Applied Thermal Engineering 122 (2017) 429–438

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate /apthermeng

Research Paper

Active substances study in fire extinguishing by water mist withpotassium salt additives based on thermoanalysis and thermodynamics

http://dx.doi.org/10.1016/j.applthermaleng.2017.05.0531359-4311/� 2017 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: hanzhiyue@bit.edu.cn (H. Zhiyue).

Zhang Tianwei a,b, Liu Hao b, Han Zhiyue a,⇑, Du Zhiming a, Wang Yong b

a State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 10081, Chinab The Chinese People’s Armed Police Forces Academy, Hebei, Langfang 065000, China

h i g h l i g h t s

� The MECs of six potassium salts were tested to enriched the database of water mist.� The active substances are creatively obtained by thermoanalysis experimental method.� The quantitative analysis is obtained through thermodynamics method.� Extinguishing occurred by water, K cations, flame radical, and other anions.

a r t i c l e i n f o

Article history:Received 5 December 2016Revised 5 May 2017Accepted 11 May 2017Available online 13 May 2017

Keywords:Cup-burnerPotassium salt additivesFire extinguishingWater mistActive substanceThermoanalysisThermodynamics

a b s t r a c t

The active substances during the process of fire extinguishing by water mist with potassium salt addi-tives was studied. The minimum extinguishing concentration (MEC) experiment results showed thatK2CO3 have the most benefit in improving the fire extinguishing efficiency of pure water with the improv-ing rate of 37.6%, 47.2% and 64.8% which the mass percent was 1%, 2% and 5%, respectively. Other potas-sium salt additives followed the order by: K2C2O4 > CH3COOK > KNO3 > KCl > KH2PO4, and the reasonattributed to the different active substances decomposed from flame temperature by different kinds ofpotassium salt additives. Thermoanalysis and characterization of the pyrolysis products of potassium saltsolutions at flame temperature were analyzed through TG-DSC, XRD, and SEM, the results showed theKOH was the main product in the high extinguishing performance of potassium salts in flame chemicalreactions. Thermodynamics analysis by HSC CHEMISTRY showed the K2CO3 could provide 4.85% KOH inequilibrium substances, which well above other potassium salts, and other active substances good for fireextinguishing benefited from the intermediate product during the process of the KOH reacted with theflame free radicals.

� 2017 Elsevier Ltd. All rights reserved.

1. Introduction become the research focus of fire science. Water is a main method

In recent years, as the rapid development of the economic devel-opment, energy has become an important guarantee for the devel-opment of energy issues and even directly restricts the economicdevelopment. With the increase of population density and building,as well as the increased fuel consumption, fuel structure has beenchanged, gaseous fuel such as coal gas, liquefied petroleum gasand natural gas gradually becomes the main fuel for city life [1].

However, the widespread use of natural gas increasing the riskof natural gas leakage and fire and explosion accidents. CH4 firethreatening the life security of the public and causing great dam-age to the material property. How to solve the CH4 fire better

for the treatment of fire since ancient times. With the continuousdevelopment of society, the fire form is constantly updated, andwith the progress of science and technology, the form of water isalso constantly innovating. The method of using water to treat firehas been greatly improved, and the water mist is the most repre-sentative of fire treatment means [2].

The addition of additives in fine water mist can enhance the fireextinguishing efficiency of pure water according to various studieson the fire extinguishing efficiency of additives [3–9]. In theseresearches, the water mist additives include KCl, NaCl, CH3COOK,LiI, NiCl2, CoCl2, NaHCO3, NaOH, CaCl2, MgCl2, FeCl2, and MnCl2.Experiments showed that the additives with the highest efficiencyare the compounds of Fe and K [10]. Toxicity and cost problemsresulted in the limited application of the Fe compound as anadditive [11], which made potassium salts the focus of the presentresearch.

430 Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438

The potassium salts decomposed at the flame temperature, andthe fire extinguishing occurred with the reaction between the gen-erated substances and flame radical by the chain termination reac-tion. The detailed methane pyrolysis process was GRI3.0, whichcontained 53 species and 325 reactions [12]. Warnatz [13] pro-posed that due to the slow chemical reaction rate of N in the CH4

oxidation process, some C species has been rapid oxidation ofCOx before reactive with some N species. Therefore, the productionof some N containing species is much smaller in the whole reactionprocess, which most detailed chemical reaction mechanism con-taining N species could negligible, while OH, H, O, CH, CH2 weremainly components during methane pyrolysis. Venkata [14] alsoconfirmed COx and H were the main species in methane decompo-sition. Based on a premixed flame burner, Mitani [15] proved thatalkali metal salt additives can induce chemical extinguishment byeliminating free radical came from CH4 pyrolysis through gas-phase homogeneous reaction. Zhang [16] reported that gaseousKOH could inhibit OH and H free radical in methane pyrolysis, gen-erated intermediate transition KO and K, which combined with OHor H to generate gaseous KOH again. The content of the free radi-cals in the flame propagation process was decreased by catalyticreaction and indeed inhibited the pyrolysis reaction.

