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

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Fuel Processing for Fuel Cells 43
requires knowledge of the amount of catalytically active surface area as
discussed above. The total catalytically active surface area of a metal
catalyst is determined experimentally, for example, by chemisorption
measurements. The effect of internal mass transfer resistance of the catalyst,
dispersed in the usually applied porous washcoat, can be included
by an effectiveness factor (Equation (34); Hayes and Kolaczkowski, 1997;
Papadias et al., 2000). However, more accurate models such as the Dusty
Gas Model often need to be applied for an accurate description of the
local reaction rate. For more detailed models for transport in porous
media, readers are referred to the chapter 6 by Kee et al. within this
book and to the relevant literature (Deutschmann et al., 2001; Kee et al.,
2003; Keil, 1999, 2000; Mladenov et al., 2010).
Even though the implementation of elementary reaction mechanisms in
fluid flow models is straightforward, an additional highly nonlinear coupling
is introduced into the governing equations leading to considerable
computational efforts. The nonlinearity, the very large number (thousands)
of chemical species occurring in the reforming of logistic fuels and even in
fuel surrogates, and the fact that chemical reactions exhibit a large range of
time scales, in particular when radicals are involved, render the solving
of those equation systems challenging. In particular for turbulent flows, but
sometimes even for laminar flows, the solution of the system is too CPU
time-consuming with current numerical algorithms and computer capacities.
This calls for the application of reduction algorithms for large reaction
mechanisms, for instance by the extraction of the intrinsic low dimensional
manifolds of trajectories in chemical space (Maas and Pope, 1992), which
can be applied to heterogeneous reactions (Yan and Maas, 2000). Another
approach is to use ‘‘as little chemistry as necessary.’’ In these so-called
adaptive chemistry methods, the construction of the reaction mechanism
only includes steps that are relevant for the application studied (Susnow
et al., 1997).
6.4 Modeling the dynamics of monolithic catalytic reformers
The catalyst-coated monolithic structure given in Figure 3.1 shall serve as
an example of modeling a fuel processor (Maier et al., 2011a). An efficient
approach for modeling such monolithic structures is based on the combination
of simulations of a representative number of single channels with
the simulation of the temperature profile of the solid structure, treating
the latter one as a continuum (Tischer and Deutschmann, 2005; Tischer
et al., 2001). This approach has been implemented, for instance, in the
computer code DETCHEM MONOLITH (Deutschmann et al., 2008), which
can be used to model the dynamic behavior of catalytic monoliths. The
code combines a transient three-dimensional simulation of a catalytic