application across the oil industry as a whole will demand a scaling up of manufacturing operations. Similar technical considerations will apply to sequestration in depleted oil and gas wells.
Ocean sequestration
The oceans already contain some 140 000 Billion tonnes of CO 2 , an amount that dwarfs the annual human production of around 22 Billion tonnes. The amount of carbon that would double the load in the atmosphere would increase the concentration in the deep ocean by only two percent. Ocean sequestration occurs naturally (some 90% of current emissions will eventually be absorbed by the sea), and sequestration studies have therefore concentrated on accelerating the process. Both shallow and deep level sequestration are potentially possible.
Shallow ocean sequestration
Injection of CO 2 into shallow ocean layers may be used to increase the growth of phytoplankton and thus stimulate the ocean food chain. Iron additions to the ocean may be used to stimulate biological growth in areas that would otherwise be low in phytoplankton and hence higher forms of life. Although this approach has been used experimentally, there are few data to show that the CO 2 absorbed is actually sequestered for long periods.
Deep ocean sequestration
The deep oceans have enormous sequestration potential because of their vast size and the favourable physical conditions (high pressure, low temperature, low life content) that operate at depths of greater than around 800m. Two broad classes of deep ocean sequestration are under consideration:
- The direct injection of liquid at depths in excess of 1000m from static or moving pipes. The liquid CO 2 would form lakes, or combine with water to form CO 2 clathrate, (an ice compound, in which 44 water molecules form a lattice that traps up to 8 CO 2 molecules in small 'cages'). Deep ocean sequestration using liquid CO 2 could be scaled up to handle large tonnages of CO 2 , but the ecological impact of this approach is unknown.
- Production of ice-encapsulated solid CO 2 projectiles of 100-1000 tonnes, which would free-fall into deep ocean and bury themselves under sediment, allowing slow combination. This approach appears to offer less risk to marine organisms, but its cost and complexity will be higher than liquid injection.
Both of these approaches would require new technologies for handling and deploying large amounts of gaseous, and liquid and/or solid CO 2 .The main barriers to use are likely to be ecological and economic, rather than technological.
General Development Issues
Before CO 2 can be sequestered from power plants or industrial sources, it will have to be captured as a relatively pure gas. With current sequestration systems (e.g. enhanced oil recovery), CO 2 capture is estimated to amount to around 75% of total sequestration costs (including capture, storage, transport, and sequestration). Although separation and capture systems are fairly well developed, as part of industrial processes such as synthetic ammonia production, hydrogen production, and cement production, they are not developed enough for large-scale sequestration. Opportunities for significant cost reductions exist since very little R&D has been devoted to this area.
The most likely options currently identifiable for CO 2 separation and capture include the following:
- Absorption (chemical and physical)
- Adsorption (physical and chemical)
- Low-temperature distillation
- Gas separation membranes
- Mineralisation and biomineralisation
In most cases for development in the short-medium term, CO 2 will be captured close to its point of generation (e.g. at the power plant).
The large-scale development of shallow and deep ocean sequestration activities on an industrial scale will have to await significant ecological studies. The main manufacturing challenges are believed to be similar to those that pertain for other deep-water operations in the oil industry. Existing offshore companies and contractors are therefore well placed to participate in this field as and when it becomes commercially viable.
Development Areas where TWI can help
The transport of supercritical CO 2 and disposal in mature reservoirs faces major investment hurdles. Significant cost savings will result if redundant subsea oil & gas production assets can be reused for CO 2 transportation and injection. Such assets include downhole and well-head equipment, platform structures and in particular, pipelines reused to transport supercritical CO 2 .
Pipelines are 'corrosion-critical' and have a limited design life of 15 to 25 years. Before the change of service from oil & gas production to CO 2 transport, pipeline operators will need to make a run/rerate/repair/replacement decision, in the context of:
- - ageing pipelines will be internally corroded following oil & gas production service;
- - any underwater pipeline repairs will be expensive and difficult;
- - the comprehensive inspection of some pipelines may be impossible; and
- - remaining life assessment procedures for corrosion, are conservative.
Based on operating conditions for CO 2 pipelines in North America and elsewhere it is clear that for the reuse of redundant pipelines for CO 2 service, integrity concerns will need to be addressed. These include:
- - normal CO 2 transportation pressure may be higher than the original design
- - higher CO 2 transportation temperature may cause pipeline buckling failure
- - CO 2 leakage causes cooling which may lead to catastrophic pipeline fracture
- - carbonate stress cracking in CO 2 -containing systems may lead to failure
- - CO 2 may form carbonic acid promoting general corrosion
current safety assessment decisions do not assist the difficult cost-risk investment decision-making process.
For more information, please contact: power@twi.co.uk
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