How Carbon Capture and Storage works
Carbon Capture and Storage (CCS) involves capturing the carbon dioxide (CO2) that would normally be emitted from burning fossil fuels, for instance in a power plant, thus preventing the greenhouse gas entering the atmosphere, and storing it deep underground. In combination with low-carbon and clean fuel technology, this would allow power stations to still burn fossil fuels while the world develops and expands its use of non-fossil energy sources. Greater use of CCS could be achieved without the need for rapid change in the energy supply infrastructure. Thus CCS could help ease the transition to a future carbon-free energy system.
There are two main options for Carbon Capture and Storage: in oceans and in geological formations. Ocean storage is losing favour due to ecological and legal issues. The most viable and environmentally acceptable storage is geological storage (also called sequestration). CCS in geological formations involves capturing CO2 and then injecting it into rock layers. There are three main storage options, these are shown in Figure 1 and listed here::
- unmineable coal seams, which can also displace trapped methane which can then be used as fuel
- deep saline aquifers (porous rock layers containing salty water deep underground)
- depleted or near-depleted oil and gas fields; helping to increase pressure and making extraction easier and so assisting recovery of oil or gas from near-depleted fields to give enhanced output of reserves (enhanced oil/gas extraction)
In enhanced oil extraction/recovery, the injected CO2 is miscible with the oil and has the dual affect of making the oil more liquid and expanding it. This recovers oil which would otherwise remained untapped by forcing it out and making it able to flow through smaller pores. The CO2 can then be recovered from the oil at the surface for reinjection.
Figure 1 - Carbon storage options
1. CO2 pumped into disused coal fields
2. CO2 pumped into and stored safely in saline aquifers
3. CO2 pumped into oil or gas fields
A number of projects around the world are currently ongoing proving the technically feasiblity of the process but large-scale commercialisation is not yet economically viable due to the extent and cost of equipment needed.
