SOFC Modeling from Micro- to Macroscale: Transport Processes and Chemical Reactions

Sammanfattning: 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 three-phase-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.