Journal Of Hazardous Materials Volume 341 Issue 2018 [doi 10.1016_j.jhazmat.2017.07.040] Ni, Xiaomin; Zhang, Shaogang; Zheng, Zhong; Wang, Xishi -- Application Of Water@silica Core-shell Particles F.pdf

Accepted Manuscript Title: Application of Water@Silica Core-Shell Particles for Suppressing Gasoline Pool Fires Authors: Xiaomin Ni, Shaogang Zhang, Zhong Zheng, Xishi Wang PII: DOI: Reference:

S0304-3894(17)30548-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2017.07.040 HAZMAT 18732

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

11-4-2017 18-7-2017 19-7-2017

Please cite this article as: Xiaomin Ni, Shaogang Zhang, Zhong Zheng, Xishi Wang, Application of Water@Silica Core-Shell Particles for Suppressing Gasoline Pool Fires, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.07.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Application of Water@Silica Core-Shell Particles for Suppressing Gasoline Pool Fires

Xiaomin Nia,*, Shaogang Zhanga, Zhong Zhengb, Xishi Wanga

aThe

State Key Laboratory of Fire Science,

University of Science and Technology of China, Hefei 230027, P.R. China

bDepartment

of Electronic Science and Technology,

University of Science and Technology of China, Hefei 230027, P.R. China

*

Corresponding author, Email: [email protected], Tel: 86-551-63606430-802, Fax: 86-551-63601669 1

Highlights

•

A new type of fire suppressants with water@silica core-shell structures was fabricated as through a simple stirring method.

• The capsular particles showed excellent performance in extinguishing gasoline pool fires in terms of time and agent mass. • The work presented a novel route to produce small sized water droplets and store them without coalescence for long time.

Abstract A new type of dry powders with capsular structure was fabricated for fire suppression, in which the content of water approached 60%. The capsules with the size of 3~5 μm consisted of liquid core and solid shell, where the core was water droplet and the shell was assembled silicon dioxide particles with surface hydrophobic modification. The shell of close-packed silica particles surrounding each water droplet provided the structural rigidity of the capsules and enabled their application as powder fire suppressants. Two different scaled real fire tests showed that thus-prepared solid powders could extinguish 0.21 MW gasoline pool fire in 2.0 s with agent mass of 0.055 kg, and 1.0 MW gasoline pool fire in 5.0 s with agent mass of 0.49 kg. Such fire extinguishing performance greatly outperformed the conventional monoammonium phosphate (ABC) powders, neat silica powders and water mist, with significantly reduced fire extinguishing time and mass of agent consumed. Mechanism of the core-shell particles in fire suppression was discussed based on established theories and experimental results.

2

Keywords: fire suppressant; capsular particles; water; silicon dioxide; gasoline pool fire

1. Introduction Gasoline is one of the most commonly known liquid fuels, which is highly ignitable and volatile. Gasoline has a comparatively low flash point of about -65°C and the ignition temperature of about 232°C. While burning gasoline has a temperature above 945°C, which can heat objects in the fire area above its ignition temperature [1]. Meanwhile, gasoline is so volatile that a large amount of vapor is quickly generated from the liquid surface. The flammable range of gasoline is only 1.4% to 7.1% [2]. When the gasoline vapor in an enclosure is ignited, it will burn explosively and causes extensive damage. Gasoline vapor is heavier than air. It tends to flow downhill and downwind from liquid gasoline, making it possible for explosive mixtures to collect in low points such as pipe trenches or terrain depressions. Usually, a crash is often followed by a gasoline fire. At present, gasoline is widely used in cars, aeroplanes and some machines. The distressing and increasingly frequent incidence of fatal fires in employing gasoline [3], has repeatedly directed attention to the need for an effective fire suppressant for gasoline fires.

