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Gasoline fire extinguishing by 0.7 MPa water mist with multicomponent additives driven by CO2
Process Safety and Environmental Protection 129 (2019) 168–175
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Gasoline fire extinguishing by 0.7 MPa water mist with multicomponent additives driven by CO2 Dong Lv a,b , Wei Tan a , Guorui Zhu a , Liyan Liu a,∗ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300354, China Tianjin Fire Research Institute of MEM, Tianjin, 300381, China
a r t i c l e
i n f o
Article history: Received 17 March 2019 Received in revised form 4 July 2019 Accepted 4 July 2019 Available online 11 July 2019 Keywords: Gasoline fire Water mist Fire suppression Multicomponent additives
a b s t r a c t Water mist has a good three-dimensional cooling effect; however, its extinguishing effect on oil fires is less than that of foam extinguishing agents. This study aimed to enhance the gasoline fire extinguishing efficiency of water mist by adding KBr, Tween-80, and a dissolved CO2 solution as multicomponent additives with different suppression mechanisms. The fire extinguishing time was measured to evaluate the extinguishing efficiency of different additives. The test showed that KBr mainly works near the flame base and makes an unstable gap between the flame base and the gasoline surface; the Tween80 mist and dissolved CO2 mist make the flame height lower under the test conditions. By analyzing the different extinguishing phenomena, temperature and efficiency of each additive, the multicomponent additives were proposed to enhance the extinguishing efficiency based on different mechanisms. The results showed the water mist with KBr, Tween-80 and dissolved CO2 resulted in a remarkable improvement in the ability to extinguish a gasoline fire compared with pure water mist. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction A foam extinguishing agent is an appropriate choice to control a liquid plane fire, such as a pool fire. Nevertheless, for a threedimensional fire source, such as a fire including a pool fire and a solid fire at different heights, it is difficult for a foam extinguishing agent to cover the fire source completely, which will affect its extinguishing effect. Water mist can fill the fire space from top to bottom, which is a very important advantage for extinguishing three-dimensional fires. However, compared with foam extinguishing agents, pure water mist has a lower efficiency for extinguishing a pool fire (Iii et al., 2000; Ni and Chow, 2011). To improve the extinguishing efficiency, much work has been conducted in different methods. Some researchers devoted to improve the efficiency by optimizing the parameters of extinguishing agents (Harding et al., 2016; Zhen et al., 2016), some tried different additives with different mechanism in extinguishing agents. For example, it was reported some additives added in water mist can apparently improve the extinguishing efficiency by breaking the flame reaction chains. Iron pentacarbonyl and ferrocene have been indicated to be effective inhibitors on flame reaction
∗ Corresponding author. E-mail address: [email protected] (L. Liu).
chains (Koshiba et al., 2015, 2016; Rumminger and Linteris, 2000; Yu et al., 2008). Yu (Yu et al., 2008) has suggested saturated vapor of ferrocene is more effective than nitrogen for alcohol fire suppression under some conditions. However, the toxicity of these additives should not be ignored. According to the MSDS, the LD50 parameter (mouse oral) of iron pentacarbonyl and ferrocene are approximately 40 mg/kg and 600 mg/kg, respectively. Potassium free radicals show a capacity to capture free radicals and inhibit flames (Zhang et al., 2016a, b; Zhang et al., 2017). In addition, some research has shown halides, such as Cl, can capture the free radicals generated in burning reactions (Cao et al., 2016, 2015; Zheng et al., 1997). However, Cl− is more corrosive compared with its congeners. In addition, iodides are more expensive and have less chemical stability than bromides. Combined with the above analysis, KBr was evaluated as a component of the fire extinguishing agent in this work. Surface active agents have been proven to be effective in fire suppression. Some works have focused on surfactants additive agents for suppressing pool fires (Koshiba et al., 2016; Zhou et al., 2006). Generally, foams suppress oil fires effectively. Foams can cover the oil surface and surfactants can emulsify the oil surface, which slows the burning rate. In this study, Tween-80 was used as a component of the fire extinguishing agent. CO2 is often applied as a diluent to extinguish flames (Baev and Bazhaikin, 2014; Jung et al., 2014; Min and Baillot, 2012). CO2 has
https://doi.org/10.1016/j.psep.2019.07.002 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
D. Lv et al. / Process Safety and Environmental Protection 129 (2019) 168–175
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a low cost and a high efficiency of creating an inert combustion environment. Specifically, CO2 will dissolve in water and form one phase mixture under pressure. Dissolved CO2 can then be simultaneously used as an extinguishing agent and a power resource to drive the mixture spray out. In this study, the different extinguishing mechanisms and the synergistic effect of KBr, Tween-80 and dissolved CO2 mist were studied to determine suitable water mist additives for extinguishing gasoline fires.
Tween-80 was applied as an emulsifier to decrease the gasifying rate. When a water mist with Tween-80 is dropped on a gasoline surface, it emulsifies the gasoline to form a water in oil (W/O) mixture. The gasifying rate depends on the liquid’s saturated vapor pressure. Consider a water mist with Tween-80 as the solvent and gasoline as the dissolvent; refer to Raoult’s law (Bancroft and Davis, 2002):
2. Extinguishing mechanism
P=
The fire extinguishing mechanisms of KBr, Tween-80 and highpressure dissolved CO2 solutions are different. The follow sections will discuss this topic. 2.1. KBr’s extinguishing mechanism The combustion chain reactions, such as hydrocarbons burning, gasoline burning, etc. produce OH and H radicals (Kong et al., 1998). These radicals are intermediate products that help to maintain the combustion chain reactions. KBr can decrease the radicals’ concentration due to interrupting the combustion chain reactions. The reaction mechanism of KBr suppressing the fire is explained though the following process (Birchall, 1970; Zhang et al., 2016a): KBr + H → HBr + K
(1)
KBr + OH → KOH + Br
(2)
The molar reaction enthalpies of reaction (1) and (2) are approximately 3.9 kcal and 9.5 kcal, so, reaction (1) may be easier to achieve than reaction (2). The heat of flame combustion affords enough energy for these reactions. Once K and KOH are generated, the following reactions continue (Hynes et al., 1984; Jensen and Jones, 1982; Slack et al., 1989): K + OH + M → KOH + M
(3)
Where M is an inert third body. KOH + H → H2 O + K
(4)
K + O2 +M → KO2 +M
(5)
KO2 +H → KO + OH
(6)
Meanwhile, studies (Cao et al., 2015; Wilson et al., 1969; Zheng et al., 1997) have proposed that the Cl radicals can inhibit the chain reaction. Similar reactions may occur as follows once Br and HBr are generated. Br + Br + M → Br2 +M
(7)
Br2 +H → HBr + Br
(8)
HBr + H → H2 +Br
(9)
Another reaction may occur as follows: HBr + OH → H2 O + Br
(10)
Comparing reaction (9) with (10), the molar reaction enthalpies of reactions (9) and (10) are about -16.65 kcal and -31.4 kcal, respectively. So, reaction (10) may work better than reaction (9) for flame extinguishing. 2.2. Tween-80 s extinguishing mechanism If a water mist with KBr cannot extinguish the gasoline fire thoroughly, the gasoline rapidly becomes gasified in high temperature conditions and the gasoline vapors will be ignited continuously by the unextinguished part of the flame. The fresh ignited vapors make more gasoline vapor to burn, which constitutes a combustion cycle.
