hydrogen fire hypothesis - Department of Aerospace Engineering
hydrogen fire hypothesis - Department of Aerospace Engineering
AVIATION FIRE DYNAMICS
CHALLENGES WITH HYDROGEN
FIRES AND THEIR SUPPRESSION
HYDROGEN: BACKGROUND INFORMATION
Hydrogen is the lightest element and its monatomic form (H1) is the most
abundant chemical substance.
At standard temperature and pressure, hydrogen is
a colorless, odorless, tasteless, non-toxic, nonmetallic,
highly combustible diatomic gas with the molecular formula H2.
Hydrogen readily forms covalent compounds with most non-metallic
elements and is present in the water molecule and in most organic
compounds. So, naturally occurring atomic hydrogen is rare on earth.
Hydrogen gas produces water when burned, a property which later gave it
its name (in Greek, hydrogen means "water-former“)
Hydrogen poses a number of hazards to human safety, from
potential detonations and fires when mixed with air to being
an asphyxiant in its pure, oxygen-free form.
In addition, liquid hydrogen is a cryogen and presents dangers (such
as frostbite) associated with very cold liquids.
Hydrogen dissolves in many metals, and, in addition to leaking out, may
have adverse effects on them, such as hydrogen embrittlement leading to
cracks and explosions.
Ideally, no oxygen should be present in the liquid hydrogen tanks in a fuel
cell vehicle, but trace amounts of air may contaminate the hydrogen supply.
Hydrogen is highly flammable and requires only a small amount of energy to
Hydrogen gas forms explosive mixtures with air and with chlorine.
Hydrogen requires only one 10th as much energy to ignite as gasoline does.
A spark of static electricity from a person's finger is enough to set it off.
EXPOSIVE LIMITS OF COMBUSTIBLE GASES
WHEN MIXED WITH AIR
% Gas in Mixture With Air
With 4%-75% hydrogen, very little energy is required to ignite
hydrogen. The speed of a hydrogen fire is ten feet per second to 6750
4% Barely burn and even then only upward
5% Fire starts burning sideways
18% Fire will now burn faster than the speed of sound
45% Burning at 6750 ft./sec -including shock wave
74% Barely burn
75% Will not burn-no air
Hydrogen gas leaking into external air may spontaneously ignite. Moreover,
hydrogen fire, while being extremely hot, is almost invisible, and thus can
lead to accidental burns
If leaking from a pipe at a high enough pressure, hydrogen gas can selfignite without the aid of an external energy source
Hydrogen has a much wider flammability range and detonability limits
compared to most hydrocarbons
A hydrogen flame poses special dangers beyond those posed by
hydrocarbon flames because human senses cannot easily detect it.
If you come upon a hydrogen fire, you will not see it – even up close. You
might see sparkles, dust particles briefly burning
You won’t feel the heat of the hydrogen fire because very little infrared
radiation (which gives us the sensation of heat) is present
The physical flame is stronger than a hydrocarbon flame.
The mixtures spontaneously explode by spark, heat or sunlight.
Hydrogen flames can burn in a strong wind and be stretched out away from
its source a number of feet
THE HINDENBURG FIRE
Hindenburg disaster took place on Thursday, May 6, 1937
German passenger airship LZ 29 Hindenburg caught fire and was destroyed
during its attempt to dock
Of the 97 people on board (36 passengers, 61 crew), there were 35 fatalities;
there was also one death among the ground crew
The actual cause of the fire remains unknown, although a variety of
hypotheses have been put forward for both the cause of ignition and the
initial fuel for the ensuing fire
STATIC SPARK HYPOTHESIS
ENGINE FAILURE HYPOTHESIS
INCENDIARY PAINT HYPOTHESIS
HYDROGEN FIRE HYPOTHESIS
Paint was both the point of ignition and the sole cause of the rapid spread
of the fire.
“It did not look like a hydrogen fire”
The hydrogen within the airship is held blameless for both the start of the
fire and its subsequent advance
The chemical composition of the Hindenburg paint is known, and it
burns too slowly.
Offering support for the hypothesis that there was some sort of
hydrogen leak prior to the fire is that the airship remained sternheavy before landing.
Hindenburg's skin was covered with the extremely flammable cellulose
nitrate or cellulose acetate, added to help with rigidity and
The skin was also coated with flecks of aluminum, a component of
rocket fuel, to reflect sunlight and keep the hydrogen from heating and
These substances caught fire from an electric spark that caused the skin
At this point the hydrogen became the fuel to the already existing fire
The Challenger flew nine successful missions before that fateful day of the
disaster in 1986
The Challenger was scheduled to carry some cargo, the Tracking Data Relay
Satellite-2, as well as fly the Shuttle-Pointed Tool for Astronomy (SPARTAN203)
The Challenger had 7 crew members and it was the first flight of a new
program called TISP (Teacher In Space Program).
