Steam reforming Carbon formation

Carbon produced by these latter reactions is formed in the catalyst pores, making it much more difficult to remove, and potentially causing physical breakage. Operating steam to carbon ratios are chosen above the minimum required in order to make carbon formation by these reactions thermodynamically impossible (3). Steam is another potential source of contaminants. Chemicals from the boiler feedwater or the cooling system are poisons to the reformer catalyst, so steam quality must be carefully monitored. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Steam reforming is the reaction of steam with hydrocarbons to make town gas or hydrogen. The first stage is at 700 to 830°C (1,292 to 1,532°F) and 15-40 atm (221 to 588 psih A representative catalyst composition contains 13 percent Ni supported on Ot-alumina with 0.3 percent potassium oxide to minimize carbon formation. The catalyst is poisoned by sulfur. A subsequent shift reaction converts CO to CO9 and more H2, at 190 to 260°C (374 to 500°F) with copper metal on a support of zinc oxide which protects the catalyst from poisoning by traces of sulfur. 

There aie a number of major indusuial problems in the operation of the steam reforming of metlrane. These include the formation of carbon on the surface of the catalyst, the sulphidation of the catalyst by the H2S impurity in commercial natural gas, and die decline of catalytic activity due to Ostwald ripening of the supported catalyst particles by migration of catalyst atoms from the smaller to tire larger particles, as the temperamre is increased. A consideration of tire thermodynamics of the principal reaction alone would suggest that the reaction shifts more favourably to the completion of the reaction as the temperature is increased. 

In the steam reforming reaction, /rC02/p C0 = 2.08 (hr = 5.64) at this temperamre, and at lOOOK the results are hr A = 0.55 for carbon formation, and the PCO2/p CO ratio is 0.155 (hr A = 1.16), and tlrus the tendency for carbon formation passes from zero to unity in tlris temperature range. The presence of CO2 is not indicated in this reaction as given above, but its partial pressure can be obtained from data for the concunent reaction... 

The steam reforming catalyst is very robust but is threatened by carbon deposition. As indicated in Fig. 8.1, several reactions may lead to carbon (graphite), which accumulates on the catalyst. In general the probability of carbon formation increases with decreasing oxidation potential, i.e. lower steam content (which may be desirable for economic reasons). The electron micrograph in Fig. 8.4 dramatically illustrates how carbon formation may disintegrate a catalyst and cause plugging of a reactor bed. 

It is important to note that the selectivity of sulfur-passivated catalysts towards steam reforming is greatly enhanced because carbon formation is effectively suppressed. The decrease in activity can to largely be compensated for by selecting inherently more active catalysts and by operating at higher temperatures. Unfortun-... 

An addition of the supplemental steam shifts the reforming reaction equilibrium away from carbon formation. 

Snoeck, J.-W., Froment, G. F., and Fowles, M. 2002. Steam/C02 reforming of methane. Carbon formation and gasification on catalysts with various potassium contents. Ind. Eng. Chem. Res. 41 3548-56. 

Another cause of activity loss is carbon deposition, which can be avoided if a high steam to carbon (S/C) ratio is employed [45, 46], However, economic evaluations indicate that the optimum S/ C ratio tends to be low. The presence of tars in the reforming reactor enhances coking and it is the main cause of carbon formation in reforming a gas from biomass thermal conversion [29]. 

One of the possible problems in a steam reforming membrane reactor is the formation of carbon, either by cracking of methane (Reaction 4) or the Bou-douard reaction (Reaction 5). 

Whether a driving force for carbon formation exists is dictated by thermodynamics, and so it is dependant on reaction temperature and pressure, and the H/C and O/C ratios in the system (Fig. 14.6). By removing hydrogen from the reaction zone, the H/C ratio decreases and the system moves closer to the thermodynamic area where carbon formation is likely. The arrow in Fig. 14.6 labeled SMR is equal to a reforming mixture with a steam-to-carbon ratio of 3. When hydrogen... 

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. 

Carbonaceous compounds can also form in the absence of a catalyst by free-radical, gas-phase condensation reactions. The formation of this pyrolytic carbon is known in steam-reforming reactors where it can be controlled to some extent by minimizing the free volume within the reactor chamber. This type of carbon does not form readily with methane but can be severe with larger hydrocarbons. The compounds formed by free-radical reactions tend to be quite different from the graphitic carbon formed by metal catalysts. For example, Lee et al. showed that the compounds formed by passing pure, undi-... 

It was observed that the water content does not influence ethylene formation. When 5% Rh is added to alumina, the main steam reforming reaction occurs above 460 °C and the products include hydrogen, carbon dioxide, carbon monoxide and methane. 

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