P ∗ · nA nA + nB
(11)
where P - the vapor pressure of the emulsified gasoline, P ∗ - the vapor pressure of gasoline, nA - the mole fraction of gasoline, nB - the mole fraction of water mist with Tween-80. Water mist with Tween-80 (nB ) in gasoline decreases the vapor pressure (P) of the fuel, which increases the boiling point and therefore decreases the gasifying rate. Meanwhile, the generated water vapor with gasoline decreases the concentration of gasoline vapor, which decreases the combustible source transported to the flame. If gasoline is sprayed with Tween-80, foams may form. Foam will separate the flame from the fuel and decrease the gasifying rate, which will be very helpful in fire extinguishing.
2.3. Dissolved CO2 solution extinguishing mechanism Pressure will increase the solubility of CO2 in water. One volume of water can dissolve 6.4 volumes of CO2 at 0.7 MPa, 20 ◦ C. When a water mist with dissolved CO2 is sprayed out from a nozzle, the dissolved CO2 will escape because of the lost pressure, which is an endothermal process. The decomposition heat of H2 CO3 is about 19.4 kJ/mol, 25 ◦ C. Thus, inerting and cooling effects help the water mist extinguish the fire.
3. Experimental apparatus and methods The experiment was conducted in a large indoor combustion test field. The water mist extinguishing system is shown in Fig. 1. It consists of a CO2 bottle and an air compressor, a 5 L water storage tank, a spray device held by an adjustable holder with a range of 1.5 m–3 m, and a gasoline pan 20 cm in diameter and 12 cm in height. Three thermocouples, #1, #2 and #3, were fixed over the center of the gasoline pan with a height of 10 cm, 20 cm and 30 cm above the gasoline surface, respectively. The water with or without additives was added into a water storage tank through the funnel. The water tank was pressurized with air or CO2 to 0.7 MPa. If using CO2 as the driver gas, we maintained the pressure for more than 40 min to allow it to dissolve fully. Different spray heights and additives components were evaluated in this work. The fire extinguishing time and temperature data were recorded to evaluate the extinguishing efficiency. The water mist with a diameter of 211 m (DV0.9 ) and a flow rate of 1.31 L/min was applied, tested by CNCF (China National Center for Quality Supervision and Test of Fixed Fire-fighting System and Fire-resisting Building Components) under 0.7 MPa. Each trial used 200 ml of new gasoline. The mist was sprayed after 15 s of burning. KBr, Tween-80, and CO2 were tested as additives with different constituents. Pure water mist was tested as a contrast which to compare the additives’ extinguishing effects. Every test in the manuscript was repeated for at least 3 times.
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Fig. 1. Schematic of the water mist extinguishing system.
Table 1 Extinguishing time of water mist with and without additives. Scenario No.
Materials
Driver gas (0.7 MPa)
Spray height (m)
Mean extinguishing time (s)
1 2 3 4 5 6 7 8 9 10 11 12
Pure water Pure water 10% KBr 10% KBr 5% KBr 5% KBr 0.5% Tween-80 0.5% Tween-80 2% Tween-80 2% Tween-80 CO2 aqueous solution CO2 aqueous solution
air air air air air air air air air air CO2 CO2
2 3 2 3 2 3 2 3 2 3 2 3
– – 81 s – – – 32 s – – – – –
Note: “-” means the fire cannot be extinguished in 90 s under the test conditions.
4. Results and discussion 4.1. Extinguishing effect of each component 4.1.1. Extinguishing time The extinguishing times of the water mist with and without additives are listed in Table 1. The water mist with 10% KBr and 0.5% Tween-80 extinguished the fire in 81 s and 32 s at a 2 m spray height, respectively. None of these mists could extinguish the fire at a 3 m spray height under the test conditions. Pure water mist and the CO2 aqueous solution mist could not extinguish the fire at either spray height. This result shows the extinguishing effects of those components are limited by themselves. A higher concentration of a KBr aqueous solution provided more KBr crystals and decomposition products at high temperatures and they were useful to extinguish the fire. That’s why scenario No. 3 is better than scenario No. 5 in Table 1. A notable phenomenon was that 2% Tween-80 mist could not extinguish the fire at a 2 m height, while 0.5% Tween-80 mist could extinguish it under the same test conditions. A similar phenomenon was recorded by Bagdassarov et al. (Bagdassarov et al., 2000). This phenomenon may result from more gasoline being dissolved in the emulsified mixture when the concentration of the nonionic surfactants increased.
4.1.2. Temperature and flame appearance Compared with pure water mist, the water mist with additives had a higher efficiency in suppressing temperatures. The cooling effects of water mist with each component are listed in Table 2. The temperature increased to more than 600 ◦ C in approximately 30 s without any suppression at a 10 cm height above the gasoline surface (thermocouple #1). When the flame was suppressed by pure water mist, its temperature obviously decreased. The water mist produced a dramatic cooling effect on the gasoline fire, and the cooling extent was approximately 500 ◦ C. The temperatures without any suppression and that suppressed by pure water mist are shown in Fig. 2 and Fig. 3. When the gasoline fire was suppressed by pure water mist, the peak temperature at thermocouple #1 was more than 450 ◦ C, and at thermocouple #2 it was still more than 100 ◦ C. That means if pure water mist cannot extinguish the fire, the gasoline will gasify rapidly due to the high environmental temperature. Under the same test conditions, a pure water mist could not extinguish the fire; water mist only made the flame fluctuate, as shown in Fig. 4. The flame can cover the thermocouple #1 continually and cover the thermocouple #2 intermittently, which resulted in them maintaining relatively high temperatures. KBr mist improved the extinguishing ability compared with pure water mist. The mean temperate tested at the three thermocouples were lower than those suppressed by pure water mist. In
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Table 2 Cooling effects of water mist with and without additives at a 2 m spray height. Component
Driver gas (0.7 MPa)
None Pure water 5% KBr aqueous solution 10% KBr aqueous solution 0.5% Tween-80 aqueous solution 0.7 MPa CO2 aqueous solution
– air air air air CO2
Mean maximum temperature #1
#2
#3
685 416 151 72 256 172
669 122 45 284 62 34
445 56 29 46 46 24
Fig. 5. Temperature of burning suppressed by 10% KBr mist. Fig. 2. Temperature of burning without any suppression.