Previous studies on active substances in fire extinguishing bypotassium salts mainly through the change of OH radical concen-tration in flame doped with K element, fit the chemical reactionrate function and inferred possible mechanisms by designingdifferent burner. Firedman [17] studied the extinguishing effectof K stream through a clashing flame experiment and demon-strated that KOH is the extinguishing active substance in potas-sium salts by combining computation of equilibrium productsand possible reaction pathways. Based on the chemical equilibriumcomputation with the updated database, Machale [18] furtherconfirmed that the active product in the extinguishing reactionof potassium salts is gaseous KOH. However, due to the difficultyin measuring the concentration of OH radical in the experiment,the accuracy and repeatability was uncertain. Furthermore, for lackof uniform experimental design, the results obtained from differentresearchers are quite different. For example, the flame reactiondoped with potassium salts, Slack [19] reported the fitting resultsof the rate constants for the critical reactions Kþ OHþM ¼KOHþM was 5 � 10�32 cm6/molec2 s�1 (T = 2000 K), while Patrickand Golden [20] calculated the same rate constants was1.9 � 10�25 cm6/molec2 s�1 (T = 2000 K); Husain [21] got the fit-ting results of the rate constants expression for the ractionKþ O2 þM ¼ KO2 þM was 1.89 � 10�22 T�2.68, while Silver [22]gave the fitting result was 1.19 � 10�10 T�0.56. These studies

chimney Laser particle

stainless steel cup

circ

water mist generator

axial flow fan

Fig. 1. Cup-burne

showed that the extinguishing active substance of potassium saltsmay theoretically be KOH, which has not been verified by acompelling experiment yet.

In the present study, six common potassium salts K2C2O4,CH3COOK, K2CO3, KNO3, KCl, and KH2PO4 were chosen, a reliableand repeatable experimental method based on classic thermoanal-ysis in proving active substances during extinguishing process bywater mist with potassium salt additives was reported. In addition,the minimum extinguishing concentration (MEC) of thesepotassium salts were tested by an improved cup burner was alsoinvestigated. Furthermore, the relationship between the extin-guishing efficiency and the amount of active substances wasproposed through thermodynamics by HSC CHEMISTRY6.

2. Experimental equipment and method

2.1. Cup-burner experiment

Fig. 1 shows the cup burner device. The experiment method isthe same as Ref. [7].

2.2. TG-DSC experiment

Thermoanalysis of chemical extinguishing substances was usedto measure the dependence of the characteristic parameters of thesubstances on temperature. Thermoanalysis can provide thermo-dynamic parameters and chemical reaction kinetic parameters ofmaterials. At present, the most common method of studying ther-mostability of substances include differential scanning calorimetry(DSC), thermogravimetry (TGA), and differential thermal analysis(DTA). A chemical extinguishing agent only decomposes into extin-guishing active substances under burnt temperature and therebydevelops its chemical extinguishment capacity after the active sub-stances have already reacted with flame radicals. Thermoanalysishas been highly appreciated by Chinese and foreign scholars instudying the decomposition mechanism of chemical extinguishingsubstances and possible extinguishing active substances.

TGA and DTA of six potassium salts in atmosphere were imple-mented by TGA-DSC synchronous tester of Switzerland METTELERCompany. The reference compound was a-Al2O3, and the air inflowrate was 50.0 mL min�1.

2.3. XRD and SEM experiment

The cup burner test system was improved to collect the pyrol-ysis products of water mist after ultrasonic atomization under

size analyzer

ulation pump

rotameter

cylinders

connect line

r apparatus.

Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438 431

different heating temperatures. The chimney unit was replaced bya tube type resistance furnace for electric heating. The highestheating temperature, total heating power, and temperature controlaccuracy were 1100 �C, 20 kW, and ±1 �C, respectively. TEX-C100temperature controller was used. The tube type resistance furnacehas one reaction tube, in which fogdrop gained from ultrasonicatomization can directly induce conversion reaction. This reactiontube was made of materials with high temperature resistance, cor-rosion resistance, good heat transmission, and temperature changetolerance. An axial flow fan and a product collection bottle were setat the top of the tube type resistance furnace. Collected target pow-der was analyzed by D8-Advance X-ray diffractometer, and its sur-face microstructure was observed by the S4700 cold field emissionSEM.

3. Results and discussion

3.1. MEC of potassium salt solutions

Theoretically, the MEC of water mist containing potassium saltsis a fixed value. However, in this experiment, the concentration ofwater mist containing additives was adjusted by changing theworking voltage of the atomization modules and their number.Therefore, concentration changed discontinuously, and the upperand lower limits of the MEC were observed. Extinguishing concen-tration using the upper and lower limits may extinguish fires suc-cessfully or unsuccessfully. Test results of MEC of six potassiumsalt solutions are listed in Table 1.

