Post-combustion capture

In a conventional combustion process, such as a power station boiler, the carbon dioxide is contained in the exit flue gas. The concentration is generally fairly low and ranges from about 4 vol.% for a combined-cycle system operating on natural gas to 12-14 vol.% for a traditional boiler fired by pulverized coal. The exhaust can be scrubbed with an amine solution, typically monoethanolamine, which is then heated to release the absorbed carbon dioxide. The low concentration of carbon dioxide in the exit gas means that a huge volume would have to be handled and this would entail the installation 

Principal routes for managing carbon dioxide emissions from power stations and industrial sources.  

In recent years, the efficiency of electricity plants that operate with pulverized coal has improved steadily from about 35% to close to 40%, since it has been possible to raise the temperature and pressure of the steam, consequent upon the development of new alloys for turbine blades. The latest state-of-the-art, coal-fired plant, which uses supercritical pressure steam conditions, has an efficiency as high as 46%. Some new coal stations of this type are equipped with carbon capture, while others are being built capture ready (/.c., with appropriate land and other facilities for the required technology to be installed later). The reason for the latter option is that carbon dioxide capture adds to the capital and operating costs and will not lightly be undertaken until demanded by legislation or the economics become favourable. 

---

Post-combustion capture follows the conventional application of a specific purification unit applied for a particular pollutant removal (C02 in this case). Figure 3 illustrates a typical block diagram of the post-combustion process that offers a great feasibility and versatility in terms of operating conditions and process integration. 

Figure 3. Simplified scheme of a fossil-fuel power plant using a post-combustion capture unit [5].

The technologies currently available for post-combustion capture are classified into five main groups absorption, adsorption, cryogenics, membranes and biological separation. The most mature and closest to market technology and so, the representative of first generation of postcombustion options, is capture absorption from amines. 

Post-combustion capture using chemical absorption by aqueous alkaline amine solutions has been used for C02 and H2S removal from gas-treating plants for decades [6]. Amines react rapidly, selectively and reversibly with C02 and can be applied at low C02 partial pressure conditions. Amines are volatile, cheap and safe in handling. They show several disadvantages as they are also corrosive and require the use of resistant materials. Furthermore, amines form stable salts in the presence of O2, SOx and other impurities such as particles, HC1, HF and organic and inorganic Fig trace compounds that extremely constrain the content of those compounds in the treated gas. 

Selectivity for C02 it represents the C02 uptake ratio to the adsorption of any other gas (typically nitrogen for post-combustion capture, and methane for natural gas). It is an essential evaluation criterion, and affects the purity of the adsorbed gas, which will significantly influence the sequestration of C02. The simplest method to estimate the selectivity factor is to use single-component adsorption isotherms of C02 and nitrogen. 

In the field of electricity generation, in particular, intensive research is carried out on capture processes, as this industrial sector contributes heavily to worldwide greenhouse-gas emissions. Basically, three concepts for C02 capture in electricity generation can be distinguished post-combustion capture, pre-combustion capture and oxyfuel (see Fig. 6.1 for a detailed description of the various capture processes, see, for example, BMW A (2003) IPCC (2005)). 

Carbon dioxide capture and storage is especially studied in connection with electricity generation, where different technology routes - post-combustion capture, pre-combustion capture and oxyfuel combustion - are under research and development. Carbon dioxide capture is also crucial in connection with hydrogen production... 

Treated flue gas contains C02, N2, 02. Since N2 and 02 form hydrate crystals at approximately the same conditions the treated flue gas is considered a C02/N2 mixture. Thus in post-combustion capture from power plants the task is to separate C02 from a C02/N2 mixture in which the C02 molar content is approximately 15-20 %. Pre-combustion capture involves separation of C02 from a mixture with H2 in which the C02 molar concentration is approximately 40 % (Klara and Srivastava, 2002 Kang and Lee, 2000 Englezos and Lee,... 

Steam power plants with highest efficiencies and post combustion capture of GO2. 

Many criteria must be considered when choosing or developing absorbents for C02 removal, including selectivity, cost, and stability. There are mainly two C02 separation issues (1) For post combustion capture from flue gas (e.g., coal-fired power station), a major obstacle is the low pressure of the flue gas (1 atm), with just 15 % C02 concentration together with other component gasses, predominantly N2. 

The principle of post-combustion capture is shown in Figure 11.9. The exhaust gas from the NGCC plant is sent to the C02 capture unit, which typically utilizes chemical solvents such as amines or alcohols.7"13 Chemical absorption of C02 using solvent is preferred because of the low concentration of C02 ( 5%) and low pressure in the flue gas. The solvent loaded with C02 is regenerated typically in a reboiler using low-quality steam. The C02 released from the solvent can be sent to a sequestration plant. 

Pre-combustion capture is when the carbon dioxide is separated from the hydrogen before it is combusted in a gas turbine. Post-combustion capture is the more usual process where the fuel is burnt first and the carbon dioxide is extracted from the exhaust gas. See Section 3.2, Chapter 3. 

Figure 3.2 (a) Post-combustion capture at a plant in Malaysia that employs a... 

All three major processes - post-combustion capture, oxy-fuel combustion, pre-combustion capture - require a step that, variously, involves the separation of carbon dioxide, oxygen or hydrogen from a bulk gas stream (flue gas, air or syngas, respectively). These separations can be accomplished by means of physical/chemical solvents, membranes, solid sorbents or cryogenic processes. 

Davy R. Development of catalysts for fast, energy efficient post combustion capture of COj into water an alternative to monoethanolamine (MEA) solvents. Energy Procedia 2009 1 885. 

Nguyen, P.T., Lasseuguette, E., Medina-Gonzalez,Y., Remigy, J-C., Roizard, D., Favre, E. 2011. A dense membrane contactor for intensified COj gas/liquid absorption in post-combustion capture. J. Membr. Sci. 377 261-272. 

Post-combustion capture involves separating the carbon dioxide from other exhaust gases after combustion of the fossil fuel. Post-combustion capture systems are similar to those that already remove pollutants such as particulates, sulfur oxides, and nitrogen oxides from many power plants. 

Each of these capture options has its particular benefits. Post-combustion capture and oxyfuel have the potential to be retrofitted to existing coal-fired power stations and new plants constructed over the next 10-20 years. Pre-combustion capture utilizing IGCC is potentially more flexible, opening up a wider range of possibilities for coal, including a major role in a future hydrogen economy. 

Development of Amine-blend Systems for CO2 Post-Combustion Capture... 

Until now, the reports of adsorptive separation of CO2 and other gases in a mixture by using MOFs, conducted by experimental separation process, are limited. Among various characterization methods in separation, the breakthrough experiment and gas chromatographic separation are simple and straightforward in the evaluation of the separation performance of a MOF toward a gas mixture. On the other hand, as previously mentioned, reported CO2 separation in MOFs mainly includes CO2/N2 separation for post-combustion capture, CO2/H2 from synthesis gas for pre-combustion capture, and O2/N2 and CO2/CO separation for oxy-combustion capture, which will be detailed as following. 

M. R. M. Abu-Zahra, P. H. M. Feron, P. J. Jansens, E. L.V. Goetheer, New process concepts for CO2 post-combustion capture process integrated with co-production of hydrogen. Inter. J. Hydro. Energy, 34, 3992-4004 (2009). 

---