Fig. 3. Temperature of burning suppressed by pure water mist.
addition, instead of appearing at thermocouple #1 (10 cm above the oil surface), the maximum temperature suppressed by 10% KBr mist appeared at thermocouple #2 (20 cm above the oil surface), as shown in Fig. 5. The temperatures at thermocouple #1 were lower than that at thermocouple #2, which was different from that the temperature suppression observed with pure water mist or other mists, such as Tween-80 and CO2 aqueous solutions. The reason is that the 10% KBr mist made the flame frequently separate from the oil surface and form an unstable gap, as shown in Fig. 6. Thus, the position of thermocouple #1 will be lower than the flame base intermittently, and thermocouple #2 was covered in flame continually, causing the temperature at thermocouple #1 to be lower than that at thermocouple #2. The reason why an unstable gap formed when it was suppressed by 10% KBr mist may be a result of the following: before reacting with OH and H free radicals in flames, KBr needs to crystalize from its aqueous solution. When the KBr mist falls down from a flame top
Fig. 4. Fluctuating flame under water mist.
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Fig. 6. Flame separates from the gasoline surface frequently when suppressed by 10% KBr mist.
Fig. 7. 0.5% Tween-80 mist effects on gasoline fire extinguishing.
to bottom, it will crystallize and decompose. Thus, more KBr crystals and its decomposition products will appear near the bottom of the flame. A 5% KBr mist cannot produce enough decomposition products to extinguish the flame near the gasoline surface. The other unusual phenomena were that the mean temperature of #1 to #3 suppressed by 10% KBr mist was higher than that suppressed by 5% KBr mist. This result may occur because increasing the KBr concentration decreases the saturated vapor pressure of the mist and then decreases the gasification speed of water in the KBr aqueous mist, ultimately decreasing the cooling effect of the water mist. The 10% KBr mist almost extinguished the flame near the oil surface intermittently. However, the flame over the gasoline surface still existed. This phenomenon resulted from the mean maximum temperature near the gasoline surface (at thermocouple #1) being 129 ◦ C. The higher temperature led to a higher gasification of the gasoline; then, the vapor continually fed the flame over the gasoline surface. The flame height suppressed by 0.5% Tween-80 mist was lower than that suppressed by the KBr mist, which may be a result of this emulsifying effect. Emulsification will decrease the gasification rate, as analyzed above. The temperature suppressed by the CO2 aqueous solution mist resulted in a lower flame height and temperature. Tween-80 mist and the CO2 aqueous solution mist mainly resulted in the flame having a lower height. These results were different from those obtained with the 10% KBr mist, which worked on the bottom of the flame and formed a gap between the flame base and gasoline surface, as shown in Fig. 7 and Fig. 8. A synergistic extinguishing effect should appear if the multicompo-
Fig. 8. 0.7 MPa CO2 aqueous solution mist effects on gasoline fire extinguishing.
nent additives of KBr + Tween-80 + dissolved CO2 mist were applied together. 4.2. Synergistic extinguishing effect of multicomponent mists According to the above analysis, KBr mist can inhibit the flame by breaking the burning reaction chain and mainly works near the flame base. Tween-80 mist can decrease the gasification rate of the gasoline by emulsification effect, and a CO2 aqueous solution mist decreases the flame height by inerting and absorpting heat, which lowers the flame height. In addition, when a mixture of dissolved CO2 + Tween-80 aqueous solution is sprayed out from a nozzle, CO2 will expand in the mist and form fine foam because of the loss of pressure and reduction in the surface tension induced by Tween80. Thus, in addition to the emulsifying effect of Tween-80 and the inerting and cooling effects of CO2 , the formed foams can cover the gasoline surface and separate gasoline from the flame to decrease the gasification rate. At the same time, the formed foam from the mist will enlarge the contact area with the flame when it drops down, which is helpful for decreasing the flame temperature. The fire extinguishing effect of KBr + Tween-80 + dissolved CO2 multicomponent solutions are shown in Table 3. As shown in Table 1, at a 2 m spray height, 5% KBr mists could not extinguish the fire, and 0.5% Tween-80 and 10% KBr mists needed 32 s and 81 s to extinguish the fire, respectively. However, neither could extinguish the fire at a 3 m spray height. Compared with the components alone, the combination of KBr + Tween-80 as additives in water showed a good efficiency for extinguishing the flame. As shown in Table 3, 5% KBr + 0.5% Tween-80 mist could extinguish
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Table 3 Extinguishing times of gasoline fire suppressed by multicomponent mists. Scenario NO.
13 14 15 16 17 18 19 20
Component KBr
Tween-80
CO2
5% 5% 10% 10% 5% 5% 10% 10%
0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5%
0 0 0 0 0.7 MPa2 0.7 MPa 0.7 MPa 0.7 MPa
Height (m)
Driver gas (0.7 MPa)
Mean extinguishing time (s)
2 3 2 3 2 3 2 3
air air air air CO2 CO2 CO2 CO2
21 s -1 16 s – 4s – 3s 35 s
Note. 1 “-” means the flame cannot be extinguished in 90 s under the experimental conditions. 2 CO2 saturated solution under 0.7 MPa.
Fig. 10. temperatures at a 20 cm height under different conditions. Fig. 9. Flame temperatures suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist.
the fire in 21 s at a 2 m height, and 10% KBr + 0.5% Tween-80 mist could extinguish the same type of fire in 16 s. As shown in Table 3, two kinds of multicomponent mists, 5% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist, and 10% KBr+ 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist, extinguished the fire in 4 s and 3 s, respectively. These two multicomponent mists showed excellent extinguishing efficiency compared with pure water mist. However, when the nozzle was at a 3 m height over the gasoline surface, the former mist did not extinguish the fire, and the latter mist extinguished it in 35 s. This may be a result of the higher concentration of KBr providing more effective free radicals inhibitors to contribute to flame extinguishing. An extinghuish time of 35 s (at 3 m spray height) is still too long compared with the 4 s and 3 s extinguishing time at a 2 m spray height. That is, probably because a higher spray height causes less mist to fall into the flame. A faster flow rate may reduce the extinguishing time because more mist can reach the flame. The temperature of one test suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist (Scenario NO. 19) is shown in Fig. 9. The mean temperature of this kind of scenario at the three thermocouples, respectively, decreased at approximately 255 ◦ C, 57 ◦ C and 16 ◦ C compared with temperatures obtained from a fire suppressed by pure water mist. The temperatures at the #2 thermocouple (middle of the three thermocouples) were selected to compare the cooling effect of different compound water mists as shown in Fig. 10. According to Fig. 10, water mist with and without additives can significantly cool the flame temperature by approximately 500 ◦ C. Furthermore, the additives such as KBr and Tween-80 mainly interrupted the chain reaction and the reactant feeding process. Thus,
the mists with those additives further decreased the flame temperatures compared with pure water mist. As mentioned above, increasing the KBr aqueous solution concentration decreased the saturated vapor pressure and then decreased the gasification speed of water in the KBr aqueous solution. As a result, the cooling effect of the water with a multicomponent composition consisting of 10% KBr+ 0.5% Tween-80 was slightly less than the same multicomponent composition consisting of 5% KBr+ 0.5% Tween-80. KBr + Tween-80 + dissolved CO2 solution showed a prominent extinguishing effect. At 2 m of spray height under the experimental conditions, 5% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 solutions mist and 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 solutions mist extinguished the gasoline fire in 4 s and 3 s, respectively. A series of pictures obtained during the extinguishing process for 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 at a 2 m spray height is shown in Fig. 11. According to Fig. 11 (c) to Fig. 11 (f), the fire was extinguished efficiently under the experimental conditions. In Fig. 11 (c) and (d), the flame near the gasoline surface was almost distinguished, but the flame over the surface still existed, which was similar to the scenario suppressed by 10% KBr mist. The difference between them was that the 10% KBr mist could not extinguish the fire, while the 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 multicomponent mist extinguished it in 3 s. The extinguishing effect of the multicomponent additives can be explained by the follow reasons. First, crystallization and decomposition of KBr makes the flame base extinguish intermittently. Second, the multicomponent mist decreased the gasoline’s volatilization rate, which decreased the fuel feeding the flame over the gasoline surface. The emulsifying effect of the Tween-80 and fine foam formed by CO2 escaping from the Tween-80 mist covered the gasoline surface and both contributed to decreasing the volatilization rate. Third, there was a cooling effect on the flame. In addition to the water mist’s cool-
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Fig. 11. The extinguishing process of a gasoline fire suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 solution mist (the interval time between (c) to (f) was 1/24 s, and the total extinguishing time (from starting the spray) was approximately 3 s.).