Table 1MEC of water mist with potassium salt additives.

Agent Upper limit of MEC(mass percent, %)

Lower limit of MEC(mass percent, %)

Pure water 12.11 10.071% K2CO3 7.56 6.982% K2CO3 6.39 5.085% K2CO3 4.26 3.951% K2C2O4 8.38 7.782% K2C2O4 7.96 7.255% K2C2O4 4.51 3.751% CH3COOK 8.85 6.762% CH3COOK 6.05 5.755% CH3COOK 5.04 4.631% KNO3 10.8 8.62% KNO3 8.34 6.715% KNO3 5.05 3.431% KCl 9.34 7.132% KCl 7.95 6.845% KCl 6.97 4.621% KH2PO4 9.16 7.682% KH2PO4 10.12 8.485% KH2PO4 10.63 8.91

Table 2Thermal decomposition data of potassium salts in air.

Potassium salts Temperature range (�C) Thermal decom

K2CO3 62.54–125.27 K2CO3 � 2H2O !K2C2O4 84.13–110.32 K2C2O4 �H2O !

510.92–613.34 K2C2O4 ! K2COCH3COOK 313.07–606.85 2CH3COOK ! K

K2C2O4 ! K2COKNO3 602.44–740.82 2KNO3 ! 2KNO

4KNO2 ! 2K2OKH2PO4 207.98–271.01 2KH2PO4 ! K2

291.76–358.78 K2H2P2O7 ! 2K

In conclusion, potassium salt additives can improve the fireextinguishing efficiency of water mist. The degree of improvementin the fire extinguishing efficiency of the said additives is ranked asfollows: K2CO3 > K2C2O4 > CH3COOK > KNO3 > KCl > KH2PO4. Addi-tives influence fire extinguishing efficiency mainly by increasingchemical actions and reducing the competition between the evap-orative capacities of pure water. The fire extinguishing efficiency ofK2CO3, K2C2O4, CH3COOK, KNO3, and KCl solutions increased withincreasing concentration. This effect indicates that the degree ofincrease in chemical actions is greater than the weakening of theevaporative capacity of pure water. The fire extinguishing effi-ciency of KH2PO4 solution decreased with increasing concentra-tion. Hence, the chemical fire extinguishing capacity of the twosalts is weak. Increasing concentration dominates the competitiverelation for reducing the evaporative capacity of pure water.

3.2. Thermal analysis of potassium salts

TG-DSC results of the six potassium salts at a temperature rateof 10 �C�min�1 are shown in supplementary materials and theresults are shown in Table 2.

Table 2 shows that the final decomposition products of K2CO3,K2C2O4 and CH3COOK are all K2CO3, and anhydrous K2CO3 withinthe temperature range of 25–800 �C is very stable and showsalmost no loss of weight until the compound reached its 901 �Cmelting point, in which it decomposed until the K2O boiling pointtemperature [23]. Kuang [24] showed that K2CO3 had no suppres-sion efficient compare to KHCO3 mostly because of stability. Hut-tinger [25] concluded that K2CO3 could easily react and producegaseous KOH only combine with water vapor under high tempera-ture. Based on the above deduction, it is difficult for solid K2CO3 todecompose in the general fire temperature.

DSC curve for K2C2O4 shows the two endothermic peaks andone exothermic peak. The corresponding TG curve shows twoweight loss platforms. The first endothermic peak is within84.13–110.32 �C, and the peak temperature is 102.16 �C. Theendothermic peak area or decomposition enthalpy is 228.17 J g�1.According to the platforms on the TG curve and the weight losspercentage, the deduced weight loss rate is 10.68%. The secondendothermic peak is within 374.25–411.34 �C, and the peak tem-perature is 390.37 �C. The TG curve reveals that no weight lossoccurred during this process. According to the test results of theX-4 microscopic melting point apparatus, the peak temperatureis not the melting temperature and may be the endothermic peakcaused by crystal transfer of K2C2O4. The exothermic peak isbetween 510.92–613.34 �C, and the peak temperature is592.54 �C. The TG curve reflects K2C2O4 decomposed within thisrange, releasing abundant heat and the enthalpy is 446.90 J g�1.Ren [26] reported the decomposition of K2C2O4 also had two steps:the first step corresponding the temperature range of 67.19–123.44 �C, with weightlessness rate of 9.83%; the second step

position procedure Weight loss (%)

Theoretical value Actual value

K2CO3 þ 2H2O 20.69 21.15K2C2O4 þH2O 9.783 10.683 þ CO 16.87 15.702C2O4 þ C2H6 15.31 13.783 þ CO 16.87 13.012 þ O2 60.54 66.72þ 2N2 þ 3O2

H2P2O7 þ H2O 6.62 6.9PO3 þH2O 7.09 7.64

432 Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438

corresponding the temperature range of 565.04–689.84 �C, withweightlessness rate of 15.02%. The results quite agreement withthe present study testified the decomposition process of K2C2O4,and the experimental error is within the allowable range.

