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Projektinfo 01/2014

CO2 capturing using limeExisting power plants can reduce emissions by about 90 per cent using carbonate looping – and at acceptable costs

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 in-dustry 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 genera-tion 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.

This research project is funded by the

Federal Ministry for Economic Affairs and Energy (BMWi)

Two fluidised bed reactors limit energy lossesWith 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 cir-culating 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 (Fig. 4). 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 re-quired 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 effi-ciency 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 op-erating temperature in the calciner ranges between 900 and 950 °C.

Quicklime is suitable for desulphurisationWith carbonate looping, the lime is generally re-circulated (Fig. 1). However, quicklime has to be continually removed and replaced with new calcium carbonate (make-up). This is because the quicklime becomes less able to bind CO2 as the number of carbonisation and calcina-tion cycles increases.The reaction speed in the carbonator is determined by the interactions between the lime particles and the CO2. By means of a rapid, purely chemical reaction, the CO2is initially bound to the particle surface by forming a cal-cium carbonate (CaCO3) layer. The reaction then contin-ues more slowly, since the CO2 has to first diffuse under this layer in order to come into contact with the lime. The pores of the lime particles gradually sinter. After many cycles, the reactivity reduces to about 15 to 20 per cent (Fig. 2). In order to enable good CO2 capture rates, the process is continually fed with fresh limestone (CaCO3), which is cheaply available as a natural raw ma-terial.

In the calciner, the limestone is calcined to form quicklime (CaO). The need for fresh lime is increased by the reaction of the material with sulphur dioxide (SO2). This produces gypsum (CaSO4). This reaction is irreversible with the prevailing temperatures.SO2 and CO2 do not react with the lime independently from each other but mutually influence each other’s ability to be bound in: sulphur dioxide reduc-es the binding of carbon dioxide whereas the alternating calcination and carbonisation increases the absorption of SO2. This means that the quick-lime can also be used for desulphurisation after the carbonate looping. The flue gas desulphurisation (FGD) plants installed in power plants are nor-mally operated with lime.In principle, it is also possible to regenerate the quicklime that is deacti-vated after many cycles in order to re-add it to the carbonate looping. For this purpose the quicklime must be treated with water vapour. The forma-tion of calcium hydroxide increases the pore surface so that the reactivity is

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Fig. 1 The carbonate looping process can be conducted in two coupled fluidised bed reactors.

Fig. 2 The pores increase after many cycles, which reduces the absorption of CO2 and increases the absorption of SO2.

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increased again. After capturing in the calciner, a pure CO2 gas flow is pro-duced. As with all known capturing processes, this must be compressed for the transport. Besides the use of pure oxygen for the combustion in the calciner, this is the second aspect that significantly reduces the efficiency of the overall power plant process. When compressing to a pressure of 100 bars at a temperature of 30 °C, a loss of efficiency of 2.5 to 4 per cent can be expected in accordance with the process. Installing carbonate looping in a power plant therefore lowers the overall efficiency by about 5 to 7 per cent. This is a good value when compared with other capturing processes.Engineers have been researching the process in recent years with a 1-mega-watt pilot plant at TU Darmstadt, whereby they have achieved CO2 binding rates of more than 80 per cent in the carbonator. Because the CO2 released during the combustion with coal in the calciner is completely captured, this means that more than 90 per cent is captured overall.

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Aspen plus simulation model has been further developedIn addition to the pilot plant, the project consortium has also further developed a process model using the Aspen Plus software. The programmers have supplemented the missing components in the circulating fluidised bed using a fluidised bed reactor code. They have adapted the program to the design of the built plant and validated it with the experimental results, whereby the calculated CO2 absorption rates closely match the experimental re-sults. It is intended to validate and optimise the process model more comprehensively during the next project phase. It is also planned to develop 3D models for the fluidised bed reactor. In order to validate these models, it is also planned to incorporate profile measurements in the reactors.

Testing the carbonate looping in flexible operationBased on the previous findings, the 1-megawatt system at TU Darmstadt has been undergoing optimisation since the beginning of 2014.During the next few years, the engineers will be con-ducting experimental series to test the behaviour with many load changes. This is in order to gain knowledge to optimally design the technology for flexible power plant operation. This is because in future coal-fired power plants will be carrying out increasingly frequent and quicker load changes in order to balance out the fluctu-ating injection of renewable energies. This also varies the flue gas flow and the requirements for the two fluid-ised bed reactors in carbonate looping systems. Furthermore, not just black coal but also lignite is being deployed as a fuel. The next step will then be the con-struction and operation of a pilot plant with a 20-mega-watt thermal capacity.

Fig. 4 The CO2 and lime are separated again in the calciner.

Fig. 1 The carbonate looping process can be conducted in two coupled fluidised bed reactors.