ing effect, an endothermic process occurs when CO2 escapes from the water mist. At the same time, the fine foam resulted from the CO2 escaping from the multicomponent mist, thus enlarging the contact area between the mist and flame. The multicomponent KBr + Tween-80 + dissolved CO2 mist integrates the advantages of each component and produced an evidently superior extinguishing effect compared with pure water mist or water mist combined with each component alone. 4.3. Balance analysis of fire extinguishing According to Liu et al (Liang et al., 2015; Liu et al., 2007), the energy balance of a flame under water mist can be indicated as Eq. (13) •
mf · Hc = me · A · Lvf + Qair + Qmist
(13)
Where mf - Burning gasoline mass rate in flame, kg/s; •
me - Evaporation rate of the gasoline, kg/m2 s; A - Evaporation area of the gasoline, m2 ; Hc - Heat of combustion of the gasoline, kJ/kg; Lvf - latent heat of evaporation of the gasoline, kJ/kg; Qair - Heat dissipated to the surrounding air, kJ/s; Qmist - Heat dissipated to the water mist, kJ/s. While, mf , evaporation rate of the gasoline, is affected significantly application of water mist. The cooling effect will decrease
mf , and the flame will be lower at the moment when water mist come into the flame. However, the gasoline was not drop drasti•
cally at the moment. me mainly affect by the gasoline temperature and, for the low suppressing effect of pure water mist, the gasoline still keeps a high temperature close to before. Thus, there will be unburned gasoline vapor exist instantaneously, and was ignited soon by the survived fire in water mist. A big flame appeared suddenly, and the big flame heat the oil to keep a high temperature and was suppressed to a small survived flame. Thus, Eq. (13) is hold in a long process, but it is not hold instantaneously. This may be the reason of the fluctuating flame under water mist. But to some water mist with more efficiency on fire suppression, such as water mist containing Tween-80 or dissolved CO2 , the gasoline flame under these kinds of water mist is much lower (about 200 ◦ C lower than suppressed by pure water mist) than that under •
pure water mist. The lower the lower mf decreased me and may keep a new balance. The low mist cannot extinguish by the water mist. Back (Iii et al., 2000) and Liang (Liang et al., 2015) pointed out the similar phenomenon that water mist have difficulty to extinguish a small fire. When a large fire was suppressed to a very small fire, the oxygen consumption decreased dramatically and kept a new balance of Eq. (13). At this moment, if the water mist stop, the surviving fire will change to a whole surface fire rapidly. TWEEN-80 and dissolved CO2 could form little bubbles when the water mist sprays out from the nozzle. The little bubbles can float on the surface of the gasoline and decrease the parameter of A
D. Lv et al. / Process Safety and Environmental Protection 129 (2019) 168–175 •
in Eq. (13). Thus, the decreased me · A · Lvf will reduce the mf · Hc , and finally make the flame smaller. KBr decreased mf to burn by breaking the flame reaction chains, •
which decreased the me · A · Lvf and finally feedback to mf . The unstable gap between flame bottom and gasoline surface caused by KBr, the lower flame caused by TWEEN, CO2 and water mist, and the decreased A caused by the fine foam covering the gasoline surface, constitute a mechanism of mutual feedback and influence to extinguish the flame. 5. Conclusion Pure water mist has a good cooling effect but has limited extinguishing effects on oil fires. To enhance its gasoline fire extinguishing efficiency, an experimental apparatus was adopted consisting of a gasoline fire source and a solid-cone spray system producing a water mist diameter of 211 m (DV 0.9 ) and a flow rate of 1.31 L/min under 0.7 Mpa; then, KBr, Tween80 and dissolved CO2 were researched as additives in this work. Under the test conditions, KBr was found to mainly work near the flame base and to form an unstable gap between the flame base and the gasoline surface; the Tween-80 mist and dissolved CO2 mist make the flame height lower. By analyzing the different extinguishing phenomena, temperature and efficiency of each additive, multicomponent additives of KBr + Tween-80 + dissolved CO2 are proposed from the findings of this research. The multicomponent mist shows a better cooling effect compared with the pure water mist. The mean maximum temperatures of the flames further decreased 255 ◦ C, 57 ◦ C, and 16 ◦ C at thermocouple #1-#3, respectively when suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 mist instead of pure water mist. As to the extinguishing effect, KBr + Tween-80 + dissolved CO2 mist works more efficiently than pure water mist or water mist with any single component of the additives. For example, at 2 m of spray height under the experimental conditions, 5% KBr + 0.5% Tween80 + 0.7 MPa CO2 mist and 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist could extinguish the fire in 4 s and 3 s, respectively. These two multicomponent mists outperformed the 10% KBr mist (81 s), 0.5% Tween-80 mist (32 s), and 0.7 MPa CO2 solution mist and pure water mist (neither of which could extinguish the fire in 90 s). However, at a 3 m spray height, water mist used in combination with any of the single component additives could not extinguish the gasoline fire, but 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 could extinguish the fire in 35 s. To reduce the extinguishing time at a higher spray position, a faster flow rate will be researched in future work. Acknowledgements This work is supported by the Key Project of Chinese Ministry of Public Security (2017JSYJA13) and the Natural Science Foundation of China (21606164).
