https://doi.org/10.3390/en15020425
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The previous section only considered economic viability without taking into account climate change implications. The actual CO2 emission reductions are not considered in CO2 capture cost estimation. This section deals with the cost of actual CO2 emissions reduction. It is pertinent to re-emphasise that the CO2 avoided cost in this study does not include CO2 transport and storage cost as in [29,52,54,55].
The results of the cost of actual CO2 emission reductions are presented in Figure 13 and Figure 14 for the standard and lean vapour compression CO2 capture processes from the cement plant’s flue gas, respectively. The red dot represents where the optimum CO2 capture cost was achieved. It is used to make a comparison with optimum CO2 capture cost and the optimum CO2 avoidance cost, that is when the actual CO2 emissions reduction is considered.
Figure 13. Energy and heat exchanger costs trade-off analysis at different ΔTmin for different heat exchanger types in a standard CO2 capture from cement flue gas with consideration of actual CO2 emissions reduction (red dot is the ΔTmin where optimum CO2 capture cost is achieved, which can be different from the CO2 avoided cost optimum the ΔTmin).
Figure 14. Energy and heat exchanger costs trade-off analysis at different ΔTmin for different heat exchanger types in an LVC CO2 capture from cement flue gas with consideration of actual CO2 emissions reduction (red dot is the ΔTmin where optimum CO2 capture cost is achieved, which can be different from the CO2 avoided cost optimum the ΔTmin).
In the standard CO2 capture process, the optimum CO2 avoided cost was evaluated to be at ΔTmin of 10 °C and 4 °C in the FTS-STHX and PHE scenarios, respectively. Meanwhile, in CO2 capture cost estimation, the cost optimum ΔTmin is 13 °C and 7 °C in the cases of the FTS-STHX and PHE, respectively. In the LVC CO2 capture process, a ΔTmin of 4 °C was also estimated as the cost optimum CO2 avoided cost, while it was 7 °C in the case of the FTS-STHX. The CO2 capture cost optimum ΔTmin in the LVC process were 10 °C and 5 °C in the cases of the FTS-STHX and PHE, respectively.
The ΔTmin in the lean/rich heat exchanger has a significant impact on the steam consumption in the reboiler, as shown in Table 10, Table 11, Table 12 and Table 13 as well as in Figure 6. Thus, the higher the ΔTmin, the higher the steam requirement, which also implies the higher the indirect CO2 emissions due to production of steam by combustion of natural gas. The actual CO2 emissions reduction achieved by using an STHX as the lean/rich heat exchanger is a bit higher than if the PHE is applied. This is because of the higher electrical energy consumption in the case of the PHE compared to the STHX. It is due to the higher pumping duties by the rich pump and lean pump to pump the lean and rich amine streams through the small channels of the PHE. However, considering the cost optimum ΔTmin of 4 °C in the case of using the PHE in both CO2 capture processes compared to the case of the FTS-STHX, the PHE absolutely dominates in performance economically and in CO2 emissions reduction efficiency. If the PHE is selected, its cost optimum ΔTmin or even if 5 °C is specified for the lean/rich heat exchanger, it will achieve about 1.2% and 1.0% more CO2 emissions reduction more than its counterpart in the standard CO2 capture process and in the LVC CO2 capture configuration, respectively.
The optimum CO2 avoided costs of the PHE cases are EUR 82/tCO2 and EUR 73/tCO2 in the cases of the standard and LVC CO2 capture processes, respectively. The actual CO2 emissions estimated are approximately 65% and 68%, respectively. For the FTS-STHX cases, the estimated optimum CO2 avoided costs are EUR 88/tCO2 and EUR 77/tCO2 in the standard and LVC capture processes, respectively. The actual CO2 emissions reduced were estimated to be around 64% and 67%, respectively.
The results reveal the significance of performing cost optimisation of the lean/rich heat exchanger based on ΔTmin trade-off analysis between energy cost and capital cost (especially heat exchanger cost). This work is therefore more complete than our previous work [3] where the conventional ΔTmin of 10 °C was specified for all the specific heat exchanger types. It also emphasises the importance of this study.
Another important observation is that even though the electricity consumption of the lean vapour compression CO2 capture process is higher than that of the standard process, the significant reduction in steam consumption meant it achieved better actual CO2 emissions reduction and less CO2 avoided costs. Therefore, the lean vapour compression configuration gives a more economic and a more environmentally friendly outcome.
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