Fig. 2 The pores increase after many cycles, which reduces the absorption of CO2 and increases the absorption of SO2.

Fig. 3 Simplified schematic showing the structure of the 1-megawatt experimental plant at TU Darmstadt.

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BINE Projektinfo 01/2010BINE-Projektinfo 01/2014

The market for lime The idea of utilising lime in power plants and then selling it on to the cement industry dates back to the 1980s. At that time, German power plant operators began using lime to filter sulphur dioxide (SO2) from the flue gases. Amongst other things, sulphur dioxide is deemed to be responsible for acid rain, which damages for example the forests. The comprehensive use of flue gas desulphurisation (FGD) plants has reduced the SO2 emissions from power plants by about 95 per cent.A limestone-based process has become widely used in power plants for capturing SO2 from flue gases. In FGD plants, sulphur dioxide reacts with lime and oxygen to form FGD gypsum. This calcium sulphite is chemically identical to natural gypsum. In particular, it is further processed by the cement and gypsum industry for producing gypsum panels. According to the German gypsum federation, the Bundesverband der Gipsindustrie, FGD gypsum meets around half of Germany’s gypsum requirements.

With carbonate looping, quicklime is produced as a waste product. This is required in cement plants. Until now the factories have produced it themselves. This process stage is one reason for the high energy requirement and the high CO2 emissions in the cement industry. Quicklime is produced by calcining the CO2 contained in naturally occurring limestone using considerable energy. This means that there will presumably be a market for the quicklime once carbonate looping has become commercially implemented. This would lower the emissions and energy consumption at cement plants.

The sale of quicklime enables power plant operators to reduce the specific costs for capturing CO2 from flue gases. That would reduce the previously forecast costs of 15 euros per tonne of captured CO2. That is an important argument in terms of carbonate looping’s marketing prospects. Until now the price per tonne of CO2, which within the European Union is determined by a carbon permit market, has been less than 5 euros. Since fewer carbon permits will be issued in future, the price is expected to rise.

Project participants >> Project coordination: Department of Energy Systems and Technology (EST) at the Technische Universität

Darmstadt, Prof. Dr.-Ing. Bernd Epple, bernd.epple@est.tu-darmstadt.de, www.est.tu-darmstadt.de>> Support with the plant design and operation: Fisia Babcock Environment GmbH, Gummersbach,

Germany>> Support with the plant design and operation: Alstom Carbon Capture GmbH, Wiesbaden, Germany>> Provision and analysis of the limestone: Rheinkalk GmbH, Wülfrath, Germany>> Provision of the technical gases: Linde AG, Pullach, Germany>> Provision of coal: Grosskraftwerk Mannheim AG, Germany>> Advice with the incorporation in coal-fired power plants: E.ON New Build & Technology GmbH,

Gelsenkirchen, Germany>> Advice with the incorporation in lignite-fired power plants, provision of lignite:

RWE Power AG, Essen, Germany

Links and literature >> www.kraftwerkforschung.info/en/co2-scrubbing-post-combustion-capture/ >> Ströhle J. u. a.: Carbonate looping experiments in a 1 MW

th pilot plant and model validation.

In: Fuel. (2014) In Press, Corrected Proof, http://dx.doi.org/10.1016/j.fuel.2013.12.043 >> Ströhle, J. u. a.: CO2-Abscheidung aus Kraftwerksabgasen mittels Kalkstein. Teilprojekt: Untersuchungen

im Technikumsmaßstab. Schlussbericht. FKZ 0327771C. TU Darmstadt. EST (Hrsg.). Juli 2013, 104 S. >> Rodríguez, N. u. a.: Comparison of experimental results from three dual fluidized bed

test facilities capturing CO2 with CaO. In: Energy Procedia. Vol. 4 (2011), p. 393 – 401, http://dx.doi.org/10.1016/j.egypro.2011.01.067

More from BINE Information Service>> This Projektinfo brochure is available as an online document at www.bine.info under

Publications/Projektinfos. You can find further literature there as well.

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ImprintProject organisation Federal Ministry for Economic Affairs and Energy (BMWi)11019 Berlin Germany

Project Management Jülich Forschungszentrum Jülich GmbHDr Wolfgang Körner52425 Jülich Germany

Project number 03ET7018

ISSN0937 - 8367

PublisherFIZ Karlsruhe · Leibniz Institute for Information Infrastructure Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany

AuthorChristina Geimer

Copyright Cover image, Fig. 1, 3: EST, TU DarmstadtFig. 2: Grasa, G. S. et al.: Reactivity of highly cycled particles of CaO in a car-bonation/calcination loop. In: Chemical Engineering Journal. Vol. 137 (2008), Issue 3, p. 561–567, http://dx.doi.org/10.1016/j.cej.2007.05.017Fig. 4 BINE Information Service

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