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).