The prediction of spontaneous ignition hazards resulting from the hot stacking' of process materials

Abstract

A change in the normal routine at a factory, which produced a cellulose-based product, necessitated taking the individual blocks of material from a hot curing process (ca. 200 C) and packing them before they had cooled to an appreciable extent. Spontaneous ignition took place in the packaged material some hours later. Considerable damage to buildings and other losses were incurred. The ambient temperature within the store was not sufficiently high to cause thermal ignition. The problem proved to be a special but not necessarily unusual case in which the onset of ignition was governed by the initial temperature of assembly of the packaged material. These circumstances have been recognized in the fiberboard manufacturing industry for example, such that US legislation requires the cooling of newly manufactured boards below a specified temperature before stacking takes place. The authors discuss the theoretical background to this type of problem, based on an adaptation of thermal ignition theory with conductive heat transport (Frank-Kamenetskii conditions). They obtained the appropriate kinetic and thermochemical parameters for exothermic reaction in a cellulosic material in order to apply the theory. They were then able to calculate a maximum packing temperature to expedite safe but efficient storage or transport. Themore » numerical calculations to solve the spatial and time dependent energy conservation equations by use of the finite difference method required a three dimensional grid, set up as 20{sup 3} mesh points, equivalent to cube-shaped, packaged material in the practical application. Most of the calculations were performed on a personal computer.« less