Experimental study of the effect of water spray on the spread of smoldering in Indonesian peat fires
Introduction
Forest fires in Indonesia remain a major environmental problem. The Indonesian archipelago has a lot of scattered hotspots, caused by wild land and peat fires. For example, on December 15, 2015, NASA Fire Information for Resource Management identified around 96,937 hotspots on Sumatra Island [1]. One of the main contributors of forest fires in Indonesia is peatland fire. Indonesia, with 188.2 million ha area, is comprised of both dry lands and swamps. The swamp area itself, which is 33 million ha, consists of 20.6 million ha of peatlands or around 10.8%. Most of the peatlands are located in three major islands, which are Sumatra (35%), Kalimantan (32%), and Papua (30%). The remaining 3% are located in Sulawesi and spread across confined areas [2], [3].
Peat fire is an example of smoldering fire which occurs at low-temperature and generates incomplete combustion. Smoldering combustion produces toxic gases and particulates at a higher rate than flaming combustion. Several trials for fire suppression on peatlands have been conducted using methods such as quarrying, water spray, foam spray, and artificial rain. Much like fire suppression in coal, Colaizzi [4] showed that the extinction process is costly, and often fails to produce the desired result.
According to Hadden [5], there are three main in situ mechanisms for fire suppression in smoldering fire: through cooling, smothering, and burn-out. The first two mechanisms are based on the physical and chemical processes which play a role in smoldering combustion. For cooling, the temperature of peat is reduced to below the critical re-ignition temperature by adding a suppression agent below the surface. Suppression agents can be in the form of gas or liquid [5]. The smothering mechanism has been identified by Ohlemiller [6] and shows that when oxygen concentration is reduced to below a critical value, oxidation reactions will stop. However, higher concentration of oxygen in the atmosphere would increase the smoldering spread rate, and the critical oxygen concentration is influenced by several parameters, such as the moisture and inorganic content within the peat bed [7], [8]. The air that is used as the oxidizer produces a buoyancy effect on the burned fuel during the smoldering process [9]. In this research, a water mist suppression system is used to see the effect of suppression by forced cooling for peatland. There are many variables that could affect the performance of the water mist system. Xiaohui et al. [10] studied the performance of water mist in an enclosed space by using nozzles with different characteristics. He found that increasing the spray cone angle and decreasing the flow rate would increase the extinguishing time [10]. Pressure, droplet size, and density of discharged water determine the water mist system performance.
Peat fires, which means smoldering fires, tend to move slowly, have a low and stable temperature, and are flameless [6]. Smoldering is a combustion process that includes heterogeneous chemical reactions, and also heat, mass, and momentum transfer [11]. Ohlemiller [6] and Rein et al. [12] explained in general that smoldering has four stages while it is spreading, resulting in the structure of a smoldering front. The smoldering front therefore consists of four zones: the preheat zone, the evaporation zone, the burning zone, and the char and ash zone [6]. These are the explanations of smoldering front:
- 1.
Preheating of Virgin Fuel Zone
The zone where the virgin fuel is preheated is an area where the initial temperature is increasing, from the initial fuel temperature until the evaporation point, thus the temperature of the virgin fuel will increase without the creation of gas.
- 2.
Evaporation Zone
The evaporation zone is an area where the endothermic process happens within the temperature range of 80–100 °C and produces water vapor.
- 3.
Burning Zone
Burning zone is an area where the oxidation and pyrolysis processes occur and release the net heat.
- 4.
Char and Ash Zone
The char and ash zone is an area where the smoldering process stops and creates char along with ash. Ash is made from remnants of mineral fuels, while char is made from incomplete combustion.
Tropical peatlands have an important role in the ecosystem when considering both local and global perspectives [13]. Thus, minimizing the negative environmental and socioeconomic consequences of peat fires becomes necessary. An integrated strategy for preventing the occurrence of peat fire as well as measures to control the fire when hotspot are identified should be integrated into local development strategies in the areas where natural peatlands are located [14]. Because the fire risk increases if the water table falls below 0.40 m [15], maintaining the water table above the critical value by canal blocking strategies is a potential option for hydrological restoration of degraded tropical peatlands [13]. Nevertheless, once the peat fires occur in the degraded peatlands, measures of suppressing the fire should be conducted effectively. These measures include direct attack by sending fire brigades equipped with portable pumps, hoses, and spraying nozzles, or even aerial water bombing using helicopter or aircraft. However, for those methods, the amount of water required for peat fire extinction and cooling currently has not been well established.
The objective of this research is to study the effect of water mist spray intensity on the suppression of smoldering combustion of Indonesian peat. Samples of peat from South Sumatra and Papua were tested and compared, and experimental studies on the smoldering combustion behaviour of Indonesian peat were conducted. It is expected that a minimum amount of water spray is required for effective suppression of smoldering in peat fires.
Section snippets
Peat sample preparation
Two sample types of peat for this research were taken in different locations as shown in Fig. 1. The first sample was taken in Kabupaten Ogan Komering Ilir (OKI), South Sumatra Province, Indonesia, with coordinates S: 03°24′00.7″; E: 104°52′39.1″. The second sample was taken in Kabupaten Sarmi, Papua Province, with coordinates S: 01°55′14.11″; E: 138°6′17.35″. Both samples were taken at a depth of 60–120 cm with undisturbed sampling methods in order to obtain samples with stable conditions until
Smoldering experiment
Characterization of the smoldering combustion of the peat was conducted in advance to facilitate the research analysis on water mist suppression. The comparison of the temperature distribution trends can be seen in Fig. 5(a) and (b) for Papuan and South Sumatran peat respectively. There is a very noticeable difference of the required time to heat up the peat without any influence from the igniter. At approximately 50 min, the heating process for Papuan peat is still strongly influenced by the
Conclusion
A series of smoldering combustion experiments, characterization, and suppression of Indonesian peat have been conducted in laboratory scale. Peat samples from different locations were ignited while the mass and temperature change in the apparatus were measured. This experiment investigates the smoldering combustion phenomenon of peat fires and also the extinguishing process by water mist suppression application.
The experiment found that the characteristics of peat strongly affects the
Acknowledgements
The authors would like to thank the financial support provided by Universitas Indonesia through the PITTA 2017 funding scheme under Grant no. 857/UN2.R3.1/HKP.05.00/2017 managed by the Directorate for Research and Public Services (DRPM) Universitas Indonesia.
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