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The flue gas is cleaned of CO2 in the Carbonator. There the CO2 reacts with lime to form a solid.
© EST, TU Darmstadt
Existing power plants reduce emissions
Projektinfo 01/2014

The CO2 and lime are separated again in the calciner.
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CO2 capturing using lime

CO2 can be separated from the flue gases from power plants with the aim of preventing climate-harming emissions. However, the processes developed for this have been very expansive until now. When capturing CO2 using quicklime, on the other hand, the costs are much lower – around 15 euros per tonne of CO2. These plants for capturing CO2 from exhaust gases can be retrofitted in existing power plants. The Technische Universität Darmstadt has demonstrated the process using a one-megawatt pilot plant. The solid material produced – quicklime – could be interesting for cement plants, since this would reduce their energy consumption.

In order to reduce the emissions from coal-fired power plants, researchers and industry are developing processes to capture CO2 from flue gases. The disadvantage of the processes, however, is that they considerably reduce the efficiency of the power plants by 8 to 14 per cent. The loss of efficiency with carbonate looping, on the other hand, is only about 5 per cent. According to one forecast, this creates savings in relation to the electricity generation costs: with flue gas scrubbing using monoethanolamine (MEA scrubbing), the electricity generation costs are about 55 euros per megawatt-hour. A good 40 euros are forecast for carbonate looping. Although equally small electricity generation costs amounting to roughly 42 euros are expected when combusting coal in a pure oxygen atmosphere (oxyfuel), this technology cannot be retrofitted in existing power plants but can only be installed in new ones. These figures relate to coal-fired power plants with a 1,050-megawatt electrical capacity, an efficiency of 45 per cent and a CO2 capture rate of 80 per cent.

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Two fluidised bed reactors limit energy losses

With carbonate looping, carbon dioxide (CO2) from the flue gases of fossil-fuelled power plants can be made available for geological storage or further use, whereby the CO2 is first bound using limestone and then released.

As is usual, the flue gas from the power plant is first of all desulphurised in the flue gas desulphurisation (FGD) plant. It then comes into contact with lime (CaO) in a carbonator (cover image). This part of the plant is a circulating fluidised bed reactor. This means that the flue gas is fed through a porous plate into a vertical cylinder or cuboid, which is filled with lime. The gas flows through the lime, loosens it, and swirls it around. This increases the reaction-capable surface of the lime. The lime reacts with the CO2, releasing a large amount of heat. This energy can be used via heat exchangers for generating steam in power plants. In order to maintain the ideal reaction temperature of 650 °C, the carbonator has to be cooled. The cleaned flue gas flows from the carbonator into a heat exchanger, where it is cooled. It is then freed of dust in a filter before being released to the environment.

Following the carbonisation, the CO2 is bound in the lime to form solid calcium carbonate (CaCO3). This is centrifugally separated from the flue gas in a cyclone separator. The solid material is then fed into a second fluidised bed reactor, which is called a calciner. The CO2 is separated from the calcium carbonate by adding heat. This process is known as calcination and is widely used in cement plants.

In the experimental plant, coal is combusted with pure oxygen (oxyfuel atmosphere) in order to provide the required heat. Combustion in air would unnecessarily strongly dilute the CO2 volume flow being captured as a result of the contained nitrogen. Until now, relatively large amounts of energy have been required to separate pure oxygen from the air. This is where the largest efficiency loss occurs with carbonate looping: in relation to the overall power plant, this amounts to about 3 per cent. The heat required for the calcination can, however, be decoupled and reused for generating steam since it is released at a high temperature level. The optimum operating temperature in the calciner ranges between 900 and 950 °C.

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Addresses

Project coordination
TU Darmstadt, EST

Analysis of the limestone
Rheinkalk GmbH

Technical gases
Linde AG

Provision of coal
GKM AG

Incorporation in coal-fired power plants
E.ON Technologies GmbH

Incorporation in lignite-fired power plants, provision of lignite
RWE Power AG

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BINE-Projektinfo 01/2014
(PDF, 4 pages, 1.2 MB)