High Efficiency Desulfurization of Synthesis Gas

Abstract

Mixed metal oxides containing CeO 2 and ZrO 2 are being studied as high temperature desulfurization sorbents capable of achieving the DOE Vision 21 target of 1 ppmv or less H 2 S. The research is justified by recent results in this laboratory that showed that reduced CeO 2 , designated CeO n (1.5 < n < 2.0), is capable of achieving the 1 ppmv target in highly reducing gas atmospheres. The addition of ZrO 2 has improved the performance of oxidation catalysts and three-way automotive catalysts containing CeO 2 , and should have similar beneficial effects on CeO 2 desulfurization sorbents.An electrochemical method for synthesizing CeO 2 -ZrO 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 2 were prepared. XRD showed the product to be a solid solution at low ZrO 2 contents with a separate ZrO 2 phase emerging at higher ZrO 2 levels. Unfortunately, the quantity of CeO 2 -ZrO 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 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 2 S concentrations (<~10 ppmv) and a thermal conductivity detector (TCD) for higher concentrations of H 2 S.Larger quantities of CeO 2 -ZrO 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 2 addition on desulfurization capability, the physical properties of the sorbent mixtures must be similar. That is, a CeO 2 -ZrO 2 mixture from source A would not necessarily be superior to pure CeO 2 from source B if the properties were dissimilar. Therefore, current research is concentrating on CeO 2 and CeO 2 -ZrO 2 mixtures prepared in this laboratory using the coprecipitation procedure. The structure of these sorbents is similar and the effect of ZrO 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 2 in CeO 2 . Reduction tests using an electrobalance reactor have confirmed that CeO 2 -ZrO 2 mixtures are more easily reduced than pure CeO 2 . Reduction of CeO 2 -ZrO 2 begins at a lower temperature and the final value of n in CeO n (1.5 <n < 2.0) is smaller in CeO 2 -ZrO 2 than in pure CeO 2 . 700 o C desulfurization tests have shown that both CeO 2 and CeO 2 -ZrO 2 sorbents are capable of reaching the target sub-ppmv H 2 S level in high...