Removal of carbon dioxide (CO2) from the flue gas of coal fired power stations is an essential requirement for future operation of coal-fired power stations. Several carbon capture methods have been developed, but all are complex and have a high parasitic energy load. Cryogenic carbon capture offers a simple method at a lower cost.
Current methods in an advanced stage of development or employment include solvents, pressure swing, membrane and activated carbon systems. All have a relatively high energy penalty, commonly known as parasitic energy. Current advances in existing and emerging CO2 capture and storage (CCS) technologies are mostly driven by the need for reductions in the energy penalty and costs of CCS. Cryogenic CO2 capture (CCC) is one of the more innovative and promising post-combustion technologies, which in early analyses, exhibits an apparent energy and environmental advantage compared to alternatives. This process relies on the phase change characteristic’s of CO2, which allow separation from flue gas in the form of a liquid or solid component. Although CCC techniques are still in the research phase, a variety of promising systems have been proposed. The technique uses well established uses well established system components.
CCC is used in other processes such as removal of CO2 from natural gas and process gas, and also in the recovery of CO2 in commercial CO2 production processes, and is a fairly well established process in other applications.
CO2 phase change behaviour
CO2 can exist in solid, liquid and gaseous states, as shown in the phase diagram (Fig. 1), depending on temperature and pressure. The triple point of a gas is the temperature and pressure at which all three phases coexist. The triple point of CO2 occurs at 5,1 atm and 56,7°C.
At normal atmospheric pressure there is no temperature at which the liquid phase of CO2 exists. Different gases in a mixture will exhibit different triple points and at a specific temperature different gases will be in the vapour and liquid or solid phase. Cryogenic separation methods rely on the fact that at a specific temperature and pressure, one gas will be in a liquid or solid state and another will be in a gaseous state.
All three phase changes are important to CCC.
Partial pressure and phase change
The phase diagram shown is that for 100% CO2 . The process is slightly more complicated for mixtures of gases. Flue gas consists of a mixture of CO2, nitrogen and other gases. The deposition point of CO2 for this mixture will depend on the partial pressure of the gas and not values of P and T for phase change of a component of the mixture depends on the partial pressure of the component. The partial pressure is roughly equal to the total pressure multiplied by the percentage of the component gas. For a 20% CO2 mixture the partial pressure will be approximately 20% of the total gas pressure, and at a pressure close to the critical point the partial pressure will be 1 atm and the deposition temp -78,5°C. The dependence of the deposition, or frosting, temperature on the percentage of CO2 in a mixture at a pressure of 1 atm is given in Table 1.
|CO2 concentration (%v/v)||100||10||1||0,1|
|Deposition Temp. (°C)||-78,5||-103,1||-121,9||-136,7|
This has an effect on the processes used. For liquefaction to take place at a 20% CO2 concentration would require a minimum total pressure of approximately 25 atm. The STL process however requires only a pressure of 5,4 atm as the component is 100% CO2. Deposition requires lower pressure but also lower temperatures, but the subsequent STL process required takes place at lower pressure as well, and the choice of system will depend on the comparative costs of pressurisation energy vs cooling energy. Recovery of cooling energy from the extracted products can have a major influence on comparative costs.
The CCC Process
Many different systems using this process have been proposed and researched , but all follow the basic layout as shown in Fig 2. Typical systems add additional stages to those shown here, or use other methods to implement the processes required, but the basic principle of operation remains the same.
The flue gas is dried to remove water and is compressed. The compressed gas is cooled, either by the extracted gases/liquids, or by external cooling. This is followed by an optional pollutant removal stage. The compressed and cooled gas then goes through an expansion stage which further lowers the temperature to the point where either liquid or solid CO2 is formed. The separation stage, recovers the solid or liquid CO2 from the gaseous N2. Solid CO2 under pressure is fed to the heat recovery stage where solid CO2 is melted to form liquid CO2 for storage or onward transfer. Liquid CO2 does not undergo any phase change but changes temperature. There is a variety of different liquefaction/desublimation and extraction devices and methods, also known as separation, described in the literature.
