Hydrogen Safety Properties

Hydrogen is the simplest element, has three isotopes: hydrogen at wt 1.008 (H), deuterium at wt 2.0141 (D), and tritium at 3.0161 (T). Hydrogen is very abundant, being one of the atoms composing water. Whereas hydrogen atoms exist under certain conditions, the normal of pure hydrogen is the hydrogen molecule, H2, which is the lightest of all gases [190]. The hydrogen molecule exists in two forms, ortho-hydrogen and para-hydrogen, depending on the nuclear spins of the atoms.

A phase diagram of hydrogen is shown in Figure 2.1. In normal conditions (20°C, 0.1 MPa) hydrogen is a colourless, tasteless, non-poisonous, and flammable gas. At low temperature, hydrogen is a solid with a density of 70.6 kg/m3 at -262°C, and a gas at higher temperature with a density of 0.089886 kg/m3 (i.e. 7% of the density of air) at 0°C and a pressure of 0.1 MPa. Hydrogen as a liquid in a small zone between the triple and critical points with a density of 70.8 kg/m3 at -252.87°C (Appendix A). As temperature decreases, the hydrogen gas can be transformed into liquid state, which requires an energy in amount of 670 J/g [190], iol 10*

itf1

Te jtip e rature (K)

Fig. 2.1 Simple phase diagram of hydrogen [213]

Nothing what humans do is without risk, consequently, also each energy poses its specific safety risks which have to be taken care of. Hydrogen can be safer than conventional fuels in some situations, and more hazardous in others [12]. The relative safety of hydrogen compared to that other fuel must therefore take into consideration the particular circumstances of its

H Metal

Liquid metal

H Metal

Liquid metal

Te jtip e rature (K)

Fig. 2.1 Simple phase diagram of hydrogen [213]

accidental release. Hence a meaningful comparison must be based on comparing all relevant situations. Cadwallader and Herring [36] quote the National Hydrogen Energy Association as having qualitatively determined that methane was less dangerous than hydrogen and that hydrogen was less dangerous than propane. The following subsection gives a brief overview of the hydrogen safety properties, and compared with those of methane, propane and gasoline.

2.2.1.1 Leak Propensity

Hydrogen gas has the smallest molecule and has a greater propensity to escape through small openings than liquid fuels or other gaseous fuels. For transfer through a membrane the relative rate is governed by the relative diffusion coefficients of the materials. For subsonic releases through openings the rate is dependent on whether the flow is laminar or turbulent. For laminar flow the relative molar leak rates of two gases are theoretically inversely proportional to the ratio of their dynamic viscosities. For turbulent flow the molar leak rates are theoretically inversely proportional to the square root of the relative gas densities. For sonic releases the molar leak rates are proportional to the sonic velocity of the gases. For perfect gases the ratio of molar flow rates equals the ratio of volumetric flows.

Predicted theoretical flow rates of methane and propane relative to hydrogen are given in Table 2-1. The high pressure systems of hydrogen storage the flow from any leaks is likely to be sonic [12]. Therefore hydrogen would leak approximately 3 times faster than natural gas and 5 times faster than propane on a volumetric basis. However the energy density of hydrogen is lower than that of methane or propane such that for sonic flow its energy leakage rate would be 0.34 times that of methane and 0.2 times that of propane.

Leaks of liquid hydrogen evaporate very quickly since the boiling point of liquid hydrogen is so extremely low. Hydrogen leaks are dangerous in that they pose a risk of fire where they mix with air. However, the small molecule size that increases the likelihood of a leak also results in very high buoyancy and diffusivity, so leaked hydrogen rises and becomes diluted quickly, especially outdoors. This results in a localized region of flammability that disperses quickly. As the hydrogen dilutes with distance from the leakage site, the buoyancy declines and the tendency for the hydrogen to continue to rise decreases [12]. Very cold hydrogen, resulting from a liquid hydrogen leak, becomes buoyant soon after is evaporates.

Table 2-1. Leakage properties of hydrogen and other fuels [12].

Leakage Properties

Hydrogen Methane Propane

- Diffusion coefficient in air at NTP (cm2/s)

0.61

0.16

0.12

- Viscosity at NTP (g/cm.s x 105)

89

11.7

80

- Density at NTP (kg/m3)

0.08938

0.6512

1.87

- Ratio of specific heats, Cp/Cv at NTP

1.308

1.383

1.14

Relative leak rate (Subsonic flow):

- Diffusion

1

0.26

0.20

- Laminar flow

1

7.60

1.11

- Turbulent flow

1

0.35

0.21

Relative leak rate (Sonic flow):

1

0.34

0.20

2.2.1.2 Hydrogen Embrittlement

Prolonged exposure to hydrogen of some high strength steels can cause them to loose their strength, eventually leading to failure. This effect is termed hydrogen embrittlement (HE). The study of HE mechanisms [56] includes large number of pertinent variables such as time of exposure to hydrogen, stress state, pressure, temperature, hydrogen concentration, purity of hydrogen, mechanical properties of the metal, and so on. According to [56] HE is divided into three classes: hydrogen reaction embrittlement, internal hydrogen embrittlement, and environmental hydrogen embrittlement (Appendix A). Liquid hydrogen (known as cryogenic liquids) poses additional brittle failure called low-temperature embrittlement. The increase in strength as the temperature is lowered does not make all material satisfactory for use in cryogenic applications. If the structural materials lose ductility or become brittle, they can break suddenly and unexpectedly under normal stress conditions. Proper choice of materials to avoid these risks is required.