DSC curve for CH3COOK shows the endothermic peak and twoexothermic peaks. The corresponding TG curve has two weight lossplatforms. The endothermic peak is within 285.16–316.74 �C, andthe peak temperature is 303.56 �C. The TG curve reveals that noweight loss occurred during this process. According to the testresults of the X-4 microscopic melting point apparatus, the peaktemperature is the melting point of CH3COOK. The TG curvereflects CH3COOK decomposed within the range of 313.07–606.85 �C with two corresponding exothermic peaks on the DSCcurve, which shows that the thermal decomposition of CH3COOKis completed in two steps. The first exothermic peak temperatureis 416.77 �C, with the weight loss rate of 13.78%, and the exother-mic enthalpy is 1220.60 J g�1. The second exothermic peak temper-ature is 488.68 �C, with the weight loss rate of 13.01%, and theexothermic enthalpy is 926.52 J g�1. Fleming [27] showed thatthe aqueous of CH3COOH had better performance of extinguishingthan pure water illustrated the CH3COOH could decomposed inflame temperature and demonstrated the results of Huttinger.

DSC curve for KNO3 shows the three endothermic peaks and thecorresponding TG curve has one weight loss platform. The firstendothermic peak is within 128.47–150.16 �C, and the peak tem-perature is 130.92 �C. The TG curve reveals that no weight lossoccurred in this process. According to the test results of the X-4microscopic melting point apparatus, the peak temperature is notthe melting temperature but may be the endothermic peak causedby crystal transfer of KNO3. The second endothermic peak is within300.14–342.16 �C, and the peak temperature is 326.5 �C. Accordingto the test results of the X-4 microscopic melting point apparatus,the second peak temperature is the melting point of KNO3, and themelting enthalpy is 67.04 J g�1. The TG curve reflects the decompo-sition of KNO3 within the range of 602.44–740.82 �C with one cor-responding endothermic peak on the DSC curve, a peaktemperature of 681.46 �C, and weight loss rate of 66.72%. The reac-tion absorbs a lot of heat when the endothermic enthalpy is1867.55 J g�1. Zhang [28] described the decomposition process ofKNO3 in oil flame environment and pointed out that K2O, whichgenerated from the decomposition reaction was the main sub-stance in fire suppressing. While the experiment results of Wang[29] showed that the decomposition process of KNO3 would beactivate in the presence of C condition, which demonstrated thedecomposition process and products.

DSC curve for KCl shows that the endothermic peak is within758.61–779.18 �C, and the peak temperature is 769.62 �C, indicat-ing that the endothermic peak is the melting point based on themelting point of KCl (771 �C) [23]. The TG curve indicates thatKCl is very stable, and almost no weight loss occurred before themelting point. When the KCl reached the melting point, due tothe high saturated vapor pressure of KCl, a part of the KCl volatilegas was blown off from the crucible as driven by the purge gas,which led to the loss of weight. However, unlike solid K2CO3, KCl

Table 3Decomposition thermodynamic data of potassium salts.

Potassium salts Fitting curve

K2CO3 y = 8.91777–6.81801xK2C2O4 y = 18.91288–10.66307x

y = 51.27881–53.90384xCH3COOK y = 29.32699–27.88944x

y = 8.09588–14.52782xKNO3 y = �0.36433–10.83982xKH2PO4 y = 15.26101–12.89462x

y = 3.4327–8.37709x

showed a better extinguishing efficiency by Joseph [30]. Rosser[31] explained this chemical extinguishment effect of some saltsby a volatility experiment using alkali metal salts, perfectlyexplained why the material is stable but also has a certain fireextinguishing efficiency.

DSC curve for KH2PO4 shows the two endothermic peaks andthe corresponding TG curve has two weight loss platforms, whichindicate that the thermal decomposition of KH2PO4 is completed intwo steps. The first endothermic peak temperature is within207.98–271.01 �C, with the peak temperature is 235.85 �C, weightloss rate is 6.9%, and endothermic enthalpy is 274.09 J g�1. The sec-ond endothermic peak temperature is within 291.76–358.78 �C,whose peak temperature is 336.48 �C, weight loss rate is 7.64%,and endothermic enthalpy is 30.68 J g�1. The TG curve shows thatthe thermal decomposition of KH2PO4 was completed at 360 �C,and no mass loss was observed at the test temperature of 800 �C.Du [32] also reported the decomposition process of KH2PO4 withtwo steps, one with the temperature range of 190–270 �C, andthe other was 285–470 �C, which was not too different from thepresent study. Zhang [2] concluded the suppression efficiency ofKH2PO4 by the covering effect of the stable white glassy stateKPO3, which generated from the decomposition of KH2PO4.

