It is possible, as a result of the media attention given to climate change, that greenhouse gas emissions and global warming provide the impression that carbon is an undesirable element altogether.

Carbon is a vital component of all living matter, forming around 18% of its mass – almost 100 times greater than the Earth’s average carbon content of only 0.19%.

In fact, living organisms extract carbon from their inorganic environment and after they die it must be recycled.

Natural processes cause the movement of carbon between four major stores or reservoirs:
- The atmosphere
- The terrestrial biosphere (including freshwater systems, soil carbon and inorganic material)
- The oceans (including dissolved inorganic carbon & marine biota)
- Marine sediments (including fossil fuels and sedimentary rock)

Many factors in this very complex system have only been lightly researched, but it is known for certain that the Carbon Cycle is a closed system with a finite quantity of carbon in circulation.

The system demonstrates an enormous capacity to absorb excesses and this is evident from data monitoring the level of atmospheric carbon dioxide that has been recorded since late in the 19th century.

The concentration of carbon dioxide has risen by nearly 30%, but calculations based on fossil fuel consumption and de-forestation predicted double this increase.

Carbon Cycle
The amount of carbon being exchanged in each process, measured in gigatons of carbon (Gt C) determines whether the specific sink is growing or shrinking. For instance, the ocean absorbs 2.5 Gt C more from the atmosphere than it gives off to the atmosphere.

All other things being equal, the ocean sink is growing at a rate of 2.5 Gt C per year and the atmospheric sink is decreasing at an equal rate. But other things are not equal. Fossil fuel burning is increasing the atmosphere’s store of carbon by 6.1 Gt C each year, and the atmosphere is also interacting with vegetation
and soil.

Furthermore, there is changing land use and the indicated figures are annual averages over the period 1980 to 1989, not reflecting seasonal fluctuations.

Analysis of ice cores from Antarctica reveals that the concentration of atmospheric carbon dioxide was maintained in the range of 260-280 parts per million (ppm) during a 10,000 year period, until the widespread use of coal energy began in the late 18th century. By 1973 the carbon dioxide level had risen by 50 ppm, while another 50 ppm increase was recorded in only the next 33 years

The greenhouse effect
Carbon dioxide and other gases have the property of absorbing infrared radiation that is re-radiated by the Earth’s surface trapping it as heat in the atmosphere.

Without this ‘greenhouse’ effect, the world’s surface would experience huge temperature fluctuations and our planet would be too cold for us to inhabit.

It is apparent that the accelerated emission of carbon dioxide since 2000 exceeds the capacity of the Carbon Cycle to regulate it’s concentration in the atmosphere.

Assessments by the Intergovernmental Panel on Climate Change (IPCC) suggest that the Earth’s climate has warmed between 0.6 and 0.9 degrees Celsius over the past century and that human activity affecting the atmosphere is ‘very likely’ an important driving factor.

The IPCC’s Fourth Assessment Report (Summary for Policymakers) states, “Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

The expected future trends will see global emissions growing by 1.8% p.a. until 2030, with the bulk of this increase generated by the developing industries of China and India where growth could top 2.6% annually.

Carbon sequestration
This term refers to the removal of carbon dioxide from the atmosphere and its artificial storage, or the enhancement of naturally occurring sequestration processes.

Often referred to as Carbon Capture and Storage (CSS), sequestration is intended to mitigate global warming by capturing CO2 from large point sources and thereby preventing its release into the atmosphere.

Four types of sequestration processes have been proposed, notably geologic sequestration, ocean sequestration, terrestrial sequestration, and mineral storage.

Geologic sequestration
- Injecting CO2 directly into underground geological formations. This technology is already proven as Enhanced Oil Recovery (EOR) and widely used in oil production from declining fields in the US.

- Adsorbing CO2 into the surface of un-minable coal seams.

- Dissolving CO2 into saline aquifers, some experience of chemical waste storage.

Ocean sequestration
- ‘Dissolution’ injects CO2 by ship or pipeline into the water column at depths of 1000m or more, and the CO2 subsequently dissolves.

- ‘Lake’ deposits CO2 directly onto the sea floor at depths greater than 3000m, where CO2 is denser than water and is expected to form a ‘lake’ that would delay dissolution of CO2 into the environment.

- Convert the CO2 to bicarbonates (using limestone)

- Store the CO2 in solid clathrate hydrates already existing on the ocean floor, or growing more solid clathrate.

Terrestrial sequestration
Storing CO2 in soils and vegetation is a major natural ‘carbon sink’.

Mineral storage
Producing stable carbonates by reacting CO2 with abundant metal oxides.

Known geological formations are estimated to offer storage capacity for at least 2000 Gt of CO2 and are currently seen as the most promising sites. Annual anthropogenic emissions of CO2 are estimated at 30 Gt.

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 1000–3000m. 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, which may in turn rule out deep ocean storage as an option.

CO2 capture
Several technologies exist or have been proposed for the process of CO2 capture. These include:

- Post-combustion capture removes CO2 from the flue gas after the combustion of the fossil fuel. Already used for the commercial production of CO2 and other industrial applications this technology is well understood.

