1
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 Hydrogen is not more dangerous or
safer compared to other fuels (energy carriers).
 Hydrogen safety fully depends on how professionally it is handled at
the design stage and afterwards.
 Hydrogen leak is difficult to detect: colourless, odourless and
tasteless gas, dimmest flame of any fuel in air
 High pressures (potential hazards to humans, equipment, structures):
hydrogen systems are used at pressures up to 100 MPa = 1000 bar
 Low temperatures (cold burns, etc.): down to -253oC = 20 K (liquefied
hydrogen = LH2)
 Hydrogen is mostly found in combination with other elements in form of
e.g. water, methane, biomass, etc.
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H
 Atomic hydrogen (H, mass 1.008 g/mol) is formed
by a nucleus with one unit of positive charge (proton) and one electron
(negatively charged “probability cloud” surrounding the nucleus like a
fuzzy, spherical shell). H atom is electrically neutral.
 Atom is highly reactive (embrittlement: atomic H solution in metals).
 The proton is more than 1800 times more massive than the electron.
Neutron can be present in the nucleus. Neutron has the same mass as
proton and does not carry a charge.
 The radius of the electron’s orbit (the size of the atom), is 100,000
times as large as the radius of the nucleus: 10-10 m (1 angstrom).
 Three isotopes: protium (99,985% in nature) - proton in the nucleus;
deuterium (0.015%) - proton and a neutron; tritium - proton and two
neutrons (generates β rays). Atomic mass 1, 2 and 3 respectively.
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H H
 In normal conditions hydrogen is a gas formed by
diatomic molecules, H2 2.016 g/mol), in which two hydrogen atoms
have formed a covalent bond. Molar volume of any ideal gas (1 atm,
0oC): 22.4 L/mol. Leak 2000 L/min = 2000/22.4*2 /60 = 2.98 g/s.
 Molecular hydrogen is a stable form compared to atomic hydrogen
because the atomic arrangement of a single electron orbiting a nucleus
is highly reactive. For this reason, hydrogen atoms naturally combine
into pairs.
 Hydrogen is colourless, odourless and insipid (tasteless). That is
why its leak is difficult to detect. Compounds such as mercaptans, which
are used to scent natural gas, cannot be added to hydrogen for use in
PEM (proton exchange membrane) fuel cells as they contain sulphur
that would poison the fuel cell’s catalyst (platinum).
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 Two forms of molecular hydrogen are
ortho-, and para-hydrogen. They are
distinguished by the relative nuclear
spin-rotation of the individual atoms.
 These molecules have slightly
different physical properties
but are chemically equivalent.
 The equilibrium mixture of ortho- and
para-hydrogen at any temperature is
referred to as equilibrium hydrogen.
 Normal hydrogen: the equilibrium
ortho-para-hydrogen mixture 75%:25%
at room temperature.
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 Boil-off is the vaporization of LH2 due to heat transfer
from surroundings to a storage tank. Hydrogen has to be released not to
exceed working pressure of the LH2 tank. Cause of H2 release!
 At lower temperatures, equilibrium favours the existence of the less
energetic para-hydrogen (LH2 at 20 K is 99,8% of para-H2).
 The para-ortho-hydrogen conversion is accompanied by a
consumption of heat, 703 kJ/kg at 20 K. No pressure growth!
 Cryo-compressed hydrogen (CCH2) storage is favourable compared to
LH2 due to exclusion of the boil-off at day-to-day driving (weekly?).
 Due to conversion of para- to ortho-hydrogen during “consumption” of
external heat the release of hydrogen from the storage tank as a result
of boil-off is practically excluded for CCH2 storage with clear safety
improvement.
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 Hydrogen is used as gas, liquid, or slush. Slush hydrogen is a mixture
of solid and liquid hydrogen at the triple point temperature (13.8 K).
 Three curves: boiling (condensation=liquefaction), melting (freezing),
sublimation (gas to solid).
 The triple point: all three phases can coexist (13.8 K, 7.2 kPa).
The vapour pressure of
slush hydrogen is low.
