Fuel Processing for Fuel Cells - Institut für Technische Chemie und ...

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46 Torsten Kaltschmitt and Olaf Deutschmann
reaction scheme, such as the one given in Section 4.2, little insight can be
gained from such a lumped mechanism. Therefore, it is recommended to
apply chemical models based on elementary-step reaction mechanisms
for understanding the reaction routes.
7.1.1 Reaction networks in the reforming of methane
For reaching a profound understanding of the reaction mechanism of
synthesis gas formation from methane by SR, POX, ATR, and DR, the
sequence and interaction of the reaction routes have to be analyzed for
the combined POX-SR-DR systems, because the conditions in any flow
reactor vary along the flow directions, covering a wide range of mixture
compositions and leading to quite diverse local reaction rates. There has
been a long discussion in literature on the reaction routes in CPOX of
methane over Rh and Pt catalysts at short contact times (Deutschmann
et al., 2001; Hickman and Schmidt, 1993a,b; Horn et al., 2007, 2006;
Schwiedernoch et al., 2003). The crucial point of this discussion was on
the formation route of hydrogen. One possible route is direct POX. The
other one is an indirect route, in which, at first, total oxidation occurs and
subsequently hydrogen is formed by SR of the remaining fuel. Detailed
reaction schemes for the CPOX of methane over platinum and rhodium,
which also include steps for SR, were published by Schmidt et al.(Hickman
and Schmidt, 1993b), Vlachos et al. (Maestri et al., 2008; Mhadeshwar and
Vlachos, 2005, 2007; Mhadeshwar et al., 2003), and Deutschmann et al.
(Deutschmann et al., 1994, 1996; Quiceno et al., 2006; Schwiedernoch et al.,
2003). A unified mechanism covering all aspects of SR and DR, POX and
total oxidation, carbon formation, and catalyst oxidation is still under
construction. Recently obtained new experimental data for DR (McGuire
et al., 2011) call for further adaption of the kinetic schemes available.
Based on a variety of experimental and modeling techniques, a general
consensus is now achieved on the reaction routes in CPOX over Rh catalysts:
In a quasi two-step process (indirect route), first CH 4 is completely
oxidized to CO 2 and steam, as long as oxygen is present close to the catalyst
surface, and then the remaining CH 4 is reformed with the steam to synthesis
gas (Hannemann et al., 2007; Horn et al., 2010; Schwiedernoch et al.,
2003). DR does not play any significant role and the surface acts as a sink for
radicals, inhibiting significant gas-phase reactions at pressures below 10
bars (Quiceno et al., 2006). The reaction is mass transport limited, in particular
in the total oxidation zone of the reactor, in which the surface reaction
rate is very fast in comparison to diffusion of oxygen to the channel wall,
which leads to almost zero oxygen concentrations at the gas–catalyst interphase.
The decisive proof for this reaction route came from the application
of in situ techniques to determine spatially resolved species and temperature
profiles inside the reactor. The optically accessible catalytic channel