The Carbon Cycle

Carbon is constantly entering the atmosphere in the form of carbon dioxide, methane, and other gases. At the same time, it is being removed by green plants, the oceans, and in other ways. This is the carbon cycle. The balance in the cycle is critical in determining the Earth’s climate.

Carbon atoms are continuously being exchanged between living and dead organisms, the atmosphere, oceans, rocks, and soil. With every outward breath, we release CO2 from our lungs into the atmosphere, containing atoms of carbon from plants and animals that we have eaten. Atoms of carbon in our bodies today might previously have been in many different plants and animals – perhaps including dinosaurs and other extinct creatures.

The distribution of carbon among atmosphere, organisms, land, and oceans has changed over time. About 550 million years ago the concentration of CO2 in the atmosphere was 7,000 parts per million – more than 18 times what it is today. Where did all that atmospheric carbon go? For the most part it ended up as sedimentary rocks such as limestone. How that happened is part of the larger story of the carbon cycle.

The carbon cycle is a combination of many biological, chemical, and physical processes that move carbon around.

Carbon Dioxide Sinks and Sources

A carbon dioxide sink is something that removes CO2 from the atmosphere. For example, green plants consume CO2 during the process of photosynthesis. Burning wood and fossil fuels are sources of CO2. The oceans are both a source of CO2 and a CO2 sink. This is because CO2 in air that is in contact with the surface of the ocean dissolves in water and is therefore removed from the atmosphere. At the same time, dissolved CO2 is released into the atmosphere. The balance between these two processes depends on many factors and is changeable over time. Presently there is more CO2 dissolving into the oceans than is being released. This means that right now the oceans are a CO2 sink.

Most of the carbon on Earth is in compounds found in sediments and sedimentary rocks. Comparatively little is in the atmosphere.
		Billion Metric Tons
	Underwater sediments and sedimentary rocks	80,000,000
	Ocean water, shells, and organisms	40,000
	Fossil fuels (oil, gas, and coal)	4,000
	Organic material in soil	1,500
	Atmosphere	825
	Land plants	580

CO2 capture-and -storage project

The Sleipner field is in the North Sea, about 250 km (155 mi) west of Stavanger, Norway. It is operated by Statoil, Norway’s largest oil company. The Sleipner field produces natural gas and condensate (light oil) from the Heimdal sandstones, which are about 2,500 m (8,200 ft) below sea level. Natural gas is a mixture of gases. It is typically at least 90% methane, plus other hydrocarbons such as ethane and propane. Natural gas often also contains gases such as nitrogen, oxygen, and carbon dioxide; sulfur compounds; and water. Gas containing small volumes of these impurities can still be used as fuel, but gas with high volumes of them cannot be burned efficiently and safely.

The natural gas produced at Sleipner contains unusually high levels (about 9%) of carbon dioxide (CO2), but the customers buying the gas from Statoil need less than 2.5%.

A special platform, Sleipner-T, has been built to support a 20-m-high (65-ft), 8,000-ton treatment plant that separates CO2 from the natural gas. The Sleipner-T plant produces about 1 million tons of CO2 per year.

To encourage companies to reduce their carbon emissions, the Norwegian government imposes a carbon tax equivalent to about $50 per ton of CO2 released into the atmosphere. To avoid paying this tax, and as a test of alternative technology, all of the CO2 extracted since 1996, when gas production started at Sleipner, has been pumped back deep underground.

 It is not put back where it came from, because that would further contaminate the natural gas. Instead, it is put into a 200-m-thick (650-ft) sandstone layer called the Utsira formation, about 800 m (2,600 ft) beneath the bottom of the North Sea. The Utsira formation contains no commercial oil or gas; like most rocks deep underground, it is filled with salt water. The Utsira formation has high porosity and permeability, so the CO2 moves rapidly sideways and upward through the rock layer, replacing the water between the sand grains.

It is estimated that it would take about 600 billion tons of CO2 to fill all the pore space of the Utsira sandstone. That is equivalent to all the human-made CO2 production for over 20 years, at current rates. It is likely that CO2 sequestration will continue at Sleipner long after the abandonment of the field as a hydrocarbon producer. The Utsira formation is just one of many similar deep saline aquifers around the world that could be used to help slow down or reverse the rate at which CO2 and other greenhouse gases are released into the atmosphere.