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Process Safety and Environmental Protection journal homepage: www.elsevier.com/locate/psep
Gasoline fire extinguishing by 0.7 MPa water mist with multicomponent additives driven by CO2 Dong Lv a,b , Wei Tan a , Guorui Zhu a , Liyan Liu a,∗ a b
School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300354, China Tianjin Fire Research Institute of MEM, Tianjin, 300381, China
a r t i c l e
i n f o
Article history: Received 17 March 2019 Received in revised form 4 July 2019 Accepted 4 July 2019 Available online 11 July 2019 Keywords: Gasoline fire Water mist Fire suppression Multicomponent additives
a b s t r a c t Water mist has a good three-dimensional cooling effect; however, its extinguishing effect on oil fires is less than that of foam extinguishing agents. This study aimed to enhance the gasoline fire extinguishing efficiency of water mist by adding KBr, Tween-80, and a dissolved CO2 solution as multicomponent additives with different suppression mechanisms. The fire extinguishing time was measured to evaluate the extinguishing efficiency of different additives. The test showed that KBr mainly works near the flame base and makes an unstable gap between the flame base and the gasoline surface; the Tween80 mist and dissolved CO2 mist make the flame height lower under the test conditions. By analyzing the different extinguishing phenomena, temperature and efficiency of each additive, the multicomponent additives were proposed to enhance the extinguishing efficiency based on different mechanisms. The results showed the water mist with KBr, Tween-80 and dissolved CO2 resulted in a remarkable improvement in the ability to extinguish a gasoline fire compared with pure water mist. © 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction A foam extinguishing agent is an appropriate choice to control a liquid plane fire, such as a pool fire. Nevertheless, for a threedimensional fire source, such as a fire including a pool fire and a solid fire at different heights, it is difficult for a foam extinguishing agent to cover the fire source completely, which will affect its extinguishing effect. Water mist can fill the fire space from top to bottom, which is a very important advantage for extinguishing three-dimensional fires. However, compared with foam extinguishing agents, pure water mist has a lower efficiency for extinguishing a pool fire (Iii et al., 2000; Ni and Chow, 2011). To improve the extinguishing efficiency, much work has been conducted in different methods. Some researchers devoted to improve the efficiency by optimizing the parameters of extinguishing agents (Harding et al., 2016; Zhen et al., 2016), some tried different additives with different mechanism in extinguishing agents. For example, it was reported some additives added in water mist can apparently improve the extinguishing efficiency by breaking the flame reaction chains. Iron pentacarbonyl and ferrocene have been indicated to be effective inhibitors on flame reaction
∗ Corresponding author. E-mail address: [email protected] (L. Liu).
chains (Koshiba et al., 2015, 2016; Rumminger and Linteris, 2000; Yu et al., 2008). Yu (Yu et al., 2008) has suggested saturated vapor of ferrocene is more effective than nitrogen for alcohol fire suppression under some conditions. However, the toxicity of these additives should not be ignored. According to the MSDS, the LD50 parameter (mouse oral) of iron pentacarbonyl and ferrocene are approximately 40 mg/kg and 600 mg/kg, respectively. Potassium free radicals show a capacity to capture free radicals and inhibit flames (Zhang et al., 2016a, b; Zhang et al., 2017). In addition, some research has shown halides, such as Cl, can capture the free radicals generated in burning reactions (Cao et al., 2016, 2015; Zheng et al., 1997). However, Cl− is more corrosive compared with its congeners. In addition, iodides are more expensive and have less chemical stability than bromides. Combined with the above analysis, KBr was evaluated as a component of the fire extinguishing agent in this work. Surface active agents have been proven to be effective in fire suppression. Some works have focused on surfactants additive agents for suppressing pool fires (Koshiba et al., 2016; Zhou et al., 2006). Generally, foams suppress oil fires effectively. Foams can cover the oil surface and surfactants can emulsify the oil surface, which slows the burning rate. In this study, Tween-80 was used as a component of the fire extinguishing agent. CO2 is often applied as a diluent to extinguish flames (Baev and Bazhaikin, 2014; Jung et al., 2014; Min and Baillot, 2012). CO2 has
https://doi.org/10.1016/j.psep.2019.07.002 0957-5820/© 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
D. Lv et al. / Process Safety and Environmental Protection 129 (2019) 168–175
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a low cost and a high efficiency of creating an inert combustion environment. Specifically, CO2 will dissolve in water and form one phase mixture under pressure. Dissolved CO2 can then be simultaneously used as an extinguishing agent and a power resource to drive the mixture spray out. In this study, the different extinguishing mechanisms and the synergistic effect of KBr, Tween-80 and dissolved CO2 mist were studied to determine suitable water mist additives for extinguishing gasoline fires.
Tween-80 was applied as an emulsifier to decrease the gasifying rate. When a water mist with Tween-80 is dropped on a gasoline surface, it emulsifies the gasoline to form a water in oil (W/O) mixture. The gasifying rate depends on the liquid’s saturated vapor pressure. Consider a water mist with Tween-80 as the solvent and gasoline as the dissolvent; refer to Raoult’s law (Bancroft and Davis, 2002):
2. Extinguishing mechanism
P=
The fire extinguishing mechanisms of KBr, Tween-80 and highpressure dissolved CO2 solutions are different. The follow sections will discuss this topic. 2.1. KBr’s extinguishing mechanism The combustion chain reactions, such as hydrocarbons burning, gasoline burning, etc. produce OH and H radicals (Kong et al., 1998). These radicals are intermediate products that help to maintain the combustion chain reactions. KBr can decrease the radicals’ concentration due to interrupting the combustion chain reactions. The reaction mechanism of KBr suppressing the fire is explained though the following process (Birchall, 1970; Zhang et al., 2016a): KBr + H → HBr + K
(1)
KBr + OH → KOH + Br
(2)
The molar reaction enthalpies of reaction (1) and (2) are approximately 3.9 kcal and 9.5 kcal, so, reaction (1) may be easier to achieve than reaction (2). The heat of flame combustion affords enough energy for these reactions. Once K and KOH are generated, the following reactions continue (Hynes et al., 1984; Jensen and Jones, 1982; Slack et al., 1989): K + OH + M → KOH + M
(3)
Where M is an inert third body. KOH + H → H2 O + K
(4)
K + O2 +M → KO2 +M
(5)
KO2 +H → KO + OH
(6)
Meanwhile, studies (Cao et al., 2015; Wilson et al., 1969; Zheng et al., 1997) have proposed that the Cl radicals can inhibit the chain reaction. Similar reactions may occur as follows once Br and HBr are generated. Br + Br + M → Br2 +M
(7)
Br2 +H → HBr + Br
(8)
HBr + H → H2 +Br
(9)
Another reaction may occur as follows: HBr + OH → H2 O + Br
(10)
Comparing reaction (9) with (10), the molar reaction enthalpies of reactions (9) and (10) are about -16.65 kcal and -31.4 kcal, respectively. So, reaction (10) may work better than reaction (9) for flame extinguishing. 2.2. Tween-80 s extinguishing mechanism If a water mist with KBr cannot extinguish the gasoline fire thoroughly, the gasoline rapidly becomes gasified in high temperature conditions and the gasoline vapors will be ignited continuously by the unextinguished part of the flame. The fresh ignited vapors make more gasoline vapor to burn, which constitutes a combustion cycle.
