A model calculation of spontaneous ignition in an autoxidative powder bed has shown that the critical state can be classified into Frank-Kamenetskii and oxygen-deficient types. The first type includes an irreversible transient process to ignition at the critical point and its thermal stability is rather frail against the changes in ambient conditions. In the second type, the oxygen diffusion suffocatingly controls the rate of heat generation in the bed and the thermal stability is comparatively robust. The types can be largely discriminated by the material and the porosity of the bed.

Theoretical discussion on two types of critical state for ignition of an autoxidative powder bed

Spontaneous Ignition and Dust Explosion

The effect of aeration on the thermal stability of a self-heating powder bed is mathematically discussed. Judging from the thermal characteristics in the steady state, aeration greatly improves the thermal stability. It is also shown, however, that a steady state solution for the temperature distribution in the bed always accompanies an unstable equilibrium solution. It follows that once the steady state temperature distribution is forced through the unstable one by some external disturbances, the powder bed starts to run away towards an ignition and loses stability even after the disturbances are removed. An aerated powder bed is rather susceptible to this kind of ignition. To avoid possible risk of such an ignition, a restorable limit of the temperature distribution for a given aerated powder bed is proposed. Aeration should be controlled so that the temperature at any position in the powder bed is kept well below the restorable limit distribution.

Tatsuo Tanaka

Papers

Efficacy and Risk of Aeration to Prevent Self-heating Powder Bed from Spontaneous Ignition

Efficacy and risk of aeration to prevent self-heating powder bed from spontaneous ignition

Analyzing the centre-line temperature of a reactive dust deposit under sinusoidally varying ambient temperature