Solid oxide fuel cells (SOFC) are attractive to the energy sector owing to their efficiency and eco-friendliness. However, the efficiency of power units based on the SOFC is impeded by the need for fuel cell stacks performance assurance. Therefore, it is essential to develop new advanced SOFC power unit schemes and improve their balance-of­-plant equipment performance. By doing so, this technology will become more widely spread. In fact, the SOFC power unit efficiency can be increased by means of anode off-gas recirculation [1-3].

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 α_{f_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) α_{c_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 z_minto 0.8 using the developed method. References:

1. R.-U. Dietrich, A. Lindermeir, C. Immisch, C. Spieker, C. Spitta, S. Stenger, R. Leithner, T. Küster, A. Oberland. SOFC System Using a Hot Gas Ejector for Offgas Recycling for High Efficient /Power Generation from Propane / ECS Transactions // 57 (1) 171-184 (2013)
2. Roland Peters, Robert Deja, Ludger Blum, Jari Pennanen, Jari Kiviaho, Tuomas Hakal. Analysis of solid oxide fuel cell system concepts with anode recycling / International journal of hydrogen energy // 38 ( 2013) 6809-6820
3. M. Atanasiu. The Status of SOFC R&D in the Fuel Cell and Hydrogen Joint Undertaking Program / ECS Transactions // 68 (1) 3-14 (2015)