Romana
CO2 capture and storage (CCS)
What is Carbon Capture and Storage (CCS)?
Fig. Methods of transport and geological storage of CO2
Carbon capture and storage (CCS) is an approach to mitigate 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. Although CO2 has been injected into geological formations for various purposes, the long term storage of CO2 is a relatively untried concept and as yet (2008) no large scale power plant operates with a full carbon capture and storage system.
Carbon is emitted into the atmosphere (as carbon dioxide, also called CO2) whenever we burn any fuel, anywhere. The largest sources are cars and lorries, and non-nuclear power stations - those that burn coal, oil or gas, otherwise known as fossil fuels. To prevent the carbon dioxide building up in the atmosphere (possibly causing global warming and definitely causing ocean acidification), we can catch the CO2, and store it. As we would need to store thousands of millions of tons of CO2, we cannot just build containers, but must use natural storage facilities. Below, a diagram of possible locations for underground storage of CO2, from IPCC report.
The effects of carbon dioxide in the atmosphere are controversial. However, the average temperature of the Earth is rising, especially when measured at the poles. Note that the average Earth surface temperature correlates well with the amount of CO2 in the atmosphere (i.e. as the CO2 levels in the atmosphere have increased, the surface temperature has gone up at the same time).
In the diagram, the average temperature is in red and the CO2 content of the atmosphere is in green.
up
CO2 Capture
Capturing CO2 can be applied to large point sources, such as large fossil fuel or biomass energy facilities, industries with major CO2 emissions, natural gas processing, synthetic fuel plants and fossil fuel-based hydrogen production plants. Broadly, three different types of technologies exist: Post-combustion, pre-combustion, and oxyfuel combustion.
- In post-combustion, the CO2 is removed after combustion of the fossil fuel - this is the scheme that would be applied to conventional power plants. Here, carbon dioxide is captured from flue gases at power stations. The technology is well understood and is currently used in other industrial applications.
- The technology for pre-combustion is widely applied in fertilizer, chemical, gaseous fuel (H2, CH4), and power production. In these cases, the fossil fuel is partially oxidized, for instance in a gasifier. The resulting syngas (CO and H2) is shifted into CO2 and more H2. The resulting CO2 can be captured from a relatively pure exhaust stream. The H2 can now be used as fuel; the carbon is removed before combustion takes place.
- In Oxy-fuel combustion the fuel is burned in oxygen instead of air. To limit the resulting flame temperatures to levels common during conventional combustion, cooled flue gas is recirculated and injected into the combustion chamber. The flue gas consists of mainly carbon dioxide and water vapour, the latter of which is condensed through cooling. The result is an almost pure carbon dioxide stream that can be transported to the sequestration site and stored. Power plant processes based on oxyfuel combustion are sometimes referred to as "zero emission" cycles, because the CO2 stored is not a fraction removed from the flue gas stream (as in the cases of pre- and post-combustion capture) but the flue gas stream itself. It should be noted, however, that a certain fraction of the CO2 generated during combustion will inevitably end up in the condensed water. To warrant the label "zero emission" the water would thus have to be treated or disposed of appropriately. The technique is promising, but the initial air separation step demands a lot of energy.
- Plants that produce ethanol by fermentation generate cool, essentially pure CO2 that can be pumped underground. Fermentation produces slightly less CO2 than ethanol by weight. World ethanol production in 2008 is expected to be about 16 billion gallons or 48 million tonnes.
An alternate method, which is under development, is chemical looping combustion (CLC). Chemical looping uses a metal oxide as a solid oxygen carrier. Metal oxide particles react with a solid, liquid or gaseous fuel in a fluidized bed combustor, producing solid metal particles and a mixture of carbon dioxide and water vapor. The water vapor is condensed, leaving pure carbon dioxide which can be sequestered. The solid metal particles are circulated to another fluidized bed where they react with air, producing heat and regenerating metal oxide particles that are recirculated to the fluidized bed combustor.
A few engineering proposals have been made for the much more difficult task of capturing CO2 directly from the air, but work in this area is still in its infancy. Global Research Technologies demonstrated a pre-prototype in 2007. Capture costs are estimated to be much higher than from point sources, but may be feasible for dealing with emissions from diffuse sources like automobiles and aircraft.
