The following parameters must be calculated when developing the SOFC power units with anode off-gas recirculation: recirculation ratio, carbon deposition boundary when methane reforming is performed by an anode off-gas, equilibrium chemical composition of gas mixtures reacting in a reformer and anode channel, etc. [1, 2].
The paper contains the method for calculation of the anode off-gas recirculation ratio and synthesis gas equilibrium chemical composition which is based on simplification of both balance equation system and equilibrium constant expressions for reforming reactions. In addition, the recirculation ratio z at which the carbon deposition fails to occur in the reformer and fuel cell anode channel for temperatures ranging from 600 to 900 °С was calculated. It was shown that carbon deposition during fuel reforming would be unavoidable at low reformer operation temperatures and low SOFC fuel utilization rates even though all anode off-gas was recirculated. The SOFC power units with anode off-gas recirculation most efficient modes of operation for the low reforming temperatures and fuel utilization rates were determined given the methane reforming was carried out by a mixture of air and anode off-gas. For instance, given the average reforming temperature is 700°С, SOFC fuel utilization rate αf_out=0.6 and recirculation ratio z=0.6 it is necessary to add air to the reformer at the air supply ratio (excess air ratio) αc_rec=0.15 to effectively avoid carbon deposition in the reformer (fig. 1). By doing so, the synthesis gas with more combustibles (H2 and CO) content can be derived than in case of partial oxidation reforming. Moreover, this solution will allow reduction in heat supplied to the reformer for the endothermic reactions of steam and carbon dioxide reforming heat coverage. The SOFC electromotive force (EMF) as a function of the recirculation ratio was obtained in the range from zminto 0.8 using the developed method. References:
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