Seventy three seconds into the mission, the Challenger exploded, killing the
The commission's report cited the cause of the disaster as a the failure of an
“O-ring” seal in the solid-fuel rocket.
The faulty design of the seal coupled with the unusually cold weather, let
hot gases to leak through the joint.
Booster rocket flames were able to pass through the failed seal enlarging
the small hole. These flames then burned through the Space Shuttle
Challenger's external fuel tank and through one of the supports that
attached the booster to the side of the tank.
That booster broke loose and collided with the tank, piercing the tank's side.
Liquid hydrogen and liquid oxygen fuels from the tank and booster mixed
and ignited, causing the Space Shuttle Challenger to tear apart.
MORE ON CHALLENGER DISASTER
WHY HYDROGEN SAFETY
RESEARCH IN FIRE SAFETY FOR HYDROGENFUELED VEHICLES
Hydrogen is currently produced on an industrial scale of about 10 million
By 2040, it is anticipated that the use of hydrogen in fuel cell powered
vehicles could replace the consumption of petroleum.
To be economically competitive with the present fossil fuel economy, the
cost of fuel cells must be lowered by a factor of 10 or more and the cost of
producing hydrogen must be lowered by a factor of 4.
Moreover, the performance and reliability of hydrogen technology for
transportation and other uses must be improved dramatically. The role of
hydrogen safety to achieve it can not be underestimated.
For automotive applications it is expected that the level of safety at the
consumer interface with hydrogen is similar or exceeds that present with
natural gas usage.
Hydrogen safety research was initiated decades ago as a result of accidents
in the process industries
CFD tools are used to study the phenomena of hydrogen combustion,
deflagration-to-detonation transition and hazard and risk assessment
In spite of recent progress there are still some unresolved issues of hydrogen
SLOW SUBSONIC LEAKS
Characteristic for connections and holes in delivery pipes.
The main area of concern is private and maintenance garages, car parks,
and other automotive infrastructures
Diffusion of high pressure hydrogen through storage vessel materials to
Permeation is a slow molecular flow regime through the surface material
and must be modeled as diffusion through a solid rather than a flow.
The process of permeation involves the absorption and dissociation of
molecular hydrogen to atomic hydrogen on the internal surface
Then atomic hydrogen diffuse through the metal to the opposite surface
where atoms recombine and undergo desorption
HIGH-MOMENTUM LARGE-VOLUME RELEASE
Characteristic for accidental breaks of high-pressure tanks or activation
of pressure relief device (PRD), e.g. by external fire
Hydrogen discharges through circular orifices larger than a critical
diameter sustain stable lifted flames irrespective of the reservoir
pressure driving the release.
At smaller diameters, however, stable burning will only be achieved at
operating pressures higher than a particular value.
LIFT-OFF VELOCITY, LIFT-OFF HEIGHT,
AND BLOW-OUT VELOCITY
Flame stability is usually characterized by lift-off velocity, lift-off height, and
Defined as the mean jet velocity at which the flame becomes lifted above the
jet exit rim.
If the jet velocity is further increased, the flame moves downstream to a
position where it stabilizes.
Distance between the lifted flame base and the jet exit.
Jet velocity at which the reaction cannot be sustained and the flame is
For positions between the jet exit and the flame base, a turbulent mixing
The stability limits of turbulent jet diffusion flames are important for
operation of combustion systems and have safety implication for
handling combustible fuels
EFFECTS OF THESE FACTORS ON HYDROGEN
Hydrogen has the much higher turbulent blow-off velocity in comparison
to that of hydrocarbons.
This permits turbulent diffusion hydrogen flames to exist at velocities far
in excess to those possible for turbulent diffusion hydrocarbon flames in
The flame lift-off height of the pure hydrogen jet diffusion flame was
found to increase with the jet exit velocity.
The lift-off length and blow-off velocity have strong implications in
safety considerations and hazard/risk assessment.
DEFLAGRATIONS AND DETONATIONS
There are two types of explosions:
Characterized by a flame propagation velocity below the speed of
Typically far below 100 m/s, and relatively modest overpressures,
say below 0.5 bar.