2.2.1.3 Dispersion

Hydrogen gas is more diffusive and under most conditions more buoyant than gasoline, propane or methane and hence tends to disperse more rapidly if released. The one exception is for cryogenic releases of hydrogen where the very cold vapour cloud initially formed can be denser than the surrounding air [12].

2.2.1.4 Flammability and Ignition

Hydrogen has much wider limits of flammability in air than methane, propane or gasoline and the minimum ignition energy is about an order of magnitude lower than that of other combustibles (Table 2-2). The wide range of flammability of hydrogen-air mixtures compared to other combustibles is in principle a disadvantage with respect to potential risks. A hydrogen vapour cloud could potentially have a greater volume within the flammable range than a methane cloud formed under similar release conditions. In practical release situations the lower ignition energy of hydrogen may not be as significant a differentiation between the fuels as it first seems. The minimum ignition energy tends to be for mixtures at around stoichiometric composition (29 vol.% for hydrogen). Figure 2.2 shows that at the LFL the ignition energy for hydrogen is similar to that of methane.

FlajiuttabiJity Luvuis

FlajiuttabiJity Luvuis

Ignition Energy Gasoline

Fig. 2.2 Minimum ignition energy of hydrogen compared with that of methane [17].

Fig. 2.2 Minimum ignition energy of hydrogen compared with that of methane [17].

The minimum autoignition temperature of hydrogen is higher than that of methane, propane or gasoline (Table 2-2). However the autoignition temperature depends on the nature of the source. The minimum is usually measured in a heated glass vessel, however if a heated air jet or nichrome wire is used the autoignition temperature of hydrogen is lower than that of other fuels.

2.2.1.5 Deflagration and Detonation

Hydrogen gas can burn as a jet flame with combustion taking place along the edges of the jet where it mixes with sufficient air. In the open flammable mixtures undergo slow deflagration. Where the flame speed is accelerated e.g. by extreme initial turbulence, turbulence from obstacles, or confinement, the result is an explosion. An extreme example is a detonation where the flame speed is supersonic.

An explosion is always accompanied by a fireball and a pressure wave (overpressure). The fireball can ignite combustible materials in the vicinity or fuel released by the explosion so that a fire may follow an explosion. If the flammable mixture is partially or totally confined the explosion may propel fragments of the enclosure material over great distances. A detonation explosion is more severe than a deflagration explosion, the overpressures generated are higher and hence much greater physical damage is possible. Direct detonation of a hydrogen gas cloud is less likely than a deflagration explosion as the ignition energy required is in the 10 kJ range, the minimum concentration is higher and the detonable range is narrower than the flammable range.

A deflagration can make the transition to a detonation (called deflagration to detonation, DDT) if the concentrations in the flammable cloud are within the detonable range and the flame front can accelerate to a speed above the sonic velocity in air. This can occur if the dimensions of the cloud are large enough to provide sufficient run-up distance for the flame to accelerate, and if there are turbulence promoting structures to accelerate the flame or there are pressure wave reflecting bodies such as walls. The turbulence in an emerging high pressure hydrogen gas jet release coupled with its exceptionally high burning velocity may also provide the conditions for detonation rather than deflagration to occur on ignition.

Table 2-2. Deflagration and detonation properties of hydrogen and other fuels [46]_

Hydrogen Methane Propane Gasoline

Table 2-2. Deflagration and detonation properties of hydrogen and other fuels [46]_

Hydrogen Methane Propane Gasoline

Lower flammability limit (LFL, vol.% in air)

4

5,3

2,1

1

Upper flammability limit (UFL, vol.% in air)

75

15

9,5

7,8

Minimum ignition energy (mJ)

0,02

0,29

0,26

0,24

Auto-ignition temperature (°C):

- Minimum

585

540

487

228-471

- Heated air jet (0.4 cm diameter)

670

1220

885

1040

- Nichrome wire

750

1220

1050

Adiabatic flame temperature in air (K)

2318

2158

2198

2470

Quencing gap at NTP (mm)

0,6

2

2

2

Lower detonability limit (LDL, vol.% in air)

11-18

6,3

3,1

1,1

Upper detonability limit (UDL, vol.% in air)

59

13,5

7

3,3

Maximum burning velocity (m/s)

3,46

0,43

0,47

Concentration at maximum (vol.%)

42,5

10,2

4,3

Burning velocity at stoichiometric (m/s)

2,37

0,42

0,46

0,42

Concentration at stoichiometric (vol.%)

29,5

9,5

4,1

1,8

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