Which is the most effective method available for extinguishing smoldering fires?

Abstract

Smoldering fires in peatlands can consume large areas of peat and release important amounts of carbon to the atmosphere as they self-propagate. This chapter focuses on the use of infrared images to characterize the horizontal propagation of smoldering fires in laboratory experiments. In these laboratory experiments an infrared camera takes images of the peat surface at regular intervals during the experiment. We present methods to process and analyze these infrared images that identify the shape and position of the smoldering front, quantify the maximum energy flux, the spread rate and direction of the front and its variability to time. To demonstrate our methods we analyze images from experiments that record the smoldering of dry peats (25% moisture content, mass of water per mass of dry peat) and wet peats (100% moisture content). Infrared images are used to quantify the effect of moisture content upon the smoldering fronts. Our methods demonstrate that smoldering combustion in dry peats has a wider front (6.8 ± 1 cm for the dry peat, 2.4 ± 0.7 cm for the wet peat), a faster spread rate (4.3 ± 1 cm/h for dry peat, 2.6 ± 0.7 cm/h for wet peat), and a lower peak of radiative energy flux (7.1 ± 0.7 kW/m2 for dry peat, 10.51 ± 2.1 kW/m2 for wet peat). Our infrared image analysis is a useful tool to characterize peat fires at an experimental scale. These methods can be applied to peats with different characteristics to identify and compare smoldering propagation dynamics.

Fire Behavior and Its Effects

Smoldering fire spread was observed 24 h after the fire event started (Figure 20.3.5). At that time, the charred or partially consumed vegetation was irregularly distributed on the landscape. The lack of flames and the release of smoke from the ground indicated that 24 h later there was still active consumption of peat layers due to smoldering fire fronts. Compared to flaming fires, smoldering is considered a type of low-intensity fire (Rein, 2016), meaning that fire slowly spreads and can be expected to last for several days, releasing small amounts of energy (Keeley, 2009). The Kippure Estate section of this fire continued smoldering for 6 days.

The spread of a smoldering front can happen in all directions, either at ground surface or in deep peat layers (Rein, 2016). The spread of the fire, remaining active days after the start of the fire, supports previous studies indicating a slow speed of the smoldering fires moving at rates of less than 9 cm h−1 (Prat-Guitart et al., 2016). A close-up photo of a peat surface shows the position of the smoldering front moving at the surface of the peat (Figure 20.3.6, left). The spread to deeper peat layers could also be happening simultaneously if the moisture content of the peat was low enough to sustain downward smoldering self-propagation (Huang and Rein, 2015); however, this was not directly observed during the fire event due to the difficulties of detecting deep smoldering from the surface.

The irregular consumption of organic soil during the Wicklow Mountains Fire suggests that the distribution of the moisture content at the surface ground layers was also heterogeneously distributed (Figure 20.3.5). In drained peatlands however, the moisture distribution of the surface soil layers tends to be more homogeneous, thus the irregular consumption of the peat would indicate the moisture content distribution at the surface is strongly associated with the plant and root distribution.

Overlapping effects of the 2003 and the 2015 fires were also observed in the area: trees that were damaged in the 2003 fire were affected again by the 2015 events (Figure 20.3.6, right). The organic soil underneath the dead roots was consumed to ashes.

Abstract

Smoldering megafires are the largest and longest burning fires on Earth. They destroy essential peat land ecosystems and are responsible for 15% of annual global greenhouse gas emissions. This is the same amount attributed to all the combustion engine vehicles in the world, yet it is not accounted for in global carbon budgets. Peat fires also induce surges of respiratory emergencies in the population and disrupt shipping and aviation routes for long periods, weeks, and even months. Despite their importance, we do not understand how smoldering fires ignite, spread, or extinguish, which impedes the development of any successful mitigation strategy. Megafires are routinely fought across the globe with techniques that were developed for flaming fires, and are thus ineffective for smoldering. Moreover, the burning of deep peat affects older soil carbon that has not been part of the active carbon cycle for centuries to millennia, and thus creates a positive feedback to the climate system.

