Criterion for Spontaneous Ignition of Radiantly Heated Organic Solids

Abstract

In analytical studies of radiant ignition of organic solids, one of the important tasks is to quantitatively describe what ignition is. Experimentally, ignition is signalled by the inception of a flame or of an abrupt change in such crucial gas property as temperature. Some ignition criteria which evolved from experimental observations and intuition are the attainment of a critical: (i) temperature of the exposed surface; (ii) pyrolyzate efflux rate; (iii) char depth in the pyrolyzing slab; (iv) rate of local gas temperature in the boundary layer; and (vii) gas temperature gradient at the solid-gas interface.

In this paper, a one-dimensional mathematical model is presented for the radiantly induced spontaneous ignition of solids to examine the issue of ignition criterion. The model takes into account the transient heat conduction within the heated solid, consequent pyrolysis, and exothermic oxidation reactions in the developing natural convective boundary layer in the gas-phase.

The results show that for gas-phase ignition of organic solids, ignition is satisfactorily indicated by the criterion of gradient reversal of the gas temperature profile at the solid-gas interface. The study also shows that this criterion makes it possible to predict a number of hitherto unexplained experimental observations. Examples are: the dependence of surface temperature at ignition on the radiant flux, and the influence of slab thickness on the ignition delay. A comparison of the present predictions with some available experimental ignition data on cellulosic solids leads to confirm the validity of the gradient reversal criterion under a variety of conditions. The criteria based on surface temperature, a pyrolyzate efflux rate, and a total reaction rate in the boundary layer are shown to lead to error in determining the ignition delay of materials of different solid pyrolysis kinetic parameters. The proposed new criterion, being based on more fundamental grounds is shown to be better.