High Fuel Utilization Operation of Solid Oxide Fuel Cells - a Modeling Study
Ideally, one would like to identify a safe operating zone where the fuel utilization Uf can be maintained in a high enough range without risking Ni oxidation. In this presentation we will use simulation results from a detailed multiphysics model to outline some of the challenges of operating a SOFC at high fuel utilization and attempt to identify the above mentioned “safe operating zone”.
The multiphysics model used in this work is for a planar anode supported geometry and includes a fully coupled description of the flow and mass transfer in the air/fuel channels and the electrodes, and the current/potential distribution in the electrodes. One of the mechanisms for Ni oxidation is through reaction (1).
Ni + H2O ↔ NiO + H2 (1)
The favourability of reaction (1) at a particular location in the anode is dictated by the thermodynamics of the reaction and the local fuel composition. As the model is able to predict the fuel side composition as a function of position, the output of the model can be used to predict whether there is a risk of Ni oxidation in the anode at any given set of operating conditions.
We will present results from a parametric study over a wide range of different temperatures, cell voltage, as well as inlet fuel flow-rates and composition. An example set of results is given in Fig 1 where the inlet flow-rate is varied for a given cell voltage and temperature. The Ni oxidation reaction is thermodynamically favourable in the latter section of the anode where the partial pressure ratio PH2O/PH2 exceeds the threshold value given by the thermodynamics of the reaction and shown as a dot-dash line in the bottom plot. According to Fig 1, the threshold Uf for these conditions is ~99%. This is a much higher value than the typical 80-85% used in most SOFC designs. Another noteworthy point is that at this high fuel utlization, the last 1/3 of the cell is producing a very small fraction of the total current.