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Fuel Processing for Fuel Cells 53
1.E-02
5.E-06
8.E-03
4.E-06
Mole fraction
6.E-03
4.E-03
C 3 –C 4 olefins
PAH
3.E-06
2.E-06
2.E-03
1.E-06
0.E+00
0.00
i-C 8 H 18 C 2 H 2
C 6 H 6
0.10
z [m]
0.20
0.E+00
Figure 17 Influence of gas-phase reactions on the catalyst exhaust composition in
the postcatalyst zone of a CPOX reformer (Rh/alumina honeycomb) operated with
iso-octane at rich conditions (C/O¼1.6). The catalyst exhaust composition measured was
fed into an empty tube heated by a furnace to a temperature of 1106 K, which was the
catalyst exit temperature measured in the CPOX reactor. The figure shows the numerically
predicted distribution of carbon precursors along the empty tube, thereby clearly
revealing postcatalyst conversion in the downstream gas phase. C 3 –C 4 olefins contain 1,2-
propadiene, propene, propyne, n-butene (1-butene, 2-butene), iso-butene, and 1,3-butadiene;
PAH contains naphthalene, anthracene, and pyrene. The embedded photo shows the
tubular quartz reactor after operation. Taken from Kaltschmitt et al. (2011).
and selectivity. An adiabatic single-channel simulation will fail; it is not
able to predict even the qualitative behavior, that is, the increase of the
catalytic exit temperature with increasing flow rate as shown in Figure 18
(Hartmann et al., 2011). Instead, the entire catalytic monolith with at least
several representative channels has to be considered, including heat
transport within the solid structure of the monolith and at all monolith
boundaries. The increase in temperature with increasing flow rates can be
explained by the effect of heat losses. The total amount of heat released by
the reaction almost linearly increases with the flow rate, because the fuel is
fully converted in the first zone of the catalyst. However, since higher
temperature favors the less exothermic POX over the highly exothermic
total oxidation, a self-limiting process concerning the temperature increase
occurs. Consequently, the temperatures do not increase extraordinarily
with increasing flow rate. The total amount of heat loss to the ambience by
means of thermal conduction and radiation mainly depends on the temperature
of the solid structure, which indeed is higher, but not so much higher
as to compensate for the larger heat release effect. In short, the ratio of
chemical heat release to thermal heat loss increases with increasing flow