Carbon formation in steam reforming

There are several ways to reduce the risk of carbon formation in steam reforming reactions. Some of the approaches include ... 

Fig. 6. Carbon Formation in Steam Reforming of Butane (6). H20/C Hio= 0.5 mol/C-atom. 1 bar.

The mechanism of formation of whisker carbon has been studied over the last 30 years. It is the main route for carbon formation in steam reforming. The understanding of the mechanism has been the basis for design principles for carbon-free operation and for optimum catalyst formulation. Recent work has confirmed the importance of surface steps for carbon formation and given new ideas for promotion of the catalyst by inhibition of full dissociation of methane. 

Gold is not dissolved in nickel and the addition of even small amounts of gold to the nickel surface resulted in elimination of carbon formation in steam reforming of butane (15,22). This result could be explained by DFT-calculations and showing that gold increases the energy barrier for the dissociation of methane (23) (the DFT-calculations also showed that small additions of copper decreases the barrier). These results were verified by molecular beam scattering experiments on well-defined... 

Carbon formation and catalyst deactivation have also been observed in the SRE. Approaches similar to those discussed above for the steam reforming of hydrocarbons are also employed to suppress the carbon formation in ethanol reforming as well.1 7... 

Carbon formation on steam reforming catalysts takes place in three different forms whisker-like carbon, encapsulated carbon, and pyrolytic carbon as described in Table 2.2 [1]. Whisker-like carbon grows as a fiber from the catalyst surface with a pear-shaped nickel crystal on the end. Strong fibers can even break down catalyst particles increasing the pressure drop across the reformer tubes [4], The carbon for whisker formation is formed by the reaction of hydrocarbons as well as CO over transition metal catalysts [1], The whisker growth is a result of diffusion through the catalyst and nucleation to form a long carbonaceous fiber. 

For higher CH4 content, more steam and CO2 at the anode reduce power and fuel utilization of the stack, thus a small (0/C)Ref ratio should be preferred for high system efficiency. On the other hand, the risk of carbon formation in the reformer increases with lower (0/C)Ref, so a compromise between system efficiency and a safe operation has to be found. Thermodynamic calculation indicated that for soot free operation an (0/C)Rer above 2.5 at temperature > 625 °C has to be assured. ... 

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]. 

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-... 

These reactions are reversible, and there is a dynamic equilibrium between carbon formation and removal. Under typical steam reforming conditions, reactions (46) and (48) are carbon - removing, whilst reaction (47) leads to carbon formation in the upper part of the tube [503]. With naphtha as steam reformer feed, irreversible pyrolysis (as in a steam cracker for ethylene production) with the sequence naphtha —> olefins—> polymers—- coke will occur. The mechanism of carbon formation and the determination of the risk areas in the reformer operating conditions on the basis of relevant equilibrium data are discussed in some detail in various publications [362], [363], [418]-[420]. 

Shift Conversion. Improved LT shift catalysts can operate at lower temperatures to achieve a very low residual CO content and low byproduct formation. A new generation of HT shift catalysts largely avoids hydrocarbon formation by Fischer-Tropsch reaction at low vapor partial pressure, thus allowing lower steam to carbon ratio in the reforming section (see Section 4.2.1.1.1). 

The steam reforming of hydrocarbon feedstocks is a common industrial process which produces hydrogen for use in methanol or ammonia synthesis. A variety of hydrocarbons, e.g. natural gas or naphthas, can be used as the reactant in the steam reforming process, This use of a variety of reactant feed types places considerable demands upon the catalyst manufacturer since all hydrocarbons have different reactivities and, most importantly, disparate tendencies to generate carbonaceous deposits, ICI produce a range of catalysts for use with a number of hydrocarbon reactants. For the reforming of heavy naphtha feedstocks, which show a considerable propensity for carbon deposition, ICI provides a potassium promoted nickel based catalyst (ref 1). The object of this paper is to describe the mechanism by which alkali provides resistance to carbon formation in nickel catalysts. 

Carbon deposition is one of the luost serious problems of the steam reforming catalyst process (ref 1). The deposition of carbon on naphtha steam reforming catalysts depends ori the chemical composition of the hydrocarbon oil, the steam/carbon ratio in the feedstock, as well as the pi ocesa temperature and pressure, it is also affected by tlie presence of sulfur poisons Our past research of SNG catalysts ejiamined the nature of the carbon deposits as a function of the sulfur level on the catalyst (refs, 2 4). A small amount of sulfur was found to promote the formation of carbon that is non-reactive with steam and hydrogen under steam reforming reaction conditions. The continuous accumulation of this less reactive carbon [continuous carbon deposition (CCD)l on the catalyst surface leads to coke fouling Studies of the occurrence of CCD in our laboratory tests allow ua to predict, that coke fouling is likely to occur on the same catalyst used in real Indusl.rlal applications. 

Figure 2.32. Thermodynamic minimum S/C ratios required to prevent carbon formation in diesel, jet fuel, gasoline, and methane steam reforming.

As has been discussed in Section 2.3.6, the use of an adiabatic prereformer also helps in alleviating the risk of carbon formation in the steam reforming of higher... 

Higher hydrocarbons do not exist at equilibrium and any risk of whisker formation from these compounds can be disregarded at these conditions. Nevertheless whiskers may still form from higher hydrocarbons because at nonequilibrium conditions a potential for the irreversible carbon formation [e.g.. Reaction (11) in Table 3] may exist. The formation of whisker carbon at these conditions depends on a kinetic balance between the rate of the carbon forming and steam-reforming reactions. A simplified reaction sequence outlining the kinetic balance is shown in Fig. 8. The key step is whether the adsorbed hydrocarbon species will react to form adsorbed carbon and whiskers or react with oxygen species to produce gas. ... 

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