Thesis for degree: Licentiate of Engineering

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H 2 O. Both CO and H 2 are initially set to small values to enable the numerical calculations, but CO 2 and H 2 O are decided in accordance with the chosen methane content and SF. Figure 4.11: Reaction rate distribution in the middle of the anode for 45% CH 4 (left) and 75% CH 4 (right). The reaction rates for 45% and 60% methane contents are rather similarly distributed, but in the case of 75 % methane the steam reforming reaction rate has initially higher values and much lower values at the outlet of the cell. Also, the water-gas-shift reaction rate for high CH 4 content increases to the maximum value closer to the inlet of the cell than it does for the other two cases. For the situations 45% and 75% content of methane shown in Figure 4.11, the steam reforming reaction rate (within the anode) is high as long as a high concentration of methane is available. The reaction rate increases as the temperature and concentration of steam increase, and decreases as the concentration of methane decreases. Note that there is a difference in scale between the x- and y-axes. It is possible to change the reaction rate, either by changing the particle size of the active catalyst, catalytic material composition or the porous structure, i.e., the active catalytic area. The limitation to be considered is that the probability of carbon deposition increases where there is almost no hydrogen present. A higher risk for carbon deposition occurs when there is a high temperature gradient close to the cell inlet. This is not the case here as the gradients are not so high. 48
Figure 4.12: Mole fraction distribution in the middle of the anode along the flow direction for SF = 1 (left) and SF = 5 (right). Only the two extreme cases are shown here to visualize the effect of a change in SF. In Figure 4.12 the mole fraction for SF=1 can be viewed to the left and SF=5 to the right. It is important to verify that there is a sufficient amount of H 2 O throughout the cell or else no efficient reaction will occur and there will be a risk for carbon deposition. In the figure for the mole fractions, it can be seen that a drop of H 2 O exists slightly downstream the inlet. Instead of putting all focus on the inlet mole fraction of H 2 O, one should also consider whether there is a sufficient amount to handle this drop and adjust the inlet mole fraction subsequently. This mole fraction drop increases when a faster reaction rate is applied which was shown in the previous section. Figure 4.13: Reaction rate distribution in the middle of the anode for SF = 1 (left) and SF = 5 (right). 49
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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 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: Table 4.5: Inlet mole fractions for
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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