Capturing the greenhouse gang

Key text

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.

Related site: Climate change – the greenhouse gases 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.

Related site: CO2 mineral sequestration studies in US 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.

Related site: Acid test for the seas 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.

Animation showing 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.

Related Nova topics:

Acid test for the seas

Carbon currency – the credits and debits of carbon emissions trading

Enhanced greenhouse effect – a hot international topic

Box 1: Natural carbon capture

Many power stations and other greenhouse gas polluters have already started to pay for the establishment and management of tree plantations to 'offset' their carbon dioxide gas emissions. The idea is that if polluters continue to emit carbon dioxide, they should balance this pollution by capturing and storing it elsewhere.

Trees absorb carbon dioxide during photosynthesis. They use energy from the sun to drive a reaction between carbon dioxide and water to form carbohydrates, releasing oxygen as a waste product. Some of the captured carbon becomes stored in the trunk, branches, leaves and roots of the tree, where it might stay for decades. Even if the tree is cut down, carbon dioxide might still be kept out of the atmosphere if the wood is used to make long-life products, such as houses.

Scientists also believe that, in some conditions, the increased concentration of carbon dioxide in the atmosphere could help stimulate plant growth, accelerating its uptake. This is because the greater availability of carbon dioxide, an ingredient of photosynthesis, will act like a fertiliser. Other resources, such as water, nitrogen and trace elements, would also need to be available. Research has shown that the fertiliser effect tends to level off in natural ecosystems within a few years due to these other limiting factors. Other effects of climate change (eg. higher temperatures) can work against the fertiliser effect resulting in decreased growth in some regions.

The Kyoto Protocol, an agreement between nations designed to reduce the greenhouse gas emissions, allows countries to count carbon dioxide sequestered in tree plantations against their overall greenhouse gas emissions, but there are risks. Fire, disease, drought and poor management can all cause premature tree death. If a tree rots or burns, all its good carbon-capture work is undone because carbon dioxide, methane and other greenhouse gases are released to the atmosphere. Even carbon dioxide in well-managed, disaster-free plantations is likely to re-enter the atmosphere within decades or, at best, centuries.

Phytoplankton

Other organisms absorb and store carbon dioxide. Phytoplankton use a carbon-based compound, calcium carbonate, in their skeletal structures. They are abundant in the world's oceans and play a significant role in the global carbon cycle. Some scientists have speculated that stimulating the growth of phytoplankton by adding a micronutrient, iron, to phytoplankton 'dead spots' in the ocean could help remove large quantities of carbon from the atmosphere. When the phytoplankton die, their skeletons sink to the bottom of the ocean, where they can remain for many centuries. The impact of iron fertilisation of the oceans on carbon dioxide levels and the environment is subject to debate.

Crops

According to some scientists, even agricultural crops can help sequester carbon dioxide. As they grow, many plants form tiny, rigid particles called phytoliths. Phytoliths are made of silica and sometimes calcium oxalate or other minerals; these substances are deposited in and around cells as the plant grows, eventually forming microscopic sacks. Inside the sacks are small fragments of organic material – including carbon. Since these sacks are made of minerals that don't break down, the carbon trapped within them cannot escape. When the plant dies, its phytoliths remain in the soil, often for thousands of years, forming a long-term carbon pool.

Phytoliths are present in most soils of the world, but agricultural crops can produce them in large quantities. An Australian study found that the organic carbon trapped in phytoliths by a variety of sugar cane commonly used in New South Wales, for example, was more than 30 times higher than that observed in natural vegetation communities. Other sugar cane varieties sequester even higher amounts. Scientists suggest that the selection of crop varieties that yield high levels of phytolith carbon could make a substantial contribution to reducing the concentration of carbon dioxide in the atmosphere.