P ∗ · nA nA + nB
(11)
where P - the vapor pressure of the emulsified gasoline, P ∗ - the vapor pressure of gasoline, nA - the mole fraction of gasoline, nB - the mole fraction of water mist with Tween-80. Water mist with Tween-80 (nB ) in gasoline decreases the vapor pressure (P) of the fuel, which increases the boiling point and therefore decreases the gasifying rate. Meanwhile, the generated water vapor with gasoline decreases the concentration of gasoline vapor, which decreases the combustible source transported to the flame. If gasoline is sprayed with Tween-80, foams may form. Foam will separate the flame from the fuel and decrease the gasifying rate, which will be very helpful in fire extinguishing.
2.3. Dissolved CO2 solution extinguishing mechanism Pressure will increase the solubility of CO2 in water. One volume of water can dissolve 6.4 volumes of CO2 at 0.7 MPa, 20 ◦ C. When a water mist with dissolved CO2 is sprayed out from a nozzle, the dissolved CO2 will escape because of the lost pressure, which is an endothermal process. The decomposition heat of H2 CO3 is about 19.4 kJ/mol, 25 ◦ C. Thus, inerting and cooling effects help the water mist extinguish the fire.
3. Experimental apparatus and methods The experiment was conducted in a large indoor combustion test field. The water mist extinguishing system is shown in Fig. 1. It consists of a CO2 bottle and an air compressor, a 5 L water storage tank, a spray device held by an adjustable holder with a range of 1.5 m–3 m, and a gasoline pan 20 cm in diameter and 12 cm in height. Three thermocouples, #1, #2 and #3, were fixed over the center of the gasoline pan with a height of 10 cm, 20 cm and 30 cm above the gasoline surface, respectively. The water with or without additives was added into a water storage tank through the funnel. The water tank was pressurized with air or CO2 to 0.7 MPa. If using CO2 as the driver gas, we maintained the pressure for more than 40 min to allow it to dissolve fully. Different spray heights and additives components were evaluated in this work. The fire extinguishing time and temperature data were recorded to evaluate the extinguishing efficiency. The water mist with a diameter of 211 m (DV0.9 ) and a flow rate of 1.31 L/min was applied, tested by CNCF (China National Center for Quality Supervision and Test of Fixed Fire-fighting System and Fire-resisting Building Components) under 0.7 MPa. Each trial used 200 ml of new gasoline. The mist was sprayed after 15 s of burning. KBr, Tween-80, and CO2 were tested as additives with different constituents. Pure water mist was tested as a contrast which to compare the additives’ extinguishing effects. Every test in the manuscript was repeated for at least 3 times.
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Fig. 1. Schematic of the water mist extinguishing system.
Table 1 Extinguishing time of water mist with and without additives. Scenario No.
Materials
Driver gas (0.7 MPa)
Spray height (m)
Mean extinguishing time (s)
1 2 3 4 5 6 7 8 9 10 11 12
Pure water Pure water 10% KBr 10% KBr 5% KBr 5% KBr 0.5% Tween-80 0.5% Tween-80 2% Tween-80 2% Tween-80 CO2 aqueous solution CO2 aqueous solution
air air air air air air air air air air CO2 CO2
2 3 2 3 2 3 2 3 2 3 2 3
– – 81 s – – – 32 s – – – – –
Note: “-” means the fire cannot be extinguished in 90 s under the test conditions.
4. Results and discussion 4.1. Extinguishing effect of each component 4.1.1. Extinguishing time The extinguishing times of the water mist with and without additives are listed in Table 1. The water mist with 10% KBr and 0.5% Tween-80 extinguished the fire in 81 s and 32 s at a 2 m spray height, respectively. None of these mists could extinguish the fire at a 3 m spray height under the test conditions. Pure water mist and the CO2 aqueous solution mist could not extinguish the fire at either spray height. This result shows the extinguishing effects of those components are limited by themselves. A higher concentration of a KBr aqueous solution provided more KBr crystals and decomposition products at high temperatures and they were useful to extinguish the fire. That’s why scenario No. 3 is better than scenario No. 5 in Table 1. A notable phenomenon was that 2% Tween-80 mist could not extinguish the fire at a 2 m height, while 0.5% Tween-80 mist could extinguish it under the same test conditions. A similar phenomenon was recorded by Bagdassarov et al. (Bagdassarov et al., 2000). This phenomenon may result from more gasoline being dissolved in the emulsified mixture when the concentration of the nonionic surfactants increased.
4.1.2. Temperature and flame appearance Compared with pure water mist, the water mist with additives had a higher efficiency in suppressing temperatures. The cooling effects of water mist with each component are listed in Table 2. The temperature increased to more than 600 ◦ C in approximately 30 s without any suppression at a 10 cm height above the gasoline surface (thermocouple #1). When the flame was suppressed by pure water mist, its temperature obviously decreased. The water mist produced a dramatic cooling effect on the gasoline fire, and the cooling extent was approximately 500 ◦ C. The temperatures without any suppression and that suppressed by pure water mist are shown in Fig. 2 and Fig. 3. When the gasoline fire was suppressed by pure water mist, the peak temperature at thermocouple #1 was more than 450 ◦ C, and at thermocouple #2 it was still more than 100 ◦ C. That means if pure water mist cannot extinguish the fire, the gasoline will gasify rapidly due to the high environmental temperature. Under the same test conditions, a pure water mist could not extinguish the fire; water mist only made the flame fluctuate, as shown in Fig. 4. The flame can cover the thermocouple #1 continually and cover the thermocouple #2 intermittently, which resulted in them maintaining relatively high temperatures. KBr mist improved the extinguishing ability compared with pure water mist. The mean temperate tested at the three thermocouples were lower than those suppressed by pure water mist. In
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Table 2 Cooling effects of water mist with and without additives at a 2 m spray height. Component
Driver gas (0.7 MPa)
None Pure water 5% KBr aqueous solution 10% KBr aqueous solution 0.5% Tween-80 aqueous solution 0.7 MPa CO2 aqueous solution
– air air air air CO2
Mean maximum temperature #1
#2
#3
685 416 151 72 256 172
669 122 45 284 62 34
445 56 29 46 46 24
Fig. 5. Temperature of burning suppressed by 10% KBr mist. Fig. 2. Temperature of burning without any suppression.