- Pre-combustion technology is used extensively in the production of fertiliser, chemicals, gaseous fuels and power, by partial oxidation in a gasifier.

- Oxy-fuel combustion burns the fossil fuel in oxygen rather than air, thus eliminating the nitrogen from the flue gas that can be cooled, transported and stored as CO2.

- Ethanol plants using fermentation do produce nearly pure CO2, that can be easily pumped underground.

- Chemical looping combustion is still under development and uses metal oxide as a solid oxygen carrier.

- Direct capture from the atmosphere is a novel concept that would be useful to absorb CO2 from diffuse sources, such as vehicles.

The Cost of Sequestration
Fossil fuelled power plants are an obvious target for the deployment of CCS technology, especially because of our high dependence on fossil fuels for electricity generation and the vast availability of un-tapped coal energy resources.

In theory CCS systems could reduce CO2 emissions by as much as 90%, depending on plant specific details.

Unfortunately, the application of CCS in power generation results in a considerable increase in the energy required to both process and compress, the captured CO2. This translates into significantly increased, specific fuel consumption and therefore partially negates the climate mitigation potential. The increased fuel for a coal-fired plant with CCS is expected to be around 25% and for a gas-fired plant, about 15%.

The total cost penalty including additional fuel, storage and other system costs will probably increase the cost of electric power by 30-60%.

Another disadvantage resulting from the increased fuel consumption is a reduction in air quality, due to greater emission of air pollutants.

Carbon sequestration projects
As of 2007, four industrial scale sequestration projects were in operation.

Sleipner, located in the North Sea, is the oldest project in operation since 1996 and has stored a total exceeding 1,000,000 MT of excess CO2. The Snøhvit gas field in the Barents Sea meanwhile, stores 700,000 MT p.a. separated from the production gas before distribution.

The Weyburn project in Saskatchewan, Canada is the largest project to date, with CO2 used for enhanced oil recovery (EOR) and around 1,500,000 MT p.a. injected for this purpose. The fourth such project is the Salah initiative in Algeria, a high CO2 natural gas reservoir that re-injects about 1,200,000 MT annually.

Meanwhile, the world’s first CCS coal plant began operating in Germany’s industrial area of Schwarze Pumpe in September 2008, to serve as a prototype for future full-scale plants. The mini plant is rated at 30 MW and produced 240 MY per day of CO2 that is trucked off-site and injected into an empty gas field.

In Australia, the southern hemisphere’s first demonstration geo-sequestration plant in South Western Victoria was funded jointly by government and industry, and aims to store 100,000 MT of CO2 from a natural gas stream in a depleted reservoir.

Conclusion
Data suggest that the intensive worldwide efforts to implement carbon dioxide sequestration are more than justified. The Earth’s surface temperature has been shown to be unusually stable during the past 10,000 years, a fact that science has as yet still not fully explained.

Strong correlation is indicated between atmospheric CO2 concentration and temperature, but were the past dramatic temperature drops triggered by the accumulation of greenhouse gases? Perhaps it would be prudent not to simply wait and see.

Several projects are under development:

1. The Alberta Saline Aquifer (ASAP) project will sequester 1,000 MT / day in 2010 peaking at 10,000 MT / day by 2015.

2. The Integrated CO2 Network (ICO2N) members have proposed a system for capture, transport and storage development in Canada.

3. The Wallula Energy Resource
Centre in Washington has
proposed a coal-fuelled Integrated Gasification Combined cycle electricity generation plant. Around 65% of the CO2 will be captured using pre-combustion technology and sequestered in underground basalt formations.

4. The Southeast Regional Carbon Sequestration Partnership (SECARB), funded by the US Department of Energy (DOE), will sub-contract the first intensively monitored, long term US project to the Bureau of Economic Geology at The University of Texas. In the Tuscaloosa-Woodbine geologic system between Texas and Florida, the potential to store CO2 is estimated at 200 billion MT, the equivalent of 33 years of total US emissions at current rates.

5. Slated as the worlds first CCS power plant, the Future Gen will no longer be funded by the DOE. Instead the DOE intends to fund an IGCC project with integrated CCS of at least 250MW capacity.

6. The Big Sky Carbon Sequestration Partnership at Montana State University (MSU) was recently awarded $66.9m funding for a major project to inject over one million tonnes of CO2, into the sandstone layer beneath south-western Wyoming. Previous studies estimate the region’s ultimate capacity at 200 billion MT. The partnership and MSU will test sequestration methods and develop the requirements for carbon management in the region.

7. Utility company Luminant’s pilot CCS project, at its Big Brown Steam Electric Station, an existing coal fired power plant in Fairfield Texas, converts CO2 into baking soda to be stored in mines or landfills or sold commercially.

8. GreenFuel Technologies is piloting biological systems using algae to capture the CO2 that is converted either to fuel or feed.

9. Four synthetic fuel projects in the US have announced plans to incorporate CCS that will market the CO2 extracted for EOR applications.

10. In the Netherlands a Zero Emissions Power Plant of 68 MW capacity is planned to be operational in 2009.

11. The UK government has launched a tender process for a CCS demonstration project, to be operational by 2014 using post-combustion capture technology on coal-fired generation of 300-400 MW capacity.