'Clean' coal plants get go-ahead

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

The leading method currently being developed is a system called Integrated Gasification Combined Cycle (IGCC), which involves generating electricity from gas turbines as well as steam-powered ones:

AIR SEPARATION

Similar to the oxyfuel process, the Air Separator removes nitrogen from the air that is pumped into the unit.

Once the nitrogen has been removed, the remaining product is an almost pure stream of oxygen.

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GASIFIER

Unlike the oxyfuel system, the coal is not combusted at this stage.

Instead, it is blasted into a gasifier and reacts with oxygen and steam to form a "syngas" that primarily consists of carbon monoxide (CO) and water (H2O).

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PARTICLE REMOVAL

The syngas is then passes through the first filtering process, where small particles called "fly ash" are removed.

Electrostatic precipitators can remove more than 99% of particulates from the gas.

The system works by creating an electrical field to create a charge on particles which are then attracted by collection plates.

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SHIFT REACTOR

The Shift Reactor injects steam, causing a chemical reaction that converts the CO into hydrogen (H2) and carbon dioxide (CO2).

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SULPHUR REMOVAL

Next, sulphur is removed from the syngas. This process can occur either before or after the Shift Reactor.

This stage usually involves a mixture of limestone and water, which reacts with the sulphur dioxide (SO2) in the syngas to form gypsum (a calcium sulphate), a material that can be used in the construction industry.

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CO2 ABSORBER

At this stage, the syngas stream is typically passed though a liquid sorbent (the CO2 absorber), which captures the CO2 from the gas stream.

Once the CO2 is captured, the sorbent is moved to the desorber to be "regenerated".

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CO2 DESORBER

"Regeneration" in the CO2 Desorber generally involves heating the sorbent, which releases the CO2 ready for compressing and transporting.

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

One advantage that IGCC has over post-combustion CCS technologies is that the gas stream has a much greater concentration of CO2.

As a result, IGCC uses less energy to remove the CO2, improving the plant's overall efficiency.

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GAS TURBINES

Once the CO2 has been removed, the syngas consists primarily of hydrogen.

This is then used to power gas turbines to generate electricity.

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HEAT RECOVERY

The excess heat from the combustion of the syngas is captured to generate steam.

This in turn is used to power a steam turbine, which also generates electricity.

This additional use of the excess heat improves the overall efficiency of the IGCC power station.

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WATER VAPOUR AND EXCESS AIR

At the end of the process, the residual water vapour and air is released into the atmosphere.

Although most of the process equipment required for pre-combustion technology is readily available, further development of the gas turbine and gasifier is needed to make it commercially viable.

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