International scientists and engineers are working with reactive metal-hydride hydrogen storage materials in their laboratories. Activities ranging from new materials discovery, to properties characterization, and engineered system development are being performed to develop safe and economically favorable on-board and off-board storage for hydrogen-powered vehicles and stationary applications of the future. The materials must be able to quickly and efficiently absorb hydrogen during the fueling process and then release it appropriately during the duty cycle of the vehicle or stationary system.
Metal hydrides are designed to be reactive with hydrogen gas. This characteristic is also typically associated with reactivity to other gases and liquids, such as humid air. Like any new energy storage technology, the behavior and hazards must be understood and managed to enable safe and effective utilization. Improper storage or use of these materials could result in unfavorable behavior and exposure, so care is needed to maintain a safe and effective operating environment. Most critically, a robust inert gas environment in the form of a laboratory glove box is required to avoid safety events and protect the scientists and their equipment. Ten safety event records in our database illustrate the kinds of incidents that occur and the necessity of careful laboratory practices for handling and operation of metal hydride systems.
Lessons have been learned from these safety events in laboratories where metal hydrides are stored and handled (see alphabetical listing below). Seventy percent of these events are categorized as resulting from equipment failure according to their “owners.” But it’s interesting to note that procedural issues played a hand every single one of them, which may indicate that best practices for working with reactive metal hydrides have not been clearly communicated and understood in some laboratories. Best practices for safe storage and handling of metal hydrides are described in H2BestPractices.
Setting: Laboratory
Description: A university researcher reported that a fire resulted when he scraped lithium aluminum hydride (LiAlH4) out of the glass jar in which it was contained (see attached photo). The jar had been in the laboratory since 2005 (about 6 years), so the LiAlH4 was old. The researcher was using a dry metal spatula to scrape the LiAlH4 out of the jar. A quick review of the manufacturer's Material Safety Data Sheet (MSDS) for LiAlH4 informed the researcher of its moisture sensitivity, but there was no indication of friction causing a fire. However, the supervising faculty member reported personal knowledge that friction can cause ignition of LiAlH4.
Lessons Learned: As stated on the MSDS and also on the container labels, LiAlH4 should be handled under argon. LiAlH4 is advertised and sold as a powder. If the researcher had to scrape it out of the jar, then it was no longer a powder, which seems indicative of past reaction that may have been due to exposure to atmospheric moisture.
The manufacturer stated that they do not have any first-hand data suggesting that friction alone could cause ignition. All of their handling of LiAlH4 is performed inside a glove bag under an argon atmosphere, so they have never had a fire during the packaging process. They recommend handling LiAlH4 under argon in a glove box or glove bag to minimize oxygen and moisture contact and, therefore, minimize the chance of a fire.
The university ES&H department did some searching online and found several relevant websites that provide confirmation that friction alone in the presence of air may be able to ignite LiAlH4.
Setting: Laboratory
Description: A guest student was weighing out less than 200 mg of sodium hydride. The material reacted with moisture in the air, producing hydrogen. The heat of the reaction ignited the hydrogen on the end of the spatula being used to transfer the material and at the mouth of the bag holding the stock material (approximately 48 to 50 g). The student attempted to smother the flames with a cotton lab coat hanging nearby. He quickly determined that the lab coat was insufficient to smother the flames and entered the adjoining lab to get a fire extinguisher and warn other lab workers in the area. The other lab workers exited the lab, warned others in the area, pulled the fire alarm and called the laboratory shift supervisor. The student extinguished the fire with the fire extinguisher, then left the building with the other occupants.
Members of the onsite fire department responded and entered the building at 16:04. They noted the absence of smoke in the hallways, dissipating smoke in the lab and no apparent flames. The Hazardous Materials Response Team was assembled to develop a plan for mitigation. Building occupants were escorted back into the building to power down critical experiments and at 18:00 all occupants were given the opportunity to re-enter the building to collect personal items and leave for the day. At 19:20, the Hazardous Materials Response Team entered the laboratory to execute the mitigation plan, removing the materials from the area and wiping down the area with mineral oil. There were no injuries resulting from this event.
