Container Specifications for Reactive Type Storage (25.3)

Criticality: Medium
Progress: Not Addressed
Score: 24
DOT Relevance: §178

Description of Key Area

Metal hydride-based hydrogen storage systems are presently being commercialized. While there are many different materials that may be used as hydrogen storage materials, they can be divided into two distinctive categories: rechargeable and non-rechargeable. The term rechargeable is used to describe a system which can be refilled by introducing hydrogen to the depleted system without the need to add or remove any other reactant or by-product and the systems are designed to retain all material other than hydrogen. Non-rechargeable describes systems where to refill the system, the hydrogen-depleted material or by-products must be removed and fresh hydrogen-containing materials replenished. These systems are therefore designed to allow removal and addition of material other than just hydrogen. This part will discuss non-rechargeable systems; rechargeable systems are discussed in Item 25.2.

Non-rechargeable systems may contain a mixture of hazardous materials that are selected such that they combine or react to produce hydrogen gas. These systems may contain mixed solids, mixed liquid and solid phases, liquids with dissolved solids, gaseous and liquid or solid phases, etc. They may also contain gaseous hydrogen during part of the time or at all times in at least part of the packaging.

A number of different types of non-rechargeable systems have been developed and/or proposed. Three examples include:

1. Reacting alkali (e.g., sodium and potassium) or alkaline earth metals (e.g., calcium) or their hydrides with water to produce hydrogen gas. For example with sodium metal the reaction would be:
   Na_(s) + H₂O_(l) → ½ H_2(g) + NaOH_(aq)
   Various method of containing reactants and controlling the reaction have been proposed, such as making a slurry of the metals in an organic or inorganic oil and controlling the addition of water and encapsulating the solids in with a non-reactive coating and placing them in a container with water—the encapsulating coating is then mechanically breached as required to allow reaction and liberate gaseous hydrogen.
2. Reacting ammonia with an aluminum hydride, such as LiAlH₄, to produce hydrogen gas and various amines and amides as by-products. In one system developed for and tested by the military, the ammonia, which is contained in one pressurized compartment, passes through a one-way valve into a second compartment that contains the solid hydride phase. In the second compartment the reaction occurs, producing gaseous hydrogen. The hydrogen gas pressure is used to control the rate of ammonia passing into the second compartment and thus the rate of reaction.
3. Reacting sodium borohydride catalytically with water to produce hydrogen and sodium borate. The reaction is:
   NaBH_4(aq) + 2 H_­2O_(l)–(cat.) → 4 H_2(g) + NaBO_2(aq)
   If not taken to completion, the reaction by-product could be NaB(OH)₄. In one version of this type of system, the aqueous solution of sodium borohydride is stabilized by buffering the solution to a high pH, typically in the 12 to 14 range. The stabilized solution is then passed through a second chamber containing the catalyst where the reaction rapidly occurs, liberating gaseous hydrogen. The spent solution is collected in a third chamber.

For safety and proper operation, non-rechargeable hydride systems will need to be able to contain both the reactive phases and the produced by-products, while “idle” and while producing hydrogen. There must be a means to remove the by-products and replenish the reactive species. These systems may therefore contain multiple compartments, one-way flow valves, multiple PRDs, manifolds, etc. Since the materials are selected to react to produce hydrogen, a flammable gas, current regulations might prohibit containing these materials in a single package. The hazardous materials regulations contain packaging requirements and packaging specifications for the various hazard classifications, however the current requirements and specifications may not be suitable for non-rechargeable or “reactive” hydride-based hydrogen storage systems.

Due to the variety of non-rechargeable hydride systems being investigated and the early stage of most of their development, it is not likely that a single set of packaging specifications could be developed to appropriately cover them all. A more appropriate approach would be to treat them as articles and develop general guidelines as found in Subpart E- Non-bulk Packaging for Hazardous Materials Other Than Class 1 and Class 7 of 49 CFR 173. Consideration will also need to be given to allowing exceptions to current restrictions on mix content hazards, as discussed in Item 18.4 of this report.

Discussion of Criticality

This item is considered to be of medium criticality. Non-rechargeable hydride-based hydrogen storage systems are likely to contain a mixture of hazardous materials. Due to the variety of potential materials and system designs, specific packaging specifications will need to be general, emphasizing demonstration of safety through testing and material compatibility. Considerations will need to be given for compatibility of packaging to the reactive species and the produced by-products. The ability of the packaging to contain all of the material (solid, liquid and gas as appropriate) while “idle” and while producing hydrogen must both be considered. The packaging must also be able to safely prevent and/or control any potential “run-away” reaction. At this time, there are few systems in commercialization. Over the next few years, more are expected to become commercial, however at this time which technologies and system designs likely to reach that stage is uncertain.

In the near-term it is expected that special permits may need to be granted to allow exceptions to regulatory prohibitions to combining certain reactive hazards within a single package, see Item 18.4 of this report. Review of these special permits could include system design for material of construction compatibility, reaction containment, etc. When the scope of technologies that are likely to be commercialized becomes apparent, it would be appropriate for exceptions to prohibitions of certain mixed hazards to be included into the 49 CFR. At that time, it would also be appropriate to include a general packaging section into Subpart E of §173, similar to that for wet batteries (§173.159).

Discussion of Progress

In the last several years, the US DOT has included in the hazardous materials table listing NA 9279, Hydrogen absorbed in metal hydride and UN 3468, Hydrogen in a metal hydride storage system. Both of these listing assign a hazard classification to the systems of 2.1 flammable gas. Currently these identifications can only be used with approval from the OHMS after review and approval of the packaging. No packaging instructions have been adopted in either the US regulations or the international Model Regulations. The OHMS has issued several special permits for metal hydride hydrogen storage systems, essentially approving the packaging and exempting them from §173.301(d).

Currently there are no known special permits issued for non-reversible hydride-based hydrogen storage systems. However there are numerous companies and organizations that are developing various types of systems. Systems of this type have been tested by the military, government laboratories and corporations over a number of years. Commercially available products are expected to become available within the next few years. DOE—through its Hydrogen, Fuel Cells and Infrastructure Technologies Program—has established three hydrogen storage research Centers of Excellence, one on metal hydride (i.e., rechargeable) and one on chemical hydride (i.e., non-rechargeable) materials and systems.

Recommendations

For non-rechargeable or chemical hydride-based systems, there has been little work on developing system standards. This is partly due to the broad range of materials and system designs and the fact that most are currently proprietary and not commercially available. It is recommended that the DOT review current proposed chemical hydride systems against current regulations to start developing requirements and guidelines for potential special permits to regulations that would prohibit the systems. Experience from this effort could be used for eventual inclusion of possible new entries to the Hazardous Materials table (§172.101) and/or development of packaging specifications, specifically for Subpart E of §173. The review should include persons from industry, the DOE Centers of Excellence and the DOT.

To help ensure that any standards being developed for hydride-based hydrogen storage systems meet the need of OHMS, it is recommended that the OHMS assign personnel or contractors to actively participate on applicable standards development committees. These might include ISO, IEC, CGA, and UL committees.