HIGH EFFICIENCY DESULFURIZATION OF SYNTHESIS GAS
Mixed metal oxides containing ceria and zirconia have been studied as high temperature desulfurization sorbents with the objective of achieving the DOE Vision 21 target of 1 ppmv or less H{sub 2}S in the product gas. The research was justified by recent results in this laboratory that showed that reduced CeO{sub 2}, designated CeOn (1.5 < n < 2.0), is capable of achieving the 1 ppmv target in highly reducing gas atmospheres. The addition of ZrO{sub 2} has improved the performance of oxidation catalysts and three-way automotive catalysts containing CeO{sub 2}, and was postulated to have similar beneficial effects on CeO{sub 2} desulfurization sorbents. An electrochemical method for synthesizing CeO{sub 2}-ZrO{sub 2} mixtures was developed and the products were characterized by XRD and TEM during year 01. Nanocrystalline particles having a diameter of about 5 nm and containing from approximately 10 mol% to 80 mol% ZrO{sub 2} were prepared. XRD analysis showed the product to be a solid solution at low ZrO{sub 2} contents with a separate ZrO{sub 2} phase emerging at higher ZrO{sub 2} levels. Unfortunately, the quantity of CeO{sub 2}-ZrO{sub 2} that could be prepared electrochemically was too small to permit desulfurization testing. Also during year 01 a laboratory-scale fixed-bed reactor was constructed for desulfurization testing. All components of the reactor and analytical systems that were exposed to low concentrations of H{sub 2}S were constructed of quartz, Teflon, or silcosteel. Reactor product gas composition as a function of time was determined using a Varian 3800 gas chromatograph equipped with a pulsed flame photometric detector (PFPD) for measuring low H{sub 2}S concentrations from approximately 0.1 to 10 ppmv, and a thermal conductivity detector (TCD) for higher concentrations of H{sub 2}S. Larger quantities of CeO{sub 2}-ZrO{sub 2} mixtures from other sources, including mixtures prepared in this laboratory using a coprecipitation procedure, were obtained. Much of the work during year 02 consisted of characterization and desulfurization testing of materials obtained from commercial sources. Most of the commercial CeO{sub 2} and CeO{sub 2}-ZrO{sub 2} materials were capable of reducing H{sub 2}S concentration from 5000 ppmv in highly reducing feed gas to less than 1 ppmv in the product gas. However, to properly evaluate the effect of ZrO{sub 2} addition on desulfurization capability, the physical properties of the sorbent must be similar. That is, a CeO{sub 2}-ZrO{sub 2} mixture from source A would not necessarily be superior to pure CeO{sub 2} from source B if the properties of the pure CeO{sub 2} were superior. Therefore, research during year 03 concentrated on CeO{sub 2} and CeO{sub 2}-ZrO{sub 2} mixtures prepared in this laboratory using a coprecipitation procedure. The structural properties of these sorbents were similar and the effect of ZrO{sub 2} addition could better be separated from the structural effects. X-ray diffraction tests of the sorbents prepared in house confirmed the formation of a solid solution of ZrO{sub 2} in CeO{sub 2}. Crystallite sizes ranged from 12.7 to 18.8 nm and surface areas from 75 to 85 m{sup 2}/g. Reduction tests using an electrobalance reactor confirmed that CeO{sub 2}-ZrO{sub 2} mixtures were more easily reduced than pure CeO{sub 2}. Reduction of CeO{sub 2}-ZrO{sub 2} began at a lower temperature and the final value of n in CeO{sub n} (1.5 < n < 2.0) was smaller in CeO{sub 2}-ZrO{sub 2} than in pure CeO{sub 2}. Sorbent performance during desulfurization testing was judged both by the minimum H{sub 2}S concentration achieved during the so-called prebreakthrough period and by the duration of the prebreakthrough period. The end of the prebreakthrough period was defined as the time when the H{sub 2}S concentration in the product gas exceeded 1 ppmv. Both CeO{sub 2} and CeO{sub 2}-ZrO{sub 2} sorbents produced in house were capable of reaching the target sub-ppmv H{sub 2}S level in highly reducing gases for extended time periods. H{sub 2}S concentrations were reduced to levels approaching the minimum detectable limit of the PFPD detector, approximately 100 ppbv for time periods corresponding to as much as 60% sorbent sulfidation. The critical test of the sorbents was their performance when the reducing power of the feed gas was decreased by the addition of CO{sub 2} as an oxidant. While sub-ppmv levels of H{sub 2}S were still achieved using both CeO{sub 2} and CeO{sub 2}-ZrO{sub 2} sorbents when the feed gas contained as much as 1% CO{sub 2}, the duration of the prebreakthrough time decreased as the CO{sub 2} concentration increased.
- Research Organization:
- Louisiana State University (US)
- Sponsoring Organization:
- (US)
- DOE Contract Number:
- FG26-00NT40813
- OSTI ID:
- 833407
- Resource Relation:
- Other Information: PBD: 1 Mar 2004
- Country of Publication:
- United States
- Language:
- English
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