Physicochemical features of the effect of special water-based fire retardants on forest materials

https://doi.org/10.1016/j.firesaf.2021.103371Get rights and content

Abstract

Specialists in forest fire extinguishing consider special fire retardants (solutions, suspensions, and emulsions), in addition to water, to be the promising. When discharged from aircrafts, these agents show greater efficiency in suppressing combustion and preventing ignition of forest materials compared to water. However, such agents have been developed through trial and error so far, because no general theory of the effect of aqueous suspensions, solutions, and emulsions on combustion has been developed yet. In addition, there have not been any experimental data to develop physical and mathematical models of processes when a combustion front acts on forest materials after their wetting by special aqueous compositions. In this paper, we studied the wetting behavior of water and four special fire retardants (bentonite suspension, bischofite solution, fire-extinguishing solution FES-5, and foam agent emulsion) on surfaces of leaves, branches, and needles. The lifetimes and evaporation rates of droplets of fire retardants from the leaf surface were determined at 50–110°С. The differences were established in the evaporation rate of droplets of typical fire retardants and water without additives. With identical initial droplet sizes of fire-extinguishing suspension, emulsion and solutions, their evaporation times differed significantly.

Introduction

According to expert estimates [1,2], the problem of forest fires has remained extremely urgent for many recent decades despite rather great attention to the creation of fire-extinguishing systems and equipment in many developed countries [3,4]. Examples of almost annual large forest fires in the state of California (USA) causing enormous economic damage and leading to human casualties show that even such a technologically and scientifically developed country as the United States cannot effectively solve this problem. Such a situation is typical of all countries with large forested areas. For example, six countries in Europe are mainly exposed to forest fires: Spain, Italy, Greece, Cyprus, Portugal, and France. Hundreds of forest fires are recorded on thousand hectares annually in the Russian Federation [1,2,5]. The last fire is usually suppressed when the period of intense seasonal rains starts. The low efficiency of forest fire control on all inhabited continents of the Earth is due to many objective and subjective reasons. However, one of the main reasons is a lack of understanding of the physics and chemistry of forest burning and their extinguishing using water [6] and special aqueous agents (for example, graphite [7], foam and gel [8], water emulsion [9,10], foam agent and salt solutions with NaCl [9]). The efficiency of these agents in most cases is justified [9,10], but so far there has been no complete understanding of the mechanisms of combustion suppression by special compounds, for example, whether the additives to water affect the chemical processes of forest material (FM) combustion. In other words, it is still unknown whether the additives act as fire retardants or special solutions, emulsions, and suspensions just intensify the physical processes that occur during the interaction of such combustion retardants with single FM elements. These physical processes involve wetting and liquid evaporation when fire retardants interact with FM heated to high temperatures (higher than thermal decomposition temperatures).

Water as the main forest fire extinguishing agent has a significant drawback: its low wetting ability on the FM due to its high surface tension [10]. To reduce surface tension and increase wetting, surfactants enhancing the fire-extinguishing effect due to covering the FM elements are introduced into water [[10], [11], [12], [13]]. As a result, the time for extinguishing forest and peat fires is reduced [11]. To date, the spreading and evaporation of surfactant solutions on inorganic surfaces, such as silicon [14], polytetrafluorethylene [15,16], aluminum [17,18], and glass [18], has been thoroughly studied. However, wetting of plant surfaces (e.g., ginkgo leaves [19] and wood [20]) is much more complicated.

Laboratory and field test results of new wetting compositions (W-50, W-52, W-56, W-10, W-11, and W-32) for extinguishing forest fires are presented in Ref. [10]. These compositions are mixtures of anionic and nonionic surfactants, organic solvents and hydrotropic agents. A 1% W-50 solution penetrated through a layer of loose rotting wood 68 times faster than water. Water droplets were absorbed 40 times slower compared to the solutions of wetting compositions. The field tests of the compositions with the best wetting properties according to laboratory experiments were conducted to analyze their ability to create fire barriers and their effectiveness in fighting ground fires. The W-52 composition was found to reduce the burnt area and the loss caused by an increase in the amount of deadwood after soil cover fires compared to water [10].

The sorption ability and wettability of loose and pressed peat were studied in Ref. [11]. The nonionic and anionic surfactants exhibited various properties of foaming and moisturizing peat. The preliminary results allowed us to determine the surfactant components to obtain a wetting agent composition. The wettability and adsorption characteristics were tested, and the foaming ability was evaluated [11]. The effectiveness of the wetting solution obtained in this way was confirmed in the laboratory and in fire extinguishing [11].

Recently, the authors established dependences of the wetting time of powdered FM on the surface tension of solutions containing surfactants [12]. The surface tensions of liquids depending on their concentrations were determined in the laboratory studies. In addition, the authors measured the time and mass wetting rate of solutions with different concentration on four FM samples (green moss, lichen, forest cover and peat). At a certain surfactant concentration, the surface tension ceased to decrease while the wetting rate continued to increase. Different types of FM were wetted at different rates [12]. The wetting time of different surfactant solutions with identical surface tension on the same FM can differ by ten or more times. The effect of organosilicone surfactant (C13H34O4Si3 Silwet L-77) and pesticide (C3H8NO5P glyphosate) in an aqueous solution on wetting of leaf surfaces was studied in Ref. [13] by applying induction charges to droplets of this solution. The conductivity and polarizability of these compounds increased with increasing concentrations while the surface tension (σ) decreased, which ultimately led to a decrease in the static contact angle (θ) on the hydrophobic surfaces of Brassica campestris leaves [13]. The decrease in σ and θ made the wetted area of droplets larger. When the concentration of L-77 and EC increased from 2.5 ml/l and 7.5 ml/l to 10 ml/l and 15 ml/l, respectively, the wetted area changed from 9.26 mm2 at θ = 65.29° to 13.29 mm2 at θ = 43.1°.

