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Hydrogen Behaviour

The accidental release of hydrogen in confined environment differs from the open atmosphere and semi-confined cases in the fact that the leakage is located in a room. Then, the released hydrogen gets mixed with the room atmosphere, building up there or dispersing outwards through venting holes.

Depending on the storage system, hydrogen leaks as liquid or gaseous phase. For leaks involving LH2, vaporization of cold hydrogen vapour towards the atmosphere may provide a warning sign because moisture condenses and forms a fog. This vaporization process usually occurs rapidly, forming a flammable mixture. On the other hand, for GH2 leaks, gas diffuses rapidly within the air.

The hydrogen gas released or vaporised will disperse through the environment by both diffusive and buoyant forces. Being more diffusive and more buoyant than gasoline, methane, and propane, hydrogen tends to disperse more rapidly. For low-momentum, gaseous hydrogen leaks, buoyancy affects gas motion more significantly than diffusivity. For high-momentum leaks, which are more likely in high-pressure systems, buoyancy effects are less significant, and the direction of the release will determine the gas motion; on the other hand, a jet is established, which reduces its inertia while it mixes.

Conversely, saturated hydrogen is heavier than air at those temperatures existing after evaporation. However, it quickly becomes lighter than air, making the cloud positively buoyant. At the end, localized air streams due to ventilation will also affect gas movement. Therefore, in all cases, a light gas cloud is developed near to the leak. It is rich in hydrogen, which is less dense than air in the room. This density difference induces a vertical buoyant force, making the hydrogen-rich cloud rise up and the heavier atmosphere air drop down. A region which is richer in hydrogen is developed and a buoyant plume is established. This plume mixes with the surrounding atmosphere but in a non-homogeneous way. When the plume impinges the top of the enclosure, it spreads throughout the ceiling and stagnates there. Depending on the release location and the geometrical aspect ratio (slenderness) of the building, the inertia forces would be able to drive the atmosphere to either well-mixed or stratified conditions.

In the medium and long-term, other mixing phenomena could appear and change the atmosphere conditions. Other releases of hydrogen could push the hydrogen-rich cloud downwards favouring homogeneous conditions. Heat transfer (mainly by convection) between room atmosphere and walls could induce secondary circulation loops, thereby enhancing the mixing processes.

All of these phenomena yield the final distribution of hydrogen within the confined environment: well-mixed, stratified, locally accumulated, etc. Moreover, the presence of some systems could change these conditions, normally helping the mixing process. They are: venting systems, connections to other rooms, fan coolers, sprays, rupture disks, etc. In order to deal with accident events, some mitigation systems have been developed as dilution systems (by injection of inert gas), igniters (which burn flammable mixtures) and recombiners (which oxidise hydrogen in a controlled way) etc. Valuable devices are the Passive Autocatalytic Recombiners (PAR), which reduce the hydrogen mass by inducing the reaction with oxygen at low hydrogen concentrations, using palladium or platinum as catalysts.

In summary, six stages of a release in a confined environment can be identified the phenomena after a release in a confined environment can be grouped together as:

(1) leakage;
(2) jet;
(3) buoyant plume;
(4) homogeneous/stratified dispersion;
(5) convective and venting phenomena; and
(6) mitigation systems (if any).


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Page last modified on December 05, 2008, at 05:27 PM