CO2 scrubbing (post-combustion capture)
CO2 scrubbing is also referred to as the post-combustion capture process. Most common here is scrubbing of the flue gas using a chemical absorption medium or, as an alternative, dry CO2 adsorption downstream of a conventional fossil-fuelled power plant – after the actual combustion process including the downstream flue gas purification systems (dust filter, flue gas desulphurisation, denitrification).
In the case of wet chemical scrubbing, additional increases in efficiency can be expected mainly as a result of the ongoing development of the solvents used. An important research goal here is the improvement of the stability of these solvents with regard to degradation processes as a result of chemical reactions with components contained in the flue gas.
Further key areas in research and development work on the post-combustion capture process include process integration and optimisation for applications in large-scale power plants. It is expected that this technology will be commercially available by 2020 at the earliest. This means that power plants built before 2020 will not include CO2 capture, but can be prepared for retrofitting ("capture ready"). It is currently assumed that the post-combustion process is the only CO2 capture process that is suitable for retrofitting.
- Development and testing of optimised amine-based scrubbing liquids
- Adaptation of scrubbing columns and their internal components to solvents and to the flue gas components and conditions that are specific to power plants
- Investigation of alternative processes that are still based on amines as solvents (e.g. use of fixed amines, amino salts and membranes for separating the gas and liquid phases)
- Optimisation of the integration of the process into the heat circuit (both for retrofitting and for new plants)
- Demonstration of amine scrubbing on a coal-fired plant – as a step along the way to a commercial plant
Scrubbing with inorganic solvents
- Carrying out a feasibility study to identify potential
- Development of scrubbing processes with alkaline solvents
- Testing of a selected process in a pilot plant
- Carrying out a feasibility study to identify potential
- Construction and operation of a pilot plant to investigate solvents with regard to stability, consumption and subsequent uses
The most promising post-combustion processes from today's perspective are the chemical absorption processes, which are often referred to as chemical scrubbing processes. Amine-based, ammonia-containing or alkali-containing solutions are currently being favoured as possible solvents. These solvents are used to absorb the carbon dioxide contained in the flue gas. The CO2 is then removed from the CO2-containing solvent by means of a regeneration process. The regeneration of the solvent is driven by a temperature or pressure change. The solvent is returned to the circuit and the captured CO2 is treated for transport and subsequent storage.
Absorption in liquid solvents is a widely used CO2-separation technology that has proven itself in industrial applications and can be used to achieve high purities and degrees of separation. The solvents may be either organic or inorganic.
The inorganic solvents are alkali solutions. The amine-based separation process is currently commercially used in various chemical processes (e.g. ammonia production), and monoethanolamines (MEAs) have proven themselves as solvents here. The actual challenge is to apply this process to power plant flue gases. Further possible scrubbing liquids are currently being investigated in connection with various power plant processes and flue gas compositions.
Another option is ammonia scrubbing, which is being tested and optimised in test plants by Alstom under the name "Chilled Ammonia" process. The work of the industrial companies Fluor, Mitsubishi Heavy Industries, Dow Chemical, Cansolv and BASF in the area of solvents should be noted here. The main focus of this work is the improved stability and increased effectiveness of solvents, alongside the reduction of the energy consumption required for desorption. The aim is to reduce the costs, which are currently still very high.
The development of more efficient solvents for use in power plants is becoming increasingly important. The goal is to reduce the energy required for regeneration, to lower the solvent use and to save on costs. Also included here is the development of theories that explain the degradation mechanisms that occur (oxidative and thermal degradation, reaction with sour gases, side reactions with CO2).
A significant disadvantage of amine-based scrubbing is the high amount of energy required for desorption of CO2-containing solvent, which leads to huge losses in efficiency. As shown by research work currently in progress, the use of so-called sterically hindered or tertiary amines (aMDEAs, MDEAs, KS-1) with activators to speed up reactions appears promising, as they require less energy for desorption and for pumps and compressors. In addition, the risk of corrosion can also be reduced. However, these substances have disadvantages for the absorption process itself. A lot of experience is available for the use of aMDEA solvents, as they are mainly used for the removal of CO2 from natural gas. BASF's reference list alone includes 230 plants where the aMDEA solvent that is produced is in use.
The use of carbonates is another attractive option, as they boast high thermal stability, are stable with regard to oxygen and have relatively low heats of absorption and desorption. A disadvantage is the relatively low rate of reaction, although this can be increased by using additives.