The static contact angle was found to depend exponentially on the sandpaper grain size used for the processing of wood species in Ref. [21]. This is characterized by three regions: 1) decrease in the static contact angle due to an increase in adhesion; 2) reaching its minimum value; 3) a slight increase in θ. Acidity was found to affect the work of adhesion and wettability of North American species’ wood [22]. The contact angle of water droplet ranged from 60° for Sitka spruce and up to 74° for Douglas fir. In addition, the surface wettability of Cathay poplar and Scots pine decreased after their heat treatment [23].

The effect of wetting properties of fire-retardant solutions (Borax (Na2B4O7), DSHP (Na2HPO4), DAHP ((NH4)2HPO4) and DW) on the fire resistance of pine and poplar was studied in Ref. [20]. The best fire resistance was intrinsic to poplar. A high concentration of fire-retardant solutions led to significant differences in wetting angles (from 50° to 80°). After cutting and planing the surface of a poplar, its roughness at a high concentration of fire-retardant solutions had a strong effect on wettability [20].

The most effective process for extinguishing forest fires was found to be the liquid flow injection into the FM thermal decomposition zone, which, as a rule, is located at a depth of 10–15 mm from the FM/hot air interface [24]. In this case, the pyrolysis of combustible materials was suppressed, the fuel flow (CH4, CO, C2H6) stopped reaching the chemical reaction zone, and the combustion did not continue. In addition, water vapors, which were formed during the evaporation of droplets moving deep into FM, cooled the FM elements that passed the initial stages of thermal decomposition. As a result of lowering the temperature of these elements, the heat flux supplied to the pyrolysis zone decreased, which contributed to the pyrolysis suppression.

The extinction of a model fire source by an aerosol cloud of potassium hexacyanoferrate(III) K3[Fe(CN)6] aqueous solution was studied in Ref. [25]. The short-term impact of the aerosol cloud on the flame front of a surface forest fire was found to lead to the suppression of the gas-phase combustion and even to its complete disappearance in the case of the A-class model fire source (wood burning). When extinguishing a fire with an aerosol aqueous solution [25], the volumetric flow rate of this extinguishing agent was 30 times lower than the standard flow rate of pure water due to the coating of the fuel surface with deposited salt. The degree of burnup of the model source was at least 80% when extinguished with water and about 50% when extinguished with K3[Fe(CN)6] aqueous solution. It was proposed to use 40% aqueous solution of CuCl2 as a fire retardant [26]. The duration of the B1-class fire source extinguishing by the aerosol of copper(II) chloride solution was 0.6 s, which is 26 times more effective than extinguishing by an aqueous aerosol.

Despite many conducted studies [11,24,25], the practical application of all special materials for the preparation of solutions, emulsions and suspensions to suppress FM combustion or contain forest fires is hampered by their high cost. For example, the cost of 1 t of bentonite powder lies between US$ 100–1000. Therefore, there is a problem of an adequate estimate of the costs of implementing FM combustion suppression technologies based on such materials and the effectiveness of these technologies. It is difficult to conduct such an estimate based only on the results of the field tests as the field test conditions cannot always be strictly controlled, e.g., wind direction and speed can vary, FM moisture content depends on many factors, etc. For this reason, it is necessary to understand the mechanisms of the influence of additives to water on the efficiency of combustion suppression and fire containment. To fully describe the mechanisms of combustion suppression and FM thermal decomposition, information is needed on the interaction between droplets of special fire retardants and the FM elements (leaves, branches and needles). So far, the wetting, spreading and evaporation characteristics of water and special fire retardants on typical FM elements have not been published.

In this paper, we strive to obtain new knowledge about the wetting and evaporation mechanisms of water and four special water-based fire retardants on the surfaces of leaves, branches and needles. This is expressed in terms of the dependences of the contact angle, droplet height, liquid evaporation rate on droplet volume and surface temperature.

Section snippets

Experimental setup

A scheme of the experimental setup for study of wetting and droplet evaporation of fire retardants on the FM surfaces is presented in Fig. 1. The schlieren optical method was used to conduct the experiments [27,28].

The working section consisted of a GNL18/M goniometer (THORLABS), a silicone heater (Marion), a copper substrate 50 mm in diameter and 3 mm thick. The silicone heater was connected to a power source (SUNTEK laboratory autotransformer). Using a laboratory autotransformer, the FM

Wetting behavior of water and special water-based fire retardants on forest material surfaces

Fig. 2 presents typical schlieren images of droplets on the FM surfaces.

The dependences of contact angle, diameter, and height of the droplet placed on the leaf surface on its volume are presented in Fig. 3. It should be noted that in experiments, we reproduced the conditions for the interaction of water droplets and special agents during their movement through the highly porous structure of the FM irrigated by liquids. When the droplet falls onto the FM, it either hits the FM element surface,

Conclusion

  • (i)

    The fire retardants were conditionally divided into two groups according to the experimental results on the wettability properties. Water, bentonite suspension and bischofite solution are referred to the first group of fire retardants, which do no wet the FM surface. The second group includes foam agent emulsions and FES-5, which do wet the FM surface. Nevertheless, we found that wetting does not affect the FM combustion suppression. Both wetting and non-wetting liquids provide a multiple

Credit author statement

We confirm that this manuscript has not been published elsewhere and is not under consideration by another journal. All authors have approved the manuscript and agree with submission to Fire Safety Journal. All info is original.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Research was funded by the Russian Science Foundation (project 18–19–00056).

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