Coking and sulfur poison

As discussed earUer in Section 2.1.2, conventional cermet anodes suffer from issues of redox stability, coking and sulfur poisoning. In an effort to develop high durabiUty anodes that overcome these limitations the modification of typical cermet materials to offer greater resistance to coking and sulfur poisoning has been considered. Elsewhere novel ceramic anodes have been proposed to achieve the same objective. Each of these potential materials solutions are attractive and will be addressed in turn. 

Results from these and other laboratories indicate that coke is ubiquitous on any metal surface (that has not been recently atomically cleaned). At low coverages, i.e., less than half a monolayer, coke exists as isolated 2-dimensional islands. As the coverage increases, these islands may coalesce into nonuniform graphitic clumps. Even at monolayer coverages of C, the adsorption of, say, benzene is noticeable. This is in contrast to the behavior (described below) when sulfur is adsorbed very small coverages of sulfur may render the surface incapable of adsorption, probably because the sulfur is uniformly distributed while C is present in 3-dimensional clumps. Clearly, qualitatively different selectivities are to be expected during the deactivation by coke and by poisons. 

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. 

Metal oxides, sulfides, and hydrides form a transition between acid/base and metal catalysts. They catalyze hydrogenation/dehydro-genation as well as many of the reactions catalyzed by acids, such as cracking and isomerization. Their oxidation activity is related to the possibility of two valence states which allow oxygen to be released and reabsorbed alternately. Common examples are oxides of cobalt, iron, zinc, and chromium and hydrides of precious metals that can release hydrogen readily. Sulfide catalysts are more resistant than metals to the formation of coke deposits and to poisoning by sulfur compounds their main application is in hydrodesulfurization. 

The residual portion of feedstocks contains a large concentration of contaminants. The major contaminants, mostly organic in nature, include nickel, vanadium, nitrogen, and sulfur. Nickel, vanadium, and sodium are deposited quantitatively on the catalyst. This deposition poisons the catalyst permanently, accelerating production of coke and light gases. 

There are three major gas reformate requirements imposed by the various fuel cells that need addressing. These are sulfur tolerance, carbon monoxide tolerance, and carbon deposition. The activity of catalysts for steam reforming and autothermal reforming can also be affected by sulfur poisoning and coke formation. These requirements are applicable to most fuels used in fuel cell power units of present interest. There are other fuel constituents that can prove detrimental to various fuel cells. However, these appear in specific fuels and are considered beyond the scope of this general review. Examples of these are halides, hydrogen chloride, and ammonia. Finally, fuel cell power unit size is a characteristic that impacts fuel processor selection. 

Nickel aluminate, a spinel, has long been known to trap nickel. Metals like arsenic(19), antimony(20-21) and bismuth(20) are known to passivate transition elements and can be used to decrease and coke make. Sulfur is also a known inhibitor for nickel therefore, higher sulfur-containing crudes may be a little less sensitive to nickel poisoning. In our work we also found that nickel at low concentrations is actually a slight promoter of the cracking reaction when incorporated into a molecular sieve (Figure 17). 

Metal location is but one of a number of applications for scanning electron microscope studies in catalysis. Other applications are the study of the morphology of platinum-rhodium gauzes used in the oxidation of ammonia and the poisoning of catalysts, in which the scanning electron microscope results show the location of poisons such as compounds containing sulfur, phosphorus, heavy metals, or coke relative to the location of the catalytic components. 

Using fixed dolomite guard beds to lower the input tar concentration can extend Ni catalyst lifetimes. Adding various promoters and support modifiers has been demonstrated to improve catalyst lifetime by reducing catalyst deactivation by coke formation, sulfur and chlorine poisoning, and sintering. Several novel, Ni-based catalyst formulations have been developed that show excellent tar reforming activity, improved mechanical properties for fluidized-bed applications, and enhanced lifetimes. 

The recent accomplishments of near-edge X-ray absorption spectroscopy in catalysis studies are already quite impressive, in particular if one considers the limited availability of suitable X-ray spectrometers. Developments of catalytic interest have concerned the Shell Higher Olefin process, size effects, metal-support interaction, mono- and bimetallic catalysts (in particular the PtRe/Al203 system), the reactivity of supported metal catalysts, dynamical and in situ catalyst studies, and a variety of oxide and sulfide catalysts. Other catalytic problems are now coming within easy experimental reach, such as the study of sulfur poisoning and the nature of coking. 

In summary, extraction with carbon dioxide, pyridine and sulfur dioxide can remove the coke from catalyst. The amount of coke removed depends on the extraction temperature, pressure and duration. Consecutive extractions with two solvents appear to remove more coke than the individual solvents do. Adsorption of certain solvents on the catalyst during extraction can poison the catalyst. Therefore, if poisoning solvents are used for decoking, their remains must be removed from the extracted catalyst to restore the catalyst activity. 

The catalyst is sensitive to sulfur and arsenic poisoning (the Utter being a permanent poison). Natural gas must, therefore be desulfurized. Carbon and coke deposits also damage the catalyst and must be removed by steam or by burning off with air. 

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