PROPERTIES OF HYDROGEN

Hydrogen gas is colorless, odorless, tasteless, non-toxic, and undetectable for human senses. If released in a confined area, hydrogen can cause suffocation by dilution of the oxygen content Gaseous hydrogen at its boiling point (20 K) is heavier than air.

At a temperature > 22 K, it becomes buoyant and tends to rise in the ambient air.

Hydrogen coexists in two phases, para and ortho hydrogen, whose partition depends on the temperature. At low temperatures, < « 80 K, the para phase presents the more stable form.

Hydrogen exhibits in part a positive "Thomson-Joule effect" meaning a positive temperature change upon pressure decrease. The effect is found for hydrogen at temperatures > 200 K, for example, an increase of 6 degrees when released from 20 MPa to ambient conditions.

Mixtures of hydrogen with oxygen are flammable over a wide range of concentrations, 4 - 7 5 vol%. A stoichiometric hydrogen-air mixture contains 29.5 vol% H2- Despite its relatively high autoignition temperature, the minimum energy required for an ignition (0.02 mJ) is very low, further reduced by increasing temperature or pressure or oxygen content Catalytically active surfaces can ignite hydrogen-air mixtures even at much lower temperatures. The hydrogen flame is nonluminous, comparatively hot, but hardly radiates any heat.

Liquid hydrogen (LH2) has the advantages of economic transportation in large amo-unts, simple conversion technology, extreme cleanness, convenient consumption. Major drawbacks are given by the enormous energy consumption of liquefaction which is around 30 % of its heat of combustion and the unavoidable boiloff losses during storage and hand-ling, which can total 40 % of its available combustion energy [105]. Most experience in handling LHa has been gained for decades in the space programs of the USA, the Russian Federation, Europe, Japan, and China.

Slush hydrogen (SLH2) is a homogeneous mixture of liquid and solid H2- A fraction of 50 wt% results in a 16 % higher density and an 18 % higher heat capacity compared with the liquid phase. Both effects reduce volume and thus tankage weight in possible applications, e.g., in aircraft,- improving transportation efficiency. A reduced vaporization loss is given at the expense of a storage at higher pressures up to 1.3 MPa.

Table 8-1: Properties of hydrogen, from [37, 88, 101]

Molecular weight Gas constant

Normal boiling point

Heat of combustion, gross (including thermal energy of water vapor)

Heat of combustion, net Latent heat of vaporization

Vapor density @ Standard Temperature Pressure (STP) Vapor density @ Normal Boiling Point (NBP)

Liquid density @ NBP

Slush density @ Triple Point (TP) (50 wt% solid) Solid density @ TP

Liquid-to-gas expansion ratio

Specific heat capacity of gas @ STP Dynamic Viscosity of gas @ STP Thermal conductivity of gas @ STP Diffusivity of gas @ STP

Adiabatic speed of sound in air @ STP Stoichiometric composition in air Limits of flammability in air Limits of detonability in air

Minimum ignition energy in air (stoich.) Auto-ignition temperature in air

Flame temperature (stoich.) Flame temperature (maximum) Emissivity

Quenching distance in air @ STP (stoich.) Laminar burning velocity in air (stoich.) Laminar burning velocity in air (maximum) Flame front velocity (spherical gas cloud) Detonation velocity (Chapman- Jouguet)

The subatmospheric vapor pressure of (slush) hydrogen at 13.8 K can cause the total system pressure to drop below that of the local atmosphere, requiring precautions to avoid air entrainment into the system, which would solidify the air and warm up the system.

Another safety concern is electric charge accumulation in SLH² flows since it involves solid particles; this effect needs further investigation. NASA has studied SLH2 characteristics under conditions of production, storage, and transfer [81, 109].

Various property data of hydrogen are summarized in Table 8-1. A more detailed description of the properties is given, e.g., in [45, 120].

8.2. SAFETY MEASURES IN HANDLING (CRYOGENIC) HYDROGEN 8.2.1. Physiological Hazards

Direct contact with liquid hydrogen or surfaces on very low temperature causes cryogenic "burns", i.e., freezing of living tissue, except for very brief contact periods where the temperature difference between cryogen and skin is still high (film boiling regime) and heat transfer small. Prolonged exposure to cold temperatures after a large spill lowers the body temperature resulting in hypothermia, organ dysfunction, and respiratory depression [43].

Vaporization of released liquid hydrogen affects the local atmosphere by diluting the oxygen in the air. A fraction of less than 19.5 % is considered by NASA to be dangerous to humans causing asphyxication; less than 8 % will be lethal within minutes.

Physiological effects as anticipated upon the explosion of a flammable gas mixture are listed in Table 8-2; the effects arising from the accidental release and combustion of hydrogen are treated in section 8.4..

Table 8-2: Physiological Effects of blast overpressures, from [43]

Maximum overpressure

[kPa] Effect on personnel

7 35

100 240 345 450

Knock personnel down Eardrum damage

Lung damage Threshold for fatalities

50 % fatalities 99 % fatalities

8.2.2. Safety Measures

Safety of a system must be considered in all phases from design to fabrication and its operation, sufficient instrumentation, safety analysis to assess consequences of untoward

Primary Safety Measures

Secondary Safety Measures

Tertiary Safety Measures

Basic Safety by Design, Material, Manufacture,

Quality Assurance, Repeated Non-Destructive Examination, Operation-Manuals. Qualified Personal, Accident Management

Prevention of [nciderits

Prevention of Accidents

Mitigation of

Acc.Consequences Fig. 8-1: Safety approach for handling hydrogen in the industries, from [10]

events, plant maintenance and documentation, well-trained and knowledgeable personnel [43]. The investigation of the combustion behavior of hydrogen connected with the pressure development under accident conditions is the basis of all safety-related considerations.

NASA experience with hydrogen began in the 1950s, when hydrogen was considered the principal liquid rocket fuel. Safety engineering soon started to address hydrogen hazards and to develop procedures for the safe operation of equipment and facilities. NASA also determined the need for rigorous training and certification programs for personnel.

The large-scale industrial application of hydrogen has led to a safety concept with a scale of safety measures aimed at minimizing risks by preventive safety measures and by mitigation measures. A basic safety approach that can be applied to hydrogen energy technologies is outlined in Fig. 8-1. Some examples are [45]

1. for primary measures (design):

welding as a basic principle of construction, metal gaskets for connectible parts, selection of proper materials, waste or leakage or relief valve piping leading into the open atmosphere, sufficient ventilation in closed buildings, handling of LH2 in closed systems only, leak detection, regular check, training of personnel;

2. for secondary measures (avoidance of ignition sources):

prohibition of open fire, avoidance of electric or mechanically generated sparks, avoidance of electrostatic charge, flame barrier or N£ inertialization system in large plants;

3. for tertiary measures (mitigation of consequences):

quick shutdown system with cutoff of H2 feed lines, explosion-proof construction of

components (e.g., stack), avoidance of a transition from deflagration to detonation, safety distance, flare rather than extinguish hydrogen fire.

Measures to be applied are defined and fixed in rules and regulations or in respective recommended industrial standards or company internal rules.