CO2 Transport
After capture, the CO2 must be transported to suitable storage sites. This is done by pipeline, which is generally the cheapest form of transport. In 2008, there were approximately 5,800 km of CO2 pipelines in the United States. These pipelines are currently used to transport CO2 to oil production fields where the CO2 is injected in older fields to produce oil. The injection of CO2 to produce oil is generally called "Enhanced Oil Recovery" or EOR. In addition, there are several pilot programs in various stages to test the long-term storage of CO2 in non-oil producing geologic formations. These are discussed below.
COA conveyor belt system or ships can also be used. These methods are currently used for transporting CO2 for other applications.
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.
up
1. Geological storage
Also known as geo-sequestration, this method involves injecting carbon dioxide, generally in supercritical form, directly into underground geological formations. 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). This option is attractive because the storage costs may be partly offset by the sale of additional oil that is recovered. Disadvantages of old oil fields are their geographic distribution and their limited capacity, as well as that the subsequent burning of the additional oil so recovered will offset much or all of the reduction in CO2 emissions.
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. To keep the cost of storage acceptable the geophysical exploration may be limited, resulting in larger uncertainty about the aquifer structure. Unlike storage in oil fields or coal beds no side product will offset the storage cost. Leakage of CO2 back into the atmosphere may be a problem in saline aquifer storage. However, current research shows that several trapping mechanisms immobilize the CO2 underground, reducing the risk of leakage.
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.
up
2. Mineral storage
Carbon sequestration by reacting naturally occurring Mg and Ca containing minerals with CO2 to form carbonates has many unique advantages. Most notably is the fact that carbonates have a lower energy state than CO2, which is why mineral carbonation is thermodynamically favorable and occurs naturally (e.g., the weathering of rock over geologic time periods). Secondly, the raw materials such as magnesium based minerals are abundant. Finally, the produced carbonates are unarguably stable and thus re-release of CO2 into the atmosphere is not an issue. However, conventional carbonation pathways are slow under ambient temperatures and pressures. The significant challenge being addressed by this effort is to identify an industrially and environmentally viable carbonation route that will allow mineral sequestration to be implemented with acceptable economics.
up
Leakage
A major concern with CCS is whether leakage of stored CO2 will compromise CCS as a climate change mitigation option. For well-selected, designed and managed geological storage sites, IPCC estimates that risks are comparable to those associated with current hydrocarbon activity. CO2 could be trapped for millions of years, and well selected stores are likely to retain over 99% of the injected CO2 over 1000 years. For ocean storage, the retention of CO2 would depend on the depth; IPCC estimates 30–85% would be retained after 500 years for depths 1000–3000 m. Mineral storage is not regarded as having any risks of leakage. The IPCC recommends that limits be set to the amount of leakage that can take place.
It should also be noted that at the conditions of the deeper oceans, (about 400 bar or 40 MPa, 280 K) water–CO2(l) mixing is very low (where carbonate formation/acidification is the rate limiting step), but the formation of water-CO2 hydrates is favorable. (a kind of solid water cage that surrounds the CO2). [3]
To further investigate the safety of CO2 sequestration, we can look into Norway's Sleipner gas field, as it is the oldest plant that stores CO2 on an industrial scale. According to an environmental assessment of the gas field which was conducted after ten years of operation, the author affirmed that geosequestration of CO2 was the most definite way to store CO2 permanently. [4]
Example CCS projects
As of 2007, four industrial-scale storage projects are in operation. Sleipner is the oldest project (1996) and is located in the North Sea where Norway's StatoilHydro strips carbon dioxide from natural gas with amine solvents and disposes of this carbon dioxide in a deep saline aquifer. The carbon dioxide is a waste product of the field's natural gas production and the gas contains more (9% CO2) than is allowed into the natural gas distribution network. Storing it underground avoids this problem and saves Statoil hundreds of millions of euro in avoided carbon taxes. Since 1996, Sleipner has stored about one million tones CO2 a year. A second project in the Snøhvit gas field in the Barents Sea stores 700,000 tones per year.
The Weyburn project is currently the world's largest carbon capture and storage project. Started in 2000, Weyburn is located on an oil reservoir discovered in 1954 in Weyburn, southeastern Saskatchewan, Canada. The CO2 for this project is captured at the Great Plains Coal Gasification plant in Beulah, North Dakota which has produced methane from coal for more than 30 years. At Weyburn, the CO2 will also be used for enhanced oil recovery with an injection rate of about 1.5 million tonnes per year. The first phase finished in 2004, and demonstrated that CO2 can be stored underground at the site safely and indefinitely. The second phase, expected to last until 2009, is investigating how the technology can be expanded on a larger scale.
