Carbon Formation and Pre-Reforming

One of the most critical issues in fuel cell systems is the risk of carbon formation from the fuel gas. This can occur in several areas of the system where hot fuel gas is present. Natural gas, for example, will decompose when heated in the absence of air or steam at temperatures above about 650°C via pyrolysis reactions of the type 

Similar reactions can be written out for other hydrocarbons. Higher hydrocarbons tend to decompose more easily than methane and therefore the risk of carbon formation is higher with vaporised liquid petroleum fuels than with natural gas. Another source of carbon formation is from the disproportionation of carbon monoxide via the so-called Boudouard reaction  

This reaction is catalysed by metals such as nickel, and therefore there is a high risk of it occurring on steam reforming catalysts that contain nickel and on nickel-containing stainless steel used for fabricating the reactors. Fortunately, there is a simple expedient to reduce the risk of carbon formation from reactions 8.8 and 8.9, and that is to add steam to the fuel stream. The principal effect of this is to promote the shift reaction 8.5, which has the effect of reducing the partial pressure of carbon monoxide in the fuel gas stream. Steam also leads to the carbon gasification reaction, which is also very fast  

The minimum amount of steam that needs to be added to a hydrocarbon fuel gas to avoid carbon deposition may be calculated. The principle here is that it is assumed that a given fuel gas/steam mixture reacts via reactions 8.3, 8.4, and 8.5 to produce a gas that is at equilibrium with respect to reactions 8.3 and 8.5 at the particular temperature of operation. The partial pressures of carbon monoxide and carbon dioxide in this gas are then used to calculate an equilibrium constant for the Boudouard reaction 8.9. This calculated equilibrium constant is then compared with what would be expected from the thermodynamic calculation at the temperature considered. If the calculated constant is greater than the theoretical one, then carbon deposition is predicted on thermodynamic grounds. If the calculated constant is lower than theory predicts, then the gas is said to be in a safe region and carbon deposition will not occur. In practice, a steam/carbon ratio of 2.0 to 3.0 is normally employed in steam reforming systems so that carbon deposition may be avoided with a margin of safety. 

A particular type of carbon formation occurs on metals, known as carburisation, leading to spalling of metal in a phenomenon known as metal dusting . Again, it is important to reduce the risk of this in fuel cell systems, and some developers have used copper-coated stainless steel in their fuel gas preheaters to keep the risk to a minimum. 

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