Thesis for degree: Licentiate of Engineering

cathode are assumed to be similar to the case of the reactor bed. The aim of this case study is
to examine whether the kinetics used for previous models [1] fulfill these criteria so no
limiting effects occur for the heat and mass transport.
The macroscale (2D) computational fluid dynamics (CFD) model of an intermediate
temperature anode-supported SOFC operating on 30% pre-reformed natural gas is the base
for the calculations performed here. First, the criteria for the different domains are described
and defined below. Then, the results of the analysis for the SOFC are presented in the next
chapter.
3.3.1 Interparticle transport
The largest scale in this analysis is for the interparticle transport which is also sometimes
called intrareactor scale because it applies to gradients within the reactor as a whole.
Transport phenomena can occur both radially and axially within the reactor and these are hard
to control and evaluate. For the SOFC the axial direction refers to the main flow direction (xdirection)
and the radial direction refers to the direction normal to the main flow direction (ydirection).
But if neglected, radial temperature gradients can force the reaction rates to be
thousandfold greater for parts of the reactor often close to the axis [49]. For the SOFC this
would occur in the anode and near the electrolyte close to the inlet of the cell and would mean
a risk for disturbing “hot spots”. This can be checked by radial dispersion [50]:
(3.38)
where Bi R is the Biot number based on the reactor diameter, ΔH the enthalpy change of
reaction, r r the reaction rate, R o the reactor diameter, k e the thermal conductivity for the solid
porous media, T w the temperature at the solid surface and γ the dimensionless activation
energy. The axial dispersion is a less frequent limitation in a severe manner. Axial
temperature gradients and axial conduction are possible to neglect if the length-to-particle
diameter ratio is large enough (L/d p > 30) which is the case for SOFCs [50]. The criterion for
the limitation for the temperature gradient across the reactor diameter is defined as [49]:
(3.39)
where the parameters are the same as for equation (3.38) except the Biot number, R the gas
constant and E a the activation energy.
Mears [49] described an approach to adopt a differential reactor and this seems to be a very
useful approach for the SOFC reactor beds. A differential reactor consists of different
amounts of catalyst throughout the reactor bed to compensate for unfavorable effects such as
extremely high reaction rates in parts of the reactor. For the SOFC one would wish to level
out the reactions and the electricity generating action over the whole bed. This can be
achieved by either increasing the amount of catalyst or to use finer particles to increase the
reaction rate. However, the SOFC has contradicting needs for the reaction rates depending on
whether the focus is on the methane steam reforming reaction or electrochemical reactions.
For the steam reforming reaction, the reaction rate is very high at the inlet and then gradually
decreases in the flow direction for the cell. But the reaction rate for the electrochemical
reactions requires a higher reaction activity right at the inlet for a limited area which would
increase if more catalyst material was provided or finer particles were present. By adjusting
the reaction rate activity for its needs, severe effects of temperature or concentration gradients
could be minimized.
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