Safety measures must be
taken during operations
to prevent air leakage
into the system that could
create flammable mixture.
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 The highest temperature, at which a hydrogen vapour can be liquefied,
is the critical temperature of 33.145 K (“critical point”). The
corresponding critical pressure is 1.3 MPa (density is 31.263 kg/m3).
 Above the critical temperature it is impossible to condense hydrogen
into liquid just by increasing the pressure. All you get is cryocompressed gas.
 The normal boiling and
melting points (pressure
101,325 kPa = 1 atm) are:
NBP = 20.3 K;
NMP = 14.1 K
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 Liquid hydrogen (LH2) at NBP has a density of 70.78 kg/m3. The
specific gravity is 0.071 (the reference substance is water with specific
gravity of 1). Thus, LH2 is 14 times less dense than water.
 Ironically, every cubic meter of water (made up of hydrogen and
oxygen) contains 111 kg of hydrogen whereas a cubic meter of LH2
contains only 70.78 kg of hydrogen. Thus, water packs more mass of
hydrogen per unit volume, because of its tight molecular structure, than
hydrogen. This is true of most liquid hydrocarbons “H2 carriers”.
 The higher density of the saturated hydrogen vapour at low
temperatures may cause the cloud to flow horizontally or
downward immediately upon release if liquid hydrogen leak occurs.
These facts have to be accounted for by first responders during
intervention at an accident scene.
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 Safety concern of LH2: low temperature
(NBP=20 K) (exception of Helium NBT=4.2 K, NMT=0.95 K): gases will
condense or solidify (Nitrogen: NBP=77.36 K, NMP=63.15 K;
Oxygen: NBP=90.2 K - liquid O2, NMP=54.36 K – solid O2).
 Leaks of air or other gases into direct exposure with liquid hydrogen can
lead to several hazards:



The solidified “other” gases can plug pipes and orifices and jam valves.
In a process known as cryo-pumping the reduction in volume of the condensing
gases may create a vacuum that can draw in “other” gases, e.g. air (oxidiser).
Large quantities of “other” material can accumulate due to “draw in” and displace
LH2 if the leak in persists for long periods. At some point, should the system be
warmed for maintenance, these frozen “other” materials will re-gasify possibly
resulting in high pressures or explosive mixtures. These other gases might
also carry heat into LH2 and cause enhanced evaporation losses or
“unexpected” pressure rise.
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 Oxygen (O2) enrichment can increase the flammability and even lead
to the formation of shock-sensitive compounds.
 Oxygen particulate in cryogenic hydrogen gas may even detonate.
 Vessels with LH2 have to be periodically warmed and purged to keep
the accumulated O2 content to less than 2%.
 Caution should be exercised if carbon dioxide (CO2) is used as a purge
gas. It may be difficult to remove all CO2 (mass 44 g/mol) from the
system low points where the gas can accumulate.
 Usually the condensation of atmospheric humidity during release to
atmosphere (spill of 5.11 m3=361.8 kg of LH2 in 38 s) will add water to
the mixture cloud, firstly making it visible (visible cloud shape is close
to flammable cloud), and secondly increasing the molecular mass of
the mixture.
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 The volume of LH2 expands with the addition of heat significantly more
than can be expected based on our experience with water. The
coefficient of thermal expansion at NBP is 23 times that of water at
ambient conditions.
 The significance for safety arises when cryogenic storage vessels have
insufficient ullage space to accommodate expansion of the liquid. This
can lead to an over pressurisation of the vessel or penetration of LH2
into transfer and vent lines.
 The phase change of LH2 to gaseous hydrogen (GH2), and yet
another volume increase for GH2 to warm from the NBP (20 K) to
293.15 K gives the expansion ratio of 847. This results in a final
pressure of 177 MPa (initial 0.101 MPa) if LH2 is in a closed vessel.
Pressure relief devices (PRD) should be installed as a safety measure.
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Hydrogen: 0.083 kg/m³; Methane: 0.6512 kg/m³;
Propane: 1.870 kg/m³; Gasoline vapours: 4.25 kg/m³.
