Effect of natural convection on spontaneous combustion of coal stockpiles

Abstract

Spontaneous combustion may occur in a coal stockpile when the heat generated within the pile cannot be dissipated at near ambient temperature. Under practical conditions, natural convection enhances the rate of heat removal from the bed and shifts the ignition to lower particle sizes (higher reactivities). Analysis of three limiting cases of a one-dimensional model yields criteria predicting the conditions under which ignition occurs as well as those for which a low-temperature (extinguished) state exists for all particle sizes. A simple asymptotic relation predicts the ignition point at large Rayleigh numbers.

Natural convective boundary-layer flow in a heat generating porous medium with a prescribed wall heat flux

“…The transformation given by (8), resulting in equation (10) at leading order, could give a possible behaviour for x large as this equation has a solution for p > 5 (figure 2b). We note that this gives θ 0 of O(x −1/p−2 ) decreasing to zero as x increases as seen in figure 4.…”

“…The local heating is the dominant effect for x large, with the flow being described by (8,10,11) for p < 2 and by (33, 36, 37) for the transitional case p = 2. For p ≥ 5 there is also a boundary-layer solution for all x > 0 but now the local heating plays an increasingly minor role as x increases (figures 4, 5).…”

“…An alternative way in which a convective flow can be set up is through local heat generation within the porous material. Such a situation can arise through radioactive decay or through, in the present context, a relatively weak exothermic reaction taking place within the porous material, as can happen, for example, in the spontaneous combustion of organic materials [4,5,6] or coal stockpiles [7,8].…”

The effects of wall transpiration on the unsteady free convection boundary-layer flow near a stagnation point in a heat generating porous medium

“…This can set up a convective flow within the material which may help to dissipate the heat or alternatively can lead to enhanced heat generation and even thermal runaway with possibly disastrous consequences. Examples include the spontaneous ignition in stock piles of coal [1][2][3][4], in bagasse (the cellulose waste left after the extraction of sugar from sugar cane) [5,6] or in the wetting of cellulosic materials [7][8][9]. Previously we have modelled this problem assuming local heat generation at a rate proportional to ðT À T 1 Þ p , where T 1 is the (constant) ambient temperature and the exponent p !…”

Modeling of the Crossing Point Temperature Phenomenon in the Low-temperature Oxidation of Coal

“…In this study, the gas flow in the reactor vessel was managed as laminar flow in porous media. Because the diameter of fine particles of coal is quite small (the average diameter is about 0.24 mm), the characteristics of the gas stream inside the pile of coal fineparticles can be represented using Darcy's law (Brooks et al, 1988;Salinger et al, 1994;Krishnaswamy et al, 1996;Moghtaderi et al, 2000;Akgun & Essenhigh, 2001). The spontaneous combustion of coal was simulated as a reaction of surface chemicals in which the oxidation reaction appeared on the surface of coal in the porous media.…”