The recent shelving of plans for a new coal-fired power plant at Kingsnorth in Kent comes as a blow for the development of new carbon capture and storage technologies (CCS).

The new plant was to be fitted with the latest in CCS technologies that are set to revolutionise coal power. Current EU rules will force Britain to close its most polluting plants from 2015 onwards, with the existing plant at Kingsnorth being one of them. If plans to replace these sources of ‘dirty’ power are not established within the next few years Britain could suffer major power cuts from 2017.

The aim of CCS is to directly remove CO2 from industrial or utility plants and store it in secure reservoirs, a process often termed as carbon sequestration. This allows the continued use of fossil fuels while at the same time reducing CO2 emissions.
Carbon can be sourced from three types of anthropogenic processes: industry, such as in ammonia manufacturing; power production, such as that from fossil fuels; and fuel decarbonisation such as the production of hydrogen fuels from biomass. There are three categories of carbon capture from these sources and these are described in more detail to the right.

Thousands of gigatons of CO2 can be successfully stored for hundreds to thousands of years in geologic and ocean sinks. One major advantage of the storage process is that CO2 injection is already a well established technology. There are three types of geologic storage: unminable coal seams, deep saline formations and depleted oil and gas reservoirs.

Storage in unminable coal seams consists of the CO2 being absorbed by the coal as it diffuses through its porous structure. Using this method there is a potential for 7.2 billion tons of CO2 storage worldwide. Deep saline formation storage is where the CO2 can be injected below 800m into subterranean and subseabed reservoirs to force the gas into a dense, or liquid, phase where it can be safely stored without diffusion back up through rock to the surface. In depleted oil and gas reservoirs the CO2 can be injected into old fuel reservoirs and aid in Enhanced Oil Recovery (EOR) to maximise efficieny of oil fields.

Oceans are the largest potential sink for CO2 storage. Liquid CO2 is injected into the water column at depths of 1000-3000m, or at depths greater than 3000m. At these depths CO2 becomes heavier than seawater and therefore drops to the bottom of the ocean to form a “CO2 lake.”

FLUE GAS SEPARATION
The Principle:
Chemical absorption of CO2 – this can then be used commercially for e.g. carbonating beverages

The Process:
1. CO2 is removed from emissions by bubbling the flue gas through a liquid solvent in an absorber column. CO2 is absorbed in the solvent.
2. The solvent passes through a regenerator unit where a conterflow of steam at 100-120oC strips CO2 from the solvent.
3. The water vapour condenses leaving a stream of highly concentrated CO2. This can be siphoned off and sold for commercial processes.
4. The liquid solvent can be recycled after cooling to 40-65oC.

Disadvantage:
The entire process requires a thermal input from the power plant, thus reducing thermal efficiency of the plant.

OXYFUEL COMBUSTION
The Principle:
Burning fossil fuels in pure or enriched oxygen rather than air. This reduces the presence of excess air gases leaving mostly CO2 and H2O.

The Process:
1. Oxygen is separated from nitrogen in an Air Separation Unit (ASU) to form liquid oxygen, gas nitrogen and gas argon.
2. Flue gas is combusted in this pure or enriched oxygen.
3. The resulting water vapour can be compressed and piped directly to the storage site.

Disadvantage:
The ASU alone can cosume up to 15% of the electrical input of a power plant.

PRECOMBUSTION CAPTURE
The Principle:
The efficiency of CO2 separation is greatly increased when captured at high pressure before combustion and subsequent dilution in air.

The Process:
1. Coal is gasified to produce a gas composed of carbon monoxide, CO, and hydrogen, H2.
2. CO is reacted with water to produce CO2 and H2.
3. CO2 can be captured while the H2 can be sent to turbines to produce electricity.

Disadvantage:
Electricity generation is cheaper by conventional coal power than coal gasification providing little incentive to further the investment in this technology.

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