Highly efficient fire suppressants could fast control the fire and greatly reduce the loss. Since the phase out of halon, water mist as one of the halon replacer has been paid great attention for its high efficiency and environmental friendliness [4]. As naturally clean agent, water mist would not decompose or produce any toxic products when exposed to flame. In comparison to conventional water spray, water mist with much smaller droplet size (with 99% of the volume of droplets with diameters less than 1000 microns) showed much higher efficiency in fire suppression. But water mist is not so efficient in extinguishing small liquid fuel fires in the open space because a small fire may not be able to generate enough heat for the transformation of water droplets into vapor to displace sufficient oxygen [5,6]. It is noted that even if a liquid fire is extinguished by water mist, re-ignition may occur at any time for the poor covering effect of water mist from contacting oxygen [7]. To prevent re-ignition after 3

extinguishment, water mist must be applied for sufficient time to allow hot objects in the fire area to cool below the ignition temperature of gasoline. This is time consuming and may cause water damage. Droplet size greatly affects the fire suppression capability of water mist [8]. It is difficult for water mist with small droplet size and low momentum to penetrate the flame fume to extinguish the fire. While water mist with big droplet size and high momentum would usually spoil out the liquid fuel or raise its level in a container and so result in larger combustible area. Production of water mist with proper droplet size was crucial for successful fire extinguishment. Conventionally, production of water mist depends on specific technique, such as specially designed nozzles and pressure. This makes the practical application of water mist become conditional. In comparison to water mist, dry powders as fire suppressants even showed superior fire extinguishing capabilities on a mass basis while consuming minimal space [9,10]. Furthermore, dry powders consisting of small particles could be easily discharged to the flame zone without any special nozzle.

Considering the shortcomings of conventional methods in producing water mist and the advantages of dry powders, here we reported a simple stirring route to produce a new type of water@silica capsular particles as fire suppressant. The water droplets with size of several microns were capsulated by silicon dioxide nanoparticles to form core-shell structures. Under the protection of outside hydrophobic solid shell, the particles with a high content of water approaching 60% behaved like dry powder fire suppressants with good flowability, which was named as “dry water” (denoted as DW). Details of the preparation, physicochemical properties and fire suppression performance of the DW suppressants were studied. Mechanism of the DW particles in fire suppression was discussed based on established fire suppression theories and experimental results.

2. Experimental 2.1 Sample Preparation All chemicals and reagents were used as received from commercial sources without 4

further purification. Hydrophobic SiO2 particles were purchased from Degussa. In a typical experiment, 100.0 g distilled water was mixed with 25.0 g SiO2 and stirred at the rate of 13000 r/min for 30 s. The resulted samples were free-flowing white particles.

2.2 Characterization Structure and morphology of the samples were characterized by X-ray diffraction (XRD, Philips X’Pert), ZEISS Axioskop2 plus optical microscopy, and scanning electron microscopy (SEM, QUANTA 200FEG). The Fourier transform infrared (FTIR) spectrum of KBr wafer was recorded using a Nicolet 6700 Fourier Transform Spectrometer. Thermogravimetry-differential scanning calorimeter (TG-DSC) curve was acquired through the SHIMADZUDTG-60H instrument.

2.3 Fire tests Generally, the standard testing procedures for evaluating the fire extinguishing performance of dry powders includes ISO 7202-2012, NFPA 17, ANSI/UL 299, and etc. However, in these standards, the experimental setup is quite large, which is expensive and time consuming to build. For the limit of our experimental conditions, large fire tests following these standards were difficult to undertake. Two laboratory scaled tests were designed with relatively small fire sources and less agents to reduce the cost. But the test methods were basically in conformance with the regulations in international standards.

