Reactive Hydrogen Storage May Be Mixtures (18.4)

Reactive Hydrogen Storage May Be Mixtures (18.4)

Criticality: High
Progress: Addressed, Not Adequately
Score: 20
DOT Relevance:

Description of Key Area

DOT hazardous materials regulations for transportation are written to provide a minimum level of safety when transporting materials that are dangerous and could pose a hazard if not appropriately controlled. Materials are divided by their potential hazard classification, defined in 49 CFR 173 of the hazardous materials regulations. This part also contains criteria to be followed for packaging the materials, including prohibitions on combining certain types of materials that could react together. The current packaging specifications, and especially the prohibitions on mixtures, may not be appropriate for metal hydride-based hydrogen storage systems and may forbid their transport as currently written.

There are two broad types of metal hydride hydrogen storage systems that are being developed and need to be considered, rechargeable and non-rechargeable systems; where rechargeable systems contain a reversible metal hydride and are refilled by applying hydrogen to the system and non-rechargeable systems are refilled by removing the spent hydrogen-depleted material and replacing it with fresh hydrogen-containing material. Any regeneration of the hydrogen-containing material from the hydrogen-depleted material in non-rechargeable systems is done independent of the storage system.

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) → H2(g) + NaOH(aq)
    Various method of containing reactants and controlling 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 waterthe encapsulating coating are then mechanically breached as required to 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 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.
  3. Reacting sodium borohydride catalytically with water to produce hydrogen and sodium borate. The reaction is:
    NaBH4(aq) + 2 H2O(l) (cat.) → 4 H2(g) + NaBO2(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.

Rechargeable metal hydride systems will normally contain gaseous hydrogen and a solid phase. Reversible hydrogen storage materials, by design, decompose under the operational conditions of the storage system to release gaseous hydrogen through a reversible decomposition reaction. By changing conditions, such as increasing the hydrogen gas pressure or reducing temperature, the reverse formation reaction occurs producing the hydrogen-containing solid phase once more. This process is normally endothermic (absorbing heat) for the decomposition reaction and exothermic (producing heat) for the formation reaction.

Metal hydride-based hydrogen storage systems are more complex than simple storage containers. For example two engineered features that reversible systems will likely contain for proper and optimal operation include a manner to transfer heat between the contained solid phase and an external heat sink and a method of preventing the solid phase from being redistributed within the container. Non-rechargeable system may include multiple compartments, heat exchangers, valve and manifolds, etc. Such engineered features are normally not found in simple packaging for transport and may mitigate potential hazards in case of an accident by minimizing release of material or restricting the ability of them to react together or with air.

When the requirements of 49 CFR 173 are examined for the example hydride-based hydrogen storage systems, several conflicts are readily apparent. First, the systems contain materials that react to produce hydrogen, a flammable gasnot allowed by 173.21(e), 173.21(g) and 173.24(e)(4). Second, the systems may not use DOT specification packaging as required 173.24(c) nor be able to use a metal cylinder (even though they may have hydrogen gas present) as required by 173.301(a)(1). If not properly designed and engineered, they might also not be allowed by paragraph 173.301(d).

Discussion of Criticality

The hazardous materials table currently includes two listings: NA 9279, Hydrogen absorbed in metal hydride and UN 3468, Hydrogen in a metal hydride storage system that may be used for the rechargeable (reversible) systems. These identifications can only be used with approval from the OHMS after review and approval of the packaging since no packaging instructions have been adopted in either the US regulations or the UN Model Regulations. The OHMS has issued several special permits for metal hydride hydrogen storage systems that are identified by either one or both of these identifiers.

The two identifiers, UN 3468 and NA 9279, are not suitable for most of the non-rechargeable systems. They might be best identified as mixed hazard systems based on the contained materials, reactants and by-products. Paragraphs 173.155, 173.156 and 173.222 (for Exceptions for Class 9 (miscellaneous hazardous materials), Exceptions for ORM materials and Dangerous goods in equipment, machinery or apparatus, respectively) might be applicable to these systems. Guidelines for packaging and testing of these systems will need to be developed. However it will be necessary to develop exceptions to certain restrictions, as listed previously, to allow these systems for transport.

While it is considered critical that packaging instructions be developed for hydride-based hydrogen storage systems, it is also recommended that the packaging instructions be designed so as to not prohibit new and innovative designs. This technology is relatively new and is evolving. New advanced materials and designs are expected. The packaging instructions should therefore be performance-based and avoid being too prescriptive, while ensuring a minimum level of safety.

Discussion of Progress

In the last several years, the US DOT issued hazardous materials table listing NA 9279, Hydrogen absorbed in metal hydride and the UN SCETDG approved entry UN 3468, Hydrogen in a metal hydride storage system to the List of Dangerous Goods. Both of these listings 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. DOEthrough its Hydrogen, Fuel Cells and Infrastructure Technologies Programhas 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.

ASME's Boiler and Pressure Vessel project team on hydrogen tanks is addressing metal hydride vessel design in a code case to Section VIII-1.


Currently rechargeable metal hydride hydrogen storage systems can be transported upon review and approval by the OHMS of the packaging using NA 9279 and UN 3468 identifiers and descriptions. It is recommended that the OHMS develop a minimum set of design and test criteria for packaging of systems that meet the UN 3468 and NA 9279 hazard descriptions and that meet the rechargeable system definition used in this report. These criteria should be provided to potential manufacturers and offerors for use in their design and testing of the storage systems and would help ensure consistency in application of rigor in determining the minimum level of safety. It is preferred that these criteria be performance-based. Ideally they would be based on ISO 16111, underdevelopment by an international committee of experts.

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 possible new entries to the Hazardous Materials table (172.101) and packaging specifications. The review should include persons from industry, the DOE Centers of Excellence, and DOT.

To help ensure that the 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.

From experience obtained from systems approved under these guidelines, they could, at an appropriate future time, be refined and used as a basis for a New Rule Making Proposal for conversion into regulations and incorporated into 49 CFR 173.