Abstract

Smoldering mining dumps and their high temperatures represent a high energetic potential, which has not been utilized so far. In the scope of a joint research project the geothermal utilization of smoldering mining dumps has been investigated. In this context, a pilot plant consisting of three borehole heat exchangers was installed on a mining dump in the western Ruhr area in Germany. Several Thermal Response Tests as well as long-term tests were carried out. The results show that a total heat output of 8 kW with system-temperatures up to 50 °C could be achieved. Thus, the output is much higher compared to common subsurface geothermal systems. In addition to the field tests several laboratory tests and theoretical investigations were carried out. As a result, the temperatures inside the dump as well as the thermal properties of the dump material have been identified as the most important factors when planning a geothermal plant on a smoldering mining dump.

Smoldering Combustion

Smoldering combustion is a self-sustaining, slowly propagating, low-temperature flameless combustion process in which solid fuel first undergoes thermal decomposition (pyrolysis), producing volatiles and carbonaceous char (Drysdale, 1985). Subsequently, the char oxidizes, providing the heat required for propagation of the smolder process. In a wildfire, smoldering occurs primarily after passage of the visible flame. Heat transfer in smoldering combustion is by conduction, convection, and radiation. See Miyanishi (2001) for an overview of the heat transfer models proposed for the propagation of smoldering combustion.

Smoldering combustion is an important process in only some ecosystems affected by wildfires. It occurs within the fermentation and humus (F and H) layers of the organic soil (i.e., duff), which lie between the litter layer and the A horizon of the mineral soil in some forests. Duff is present primarily within conifer-dominated forests with cool to cold climates, such as boreal and subalpine forests and Douglas fir forests of the Pacific Northwest (Miyanishi, 2001). Smoldering combustion may also occur in dead fallen tree boles and within thick layers of litter impregnated with roots, called root mats, in some humid tropical forests; see Miyanishi (2001) for a summary of this work. Smoldering is not an important process in most dry, warm savanna, woodland, and forest ecosystems (e.g., Mediterranean pine forests, ponderosa pine savannas). These ecosystems often have only a litter layer, which is consumed in flaming combustion.

Only porous materials that form a solid carbonaceous char when heated can undergo self-sustained smoldering combustion (Drysdale, 1985). Duff consists primarily of cellulose, hemicellulose, and lignin (Mason, 1976); of the three components, lignin is the most resistant to thermal decomposition and thus it is the main component in duff that produces char (Johnson, 1992; Miyanishi, 2001). The surface oxidation of char provides the heat necessary to cause further thermal degradation of adjacent virgin duff (Drysdale, 1985). The propagation of smoldering combustion requires that volatiles be progressively driven out ahead of the zone of active combustion (where char is oxidized) to expose fresh char that will then begin to oxidize. Heat losses from the reaction zone must not be too high because sufficient heat for the endothermic pyrolysis of the virgin duff must be supplied from this reaction zone (Drysdale, 1985).

Smoldering combustion is controlled by three properties of duff: bulk density, moisture content, and depth (Miyanishi, 2001). Bulk density (kgm−3) affects the penetration of oxygen through duff, which affects the rate of oxidation of char (Palmer, 1957; Ohlemiller, 1985; 1990). The moisture content of duff affects how much heat is required to evaporate the water before it can start to raise the temperature of the virgin duff high enough to drive off volatiles and produce char (Palmer, 1957; Miyanishi, 2001). The depth of duff affects heat losses from the oxidation zone because the duff itself acts as insulation; the thinner the duff layer, the greater the proportional heat loss from the system (Palmer, 1957; Bakhman, 1993; Miyanishi, 2001). In addition, the effects of these properties interact; for example, the thinner the duff, the lower the moisture content threshold for the propagation of smoldering (Fig. 1) (Jones et al., 1994; Miyanishi and Johnson, 2002). See Miyanishi and Johnson (2002) for a more detailed discussion of the significance of each of these three factors in smoldering combustion of duff.

