Cost Optimum Temperature Approach—CO2 Capture Cost Analysis

The results of the trade-off analysis between energy cost and capital cost at different ΔTmin based on CO₂ capture cost are presented in Figure 7, Figure 8, Figure 9 and Figure 10. This is to investigate cost reduction potential and assess if significant cost reduction can be achieved through energy cost and heat exchanger cost trade-off analysis. To make the result comprehensive, the analysis was performed for two different flue gasses of two different industrial processes with different flue gas flow rates and different CO₂ compositions, as stated earlier. Two different CO₂ capture configurations, the conventional or standard process and the lean vapour recompression configurations were also used for this study. In the four figures, the left vertical axis represents the values of the STHXs, while the right vertical axis is for the PHE.

In the CO₂ absorption from the NGCC power plant flue gas cases, the cost optimum a ΔTmin of 12 °C was estimated for both the FTS-STHX and UT-STHX processes in the standard process. The cost optimum ΔTmin for the FH-STHX and PHE are 14 °C and 6 °C, respectively. In the lean vapour compression configuration, all the STHXs have the same cost optimum ΔTmin of 9 °C, while the PHE optimum is 5 °C.

These results revealed the significance of both the lean/rich heat exchanger function and the specific type of heat exchanger selected for the lean/rich heat exchanger on the cost of the capture process. The more expensive a specific heat exchanger type is, the higher the ΔTmin that will achieve the cost optimum capture cost. Additionally, the less expensive the heat exchanger is, the lower the ΔTmin that will give the cost optimum CO₂ capture cost. While the cost savings at the optimum ΔTmin is marginal in terms of CO₂ capture cost in this work, the absolute value is significant, especially in the NGCC power plant capture system where over one million tons of CO₂ is captured annually. In the standard process for the NGCC power plant capture system, an annual total cost saving of EUR 165,000/year to EUR 311,000/year was estimated depending on the specific heat exchanger type. Meanwhile, in the lean vapour compression process, the FTS-STHX and FH-STHX could only achieve EUR 97,000 and EUR 74,000, respectively, in CO₂ capture cost at the optimum ΔTmin. This is because all the STHXs cases’ optimum ΔTmin was only one degree (1 °C) from the base case ΔTmin. The UT-STHX case which also had its optimum at 9 °C achieved a cost saving of EUR 171,510. However, the lean vapour compression process with PHE achieved a cost reduction of EUR 819,530 at the cost optimum ΔTmin.

In the cement plant standard capture processes, the cost optimum trade-off of both the FTS-STHX and the UT-STHX was 13 °C, while it was 7 °C for the PHE. In the lean vapour compression capture process, 10 °C, which is the base case ΔTmin, remained the cost optimum for the two STHXs. The cost optimum ΔTmin for the PHE system of lean vapour compression was 5 °C. The cost reduction in the two STHX processes based on standard capture configuration was marginal. However, the capture cost optimum ΔTmin achieved EUR 253,570 and EUR 483,700 in the standard and lean vapour compression CO₂ capture processes, respectively.

A general observation is that as the ΔTmin decreases from 10 °C to 5 °C, the resulting increase in the heat exchanger area makes the capital cost dominate, causing the capture cost to rise steeply in the cases of all the STHXs in both capture configurations. In the standard process, above a ΔTmin of 14 °C, the capture cost begins to increase steeply, indicating the dominance of steam cost, especially in the cement plant capture system. In the lean vapour compression systems, the impact of moving from one ΔTmin to the next is more significant. The energy cost and heat exchanger cost trade-off trends of the standard CO₂ capture system for both the NGCC power plant and cement plant capture systems are similar, likewise in the lean vapour compression configuration capture process for industrial capture processes.