Fig. 3. Temperature of burning suppressed by pure water mist.
addition, instead of appearing at thermocouple #1 (10 cm above the oil surface), the maximum temperature suppressed by 10% KBr mist appeared at thermocouple #2 (20 cm above the oil surface), as shown in Fig. 5. The temperatures at thermocouple #1 were lower than that at thermocouple #2, which was different from that the temperature suppression observed with pure water mist or other mists, such as Tween-80 and CO2 aqueous solutions. The reason is that the 10% KBr mist made the flame frequently separate from the oil surface and form an unstable gap, as shown in Fig. 6. Thus, the position of thermocouple #1 will be lower than the flame base intermittently, and thermocouple #2 was covered in flame continually, causing the temperature at thermocouple #1 to be lower than that at thermocouple #2. The reason why an unstable gap formed when it was suppressed by 10% KBr mist may be a result of the following: before reacting with OH and H free radicals in flames, KBr needs to crystalize from its aqueous solution. When the KBr mist falls down from a flame top
Fig. 4. Fluctuating flame under water mist.
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Fig. 6. Flame separates from the gasoline surface frequently when suppressed by 10% KBr mist.
Fig. 7. 0.5% Tween-80 mist effects on gasoline fire extinguishing.
to bottom, it will crystallize and decompose. Thus, more KBr crystals and its decomposition products will appear near the bottom of the flame. A 5% KBr mist cannot produce enough decomposition products to extinguish the flame near the gasoline surface. The other unusual phenomena were that the mean temperature of #1 to #3 suppressed by 10% KBr mist was higher than that suppressed by 5% KBr mist. This result may occur because increasing the KBr concentration decreases the saturated vapor pressure of the mist and then decreases the gasification speed of water in the KBr aqueous mist, ultimately decreasing the cooling effect of the water mist. The 10% KBr mist almost extinguished the flame near the oil surface intermittently. However, the flame over the gasoline surface still existed. This phenomenon resulted from the mean maximum temperature near the gasoline surface (at thermocouple #1) being 129 ◦ C. The higher temperature led to a higher gasification of the gasoline; then, the vapor continually fed the flame over the gasoline surface. The flame height suppressed by 0.5% Tween-80 mist was lower than that suppressed by the KBr mist, which may be a result of this emulsifying effect. Emulsification will decrease the gasification rate, as analyzed above. The temperature suppressed by the CO2 aqueous solution mist resulted in a lower flame height and temperature. Tween-80 mist and the CO2 aqueous solution mist mainly resulted in the flame having a lower height. These results were different from those obtained with the 10% KBr mist, which worked on the bottom of the flame and formed a gap between the flame base and gasoline surface, as shown in Fig. 7 and Fig. 8. A synergistic extinguishing effect should appear if the multicompo-
Fig. 8. 0.7 MPa CO2 aqueous solution mist effects on gasoline fire extinguishing.
nent additives of KBr + Tween-80 + dissolved CO2 mist were applied together. 4.2. Synergistic extinguishing effect of multicomponent mists According to the above analysis, KBr mist can inhibit the flame by breaking the burning reaction chain and mainly works near the flame base. Tween-80 mist can decrease the gasification rate of the gasoline by emulsification effect, and a CO2 aqueous solution mist decreases the flame height by inerting and absorpting heat, which lowers the flame height. In addition, when a mixture of dissolved CO2 + Tween-80 aqueous solution is sprayed out from a nozzle, CO2 will expand in the mist and form fine foam because of the loss of pressure and reduction in the surface tension induced by Tween80. Thus, in addition to the emulsifying effect of Tween-80 and the inerting and cooling effects of CO2 , the formed foams can cover the gasoline surface and separate gasoline from the flame to decrease the gasification rate. At the same time, the formed foam from the mist will enlarge the contact area with the flame when it drops down, which is helpful for decreasing the flame temperature. The fire extinguishing effect of KBr + Tween-80 + dissolved CO2 multicomponent solutions are shown in Table 3. As shown in Table 1, at a 2 m spray height, 5% KBr mists could not extinguish the fire, and 0.5% Tween-80 and 10% KBr mists needed 32 s and 81 s to extinguish the fire, respectively. However, neither could extinguish the fire at a 3 m spray height. Compared with the components alone, the combination of KBr + Tween-80 as additives in water showed a good efficiency for extinguishing the flame. As shown in Table 3, 5% KBr + 0.5% Tween-80 mist could extinguish
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Table 3 Extinguishing times of gasoline fire suppressed by multicomponent mists. Scenario NO.
13 14 15 16 17 18 19 20
Component KBr
Tween-80
CO2
5% 5% 10% 10% 5% 5% 10% 10%
0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5%
0 0 0 0 0.7 MPa2 0.7 MPa 0.7 MPa 0.7 MPa
Height (m)
Driver gas (0.7 MPa)
Mean extinguishing time (s)
2 3 2 3 2 3 2 3
air air air air CO2 CO2 CO2 CO2
21 s -1 16 s – 4s – 3s 35 s
Note. 1 “-” means the flame cannot be extinguished in 90 s under the experimental conditions. 2 CO2 saturated solution under 0.7 MPa.
Fig. 10. temperatures at a 20 cm height under different conditions. Fig. 9. Flame temperatures suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist.
the fire in 21 s at a 2 m height, and 10% KBr + 0.5% Tween-80 mist could extinguish the same type of fire in 16 s. As shown in Table 3, two kinds of multicomponent mists, 5% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist, and 10% KBr+ 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist, extinguished the fire in 4 s and 3 s, respectively. These two multicomponent mists showed excellent extinguishing efficiency compared with pure water mist. However, when the nozzle was at a 3 m height over the gasoline surface, the former mist did not extinguish the fire, and the latter mist extinguished it in 35 s. This may be a result of the higher concentration of KBr providing more effective free radicals inhibitors to contribute to flame extinguishing. An extinghuish time of 35 s (at 3 m spray height) is still too long compared with the 4 s and 3 s extinguishing time at a 2 m spray height. That is, probably because a higher spray height causes less mist to fall into the flame. A faster flow rate may reduce the extinguishing time because more mist can reach the flame. The temperature of one test suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist (Scenario NO. 19) is shown in Fig. 9. The mean temperature of this kind of scenario at the three thermocouples, respectively, decreased at approximately 255 ◦ C, 57 ◦ C and 16 ◦ C compared with temperatures obtained from a fire suppressed by pure water mist. The temperatures at the #2 thermocouple (middle of the three thermocouples) were selected to compare the cooling effect of different compound water mists as shown in Fig. 10. According to Fig. 10, water mist with and without additives can significantly cool the flame temperature by approximately 500 ◦ C. Furthermore, the additives such as KBr and Tween-80 mainly interrupted the chain reaction and the reactant feeding process. Thus,
the mists with those additives further decreased the flame temperatures compared with pure water mist. As mentioned above, increasing the KBr aqueous solution concentration decreased the saturated vapor pressure and then decreased the gasification speed of water in the KBr aqueous solution. As a result, the cooling effect of the water with a multicomponent composition consisting of 10% KBr+ 0.5% Tween-80 was slightly less than the same multicomponent composition consisting of 5% KBr+ 0.5% Tween-80. KBr + Tween-80 + dissolved CO2 solution showed a prominent extinguishing effect. At 2 m of spray height under the experimental conditions, 5% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 solutions mist and 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 solutions mist extinguished the gasoline fire in 4 s and 3 s, respectively. A series of pictures obtained during the extinguishing process for 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 at a 2 m spray height is shown in Fig. 11. According to Fig. 11 (c) to Fig. 11 (f), the fire was extinguished efficiently under the experimental conditions. In Fig. 11 (c) and (d), the flame near the gasoline surface was almost distinguished, but the flame over the surface still existed, which was similar to the scenario suppressed by 10% KBr mist. The difference between them was that the 10% KBr mist could not extinguish the fire, while the 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 multicomponent mist extinguished it in 3 s. The extinguishing effect of the multicomponent additives can be explained by the follow reasons. First, crystallization and decomposition of KBr makes the flame base extinguish intermittently. Second, the multicomponent mist decreased the gasoline’s volatilization rate, which decreased the fuel feeding the flame over the gasoline surface. The emulsifying effect of the Tween-80 and fine foam formed by CO2 escaping from the Tween-80 mist covered the gasoline surface and both contributed to decreasing the volatilization rate. Third, there was a cooling effect on the flame. In addition to the water mist’s cool-
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Fig. 11. The extinguishing process of a gasoline fire suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 solution mist (the interval time between (c) to (f) was 1/24 s, and the total extinguishing time (from starting the spray) was approximately 3 s.).