Subsonic combustion propagating through heat transfer; hot
burning material heats the next layer of cold material and ignites
Most fire found in daily life, from flames to explosions, is
Involves a supersonic exothermic front accelerating through a
medium that eventually drives a shock front propagating directly in
front of it.
flame propagation velocities, perhaps up to 2000 m/s, and substantial
overpressures, up to 20 bars
Detonations are observed in both conventional solid and liquid
explosives, as well as in reactive gases.
The velocity of detonations in solid and liquid explosives is much
higher than that in gaseous ones, which allows the wave system to be
observed with greater detail
The ability of a hydrogen-air mixture to detonate is greater than that of
For a stoichiometric hydrogen-air mixture, detonation once initiated will
propagate as long as the mixture is within the detonability limits.
The direct initiation of hydrogen-air mixture detonation is possible by just
1.1 g of tetryl.
Only 1.86 g of TNT is needed to initiate detonation in a 34.7% hydrogen-air
mixture in the open atmosphere.
For a 20% hydrogen-air mixture, however, the critical charge increases
significantly to 190 g of TNT.
Geometrical conditions such as partial confinement and obstacles in the
flame path may result in a subsonic flame to accelerate from subsonic speed
to supersonic speed, transitioning from deflagration to detonation.
The exact mechanism is not fully understood
Existing theories are able to explain and model both deflagrations and
detonations but there are no theories at present which can predict the
DDT OF HYDROGEN
A specific feature of hydrogen-air deflagrations compared to of the majority
of hydrocarbon-air deflagrations is the readiness to DDT
This occurs when mitigating confined deflagrations by the venting
A needle-like structured flame front can be induced by the venting.
The onset of detonation is directly linked to the needle-like structured flame
front formed by sudden venting.
HYDROGEN SAFETY ASSESSMENT
IN AIRCRAFT AND GROUND
HYDROGEN FUELLED AIRCRAFT
An average airline passenger would opt for the aircraft fueled with Jet A.
Aircraft fueled with liquid hydrogen (LH2) would be declared unsafe.
Mainly because of Hindenburg fire disaster
Hazard due to hydrogen leaks : Liquid <-> vapor
Greater ignitability of hydrogen relative to jet fuel
“Flameless detonations“ : pressure pulses and fire balls
NASA has sponsored a number of studies to explore the potential of LH2,
liquid methane (LCH4) and synthetic aviation-grade jet fuel (synjet) as
alternatives to petroleum derived Jet A for use in future transport aircraft
It is highly available, less costly, produces high energy and less noise
PASSENGERS WILL BE EXPECTED TO SURVIVE THE
LH2 tanks are less apt to be ruptured, thus minimizing spillage potential :
It is important to minimize the surface area-to-volume ratios of these tanks
To minimize the weight of the tank and insulation system
To decrease the opportunity for heat to leak into the cold fuel and cause it
Therefore it is preferable to locate the tanks for the cryogenic fuels in the
fuselage for a hydrogen-fueled airplane.
This arrangement has been shown to provide lowest direct operating cost, a
realistic measure of aircraft operating efficiency as well
In conventional aircrafts, fuel is tanked in the wing between the
front and rear spars
LH2 tanks are less susceptible to damage:
a) They are mounted in the fuselage where they present a far smaller
dimension to frontal impact.
b) They are protected by a significant amount of structure, both ahead
of and beneath, which must be crushed or penetrated before the
tanks are exposed.
c) They are designed for higher pressure than the rest of the fuselage
and therefore are less apt to be the point of failure in an impact
2) In the event a tank is ruptured and fuel is spilled, LH2 evaporates almost
immediately, and it dissipates into the atmosphere so rapidly. So the time
and the area exposed to flammable mixtures are both quite small compared
to the other candidate fuels
An assessment of the spreading and vaporization of the spilled liquid:
Results show that LH2 spreads the least and evaporates the quickest in all
The behavior of the fuel vapor cloud after vaporization has occurred:
Hydrogen rises rapidly in every case and therefore poses minimum risk to areas
surrounding a spill. The other fuels are non-buoyant for extended periods and
flammable clouds are blown along the ground
If the spilled fuel is ignited, the duration of the hydrogen fire would be
so brief that it would not heat the fuselage to the point of collapse, as would be
the case with the other fuels, and the heat-affected area would be substantially
The fuselage should collapse after ~40 s in a methane flame, after ~ 50 s in hydrogen
flame and after ~120 s in a JP-4 or Jet A flame.
When these fuselage collapse times are compared with the calculated burning times of
the fuel spills for the worst survivable case it is seen that LH2 will pose no problem.
The pool fire would last only 15 s.
