Explosive Oxidation of Hydrogen Sulphide: Self-heating, Chain-branching and Chain-thermal Contributions to Spontaneous Ignition

Abstract

The spontaneously explosive oxidation of hydrogen sulphide in a 290 cm3 vessel has been investigated over a temperature range 280–360°C and between pressures of 10 and 120 mmHg. Conditions for ignition have been mapped on a p,T diagram. Very fine thermocouples (25 µm Pt–Pt/Rh) have been used to detect and measure self-heating, and special emphasis has been laid on the direct measurement of the size and form of the temperature against time histories for different initial conditions or locations on the ignition diagram. The effects of reactant proportions and of added diluents (with different thermal conductivities) on the second and third ignition limits have also been studied.

Although the reaction exhibits many features traditionally associated with purely branched chain explosions, the direct temperature measurements have revealed extensive self-heating under very varied conditions of pressure and temperature. Boundaries may be drawn on the ignition diagram that separate the regions where a combined chain-thermal mechanism is responsible for explosions (I) from those which may be considered as purely thermal (II) or isothermal branched chain (III) in nature.

The measured temperature against time histories provide novel experimental support for the unified theoretical treatment of chain and thermal explosions of Gray and Yang. Critical temperature rises are smaller than would be expected on a purely thermal basis, and induction times are longer. In the chain-thermal region, the rate of self-heating immediately prior to ignition is not always rapid; indeed, temperature excesses may be relatively steady or even decreasing when spontaneous explosion takes place. We should expect similar behaviour in the hydrogen-oxygen reaction.