What is Carbon Capture and Storage (CCS)?
CCS technology captures carbon dioxide from fossil fuel power stations. The CO₂ is then transported via pipelines and stored safely, offshore in deep underground structures such as depleted oil and gas reservoirs, and deep saline aquifers. Up to 90% of carbon dioxide (CO₂) from a fossil fuel power station can be captured using CCS technology.
CCS also can be transferred and applied to industrial processes to significantly reduce CO₂ emissions in this sector. Applying CCS to the industrial sector, could significantly reduce the UK’s CO₂ emissions and help us achieve our 2050 climate change goals.
How does it work?
There are three different capture technologies that can be fitted to fossil fuel power stations: post-combustion, pre-combustion and oxyfuel. Only post-combustion and pre-combustion can only be applied to industrial processes.
Post-combustion capture of CO₂ occurs after the burning of fossil fuels . CO₂ is separated from the flue gas (a by-product of burning fossil fuels) through a process called scrubbing. Flue gas is passed through a liquid which causes a chemical reaction and separates the CO₂ ready for transportation and storage. Post-combustion capture technology can be retrofitted to existing fossil fuel power stations.
Pre-combustion capture is where CO₂ is separated or removed prior to the burning of fossil fuels. The pre-combustion technology converts fossil fuels into a gas made up of CO₂ and Hydrogen (H₂). These gases are then separated through scrubbing, just like in the post-combustion capture process. H₂ fuels the power plant and the CO₂ is captured, ready for transportation and storage. Only new fossil fuel power plants can be equipped with pre-combustion technology.
A leading process of pre-combustion capture is a system called IGCC (Integrated Gasification Combined Cycle) whereby electricity is generated through gas turbines, as well as steam-powered ones. A benefit of this emerging technology, is that less energy is used to remove the CO₂ which improves the power plants overall efficiency.
Oxyfuel technology burns fossil fuels with nearly pure oxygen. The flue gas produced, only contains CO₂ and steam, which are then separated by a cooling process; the water condenses and leaves a flue gas of almost pure CO₂. Oxyfuel technology can be applied to new and existing fossil fuel power stations.
Once pure CO₂ is captured through these technologies, it is compressed into a liquid state (similar density to crude oil) at 70 atmospheres, and is transported via pipeline and stored safely offshore.
How and where is CO₂ stored?
The UK is in a very advantageous position as we have access to offshore CO₂ storage locations, under the North Sea. CO₂ can be stored within two types of geological formations; deep saline aquifers and depleted oil and gas fields. The British Geological Survey has estimated that storage capacity in UK oil and gas field’s amounts to at least 7. 8GT. Storage capacity in UK aquifers is much less certain, and could range from about 24GT to an optimistic maximum of 240GT. All this equates to approximately 100 years of storage capacity (at UK current rate of emissions from power generation) available to the UK to store CO₂.
CO₂ can only be stored in specific geological formations. CO₂ is injected into a layer of porous rock, such as sandstone, where the gas moves between the pores in the rock, and is contained by a layer of non-porous cap rock. Having this layer of cap rock is essential to ensure that CO₂ does not migrate upwards, and leak into the ocean. CO₂ has been safely stored underground in many locations around the world. Norway has safely stored 13mt of CO2 in a saline aquifer above the Sleipner gas field in the North Sea, since 1996 with no signs of leakage. They are continuing to store CO₂ at rate of 1mt tonnes per annum.
The permanent storage of CO₂ is controlled and monitored through measures outlined in the EU Directive on Geological Storage of CO₂ (CCS Directive). Under the CCS Directive, the operators are not allowed to inject CO₂ into a storage site, without proving it is safe to do so.
Watch the British Geological Society's 'What is CCS?' video