The removal of CO2 from flue gases consists of an absorption step combined with a regeneration step. In the absorption step, the CO2 is removed from the flue gas. In the regeneration step, the CO2 is removed from the solvent or other CO2 carrier medium. A highly concentrated stream of CO2 is produced here, which can then be used in CO2 liquefaction. Absorption can be implemented using fluid solvents. These solvents can be divided into organic solvents, generally amine, and inorganic solvents such as alkali and alkali-earth solutions and suspensions.
Also possible is dry sorption using alkali-earth materials such as calcium oxide.
CO2 capture using amine scrubbing (organic solvents) is well-established for certain industrial applications. There are already plants around the world with CO2 capture using amine scrubbing. However, these are smaller plants that generate CO2 as a product for the food industry or to support oil or gas production (enhanced oil/gas recovery, EOR/EGR). Larger and higher-performance CO2 capture units are necessary for power plants, and in particular coal-fired power plants, with their specific gas compositions and conditions and their very large volumetric flows. There is still need for a considerable amount of research and development work here. In addition, regeneration of the solvent is very energy-intensive and leads to significant losses of efficiency.
According to current knowledge, these losses can be as much as one third.
In the case of CO2 capture with amines, the solvent is sprayed into the flue gas stream. The CO2 is bound in a chemical reaction. After absorption, the CO2-containing amine is thermally regenerated at around 100 to 130 °C. Low-pressure steam is drawn from the power plant's water-steam-circuit for this purpose. After regeneration in the desorber, the solvent is used again for absorption.
As an alternative to amines, inorganic scrubbing solutions or suspensions can also be used as solvents. However, wet flue gas scrubbing, where the flue gas comes into contact with a liquid in the absorber, remains the basic process principle. This type of scrubber solution has not yet been used in CO2 capture from flue gases. They are being used at the moment for gas treatment in the chemical industry. They may also have advantages in CO2 capture from flue gases – as a result of the stability of the solvents, and also because the formation of undesired by-products is low.
Another option is dry CO2 absorption, which is similar to dry gas purification aimed at minimising pollutants. Alkali-earth metals are well-suited here, with calcium generally preferred. If the appropriate reaction conditions are present, CO2 capture from the flue gas takes place and is accompanied by carbonate formation. The flue gas is contacted with solids in the case of dry gas purification. The gas and solids form a gas-solid suspension. Refer also to carbonate looping.
7 current research projects
Development of a scrubbing process to remove CO2 from flue gas from power plants fired by coal and natural gas Optimisation of selected properties of sorbents commercially available for this purpose (post-combustion capture)
Organisation carrying out research: Siemens AG - Power Generation - Dep. EIB12
Project number: 0327778
Ergänzungsvorhaben: Entwicklung eines Verfahrens zur Regeneration und Vermeidung von Desaktivierung aminosäuresalzbasierter Waschmittel zum Einsatz in CO2 -Wäschen für Kohle -und Ergaskraftwerke
Project number: 0327778A
Construction and operation of a pilot CO2 scrubber plant to test new, optimised solvents under real operating conditions in a lignite-fired power plant
Organisation carrying out research: RWE Power AG - Research and developement - New technologies
Project number: 0327793A
Folgeprojekt: Betrieb der CO2-Wäsche-Pilotanlage zur weiteren Erprobung eines neuen optimierten Lösungsmittels unter realen Betriebsbedingungen an einem braunkohle-gefeuerten Kraftwerk
Project number: 0327793D
Folgeprojekt: Versuchsbegleitung & Nachrüstungen der CO2-Wäsche-Pilotanlage
Project number: 0327793E
Optimisation of gas scrubbing process using simulation and tests, including boost to project package for investigation of materials
Organisation carrying out research: BASF SE - Abt. CZA/ML, Bau: E 100
Project number: 0327793C
Folgeprojekt: Betrieb einer CO2-Wäsche-Pilotanlage - Langzeittests neuer, optimierter Lösungsmittel unter realen Betriebsbedingungen an einem braunkohle-gefeuerten Kraftwerk: Simulation, Versuchsauswertung, Analytik, Reclaiming
Project number: 0327793F
Evaluation of integration, dynamics and optimisation of downstream gas scrubbing
Organisation carrying out research: Technische Universität Hamburg-Harburg - Institute of Energy Systems (IET)
Project number: 0327785
Chemical absorption process for CO2 capture from flue gases
Organisation carrying out research: Universität Duisburg-Essen - Chair of Environmental Process Engineering and Plant Design
Project number: 0327786A
Process engineering investigation and development of amine scrubbing processes for CO2 capture from power plant flue gases
Organisation carrying out research: Universität Stuttgart - Faculty 4: Energy Technology, Process Engineering and Biological Engineering - Institute of Combustion and Power Plant Technology (IFK)
Project number: 0327786B