Lessons Learned: Some hydride materials (e.g., sodium alanates) may be rapidly exothermic, even pyrophoric, if exposed to water or humid air or slowly exothermic, even pyrophoric, if exposed to oxygen. Reactive materials, including fine metal powders, should be handled (as in this incident) in an inert atmosphere such as a glove box. The protocol for handling these materials should be incorporated into a standard operating procedure and appropriate safety training conducted for laboratory personnel, including guest staff.
Setting: Laboratory
Description: No description given
Lessons Learned: Metal hydride materials of a composition which is not well characterized should be handled with procedures that assume a "worst case" for that class of materials, intermediates or precursors.
Laboratory procedures should be in written form and should be adopted only after performing a safety vulnerability analysis and adopting appropriate risk mitigation steps.
Working with small amounts of material does not provide assurance of safety.
The method described to seal samples that are highly reactive upon exposure to air is not recommended. An alternative method, that is now being used for packaging aluminum hydride samples for offsite shipment is as follows: Aluminum hydride powders (0.5 g - 1.0 g) are sealed in glass bottles with a cap. The bottles are sealed in a thick plastic bag under Ar using a plastic bag sealer in the glove box. The sample bags are then removed from the glove box and sealed under vacuum in a much larger plastic bag using a vacuum sealer. The purpose of the large evacuated bag is to contain any evolved hydrogen gas if the material begins to decompose and the internal bottle & bag rupture. It is a good idea to calculate the maximum volume of H2 (at 1-atm) that could be released by the sample to determine if the outer bag is sufficiently large to contain the evolved gas. The package is then inserted into a cardboard box of sufficient volume (about 1-liter) to accommodate the expanded outer bag.
Setting: Laboratory
Description: An experienced researcher with 30+ years of laboratory experience (including working with air-sensitive compounds) was disposing of a small vial of catalyst and hydride powder left in the laboratory by a post-doc. The researcher emptied the vial into a container of mineral oil inside a glove box, but a small amount of the hydride powder adhered to the wall of the vial. The vial was then removed from the glove box and brought over to a tall waste jar in the laboratory that contained isopropanol. (Isopropanol is the first (slowest-acting) pacifier used when deactivating pyrophoric hydrides.) The vial was opened and inverted over the isopropanol jar and the residue powder was tapped into the jar. There was a "small flash of flame" that quickly extinguished itself.
Lessons Learned: The project team concluded that the jar contained a sufficient vapor pressure of isopropanol to ignite when it came into contact with the decomposing hydride. The lesson learned was that hydrides react rapidly in air and can lead to combustion of any organic vapor that might be present nearby. Thus, the project team adopted a procedure that all hydrides must be submerged in mineral oil before they are removed from the glove box to prevent exposure to air before isopropanol treatment. Since this procedure was adopted for pacification/disposal of hydrides, there have been no more incidents or near-misses of this type in the laboratory.
Setting: Laboratory
Description: A metal hydride storage system was refilled using compressed hydrogen in a closed lab environment. The tank system is an in-house development and is optimized for high hydrogen storage density and use with an air-cooled fuel cell. The system is equipped with a pressure relief valve that opens gradually at 35 bar to protect the tank from overpressure conditions. The tank itself is designed to adsorb 400 g of hydrogen at a pressure less than 15 bar.
For refueling, the secondary pressure on the compressed hydrogen supply container was set to 20 bar and the adsorption of the hydride was started without hydrogen flow limitation. Due to the exothermic nature of the hydride upon recharge, as expected a sharp increase in tank temperature was measured. The tank was uncooled because the temperature increase leads to an increase in plateau pressure, which automatically reduces hydrogen uptake.
After 3 min 30 sec, the overpressure device was triggered and hydrogen was released into the lab. The hydrogen supply from the compressed gas cylinder was cut immediately and the window was opened. The operator was on site at the time of the incident. The amount of hydrogen released is hard to quantify, but we estimate something in the range of 200-1,000 normal liters. The overpressure device closed after some minutes.
Lessons Learned: Mechanical pressure gauges tend to be imprecise if only used in a narrow portion of the full scale. Digital transducers, although slightly more expensive, offer much more precision. The event happened because the set pressure was only 10% of full scale, and the error of the mechanical gauge was over 5%.
Setting: Laboratory
Description:
Lessons Learned:
Some of the key lessons learned from these safety events are summarized below.
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