The small scale fire tests, were conducted in a 27 m3 (3 m × 3 m × 3 m) confined space with natural ventilation. Details of the experimental apparatus were schematically shown in Fig. 1. Gasoline was contained in the oil pan with the diameter of 0.50 m. A thermocouple tree, containing six thermocouples with interval of 0.20 m and the lowest thermocouple 0.25 m above the fuel pan, was set up to measure the flame temperatures. In each test, 200.0 g powders were added into a tank with the volume of 1000 ml and pressurized by nitrogen to a pre-assigned value of 5

0.50 MPa. Before powder discharging, 400 ml gasoline was added into the pan, ignited and pre-burned for 30 s. The flame power was estimated as 0.21 MW. The distance from the extinguisher nozzle to the pan center was set as 1.0 m. The valve of the powder tank was turned off as soon as the fire was extinguished. The weight of the tank was measured before and after each test to determine the total mass of suppressants consumed for fire extinguishment. Each test was repeated at least three times to get a converged result. The fire suppression process was recorded by a video camera.

The bigger scale fire tests were conducted in a 1000 m3 (10 m × 10 m × 10 m) large space hall with natural ventilation to further evaluate fire extinguishing performance of as-prepared powders. 500 g powders were contained in a hanged powder extinguishing equipment and pressurized by nitrogen to 1.0 MPa. The circular oil pan with the diameter of 0.95 m contained 21.0 L gasoline. The flame power was estimated as 1.0 MW. The perpendicular distance from the hanged extinguisher nozzle to the fuel surface was 1.5 m. The gasoline was ignited and pre-burned for 30 s before powder discharging. After the fire was extinguished, the agent mass consumed was measured and recorded. Considering the agent flow calibration uncertainty and measurement variance, the relative expanded uncertainty of the fire extinguishing time and the agent mass consumed in firefighting was estimated as ±15% and ±10%, respectively.

3. Results and Discussion 3.1 Structure and Morphology Fig.2 showed a typical XRD pattern of as-prepared “dry water” particles, in which only a broad peak with 2θ centered at about 22°was seen. It indicated that the material was amorphous SiO2 [12]. Fig.3 presented FTIR spectrum of the sample, where the peaks could be reasonably ascribed to SiO2 and H2O [13]. The broad band around 3450 cm-1 and the peak at 1640 cm-1 was due to stretching vibration and the 6

bending vibration of water molecules adsorbed on the silica surface. The strong band at 1112 cm-1 corresponded to the asymmetric vibration of Si-O-Si. The absorption bands at 800 cm-1 and 485 cm-1 were assigned to the stretching vibration and bending vibration of Si-O-Si bond, respectively.

Morphological images of the sample were shown in Fig. 4. As revealed by Fig. 4a, the sample consisted of free-flowing powders. Optical microscopy image of Fig. 4b showed that the particles were spherical and ellipsoidal with the size in the range of 10~30 μm. SEM images of Fig.4c and d gave more details of the particles, revealing that the particle shell was made up of loosely arranged small particles with the diameter of 30~50 nm.

3.2 Fire extinguishing performance

A series of laboratory scale fire extinguishing tests were conducted to assess the performance of as-prepared DW suppressants (denoted as sample A). For comparison, hydrophobic silica particles (denoted as sample B), neat water mist with the mean size of 30~40 μm (denoted as sample C), and commercially available ABC dry powders (NH4H2PO4 50 w.t.%, (NH4)2SO4 25 w.t.%, and inert additives 25 w.t.%, denoted as sample D), were also tested under the similar conditions. Table 1 listed the agent composition, particle size distribution, flame power, fire extinction time and agent mass consumed in the two kinds of fire tests. It was shown that sample A exhibited much better performance than the three counterparts. The capsular particles could fast extinguish 0.21 MW gasoline pool fires in 2.0 s with 54.7 g agent consumed and no re-ignition occurred. While neat silica powders could not extinguish the fire even with all of the originally added agent was nearly consumed out (187.5 g). The fire was just suppressed but not extinguished. Water mist with droplet size of 30-40 μm could extinguish the fire, but a long time of 12 s and a large amount of water of 272.6 g was consumed. Commercial ABC dry powders could not extinguish the fire even with 191.3 g power consumed. Such results revealed that fire extinguishing performance of the capsular particles was much superior to that of the three counterparts in terms of 7

the fire extinguishing time and the agent mass required. Typical snapshots from the fire suppression process of the tests with different samples were shown in Fig. 5.

