https://doi.org/10.1016/j.seppur.2021.118959
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Advanced process configurations have been assessed for inlet FG conditions corresponding to a typical European cement plant using the Best Available Technology (BAT), as defined for the EU-H2020 project CEMCAP with the collaboration of the European Cement Research Academy (ECRA) [33]. The inlet FG conditions are provided in Table 1.
For the sake of simplicity, only species derived from CO2, NH3 and H2O have been considered in the process simulations. On the one hand, NOx has been assumed to enter the capture process in the form of NO, which is not soluble in the solvent [52], while SO2 has been considered to be removed completely in the FG pre-conditioning section [19], [51]. On the other hand, full removal of NH3 by acid washing has been assumed in the FG post-conditioning section [6], [51]. Consequently, the FG pre-conditioning section and the last column before the stack in the FG post-conditioning section have been simulated as a DCC and as a Direct Contact Heater (DCH), respectively, still accounting rigorously for the heat integration and cooling needs. The flowrate of aqueous NH3 (or caustic) make-up, of aqueous H2SO4 make-up and of pure solid ammonium sulphate ((NH4)2SO4) can still be computed from simple straightforward mass balances.
Table 1. Inlet FG characteristics.
| Variable | Units | Value |
|---|---|---|
| Total flowrate | [kg/s] | 107 |
| Temperature | [°C ] | 110 |
| Pressure | [bar ] | 1.1 |
| Composition | [mole frac.] | |
| yair | 0.730 | |
| yCO2 | 0.180 | |
| yH2O | 0.090 | |
Simulations have been performed in Aspen Plus, Version 8.6, using the rate-based model described, implemented and validated earlier [14]. Equilibrium-based simulations of the full capture process have been combined with rate-based simulations of the CO2 absorber and of the FG-WW column. Only those simulations leading to results meeting the specifications and constraints indicated in Table 2 were considered as representative of feasible process operation hence used for the assessment of the advanced configurations.
The CO2 capture efficiency is fixed to 0.9 in all cases in order to allow for a fair comparison among different process configurations. Additionally, all advanced configurations are designed to avoid the formation of solids in the process. The specifications of the CO2 gas stream at the battery limits of the capture plant have been set to those defined for transport by pipeline [26], [33], [39]. The CO2 compression section is also simulated in Aspen Plus, Version 8.6, by means of the configuration provided by Manzolini et al. [53] and using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) as thermodynamic model, which enables reliable results at supercritical conditions but does not include NH3 as one of the components of the thermodynamic package. Therefore, components in the CO2 gas stream other than CO2 and H2O, i.e. NH3, have been neglected, so as specifications and constraints regarding temperature, composition and avoidance of solid formation are applied to the CO2 gas stream entering the CO2 compression section, which warrants their fulfilment downstream. The maximum NH3 concentration allowable in the liquid streams purged from the process, i.e. 150 ppmwt, has been selected as an intermediate value from maximum values for European ammonia production facilities using the BAT, where it ranges between 80 and 175 ppmwt [54]. Such limited pollution with biodegradable NH3 allows for simple sewage water treatment that can be used to produce the process water make-up required in the capture process. In addition, the temperature of aqueous NH3 liquid streams has been limited to 150 °C or below in order to assure the lack of any corrosion on typical materials used in the construction of the capture process units, i.e. stainless steel.
Table 2. Specifications and constraints considered in all process simulations of NH3-based capture processes.
| Overall CO2 capture process | |
| CO2 capture efficiency (ψ) [-] | 0.9 |
|---|---|
| Formation of solids | Avoided |
| NH3 concentration in the FG at the stack [ppmv] | ≤ 10 |
| Apparent NH3 concentration in purge liquid streams [ppmwt] | ≤ 150 |
| CO2 gas stream for transport by pipeline to storage | |
| Pressure [bar] | 110 |
| Temperature [°C ] | 28 |
| Temperature of aqueous NH3 liquid streams [°C ] | <150 |
| CO2-WW section | |
| CO2 gas stream to compression | |
| Temperature [°C ] | 35 |
| NH3 concentration (yNH3CO2comp,in) [ppmv] | <50 |
| CO2 purity [vol%] | >99 |
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