https://doi.org/10.1016/j.clet.2021.100249
“Table 6 summarizes the simulation results for the MEA cases, and Table 7 summarizes those for the a-MDEA cases. From left to right in the tables, the proposed process modifications are added to the previous case, starting with the base case configuration. The process design becomes more energy efficient but also more complex as more equipment is added to the base case configuration. Fig. 3, Fig. 4 focus on the reboiler energy consumption values for different configurations respectively, MEA and a-MDEA cases. An incremental decrease in reboiler energy consumption is observed for the cases associated with both solvents.”
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Table 6. Simulation results for the selected configurations with MEA solvent.
| PCC MEA configuration | Base case | + AIC | + AIC & PEA | + AIC & LVR | + AIC, LVR & PEA (Split ratio: 0.5) |
|---|---|---|---|---|---|
| Total cooling demanda (GJ/h) | 452 | 444 | 494 | 448 | 479 |
| Solvent recirculation flow rate (m3/h) | 1031 | 1032 | 1018 | 1032 | 1049 |
| Lean/rich solvent loading (mole CO2/mole amine) | 0.21/0.6 | 0.22/0.59 | 0.23/0.59 | 0.21/0.59 | 0.23/0.6 |
| Boil-up temp. (oC) | 121 | 121 | 120 | 121 | 119 |
| Reboiler steam-to-CO2 ratio (kg/kg) | 1.46 | 1.40 | 1.35 | 1.29 | 1.24 |
| Energy savings from benchmark (%) | – | 3.8 | 7.6 | 11.8 | 15.3 |
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Total cooling demand is the summation of QDCC, QWash, QAIC, QCond, and QLAC.
Table 7. Simulation results for the selected configurations with a-MDEA solvent.
| PCC a-MDEA configuration | Base case | + AIC | + AIC & PEA | + AIC & LVR | + AIC, LVR & PEA (Split ratio: 0.5) |
|---|---|---|---|---|---|
| Total cooling demanda (GJ/h) | 387 | 401 | 443 | 429 | 422 |
| Solvent recirculation flow rate (m3/h) | 1063 | 1066 | 1067 | 1028 | 1067 |
| Lean/rich solvent loading (mole CO2/mole amine) | 0.11/0.57 | 0.12/0.56 | 0.13/0.56 | 0.11/0.56 | 0.13/0.57 |
| Boil-up temp. (oC) | 117 | 116 | 116 | 117 | 115 |
| Reboiler steam-to-CO2 ratio (kg/kg) | 1.35 | 1.31 | 1.29 | 1.24 | 1.18 |
| Energy savings from benchmark (%) | – | 2.8 | 4.1 | 8.3 | 12.4 |
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Total cooling energy is the summation of QDCC, Qwash, QAIC, Qcond, and QLAC.
Fig. 3. Reboiler Energy Consumption for the selected configurations with MEA solvent.
Fig. 4. Reboiler Energy Consumption for the selected configurations with a-MDEA solvent.
In addition, several other parameters were monitored closely to keep them in an acceptable range. These parameters are the capture efficiency, the amine solvent recirculation flow rate, and the boil-up temperature at the reboiler. Moreover, lean amine CO2 loading should be in the range of 20–25% for MEA and 10–14% for a-MDEA based on a study by (Amann and Bouallou, 2009). The savings were estimated by comparing the reboiler energy index as the main indicator in the modified configurations and the base case configuration for each solvent. It is worth mentioning that the recirculation rate of solvent (on a CO2-free basis) was equally set in each simulation case and the slight differences in the flow rates are mainly due to the amount of CO2 absorbed in the solvent.
By increasing the process complexity after adding the selected process modifications, the reboiler energy consumption decreased significantly, and consequently, the steam-to-CO2 ratio declined. The best performance-modified process and conventional process values found in the literature for the reboiler energy consumption index in the base case configurations were 2.2 and 3.8 MJ/kg CO2 for a-MDEA (Lee et al., 2016) and MEA (Zhao et al., 2017), respectively. As Table 6 and Fig. 3 portrays, this value for MEA can be reduced from 3.14 to 2.66 MJ/kg CO2. This shows a potential saving of up to 15% and confirms the promising results offered by the proposed process modifications. At the same time, the declining trend in the steam-to-CO2 ratio in the reboiler (from 1.46 to 1.24 kg steam/kg CO2 captured) favors modifying the process configuration.
As can be observed in Table 7 and Fig. 4, the a-MDEA cases follow a downward trend for energy consumption, similar to the MEA cases. This time, the reboiler energy consumption index decreased from 2.90 to 2.54 MJ/kg CO2. In the same manner, the steam-to-CO2 ratio decreased from 1.35 to 1.18 kg steam/kg CO2 captured (17%). In comparison to the base case a-MDEA process, potential energy savings of 3.8%–16% can be obtained, supporting the benefits of the suggested modifications.
The following conclusions can be made from comparing the results presented in Table 6, Table 7:
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Process modifications for MEA result in more savings in the reboiler than those for a-MDEA.
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The reductions in energy consumption of MEA are influenced more distinctively by moving towards complicated process configurations as compared to those of a-MDEA.
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As expected, the regeneration energies for a-MDEA cases are lower than those for MEA cases.
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Cooling demand for a-MDEA is lower due to the release of less heat of absorption in the absorber. In the stripper, less cooling is needed in the condenser because less steam is used in the reboiler.
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a-MDEA is easier to regenerate than MEA. Hence, the boil-up temperature in the reboiler is lower in the case of a-MDEA.
Our findings are in good agreement with the values reported in the literature. Among the proposed process modifications, LVR was reported to give one of the highest savings in reboiler energy consumption (Cousins et al., 2011). (Amrollahi et al., 2012) showed the possibility of decreasing reboiler energy consumption remarkably with the combination of AIC and LVR. They identified this arrangement as the best optimized case among the combinations of AIC, split flow, and LVR.
In terms of reboiler’s energy consumption index (and total energy demand, net external cooling and heating demands), our results also are in agreement with those from an up-to-date study performed by (Otitoju et al., 2021). They performed a techno-economic assessment on PCC for a large-scale NGCC (250 MWe). NGCC’s flue gas compositions are one the closest to those of flue gases emitted by OTSGs (in SAGD oil sand operations, as considered in this research work). (Otitoju et al., 2021) probed into AIC (absorber intercooler) and Advanced Flash Stripper (AFS) and managed to perform a comprehensive validation over a wide range for the main KPIs (rich amine’s CO2 loading, reboiler energy consumption, and capture rate/level). Instead, they limited their emphasis to PZ, not a blend/advanced amine solvent nor combined configurations. At 30% and 40 wt.% of PZ, as the solvent for NGCC-PPC, they estimated potential savings of 33%–49% in total energy demand. They succeeded in brining the capture costs down to 40, 38, (and finally 35$/tonne CO2 for the case of AFS with 40% PZ) at an estimation level. In a study by (Zhao et al., 2017) using MDEA/PZ solvent, reboiler energy consumption of 2.24 MJ/kg was obtained after combining AIC, RSF, and SIH; however, at the cost of an increased process complexity.
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