Smoldering combustion of duff causes at least three important ecological effects: (1) mortality of seeds stored in the duff; (2) partial or complete mortality of the vegetative parts of plants (i.e., roots and rhizomes); and (3) exposure of mineral soil. All three in turn affect the post-fire recruitment of herbaceous and woody plants. Seeds stored in duff are typically killed in the smoldering process; either the heat transferred through the duff is enough to kill the seeds or seeds are consumed in the process. See Chapter 12 for a detailed case study of the application of heat transfer models to seed survival within woody fruits. Smoldering combustion may also partially or completely kill the roots and rhizomes of trees, which can reduce or inhibit the post-fire recruitment of tree populations that sprout from basal buds. The partial or complete mortality of the tree roots of sexually reproducing tree populations may contribute to overall tree mortality. Finally, several studies have demonstrated the importance of exposed mineral soil to the post-fire recruitment of sexually reproducing tree populations. This is discussed in detail later in this chapter.

Organic Compounds

The smoldering of mosquito coils will emit VOCs including methylene chloride, cis-1,2-dichloroethene, chloroform, 1,2-dichloroethane, benzene, toluene, ethylbenzene, m,p-xylene, styrene, and o-xylene. Their emission rates of methylene chloride, cis-1,2-dichloroethene, chloroform, 1,2-dichloroethane, benzene, toluene, ethylbenzene, m,p-xylene, and o-xylene are 972.3, 181.7, 67.3, 8.8, 324.2, 592.7, 5.8, 7.3, and 1.2 μg h− 1, respectively. The gaseous carbonyl compounds were also quantified in mosquito coil smoke. The estimated emission rates of formaldehyde, acetaldehyde, acetone, acrolein, propanaldehyde, crotonaldehyde, 2-butanone, glyoxal, o-tolualdehyde, 4-methyl-2-pentanone, and methylglyoxal were 3.10, 1.97, 1.04, 0.60, 0.49, 0.31, 0.72, 0.15, 0.10, 0.56, and 0.79 mg h− 1, respectively. The simulation, under the conditions of a room volume of 50 m3 and an air exchange rate of 2 h− 1, showed the estimated average concentrations of formaldehyde, acetaldehyde, and acrolein ranging from 1.51 to 25.0 μg m− 3, 3.34 to 9.20 μg m− 3, and 0.28 to 5.21 μg m− 3, respectively. It is noteworthy that the acute reference exposure level (REL) for acrolein developed by the US Office of Environmental Health Hazard Assessment is 0.19 μg m− 3 only. The particulate phase PAHs found in mosquito coil smokes included acenaphthene, fluorine, phenanthrene, anthracene, fluoranthene, pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, and benzo[g,h,i]perylene. Their rates were 2.90, 9.63, 13.51, 0.69, 20.15, 5.19, 0.021, 0.021, 0.31, and 0.54 g h− 1, respectively.

2.1.5 Zinc borate and carbon

Afterglow/smoldering combustion is a nonflaming combustion of carbon (char). Some flame retardant polymers systems are prone to afterglow/smoldering combustion; borates are known to be afterglow/smoldering combustion inhibitors. It is generally believed that borates can form a glassy layer on the surface of the char and inhibit the nonflaming oxidation. It was also proposed that borates can remove free electrons from the active sites with the result that the sites are no longer capable of reacting. In addition, the inhibiting borate molecule may be covering the active sites so that, geometrically, oxygen is not able to attack the surface [35].

Polyolefin: Nakasaki [36] first reported the benefit of using a combination of Firebrake ZB and expandable graphite in EVA (Table 7).

Table 7. Flame EVA with Expandable Graphite [35]

Examples (parts by wt)1234ComponentsEVA (15% VA, MFR 1.5 9/10 min)100100100100Expandable graphite010010Zinc borate002020PropertiesUL-94 (3.2 mm)NRNRNRV-0Oxygen index (%)21.025.020.027.5

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