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 The main hydrogen safety asset, i.e. its highest on The Earth buoyancy,
confers the ability to rapidly flow out of an incident scene, and mix with
the ambient air to a safe level below the flammability limit.
 Indeed, hydrogen density is 0.0838 kg/m3 (at normal temperature and
pressure (NTP) defined by IUPAC (International Union of Pure and
Applied Chemistry): 20 C and 105 Pa (previous definition with 1 atm is
discontinued – difference between two definitions is negligible for safety
engineering applications) which is far below air density 1.205 kg/m3.
 Contrary, heavier hydrocarbons are able to form a huge combustible
cloud, as in cases of disastrous Flixborough (1974) and Buncefield
(2005) explosions. In many practical situations, hydrocarbons may
pose stronger fire and explosion hazards than hydrogen. Hydrogen
high buoyancy affects its dispersion more than its high diffusivity.
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 Pure hydrogen is positively buoyant above a temperature of 22 K, i.e.
over almost the whole temperature range of its gaseous state.
 Outdoors only small fraction of released hydrogen would be able to
deflagrate. Indeed, a hydrogen-air cloud evolving from the release
upon the failure of a storage tank or pipeline liberates only a small
fraction of its thermal energy in case of a deflagration, which is in the
range 0.1-10% and in most cases below 1% of the total energy of
released hydrogen (Lind, 1975). This makes safety considerations of
hydrogen accident with large inventory at the open quite different from
that of other flammable gases with less or no consequences at all.
 Caution should be taken in applying GH2 buoyancy observations to
releases of hydrogen vapours at cryogenic temperatures. LH2 release
vapours at low temperature can be denser than air (plus H2O!).
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 Diffusivity of hydrogen is higher compared to other gases due to small
size of the molecule: diffusion coefficient of hydrogen in air is
6.8 cm2/s (methane 0.196 cm2/s; propane 0.0977 cm2/s).
 Hydrogen effective diffusion coefficient through gypsum panels is
found to be 1.45 cm2/s at 22 C. The interior of most garages in the U.S.
have large surface areas covered with gypsum panels. The diffusion
process should not be overlooked in the hazard assessment of release
of hydrogen in garages or enclosures lined with gypsum panels.
 The low viscosity of hydrogen and the small size of the molecule
cause a comparatively high flow rate if the gas leaks through
fittings, seals, porous materials, etc. This negative effect is to a
certain extent offset by the low energy density (volumetric) of
hydrogen in comparison with e.g. methane or other hydrocarbon gases.
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 Many safety problems related to interaction of hydrogen
with materials involve welds or the use of an improper material.
 Hydrogen is non-corrosive yet a reason for hydrogen embrittlement the process by which various metals, most importantly high-strength
steel, become brittle and fracture following exposure to hydrogen.
 Embrittlement involves a large number of variables: temperature and
pressure; purity, concentration, exposure time; stress state, physical/
mechanical properties, microstructure, surface conditions, nature of the
crack front, etc. The material must have certain minimum values of
these properties over the entire range of operation, with appropriate
consideration for non-operational conditions such as a fire.
 There is an atomic solution of hydrogen in metals. Permeated H atoms
recombine to molecule on the surface to diffuse into surrounding gas.
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 Compressed gaseous hydrogen (CGH2) behaves non-ideally. The AbelNoble equation of state is applied instead of the ideal gas state
equation. The density then can be calculated from the equation
p

bp  RH 2T
b=0.007691 m3/kg is the co-volume
constant, RH2 is the gas constant.
 The ideal gas equation
overestimate the density by:
1% (1.57 MPa), 10% (15.7 MPa),
30% (48 MPa), 40% (64 MPa),
50% (78.6 MPa).
Implication for separation distances.
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R T
 Speed of sound in gaseous hydrogen is 1294 m/s at 20oC
C g
and 355 m/s at NBP (20.3 K at pressure 101,325 Pa).
M
 The specific heat at constant pressure of liquid para-hydrogen is
cp=9.688 kJ/kg/K. This is more than double that of water and greater
than 5 times that of liquid oxygen at its NBP.
