Carbon capture and storage
This article explains the process of Carbon Capture and Storage (CCS).
Carbon Capture and Storage (CCS) is the term for a series of linked processes which have the overall objective of decreasing carbon emissions (CO2) into the atmosphere and thereby helping to reduce global warming. CCS ‘captures’ the emissions of waste gases (comprising mostly CO2) from major industrial sites such as steelworks and oil refineries. The waste emissions are largely made by the combustion of fossil fuels (namely oil, natural gas, and coal) or industrial processes such as fermentation (as in distilleries), the production of cement or of ammonia. These linked processes are:
- Capturing/separating the CO2 from waste industrial gases at its emission source.
- Rendering the CO2 into either a liquid or a powder form to enable it to be transported to a secure permanent storage location.
- Finally, permanently storing the CO2 deep underground in suitable carbon ‘reservoirs’ so that it cannot leak out and return to the atmosphere.
Carbon Capture and Usage - a different approach
CCS captures and then stores the CO2 where it does not contribute further to climate change. An alternative approach, called Carbon Capture Usage (CCU) or Carbon Capture Usage and Storage (CCUS) is to treat the CO2 as a by-product and use it as a commercial ingredient in the production of items such as plastics, chemicals and synthetic fuels or in the production of low carbon energy.
The implementation of either CCS or CCUS schemes requires hugely expensive infrastructure costs.
There are no operational CCUS sites in the UK at the moment. However, the UK Government has approved funding for the development of two sites in the north of England with a goal of completion by 2030.
Capturing CO2
Carbon capture (CC) is common to both CCS and CCUS and involves separating CO2 from industrial processes or energy generation. CC technology is claimed to capture up to 90% of the C02 released through the consumption of fossil fuels.
There are three main types of CC:
- Post-combustion capture: This is where the CO2 is separated from a mixture of gases at the end of the industrial or energy production process.
- Pre-combustion capture: This is where the CO2 is separated from the fossil fuel being used before the final stage of combustion.
- Oxyfuel technology: This is where pure oxygen is used in place of air to burn fossil fuel. Due to this, only CO2 and water vapour is given off in combustion. The CO2 is then separated from the water by cooling and condensation.
Uses made of CO2 following capture
CCS does not make any economic use of CO2; once captured it is rendered into a form suitable for transport to a permanent storage site. CCUS, by comparison, transforms the CO2 into a useful product of which there are two categories:
- Non-conversion (or direct use): where it is used directly in a commercial process such as being used in greenhouses to reduce the time it takes to produce crops.
- Conversion (or non-direct use): where it is converted into a product which can be used commercially such as procuring fuel.
The conversion of CO2 into a useful product often requires significant amounts of energy which, in itself, can result in the production of more greenhouse gas emissions
How is CO2 transported and stored?
After the CO2 has been captured, it is compressed into a liquid form and transported by pipeline, ship or road tanker to a location for storage or usage.
There are three ways of storing CO2 but only the first one below (geological storage) is used in the UK at present.
- Geological storage: CO2 is stored deep under the seabed or on land. It is injected directly into sedimentary rock formations found in old oil fields, gas fields or saline formations
- Ocean storage: CO2 is injected into the sea and then either left to disperse or be trapped in a specific location. This method is believed to be detrimental to the environment and is still being researched.
- Mineral storage: here, the CO2 is held in storage sites to bind chemically to the surrounding rocks which contain naturally occurring magnesium and calcium.
Find out more about the wider subject of Carbon Capture, Storage and Usage.