There are two types of cryogenic system for CO2 removal:
In these systems temperature and pressure are used to produce liquid CO2, which is then extracted and stored or transported to a usage site. There are a number of designs in the literature, but most seem to have been intended for future oxycombustion systems or IGCC power generation. CO2 liquid production does not require the same degree of cooling as desublimation, but requires a much higher pressure. Fig. 2 shows the diagram of a proposed liquid CO2 recovery system.
Flue gas is compressed, passed through two stages of cooling, one of which is derived from the exit liquid CO2 and the other from an external source, and passes to a first separator, where most of the CO2 is liquefied and removed. The remaining CO2,which is now at a lower concentration and hence partial pressure , requires a higher total pressure to achieve liquefaction, and this is provided by the second stage of compression. The liquid CO2 is passed through two heat exchangers to provide additional chilling of the compressed flue gas, and exits under pressure to a disposal unit.
Desublimation processes separate CO2 as a solid frost at conditions below the sublimation temperature. Unlike liquefaction processes, desublimation systems avoid energy-intensive gas compression stages by operating at close to atmospheric pressure. The desublimation processes can achieve CO2 capture ratios of virtually 100% and remove water and CO2 simultaneously, thus avoiding the need for product drying stages .
An example of desublimation based system is given in Fig. 4 .
The process dries and cools flue gas from existing systems, modestly compresses it and cools it to a temperature slightly above the point where CO2 desublimates. The gas may then pass through a stage of pollutant removal which uses the same principles of phase change of the pollutants. The gas is then expanded to further cool it, precipitating an mount of CO2 as a solid that depends on the final temperature.The solid CO2 is pressurised, and reheated by cooling the incoming gases to form a pressurised liquid. The final result is the CO2 in a liquid phase and a gaseous nitrogen stream. CO2 capture efficiency depends primarily on the pressure and temperature at the end of the expansion process. At 1 atm, the process captures 99% of the CO2 at -135°C and 90% at -120°C. These are relatively mild conditions as compared to competing processes, most of which are not reasonably capable of achieving 99% CO2 capture.The captured CO2 has virtually no impurity in it.
The cooled, compressed gases make it possible to extract SO2, NO2, HCl, and Hg (among other things) in condensed-phase forms with efficiencies that exceed current best available control technologies . NO does not condense as readily as the previous gases and will need alternative treatment, but pressure and temperature regimes of this process offer alternative means of removing NO which may reduce costs as well. A green-field, fully integrated plant can redirect the capital, operating cost, and footprint resources currently dedicated to SOX, NOX, and Hg control and redirect these toward the carbon capture system .
The CCC process outperforms solvent-based systems, primarily because these require a large mass of solvent to be cyclically heated and cooled (or in some cases pressurised and depressurised) to produce a comparatively small amount of CO2. The energy invested in the cyclical heating and cooling represents a major energy sink in the process. The overall energy and economic costs appear to be at least 30% lower than most competing processes that involve air separation units (ASUs), solvents, or similar technologies .
Most of the systems mentioned in the literature are development pilots and have not yet been commercialised or tested out in real application. Companies in the natural gas sector have developed and implemented working CCC systems whose main application is the removal of CO2 from natural gas.
 D Clodic: “CO2 capture by anti-sublimation:Thermo-economic process evaluation”, 4th Annual Conference on Carbon Capture and Sequestration, May 2005.
 K Zanganeh, et al: “CO2 Capture and Development of an Advanced Pilot-Scale Cryogenic Separation and Compression Unit”, Energy Procedia 1, 2009.
 S Burt, et al: “Cryogenic CO2 Capture for Improved Efficiency at Reduced Cost”, AIChE Annual Meeting, November 2010.
 S Burt, et al: “Cryogenic CO2 capture to control climate change emissions”, Brigham Young University, 2009.
 K Osman: “Review of carbon dioxide capture and storage with relevance to the South African power sector”, South African Journal of Science, 2014.
 M Soleimani: “Carbon Dioxide Separation From Flue Gases: A Technological, Review Emphasizing Reduction in Greenhouse Gas Emissions”, The Scientific World Journal, February 2014.
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