leakage of potassium hydroxide from a water electrolyser

The incident occurred at an alkaline electrolyser in a renewable hydrogen production plant. It consisted of an explosion which brough to the release of potassium hydroxide solution (KOH) from the electrolyser stacks.
An abrupt leak at the stack lead to a short circuit and the release of alkaline at 30 bar and 70°C. This release caused the smokescreen of the production room. The equipment shut down and automatically went into safety mode in accordance with the integrated safety system.
No employees were in the electrolyser room and outside no impacts were detected.

The INITIATING CAUSE was the damaging of a electrolyser membrane and the explosive combination of oxygen and hydrogen.

The investigation revealed the following:
(1) The assembly of the cells to stack caused mechanical deformation at the micro lines took place.
(2) This induced to flow restriction: because of the deposits coming from the activation of the electrodes, it came to a blockade of alkaline lines.
(3) This resulted in local high temperatures, which destroyed the membrane.
(4) The mixture of hydrogen and oxygen with the presence of electricity leaded to the explosion
(5) The frame of the stack was damaged and alkaline was released to the electrolyser room.

In absence of further information of the reasons for the mishap during assembly, the ROOT CAUSES could be identified in shortcomings of the electrolyser design and of installation procedures.

Alkaline electrolyser

Operation started in December 2019. A first incident occurred one year after, consisting in a fire in the vent line (June 2020).

DESCRIPTION OF THE FACILITY
This power-to gas plant was coupling an hydro-electric plant with the production of renewable hydrogen.
It included a cooler, a compressor, electrolysis with purification and drying unit, a hydrogen storage, a trailer filling station and a research facility.
The electrolyser was an alkaline electrolyser:
Nominal power: 1 MW
Production capacity: 200 Nm³/h hydrogen (ca. 450 kg/d).
The electrolytic fluid: demineralized water with a 30% concentration of potassium hydroxide (KOH).
The electrolyser delivered hydrogen at 20 bar and 80°. After the purified step to a 5.0 purity by removing contaminations and humidity, it is compressed to 300 bar and stored on site.
The storage had a nominal capacity of 1,2 tons hydrogen and had three trailer filling point to fill hydrogen trailer at 200 bar, with a nominal delivery capaciry of 1500 kg/d.

Nobody was injured in the event and there was no damage on the environment.
Damage costs related only the the late restart of the facility, which took place three years later with a new and different stack. For all that time, the unit was not full operative.

Despite very limited consequence, the incident has been a lesson learnt for the electrolyser producers to improve the design of the stack (to lower the risk of overheating) and the preventive measures, by ensuring that it is a better information on the status of the operation before accessing it and by better controlling personnel location.

The stack producer informed all other sites, which used this stack.
A new construction method should prevent mechanical deformation of the micro-lines. The deposits of the activation of the electrodes cannot totally be avoided but flushing and a better control of the operation is expected to improve the situation.
The new stacks design consisted in a cabinet with lamellar structure so that the heat was not blocked in the cabinet.
An additional preventing measures aiming at avoiding unnecessary presence of personnel during operation was that the electrolyser room can only accessed through the process control system room. In this way, the operator can get information about the status of the plant, which was also indicated by lights outside the access door. Finally, to have a better knowledge of who is on site, board with presence cards was installed.