(475d) Analysis of Membrane Processes for CO2 Removal and H2 Reuse for Solid Oxide Fuel Cells

In order to enhance fuel utilization in solid oxide fuel cells (SOFCs), part of the anode exhaust may be recycled to join the anode inlet stream. However, the CO₂ present in the exhaust would dilute the fuel concentration and lower the efficiency of the SOFCs. Few papers have considered removing CO₂ from the recycle stream, and none have reported on using a membrane process for this task. A CO₂-selective facilitated transport membrane was developed with fluoride- and hydroxide-containing species for the CO₂/H₂ separation, and the membrane exhibited a good stability, showing a CO₂ permeance of 108 GPU and a CO₂/H₂ selectivity of 106 at 120°C. Moreover, the membrane was scaled up to 21ʺ in width, which had consistent transport results as the lab-scale samples.

Herein, a process analysis was done on using the developed membrane for an SOFC system. First, due to its wide availability, air was used as the sweep gas. Furthermore, a vacuum was proposed for the permeate side to further enhance the driving force and allow for CO₂ capture. Also, as oxygen is absent, amine-containing membranes with better transport performances could be used for the vacuum process at 120°C. At 99% H₂ recovery, the air sweep process was estimated to remove 44.7% of CO₂ from the recycled anode exhaust, whereas the vacuum process could boost the removal to 76.9%. A less stringent H₂ recovery requirement could yield a higher CO₂ removal. For instance, a CO₂ removal of over 90% could be achieved when the H₂ recovery was relaxed to 96.7% for the vacuum operation. In addition, a high-level techno-economic analysis was conducted, without including the H₂ reused value. The CO₂ removal costs corresponding to the air sweep and vacuum processes were $62.9/tonne and $53.3/tonne, respectively, even with the added cost of the vacuum pump for the amine membrane process. The cost is even lower at $35.4/tonne if the captured CO₂ is utilized for enhanced oil recovery.