Flue Gas Drying
The drying of flue gas for post-combustion CO2 capture process incurs high energy demands and is thus often considered impractical.22-24 If flue gas can be dried, however, the options available for CO2 capture broaden considerably. In our process, flue gas drying is carried out in three stages, first using cooling water from the plant, then taking advantage of Joule-Thomson cooling from the sub-ambient process, and finally, an adsorbent dryer bed. We expect the water from the first two drying stages to be recovered and sent to a cooling tower where it can be returned to the desired conditions. We assume that cooling water at 30oC is used to remove the water before compression through the application of a direct contact chiller (DCC). The removal of as much water as possible before compression decreases the total volume of gas that must be compressed, which ultimately reduces the volume of gas to be compressed by ~10%. Removal of water is also vital when using axial compressors, as the condensation of water inside the compressor may damage the airfoils.
Importantly, the sub-ambient process we considered integrates water removal with feed compression and cooling via boiler feed water preheating. This decreases the cost of dehydration that would otherwise be expected if the equivalent energy needed to come from an external source. After the direct contact chiller, compression, and cooling the mole fraction of water in the flue gas stream is estimated in the Aspen model to be ~0.5%, as shown in Table S1. After the flue gas has been compressed to the target pressure, it is cooled to 2oC using available cold N2 enriched products as mentioned previously. This additional cooling followed by a knock out decanter results in 528 ppm residual water left in the stream to be removed via the adsorbent. With such a low concentration of water in the flue gas, it is assumed an adsorption dryer will remain essentially isothermal during drying. If the heat integration was unavailable, the economic and energy penalty of the drying would be associated with the cost of drying 10-12 times as much water. Upon completion of its expansion to near atmospheric pressure, the N2-rich product stream is used as a dry sweep gas for the regeneration of the dryer bed and then sent to the stack.

Sub-Ambient Heat Exchange

Extensive heat integration throughout the process is critical to avoid an expensive external refrigeration utility. The compression of the flue gas upstream is primarily responsible for the process’ ability to operate at a steady state with no external refrigeration, as conceptualized in Figure 1. The more the flue gas is compressed, the more high-quality heat removal becomes available from the multistage expansion of the N2 rich byproduct. The base case considers two-stage expansion to produce the required heat removal so the N2-rich product passes through the heat exchanger network three times as seen in Figure 2. This expansion-cooling train is responsible for the majority of the following steps: cooling the inlet gas entering the dryer unit, knocking out additional water, cooling the inlet gas to the separation unit, cooling the compressed CO2 enriched product after recompression to the liquefaction condition, and liquefying the CO2 enriched product. In all cases considered, the minimum temperature approach of the process heat balance was determined by the combination of the primary separation final purity and recovery and the liquefaction temperature and pressure. All cases discussed in the following discussion had a fixed recovery of 92% in the primary separation system to enable an overall system recovery of 90%.