Capturing the greenhouse gang
Key text
This topic is sponsored by the Australian Government Department of Climate Change.
The search is on for ways to capture greenhouse gases and store them out of harm's way.
There is a price on the head of the Greenhouse Gang. The increasing atmospheric concentration of greenhouse gases such as carbon dioxide, methane and nitrous oxide is causing an 'enhanced greenhouse effect', also known as global warming or climate change. Governments and industry are prepared to pay big money for their capture.
A well-known member of the gang is carbon dioxide. It is the biggest contributor (70 per cent) towards the enhanced greenhouse effect, followed by methane at about 20 per cent. Carbon dioxide has been escaping into the atmosphere at an increasing rate for more than two hundred years. Its concentration is now 38 per cent higher than in 1750 mainly due to the burning of fossil fuels such as coal, oil and natural gas, although land use change has also been a significant contributor.
Provides an overview of greenhouse gases.
(British Broadcasting Corporation, UK)
Fossil fuels still burning
Australia, which is strongly reliant on fossil fuels for energy, has aided the great escape. Today, more than 70 per cent of the country's greenhouse gas emissions are caused by people's demand for products, transport and electricity, a demand which is met by the energy sector – mainly through the burning of coal and natural gas for electricity and oil for transport.
Despite their implication in climate change, however, fossil fuels are set to underpin world energy consumption for years to come. In Australia, several more coal-fired power stations will likely be built. In China, at least 500 such stations are reportedly planned or under construction, with a new one completed every week. Even in Europe, where policies aimed at mitigating climate change are strongest, new coal power stations are on the drawing board. Under one energy scenario developed by the International Energy Agency, worldwide demand for coal, already high, could grow by as much as 73 per cent by 2030. Under the same scenario, the consumption of oil would increase by more than a third.
If these predictions are realised, greenhouse gas emissions will rise alarmingly, increasing the risk of catastrophic climate change. We desperately need alternative, less-polluting energy sources, but developing these and scaling them up to the level required will probably take decades. Meanwhile, the impact of highly polluting fuels like coal must be reduced. The search is on for ways to effectively capture and store carbon dioxide.
Carbon capture and storage (CCS)
Carbon dioxide can be captured in many different ways. Nature has been doing it for millions of years, helping to maintain the concentration of greenhouse gases at reasonably stable levels. Trees and other carbon-fixing organisms could be deployed to sequester carbon dioxide (Box 1: Natural carbon capture).
Biological CCS mechanisms could be complemented by engineered technologies, particularly in power stations, where emissions are concentrated at a single point. The first step is to catch the carbon dioxide, preferably before it leaves the chimney stack. Several techniques already exist to do this, they are expensive, consume considerable quantities of energy, don't catch everything, and so far have not been deployed on a large scale (Box 2: Clean technologies for fossil fuels) but they do offer the prospect of long-term storage of the extracted carbon dioxide gas.
Mineral carbonation
One storage possibility is mineral carbonation, which is the fixation of carbon dioxide in the form of inorganic carbonates. It involves the reaction of carbon dioxide with suitable minerals such as silicates to form highly stable carbonates of calcium, magnesium or iron. Carbonation takes place in nature, but only very slowly; artificial carbonation would involve measures to greatly speed it up. In one scenario, carbon dioxide captured at a power station would be piped to a mineral carbonation plant, where it would be combined with suitable minerals obtained either from nearby mines or from industrial processes, such as steel smelting, to form carbonates. These carbonates would be disposed of at the mine site, or possibly used as soil enhancers or in roads.
Describes the concept of mineral sequestration.
(National Energy Technology Laboratory, Department of Energy, USA)
Implementing mineral carbonation on a large scale, however, faces many obstacles. The technical challenges of accelerating the reaction rate are considerable and might require significant inputs of energy. The monetary cost is also likely to be high and, in many locations, the natural supply of suitable minerals is low. On the plus side, a mineral carbonate sink, once formed, will be almost completely permanent and will require little if any monitoring.
An Australian scientist has proposed a variation on the mineral carbonation idea that would involve the use of magnesium carbonate in the manufacture of cement. Most of today's concrete-based structures are held together by Portland cement, the key ingredient of which is calcium carbonate. Its manufacture is responsible for about seven per cent of all human-induced carbon dioxide emissions, partly because of the energy needed to heat kilns to the required 1450ºC and partly because the chemical reaction involves the release of carbon dioxide. The temperature required to manufacture magnesium carbonate-based cement is lower – about 650ºC, making it a less energy-intensive process. As with calcium carbonate-based cement, large quantities of carbon dioxide are released during manufacture, but most are re-absorbed by carbonation during setting and hardening. Carbonation occurs more quickly in magnesium carbonate-based cement than in Portland cement and adds to the strength of the material.