For LCH4 the pool fire would last 38 s, uncomfortably close to the 40 s calculated for
Although the pool burning times were not calculated for JP-4 and Jet A, it is obvious
from accident experience that fuselage collapse would occur in event those fuels were
LH2 was found to be a safer fuel than any of the other candidates.
The passengers on board, and people and property in the immediate
surroundings, would be exposed to less hazard if a crashed aircraft
were fueled with LH2.
BLAST WAVES AND FIREBALLS GENERATED BY
HYDROGEN FUEL TANK RUPTURE
This paper describes and analyzes the results of Type 4 hydrogen fuel tank
fire exposure tests.
Project explored fire safety issues with an on-board hydrogen storage tank
Aimed at replacing compressed natural gas containers with hydrogen fuel
TYPE-4 HYDROGEN CYLINDER RUPTURE
Comprised mainly of a high-density polyethylene inner liner, a carbon
fiber structural layer, followed by a fiberglass protective layer
Composite material on the surface of the tank ignited approximately 45
seconds after the bonfire exposure.
After 6 minutes and 27 seconds, the cylinder failed catastrophically
through the bottom, launching the 30.9 lb main portion 270 ft. from the
Blast pressures were 300 KPa at 6.3ft. and 41 kPa at 21.3 ft.
A typical compressed hydrogen tank, when exposed to a bonfire,
presents safety challenges.
The consequence of a rupture is catastrophic.
In the test, blast pressures of 6 psi were measured 21 ft. away from the
tank, and debris was propelled more than 250 ft.
The test results suggest that bonfire protection and pressure relief
sensing for Type 4 hydrogen tank will require some sophistication.
HYDROGEN SAFETY MEASURES
Ventilation is useful for preventing the formation of explosive atmospheres
when hydrogen is dealt with in confined space.
The basic principle of ventilation is to bring fresh outside air inside the room
and remove the inside air. The main advantages of using ventilation in
hydrogen systems are:
It prevents the accumulation of explosive gas inside the room.
It dilutes the explosive gas by bringing fresh air in.
It limits the resident time of explosive atmospheres.
Ventilation can be of two types:
Selection of appropriate type depends on many factors.
Natural ventilation functions due to temperature differences and winds.
Hydrogen buoyancy and diffusion characteristics should be taken into
account while selection the size of opening and their positions.
Although natural ventilation is a cheaper option than the mechanical
ventilation, it is less controllable and is affected by weather conditions.
On the other hand mechanical ventilation can give constant and controlled
The sizing of mechanical ventilation is not a big concern. It can be sized
based on the previous experiences, based on calculations or based on
hydrogen leak detection sensitivity.
The main disadvantage of this type of ventilation is that the gas cloud and
air will be more turbulent and in this case if the gas cloud is ignited then the
explosions will be more severe.
Using a combination of natural and mechanical ventilation can be a solution
in some cases.
AUTOMATIC SHUTDOWN SYSTEM
It is defined as the replacement of a sufficient proportion of oxygen
contained in a gaseous atmosphere by an inert gas, to avoid the ignition of
It is an important way to prevent the formation of explosive atmosphere.
However, it can be dangerous for the workers due to the asphyxiating
property of inert gas.
The main inerting methods include:
Pressure swing method
Volume swing method
SCHEMATIC OF HYDROGEN-AIR-INERT MIXTURES:
The conditions of a hydrogenated atmosphere to be subjected to inerting
can be derived from the triangular diagram
The three colored zones in the diagram represent:
Red zone: It corresponds to the explosion area
Orange zone: It corresponds to the mixtures which are not explosive but which may
explode if they are mixed with air.
Green zone: It corresponds to mixtures which are not explosive and which will not
explode if they are mixed with air.
A recombiner is a device that promotes the recombination of hydrogen and
oxygen to form water.
It serves to avoid, remove or at least slow down the formation of flammable
mixtures formed due to leakage of hydrogen.
Recombiners can be classified as:
Use heat to initiate conversion
Use the property of hydrogen that, in presence of catalysts like platinum and
palladium hydrogen reacts with oxygen at low temperatures in an exothermic
While using recombiners appropriate measures should be taken to keep the
system temperatures below the self-ignition temperature.
CONTROL OF IGNITION SOURCES
Electrical equipments, hot surfaces, open flames, static electricity are
the possible ways of ignition of hydrogen and propagation of the flame.
By controlling these sources explosions can be avoided
DETECTION OF HYDROGEN LEAKS
The major hazards due to an unwanted release of hydrogen are connected
with the possibility of building up explosive conditions.
Hydrogen is more dangerous due to its high flammability and low ignition
energy. Thus appropriate detection measures should always be taken into
The main criteria for selecting a hydrogen sensor are listed below.