Related sites

• CO₂ tree capture – how much carbon dioxide do trees really capture? (Catalyst, 19 April 2007, Australian Broadcasting Corporation)

• Forests, wood and Australia's carbon balance (Forest and Wood Products Australia)

• New Scientist
  • 'Plantstones' could help lock away carbon (7 January 2008)
  • CO₂: Don't count on the trees (27 October 2007)
  • Climate myths: Higher CO₂ levels will boost plant growth and food production (16 May 2007)
  • Tree farms won't halt climate change (28 October 2002)

• Iron fertilisation of the oceans (Nova: Science in the news, Australian Academy of Science)

• Farming the climate (ScienceAlert, 9 February 2007)

Box 2: Clean technologies for fossil fuels

Pre-combustion

Integrated gasification combined cycle (IGCC) is an example of lower emission technology, which uses coal to synthesise gas (syngas). The reaction combines the carbon in coal with water to produce a mixture of hydrogen and carbon monoxide. The mixture is then combusted with air or oxygen to drive a gas turbine to generate electricity; the exhaust heat is converted into steam, which then drives a steam turbine which will produce additional electricity.

IGCC improves the efficiency with which coal is converted to electricity and therefore lowers greenhouse gas emissions per unit energy. Further emission reductions can be obtained by removing carbon dioxide from the mix of waste gases.

In a pre-combustion capture plant, the syngas is further reacted with water to convert the carbon monoxide to carbon dioxide and hydrogen. The carbon dioxide is captured and the hydrogen is used as a fuel in a gas turbine to generate electricity. The costs are likely to be high and there no IGCC plants generating electricity with pre-combustion carbon capture currently in operation.

The hydrogen produced in this process could also be used in fuel cells for power generation and in vehicles.

Post-combustion

Post-combustion technologies focus on capturing carbon dioxide after the fuel has burned but before it is released from the power station. Using current technology, the waste gas from the combustion process must first be cleaned of impurities such as sulfur oxides, nitrous oxides and particulate dust. It is then piped to a tall, cylindrical vessel called an absorber and mixed with a solvent at a temperature of about 50ºC. Carbon dioxide molecules bind to the solvent and the cleaned flue gas can then be released into the atmosphere. The carbon dioxide/solvent mixture is sent to a stripper – another tall, cylindrical vessel – and the carbon dioxide removed in the presence of heat. The solvent can be used again, while the carbon dioxide is compressed and sent to long-term storage.

Several post-combustion scrubbing pilot plants are in operation around the world, but the use of the technology at a large scale is held back by its high cost, particularly due to its effect on energy efficiency.

Oxy-fuel combustion

In conventional coal-fired power stations, coal is burned in air, which contains about 20 per cent oxygen. Oxy-fuel combustion uses a gas mix containing about 97 per cent oxygen and recirculated flue gas, which is composed almost entirely of carbon dioxide and water. This process reduces the net volume of flue gas produced while increasing the concentration of carbon dioxide in the flue gas. One advantage of this is that the flue gas is ready for sequestration without the need for post-combustion scrubbing. Another is that the lower volume of flue gas means a lower heat loss and therefore a more efficient process. The disadvantage is the financial and energy cost of producing the oxygen used in the combustion process.

Processes for carbon dioxide capture (©CO2CRC 2008) (Click on image for a larger version)

Related sites

• Clean energy special: A greener goal for coal (New Scientist, 3 September 2005)

• Clean coal technologies (Australian Coal Association)

• CO2 capture and storage (Coal21, Australia)

• World Coal Institute
  • Environmental impacts of coal use
  • Integrated gasification combined cycle (IGCC)

• About geosequestration: Capture (CO2CRC, Australia)

• Post-combustion carbon capture from coal-fired plants – solvent scrubbing (International Energy Agency Clean Coal Centre, UK)

• Callide oxyfuel project (CS Energy, Australia)

Activities

• Ocean Explorer (National Oceanic Atmospheric Administration, USA)
  • It looks like champagne – students will learn about supercritical carbon dioxide and carbon dioxide sequestration.

• Climate Change Educator Program (Wild BC, British Columbia, Canada)
  • Carbon sinks and storage – students learn about the carbon cycling system.

• CSI: Climate Status Investigations (Keystone Center, USA)
  • Geologic sequestration – students learn how geologic sequestration can be used as a technique to reduce carbon dioxide in the atmosphere.
  • Trapping CO₂ – students investigate the capture of carbon dioxide.

