Primary Separator Feed Pressure

The level of flue gas compression before the separation affects two main performance metrics. First and most importantly, higher compression requires more energy. Figure 3a shows the relationship between compression and process energy requirement (kWh/tonneCO2) on the capture process. As more compression is required from the process, the compressors become larger and require more energy, resulting an expected increase in energy cost per tonne of CO2. The trend, normalized to the baseline power requirement of 403.1 kWh/tonneCO2, appears fairly linear, which may be expected. However, depending on the separation process and materials, additional compression may allow for a more effective separation (higher CO2 purity, recovery, and productivity/fluxes) at the cost of this additional energy demand but an ultimately lower cost of CO2 capture.
Increases in the required compression also necessitate an increase in the size of three of the four compressors in the system resulting in a higher capital cost (Figure S9). This cost may be partially offset with the size reduction of other system components, like piping, whose size is in part determined by the volumetric flow rate; this is not accounted for in this preliminary cost study. Compression is a vital part of the ability of the process to operate without an external cooling utility, as previously discussed. Beyond a certain level of compression, the amount of cooling available via Joule-Thomson cooling cannot provide the required heat removal. In this scenario, there is no simple way to adjust the process design (e.g., changing the expander pressure ratio will not allow for more cooling to become available) to provide more cooling to the process. While we do not observe this in cases where the operating pressure is 6 bar or above it is conceivable such situations may arise.

Primary Separator Operating Temperature

Figure 3b shows the effect of decreasing the operating temperature of the separator on the overall energy demand of the process. As the operating temperature decreases the overall system energy requirements decrease, while the total amount of heat removal required increases. This increase in required cooling ultimately does not impact the energy demand of the process on the plant due to the significant levels of heat integration and recovery. Across all separation system temperatures considered we estimated that there was at least 18.5 MW of excess heat removal capabilities available at low qualities (-5 to 30°C). The decrease in energy instead is due to a slight reduction in the required energy of the liquefaction compressor (in COMP3 in Figure 2) as the compressor receives a lower temperature inlet gas. The economic analysis for the process shows a similarly small reduction in the cost of capture with lower temperatures, where the cost of the additional heat exchanger area is offset almost entirely by the reduction in monetized energy demand on the plant (Figures S10 and S11).

Primary Separator Vacuum Pressure

The vacuum pressure used to recover the CO2-rich product leaving the separator was varied from 0.05 to 0.3 bar. With these moderate levels of vacuum we assumed a vacuum efficiency of 0.8 for an isothermal vacuum pump. Since the vacuum pump is somewhat isolated from the rest of the process, a variation of the vacuum pressure results follows a logarithmic profile with respect to energy as shown in Figure 3c. The lower the purity at the designated recovery, the more the volume of gas that needs to be removed via the vacuum, resulting in higher cost (see Figure S12).