Although the above experiments clearly ranked the fire suppression effectiveness of four samples, fire tests in relatively large scale gave more direct check on the firefighting capacity of agents. Therefore, a bigger sized gasoline pool fire test was conducted with the flame power of about 1.0 MW. As-prepared “dry water” powders could extinguish the fire in 4.8 s with 487.4 g consumed. While the other three samples of silica, water mist and commercial ABC dry powders all failed to extinguish the big fire. Typical snapshot images of the big fire test using the DW powders were shown in Fig.6.

The above two different scaled fire experiments revealed that neither water mist nor silica particles of the same amount used alone could extinguish the fire. But when the two components were combined and transformed into capsular particles, the fire could be fast extinguished. The composite greatly outperformed each component.

3.3 Fire Suppression Mechanism

Successful fire suppression required that one or more of the four factors of fuel, oxygen, heat and chain reaction which tend to propagate a fire be suppressed. It was assumed that solid particulates functioned as fire suppressants through several mechanisms, including chemical inhibition of the chain reactions via the catalytic combination of active species, heat absorption and cooling by decomposition and vaporization of the solid particles, oxygen dilution in the flame regions by the inert gases produced and the chemical reaction of the particles and active species [14,15]. Here, for the capsular particles containing water and silicon oxides, they would not decompose in the flame and produce any species to stop the combustion reactions. So, it was considered that as-prepared capsular particles should have the following three mechanisms in extinguishing the gasoline pool fires: cooling effect, dilution of oxygen, and heterogeneous chemical effect. 8

When discharged into the flame, the capsular particles would be heated and the inner water cores would be released out. As revealed by the TG-DSC curve of sample A from room temperature to 800oC in Fig. 7, the particles were fast dehydrated and the mass loss stopped at about 140oC. During this dehydrating process, a big exothermal peak was exhibited, which could be ascribed to the evaporation of water. Here, it was assumed that the “water cores” played an important role in extinguishing the gasoline pool fires.

The suppression mechanism of water mist on fire was believed to be basically physical, through heat extraction on the flame by water evaporation, oxygen displacement and attenuation of thermal radiation by increased water vapor concentration [16]. As shown by Fig.7, the exothermal peak from water evaporation contributed to decrease the flame temperatures. Meanwhile, the resulted water vapor diluted the oxygen concentration around the flame. Oxygen dilution was the main mechanism of suppressing fires by water mist in an enclosed space. In the enclosure, the oxygen concentration around the flame may be greatly diluted by the resulted water vapor and then satisfied fire suppression performance would be exhibited. But under the conditions with adequate oxygen, the effect of oxygen dilution became weak. In the present tests with small fires relative to the compartment with adequate supply of oxygen, the oxygen dilution effect was minor [17]. So the cooling effect from water was considered the dominant mechanism.

But different to regular water mist, here, water droplets were encapsulated in the solid silica particles shells, which made them easier to reach the flame zone to exert its suppressing effect. An important practical issue in employing highly effective suppressants was getting sufficiently high concentrations into the flame zone. For regular neat water mist, most of the water droplets evaporated when contacting with the flame edge and only a small amount of water droplets could penetrate into the flame zone. But for the water droplets loaded in the silica particles, they could reach the flame zone more easily. Furthermore, the core-shell particles with a proper density 9

could fast distribute and suspend in the flame zone. Upon heating, the in-situ desorbed water molecules would pavilion the flame and form a water vapor atmosphere, which contributed to the flame suppression. Thus the cooling effect and oxygen dilution of the water droplets could be fully experienced by the flame. As a result, excellent fire extinguishing performance was exhibited by the capsular particles, but failed for the regular water mist. For the limitation of instrument, the oxygen concentrations in the flame could not be measured accurately. The discussion on the contribution of the water cores for fire extinguishment was still a hypothesis requiring justification at the moment.

