Optimizing the design of a CO2 electrolyser for producing formate to improve flow distribution and gas bubble flow

Abstract

The worlds energy demand is rising as a result of industrial activity and advances in countries around the world. Fossil fuel sources are used to meet this demand for much of the world [1]. The way in which energy is handled causes pollution e.g., CO2 emissions, inducing climate change, therefore a more sustainable industry is desired [2, 3]. Carbon Capture and Utilization (CCU) is a way to use CO2 from the atmosphere or which would end up in the atmosphere, limiting its influence on the climate. Sustainable fuels or chemicals can be produced from the captured CO2, reducing the use of fossil fuels and fossil feedstock [4, 5].
Formic Acid is a favourable product from CO2 electrolysis. Only 2 electrons are required for one molecule Formic Acid, which results in a high normalized price ($/electron) [6]. COVAL Energy works on scaling-up and commercialization of CO2 electrolysis to Formic Acid. Coval has already an operational reactor on lab scale with promising performance, a conversion efficiency up to 90%. Although the reaction kinetics are limited by a poor distribution of reactants on the electrode surface. Previous research by Jos van der Maas showed a highly uneven flow distribution in the cell reactor [7]. Gas bubbles are formed inside the reactor, which likely limit the reaction. Consequences of this poor distribution of reactants and these bubbles are a lower energy efficiency and higher electrical resistance.
The main question of this thesis is: How can the current knowledge of electrolysers be applied to improve the Coval electrolyser design regarding flow distribution and mitigation of the impact of dissolved gas production? Experiments were performed to gain knowledge about the bubble behaviour in the Coval electrolyser. Computational Fluid Dynamics (CFD) was used to screen design ideas on liquid distribution. Promising designs were assessed in a flow and gas bubble visualisation experimental setup, in which also the bubble behaviour was investigated. Finally, prototypes to be tested in the high pressure reactor were produced.

Two auspicious designs were found. The first design has channels in the inter electrode gap. The results showed that with channels the velocity variation over the cell width was reduced. The second design has a manifold like geometry in a calm zone before and after the interelectrode gap. This resulted in an almost constant velocity over the cell width for the entire cell. Additionally, a larger inlet and outlet pipe to and from the cell improves flow homogeneity and reduces pressure dorp in these pipes. The gas fraction in the cell can be kept low by sloping the top part of the cell towards the outlet.
A next step is to assess both designs in the high pressure reactor. Particular attention should be paid to the manufacturing of the design to avoid leakage and mechanical deterioration. It is recommended to measure pressure drop over the cell for better assessment of the designs.