Page last updated at 22:09 GMT, Wednesday, 3 September 2008 23:09 UK
'Clean' coal plants get go-ahead



The pilot carbon capture and storage (CCS) power plant at the first coal-fired plant in the world, Schwarze Pumpe in northern Germany, uses "oxyfuel" technology.

Oxyfuel combustion involves burning the coal in nearly pure oxygen rather than the air/coal mix currently used in conventional power plants.

Steam turbineCO2 compressorSulphur removalParticle removalFuel injectionAir separationCooler and condenserBoiler

AIR SEPARATION

In this first stage, the Air Separator removes nitrogen, which makes up 78% of air. Once the nitrogen has been removed, the remaining product is an almost pure stream of oxygen.

However, there is a downside; the air separation process demands a lot of energy and reduces the overall efficiency of the power plant.

FUEL INJECTION

At this point, a mixture of coal and oxygen 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).

BOILER

The combustion of the coal and oxygen generates the heat that generates the steam to power the generator.

Because the coal/oxygen mix burns at a higher temperature than a coal/air mix, it is necessary to recycle some of the flue gas, primarily consisting of CO2 and water vapour, back into the boiler to reduce the overall temperature.

STEAM TURBINE

The steam generated as a result of heating water passed through the boiler in pipes is then used to power steam turbines that generate electricity.

However, the pilot plant at Schwarze Pumpe will not use the steam to power electricity generators. Instead, the steam will be piped to a nearby industrial plant.

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.

SULPHUR REMOVAL

This stage, which usually involves a process called flue gas desulphurisation (FGD), removes sulphur dioxide (SO2), which causes acid rain if it is released into the atmosphere.

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.

COOLER AND CONDENSER

At this final filtering stage, the flue gas is cooled to condense the water vapour.

Because nitrogen was removed during the air separation process, nitrogen oxides were prevented from forming during the combustion process.

As a result, the remaining gas is an almost pure stream of CO2 (formed when oxygen and carbon atoms bonded together during combustion).

CO2 COMPRESSION

The remaining flue gas should be at least 95% CO2, which allows it to 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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SEE ALSO
Germany leads 'clean coal' pilot
03 Sep 08 |  Science & Environment
Access all areas: Schwarze Pumpe
03 Sep 08 |  Science & Environment
Pre-combustion CCS technology
03 Sep 08 |  Science & Environment
Post-combustion CCS technology
03 Sep 08 |  Science & Environment
Geological options for storing CO2
03 Sep 08 |  Science & Environment

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