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:
- 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 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 water—the encapsulating coating are then mechanically breached
as required to liberate gaseous hydrogen.
- 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.
- 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 gas—not 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. 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.
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.
Recommendations
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.
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