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.