Mojumdar [33] declared that with the increase in temperaturerate, the decomposition peak of potassium salts skews to high tem-perature and becomes much sharper gradually. The decompositionpeak temperature of potassium salts under different temperaturerates varies. Decomposition activation energy of different potas-sium salts can be acquired by the Kissinger method. The Kissingerequation is as follows:

lnb

T 02P

!¼ ln

AkREk

� �� Ek

R1T 0P

; i ¼ 1;2; . . . ;n

In the equation, b is the heating rate, �C�min�1; T’P is the peaktemperature, K; Ak is the pre-exponential factor, s�1; Ek is the acti-vation energy, kJ�mol�1; R is the gas constant, J�mol�1�K�1. The

expression ln bT 02P

� �to 1

T 02Pwas mapped as the proper mechanism

function with Ek as the slope and Ak as the intercept. The decompo-sition thermodynamic data of potassium salts are listed in Table 3.

3.3. Decomposition products analysis of potassium salts

The temperature of the heating tube was set according to theinitial decomposition temperature of different potassium saltsand was not higher than the limit temperature of the heating tube,more significantly, as 800 �C represented most of the actual flametemperature, this temperature existed in any set of experiments.Decomposition products were collected and analyzed, the experi-mental equipment was followed by Supplementary Fig. 7.

3.3.1. The characterization of decomposition products under differenttemperature

Temperature of the tube type resistance furnace wascontrolled at 800, 900, and 1000 �C, respectively. Driven by air,

r Ek/kJ mol�1 Ak/s�1

0.89412 56.67 0.51 � 105

0.98206 88.65 1.74 � 109

0.87462 448.16 4.05 � 1026

0.66278 231.82 1.52 � 1014

0.96207 120.76 0.48 � 105

0.70798 90.10 7.530.98187 107.18 5.47 � 107

0.86028 69.63 259.35

Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438 433

5% (mass concentration) ultrasonically atomized KCl water mistwas added into the heating tube, the products are collected andthe XRD analysis is shown in Fig. 2.

Through contrast with PDF standard card, particles gained fromultrasonic atomization of KCl under different temperatures provedto be the diffraction peaks of KCl. XRD spectra of the product sam-ples at different temperatures are similar with each other. Peaksfor other impurity phases were absent, and the main diffractionpeak is sharp, indicating the perfect crystal structure of the prod-ucts. According to XRD spectra in Fig. 2, KCl particles could begained from ultrasonic atomization of KCl at 1000 �C. SEM imagesof the product particles are shown in Fig. 3.

Product particles at 800 �C were typical crystalline polymerswith clear edges and corners. Although some particle clusters werepresent, the disparity remained good, and distinct cubic structurecould be detected under 2500 amplifications. Crystals appeared,and the particle size was about 5 mm. Particles at 900 �C alsoshowed good disparity, but particle size further increased, andthe cubic structure gradually became rounder, indicating thespheroidization of the particles. Particle size at 1000 �C increasedcontinuously, and particle clustering decreased. However, particlesize increased to different extents, resulting in the poor particleuniformity. Particle edges and corners were significantly round,and spherical particles could be clearly observed under 8000amplifications.

Driven by air, 5% (mass concentration) ultrasonically atomizedwater mist with other potassium salt additives was added into

(111)(200)

x103

x103

x103

40

[KCL-800-200-30.raw]

(111)(200)

40

[KCL-900-200-30.raw]

(111)(200)

10 20 30Two-T

40

[KCL-1000-200-30.raw]

Inte

nsity

(Cou

nts)

Fig. 2. XRD results of heating prod

800

Fig. 3. SEM results of heating prod

the heating tube, and the products were collected for XRD andSEM analysis. The characterization results of decomposition prod-ucts by XRD under different temperature are shown in Table 4,XRD and SEM pictures are shown in supplementary materials.

