Technology Status of Hydrogen Road Vehicles

Fischer and Eichert (1995) provide an updated critical review of the data (mostly experimental) that affect the
behavior of hydrogen in situations relevant to safety. Though general, this review repeatedly illustrates how
the behavior can be greatly affected by the initial and boundary conditions, i.e., the specifics of each accident.
Fischer et al. (1996) state that today modeling and simulation show good agreement with a variety of fast
combustion phenomena observed in experiments. One useful result would be the design of improved
detonation arrester geometries. Sanai (1996) proposes an accident analysis algorithm, based on available
experimental data and computer simulation of explosion accidents, to quickly calculate the response of humans
and structures to such accidents. Maas (1996) handles phenomena such as auto ignition, induced ignition,
minimum ignition energy, the transition from deflagration to detonation, and catalytic combustion, and
concludes that the rapid improvements in numerical simulation and computing power will soon allow good
simulation of practical combustion systems with complex geometry. Kratzel (1996) also models the processes
leading to detonation, especially in complex geometries, among the most challenging to hydrogen safety.
Table 3 is a selection of parameters from Fischer and Eichert (1995) and Kalyanam and Moore (1987).
Table 3. Safety-Related Properties of Hydrogen and Conventional Fuels
Property Unit H 2 CH 4 Gasoline Diesel
Gas density g/l NTP 0.08 0.65 4.4 -
Lower heating value kWs/g 120 50 44 43
Stoichiometric composition in air vol. % 29.5 9.48 1.75 -
Buoyant velocity in air m/s >1-0 >1-6 nonbuoyant nonbuoyant
Self-ignition temperature °C 585 540 230-470 251
Flame temperature in air °C 2045 1875 2200
Ignition limits in air vol. % 4-75 5-15 1->7 7
Minimum ignition energy mWs 0.02 0.29 0.24 -
Flame speed in air cm/s -295 -41 -40 -
Flame thermal radiation % 17-25 23-33 30-42 -
Maximum safe gap in air mm 0.08 1.2 0.7 -
Detonation limits in air vol. % 18-59 >6->13 >1->3
Detonation velocity in air km/s -1.8 -1.5 -1.6 -
Theoretical explosive yield per m 3 kg TNT 2.02 7.03 44.22 -
It is not advisable to rate the above properties according to their importance to safety. Many authors focus
on the ease of ignition (limits and minimum energy for ignition), the speed of combustion, the potential for
detonation, and so on, as of particular concern. But closer examination can easily re-dimension the apparent
gravity of these factors:
! The lower ignition limit guides the evolution of most fires, and there is little difference between the
various fuels; in fact, propane has the lowest limit.
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