The principle underlying this method is not new. It is already being used today, for example, to remove CO2 from natural gas to obtain greater gas purity. However, the other uses are subject to completely different marginal conditions than in the case of CCS, in both technical and economic terms.
Carbon capture has in fact been used hitherto-within a process chain-to make either a marketable product or a product that brings some economic benefit. This means that carbon capture is currently only being used where the costs of capture are covered by the proceeds of sales. Carbon capture in a CCS setting, by contrast, is subject to different economic marginal conditions: the product, electric power, is not made possible in the first place by this process step and is not changed. The fact is that, from a cost angle, carbon capture is initially an additional outlay. This being so, the requirement that costs be kept low is much more stringent than for previous applications. On the other hand, cost savings can be obtained wherever CO2 certificates are traded. A plant with CCS needs fewer CO2 certificates thanks to the avoided CO2 emissions. Ultimately, the level of CO2 certificate costs decides whether CCS brings economic advantages. In principle, however, it must be noted that, unlike previous applications, considerable pressure exists to minimize the costs of carbon capture.
In technological terms, CO2 scrubbing, too, faces completely new challenges. While in the past, CO2 scrubbing was deployed wherever no technical problems worth mentioning existed, much more rigorous requirements must be met when it is used in capturing carbon from the waste gas of coal- fired power plants. This concerns, first, the necessary properties of the scrubbing solution and, second, the plant technology.
In the flue gas, the CO2 has low partial pressure due to the atmospheric pressure of the flue gas itself, and due to the CO2 volume percentage of below 15%. In this situation, physical absorption agents are unsuitable, and chemical absorption agents must be used. The scrubbing solution usual in today' s processes under these conditions is a aqueous solution of monoethanolamine (MEA). The chemical equation for the absorption/desorption process is (Scheme 11.1):
Scheme 11.1 Absorption/desorption of CO2 using MEA.
In this form, however, MEA is unsuitable for the capture of carbon from the waste gas of coal- based power plants. The waste gas still has traces of various impurities even after flue gas scrubbing, and these have deleterious effects on the MEA solution. Particular mention must be made here of oxygen and SO-. These
Scheme 11.1 Absorption/desorption of CO2 using MEA.
components lead to rapid degradation of the MEA solution, so that much -too-frequent replacement of the solution would be necessary. What is more, energy consumption to recover the CO- in the desorber from the scrubbing solution is too high. This energy consumption is the crucial parameter for the overall efficiency of CO - scrubbing. In the case of MEA scrubbing, so much heat must be added here that the result is an excessive fall in the overall efficiency of power stations.
If CO2 scrubbing is to be used as part of CCS, therefore, scrubbing solutions must be developed that are more resistant to impurities in the flue gas and also have lower heat requirements for desorption. These new scrubbing solutions then have to prove their fitness under operating conditions. For this purpose, pilot plants are being erected near power stations that are fed with a small diverted flue-gas stream. In RWE's Niederaussem power station, for example, a pilot CO2 scrubbing plant was commissioned in August 2009 (see Figure 11.4). The pilot plants allow us to investigate the entire process of CO2 scrubbing, including the performance of the scrubbing solution. They have the same building height as a later commercial- scale plant in order to provide the necessary distance for the
complete absorption and desorption process, but are limited in their diameter and, hence, in their capacity to the minimum. The small amounts of captured CO2 are usually added to the power station's waste-gas stream.
The pilot plants have one final development step still open before commercial operations, viz. the inclusion of carbon capture in the overall CCS process chain. CO2 scrubbing must slot into the operating requirements of the power-plant process. On the one hand, this concerns the large mass flows of the waste - gas stream and the CO - - which differ by a factor of about 10 from previous applications. On the other, the various operating modes in a power station must be tackled, including behavior in the case of failure, for example. The transport infrastructure and storage downstream of the CO2 scrubbing system, too, are new marginal conditions, not only for carbon capture, but for the entire power station. To implement the necessary developments here, demonstration plants will be built in future, also to acquire the necessary knowledge and experience and ultimately to prove commercial deployability.
First of all, however, one major task is to develop and trial suitable scrubbing solutions in depth. In addition to resistance to impurities in the flue gas and to lowering energy consumption in desorbing the CO2 , further criteria must be taken into account. For instance, the scrubbing solution should have low steam pressure in order to reduce the loss of scrubbing agent in desorption. Reactivity with CO2 must be high, so that the absorption process takes place at high speed. Also, of course, the scrubbing solution should be as harmless as possible and low-cost.
With these aims, three different substance classes are being considered from which suitable scrubbing solutions will emerge:
• Amines: The previous standard scrubbing solution MEA, too, falls under this heading. From the numerous chemical compounds in this substance group, suitable scrubbing solutions are established using screening methods and tests. Modifications and additional substances that act as activators, oxidation stabilizers or corrosion stabilizers contribute to creating the desired properties.
• Ammonia: Example of the chemical equation for the absorption/desorption (Scheme 11.2):
Scheme 11.2 Absorption/desorption of CO2 using ammonia.
• Salts of amino acid: Example of the chemical equation for the absorption/ desorption process in this substance class (Scheme 11.3):
Scheme 11.3 Absorption/desorption of CO2 using amino acid salts.
For scrubbing solutions that can be commercially used in future CCS, all three substance classes offer interesting approaches and different positive properties, but also have specific drawbacks. Only long-term tests in pilot and demonstration plants will reliably show which scrubbing solutions and, hence, which process configurations will yield optimal results.
Compared with other processes for carbon capture, post-combustion CO2 scrubbing has the interesting feature that it is downstream of the actual power-plant or production process. This means, first, that the main process is not- or hardly--mpacted and, second, that existing plants can be retrofitted with CO- -scrubbing systems. The chief point to be considered here is the fairly large space requirement for a CO2 scrubber. A hard - coal - fired power plant with an electric output of 800 MW, for instance, needs additional space of some 16000 m2. Such a CO2 scrubber separates some 550th-1 ofCO2 from a volumetric flue-gas flow of about 600m3s-1 (standard temperature and pressure). Besides the space requirement, there are the steam needs for desorption, cooling-water needs and, possibly, further treatment of the flue gas (desulfurization) to be considered. Modern plants being erected now can be prepared for later retrofitting with CO2 scrubbers. Ultimately, the specific measures to be taken here have to be assessed for each individual project. What is important is that retrofitting is not rendered impossible because no provision was made for this in the design and erection of the plant. Plants that are designed for retrofitting are referred to as 'capture-ready'.
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