Induction period third limit

Figure 8.11 is a general representation of the explosion limits of H2S/02 mixtures. This three-limit curve is very similar to that shown for H2/02 mixtures. However, there is an important difference in the character of the experimental data that determine the H2S/02 limits. In the H2S/02 peninsula and in the third limit region, explosion occurs after an induction period of several seconds. 

First, there is an inhibiting effect of the additive on the low pressure explosions, so that the second limit pressure is reduced and the first limit is raised on addition of the hydrocarbon [329—331]. Secondly, there may be an increase in the maximum rate (of decrease of pressure) in the slow reaction [330]. Thirdly, induced explosions may occur in some cases (not with methane) at pressures outside the + O2 explosion peninsula. In most cases such induced explosions appear as one sharp explosive reaction. However, they are sometimes characterized (e.g. with CaHg at 560 °C) by an induction period during which there is a rapid pressure increase, and this is followed immediately by a very rapid pressure decrease in the system. It is probable that all the induced explosions follow this two-stage pattern. This type of explosion does not occur in H2—Oj—CH4 mixtures because methane is not as reactive as propane in... 

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