Thesis for degree: Licentiate of Engineering

5 Conclusions
The physics and the transport processes in SOFCs can be described at different length and
time scales. This constitutes a challenge for the development of multiscale models for fuel
cell simulations. In this study, a LBM microscale model was developed for the D2Q9 case
(two-dimensional nine speed case). The kinetic model was examined so that no severe
limiting effects on heat and mass transport occurred. Also, a FEM based model for an anodesupported
SOFC was developed to better understand the internal reforming reactions of
methane and the effects on the transport processes. The model was implemented in COMSOL
Multiphysics for the analysis of three different kinetic models found in the literature. An
equilibrium equation was employed for the water-gas shift reforming reaction rate. Parameter
studies were also conducted for the methane content and SF.
Five conclusions can be made in this study. First, LBM was found to be a functional method
to microscale modeling predicting the velocity profile and mass diffusion well. LBM could
handle both a simple geometry as a channel to a more complex geometry such as a porous
media. For the velocity field, the LBM was able to illustrate the flow correctly around the
obstacles. The mass diffusion for hydrogen was reduced from the inlet to the outlet as
expected and contours were seen around the obstacles where mass diffusion of hydrogen
occurred parallel to the surface. The detailed information from LBM at microscale regarding
the transport processes and chemical reactions can improve the macroscale model by
including this information for the TPB areas.
Second, it was shown that the reaction rates were very fast and differed slightly across the
three models due to the great differences of the pre-exponential value and the activation
energy. The model was found to be sensitive to variation of the steam reforming reaction rate.
Both the inlet temperature and active surface area to volume ratio showed an effect on the
reaction rates in terms of the maximum value.
Third, it was found that a fuel containing a high percentage of methane in combination with a
high inlet temperature produced a steep temperature gradient close to the cell inlet. Fourth, a
higher steam-to-fuel ratio showed a decreased risk of carbon deposition at the anode catalytic
active area.
Finally, there was no direct significant risk for heat and mass transport limitations for the
SOFC model with the kinetic parameters in this study. Care should be taken if the reaction
rate is increased since this will affect almost every criterion in the analysis. It transpired not to
be sufficient only to describe the reaction rates with a few empirical parameters. It was
necessary to develop a suitable microscale model for the SOFC. However, the global kinetic
models have still predicted valuable behaviors. The reason why the kinetics models differed
to a large extent is that they were sensitive to how the experiment was designed.
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