Recycling steam condensate for hydrogen production

Abstract

Producing green hydrogen can be done using alkaline water electrolysis. Recycling water might help to fulfil the large water demand of hydrogen production facilities. In fertiliser production, steam condensate could be a recyclable water source for hydrogen production. This condensate is relatively clean as its main contaminant is a small concentration of ammonia. This report researches the effect of adding ammonia to the electrolyte of an alkaline water electrolysis cell.

When steam condensate is used instead of ultra-pure water, it is important to find out what happens to the added ammonia and if it affects the production rate and the production efficiency, or if it degrades the equipment. The experiments in this report used three different ammonia concentrations and the electrolyte was made using potassium hydroxide. The electrolysis cell used nickel mesh electrodes and a Selemion™ anion exchange membrane. Each of the experiments were carried out at 2.15 Volts for a duration of 50 minutes.

It was found that adding 1 mmol/L ammonia to the electrolyte decreased the current density of the cell. Doubling the ammonia concentration led to an even larger decrease in current density of up to 19%. A significant effect on the Faraday efficiency was not measured. During the experiments, the ammonia was partially stripped from the electrolyte due to its high alkalinity. Another part was oxidised to produce nitrogen gas and nitrate. Some of the ammonia had not reacted after the 50-minute experiment and could be measured in the spent electrolyte. After all the experiments were carried out, the cell was disassembled. The cathode showed significant signs of degradation. However, the many starts and stops between experiments could be the primary reason for this degradation. The ammonia could have accelerated the degradation, but this was not proven.

Overall, the benefits of using steam condensate do not seem to outweigh the drawbacks. The ammonia caused a significant decrease in current density. On the long term, ammonia might cause electrode poisoning which would further lower the current density. The experiments also found nitrate in the spent electrolyte. Having additional pollutants in both the spent electrolyte and the produced gasses might introduce additional disadvantages to using an ammonia containing electrolyte. If steam condensate is used as a water source, a treatment step is advised. Aeration could be used to strip the ammonia from the electrolyte. To get a more accurate insight into the effects of ammonia on alkaline water electrolysis, additional research is necessary. In future research, the effects of ammonia should be measured more accurately, and the long-term effects should be researched.