Annex B Background to CCS
Schematic diagram showing Elements of CCS infrastructure and relationship with geological structure:
Source: British Geological Survey
What is CCS?
CCS is a 3 step process which involves capturing the CO 2 from power plants and other industrial and energy-related sources, transporting it to storage points then storing it safely in offshore depleted oil and gas fields, deep saline aquifers as well as possible sites onshore.
1. Capture
CO 2 capture is the process of removing CO 2 (carbon dioxide) produced by hydrocarbon combustion (coal, oil and gas) before it enters the atmosphere. The process will be most cost effective when it is used on large point sources of CO 2 such as power stations and industrial plants. These currently make up more than half of all man-made CO 2 emissions.
There are currently three main methods of capturing CO 2:
- Post-combustion capture - removing the dilute CO 2 from flue gases after hydrocarbon combustion
- Pre-combustion capture - removal of CO 2, prior to combustion, to produce hydrogen
- Oxy-fuel combustion capture - burning fossil fuels in pure oxygen as opposed to air resulting in an exhaust gas of mainly CO2 and water vapour
2. Transport
CO 2 capture is likely to be most economic at large point sources of CO 2 such as power stations and large industrial plants. In most cases these will not be close to a suitable underground geological store and therefore the CO 2 will have to be transported.
Transport is currently the least complicated element in the CO 2 capture and storage chain as the technology is already in existence and costs can be realistically estimated.
The main complication with CO 2 transport is that CO 2 behaves differently under varying pressures and temperatures and therefore transport of CO 2 must be carefully controlled to prevent solidification and blockages occurring.
There are currently two methods used to transport large volumes of CO 2 by industry:
- Pipeline Transport
- Ship Transport
3. Storage
CO 2 storage is the process of taking captured CO 2 and then placing in a store where it will not be in contact with the atmosphere for thousands of years. Storage of the CO 2 underground in deeply buried porous rocks beneath a layer of impermeable rock (cap rock), which acts as a seal to prevent the CO 2 from leaking out, is the most obvious option at present.
There are three main types of proposed underground storage sites:
- Depleted Oil and Gas Reservoirs
- Deep Saline Aquifers
- Deep Unminable Coal Seams
You can read more about CCS at The Scottish Centre for Carbon Storage http://www.geos.ed.ac.uk/sccs