Origin of Steam Contaminants and Degradation of Solid-Oxide Electrolysis Stacks

Two once-through steam generators and a combination of a steam generator and a gas preheater for supplying feed gases to solid-oxide electrolysis stacks were evaluated for their carryover characteristics of contaminants from the feed-water into the steam phase. The concentrations of various trace impurities in the steam were determined by sampling the steam condensates and screening them with inductively coupled plasma-mass spectrometry for 19 elements and liquid ion chromatography and continuous flow analysis for chloride and ammonium. Steam-soluble species such as boric acid undergo complete volatilization and transfer into the steam phase. During unstable evaporation in the steam generators an extensive physical carryover of alloying metal species was observed. At realistic operation conditions for steam electrolysis, the gas preheater caused a considerable release of silicon into the steam phase. Two stack experiments were performed with common preheater temperatures and showed largely increased cell voltage degradation at higher operation temperatures. The post-test chemical analysis of cell samples revealed significant concentrations of silicon in the samples that are regarded as primary cause for increased degradation. These findings could partially explain the wide spread of degradation rates reported for solid-oxide steam electrolysis experiments.

Automated summary

The carryover characteristics of contaminants from the feed-water into the steam phase were evaluated for different types of steam generators supplying feed gases to solid-oxide electrolysis stacks. The study found that steam-soluble species like boric acid completely transfer into the steam phase, while alloying metal species are carried over into the steam during unstable evaporation in the steam generators. The gas preheater may cause the release of silicon into the steam phase under realistic operation conditions.