Simulation study on preventing explosive mixture formation in PEM electrolysers’ water recovery tanks

Abstract

Investigations of safety issues related to the design and operation of high-pressure proton exchange membrane (PEM) water electrolysers have until now mainly focused on the electrolyser stack. The present study focuses on the risk of explosive mixture formation in the water recovery system, which is part of the balance of plant in water electrolyser systems. After a major electrolyser company experienced an unexpected detonation in this system, we analysed their process design as a base case: a low-pressure buffer tank where anodic and cathodic water is mixed with make-up deionised water poses a safety risk because an explosive gas mixture is likely to form there, resulting in severe accidents with potential for escalation. Several retrofit options are explored using dynamic and steady-state simulation models: these options are then evaluated based on their effects on safety, general economics and operation of the plant, in addition to ease of retrofit. The results show preventing the formation of explosive gas mixtures in the water recovery system is achievable in several ways, such as avoiding recycling cathodic water altogether, stripping it of hydrogen, flushing buffer tanks with nitrogen or oxygen, rearranging the balance of plant appropriately, or changing the control algorithms of the system.