In methane steam reforming, methane is reacted in a highly endothermic reaction with steam over a catalyst, typically based on nickel, at high temperatures (800-1000 °C, 20-30 atm)  to form CO and H2. A part of the CO formed reacts consequently with steam in the water gas shift (WGS) reaction to yield more H2 and also CO2. The gas obtained is thus a mixture of H2, CO and CO2.
The concentration of the various components depends on the reaction conditions: temperature, pressure and H2O/CH4 ratio. Syn-gas generation from methane however increases with increasing temperature and decreasing pressure. With increasing temperatures, the WGS reaction becomes also less domi nant and the main products are CO and H2 . Since the overall methane steam reforming process is highly endothermic, heat must be supplied to the system by burning a part of the natural gas used as feedstock. Steam reforming is the most widely used technology to produce not only syn-gas for methanol synthesis but also ammonia and other bulk chemicals. The stoichiometric number S obtained by steam reforming of methane is close to 3, far from the desired value of 2. This can generally be corrected by addition of CO2 to the steam refor-mefs exit gas or use of excess hydrogen in some other process such as ammonia synthesis.
Methane reforming can also be affected by a process called "coking"; this involves the formation of carbon, which may deposit in the form of soot or coke on the catalyst (greatly reducing its activity) as well as all the internal parts of the reformer and downstream equipment, resulting in possible clogging. Carbon may be formed by CH4 decomposition or CO disproportionation (Boudouard reaction).
In practice, the undesired formation of carbon is largely prevented by the use of excess steam and short residence times in the reactor. It can, however, be more problematic for the partial oxidation process operating at higher temperature than methane reforming.
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