Thesis for degree: Licentiate of Engineering

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Table 4.3: Inlet and operating conditions. Parameter Value Fuel utilization 80 % Oxygen utilization 20 % Inlet mole fraction of methane 0.171 Inlet mole fraction of hydrogen 0.2626 Inlet mole fraction of water 0.4934 Inlet mole fraction of carbon monoxide 0.0294 Inlet mole fraction of carbon dioxide 0.0436 Inlet temperature Pressure Average current density 1000 K 1 atm 3000 A/m² The results are presented in Table 4.4 and commented below for all levels of the analysis of the kinetic criteria. Table 4.4: Results from the criteria analysis. Transport domain Heat transport Mass transport Interparticle No limitation – Equation (3.40): ~10 -5 –10 -9 < ~10 -2 Interphase No limitation Bi = 2 – 4∙10 -2 < 10 No limitation Equation (3.45): ~10 -7 –10 -8 < ~1–10 -2 Intraparticle No limitation No limitation Equation (3.61): η ≈ 0.999 > 0.95 The results are homogeneous with no limitations for any of the involved scales and reactions. As the criterion is fulfilled for the heat transport at the interparticle transport level then no limitation occurs for the heat transport at interphase or intraparticle level. The main difference between the steam reforming reaction and the electrochemical reactions is that the latter ones have a somewhat higher reaction rate. These reactions occur only in a small part of the anode/cathode domain and the reaction rate is highest at the boundary between the anode and electrolyte or between cathode and electrolyte. But the criteria for the electrochemical 42
eactions are safely fulfilled. This is a good verification of the chosen parameters for the computational model examined in previous studies. However, it is interesting to reflect over which parameters can form a potential risk for limiting the transport processes. For the heat transport at all the domains, a larger enthalpy change or an increased reaction rate can increase the risk. Also, a lowering of the temperature can cause an increased risk but this is less significant. For the mass transport, an increased reaction rate or a decreased concentration of reactant can cause a higher risk. As mentioned before the effectiveness factor is affected by the characteristic dimensions of the particle which can also be confirmed here. If a change in particle diameter causes a change in the reaction rate, this may have a strong influence on the transport processes [6]. To summarize, the elimination of transport gradients which limit the reaction and catalytic kinetics is complex to study. This case study, by tools to assess the transport limiting issues, seeks to locate the limiting sources and improve these for the desired outcome. The reaction rate is the most direct risk for limitation on the transport processes. If the reaction rate is increased it will affect every criterion in the analysis and can cause severe gradients which will create transport limitations. The anode and cathode structure and catalytic characteristics have an impact on the reaction rates, especially on the steam reforming reaction, which will in turn affect the cell performance. 4.3 Macroscale model by CFD A two-dimensional model for an anode-supported SOFC has been developed and implemented in the commercial software COMSOL Multiphysics (version 3.5a). Equations for momentum, mass and heat transport are solved simultaneously. The cell geometry and SOFC operating parameters are defined in Table 4.3. It should be mentioned that this macroscale model is 2D only, and the connection between the electrodes and interconnect cannot be explicitly observed in this case. 4.3.1 Case study: Internal reforming reaction rates The flow direction is set to be from left to right for air and fuel channels as well as the anode and the cathode. It is also possible with counter flow but this is not included in this study. It should be explicitly mentioned that the length of the cell is 100 times longer than the height of the air or the fuel channel. 43
Page 1 and 2:
SOFC modeling from micro to macrosc
Page 3 and 4: Abstract The purpose of this work i
Page 5 and 6: katalytisk omvandling av naturgas o
Page 7 and 8: List of publications Journal public
Page 9 and 10: 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: Table 4.2: Parameters in the SOFC a
Page 57 and 58: Figure 4.8: Mole fraction distribut
Page 59 and 60: Table 4.5: Inlet mole fractions for
Page 61 and 62: Figure 4.12: Mole fraction distribu
Page 63 and 64: 5 Conclusions The physics and the t
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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