Design of Experiment investigation and model-based process parameter optimisation of industrial-sized electrolyte supported solid oxide electrolysis stack for downstream Fischer-Tropsch synthesis

Optimising the performance of Solid Oxide Electrolysers (SOE) in Power-to-X (PtX) systems is a key challenge to increase competitiveness and improve efficiency, especially in the production of E-Fuels and Sustainable Aviation Fuel (SAF). This work proposes a systematic approach to optimising SOE stack parameters. Experiments were carried out on an industrial-sized SOE stack to understand how process parameters affect voltage and product gas composition, which can then be used in a Fischer–Tropsch (FT) reactor. The process parameters investigated were air outlet temperature, fuel flow rate, reactant conversion rate, H2x/COy-ratio and reactant/product ratio. To determine the isolated and interactive effects of each parameter, a central composite inscribed (CCI) Design of Experiments (DoE) test plan was used. Non-linear characteristics of the stack voltage were observed as the temperature was varied and strong interaction effects were detected for the reactant conversion rate, educt content and fuel flow rate parameters on the target parameter stack voltage. The H2/CO-ratio of the product gas can be increased by increasing the H2x/COy-ratio of the inlet composition, or by decreasing the reactant conversion rate or temperature. Spatial temperature distribution in the cell and electrochemical impedance spectroscopy were measured to derive possible explanations for the observations made. Two response models were derived from test results and coupled with an GA to optimise SOE operating parameters. This resulted in several suitable SOE unit operation parameters within a PtX plant. The work provided a quantification of main and interaction effects on stack voltage and product gas composition, which can be used to reconstruct the models and use them in further work.