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stage, and the diameter of a column was set to 8 m. Table 6 shows an estimation of the footprints of the two separation techniques considered. The absorption column diameter was calculated by defining a reasonable superficial velocity of the flue gas entering the column (i.e., 2 m/s). It becomes clear that the total footprint of the CO2 capture unit is excessive to be considered feasible. A way to partially reduce the footprint could be to introduce a flue gas compression before the PSA unit. Compressing the flue gas up to 1.5 bar demonstrated to lead to a reduction in the number of necessary PSA trains of about 9 units. It was already verified that this operation would also be beneficial for the CO2 separation process. However, the final footprint would still be much larger than that of the absorption-counterpart. Not to mention the additional power consumption introduced which would severely affect the process competitiveness under an energy efficiency point of view. Table 6. Footprint analysis for the post-combustion scenario. Column diameter (m) Number of columns Footprint (m2) 4.4. Pre-combustion PSA process Absorption PSA 20,7 8,0 2 264 674 13285 The PSA process is supposed to be able to process the syngas and return two streams: a CO2-rich stream to be sent to compression and transportation; and a CO2-lean stream, rich in H2, to be fed to the gas turbine as fuel. Both streams request some purity characteristics to be fulfilled, namely CO2 and/or H2 purity and recovery. Previous studies [25] suggested that a single PSA stage would have been able to fulfill these requirements in conditions typical for a pre-combustion application. However, Casas et al. [25] simulated a gas stream which contains only H2 and CO2. When applying a realistic syngas composition, the results of the simulations became different from those expected. The PSA layout adopted in the present work is a seven-bed and twelve-step cycle and the regeneration pressure was set to 1 bar. Some demonstrative simulations were run to assess the effectiveness of the selected regeneration process. Higher regeneration pressure levels can bring an improvement on an energy point of view, although the reduced purity could partially even out the expected reduction in compression power consumption. Conversely, the separation performance decreases according to the less effective regeneration process. 1 bar appeared to be the regeneration pressure which was closer to meet both separation and energy specifications. Figure 8 shows the levels of CO2 recovery and CO2 purity obtained in the assessed PSA process by varying the Purge-to-Feed mole flow rate ratio (P/F). The values reported in the figure refer only to the PSA unit. The overall plant CO2 purity and recovery will be different since an additional flash separation process is implemented after the PSA process. Figure 8 makes clear that the PSA process is not quite able to match the specifications. Whilst the CO2 recovery can be pushed easily over the target value of 90%, the CO2 purity hardly reaches values around 85%. A further increase of the CO2 purity appears difficult to achieve and would come at the expense of the CO2 recovery, which would drastically decrease. Realizing the impossibility to reach the desired output streams characteristics within the PSA unit, the strategy was modified. A solution could have been to introduce an additional PSA stage (likewise post-combustion scenario) or better to apply a dual PSA process [56]. Considerations mainly regarding the possible footprint related to a second PSA train lead us to choose a different option. Nevertheless, the dual PSA process could result competitive and should be matter of further investigations. To comply with the selected alternative, the CO2 recovery target was set to the highest possible level, while a relatively lower value of CO2 purity was accepted. It was then introduced a further CO2 purification process downstream of the PSA unit. It consists of a double flash separation integrated in the CO2 compression process (Figure 3). Referring to Posch and Haider [35], the temperatures selected at the outlet of each heat exchanger were set respectively to -30°C and -54.5°C. The gas stream is compressed up to 30 bar before entering the flash separation unit. Implementing this additional separation step, the final result in terms of CO2 purity (PCO2=98.9%) and recovery (RCO2=89.8%) basically fulfilled the requirements. The H2 recovery (RH2=99.6%) was satisfactory as well. The operating conditions selected for the full-plant analysis are those represented by the highlighted point in Figure 8 (i.e., P/F = 0.140). This configuration was chosen because it provides a good balance between separation and energy performances. Table 7 displays the relative PSA characteristics, together with the separationPDF Image | Evaluating Pressure Swing Adsorption as a CO2 separation technique in coal-fired
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