Defining CCS Ready: An Approach to An International Definition

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Appendix B: Technology Design Options for a Capture Ready PlantB.3 Pre-combustion CaptureFor flue gases with low CO 2 concentration, it would be useful to capture CO 2 prior tocombustion. Such “pre-combustion” strives to separate the CO 2 from a process stream richin CO 2 prior to combustion of the remaining hydrogen-rich fuel.Prime candidates for pre-combustion CO 2 capture are Integrated Gasification CombinedCycle (IGCC) power plants. IGCCs generally use high-temperature oxygen-blown entrainedflow gasifiers. Syngas cooling can take place against boiler feedwater and steam in a syngascooler, or by means of a wet quench. Following this, a number of cleanup steps are used toclean the syngas of contaminants.Natural-gas-fired combined cycle power plants can also be equipped with pre-combustioncapture. For these gaseous fuels, the fuel conversion into syngas is achieved through partialoxidation and/or steam reforming. If a combination of both is used, it is generally termedauto thermal reforming. As natural gas is a clean fuel, no elaborate cleaning is required, buthumidifying and possibly steam or water injection in the combustion chamber may still berequired to reduce NO x formation.An important consideration in IGCCs with pre-combustion CO 2 capture is whether toremove sulphur compounds before or after shifting. If removed before, it is called “sweetshift” (or clean shift). An advantage of this setup is that it is easier to remove a relativelypure sulphur (H 2 S) flow without dilution by CO 2 , as the latter is only present in smallquantities. If sulphur compounds are removed after shift, the setup is called “sour shift.”Advantages of this arrangement are a more level temperature profile, and thus higherefficiency, and no need for a separate COS hydrolysis reactor as this reaction already takesplace in the shift reactors. Moreover, sour shift catalysts have a wider temperature windowto operate in than sweet shift catalysts. Avoiding sulphur removal before shifting leavesmore steam in the syngas flow, requiring less additional steam for shifting.After water gas shift, the CO 2 is removed from the syngas. Technologies used includesolvents also used for desulphurization, such as Selexol or Rectisol. For IGCCs for bothsolvents, schemes have been proposed combining sulphur and CO 2 removal (in case a sourshift arrangement would be used). The resulting hydrogen-rich fuel stream cannot be fired ina gas turbine equipped with dry low-NO x combustion chambers, due to the wideflammability limits and high flame speed of hydrogen. Therefore, diffusion flame combustionchambers are needed, the hydrogen-rich fuel stream needs to be humidified, and nitrogenneeds to be added (as is also done in IGCCs without CO 2 capture) to reduce NO xformation. Exhibit B-8 shows an IGCC equipped with pre-combustion CO 2 capture using asour shift.One drawback of pre-combustion method is that there are only a small number of IGCCplants, while the industry has much more experience in PC plants. In recent years, however,many coal gasifiers have been built for chemicals production, and the reliability of IGCCplants currently running (such as Nuon’s Willem Alexander Centrale in Buggenum, TheNetherlands) matches that of PC plants.23 February 2010 88
Appendix B: Technology Design Options for a Capture Ready PlantExhibit B-8: Pre-Combustion CO 2 Capture ProcessSource: Vattenfall. (2010). 140B.3.1 Capture Ready Design Considerations for Pre-Combustion CaptureRetrofitting with pre-combustion CO 2 capture has consequences for the complete powerplant, and consequently Capture Ready considerations apply to virtually all components.Reduced Heat Content of the Gas Turbine FuelAn important consequence of retrofitting a power plant with pre-combustion capture is thatthe heat content of the fuel gas is reduced, potentially leading to a drop in electricity output.The water-gas shift reaction is exothermic, which means that part of the heat content in theincoming flow (into the water-gas shift reactors) is released in the form of sensible heat(most likely used for steam production). This means that the heat content of the hydrogenrichfuel fired in the gas turbine decreases by around 10% compared with the non-capturecase. In other words, the gas turbine combined cycle will operate in part load if noadditional fuel is added.For a natural gas combined cycle this may not be a big Capture Ready consideration, asmost of the equipment required for the pre-combustion capture retrofit (mainly thepartial oxidation/reforming reactor) will be designed at the time of retrofit and thus can besized to guarantee full load operation of the gas turbine. Basically, only the natural gasinput needs to increase. For IGCCs it is an important consideration, however. To preventpart load operation after capture retrofit, the gasifier needs to be oversized in the originaldesign, and with the gasifier also the air separation unit and gasifier auxiliaries (such as thecoal feeding system).Syngas Cooler vs. Wet QuenchFor IGCC power plants, the choice between a convective syngas cooler (which heats andevaporates steam cycle feedwater) and a wet quench influences the shift options at the time140 Vattenfall. (2010). Illustrations. Retrieved from Vattenfall News & Reports website.23 February 2010 89
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Defining CCS Ready: AnApproach to A
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PrefacePrefaceThe Global CCS Instit
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