Operating Strategies for Fischer-Tropsch Tail Gas Recirculation on a 100 kW SOEC Reactor

Abstract

The efficient use of CO2 and its conversion into CO via high temperature electrolysis is considered as a suitable route for the de-fossilization of anthropogenic activities. Within electrochemical solid oxide cells reactors (SOCs), the co-electrolysis of H2O and CO2 at high temperature (above 800 °C) yields syngas (H2 + CO), one of the most important feedstocks for the production of synthetic fuels and chemicals, e. g. via the Fischer-Tropsch process.At the Institute of Engineering Thermodynamics of the German Aerospace Center (DLR) in Stuttgart Germany, a unique test environment (Galactica) for investigating SOC reactors has been designed and constructed with the aim to deepen the research at the 100 kW-scale reactor in commercial systems. State-of-the-Art SOCs have shown promising results that allow the scale-up to larger, industrially relevant systems sizes. With Galactica, DLR was able to demonstrate successfully how 100 kW-scale reactors are a suitable stepping stone towards multi MW-scale process systems. However, operating strategies are required in order to understand how these systems would perform when steady-state operation cannot be guaranteed and unexpected shortages on the feedstocks and electricity could arise.In this work, the coupling of a 100 kW SOC reactor (consisting of 24 single SOC stacks) with the tail gas recirculation of a Fischer-Tropsch reactor will be presented. The main motivation is to improve the overall process efficiency, increase the conversion (since tail gas hydrocarbons will be reformed) and reduce the net material consumption [1] [2]. By emulating the tail gas composition from the Fischer-Tropsch reactor, different syngas ratios (H2/CO) were investigated in Galactica, not only experimentally but also with the support of the in-house simulation framework TEMPEST [3], specialized for transient simulations on electrochemical reactors. Different operating conditions were evaluated by: (i) adding CH4 at open circuit voltage (OCV) and under current, (ii) performing an experimental ramp of the emulated Fischer-Tropsch gas composition into the inlet gases of the SOC reactor and (iii) by simulating a Feed-Forward controller in the case of H2O and CO2 shortages by varying the current and the air flow values in order to keep the operating temperature and reactor conversion in stable conditions. These results will allow to evaluate the performance of the SOC reactor with the temperature profile along the cells and the stacks, as well as with the syngas ratio behaviour.In this regard, operation strategies will be analyzed and discussed with the aim to mitigate failure conditions on SOC reactors, while operating in transient conditions in the frame of syngas production via high temperature co-electrolysis.[1] Herz, G.; Reichelt, E.; Jahn, M., Techno-economic analysis of a co-electrolysis-based synthesis process for the production of hydrocarbons. Applied Energy 2018, 215, 309-320 [2] Cinti, G.; Baldinelli, A.; Di Michele, A.; Desideri, U., Integration of Solid Oxide Electrolyzer and Fischer-Tropsch: A sustainable pathway for synthetic fuel. Applied Energy 2016, 162, 308-320. [3] Tomberg, M.; Heddrich, M. P.; Sedeqi, F.; Ullmer, D.; Ansar, S. A.; Friedrich, K. A., A New Approach to Modeling Solid Oxide Cell Reactors with Multiple Stacks for Process System Simulation. J. Electrochem. Soc 2022, 169, 054530.