 Gas constant of hydrogen is 4.1243 kJ/kg/K (this is the universal gas
constant divided by the molecular mass).
 The specific heats ratio of hydrogen at 20oC and 1 atm is g =1.39 and at
0oC and 1 atm is g =1.405.
 Thermal conductivity of hydrogen is significantly higher than that of
other gases. GH2 (W/m/K): 0.187 (NTP), 0.01694 (NBP). LH2 (W/m/K):
0.09892 (NBP). Use in superconductivity applications.
 Hydrogen is non-carcinogenic, non-corrosive (yet embrittlement!).
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 At normal temperature hydrogen is a not very reactive,
unless it has been activated somehow, e.g. by an appropriate catalyser.
Hydrogen reacts with oxygen to form water at ambient temperature
extraordinarily slow. However, if the reaction is accelerated by a
catalyser or a spark, it proceeds with high rate and “explosive” violence.
 Hydrogen burns in a clean atmosphere with an invisible flame. It has a
somewhat higher adiabatic premixed flame temperature for a
stoichiometric mixture in air of 2403 K compared to other fuels.
 This temperature can be a reason for serious injure, especially at
clean laboratory environment.
 Combustion and hot currents will cause changes in the surroundings
that can be used to detect the flame. These changes are called the
signature of the fire.
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 Stoichiometric mixture is a mixture in which both fuel and oxidiser are
fully consumed (complete combustion) to form combustion products.
 The stoichiometric composition of hydrogen-oxygen mixture is 66.66%
by volume of H2 and 33.33% of O2. The reaction equation
H2 + ½ O2 → H2O
 The stoichiometric composition of hydrogen-air mixture is 29.6% by
volume of H2 and 70.4% of air. The reaction equation is
H2 + ½ (O2 +3.76N2) → H2O +1.88N2 or
H2 + 2.38 air → H2O +1.88 N2
 Lean mixtures – concentration of hydrogen in mixture with oxidiser is
below stoichiometric.
 Rich mixtures – concentration of hydrogen in mixture with oxidiser is
above stoichiometric.
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The lower heating value (lower heat of combustion) of
hydrogen is 241.7 kJ/mol and the higher heating value is 286.1 kJ/mol.
The difference of about 16% is due to the heat of condensation of
water vapour, and this value is larger compared to other gases.
Blue – lower heating value
Blue+Red – higher heating value
DHc, kJ/g
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 Hydrogen flame is not visible in clean atmosphere.
 Real jet flame can be seen due to combustion of entrained
particulates (dust, etc.).
 Radiation emitted from a hydrogen flame is very low
(emissivity ε < 0.1) unlike hydrocarbon flames (ε ~ 1)
(ADL, 1960).
 Recent large-scale studies performed by Sandia National
Laboratory (USA) indicate that emissivity could be higher
yet at the level below 0.3.
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 Flammability range is the range of concentrations between the lower
and the upper flammability limits.
 The lower flammability limit (LFL) is the lowest concentration of a
combustible substance in a gaseous oxidizer that will propagate a flame
(… flammable envelope).
 The upper flammability limit (UFL) is the highest concentration of a
combustible substance in a gaseous oxidizer that will propagate a flame
(…air to compressor).
 The flammability range of H2 in air (4%-75% by volume ) is wider
compared to most hydrocarbons:
- methane (5.28-15);
- propane (2.2-9.6);
- gasoline (0.79-8.1);
- methanol (6-36.5).
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Table shows the scattering of the flammability limits (% by volume)
determined by different standard apparatuses and procedures applied
(Schröder and Holtappels, 2005).
DIN 51649
EN 1839 (T)
EN 1839 (B) ASTM E 681
LFL
3.8%
3.6%
4.2%
3.75%
UFL
75.8%
76.6%
77.0%
75.1%
25
Limit
Note: (T) – tube; (B) – constant volume bomb.
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 Flammability range depends on a flame propagation direction.
 In quiescent mixture a conservative value of LFL changes: from 3.9%
by volume for upward propagation (flame balls),
through 6% for horizontal,
to 8.5% for downward propagation.
