Thesis for degree: Licentiate of Engineering

Abstract
The purpose of this work is to investigate the interaction between transport processes and
chemical reactions, with special emphasis on modeling mass transport by the Lattice
Boltzmann method (LBM) at microscale of the anode of a solid oxide fuel cell (SOFC). In
order to improve the performance of an SOFC, it is important to determine the microstructural
effect embedded within the physical and chemical processes, which usually are modeled
macroscopically. Without detailed knowledge of the transport processes and the chemical
reactions at microscale it can be difficult to capture their effect and to justify assumptions for
the macroscopic models with regard to the source terms and various properties in the porous
electrodes. The advantage of an anode-supported SOFC structure is that the thickness of the
electrolyte can be reduced, while still providing an internal reforming environment. For this
configuration with an enlarged anode, more detailed knowledge of the porous domain in
terms of the physical processes at microscale is called for.
In the first part of this study, the current literature on the modeling of transport processes and
chemical reactions mechanisms at microstructural scales is reviewed with special focus on the
LBM followed by a report on the emphasis to couple conventional CFD to LBM. In the
second part, two models are described. The first model is developed at microscale by LBM
for the anode of an SOFC in MATLAB. In the LB approach, the main point is to carefully
model the diffusion and convection at microscale in the porous region close to the threephase-boundary
(TPB). The porous structure is reconstructed from digital images, and
processed by Python. The second model is developed at macroscale for the whole unit cell.
For the macroscale model the kinetic model is evaluated at smaller scales to investigate if any
severe limiting effects on the heat and mass transfer occur.
LBM has been found to be an alternative method for modeling at microscale and can handle
complex geometries easily. However, there is still a need for a supercomputer to solve models
with several physical processes and components for a larger domain. The result of the
macroscale model shows that the three reaction rate models are fast and vary in magnitude.
The pre-exponential values, in relation to the partial pressures, and the activation energy
affect the reaction rate. The variation in amount of methane content and steam-to-fuel ratio
reveals that the composition needs a high inlet temperature to enable the reforming process
and to keep a constant current-density distribution. As experiments with the same chemical
compositions can be conducted on a cell or a reformer, the effect of the chosen kinetic model
on the heat and mass transfer was checked so that no severe limitation are caused on the
processes at microscale for an SOFC.
For future work, macroscale and microscale models will be connected for the design of a
multiscale model. Multiscale modeling will increase the understanding of detailed transport
phenomena and it will optimize the specific design and control of operating conditions. This
can offer crucial knowledge for SOFCs and the potential for a breakthrough in their
commercialization.
Keywords: mass transport, diffusion, microscale, porous media, kinetics, LBM, CFD, anode
multicomponent, MATLAB.
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