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

Abstract

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 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 (R 2 > 90%); while in-depth spread rate increased linearly up to 300% from moisture contents 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.

Laboratory study on the suppression of smouldering peat wildfires: effects of flow rate and wetting agent

“…A commercially available peat was used (Shamrock Irish Moss Peat, Bord na Mona Horticulture Ltd) owing to its homogeneous properties and batch consistency in the long term. The elemental content of the peat on a dry mass basis was C/H/N/S of 54.1/5.1/1.3/0.5%, and the inorganic content (IC) was 2.5 AE 0.6% (Christensen et al 2020). In this experiment, the MC of peat is ,100% (mass of water content over mass of dried peat).…”

Spatially resolved horizontal spread in smouldering peat combining infrared and visual diagnostics

“…Following the procedure of [25], a low threshold intensity value, which could correspond to drying region, was chosen and it was kept same during the entire set of experiments.…”

Smouldering wildfires in peatlands, forests and the arctic: Challenges and perspectives

“…However, the water table of peatlands can descend, and the moisture content of organic soils is the single most important property governing the ignition and spread of peat wildfires. For example, the critical moisture content for igniting typical boreal peat samples has been measured at about 125% in the dry base where the soil mineral content is less than 10% [ 22 , 23 , 24 , 25 ]. Smouldering fires propagate horizontally and vertically through organic layers of the ground and can reach deep into the soil where large cracks or natural piping systems exist.…”

“…Once ignited, they are particularly difficult to extinguish and can persist for long periods of time (months, years), spreading deep into the ground and over extensive areas. Recent studies have quantified the maximum soil moisture content that allows smouldering spread for different types of [ 22 , 23 , 24 , 25 ], the conditions for water suppression [ 37 , 38 ] and the creation of firebreaks [ 23 ]. For most peatland megafire, firefighting with water is very challenging because of the large amounts of water needed.…”