Thesis for degree: Licentiate of Engineering

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The reaction rates for SF=1 (left) and SF=5 (right) are presented in Figure 4.13. It should be mentioned that the temperature distribution was overall quite similar for the cases. When SF=1 the temperature was overall lower for the larger part of the cell. For the case with SF=5, the lowest temperature was much closer to the inlet than for SF=1 and the three cases with varying methane content. The temperature distribution has an effect on the reaction rates. The profiles of the reaction rates are quite different. For SF=1, the maximum rate value of the water-gas-shift reaction is not reached until just upstream from the outlet but, on the other hand, for SF=5, it is reached close to the inlet. The steam reforming reaction shows the same tendency but it is slightly higher for SF=1 to begin with. The water-gas-shift reaction rate was low because the mole fraction of CO is initially so small. Furthermore, the water-gas shift reaction is connected to the steam reforming reaction, which affects the profile throughout the cell. 50
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. 51
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SOFC modeling from micro to macrosc
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Abstract The purpose of this work i
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katalytisk omvandling av naturgas o
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List of publications Journal public
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3.2 Governing equations for the mac
Page 11 and 12: Q source term (heat), W/m3 q heat f
Page 13 and 14: 1 Introduction After some time of d
Page 15 and 16: 2 SOFC modeling at smaller scales T
Page 17 and 18: etween 0.3 and 1.5 mm. In this conf
Page 19 and 20: Volume Method (FVM) which adopt the
Page 21 and 22: 2.3 Lattice Boltzmann method LBM ha
Page 23 and 24: streaming [3]. The concentration ρ
Page 25 and 26: ease the correspondence between the
Page 27 and 28: applicable for a mixture of two spe
Page 29 and 30: Figure 2.6: Effective boundary arra
Page 31 and 32: Table 2.1: Comparison of previous w
Page 33 and 34: Figure 2.7: Schematic illustration
Page 35 and 36: model is presented after the data c
Page 37 and 38: 3.2.1 Mass transport In the electro
Page 39 and 40: where h v is the volume heat transf
Page 41 and 42: Achenbach [43], it was found that t
Page 43 and 44: cathode are assumed to be similar t
Page 45 and 46: (3.48) where, besides the parameter
Page 47 and 48: for an isothermal spherical particl
Page 49 and 50: Velocity carried out to check wheth
Page 51 and 52: Figure 4.4: Velocity field [lu/ts]
Page 53 and 54: Table 4.2: Parameters in the SOFC a
Page 55 and 56: eactions are safely fulfilled. This
Page 57 and 58: Figure 4.8: Mole fraction distribut
Page 59 and 60: Table 4.5: Inlet mole fractions for
Page 61: Figure 4.12: Mole fraction distribu
Page 65 and 66: 7 References [1] Andersson M., Yuan
Page 67 and 68: [25] Latt J., Hydrodynamic Limit of
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Thesis for degree: Licentiate of Engineering

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