Explosion limits and oxidation characteristics of hydrogen

Many of the early contributions to the understanding of hydrogen-oxygen oxidation mechanisms developed from the study of explosion limits. Many extensive treatises were written on the subject of the hydrogen-oxygen reaction and, in particular, much attention was given to the effect of walls on radical destruction (a chain termination step) [2], Such effects are not important in the combustion processes of most interest here however, Appendix C details a complex modem mechanism based on earlier thorough reviews [3,4], 

Certain general characteristics of this curve can be stated. First, the third limit portion of the curve is as one would expect from simple density considerations. Next, the first, or lower, limit reflects the wall effect and its role in chain destruction. For example, H02 radicals combine on surfaces to form H20 and 02. Note the expression developed for acnt [Eq. (3.9)] applies to the lower limit only when the wall effect is considered as a first-order reaction of 

FIGURE 3.2 Explosion limits of a stoichoi metric H2—02 mixture (after Ref. [2]). 

Basically, only thermal, not photolytic, mechanisms are considered. The dissociation energy of hydrogen is less than that of oxygen, so the initiation can be related to hydrogen dissociation. Only a few radicals are required to initiate the explosion in the region of temperature of interest, that is, about 675 K. If hydrogen dissociation is the chain s initiating step, it proceeds by the reaction 

The early modeling literature suggested the initiation step 

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Boiling point, °C 138 Melting point, C -78 Flash point, C 33 Auto-ignitlon temperature, ° C 300 Relative density (water -1) 0.8 Relative vapor density (air-1) 3.0 Relative density at 20 ° C of saturated mixture vapor/air(air-1) 1.01 Vapor pressure, mm Hg at 20 C 2.3 Solubility in water, g/100 ml 2.7 Explosive limits, vol% in air 1.3-10.5 Electrical conductivity, pS/m 2.6 x 10 Relative molecular mass 88.1 Log P octanol/water 1.3 Grossformula 5 12 COLORLESS LIQUID WITH CHARACTERISTIC ODOR Vapor mixes readily with air. Reacts violently with oxidants, with risk of fire and explosion. Reacts violently with alkaline-earth and alkali metals, giving off flammable gas (- hydrogen). ... 

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