Calibration / Maintenance
DETECTION OF HYDROGEN FLAMES
Hydrogen burns with a pale blue flame and does not emit visible light or
smoke. Hydrogen flames emit significantly less heat.
Thus it is difficult to detect the flames by human perception. Therefore
hydrogen fire detectors are necessary.
Special imaging systems are required for determining the size and
location of the flame. Many times it is difficult to differentiate hydrogen
related signals and the parasitic signals. Thus to prevent wrong alarms
human analysis may be advantageous.
The hydrogen fire detectors should posses following main
Detect hydrogen flame at sufficient distance
Detect small flames
In case of fires the supply of hydrogen should be shut off as soon as
possible since the hydrogen fires are not extinguished due to the danger
of re-ignition or explosion.
The adjacent metal surfaces should be cooled by water or other means
as they can be the possible source of re-ignition.
Water can be used as a fire extinguisher. Small fires can be extinguished
by dry chemical extinguishers or with carbon dioxide, nitrogen, and
ACTIVE INERTING, SUPPRESSION AND ISOLATION
Inerting as described before is used for dilution of explosive atmosphere
to prevent ignition.
In situations where no human activity is involved all air can be replaced
by inert gas.
Typical inert gases used are nitrogen, carbon-di-oxide and some special
mixtures, for example Inergen (mainly Ar and N2, some CO2), Argonite
A chemical suppression or isolation system may also be used in
quenching the flame. But they pose more challenges. The flames may
propagate faster due to the turbulence created by suppression systems.
In isolation systems, early detection of fire and fast isolation is necessary
WATER BASED PROTECTION SYSTEMS
Water can be used to extinguish the fire. Water spray system can be
provided for hydrogen storage containers, grouped piping, and pumps
where potential fire hazard exists.
The system should deliver a uniform spray pattern over 100 percent
surface of the container, pump and adjacent piping.
EMERGENCY RESPONSE MEASURES
The emergency response plan should be based on systematic
identification of hazards, followed by evaluation risk management.
The aim should be minimization of risk to people. Communication is a
key element in any emergency response plan.
Effective communication will involve technical measures, organization,
procedures and training adapted to each other and to the overall
If communication fails, effective emergency response is not possible.
Effective emergency response will also require an organization intended
and prepared for emergency response.
DETERMINING THE SAFETY DISTANCE
The location and distance of hydrogen storage from people and other
facilities should be decided in such a way that the effects of fire,
explosion and detonation, if occurs, can be reduced to tolerable levels.
While selecting a location the effects of possible combustible cloud drift
and thermal radiations should be considered.
The hydrogen storage facilities should be located outdoors or in a
building specially built for the purpose.
Hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic,
highly combustible diatomic gas.
One cannot see hydrogen fire even up close and cannot feel the heat of the hydrogen
fire but the physical flame is stronger than a hydrocarbon flame.
Hindenburg airship and challenger space shuttle disaster : Hydrogen fire disasters
LH2 was found to be a safer fuel than any of the other candidates.
Type 4 hydrogen tank will require some sophistication.
Prevent the hydrogen from escaping by following good process maintenance
In case of leak the area should be well ventilated to prevent hydrogen build up.
Gas detectors and flame detectors can quickly identify a gas leak or the resulting
Combustible gas detection technologies:
Flame detection technologies:
UV flame detectors
Multi-spectrum IR flame detectors
Chemical suppression system and venting method should be handled with care.
Blast waves and fireballs generated by hydrogen fuel tank rupture during fire exposure” : Robert
Zalosh, Firexplo, Wellesley
“Large scale experiments to study fires following the rupture of high pressure pipelines conveying
natural gas and natural gas/hydrogen mixtures”: B.J. Lowesmith, G. Hankinson
“An Assessment of the Safety of Hydrogen-Fueled Aircraft”: G. Daniel Brewer
“Research in Fire Safety for Hydrogen-Fueled Vehicles”: Kennerly H. Digges, R. Rhoads Stephenson
“Handling and safety of Hydrogen”: Itsuki Uehara
The Hindenburg Fire: Hydrogen or Incendiary Paint?”: A. J. Dessler, D. E. Overs, and W. H. Appleby
“Hydrogen safety research”: Molkov, V.
“Hydrogen Plant Explosion and Fire”
“Twenty Hydrogen Myths” : Amory Lovins
Analysis of Published Hydrogen Vehicle Safety Research” : U.S. Department of Transportation
“A numerical study of hydrogen or helium release and mixing in partially confined spaces” : K.
Prasad, W. M. Pitts and J. C. Yang