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Comparison of Optimum ΔTmin Results with Literature

https://doi.org/10.3390/en15020425

“A lot of literature may not be available on an extensive trade-off analysis between energy cost and capital cost at different ΔTmin in a post-combustion CO2 capture process. However, a review of some of the literature is given here. The work of Tobiesen et al. [25] indicated that reducing the ΔTmin does not have a significant effect on the steam consumption in the reboiler. This is not the case in our work and some other works reviewed here. They stated that 15 °C may be a reasonable ΔTmin for a CO2 capture plant based on new technology. Their final proposition is that the ratio between the cost of energy consumption and capital cost is anticipated to increase; hence, a ΔTmin of 10 °C or less is conceivably reasonable.

In a CCP project, Choi et al. [22] specified 11 °C for their lean/rich heat exchanger ΔTmin and claimed that this is close to the cost optimum value. They also suggested that to reduce cost, the PHE should replace the STHX, and that it could probably result in a lower cost optimum ΔTmin. The results from this study affirm the latter. Besides reduction in the capital cost, which is achieved by the PHE, the cost optimum ΔTmin based on both CO2 capture cost and CO2 avoided cost is also reduced to between 4 and 7 °C, instead of the higher ΔTmin obtained as cost optimum in the cases of the STHXs.
Li et al. [59] investigated an 85% CO2 capture from the exhaust gas from a 650 MW coal-fired power plant. They estimated an optimum CO2 avoided cost for a standard MEA capture process to be 5–7 °C. The exact type of heat exchanger was not mentioned. It is important to state the specific type of heat exchanger to ensure a proper and transparent comparison with other studies [3]. The benefit from reduction in energy consumption at the lower ΔTmin was more significant compared to the increase in capital cost due to the high increase in the heat exchanger area. They concluded that due to the difficulty of manufacturing the heat exchanger to meet the requirement of such large area, the ΔTmin range of 5–10 °C will achieve the optimum process in avoided cost. In this study, the optimum CO2 avoided costs estimated for the cement flue gas CO2 capture plant was within 4–7 °C for the PHE capture scenarios and 7–10 °C in the FTS-STHX capture scenarios.
For a 90% MEA-based standard CO2 capture process, Schach et al. [55] conducted a trade-off analysis based on an LMTD and on a standardise CO2 avoided cost. Their cost optimum was an LMTD of 7.5 °C. They proposed an advanced MEA-based CO2 capture configuration which include inter-cooling of the absorber, a conventional rich-split process and desorber inter-heating. For this process, they estimated an optimum LMTD of 8 °C. The type of heat exchanger was also not stated.
Karimi et al. [11] investigated seven different configurations for 90% CO2 capture from the flue gas of a 150 MW bituminous coal power plant. They were evaluated for a ΔTmin of 5 °C and 10 °C using CO2 capture cost and CO2 avoided cost metrics. In all the configurations, a ΔTmin of 10 °C achieved the lesser CO2 capture cost and CO2 avoided cost, except in the multi-pressure configuration where 5 °C achieved a marginal reduction of USD 0.01/tCO2 in CO2 avoided cost with a ΔTmin of 5 °C.
Some other studies of an MEA-based post-combustion CO2 capture system can be found in [20,35,36,43,69]. These studies were all carried out using the U-tube and the fixed tube sheet shell and tube heat exchangers in an 85% MEA-based CO2 capture from the NGCC power plant exhaust gas. Kallevik [36] estimated the cost optimum for the UT-STHX to be 10–14 °C in a standard CO2 capture process. In a lean vapour compression CO2 capture process, Øi et al. [69] estimated the cost optimum to be 12 °C. Meanwhile, Aromada and Øi [43] estimated a ΔTmin of 13 °C as the cost optimum in an LVC process. These studies made several simplification assumptions that excluded some important parameters, and the process scope did not include CO2 compression. In a study conducted for 5 °C, 10 °C, 15 °C and 20 °C where FTS-STHX was used as the lean/rich heat exchanger in CO2 capture from NGCC power plant flue gas, Aromada et al. [35] estimated the cost optimum ΔTmin with different capital cost estimation methods to be 15 °C. Preliminary results of this work for different heat exchangers used as the lean/rich heat exchanger for CO2 capture from a cement plant flue gas without the compression section also estimated the cost optimum ΔTmin for the UT-STHX, FTS-STHX and FH-STHX to be 15 °C [20]. The cost optimum ΔTmin. if PHE is selected was evaluated to be 5 °C. The investigation was also carried out for for 5 °C, 10 °C, 15 °C and 20 °C only. Ali et al. [31] estimated 10 °C as a cost optimum using the UT-STHX as the lean/rich heat exchanger in a standard CO2 capture process from cement plant flue gas.
In the NGCC power plant CO2 capture process in this work, the optimum CO2 capture costs were achieved at a ΔTmin of 12 °C in the cases of FTS-STHX and UT-STHX. For the FH-STHX and PHE, this was 14 °C and 6 °C, respectively. Meanwhile, 9 °C and 5 °C were the optimum CO2 capture costs for all the STHXs and the PHE, respectively, in the lean vapour compression process configuration.
In the cement plant capture system, FTS-STHX and UT-STHX cases achieved their capture cost optimum at a ΔTmin of 13 °C and 10 °C in the standard and lean vapour compression processes, respectively. Meanwhile, this was 7 °C and 5 °C, respectively, in the PHE case.
In avoided cost estimates for the cement plant capture process, a ΔTmin of 4 °C was estimated as cost optimum in both the standard and lean vapour compression capture processes. Meanwhile, the two STHX achieved their optimum CO2 avoided costs at 10 °C and 7 °C in the standard and lean vapour compression CO2 capture processes, respectively.
To select PHE instead of the STHXs will result in capital cost reduction, lower energy cost and higher emissions reduction, since a lower ΔTmin results in lower steam consumption. It is therefore desirable to operate at a lower ΔTmin. Larger capital costs at lower a ΔTmin cancel out the OPEX advantage in the cases of the more expensive heat exchangers (STHXs). Higher-cost optimum ΔTmin implies that the capital cost dominates the system, and a lower-cost optimum ΔTmin indicates that energy cost dominates. While the results agree with some of the studies reviewed, to only consider energy reduction of a process only can cause a conclusion which would not evince the best possible solution to be made. Therefore, it is imperative to perform a trade-off analysis between energy cost and capital cost at different ΔTmin for every innovative solvent-based capture system if the best possible CO2 capture process economically and in respect of emissions reduction is to be achieved.”

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