Ammonia electrolyser plate topologies and its effects

Ammonia electrolysis is still quite a new process that is being learned and analysed. This was no different for me and the course of my graduation project with Entrance as a student from the Hanze University of Applied Sciences studying mechanical engineering. There is so much that can be learned in such a short time frame but can open so many possibilities to explore as more work is done.

The first part is understanding the science behind the electrolysis of ammonia. Traditionally, in an electrolyser an anode and cathode plate are charged to separate hydrogen and oxygen molecules from water to so that the hydrogen can be reused. The overall process in ammonia is the same however instead of hydrogen being separated from oxygen, in ammonia electrolysis, hydrogen is separated from nitrogen. This is a cheaper alternative in the sense that it uses less electricity to complete the electrolysis process.

As the main part of an electrolyser is the anode and cathode plate, my project focused on the topologies of the plates. This includes the effect of gravity, effect of varying heights of the plates, varying widths of the plates, a combination of both, and lastly the possibility of a spinning electrode.

The first part was to see the effect of gravity on the plume of produced hydrogen and nitrogen. The biggest difference was seen in an upright position and at 45 degrees which is to be expected. In an upright position, the plume is fully developed and is considered to be 1 or 100% developed. However, when having the electrode tilted at 45 degrees, there is a drop of about 10% down to 0.9 or 90%. This means that there is a about 10% loss in production and plume development when the plates are at 45 degrees.

Next was to see the effect of varying heights and widths. The standard model used was 4mm wide and 100mm high. Now, there are tens of thousands of variations that can be used so it was decided to take up to 300mm high and down to 2mm wide. This still produces an extremely large number of possibilities so steps of 50mm in height and 0.5 mm in width were taken. When taking each parameter separately, there is no difference in terms of mixing of nitrogen and hydrogen. It simply does not happen. The real difference is when you take a combination of the two. For example, when you take a 2mm gap and a plate height of 300mm, there then becomes significant mixing. Across the entire electrode, there becomes a constant gas fraction of 15% meaning there is always at least 15% gas fraction which indicates that nitrogen has significantly mixed with hydrogen. This first becomes apparent in the 200mm range but is quite small when compared to the result just discussed.

Lastly, the possibility of using a spinning electrode was investigated but is still an early look investigation to the effect and feasibility. A spinning electrode works by having two walls like normal, but they are cylindrical with one spinning and the other remaining stationary. For the case of this project, it was taken to have the inner wall spin and the outer wall to remain stationary. Here it was seen that the outer wall had a higher region of accumulation of gas near the outlet region which is normal as this is the direction the wall is spinning. This makes the electrolyte and produced gasses to be pushed towards the outside end of the electrode causing a higher concentration. This can be best shown with the magenta line in the figure below.

Thank you for your reading of this blog, it was a great experience learning so much about ammonia electrolysis and learning the CFD software COMSOL. I hope to see you at the New Energy Forum!

A blog by: Jordan Beltchev, fourth year student Mechanical Engineering

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