The Sleipner project is the first commercial example of CO2 storage in a deep saline aquifer, so there is a lot of interest from around the world in its success. In particular, scientists want to know how the CO2 moves inside the aquifer and if there is a risk that it could escape back to the surface.

Carbon dioxide capture, transport and storage technologies

Carbon dioxide capture and storage (CCS) in geological formations is a potentially important climate change mitigation measure in the coming decades, as geological formations can store large amounts of CO2 (as well as other gases or liquids) for thousands of years. However, CCS has not been widely used to date. The largest CO2 storage project to date (at the Sleipner field in the North Sea) has been injecting approximately 1 million tonnes of CO2 per year since 1996 into a saline formation.

         

CO2 emissions are produced from a wide variety of combustion-related and industrial processes sources.CO2 emissions from large point sources, such as power plants, refineries or cement plants, could be captured, transported and stored in several different ways.The technologies used within each step of the carbon dioxide capture and storage chain are at different stages of development. Some are mature/widely applied; some are economically feasible in some conditions, while other technologies are at the demonstration phase. Geological CCS projects (developed as CDM projects, or not) could involve   different combinations of capture, transportation and storage technologies. In turn, this could lead to a wide range of potential CDM project types.

 

CO2 capture already occurs in some energy and industrial activities. For example, CO2 separation routinely occurs in industries where CO2 is required as an input to a manufacturing process (e.g. production of urea). CO2 must be stripped from natural gas during exploitation of fields with significant CO2 content. CO2 is also extracted in refineries, ammonia plants and hydrogen plants.

 

“Pre-combustion” separation of CO2 can occur during the partial combustion of fossil fuels, used for example in the production of hydrogen or hydrogen-rich fuels. Electricity generation from Integrated Gasification Combined Cycle (IGCC) plants also requires pre-combustion separation of CO2.

 

“Postcombustion” CO2 capture from flue gases is also possible, e.g. via absorption or flue gas treatment (IPCC 2005). This can be used to capture CO2 from electricity generation plants (and indeed is the only option for CO2 capture from existing power plants), although it is energy-intensive and so entails an energy penalty. Oxyfuel combustion refers to a technology under development whereby fuel is combusted in oxygen and re-circulated flue gas, rather than air (which is mainly made up of nitrogen). The exhaust gases from oxyfuel combustion contains thus mainly CO2 and H2O (water vapour), rather than nitrogen. As the vapour can be easily condensed, the waste gas is largely CO2.

 

Transport of CO2 can be done by pipeline or ship. Commercial-scale transport of CO2 via pipeline and ship/tankers already occurs (IPCC 2005). Pipeline transport is normally of compressed (gaseous) CO2, whereas transport on ships is often of liquefied CO2, as this takes less volume. Liquefaction of gases is routinely used, e.g. for the transport of liquefied petroleum gas (LPG) or liquefied natural gas (LNG).

 

There are also different ways in which CO2 can be stored. These include various underground geological formations such as oil and gas fields (in use or abandoned), saline formations or coal seams (mineable or unmineable). Experience with storing CO2 in these types of formations varies. For example, the largest CO2 storage project to date (at the Sleipner field in the North Sea) has been injecting approximately 1 million tons of CO2 per year since 1996 into a saline formation. Other demonstrations, pilot or commercial projects, exist to inject CO2 into depleted gas fields and coal mines.

 

CO2 is being re-injected into various oil fields to increase the rate and amount of oil produced. Such enhanced oil recovery (EOR) can also use other fluids for the same purpose, notably water and steam.

 

Globally, CO2-based EOR projects inject around 40 million tonnes of CO2 per year – of which 30 million come from natural underground sources of CO2 and about 10 million tonnes is captured from industrial  plants. The use of CO2 for EOR can provide a valuable near-term opportunity for gaining storage  experience, but this needs to be done at the right time in the life of a particular field. Enhanced gas recovery and enhanced coal bed methane recovery are in development phase.