Thermal runaway

Published:  08 September, 2015

The many challenges of shaping an effective ARFF suppression strategy for lithium battery fires means that the aviation industry’s concerns are unlikely to go away for some time yet, writes Ronald M Butler, CEO of Energy Storage Safety Products International, Detroit, USA.

The reality is that lithium and lithium ion batteries are very different. Lithium batteries, or primary batteries, are single use and incapable of recharge. They contain lithium metal which is highly combustible and represents a Class D fire hazard.

The real value in lithium lies in the fact that it delivers extremely high energy densities in small configurations. Lithium batteries are used where recharge isn’t necessary or feasible but long-term energy output is required. Common applications include military use, medical applications and certain consumer electronics to name a few.

Lithium-ion (Li-ion) batteries, or secondary batteries, are rechargeable and used on an increasingly massive scale worldwide. Li-ion is used for applications that require recharge capability.  These batteries provide high energy density and can be recharged time after time. They contain no free lithium metal but do contain highly flammable electrolyte. Common applications that incorporate Li ion technology include laptops, cell phones, electric vehicles, aircraft, hospital equipment, and stationary energy storage systems to name a few.

The battery fire concern

The aviation community is increasingly concerned about batteries. The major aviation safety authorities including the US FAA and ICAO have already demonstrated their strong concerns regarding the battery fire issue. Each group has tested batteries relative to burn characteristics and baseline suppressant effectiveness and have issued strong language tempering the shipment and use of lithium batteries in aircraft. They are highly aware of the potential threats posed by lithium battery technology and that concern is shared by cargo and passenger aircraft. Real-world examples of battery fires in aviation include those ranging from laptop fires in aircraft cabins and e-cigarette fires in underbelly cargo holds to large-scale fires in cargo planes. To these organizations, the threat is real, so much so that recently the Air Line Pilots Association called for a ban on the bulk shipment of lithium batteries, not only in passenger craft, but on freighters as well.

ARFF fire fighting strategy and tactics

When designing strategies and tactics for lithium battery fires one must have a clear understanding of battery chemistry and configuration.

A fire response strategy for the lithium family of batteries should consist of two distinct branches. A strategy for managing Li-ion battery fires is different from that chosen for lithium metal battery fires. The reason for the different approaches lies mostly in the fact that Li-ion and lithium each demands a distinct type of suppression agent and action.

The two battery groups have different burn characteristics and thus require different suppression methodology. However, it may be the case that a suppressant strategy that is considered an ‘all hazards’ approach, or one that employs a suppressant and suppression methodology that is effective on class ABCD fires, may be an option.

A second group of considerations is focused on how the batteries are configured. For example, ARFF will approach a burning laptop consisting of five to seven lithium ion 18650 cells (about the size of AA batteries) in the relative expanse of an airport terminal far differently than it would multiple pallets of burning cells in an aircraft cargo hold. The fire loads for these distinct examples differ wildly, which affects how the fire is attacked, what suppression tools are employed and what fire suppressants are used.

Suppressant of choice

When developing standard operating procedures for addressing lithium battery fire issues, the suppression agent of choice is the primary consideration. This is the point when a clear understanding of suppressant effectiveness relative to lithium and Li-ion battery fires is crucial.

As mentioned, lithium (primary) batteries contain lithium metal, and as such would generally require fire suppression methods geared toward class D combustible metals. Li ion, on the other hand, is considered by the NFPA as a common combustible, which generally requires a Class A suppression approach.

A large reservoir of data that would confirm or refute the effectiveness of a wide array of suppressants on lithium and Li-ion batteries does not yet exist. However, an understanding of lithium and Li ion battery fire behaviour, coupled with knowledge of agent suppression mechanisms, may pave the way for a common-sense approach to answer these suppressant choice questions.

In order to reason through the problem, starting with battery chemistry and comparing the potential effectiveness of common, available suppressants, begins to eliminate those that don’t fit.

Since lithium (primary) batteries are considered a class “D” fire suppression hazard, then the appropriate suppressant would be effective on combustible metal fires. Again, the NFPA is clear on this, making suppressant choice decisions much more straightforward. However, questions arise as we begin to compare available class “D” suppressants.

Burning Li-ion, on the other hand, requires a more conventional fire suppression approach, and one that has cooling at its core.

