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R CO2 m of CO2 in the product stream (10) m of CO formed PCO2 CO2 volumetric concentration in the product stream 2 (11) m of H2 entering the gas turbine as fuel (12) R H2 m of H for the reference plant without CO =1 net capture 2 1R (13) entering the CO separation unit 22 CO2 for the plant implementing CO CO net 2 capture 2 The energy efficiency of the plant is evaluated through the net electric efficiency (ηnet), referred to the LHV: net = Net electrical output (14) Net fuel input The footprint of the CO2 separation technology is evaluated in terms of square meters occupied by the relative unit. The preliminary analysis carried out considers the size and the number of columns necessary for the CO2 separation process. A more thorough analysis, including all the equipment relative to the separation process, would be needed in order to obtain more reliable outputs. However, it has been considered beyond the sake of the present work, which aims to give a first assessment on the possible dimensions of the units and on the difference between the separation techniques. 4.2. Post-combustion PSA process Liu et al. [16] demonstrated that, in order to achieve the requested performance in terms of CO2 recovery and purity, the flue gas resulting from the combustion of coal needs to undergo a two-stage PSA process. The first stage considered in the current work consists in a three-bed and five-step cycle (Figure 4). Since no flue gas compression is implemented upstream the PSA unit, the flue gas enters at about atmospheric pressure. The aim of the first stage is to achieve the highest possible CO2 recovery. As a tradeoff, it is not possible to achieve very high CO2 purity. The regeneration process is carried out by decreasing the pressure to 0.1 bar. This pressure value has been suggested in many studies [14, 16, 18, 19, 23]. The regeneration pressure to be applied is dependent on the shape of the adsorbent isotherm and on the degree of vacuum to reach in order to guarantee proper bed regeneration. 0.1 bar seemed to balance the different requirements. Other values may have been considered but the advantages in terms of energy savings obtained with a higher regeneration pressure are counterbalanced by lower separation performance. The other way around with lower regeneration process. As an example, some simulations were implemented with the vacuum level set to 0.2 bar. Whilst the energy penalty could be effectively reduced of about 0.5%, the overall CO2 recovery dropped under the target value (86.8 %). The CO2 enriched-gas leaving from the blowdown and purge steps are then collected and sent to the second PSA stage, a two-bed five- step cycle (two-bed six-step if purge is implemented), where it is further purified. In order to enhance the second PSA process performance, a compression of the gas stream is implemented between the PSA stages. The gas is brought up to 2 bar before undergoing the second adsorption process. Figure 6 shows the overall levels of CO2 recovery and CO2 purity obtained in the PSA process (after the two PSA stages) by varying the Purge-to-Feed mole flow rate ratio (P/F) of the second PSA stage. It is clear from the figure that there is a tradeoff between CO2 recovery and purity. The highlighted point in Figure 6 (PCO2 = 95.1% and RCO2 = 90.2%) represents the PSA operating conditions selected for the process to be matched with the power plant. It refers to a PSA process in which the purge step has not been implemented, hence with a P/F ratio equal to zero. This configuration wasPDF Image | Evaluating Pressure Swing Adsorption as a CO2 separation technique in coal-fired
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