A mixture of hydrogen and air will burn when there is as little as 4 percent hydrogen or as much as 75 percent hydrogen in the mix4 This is a very wide flammability range.
In comparison diesel fuel vapors in air will burn over a range of 0.6 percent to 5.5 percent. With less than 0.6 percent diesel in the mixture it is too lean to ignite, and with more than 5.5 percent diesel in the mixture it is too rich. Natural gas will burn over a range of 5 percent to 15 percent.
It takes very little energy to ignite a hydrogen-air mixture—a common static electric spark may be sufficient.
As shown in Table 4, it takes less than one tenth of the energy to ignite a hydrogen air mixture as it does to ignite a mixture of gasoline vapors in air. Over much of its flammable range, common static electricity would be enough to ignite a hydrogen-air mixture. In some cases, the electrostatic charges or heating created by the flow of hydrogen from a leaking vessel would be enough to ignite the leaking hydrogen (Murphy, et al., 1995; Argonne, 2003).
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Hydrogen flames burn very cleanly, producing virtually no soot. It is the soot created by most fuel that makes a flame visible. In addition, much of the energy radiated by a hydrogen flame is in the ultraviolet range, rather than the infrared or visible ranges of the light spectrum. Therefore, a hydrogen flame is virtually invisible to the human eye in day light, though the energy being released by the flame may create a visible “shimmer” in surrounding air due to changes in the air density. At night, hydrogen flames are visible to the unaided human eye, and in daylight, they can be “seen” by an ultraviolet light sensor.5 If a hydrogen flame ignites other nearby materials, flames or smoke may also be visible from them. See Figure 13.
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