The fourth site is In Salah, which like Sleipner and Snøhvit is a natural gas reservoir located in In Salah, Algeria. The CO2 will be separated from the natural gas and re-injected into the subsurface at a rate of about 1.2 million tones per year.
A major Canadian initiative called the Integrated CO2 Network (ICO2N) is a proposed system for the capture, transport and storage of carbon dioxide (CO2). ICO2N members represent a group of industry participants providing a framework for carbon capture and storage development in Canada.
In October 2007, the Bureau of Economic Geology at The University of Texas at Austin received a 10-year, $38 million subcontract to conduct the first intensively monitored, long-term project in the United States studying the feasibility of injecting a large volume of CO2 for underground storage. The project is a research program of the Southeast Regional Carbon Sequestration Partnership (SECARB), funded by the National Energy Technology Laboratory of the U.S. Department of Energy (DOE). The SECARB partnership will demonstrate CO2 injection rate and storage capacity in the Tuscaloosa-Woodbine geologic system that stretches from Texas to Florida. The region has the potential to store more than 200 billion tons of CO2 from major point sources in the region, equal to about 33 years of U.S. emissions overall at present rates. Beginning in fall 2007, the project will inject CO2 at the rate of one million tons per year, for up to 1.5 years, into brine up to 10,000 feet (3,000 m) below the land surface near the Cranfield oil field about 15 miles (25 km) east of Natchez, Mississippi. Experimental equipment will measure the ability of the subsurface to accept and retain CO2.
Currently, the United States government has approved the construction of what is touted as the world's first CCS power plant, FutureGen. On January 29, 2008, however, the Department of Energy announced it was withdrawing funding from FutureGen, as it had originally been proposed, casting considerable doubt on the future of the project and in the view of some effectively terminating the project.
Examples of carbon sequestration at an existing US coal plant can be found at utility company Luminant's pilot version at its Big Brown Steam Electric Station in Fairfield, Texas. This system is converting carbon from smokestacks into baking soda. Skyonic plans to circumvent storage problems of liquid CO2 by storing baking soda in mines, landfills, or simply to be sold as industrial or food grade baking soda. GreenFuel Technologies Corp. is piloting and implementing algae based carbon capture, circumventing storage issues by then converting algae into fuel or feed.
Carbon Trap Technologies, L.P., (“CTT”) was formed in early 2007 to develop and to market a technology to chemically sequester carbon dioxide emissions from fossil fuel combustion, while producing useful products with significant market value.
In the Netherlands, an 68 MW oxyfuel plant ("Zero Emission Power Plant") is being planned and is expected to be operational in 2009.
In the United States, four different synthetic fuels projects are moving forward which have publicly announced plans to incorporate carbon capture and storage.
American Clean Coal Fuels, in their Illinois Clean Fuels project, is developing a 30,000 Barrel Per Day Biomass and Coal to Liquids project in Oakland Illinois, which will market the CO2 created at the plant for Enhanced Oil Recovery applications. The project is expected to come online in late 2012.
Baard Energy, in their Ohio River Clean Fuels project, are developing a 53,000 BPD Coal and Biomass to Liquids project, which has announced plans to market the plant’s CO2 for Enhanced Oil Recovery.
Rentech is developing a 29,600 barrel per day coal and biomass to liquids plant in Natchez Mississippi which will market the plant’s CO2 for enhanced oil recovery. The first phase of the project is expected in 2011.
DKRW is developing a 15,000-20,000 Barrel Per Day coal to liquids plant in Medicine Bow Wyoming, which will market it plant’s CO2 for enhanced oil recovery. The project is expected to begin operation in 2013.
Australia. The federal Resources and Energy Minister Martin Ferguson has opened the first geosequestration project in the southern hemisphere. The demonstration plant is near Nirranda South in South Western Victoria. The plant is owned by the CO2 Cooperative Research Centre. It is funded jointly by government and industry. It aims to store 100,000 tonnes of carbon dioxide extracted from a gas well. Carbon dioxide-rich gas is extracted from a reservoir via a well, compressed and piped 2.25 km to a new well. There the gas is injected into a depleted natural gas reservoir approximately two kilometres below the surface. This project is tiny by world standards as BP's Algeria plant is storing 1,000,000 tones each year.
This plant does not propose to capture CO2 from coal fired power generation. There is no project anywhere in the world storing CO2 stripped from the products of combustion of coal burnt for electricity generation at coal fired power stations.
© 2008-2011. All the rights are reserved to CO2 Club Romania