As shown in Table 1 and Table 4, fire suppression efficiency andthe behavior of potassium salts at high temperature depending onthe anion of potassium salts. The decomposition products con-tained KOH and potassium oxides(KxOy), e.g. KO2 and KO3 in watermist with potassium salt additives with higher chemical fire extin-guishing efficiency. Kong [34] and Liu [35] calculated the stabilityof alkali metal oxides through thermodynamics and showed thatthe final stable product in excess air could only be KO2. However,large number of KO3 existed in the decomposition products ofwater mist with KNO3 additive in current research could be onlyfrom further oxidation of KO2, which inferred the KO2 might alsobe the active substance besides KOH during fire suppression. Inaddition, KOH has a good stability, that means KOH can not onlyact as an intermediate product, but also stabilize in the productbecause of the poor stability of intermediate product during thethermal decomposition of potassium salt. Chen [36] reported thatwithin the range of inorganic compounds containing potassiumions, only KOH and K2CO3 had activity under low temperaturewhich could relatively easy to hydrolyze KxOy compounds in watervapor environment. Decomposition products of water mist withKH2PO4 contains a lot of KPO3, however, the reason for chemicalextinguishing efficiency is not high partly due to higher stabilityof KPO3, which causes it difficult to open up the chemical bond

(220)(311) (222) (400)

(220)(311) (222) (400)

(220)(311) (222) (400)

40 50 60heta (deg)

75-0296> Potassium Chloride - KCl

uct of water mist with 5% KCl.

900 1000

uct of water mist with 5% KCl.

Table 4Characterization results by XRD under different temperature.

Potassium salts Characterization results by XRD

300 �C 500 �C 600 �C 800 �C 900 �C 1000 �C

K2CO3 KOH, K2CO3 KOH, K2CO3 KOH, K2CO3

K2C2O4 K2C2O4 K2C2O4, KOH, K2CO3 KOH, K2CO3

CH3COOK CH3COOK KOH, K2CO3 KOH, K2CO3

KNO3 KNO3 KO3, KO2, KNO2, KOH KO3, KO2, KNO2, KOHKCl KCl KCl KClKH2PO4 KH2PO4, KPO3 KPO3 KPO3

434 Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438

and releases potassium ions in the flame temperature; on the otherhand, no KOH or KxOy found in the products, which indirectlyproved that KOH is a good active substances during fire extinguish-ing. There also has no KOH or KxOy found in the products of watermist with KCl, however, due to the low melting point of KCl, K-Clbond breaks in the form of melting, potassium ions are releasedand participate in fire extinguishing through chain terminationreaction.

Characterization results by SEM shows that the particle size ofthe products increased in different degrees with the increase ofthe temperature, which is due to the influence of temperature onthe formation and growth of crystal particles. The temperaturechanges the internal energy of the atom, that means particles withsmaller average size can be obtained under low temperature for itsbeneficial to the formation of crystal particles, which is not con-ducive to the growth of crystal particles. The increase of tempera-ture is equivalent to supply the kinetic energy of the crystalgrowth, which increases the mass transfer coefficient of the reac-tion and accelerates the growth of crystal particles, leading theparticle size grow up [37].

3.3.2. The characterization of decomposition products under differentconcentration

The temperature is the main effect factor on decomposition ofwater mist with potassium salts, while the concentration may beone of the factors on the size and uniformity of decomposition par-ticle, the phenomenon which was found in many nano materialspreparation process [38]. Guo [39] shows that the concentrationcould not change the kind of the products but the particle size.Theoretically, the solute content increases with increasing concen-tration under the same size of droplet. Once the droplet enters thereaction zone, the solvent evaporations and the solute separatedout under the flame temperature. When the solvent evaporatescompletely, the temperature of the precipitated crystal risesrapidly to the ambient temperature and has a chemical reactiontemperature condition. The higher concentration leads to the prob-ability of particle collision and combination greatly increased,which inevitably leads to large particles. Large particles indirectlyincrease the decomposition time of the chemical particles, whichis not conducive to the formation of chemical extinguishing activesubstances. In addition, the droplet diameter increases with theincrease of the viscosity and surface tension of the solution, then,smaller particle size is conducive to product when the solutionconcentration is low [40].

SEM of the decomposition products of the K2C2O4, CH3COOKand K2CO3 aqueous solution under 800 �C are shown in Fig. 4.

As shown in Fig. 4, the morphology and particle size of the pro-duct is affected by concentration, and the general trend is that theparticle size increases with the increase of solution concentrationwith more obvious agglomeration. When the mass fraction of thesolution is less than 5%, the interface between particles is clear.The phenomenon of inter particle connection becomes obviouswhen the concentration increases, and finally agglomerated

together to form a larger size particle. The experimental resultsshow that, when the concentration is low, the dispersion and uni-formity of the product particles is better, and the average particlesize is relatively small. Hamins [41] reported that the time for fullchemical reaction of alkali metal salt additives in flame reactionhad a competitive relation with the residence time of alkali metalsalt particles. If the residence time of alkali metal salt particles isshorter than the reaction time, then the fire-extinguishing agentcannot complete the chemical extinguishment. Small size is bene-ficial for chemical particle to use the least time to release the activesubstances. As shown in Table 1, K2CO3 has better suppression effi-ciency corresponding higher concentration than K2C2O4 and CH3-COOK partly due to the smaller particle size under the sameconcentration. Similarly, the pyrolysis products of KNO3, KCl andKH2PO4 solution at different concentrations have similar rulesand no more details here.

