Can we bury our carbon dioxide problem?

Once captured, carbon dioxide could be injected into depleted oilfields

The UK is struggling to decrease its carbon dioxide emissions and meet its own domestic targets of 20 percent below 1990 levels by 2010 (with the future target of 60 percent by 2050)

Fossil fuel-based energy sources, such as gas, coal and oil will remain a key part of the world and indeed the UK's energy mix over the next 50 years. There is therefore a pressing need to develop technologies that will allow us to continue their use but with minimal or zero carbon dioxide emission.

One such technology is carbon capture and storage (CCS). CCS offers the potential to isolate and store carbon dioxide produced during the combustion of fossil fuels. 

Carbon capture

Isolating carbon dioxide from the mixture of gases produced when fossil fuels are burnt is known as carbon capture. Carbon capture is already possible and carbon dioxide can be separated from flue gas, natural gas and hydrogen (in synthetic gas generated from coal gasification)

Currently there are three technologies for doing this. BP is trailing a pre-combustion method in the decarbonised Fuels Project which will be based at the Peterhead power station in Aberdeenshire and the North Sea Miller oilfield. The  Carbon dioxide is captured from a mixture of hydrogen and carbon dioxide formed from the incomplete combustion of the natural gas feedstock. The hydrogen is then combusted to create electricity. 

An alternative post-combustion method captures low pressure carbon dioxide from flue gas after combustion. This can be applied to large power plants and industrial processes. 

Additionally, the Oxyfuel combustion method  changes the actual combustion process by burning fossil fuels in the presence of pure oxygen. This results in a flue gas containing predominately carbon dioxide and water which are then easily separated. 

Challenges for chemists 

The key chemical challenge is in isolating the carbon dioxide from other gases. Currently a solvent (known as solvent scrubbing) is used which separates carbon dioxide by absorption onto a solvent such as monoethanolamine (MEA). Membrane separation technologies are also available where selectively permeable membranes allow only carbon dioxide to pass through. 

Research is still needed to develop improved capture systems. Absorbents needs to be both efficient at capturing and releasing carbon dioxide and must be easily regenerated for reuse. membranes need to be more efficient, selective and robust. Other new methods such as high temperature solid adsorbents and cryogenic approaches may also have a part to play.

Carbon storage 

Several options for carbon dioxide storage are being considered. Most of these involve pumping the gas underground or undersea into sites such as depleted or depleting oil and gas fields, deep saline aquifers and unmineable coal seams. The techniques developed for extracting oil and gas mean that we already have much of the necessary technology to do this. 

Deep ocean storage has also been suggested which involves releasing carbon dioxide into the ocean at depths below 3,000m where it will be denser than water. Another possibility would be to return the carbon to a solid phase, known as mineral carbonisation. Reacting carbon dioxide with magnesium or calcium oxide would form mineral carbonates. 

Of these options storage in depleted oil and gas fields or saline aquifers are most suitable for the UK. The basic principal entails pumping carbon dioxide under pressure into pores in sedimentary rocks in both oil and gas fields and saline aquifers under the sea-bed. 

Worldwide there are a number of planned and ongoing demonstration projects for storage in both oilfields and aquifers. There are over 50 enhanced oil recovery (EOR) projects in Texas alone. By pumping carbon dioxide into existing oil fields the oil yields are improved in addition to leaving the carbon dioxide trapped behind. The increased oil production that the technique provides is a big incentive for developing EOR. 

The best known project for carbon storage in saline aquifers is at the Sleipner West field in the Norwegian North Sea, which is operated commercially by Statoil. Carbon dioxide is separated from the natural gas and injected into an aquifer 1,000m below sea level at the rate of one million tonnes per year. 

Is it cost effective?

Because CCS requires energy, its adoption will effectively increase the use of fossil fuels. The Intergovernmental Panel on Climate Change (IPCC) estimates the cost of CCS technology would increase the energy needs of power plants by 10-40% and increase the cost of energy from plants by between 30-60%. A solution would be to use renewable sources to provide the increased capacity needed. 

The future costs of CCS are likely to decline with advances in technology and economies of scale. But costs are not at all certain and could be influenced by factors such as oil prices.

How can these costs be weighed against the potential long term costs of climate change? What will the long term cost be of not acting to reduce carbon dioxide emissions? Even if the long term benefits are acknowledged, it is clear that the potential of capture and storage will be limited unless governments adopt climate change policies that put a cost on emitting carbon dioxide - otherwise there will be no incentive to use or develop these technologies. The EU Emissions trading Scheme (EU ETS) launched last year goes some way to bringing this about but the agreement does not go beyond 2012. 

The problems to solve 

In principle, CCS looks like the panacea to our carbon emissions problem. But there are several key issues that need to be resolved before a carbon storage strategy could be endorsed. 

Firstly, are there enough geological reservoir sites to store the amount of carbon dioxide we are releasing? At the moment we can not be sure of the total volume available for storage. We need to identify more storage sites and maximise their storage potential. 

Chemists and other physical scientists will have a key roles to play in better understanding the physical properties and behaviours of carbon dioxide storage conditions. Understanding corrosion and the potential problem of carbon dioxide leakage is also crucial.

if storage in deep seas were considered there would be concerns regarding the environmental impact of changing the oceans' chemistry. They could become significantly acidified if carbon dioxide injection occurred on a large scale. Chemical scientists will need to understand this in depth before such storage mechanisms are considered. 

Conversion to chemicals 

From a chemical science perspective, carbon dioxide must be seen as a potential feedstock for the manufacture of useful chemicals. Chemical conversion of large amounts of carbon dioxide to inert or even commercially useful materials is an option that cannot be ignored. 

A number of projects are currently ongoing in the UK and internationally and include the reaction of carbon dioxide to form useful hydrocarbons and photochemical processes designed to mimic nature's pathways for converting carbon dioxide into sugars.  

At present, the energy requirements for such chemical conversions are high, meaning that there is little potential for net reduction of carbon dioxide emissions. It will need considerable chemical ingenuity before this becomes a commercial option, but chemists are taking up the challenge. 

The policy issues 

The House of Commons Science and Technology Select Committee has recently reported its findings after an inquiry into CCS. The report concludes that UK government should make CCS a requirement for statutory licensing of new power plants.

Investment in R&D

The Committee also urged the Government to increase investment in research and development of CCS by an order of magnitude. To invest in demonstration projects was recommended in order to 'pump prime' industry to give it the confidence to proceed. 

In the RSC submission to the Science and Technology Select Committee inquiry, the need for scientists to collaborate internationally on this topic was stressed to ensure that the technology is developed as soon as possible. 

Gaining acceptance 

The RSC and other groups have also stressed the importance of creating a public dialogue to discuss the risks and benefits of CCS. Unless CCS technology is understood and accepted as a safe and technologically sound option for carbon dioxide abatement, it runs the risk of being seen as a mechanism for 'sweeping carbon dioxide under the carpet' and burying the problem for future generations.