Carbon Capture

Carbon capture and storage (CCS) is a scientific concept which attempts to limit the impact of fossil fuel power stations by capturing carbon dioxide at source, and then store it in geological formations. The capture side of the process is fairly well developed, however the storage either in deep geological formations, in deep ocean masses, or in the form of mineral carbonates, is still at the early stages of development, and there is much doubt about how long the carbon dioxide would actually remained stored. In the case of deep ocean storage, there is a risk of greatly increasing the problem of ocean acidification, a problem that also stems from the excess of carbon dioxide already in the atmosphere and oceans. There is the risk that having stored the gas underground it could vent itself back into the atmosphere, with potentially catastropic consequences.

The process of capturing the carbon dioxide increases the energy used by a power station by around 25%, however it can reduce emissions by between 80 and 90%. The Intergovernmental Panel on Climate Change (IPCC) estimates that the economic potential of CCS could be between 10% and 55% of the total carbon mitigation effort until year 2100.

Although the capture stage of CCS is technically possible, capturing and compressing CO2 requires much energy, significantly raising the running costs of CCS-equipped power plants. In addition there are added investment or capital costs. The process would increase the fuel requirement of a plant with CCS by about 25% for a coal-fired plant and about 15% for a gas-fired plant. The technique is much better applied when new plants are being built rather than retrofitting to existing power stations which is far more expensive. Although much of the carbon emissions can be captured, the emissions of other pollutants can be increased in the process.

There is huge interest in CCS because while oil and gas reserves are approaching their peak, reserves of coal are much more plentiful and found in large quantities in the rapidly developing nations like China, which is rapidly building more coal-fired power stations. But coal is also the biggest emitter of carbon dioxide per kilowatt of any hydrocarbon, and all that CO2 has to go somewhere. Whatever new energy mix Europe adopts in the coming decade, the worldwide emissions of CO2 are going to rise unless there is a reliable, efficient and cost-effective method of stripping it out and locking it away somewhere that it cannot get into the atmosphere.

'Carbon capture and storage is clearly the only way that fossil fuels can be used safely,' says Jon Gibbins of the Energy Technology for Sustainable Development Group at London's Imperial College. 'It has the potential to break the link between fossil fuels and the climate. There's still no society that can solve the problem on its own but CCS does guarantee that fossil carbon stays in the ground.'

Stuart Haszeldine, of the Scottish Centre for Carbon Storage at Edinburgh University, says the main reason is the scale of the problem and the specific challenges posed by power stations, with their variable fuels and the different species that can be present in them. 'All the component parts exist, but they're often fitted onto completely different equipment and processes,' he said. 'Separating CO2 from other gases is done by people making urea or fertilisers, or for beverages, but that's on a smaller scale and the CO2 input is reasonably pure and controlled.' This is not the case in coal-fired power stations, where the flue gases are about 12 per cent CO2 and the processes are 20 to 50 times smaller than those that would be needed for a power station.

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.

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.

All these systems are being developed around the world, with several under scrutiny in the UK. And all the developers were heartened when, last year, the government announced it would run a competition to promote CCS and fully fund a commercial-scale demonstration project. The competition is to demonstrate 50MW to 100MW of capture and storage by 2014, then capture on a 400MW plant as soon as possible after — a time limit which, Haszeldine noted, is alarmingly ill-defined. However, when the full details of the competition were announced, it transpired that only one demonstration project would be funded — and limited to post-combustion technology. The fallout was immediate: BP, which was developing one of the furthest-advanced pre-combustion projects, an IGCC plant at Peterhead in Scotland which would inject its CO2 into an oilfield to help displace more oil, announced that the project would be put on hold. The company now plans to build a carbon-copy of the Peterhead operation in Abu Dhabi. The European Union has announced its development plan for CCS, and said it wants 12 demonstration plants up and running by 2015, of which the UK's could be one.

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.

According to the Congressional Research Service, "There are important unanswered questions about pipeline network requirements, economic regulation, utility cost recovery, regulatory classification of CO2 itself, and pipeline safety. Furthermore, because CO2 pipelines for [enhanced oil recovery] are already in use today, policy decisions affecting CO2 pipelines take on an urgency that is, perhaps, unrecognized by many. Federal classification of CO2 as both a commodity (by the Bureau of Land Management) and as a pollutant (by the Environmental Protection Agency) could potentially create an immediate conflict which may need to be addressed not only for the sake of future CCS implementation, but also to ensure consistency of future CCS with CO2 pipeline operations today. For a review of federal jurisdictional issues related to CO2 pipelines and reviewing agency jurisdictional determinations under the Interstate Commerce Act and the Natural Gas Act, see Adam Vann and Paul W. Parfomak, "Regulation of Carbon Dioxide (CO2) Sequestration Pipelines: Jurisdictional Issues", updated April 15, 2008.