3.4. Fire extinguishing mechanism analysis of water mist withpotassium salts

Mitani [15] reported that when the droplet size is smaller than5 mm, liquid drops serve as the carrier of powder rather than theextinguishing agent. After quick evaporation of liquid drops atflame temperature, micro solid powders or decomposition prod-ucts of powders enter into the reaction zone of the diffusion flame.In this experiment, particle sizes of both ultrasonically atomizedpure water and water mist with additives were found to be smallerthan 5 mm by the laser particle analyzer. Therefore, water was thepowder carrier in this experiment.

Based on the calculated results of thermoanalysis activationenergy, the decomposition reactions of K2C2O4 and CH3COOK aresensitive to temperature, and the salts decomposed into K2CO3

immediately at flame temperature. The possible chemical reactionsand reaction pathways of K2CO3 in vapor are as follows:

K2CO3 þH2O ¼ 2KOHþ CO2 ð1Þ

K2CO3 ¼ K2Oþ CO2 ð2Þ

2KOH ¼ K2OþH2O ð3Þ

K2OþH2O ¼ 2KOH ð4ÞVariations in standard Gibbs free energy of the above four reac-

tions at 298–1400 K are shown in Fig. 5.Fig. 5 shows that the chemical bond of K2CO3 is easier to break

in the presence of vapor. KOH can be produced directly by Reaction(1) or gained indirectly by producing intermediate product K2O(Reaction (2)) first and then through Reaction (4). Although thestandard Gibbs free energy of Reaction (4) is smaller than 0, andReaction (4) can be activated automatically without any externalconditions, the reaction must first produce K2O through Reaction(2). Provided that the standard Gibbs free energy of Reaction (2)is relatively large, and external reaction conditions are very strict,

300 400 500 600 700

250

300

350

400

450

500

550

600

ΔG

θ (k

J)

Temperature (K)

Reaction(5)Reaction(8)

Fig. 6. Standard Gibbs free energy of Reactions (5) and (8).

1% K2C2O4 2% K2C2O4 5% K2C2O4

1% CH3COOK 2% CH3COOK 5% CH3COOK

5% K2CO3 2% K2CO3 1% K2CO3

Fig. 4. SEM of the decomposition products of the K2C2O4, CH3COOK and K2CO3 aqueous solution under 800 �C.

200 400 600 800 1000 1200 1400

-200

-100

0

100

200

300

400

ΔG

θ (k

J)

Temperature (K)

Reaction(1)Reaction(2)Reaction(3)Reaction(4)

Fig. 5. Standard Gibbs free energy of reactions (1)–(4).

Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438 435

the reaction producing KOH is apt to be Reaction (1), indicating thepossibility of inducing the hydrolysis reaction of K2CO3 first thanthe decomposition reaction under vaporization conditions. Thiscondition agrees with the mechanism of carbonate reduction byC of McKee et al. [42,43]. On the other hand, the standardGibbs free energy of Reaction (4) is smaller than 0, which alsoimplies the unstable existence of K2O. K2O is converted into KOHin the presence of H2O. KOH existence was also verified by theXRD test.

The decomposition reaction (5) of KNO3 includes the formationreaction (6) of KNO2 and the decomposition reaction (7) of KNO2:

2KNO3 ¼ K2Oþ NOþ NO2 þ O2 ð5Þ

2KNO3 ¼ 2KNO2 þ O2 ð6Þ

2KNO2 ¼ K2Oþ NOþ NO2 ð7Þ

The standard Gibbs free energy of KNO2 at temperatures higherthan 298 K is inapplicable [23]. Therefore, thermodynamic calcula-tion was accomplished according to Reaction (5). Possible reactionof KNO3 in the presence of vapor is as follows:

2KNO3 þH2O ¼ 2KOHþ NOþ NO2 þ O2 ð8ÞVariations in the standard Gibbs free energy of Reactions (5)

and (8) at 298–700 K are shown in Fig. 6.In the presence of vapor, KNO3 easily decomposes into K2O,

which can automatically produce KOH through hydrolysis reac-tion. Activity of KNO3 results from the decomposition reaction(5), and KNO2 is a possible intermediate product of the reaction.Although thermodynamic calculation shows that Reaction (5) hashigh DGh, its existence can be proven by thermoanalysis. Basedon thermodynamic calculation, Liu [35] pointed out that KO2 isthe stable product of K in excessive air and can react with excessiveair to form KO3. Hence, the XRD test revealed that the final prod-ucts of the complicated decomposition reaction of KNO3 underthe existence of vapor are mixtures of KOH, KO2, and KO3.

