Q&A What is carbon capture and storage and how does it work?
Compiled by Roger East
The Guardian
What does carbon capture and storage (CCS) involve?
Separating out the carbon dioxide (CO2) emitted by power stations (or industrial processes), and transporting it to a place for indefinite storage. In the energy producing sector, the purpose of CCS is to reduce emissions from burning fossil fuel by some 85-90%, which many see as the only way to avert devastating climate impacts from coal-fired electricity production.
Does the technology exist?
Different stages have been used or demonstrated but the whole process has not yet been applied in power stations on a commercial scale.
So what's "carbon capture ready"?
It is an imprecise term used to describe the design of new power station projects, especially coal-fired ones, without which they would stand no real chance of approval in the UK. It should mean they have space and access for retrofitting carbon capture equipment once it is developed, plus access to suitable geological storage underground, and a feasibility study giving reasonable confidence that it would work. Critics say that in the absence of legislation to make it compulsory there is no obligation to retrofit CCS techology - and that it encourages a "build now, capture later" mindset that is extremely risky while the technology remains unproven.
Why might an operator not want to retrofit later?
It entails a capital expense and high running costs. Carbon capture could well absorb a quarter of the power station's generating capacity. Uncertainty about this and other costs is one of the big question marks over the viability of the whole CCS enterprise. Current estimates suggest costs of between 40 and 90 euros (£30-70) per tonne for capture, plus the (lesser) costs of transport and storage.
Can you take out carbon dioxide from the waste gas of a power station as you do for other pollutants such as sulphur dioxide?
Yes, this is what happens in one form of the technology known as "post-combustion". Using known industrial techniques, flue gases are diverted through an absorber, where the CO2 is taken up by a solvent. The next stage, heating the CO2-rich solvent to 120C in a reformer, releases the CO2 for capture.
What are the other methods of removing CO2 from a power station?
Instead of removing the CO2 after the fuel has been burnt, the fuel is turned into a gas made of carbon monoxide and hydrogen. This gas, known as syngas, is combined with steam to turn the carbon monoxide into CO2, which can be extracted and captured before the remaining hydrogen is burned to produce electricity. This method is known as pre-combustion.
Are there any other methods?
There is a third technology called oxyfuel where the fossil fuel is burned in 95% pure oxygen instead of in air, resulting in a flue gas that consists of high CO2 concentrations and water vapour. The two can be separated just by cooling and the CO2 is then condensed and compressed for storage.
Once you've captured your CO2, how do you transport it?
Pipelines are generally the cheapest way, with costs depending on distance and the availability of suitable existing networks. However, there are questions that need to be answered about how the pipelines are going to work.
What are the main prospective storage sites?
The most important potential sites for storage are saline acquifers deep underground - porous rock formations that contain saline water. Before it is injected, the CO2 is compressed to a dense fluid. This density increases as the CO2 goes deeper underground until, below 800m, it becomes so dense that it should not rise to the surface. Other options for storage are depleted gas and oil reservoirs and seams of coal that are no longer mined.
What makes a good site?
To safely store CO2, a site needs three things: enough storage space, a layer of rock, such as clay stone or shale, that seals the site and prevents gas from rising to the surface and a sufficiently stable geological environment. Under such conditions the UN's scientific expert Intergovernmental Panel on Climate Change considers it likely that 99% of the CO2 will still remain safely sequestered after 1,000 years.
Are there other storage alternatives?
Disposal deep at sea has been considered, because of the heaviness of CO2 at great depths, but this has been largely ruled out because of concerns about CO2 changing the ocean environment.
Is there enough space underground to store all the CO2
There is an estimated capacity of at least 2,000 gigatonnes of CO2, or 65 years of man-made global emissions at the rate the world is currently putting CO2 into the atmosphere.
What happens if we run out of storage space?
CCS is not a licence to continue business as usual. Its proponents say CCS simply buys us time to switch to a more sustainable energy path, investing in renewable energy and energy efficiency to bring CO2 emissions down.