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.

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. Here, 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.

Ocean storage

Another proposed form of carbon storage is in the oceans. Two main concepts exist. The 'dissolution' type injects CO2 by ship or pipeline into the water column at depths of 1000 m or more, and the CO2 subsequently dissolves. The 'lake' type deposits CO2 directly onto the sea floor at depths greater than 3000 m, where CO2 is denser than water and is expected to form a 'lake' that would delay dissolution of CO2 into the environment. A third concept is to convert the CO2 to bicarbonates (using limestone) or hydrates.

The environmental effects of oceanic storage are generally negative, but poorly understood. Large concentrations of CO2 kills ocean organisms, but another problem is that dissolved CO2 would eventually equilibrate with the atmosphere, so the storage would not be permanent. Also, as part of the CO2 reacts with the water to form carbonic acid, H2CO3, the acidity of the ocean water increases. The resulting environmental effects on benthic life forms of the bathypelagic, abyssopelagic and hadopelagic zones are poorly understood. Even though life appears to be rather sparse in the deep ocean basins, energy and chemical effects in these deep basins could have far reaching implications. Much more work is needed here to define the extent of the potential problems.

The time it takes water in the deeper oceans to circulate to the surface has been estimated to be in the order of 1600 years, varying upon currents and other changing conditions.

The bicarbonate approach would reduce the pH effects and enhance the retention of CO2 in the ocean, but this would also increase the costs and other environmental effects.

An additional method of long term ocean based sequestration is to gather crop residue such as corn stalks or excess hay into large weighted bales of biomass and deposit it in the alluvial fan areas of the deep ocean basin. Dropping these residues in alluvial fans would cause the residues to be quickly buried in silt on the sea floor, sequestering the biomass for very long time spans. Alluvial fans exist in all of the world's oceans and seas where river deltas fall off the edge of the continental shelf such as the Mississippi alluvial fan in the gulf of Mexico and the Nile alluvial fan in the Mediterranean Sea.

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."

In this process, CO2 is exothermically reacted with abundantly available metal oxides which produces stable carbonates. This process occurs naturally over many years and is responsible for much of the surface limestone. The reaction rate can be made faster, for example by reacting at higher temperatures and/or pressures, or by pre-treatment of the minerals, although this method can require additional energy. The IPCC estimates that a power plant equipped with CCS using mineral storage will need 60-180% more energy than a power plant without CCS.

Carbon Capture and Storage Projects

There is no commercial scale CCS project capturing carbon dioxide from a coal fired power plant and storing it anywhere in the world. None are expected before 2020. The IPCC suggests that to mitigate climate change and global warming, all developed countries need to reduce emissions of greenhouse gas by 25 to 40% by 2020. CCS is not able to assist in these reductions.

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 tonnes CO2 a year. A second project in the Snøhvit gas field in the Barents Sea stores 700,000 tonnes 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 tonnes 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 tonnes 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 tonnes 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.

So ultimately, Carbon Capture and Storage remains at the theoretical stage, as an unproven method of combatting climate change. Small amounts of CO2 have been injected into deep water off the California coast but there have been no large-scale experiments to test the concept. A planned pilot scheme off Hawaii was scrapped in the late 1990s after protests from local people and environmental groups. Greenpeace remains implacably against such experiments. Bill Hare of Greenpeace has commented:

"The urgency of reducing emissions of CO2 has never been greater. But just as with an emergency in a heavy passenger jet, the crew should never rush in to hasty actions that will ultimately make a very bad situation a lot worse. Ocean disposal of CO2 is one such option. The position of Greenpeace and of other groups opposed to this option was based on research into the effects of ocean disposal of CO2." Carbon capture and storage offers the hope of mitigating climate change, but as a developmental technology it is unlikely to be available in a timescale that can make a difference, and the rapid building of new coal-fired power stations around the world without CCS means that it may be too late the the technology to make any significant impact.