(151c) Investigation of Explosion-Related Key Figures of Humid Hydrogen-Oxygen Mixtures

Hydrogen is a promising energy carrier that is becoming increasingly important in the course of the energy transition. However, the explosion-related properties of hydrogen are a major challenge for its safe handling and utilization and safety concepts for typical hydrocarbon vapors need to be carefully looked at when applied to hydrogen.

The properties of hydrogen have already been investigated many times in the past, whereby hydrogen differs from other substances above all in its wide explosion range, low maximum experimental safe gap and rapid rise in explosion pressure and must therefore be considered more critically. In contrast to earlier industrial processes, hydrogen now occurs increasingly not only in the form of hydrogen-air mixtures, but also in hydrogen-oxygen mixtures. It is known that the enrichment of oxygen in combustion or explosion processes leads to a significant intensification, but there is a lack of comprehensive basic research into the specific safety characteristics of pure fuel-oxygen mixtures. The lack of that data may turn out to be quite critical when applying explosion venting solutions for plants or equipment handling hydrogen.

Due to this research gap, we carried out a research project to provide a better understanding of these mixtures. This project compares maximum explosion pressure, max. explosion pressure rise and maximum experimental safe gap of hydrogen-oxygen mixtures in a wide range of molar fractions and humidity levels, which can occur following a membrane failure in electrolyzers. In addition to the investigation of the dry binary mixture, the influence of water vapour with a possible inerting effect was also investigated. The proportion of water vapor is determined on the basis of the vapor pressure curve and is therefore temperature-dependent.

The results show that the explosion pressure of the dry stoichiometric mixture in the 5 L explosion pressure autoclave only increases slightly by around 20 % compared to the hydrogen-air mixture, but oscillations in the pressure curve already indicate instability phenomena. Due to the enormous max. explosion pressure rise, which is about 100 times faster than that of a methane-air mixture, detonation instead of deflagration is to be expected even with spherical explosion propagation of larger volumes and without inducing turbulence.

By humidifying the gas mixtures, it was also possible to show that the explosion pressure and pressure rise can be significantly reduced with increasing temperatures and thus higher water vapour contents.

In addition we looked at the potential of safely venting a hydrogen air mixture in a pipe system and the effect that explosion venting has on the combustion process.

The results of these investigations are intended to provide a basis for the safety-related handling of hydrogen-oxygen mixtures and support the development of explosion mitigation and venting solutions.

The authors believe that this presentation provides some valuable new insights for the upcoming challenges of handling hydrogen to ensure safe processes in the future.