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Flammability and Combustion Characteristics

There exists an extensive published literature on flame radiation from hydrocarbon flames and pool fires, see, for example, (DeRisJ:1979); (TienCL:1982); (MudanKS:1984); (FaethGM:1985); (ViskantaR:1987). However, there is a limited number of studies on hydrogen flame radiation, particularly on large scale.

Thermal radiation is a primary mode of heat transfer. Radiation is the dominant mechanism of heat transfer in large fires involving hydrocarbons, producing intermediate unstable radicals (e.g., O, H, OH, N, etc.) and stable non-luminous gaseous combustion products (CO2, CO, H2O, NOX, etc.) and soot particulates.

The contribution to the radiative transfer in flames can be regarded as due to luminous and non-luminous radiation. Non-luminous flame radiation originates from transitions in the molecular energy levels due to the absorption or emission of photons. Discrete absorption-emission lines of radiation are produced in the infrared spectrum as a result of transitions between quantised electronic states for monatomic gases. Energy released by the gaseous combustion products results from the transitions between the vibrational and rotational energy levels of the molecules of gas species, particularly CO2, H2O, CO, etc., producing non-luminous radiation concentrated in spectral lines. These gases do not scatter radiation significantly but they are strong selective absorbers and emitters of radiant energy.

In practical engineering systems, where pressure and geometric scales are large, pressure broadening of spectral lines cause them to overlap with each other, The resulting radiation is thus concentrated in gaseous absorption bands in infrared spectrum produced by various types of transitions between the molecular energy states, particularly the vibrational-rotational states. In luminous flames a continuum radiation in the visible and infrared is also emitted by the unburnt carbon particulates called soot that contribute greatly to the luminosity of the flames.

The actual quantity and distribution of combustion products and/or soot produced in fires depend on the type and configuration of fuel and local supply of oxygen. In contrast to hydrocarbon fuels, the hydrogen burns more cleanly in air, producing non-luminous, almost invisible, pale blue flame due to spectral water vapour bands.

In order to understand thermal radiation hazards from hydrogen flames, it is crucial to understand the relative assessment of the physical properties and combustion characteristics of hydrogen and hydrocarbon flames. Table 3-2 provides comparison of the physical properties of hydrogen with hydrocarbon methane.

Table: Physical properties of hydrogen and methane

Auto-ignition temperature520C630C
Heat of combustion (lower heating value)119.9 MJ/kg50.1 MJ/kg
(upper heating value)141.9 MJ/kg55.6 MJ/kg
Lower flammable limit (in air)4.0 vol%5.3 vol%
Upper flammable limit (in air)75.0 vol%15 vol%
Stoichiometric mixture (in air)29.5 vol%9.5 vol%
Density (@ 20C, 100kPa)0.08988 kg/m30.71 kg/m3
Diffusivity (@ 20C, 100kPa)0.61 cm2/s0.16 cm2/s
Viscosity (@ 20C, 100kPa)8.814 Pa-s11.023 Pa-s
Flame temperature (in air)2045C1875C
Minimum ignition energy (in air)0.017 mJ0.274 mJ

Hydrogen has a much wider range of flammability in air (4 % to 75 % by volume) than methane (5 % to 17 % by volume), propane, or gasoline, and the minimum ignition energy (for a stoichiometric mixture) is about an order of magnitude lower (1/16th that of methane). In many accidental situations the lower flammable limit (LFL) is more important. The LFL for hydrogen is similar to that of methane, about twice that of propane, and four times that of gasoline. In addition, the minimum ignition energy for hydrogen at the LFL is also similar to that of methane.

Hydrogen-air mixture can burn either as a jet flame at a fixed point, with combustion taking place along the edges of the jet where it mixes with sufficient air. In a stationary mixture in the open with no confinement a flammable hydrogen mixture will undergo slow deflagration. Deflagration refers to a flame that relies on heat- and mass-transfer mechanisms to combust and move into areas of unburned fuel. If the flame speed is accelerated, perhaps due to extreme initial turbulence or turbulence induced by obstacles or confinement, the result is an explosion. In the extreme case the flame speed becomes supersonic and results in detonation. Once initiated, detonation is self-sustaining (no further turbulence or confinement is required) as long as the combusting mixture is within the detonable range.

The heat of combustion of hydrogen per unit weight is higher than any other material, but hydrogen has a relatively low heat of combustion per unit volume. Thus the combustion of a given volume of hydrogen will release less energy than the same volume of either natural gas or gasoline.

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