Thermal runaway and ignition of reactions

When a highly exothermic reaction occurs rapidly at a given temperature, the heat released cannot be removed by conduction or convection to the outside. This results in a rise in temperature, which in turn causes a sharp acceleration of the reaction with more heat production. The rate may become increasingly large and then explosive. This is the theory of explosion by heat accumulation described by van T Hoff. The explosion threshold is reached when the heat flow supplied by the reaction, given by the product of rate by enthalpy, is no longer compensated for by the heat flow discharged from the reaction medium by conduction or convection. 

If we consider a certain temperature between point B and point C and the pressure is increased, after the slow combustion area we will reach a first pressure threshold Pi. This is also called the lower ignition pressure at pressures above this the mixture ignites. By further increasing the pressure we will reach the corresponding pressure threshold P2, above which there is no mixture ignition. By further increasing the pressme, we now reach threshold pressure P3, from which ignition of the mixture reappears. 

The above theory of explosion by accumulation does not explain this phenomenon. At most, we can assign the upper threshold, P3 (DCA curve), because this theory predicts only one lower level of flarrunability. The existence of branched chain reactions allows us to explain the existence of the first two critical pressures Pi and P2. 

Other factors intervene to modify the values of critical pressures, such as the presence of inert gas which lowers the pressure at both lower and upper threshold pressures of ignition. The lower area of inflammation may eventually disappear. Some substances play a positive catalytic role, for example nitrogen pentoxide for oxidation. Others have a negative role, acting as an inhibitor, for example halogens in oxidation reactions. 

The inflammation of hydrocarbons is generally explained by the existence of branched chain reactions, and we will now see how the modeling of these reactions contributes to an understanding of the ignition process. 

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