Upward propagation Horizontal propagation
Downward propagation
LFL
LFL
UFL
3.9-5.1% 67.9-75%
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LFL
UFL
6.0-7.15%
65.7-71.4% 8.5-9.45%
UFL
68-74.5%
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 Quiescent mixtures <8% - no overpressure!
 Turbulent mixture can generate in closed vessel p=2.5 bar.
9
Red dashed lines: upward, horizontal, and downward flame propagation
Pressure, bar
7
6
5
4
3
2
6.0% horizontal propagation
8.5% downward
3.9% upward propagation
27
8
Gaseq
Furno et al., 1971 (central ignition)
Shebeko et al., 1991 (central ignition)
Shebeko et al., 1990 (top ignition)
1
0
0
5
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10
15
20
25
Hydrogen concentration, % by vol.
30
35
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 Upward and downward flammability limits.
 The flammability range expands practically linearly with temperature.
 For example:
LFL decreases from
4% to 1.5% by volume
with temperature
increase from
20oC to 400oC.
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Hydrogen-air mixture
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Hydrogen-oxygen mixture (20oC and 80oC)
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 Mixture: 45% of hydrogen, 30% of air and 25% of diluent.
 Diluents: CO2, He, or N2. The mixture is flammable.
 Diluent: H2O. The mixture is inflammable.
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 The minimum ignition energy
of hydrogen is smaller than for other fuels:
 Hydrogen
0.017 mJ
 Methane
0.28 mJ
 Propane
0.25 mJ
 Gasoline
0.23-0.46 mJ
 Methanol
0.14 mJ
 A weak spark caused by the discharge of static electricity from
a human body may ignite these fuels in air.
 The ignition energy is a function of the mixture
concentration.
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
33



Flashpoint is the lowest temperature at which the fuel produces
enough vapours to form a flammable mixture with air at its surface.
Auto-ignition temperature (AIT) is the minimum temperature required to initiate combustion reaction of
fuel-oxidiser mixture in the absence of an external source of ignition. Over catalytic surfaces like
platinum, the AIT of hydrogen can be drastically reduced down to 70°C. Other hydrogen catalysts
include but not limited to carbon nanomaterials, nickel, etc.
Minimum ignition energy (MIE) is the minimum value of the electric energy, which (upon discharge
across a spark gap) just ignites the quiescent mixture in the most ignitable composition.
Maximum experimental safe gap (MESG) of flammable gases and vapours is the lowest value of the
safe gap measured according to IEC 60079-1-1 (2002) by varying the composition of the mixture. The
safe gap is the gap width (determined with a gap length of 25 mm) at which in the case of a given
mixture composition, a flashback just fails to occur.
Fuel
Flashpoint, ºC
AIT, ºC
MIE, mJ
MESG, mm
Hydrogen
< -253
510
0.017
0.08
Methane
-188
537
0.280
Propane
-96
470
0.250
0.92
Gasoline
-(11-45)
230-480
0.230-0.460
0.96-1.02
Diesel
37-110
210-370
Methanol
6
385
0.140
-
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3.6
3.4
3.2
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
7.2
6.8
6.4
6.0
5.6
5.2
4.8
4.4
4.0
3.6
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0.0
Laminar burning velocity
Expansion ratio
0.1
0.2
0.3
0.4
0.5
Expansion ratio
Laminar burning velocity
(m/s)
34
 Laminar burning velocity of stoichiometric
hydrogen-air mixture of about 2 m/s is far greater compared to most of
hydrocarbons of 0.30-0.45 m/s. Maximum burning velocity is at rich
mixture of 40.1% by volume.
 Maximum expansion coefficient at stoichiometric mixture of 29.5% by
4.0
8.0
volume.
3.8
7.6
0.6
Hydrogen volume fraction
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 Detonation is the worst case scenario.
 The detonability range mentioned in ISO/TR 15916:2004 (18-59% of
hydrogen in air) is narrowed.
 The detonation range 13-70% is reported for hydrogen-air in a 43 cm
diameter tube.
 A lower detonability limit of 12.5% was observed in the RUT facility
(Russia).
