Performance of SOFC stacks under partial internal reforming of methane
Solid Oxide Fuel Cell systems (SOFC) are developed for their electrical efficiency and versatility. Versatility stems from their tolerance towards CO and CH4, which allows for the use of various reformed fuels. Efficiency results from the direct conversion of the chemical energy of fuels into electricity and the availability of high temperature heat for internal steam reforming. In this study, short SOFC stacks provided by SOFCpower were tested under methane steam reforming conditions. The stacks were coupled with a steam generator and a Ni-based steam reformer. The effect of the stack operating temperature, the steam-to-carbon (S/C) ratio and the methane flow density on the electrical performance of the stack are investigated. The temperature of the reformer was varied between 430 and 700°C in order to tune the degree of methane pre-reforming, and so study the effect of internal reforming on the stack performance. When increasing stack temperature from 750 to 800°C, there is almost no improvement in performance since the reduction in area specific resistance (ASR) is compensated by that of the Nernst potential. Varying the CH4 flow from 1.25 to 0.5 Nml·min-1·cm-2 at 750°C, allows to increase the LHV electrical efficiency at 89% fuel utilisation (FU) from 60 to 68% but with a corresponding loss in stack power density, from 0.46 to 0.21 W·cm-2. Changing the degree of methane pre-reforming from 95% to 20% by varying the reformer temperature did virtually not change the performance of the stack, indicating that reforming continues after the steam reformer itself, at the stack inlet, over the catalytically active anode supported cells. The observed effect is then mainly thermal since the stack inlet is cooled by the reforming reaction.