Behaviour of smoldering fires during periodic refilling of wood pellets into silos

The evolution of smoldering fires in biomass stored in lab-scale silos with additional fuel material supplied repeatedly, has been studied. The direct effects of the added material on the sample are: cooling, enhanced thermal insulation, and facilitation of more intense combustion at a later stage. This article focuses on the cooling, which leads to an almost instantaneous reduction in the combustion rate. Surprisingly, this reduction does not vary only with the amount of material refilled - but also depends strongly on the stage of the smoldering process. This demonstrates that the underlying smoldering processes merely to a certain degree are regularized by the periodic refillings. Quantitatively, the functional relationship between the refilled amount and the change in combustion rate caused by the refilling was determined. The results displays four regimes, that reflect different stages of the smoldering process at the time of the refilling. Using the Arrhenius equation, we find for one of these regimes an expression for the ratio of combustion rates (immediately after to immediately before the refill) as function of the amount of added material. This expression contains only one free (undetermined) parameter. We determine the value of this parameter from the data and demonstrate that this value is consistent with a simple model for how the sudden cooling occurs spatially in the sample.