Fig. 8 gave typical flame temperature variations of the four tests, providing more information on the different cooling effect of the samples. In comparison to that of Figs.8b and d, temperatures of the thermocouples in Figs.8a and c declined much more quickly. It meant that the cooling effect of capsular particles and water mist were much bigger than that of silica particles and ABC powders, reflecting the cooling effect of water contained in samples A and C.

In addition to the cores of water, the shells of silica also contributed to fire suppression. Neat silica particles could not extinguish the fires, but they could partly suppress the flame (as shown in Fig.6b). Silica particles were inert, which would not decompose in the flame. They would neither produce any species to scavenge combustion radicals nor absorb heat (as shown in Fig.8b, the cooling effect of silica was minor). But silica particles could act as a wall or window to inhibit the fire [18]. The solid particles as an inert wall could absorb part of FFRs’ energy to fast thermal destruction or recombination of free flame radicals (FFRs) for collision with the particle surface. As described by Krasnyansky, the surface defects of particles acted as the centers of adsorption during the adsorption process and chain interruption reactions [19]. Efficiency of heterogeneous inhibition of fire extinguishing powders depended on several factors, such as surface area, adsorption properties, degree of defects, etc. The silica particles with unique porous structure, nanosize, large surface area and a lot of defects could effectively capture FFRs [20,21]. But such an effect of 10

heterogeneous inhibition alone was not strong enough to extinguish the fire. As shown by the experimental results, the fire was just suppressed rather than extinguished by neat silica particles. When combined with water to form capsular particles, the fire extinguishing capacity of silica particles was greatly improved for the synergetic effect.

For as-prepared capsular particles, the small size of water domains greatly enhanced surface area to volume ratio in comparison with the equivalent reaction in bulk water, which led to a marked increase of the contact between flame and water. From an energy perspective, the DW method presented here a very simple route to produce small sized water droplets and stored them in dispersed state without coalescence. While in conventional method for producing small sized water droplets, specially designed nozzles must be produced, which was time-consuming and costly. Furthermore, the simple stirring technology could also be used to produce DW particles containing different chemicals, offering a safe way to transport and storage of various aqueous solutions in the form of solid powders.

3.4 Stability and toxicity

Dry water is a free-flowing powder obtained by mixing ordinary water and hydrophobic nanoparticles of fumed silica at high speed in an air atmosphere. Due to the presence of nanoparticles of fumed silica on the surface of the microdroplets, the powder could be long time stored without coalescence. For as-prepared DW fire suppressants, it was observed that could be stored in tanks for up to 12 months (denoted as sample E) without coalescence of the droplets and the appearance of a bulk water phase. As shown in Table 1, Fire tests revealed that extinguishing performance of sample E was little affected by the long time storage.

Both water and silica were safe to human beings. Silica was usually used as a food additive, which served as an anticaking agent to prevent various powdered ingredients from sticking together. On one hand, there were not any toxic elements in silica and 11

water. On the other hand, the interaction between water cores and silica shells was physical and no new substances were produced in the composite. Although toxic tests had not been performed yet, the DW fire suppressant was believed to be safe for human beings.