The primary considerations when choosing a suppressant for lithium battery fires focuses on smothering and forming a barrier between the burning fuel and air. Li-ion battery fires on the other hand generally require the ability to cool and penetrate layers of containment (shipping materials, etc.).

When batteries burn, they are subject to a process called ‘thermal runaway’. Thermal runaway can simply be viewed as a build-up of heat in a cell that grows exponentially leading to further creation of heat. In other words, the cell creates more heat than it can give away. The cell is rapidly consumed as the fire spreads to adjacent cells or other equipment. As a result, the only practical fire outcomes are the exhaustion of fuel or intervention in the form of suppression.

While as a common (class A) fire hazard, Li-ion battery fires can be suppressed with water, the suppression methodology must take into account load configuration, total fire load, and other considerations. The laptop example given earlier will require far less water for suppression than a load of multiple shipping containers packed into an aircraft cargo hold. The laptop might be managed with a hand-held extinguisher, whereas the cargo will require a separate suppression strategy.

For this application, water may be the most suitable. Inert gases, or those that function in a similar fashion, such as CO2, Halon, Nitrogen, etc. will eliminate visible flame, but offer little or no cooling effect. Powders, like gases will remove the flame, but will also fail to cool the batteries. Gases and powders can be used in conjunction with a cooling suppressant, but are probably not best suited as a primary suppression agent.

Any discussion on ARFF fire response should include the strengths and weaknesses of fire fighting foams or the suite of additional suppressants employed by ARFF crews. The reader is asked to excuse the simple nature of the following discussion as it would take far too much space and time to thoroughly discuss the technical aspects of fire fighting foam. It is my goal to give a general overview of the suppressant and its power relative to lithium and Li-ion batteries.

The strength of AFFF (and other foams) is its effectiveness on two-dimensional hydrocarbon fuel fires. Its weakness, at least as it pertains to lithium battery fires, is two-fold. Firstly, foam tends to smother and limit the release of flammable vapours from burning liquids. It is not widely considered a great cooling agent. Li-ion fires, as established earlier, generally require strong cooling capacity. Secondly, foam works well on two-dimensional fires but many battery fires, certainly those in common cargo configurations, are three-dimensional in nature, which provides no environment for the ‘blanketing’ effect that allows foam to work best. It is for these reasons that fire fighting foams would not be considered excellent battery fire suppressants.

Configuration

How the burning battery is configured is of extreme importance to the ARFF response protocol. For example, if the batteries are part of air cargo and shipped in commonly-accepted configurations then the fire management strategy should be adjusted accordingly.

There are two general ways of shipping cargo in aircraft. One involves passenger aircraft that use the areas in the belly of the ship, while the other relies on dedicated freight craft to move the product.

The constraints that the ARFF unit will experience while fighting a battery fire in the underbelly areas include limited space and the potential for unstable freight. The space limitations create an often untenable fire fighting environment. This causes access issues for the ARFF teams and changes the suppression tactics, tools employed, and suppressant choice. For example, in the event of a fire in a palletised battery shipment, the location of the burning material becomes of prime importance. If the material is at the back of the plane or near freight loading doors, the job becomes somewhat easier for the firefighter. The difficult process of having to manoeuvre along the sides of the tightly-spaced cargo pallets and containers is less likely. However, if the burning battery material is in an area away from loading doors, the fire tactics will change as different tools may need to be employed, such as the ARFF rig-piercing nozzle. The tight spaces will limit the ability of some suppressants to reach the fire. This is where water and water-based suppressants will prove preferable because of their ability to not only cool but also reach longer distances and penetrate container material.

Summary

The points outlined in this article only scratch the surface relative to ARFF suppression strategies for lithium battery fires. Additionally, much research is yet to be done that addresses concerns relative to suppressant effectiveness, fire fighting tactics and other issues. However, the industry would do well to continue the discussions about lithium batteries and aircraft fire safety as well as those centred around the strategies and tactics employed by ARFF teams in response to these fires.

About the Author

Ronald M Butler is CEO of Energy Storage Safety Products International (ESSPI) of Detroit, MI USA. He is a retired fire officer of the Detroit Fire Department with experience in fire suppression systems and training systems design. ESSPI is involved in the design and development of unique fire suppression and containment systems for the transport and storage of batteries as well as the design and development of fire management systems for stationary energy storage applications. ESSPI is also deeply involved in the design and delivery of state-of-the-art training that addresses fire response issues for both the fire service and private industry. Ron can be reached at ron@esspi.com.

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