Desulfurization of hot reformate gas, which is produced by catalytic partial oxidation (CPOX) or autothermal reforming of heavy fuels, such as JP-8, is required prior to feeding the gas in a solid oxide fuel cell (SOFC). This requires development of sorbent materials with favorable sulfidation equilibria, good kinetics, and high structural stability and regenerability at the SOFC operating temperature (650-800^oC) to avoid heat exchanger penalties. Sorbent performance stability in cyclic operation of sulfidation and regeneration at such high temperatures imposes major constraints on the choice of suitable materials.  

Lanthanum or copper oxide doped cerium oxides are being studied in our lab as high-temperature regenerable sorbents for the removal of H₂S upstream of the SOFC anode.^{1, 2} An major finding of our research is that at high temperatures (>650^oC) and at high sulfidation space velocity (> 20,000 h^-1), sulfidation can be limited onto the surface of the sorbent. This surface adsorption is reversible, and thus the sulfided sorbents can be fully regenerated.  

Detailed studies of cyclic sorbent sulfidation/regeneration under different operating conditions have been carried out in our lab. We have shown that under oxidative regeneration conditions, high regeneration space velocities (> 80,000h^-1) can be used to suppress sulfate formation and shorten the total time required for complete sorbent regeneration, and this creates new opportunities for sorber/regenerator reactor designs to meet the requirements of solid oxide fuel cell systems at any scale. Alternatively, other gas mixtures, such as the anode exhaust gas can be used for complete regeneration. To prevent re-adsorption of the desorbed H₂S onto the sorbent bed, cyclic sulfidation/ regeneration tests were carried out with pre-sulfided sorbents. The surface sulfur capacity of the sorbents remained unaffected. Characterization of fresh, sulfided and regenerated samples supports the observation of full sorbent regenerability.

References  

1.      M. Flytzani-Stephanopoulos, Z. Wang and M.Sakbodin, Removal of Hydrogen Sulfide from Hot Reformate Gas using CeO₂-based sorbents, Proceedings of Collaborative Technology Alliances Conference, Power and Energy, CTAC 2005, Crystal City, VA, June 1-3, 2005

2.      Z. Wang and M.Flytzani-Stephanopoulos, Cerium Oxide-Based Sorbents for Regenerative Hot Reformate Gas Desulfurization, Energy and Fuels, web release date: 16-Jun-2005; (Article) DOI: 10.1021/ef049664a