4. Conclusion Gasoline pool fires were highly dangerous and difficult to be extinguished for its volatility and flammability. Regular water mist and dry powders showed limited capacity in extinguishing gasoline pool fires. In this article, a novel type of water@silica core-shell particles was reported for extinguishing gasoline pool fires with excellent performance. · In the core-shell structured particles, the closely assembled hydrophobic silica nanoparticles acted as the shells surrounding each water microdroplet core, which provided the structural rigidity of the capsules and enabled their application as solid powder fire suppressants. · As-prepared capsular particles could extinguish 0.21 MW gasoline pool fire in 2 s with agent mass of 0.055 kg, and 1.0 MW gasoline pool fire in 5 s with agent mass of 0.49 kg, which greatly outperformed the conventional ABC powders, neat silica powders and water mist. · The clean, nontoxic and low cost powder suppressants were fabricated through a simple stirring method. The behaved like solid powders with good flowability, which could be long-time stored in dispersed state without coalescence. In comparison to the conventional method for producing water mist, the DW method presented here was more simple and convenient. In addition, fluidity and moisture resistance of the resulted DW particles were similar to that of the conventional dry powders, which ensured their compatibility with existing powder fire suppression systems without any changes. Such DW powders are expected to be used for scalability to actual fires for their advantages of easy production, high efficiency and low cost. The present work just gave a preliminary study on the preparation, firefighting capacity and mechanism of the water@silica particles, which provided 12

information for the numerical investigation on the suppression mechanism and practical firefighting applications. More tests on the suppression efficiency and mechanism of the capsular particles are in progress.

Acknowledgements The work was supported by Fundamental Research Funds for the Central Universities (No.WK23200000037) and National Natural Science Foundation of China (No. 51574120).

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[4]

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[7] J. R. Mawhinney, B. Z. Dlugogorski, A. K. Kim, A closer look at the fire extinguishing properties of water mist, in: Proceedings of the Fourth International Symposium on Fire Safety Science, 1997, p. 47-60 [8]

M. F. Abdrabbo, A. M. Ayoub, M. A. Ibrahim, A. M. S. Feldin, The effect of water mist droplet size and nozzle flow rate on fire extinction in hanger by using FDS, J. Civil Environ. Eng. 6 (2014) 1000216.

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V. Babushok, W. Tsang, Inhibitor Rankings for Alkane Combustion, Combust Flame; 123(2000) 488-506.

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foams to dry water, Nat. Mater. 5 (2006) 865-869. [12] A. R. Maurice, H. Faouzi, Synthesis and characterization of amorphous silica nanoparticles from aqueous silicates using cationic surfactants, J. Met. Mater. Miner. 24 (2014) 37-42. [13] T. H. Liou, C. C. Yang, Synthesis and surface characteristics of nanosilica produced from alkali-extracted rice husk ash. Mater. Sci. Eng. B. 176 (2011) 521-529. [14] C. T. Ewing, E. R. Faith, J. T. Hughes, H. W. Carhart, Evidence for flame extinguishment by thermal mechanisms, Fire Technol. 25 (1989) 195-212. [15] J. K.Charles, D. Douglas, Solid Particulate Aerosol Fire Suppressants, Fire Technol. 30 (1994) 387–399. [16] R. Ananth, R. C. Mowrey, Ultra-Fine Water Mist Extinction Dynamics of a Co-Flow Diffusion Flame, Combust. Sci. and Technol. 180 (2008) 1659-1692. [17] M. Edwards, S. Watkins, J. Glockling, Low-pressure water mist, fine water spray, water source, and additives: evaluation for the Royal Navy, in: 8th International Fire Science and Engineering Conference, Edinburgh, 445 Scotland, 1999, p. 381-394. [18] M. Krasnyansky, Studies of fundamental physical-chemical mechanisms and processes of flame extinguishing by powder aerosols, Fire Mater. 32 (2008) 27-37. [19] R. Friedman, Principles of Fire Protection Chemistry National Fire Protection Association 1989, p.190. [20] C. Adam, R. G. Dunster, G. Ralf. The evaluation of non-pyrotechnically generated aerosols as fire suppressants, in: Proceedings of Halon Alternatives Technical Working Conference, Albuquerque, NM, 1995, p.473-483. [21] H. K. Chelliah, P. C. Wanigarathne, A. M. Lentati, R. H. Krauss, G. S. Fallon, Effect of sodium bicarbonate particle size on the extinction condition of non-premixed counter flow flames, Combust. Flame, 134 (2003) 261-272.

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Table Legend and Figure Captions: Figure 1

Schematic illustration of the experimental set up in a 27 m3 compartment

Figure 2

X-ray diffraction pattern of as-prepared capsular particles.

