Hydrogen Utilization: Combustion - Hydrogen Generation, Storage, and Utilization - page 137

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Hydrogen Utilization: Combustion

8.1 BASICS ABOUT COMBUSTION

Combustion of fuel such as hydrogen or hydrocarbons in air or oxygen is an
important process for many applications, including combustion engines. The
chemistry of combustion is actually very complex, involving many elemen-
tary reactions and radial species, often called radical pools. The reaction
mechanisms are highly dependent on many factors, such as temperature,
pressure, reactor, catalyst, and composition.
Hydrogen gas is highly flammable and can burn in air at a very wide range
of concentrations between 4% and 75% by volume. The enthalpy of combus-
tion for hydrogen is −286 kJ·mol −1 :

1
2

H g ( ) + O g ( ) → H O l ( ) + 286 kJ .

(8.1)

2 2 2

Hydrogen mixed with air in the concentration range of 4-75% can spon-
taneously explode by spark, heat, or sunlight, and the hydrogen autoignition
temperature, the temperature of spontaneous ignition in air, is 500°C or
932°F. Pure hydrogen-oxygen flames emit ultraviolet light and are essen-
tially invisible to the naked eye, which presents a potential safety hazard.
Hydrogen also reacts with other oxidizing elements. For example, it can react