Sulfur tolerance

The formation of sulfides is especially the case for nickel-based OCs [31], Continuous operation experiments in a circulating fluidized bed system with CH4 and nickel-based carriers demonstrated that fuels with a sulfur content below 100 ppm, H2S can be tolerated in an industrial plant [31], At higher H2S concentrations, a desulfurization step might be required before the fuel is fed to the CLC system. Copper has been demonstrated to be more tolerant of sulfur than nickel. During continuous operation, most of the sulfur was released in the fuel reactor as H2S and SO2 [32]. 

If coal is directly fed to the fuel reactor, copper- and iron-based OCs are preferred, because they are harmless if they are mixed with residual ashes [9]. In Ref. [25] it is demonstrated that in case the CLC system is fuelled with petroleum coke and ilmenite is used as OC, all the sulfur is released as H2S and SO2 in the fuel reactor. Thus, with ilmenite and petroleum coke, no sulfur poisoning problems are expected. 

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Dr. Woodward I tried to indicate in my paper that in ammonia-hydrogen plant operation, in comparison with several other catalysts in such plants, the methanation catalyst situation is really well under control. Speaking for our company, and I would guess others, it s not a particularly active research area because we have higher priorities in catalyst development. As regards methanation catalysts for SNG, I did not discuss that today and perhaps I should let some other fellows answer first. Sulfur tolerance is one area for future development. 

A. Vannice ( Exxon Research and Engineering Co.) Would a more sulfur-tolerant catalyst be a significant improvement in the overall methanation process If so, what would be the maximum tolerable sulfur concentrations in the feed stream ... 

D. Newsome (Virginia Polytechnic Institute and State University) Almost all the talks today are concerned with nickel catalyst. Is there any place for a somewhat less active but sulfur-tolerant catalyst ... 

A. Hausberger I think a sulfur-tolerant catalyst would definitely be an advantage in that the requirement for critical control of the sulfur removal system would be eliminated. If you can allow some sulfur to pass on through the methanator into the product gas, the amount of reagent or regeneration cost of the sulfur removal system would be reduced. As to what level of sulfur could be tolerated, that is a hard question to answer since I don t think that there is a sulfur-tolerant catalyst. 

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

Fanson, P.T., Horton, M.R., Delgass, W.N. et al. (2003) FTIR Analysis of Storage Behavior and Sulfur Tolerance in Barium-Based NOx Storage and Reduction (NSR) Catalysts, Appl. Catal. B Environ., 46, 393. 

With tethering technology, immobilized Rh(DiPFc) catalyst, a veiy selective hydrogenation catalyst was developed by Engelhard (26) and Chirotech (27). Rh(DiPFc)(COD)BF4 is one of most interesting homogeneous catalysts due to its chemo-selectivity and sulfur tolerant behavior. The anchored complex y-... 

Kamakoti, P., B.D. Morreale, M.V. Ciocco, B.H. Howard, R.P. Killmeyer, A.V. Cugini, and D.S. Sholl, Prediction of hydrogen flux through sulfur-tolerant binary alloy membranes, Science, 307, 569-573,2005. 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

In recent years, there have been numerous studies on alternate anode materials. The areas of interest include carbon-tolerant anode materials, sulfur-tolerant anode materials, and redox-stable anode materials. The idea is that by developing alternative anode materials and structure, the reforming and the desulfurization unit could be eliminated, which would reduce the system complexity and cost dramatically. In this section, the studies into these new, alternative anode materials will be briefly touched upon. Because the number of candidate materials studied is quite large, the amount of study on any individual candidate anode material is rather small, and not much work has been done to reproduce the results reported. Therefore, it is not possible to fully evaluate the real potentials of those new materials proposed by different groups of researchers. Therefore, the focus would be on the fundamental issues for these alternative materials, instead of on the processing and properties of a specific candidate material. 

Summary of Previous Studies on Potential Sulfur-Tolerant Anode Materials for Solid Oxide Fuel Cells... 

Gong M, Liu X, Trembly J, and Johnson C. Sulfur-tolerant anode materials for solid oxide fuel cell application. J Power Sources 2007 168 289-298. 

Sasaki K, Susuki K, Iyoshi A, Uchimura M, Imamura N, Kusaba H et al. Sulfur tolerance of solid oxide fuel cells. In Singhal SC, Mizusaki J, editors. Proceedings of the Ninth International Symposium on Solid Oxide Fuel Cells (SOFC-IX), Pennington, NJ The Electrochemical Society, 2005 2005(7) 1267-1275. 

Kurokawa H, Sholkalapper TZ, Jacobson CP, De Johghe LC, and Visco SJ. Ceria nanocoating for sulfur tolerant Ni-based anodes of solid oxide fuel cells. Electrochem... 

Choi YM, Compson C, Lin MC, and Liu M. Ab initio analysis of sulfur tolerance of Ni, Cu, and Ni-Cu alloys for solid oxide fuel cells. J Alloys Compd 2007 427 25-29. 

Mukundan R, Brosha EL, and Garzon FH. Sulfur tolerant anodes for SOFCs. Electrochem Solid-State Lett 2004 7 A5-A7. 

Aguilar L, Zha S, Li S, Winnick J, and Liu M. Sulfur-tolerant materials for the hydrogen sulfide SOFC. Electmchem Solid-State Lett 2004 7 A324—A326. 

Zha S, Tsang P, Cheng Z, and Liu M. Electrical Properties and Sulfur Tolerance of La075Sr025Cr1.xMnxO3 under Anodic Conditions. J Solid State Chem 2005 178 1844-185o ... 

Zha S, Cheng Z, and Liu M. A sulfur-tolerant anode materials for SOFCs GdjTij 4Mo06O7. Electrochem Solid-State Lett 2005 8 A406-A408. 

Choi S, Wang J, Cheng Z, and Liu M. Surface modification of Ni-YSZ using niobium oxide for sulfur-tolerant anodes in solid oxide fuel cells. J Electrochem Soc 2008 155 B449-B454. 

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

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