Figure 2. Simplified process flow diagram for a sub-ambient
hybrid CO2 capture process using pressure swing
adsorption and product liquefaction. Intermediate heat exchangers on the
primary compression train using cooling water are not included for the
sake of simplicity. A direct contact chiller (DCC), utilizing plant
cooling water is used for preliminary water removal. Blue lines in the
sub-ambient heat exchanger network indicate streams that are being
cooled, while red lines are those being heated.
Power Recovery
As noted above, compression of the feed and subsequent expansion of the
high-pressure N2 byproduct enables the cooling process.
The process is therefore highly reliant on the ability to recover the
maximum allowable energy via the expansion of this byproduct. In the
base case considered, the energy demand for compression is 298.5 MW, a
very significant parasitic load. With the inclusion of gas expansion
that number is reduced to 207.1 MW. The compression work required is the
dominant contribution to the parasitic energy demand on the coal plant.
The energy required for the drying of the feed stream and the cooling
throughout the process is provided through Joule-Thomson cooling and
heat integration, allowing for no additional external energy requirement
for the process outside of compression and pumping.
The flowsheet in Figure 2 employs five compressors (two upstream for
flue gas, three downstream for liquefaction, recycle, and reaching
pipeline pressure specifications respectively), and two expanders
(acting on the N2 enriched product). The downstream
compressors (COMP3, COMP4, and COMP5), along with the vacuum pump and
the liquid CO2 pump have no path for power recovery.
Combined, they make up approximately 63.6 MW (21.3%) of the electrical
demand of the process in the base case. The remaining 143.5 MW of energy
is the balance of the compression work that could not be recovered via
expansion of the N2 product. Thus in the base case,
~39% of the work for compression of the flue gas was
recovered via expansion.