| In the reforming process of fuels to produce synthesis gas and high-grade hydrogen for fuel cell or other applications, it is crucial to incorporate a suitable sorbent of sulfur species upstream of the fuel cell to protect the anode material from the deleterious effect of sulfur. In the case of a solid oxide fuel cell (SOFC) operating at temperatures higher than 650°C, the removal of sulfur species should be done also at high temperature to avoid the energy penalty of cooling and reheating the gas stream. The challenge is to identify a sorbent that can efficiently remove H2S and has high reactivity, good structural stability, and good regenerability at such high temperatures. Sorbent materials considered in past have failed in one or more of the aforementioned properties.; Ceria-based materials and lanthanum oxide (La2O3) were selected for this research due to the favorable sulfidation equilibria of reduced cerium oxide (Ce2O3) and La2O 3. These have been found to have regenerability problems when used as bulk oxides or mixed oxides. New preparation procedures and additives in to ceria, such as lanthana, copper oxide or zirconia, were examined to establish baseline performance characteristics of the sorbents and whether the regenerability could thus be improved. All ceria-based materials made by the urea gelation method have relatively high surface area, 40-100m2/g, after 650°C calcination for 4 h in air, while approximately 5m2/g was typical for pure La2O3 due to carbonate formation. Doping with lanthanum and zirconium improved the ceria surface area and its stability in simulated reformate gas mixtures at temperatures in the range of 650-800°C. Doping with copper oxide, however, was found to enhance the sintering of ceria. Addition of copper is beneficial at temperatures below or equal to 650°C, where copper sulfidation improves the sulfidation kinetics of ceria. Doping with lanthanum also increases the sulfidation rate, although not as high as copper doping. Low apparent activation energies (15+/-2 kJ/mol) for sulfidation were measured on all sorbent compositions, indicating that the reaction is adsorption limited.; The problem of sorbent regenerability was not solved by the above cerium oxide modifications. All bulk sorbent materials if sulfided deeply, cannot be fully regenerated by oxygen-laden gas and the process is slow. The sorbent typically loses surface area in cyclic operation. Thus, bulk sorbent sulfidation/regeneration is not practical, requiring big reactors and cumbersome schemes for sulfur recovery from the regenerator units. A different approach was then devised and followed in this thesis. This was based on the observation that sulfidation was a very fast process on ceria and lanthana and that the sorbent had a certain sulfur capacity, non-diminishing as the contact time with the gas was made shorter and shorter. The process became essentially a surface adsorption of H2S, but with the same very high removal efficiency. The adsorbed hydrogen sulfide could fully desorb in any type of gas chosen for regeneration. Thus the adsorption was fully reversible. Then, a new technology can be developed based on a swing adsorption/desorption of H2S at these high temperatures. Moreover, the reactors can be very small, as the adsorption and desorption processes are very fast. By selecting a high space velocity, the bulk of the sorbent will not be regenerated, but this is inconsequential to the process, as it operates with the sorbent surface alone.; The second part of the thesis focused on detailed studies of the adsorption of H2S on the ceria and lanthana oxides. Characterization of the surface compositions, use of presulfided sorbents, and structural investigations of the materials at various stages, and after long cyclic operation were performed to understand the chemistry and to investigate the sorbent stability in this new application as regenerable adsorbents at temperatures as high as 800°C. Kine... |
