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Fuel Processing for Fuel Cells 55
ambience is smaller for higher flow rates (Figure 18). However, the heat
loss effect clearly overlaps with the flow rate effect. At low flow rates,
more exterior channels of the monolith experience much lower temperatures
than at high flow rates and, consequently, hydrogen production is
reduced. In addition to SR, the exothermic WGS reaction also leads to
hydrogen production; this effect is rather small but still has some impact
on the final product composition and catalyst outlet temperature
(Hartmann et al., 2011). In summary, an understanding of the dependence
of hydrogen selectivity on the flow rate can only be achieved by taking
mass and heat transfer as well as detailed kinetic schemes (reaction pathways)
into account.
7.4 Understanding the dynamics of catalytic monoliths—CPOX
of methane
In reformers, a highly dynamic behavior is observed during light-off and
shut-down. This transient behavior during light-off of the reaction has
been studied in detail for CPOX of methane in a Rh-coated honeycomb
reformer. Applying the code DETCHEM MONOLITH described in Section 6,
the transient behavior of the temperature of the solid structure of the
catalyst and the processes in the single channels can be computed.
Figure 19 shows the time-resolved temperature and species profiles in a
single channel of a catalytic monolith for POX of methane for the production
of synthesis gas and the temperature distribution of the solid structure
during light-off (Schwiedernoch et al., 2003).
7.5 Model-based optimization of fuel processor design
In technical systems, overall efficiency is increased by heat integration,
which can be realized either directly or indirectly. Figure 20 shows the
schematic of a potential technical application, in which the hot exhaust
of the SOFC stack at approximately 800 C is led through the casing of
the reformer to improve the heat balance (Goldin et al., 2009). In the case of
co-feeding part of the exhaust directly to the fuel/air mixture, one has to
be aware of the fact that the oxygen contained in steam and in particular
in carbon dioxide may show different reactivity than the oxygen of O 2
(Kaltschmitt et al., 2012).
8. SUMMARY AND CONCLUSIONS
First of all, many items have to be considered when converting the
chemical feedstock available in nature such as natural gas, crude oil,
coal, and ‘‘regenerative’’ sources (biomass) into fuels that fuel cells can