 The widest detonability range of hydrogen in air 11-59% by volume is
recommended by Alcock et al. (2001).
 A conservative generalisation of available data gives the detonability
range 11-70%. This is narrower and as expected within the flammability
range of 4-75%.
 Inherently safer range of concentrations: 4-11%, or above 70%.
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The detonability limits are not fundamental
characteristics. They strongly depend on the
size of the experimental set up. A diameter of
the tube, where detonation can propagate,
should be of the order of a detonation cell
size. A detonation cell size increases with
approaching the detonability limits. Thus, the
larger is the scale of an experimental
apparatus the smaller is the lower
detonability limit (the larger is the upper
detonability limit).
Photo of cells:
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 Odorants are artificially added to propane
and methane, while gasoline has naturally a strong odour.
 No odorant can be added to hydrogen for use in PEM fuel cells due to
poisoning of the membrane.
 Odorant for hydrogen is absent. It would require specific physical
properties not to be harmful to fuel cells.
Fuel
Hydrogen
Gasoline
Propane
Methane
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Toxic
No
Yes
No
No
Odorant added
No
Natural odour
Yes
Yes
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 Hydrogen is not expected to cause mutagenicity, embryotoxicity or
reproductive toxicity.
 There is no evidence of adverse effects if skin or eyes, it cannot be
ingested (unlikely route). However, inhaled hydrogen can result in a
flammable mixture within the body.
 Hydrogen is classified as a simple asphyxiant, has no threshold limit
value (TLV), non-carcinogen. High concentrations in air can cause an
oxygen-deficient environment. Individuals may experience symptoms
which include: headaches, dizziness, drowsiness, unconsciousness,
nausea, vomiting, depression of all the senses, etc.
 A victim of asphyxiation may have a blue colour skin, and under some
circumstances, death may occur.
 Inhaling vapour or cold hydrogen produces respiratory discomfort and
can result in asphyxiation.
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39
H2, % vol
O2, % vol
0-9
21-19
No particular symptom.
9-28
19-15
Decreased ability to perform tasks; may induce early symptoms in persons
with heart, lung, or circulatory problems, night vision impairment.
28-42
15-12
Deeper respiration, faster pulse, poor coordination.
42-52
12-10
Poor judgment, slightly blue lips, dizziness, headache and early fatigue
with a tolerance time of 30 minutes.
52-62
10-8
Nausea, vomiting, unconsciousness, ashen face, fainting, mental failure,
with a tolerance time of 5 minutes.
62-71
8-6
Unconsciousness occurs after about 3 minutes and death in 8 min. 50
percent death and 50 percent recovery with treatment in 6 min, 100
percent recovery with treatment in 4 to 5 min.
71-86
6-3
Coma occurs in 40 s, convulsions, respiration ceases then death.
86-100
3-0
Death within 45 s.
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Physiological response
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40
 If hydrogen is inhaled and above symptoms observed then remove a
person to fresh air, give oxygen if breathing is difficult, or apply artificial
respiration if not breathing.
 The system design shall prevent any possibility of asphyxiation in adjacent
areas. It is recommended to check oxygen content before entering an
incident area.
 Contact with LH2 or its splashes on the skin or in the eyes can cause
serious burns by frostbite or hypothermia.
 Direct physical contact with LH2, cold vapour, or cold equipment can cause
serious tissue damage. Short contact with a small amount of LH2 may
not pose as great a danger because a protective film may form.
 The kinetic energy of high pressure hydrogen jet represents a hazard due
the possibility of high pressure injection injuries. It takes a minimum
pressure of 7 bar for liquid, vapour or gas to breach intact human skin.
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41
It can be concluded that hydrogen is not
more dangerous or safer compared to other
fuels. Hydrogen is different and has to be
professionally handled with knowledge of
underpinning science and engineering to
provide public safety as well as
competitiveness of hydrogen and fuel cell
products and infrastructure.
© HyFacts Project 2012/13
CONFIDENTIAL – NOT FOR PUBLIC USE
Funded by FCH JU (Grant agreement No. 256823)