Influence of soil conditions on the multidimensional spread of smouldering combustion in shallow layers

Smouldering peatland fires are capable of burning vast amounts of organic soils, resulting in the release of greenhouse gases into the atmosphere, as well as a significant deterioration of air quality causing in major regional crises known as haze events. Fundamental understanding of smouldering fire spread is essential for the development of mitigating technologies. In this paper, we have systematically conducted 63 experiments studying the individual and combined influence of two key factors affecting multidimensional smouldering spread in organic soils: moisture content (MC) and inorganic content (IC). Both lateral and in-depth smouldering spread were investigated using a novel shallow reactor. This shallow depth allows a greater number of experiments to be performed in a short period of time compared to deeper samples. Lateral spread was found to decrease linearly with moisture content (R2 > 90%); while in-depth spread rate increased linearly up to 300% from moisture content of 0% to 140%. Increased inorganic content linearly decreased the lateral spread rate but had little influence on in-depth spread in drier samples. Interestingly, in wetter samples, in-depth spread was in fact sensitive to inorganic content. A novel approach combining lateral and in-depth spread rates as vector components, revealed that the global spread is independent of moisture content, with an average spread rate of 8.7 and 8.4 cm/h for 2.5 and 40% IC, with changes in direction according to moisture content; going in-depth for wet soils, and laterally for dry soils. Similarly, increasing the IC encouraged downward spread for wet samples. We also report observations of a bifurcation of lateral spread, where spread would locally extinguish where the in-depth spread was greater than the lateral spread. These findings provide previously unknown insight into the relationship between lateral and in-depth spread in smouldering fires, ultimately improving the fundamental understanding of such fires.