ing effect, an endothermic process occurs when CO2 escapes from the water mist. At the same time, the fine foam resulted from the CO2 escaping from the multicomponent mist, thus enlarging the contact area between the mist and flame. The multicomponent KBr + Tween-80 + dissolved CO2 mist integrates the advantages of each component and produced an evidently superior extinguishing effect compared with pure water mist or water mist combined with each component alone. 4.3. Balance analysis of fire extinguishing According to Liu et al (Liang et al., 2015; Liu et al., 2007), the energy balance of a flame under water mist can be indicated as Eq. (13) •
mf · Hc = me · A · Lvf + Qair + Qmist
(13)
Where mf - Burning gasoline mass rate in flame, kg/s; •
me - Evaporation rate of the gasoline, kg/m2 s; A - Evaporation area of the gasoline, m2 ; Hc - Heat of combustion of the gasoline, kJ/kg; Lvf - latent heat of evaporation of the gasoline, kJ/kg; Qair - Heat dissipated to the surrounding air, kJ/s; Qmist - Heat dissipated to the water mist, kJ/s. While, mf , evaporation rate of the gasoline, is affected significantly application of water mist. The cooling effect will decrease
mf , and the flame will be lower at the moment when water mist come into the flame. However, the gasoline was not drop drasti•
cally at the moment. me mainly affect by the gasoline temperature and, for the low suppressing effect of pure water mist, the gasoline still keeps a high temperature close to before. Thus, there will be unburned gasoline vapor exist instantaneously, and was ignited soon by the survived fire in water mist. A big flame appeared suddenly, and the big flame heat the oil to keep a high temperature and was suppressed to a small survived flame. Thus, Eq. (13) is hold in a long process, but it is not hold instantaneously. This may be the reason of the fluctuating flame under water mist. But to some water mist with more efficiency on fire suppression, such as water mist containing Tween-80 or dissolved CO2 , the gasoline flame under these kinds of water mist is much lower (about 200 ◦ C lower than suppressed by pure water mist) than that under •
pure water mist. The lower the lower mf decreased me and may keep a new balance. The low mist cannot extinguish by the water mist. Back (Iii et al., 2000) and Liang (Liang et al., 2015) pointed out the similar phenomenon that water mist have difficulty to extinguish a small fire. When a large fire was suppressed to a very small fire, the oxygen consumption decreased dramatically and kept a new balance of Eq. (13). At this moment, if the water mist stop, the surviving fire will change to a whole surface fire rapidly. TWEEN-80 and dissolved CO2 could form little bubbles when the water mist sprays out from the nozzle. The little bubbles can float on the surface of the gasoline and decrease the parameter of A
D. Lv et al. / Process Safety and Environmental Protection 129 (2019) 168–175 •
in Eq. (13). Thus, the decreased me · A · Lvf will reduce the mf · Hc , and finally make the flame smaller. KBr decreased mf to burn by breaking the flame reaction chains, •
which decreased the me · A · Lvf and finally feedback to mf . The unstable gap between flame bottom and gasoline surface caused by KBr, the lower flame caused by TWEEN, CO2 and water mist, and the decreased A caused by the fine foam covering the gasoline surface, constitute a mechanism of mutual feedback and influence to extinguish the flame. 5. Conclusion Pure water mist has a good cooling effect but has limited extinguishing effects on oil fires. To enhance its gasoline fire extinguishing efficiency, an experimental apparatus was adopted consisting of a gasoline fire source and a solid-cone spray system producing a water mist diameter of 211 m (DV 0.9 ) and a flow rate of 1.31 L/min under 0.7 Mpa; then, KBr, Tween80 and dissolved CO2 were researched as additives in this work. Under the test conditions, KBr was found to mainly work near the flame base and to form an unstable gap between the flame base and the gasoline surface; the Tween-80 mist and dissolved CO2 mist make the flame height lower. By analyzing the different extinguishing phenomena, temperature and efficiency of each additive, multicomponent additives of KBr + Tween-80 + dissolved CO2 are proposed from the findings of this research. The multicomponent mist shows a better cooling effect compared with the pure water mist. The mean maximum temperatures of the flames further decreased 255 ◦ C, 57 ◦ C, and 16 ◦ C at thermocouple #1-#3, respectively when suppressed by 10% KBr + 0.5% Tween-80 + 0.7 MPa CO2 mist instead of pure water mist. As to the extinguishing effect, KBr + Tween-80 + dissolved CO2 mist works more efficiently than pure water mist or water mist with any single component of the additives. For example, at 2 m of spray height under the experimental conditions, 5% KBr + 0.5% Tween80 + 0.7 MPa CO2 mist and 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 mist could extinguish the fire in 4 s and 3 s, respectively. These two multicomponent mists outperformed the 10% KBr mist (81 s), 0.5% Tween-80 mist (32 s), and 0.7 MPa CO2 solution mist and pure water mist (neither of which could extinguish the fire in 90 s). However, at a 3 m spray height, water mist used in combination with any of the single component additives could not extinguish the gasoline fire, but 10% KBr + 0.5% Tween-80 + 0.7 MPa dissolved CO2 could extinguish the fire in 35 s. To reduce the extinguishing time at a higher spray position, a faster flow rate will be researched in future work. Acknowledgements This work is supported by the Key Project of Chinese Ministry of Public Security (2017JSYJA13) and the Natural Science Foundation of China (21606164).
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