Figure 3

FTIR spectrum of as-prepared capsular particles.

Figure 4

Morphologic images of as-prepared capsular particles: (a) free-flowing particles, (b) optical micrograph of the particles, (c) and (d) SEM images of the particles.

Figure 5

Snapshots of the fire suppression processes of the tests (flame power = of 0.21 MW) with different samples: (a) sample A (capsular particles), (b) sample B (silicon dioxides), (c) sample C (water mist), (d) sample D (ABC powders).

Figure 6

Snapshots of the fire suppression process of the test (flame power = 1.0 MW) with capsular particles

Figure 7

TG-DSC of the capsular particles

Figure 8

Variation of the flame temperature in tests with four samples as suppressants: (a) sample A (capsular particles), (b) sample B (silicon dioxides), (c) sample C (water mist), (d) sample D (ABC powders).

16

Figure 1

Fig. 1

Relative Intensity (a.u.)

Figure 2

10

20

30 40 2(degree)

Fig. 2

50

60

 H2O

 H2O

Si-O Si-OH Si-O-Si

Si-O-Si

Transmittance (a.u.)

Figure 3

500

1000

1500

2000

3000 -1 Wavenumber (cm )

Fig. 3

2500

3500

4000

Figure 4

Fig. 4

Figure 5

0s

0.5 s

1.0 s 1.5 s 1.8 s (a) Sample A (capsule particles)

2.0 s

0s

0.5 s

1.0 s 2.0 s 4.0 s (b) Sample B (Silicon dioxides)

7.0 s

0s

1.0 s

2.0 s 3.0 s 8.0 s (c) Sample C (neat water mist)

12.0 s

0s

0.5 s

1.0 s 2.0 s 4.0 s (d) Sample D (ABC powders)

7.0 s

Fig. 5

Figure 6

0s

0.5 s

3.0 s

1.0 s

4.0 s Fig. 6

2.0 s

4.8 s

Figure 7

110

10

100

Mass/% DSC/(mW/mg)

90

Mass/%

5

70 60 0

50 40

o

59.26%, 136 C

30 200

400 Temperature (oC)

Fig. 7

600

800

DSC/(mW/mg)

80

Figure 8

800 800

capsule particles

agent discharge

600

o

Temperature ( C)

o

Temperature ( C)

600

silica oxides

agent discharge

400

200

400

200

0

0 0

50

100

150

200

250

300

350

0

50

100

Time (s)

200

250

300

350

Time (s)

(a) sample A

(b) sample B 800

800

water mist

700

ABC powders

agent discharge

600

Temperature( C)

600

agent discharge

o

o

Temperature ( C)

150

500 400 300 200

400

200

100 0

0

0

50

100

150

200

250

300

350

0

Time (s)

(c) Sample C

50

100

150

Time (s)

200

250

300

(d) sample D Fig. 8

350

Table 1

Performance of the four samples in extinguishing gasoline pool fires

Table 1 Performance of the four samples in extinguishing gasoline pool fires Samples

A

Main composition

Capsular particles

Particle/droplet

Flame power

Extinguishing

Mass of agent

size (μm)

(MW)

time (s)

consumed (g)

10~30

0.21

2.0

54.7

0.03~0.05

0.21

---

187.5

30~40

0.21

12.0

272.6

70~100

0.21

---

191.3

10~30

1.0

4.8

487.4

10~30

0.21

2.2

52.1

(40wt% SiO2 + 60wt% H2O)

B

Hydrophobic SiO2 (95%SiO2 +5% surfactant)

C

Neat water mist (100wt% water without additives)

D

Commercial ABC powder (50%NH4H2PO4 + 25%(NH4)2SO4)

A

Capsular particles (40wt% SiO2 + 60wt% H2O)

E

Capsular particles stored for 1 year (40wt% SiO2 + 60wt% H2O)

17