'Clean' coal plants get go-ahead

As well as oxyfuel and pre-combustion systems, post-combustion carbon capture and storage technologies are also being developed.

The technology is favoured by a number of nations, including the UK, because it can be "retro-fitted" to existing coal-fired power stations.

FUEL INJECTION

A mixture of coal and air is blasted into the boiler and ignited. Many power stations "wash" and pulverise the coal before it is fed into the boiler.

"Washing" actually refers to a process that involves passing coal through a series of liquids with varying densities.

This removes many of the impurities found in coal (the impurities sink in the liquid, allowing them to be removed easily).

Pulverising the coal into a fine dust means more of the fossil fuel's surface area is exposed during combustion, resulting in a more efficient burning of the coal.

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BOILER

The heat generated from the combustion of the coal/air mixture creates steam, which is then pumped to the turbine.

The flue gas, a by-product of burning coal, is removed from the boiler in order to undergo a series of filtering processes.

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STEAM TURBINE

Steam powers the turbines to generate electricity, which is transmitted into the distribution grid.

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STEAM CONDENSER

Once the steam has passed through the turbine, it arrives at a condenser.

This unit uses cool water to condense the steam back into water, allowing it to be piped back into the boiler and be re-heated again.

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PARTICLE REMOVAL

This is the first of several "cleaning" processes that the flue gas will pass through.

At this point, small particles called "fly ash" are removed from the gas.

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SULPHUR REMOVAL

This stage, which usually involves a process called flue gas desulphurisation (FGD), removes sulphur dioxide (SO2).

A mixture of limestone and water is sprayed over the flue gas, which reacts with the SO2 to form gypsum (a calcium sulphate), a material that can be used in the construction industry.

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COOLER

Before the flue gas enters the CO2 absorber, it needs to be cooled. This stage, using water, lowers the temperature of the gas.

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CO2 ABSORBER

At this stage, the gas stream is typically passed though a liquid sorbent (the CO2 absorber), which reacts with the CO2, chemically binds with it and removes it from the flue gas.

Once the CO2 is captured, the sorbent is moved to a desorber to be "regenerated", which generally involves heating the sorbent to release the captured CO2.

The sorbent, now CO2-free, can be returned to the absorber to begin the process again.

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CO2 COMPRESSOR

The almost pure stream of CO2 can now be compressed into a liquid state.

Pressurising CO2 to 70 atmospheres will transform it into a liquid with a similar density as crude oil, making it easier and more efficient to transport it to a long-term storage site.

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