Ocean sequestration
A possible repository for unwanted carbon dioxide is the deep ocean. One idea is to pump the gas to a depth of about 1 kilometre below the sea surface, where it would dissolve. But this storage would probably be temporary: the gas would leak back into the atmosphere within a century or two, perhaps sooner. Another problem is the acidification effect of the dissolved carbon dioxide and the subsequent impact on marine life.
Provides an overview to ocean acidification.
(Nova: Science in the news, Australian Academy of Science)
Another possibility is to store the carbon dioxide more deeply in the ocean, below about 3 kilometres, where the pressure is high enough and temperature cool enough to change the gas into a liquid. Scientists predict that carbon dioxide injected this deeply into the sea would form a liquid lake that would only dissolve slowly, thus delaying re-release to the atmosphere and reducing the acidification effect.
A third oceanic proposal, which builds on the physics of the second proposal, is to pipe carbon dioxide into sediments below the sea floor at depths greater than 3 kilometres. The liquid carbon dioxide would dissolve slowly into the sediment, where, scientists predict, it would be held for millions of years.
The science of oceanic carbon dioxide storage is still developing and there are many uncertainties, including the practical matter of transporting captured carbon dioxide to its resting place in the ocean, the degree to which ocean acidification would occur and the effects on marine life near the point of storage.
Geosequestration
Of all the artificial carbon capture and storage ideas, perhaps geosequestration – also called carbon burial – holds most promise in Australia.
Geosequestration is the capture of carbon dioxide and its storage in porous rocks below the surface of the Earth. Old oil and gas fields provide one potential storage option. Australia's Cooperative Research Centre for Greenhouse Gas Technologies, CO2CRC, which investigates geosequestration technologies, recently began an experiment in a depleted gas field near Warrnambool on Victoria's south coast. The plan there is to compress up to 100,000 tonnes of carbon dioxide-rich gas (80 per cent carbon dioxide, 20 per cent methane) into a 'supercritical fluid' (which means that it acts partly as a gas and partly as a liquid), inject it into porous sandstone 2 kilometres below the Earth's surface, and monitor the site to ensure that the carbon dioxide is stored securely.
The Warrnambool site has been tapped in the past for its methane deposits, meaning that it has suitable rocks for the storage of carbon dioxide. The carbon dioxide is injected into a sandstone reservoir which has tiny pores between the rock grains which enable the carbon dioxide to move through the reservoir. The carbon dioxide fills the spaces between the rock grains. The carbon dioxide will be prevented from moving upwards by an overlying layer of impermeable mudstone. A well has been drilled to inject carbon dioxide into the reservoir while monitoring equipment installed in an adjacent depleted well will monitor the behaviour of the injected gas.
geosequestration at the
Warrnambool site
(©CO2CRC 2008)
(Click on image to view
the animation)
Proposals for future stages of the project include separating the methane from the carbon dioxide prior to injecting it and also injecting carbon dioxide into a deep saline aquifer via the same injection well. Deep saline aquifers might also be used to store carbon dioxide. While they are not as well understood as depleted gas fields, it is likely that they will be suitable for holding vast amounts of carbon dioxide in some areas. For example the largest storage project to date, the Norwegian Sleipner Project, has injected more than 10 million tonnes into a saline aquifer.
Still wanted
No single carbon capture and storage option will solve the problem of escalating greenhouse gas emissions. Most options are expensive and will require substantial technological development. Some are likely to be impractical on a large scale and some are likely to be seen as environmentally unacceptable (eg ocean storage). Governments are debating how to stimulate faster progress, by making polluters pay for the greenhouse gases they emit. Putting a price on the head of the Greenhouse Gang will encourage the search for cheap solutions and reduce the quantity of emissions. Research into CCS technologies will increase dramatically in coming years. Capturing the gang is part of the battle; locking them away for good is the ultimate goal.
Boxes
1. Natural carbon capture
2. Clean technologies for fossil fuels
Related Academy Links
Nova
Geoengineering - can it help our planet keep its cool?
Acid test for the seas
Carbon currency – the credits and debits of carbon emissions trading
Enhanced greenhouse effect – a hot international topic
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Posted July 2008.