• For teachers and students (Bureau of Economic Geology, USA)
  • Audience-pleasing physical models to support CO₂ outreach – students observe a series of five models to support their understanding of carbon capture and storage

Further reading

Useful sites

• Facts on CO₂ capture and storage
  Contains information on the capture and storage of carbon dioxide.
  http://www.greenfacts.org/en/co2-capture-storage/foldout-co2-carbon-storage.pdf

• Scientific facts on CO₂ capture and storage
  Provides information on carbon capture technologies.
  http://www.greenfacts.org/en/co2-capture-storage/co2-capture-storage-greenfacts-level2.pdf

Australian Broadcasting Corporation

• The carbon farmers (The Lab, 5 September 2007)
  http://www.abc.net.au/science/features/soilcarbon/default.htm

• Carbon capture and storage (Science Show, 25 August 2007)
  http://www.abc.net.au/rn/scienceshow/stories/2007/2014388.htm

• Geosequestration (Catalyst, 10 August 2006)
  http://www.abc.net.au/catalyst/stories/s1710912.htm

• Sustainable transport and power alternatives (Science Show, 17 June 2006)
  http://www.abc.net.au/rn/scienceshow/stories/2006/1663624.htm

• Ocean may be new CO₂ dumping ground (News in Science, 24 March 2006)
  http://www.abc.net.au/science/news/stories/s1599675.htm

• Carbon capture and storage (Science Show, 22 October 2005)
  http://www.abc.net.au/rn/science/ss/stories/s1483581.htm

• New blueprint for storing carbon (News in Science, 21 September 2005)
  http://www.abc.net.au/science/news/stories/2005/1464115.htm

• Greenhouse gas grave (The Lab, 23 September 2004)
  http://www.abc.net.au/science/features/gasgrave/default.htm

• Pipe dreams (Geosequestration) (Catalyst, 9 September 2004)
  http://www.abc.net.au/catalyst/stories/s1195633.htm

• Geosequestration won't rock world, expert (News in Science, 4 August 2004)
  http://www.abc.net.au/science/news/stories/2004/1168015.htm

• Oil fields could stash carbon dioxide (News in Science, 14 July 2004)
  http://www.abc.net.au/science/news/stories/2004/1153909.htm

Geological storage of carbon dioxide: How long can we keep it out of the atmosphere? (Public Lectures, 7 July 2005, Australian Academy of Science)

Transcript of a lecture given by Dr John Bradshaw discussing the geological storage of carbon dioxide.
http://www.science.org.au/events/7july05.htm#2

CO2CRC Research Programs (CO2CRC, Australia)

Contains information on research on carbon dioxide capture and storage.
http://www.co2crc.com.au/research.html

Geosequestration – putting the carbon back (Department of Primary Industries, Australia)

Presents information on geosequestration and how it works.
http://www.dpi.vic.gov.au/DPI/nrenmp.nsf/childdocs/-9B891A4E5A686707CA25746700190BAA-
D04D48123E36F3D0CA257467001F6EF1-DD732E00FFC79788CA25746E0019B138-00ADF4AAEE8581A4CA256E530001FA0B?open

Forest sinks (Australian Government Department of Climate Change, Australia)

Provides a summary of forest sinks.
http://www.climatechange.gov.au/land/forest-sinks.html

Clean coal technology (NSW Minerals Council, Australia)

A presentation on geosequestration which clearly outlines the greenhouse effect and clean coal technology.
http://www.nswmin.com.au/__data/assets/pdf_file/0016/6019/cct_hand_out_notes.pdf

Glossary

carbon-fixing organisms. Organisms that remove and store carbon dioxide from the atmosphere during photosynthesis.

carbonation. The reaction between a substance and carbon dioxide to form a carbonate. For example, naturally occurring mineral silicates react with carbon dioxide to form a stable mineral carbonate.

enhanced greenhouse effect. An increase in the natural process of the greenhouse effect, brought about by human activities, whereby greenhouse gases such as carbon dioxide, methane, chlorofluorocarbons and nitrous oxide are being released into the atmosphere at a far greater rate than would occur through natural processes and thus their concentrations are increasing. Also called anthropogenic greenhouse effect or climate change.

impermeable. A substance that cannot be penetrated. A rock or material that stops the movement of water or other liquids through it.

phytoplankton. Microscopic, photosynthetic algae that live in water. Plant-like plankton.

sequester. To store something so that it is no longer available. Carbon sequestration involves the removal or storage of carbon dioxide so that it can't be released into the atmosphere.

supercritical fluid. A substance that when placed under a certain temperature and pressure acts like both a liquid and a gas. Carbon dioxide stored underground as a supercritical fluid diffuses like a gas with a liquid-like density.