HIGH EFFICIENCY DESULFURIZATION OF SYNTHESIS GAS

Mixed metal oxides containing CeO{sub 2} and ZrO{sub 2} are being studied as high temperature desulfurization sorbents capable of achieving the DOE Vision 21 target of 1 ppmv or less H{sub 2}S. The research is justified by recent results in this laboratory that showed that reduced CeO{sub 2}, designated CeO{sub n} (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 should have similar beneficial effects on CeO{sub 2} desulfurization sorbents. An electrochemical method for synthesizing CeO{sub 2}-ZrO{sub 2} 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 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 full testing in our desulfurization reactor. Also during year 01 a laboratory-scale fixed-bed reactor was constructed for desulfurization testing. All components of the reactor and analytical systems that may be exposed to low concentrations of H{sub 2}S are constructed of quartz, Teflon, or silcosteel. Reactor product gas composition as a function of time is determined using a Varian 3800 gas chromatograph equipped with a pulsed flame photometric detector (PFPD) for measuring low H{sub 2}S concentrations ({approx}< 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, have been obtained. Characterization and desulfurization testing of these sorbents began in year 02 and is continuing. To properly evaluate the effect of ZrO{sub 2} addition on desulfurization capability, the physical properties of the sorbent mixtures 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 were dissimilar. Therefore, current research is concentrating on CeO{sub 2} and CeO{sub 2}-ZrO{sub 2} mixtures prepared in this laboratory using the coprecipitation procedure. The structure of these sorbents is similar and the effect of ZrO{sub 2} addition can be separated from other effects. X-ray diffraction tests of the sorbents prepared in house have confirmed the existence of a solid solution of ZrO{sub 2} in CeO{sub 2}. Reduction tests using an electrobalance reactor have confirmed that CeO{sub 2}-ZrO{sub 2} mixtures are more easily reduced than pure CeO{sub 2}. Reduction of CeO{sub 2}-ZrO{sub 2} begins at a lower temperature and the final value of n in CeO{sub n} (1.5 <n < 2.0) is smaller in CeO{sub 2}-ZrO{sub 2} than in pure CeO{sub 2}. 700 C desulfurization tests have shown that both CeO{sub 2} and CeO{sub 2}-ZrO{sub 2} sorbents are capable of reaching the target sub-ppmv H{sub 2}S level in highly reducing gases. Some CeO{sub 2}-ZrO{sub 2} sorbents have successfully removed H{sub 2}S to the minimum detectable limit of the PFPD detector, approximately 100 ppbv