436 Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438

The possible reaction of KCl in the vapor atmosphere is asfollows:

KClþH2O ¼ KOHþHCl ð9ÞKOH was not observed in the XRD analysis of the product, indi-

cating that the reaction conditions are strict. Based on the thermo-analysis, solid KCl melts when the temperature exceeds 1044 K,which can further accelerate the hydrolysis reaction of KCl. Onthe other hand, provided that alkali halide has high saturatedvapor pressure [10], some accelerated hydrolysis reactions maybe attributed to gaseous hydrolysis. KOH is the only possible pro-duct in KCl activation, which is consistent with the research con-clusions of McKee and Verra et al. [42,43]. Verra believed thatK2O was the product of the first-step KCl reaction under the exis-tence of gaseous vapor, but the compound can only be the interme-diate product under the existence of vapor. Hydrolysis of KClrequires high temperature. Given the high saturated vapor pres-sure of KCl, evaporation of KCl accelerates its hydrolysis reactionand offsets the disadvantage of low burnt temperature.

Based on thermoanalysis and XRD test, reaction products ofKH2PO4 under the existence of vapor were stable and were com-posed mainly of glassy KPO3. No KOH was found.

Equilibrium calculation of products of CH4/air burning systemwith 5% potassium salt solutions was implemented by HSC Chem-istry 6. Results are shown in Figs. 7 and 8.

Figs. 7 and 8 show that KOH exists as equilibrium products ofK2C2O4, CH3COOK, K2CO3, and KNO3 in the flame reaction zone.We speculated that KOH is the key component in producing extin-guishing active substances. At the same time, KNO3 can reduce the

0 200 400 600 800 1000 12000.0

0.5

1.0

1.5

2.0

C

mol

Temperature

K2CO3

KOH

0.0

0.5

1.0

1.5

2.0

(a) 5% K2CO3

(c) 5% KCl

0 200 400 600 800 1000 12000.0

0.5

1.0

1.5

2.0

C

mol

Temperature

KCl

0.

0.

1.

1.

2.

Fig. 7. Results of equili

free radical OH concentration significantly. The SEM experimentshowed that under high temperature, pyrolysis products of KNO3

follow a spheroidization trend, which is beneficial in developingthe extinguishing performance and offsetting the disadvantage oflower KOH percentage than K2CO3. Moreover, the extinguishingperformance of KNO3 observed in the cup burner experimentwas basically consistent with those of K2C2O4, CH3COOK, andK2CO3. The chemical extinguishing performance of KCl was causedby the high percentage of KCl in the equilibrium products, indicat-ing that KCl has certain volatility at high temperature, and itschemical extinguishing performance was influenced to a largeextent by the few KOH produced by gaseous hydrolysis reaction.

In previous studies, Jensen and Jones [44] had proposed themechanism of potassium salts to inhibit fire radical as follows,which M is a third body participates in chain reactions:

KOHðgÞ þH ¼ H2OðgÞ þ K ð10Þ

Kþ OHþM ¼ KOHðgÞ þM ð11ÞHynes [45] and Slack [19] showed that the suppression kinetic

mechanism containing potassium salts should also contain XO andXO2, revealed the possible reaction path and holed that the impor-tance of the reaction (12) is equivalent to the reaction (11)

Kþ O2 þM ¼ KO2ðgÞ þM ð12Þ

KO2ðgÞ þH ¼ KOðgÞ þ OH ð13ÞWilliams and Fleming [46] considered all the researchers and

put forward a suppression kinetic mechanism model containingsodium species:

0 200 400 600 800 1000 1200 C

mol

Temperature

KNO3KOH

KNO2 KO2

(b) 5% KNO3

(d) 5% KH2PO4

0 100 200 300 400 500 600 7000

5

0

5

0

C

mol

Temperature

KH2PO4KPO3

brium calculation.

KOH(g)K2O2H2(g)

K2(OH)2(g) K(g)KO(g)

K2O(g)K2O2(g)

3.69

0.0992 0.0549

0.00206 1.40E-04

8.88E-07 3.08E-07

1.91

0.0264 0.0147

9.86E-04 7.69E-05

2.32E-07 9.22E-08

0.0423 1.34E-05 7.41E-06 2.26E-05 1.56E-06 6.41E-11 3.90E-11

3.33E-08 1.02E-14 9.96E-15 4.75E-15 6.90E-16 2.00E-25 7.87E-24

5%K2CO3 5%KNO3 5%KCl 5%KH2PO4

Fig. 8. The amount of equilibrium species by different potassium salts.

Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438 437

NaOHðgÞ þH ¼ H2OðgÞ þ Na ð14Þ

Naþ OHþM ¼ NaOHðgÞ þM ð15Þ

NaOHðgÞ þ OH ¼ NaOðgÞ þH2OðgÞ ð16ÞConsidering the similarity of sodium and potassium, the active

substances from the decomposition products of water mist withpotassium salts for chemical kinetics mechanism should includeat least the species KOH, KO2 KO and K, and this conclusion canbe expressed by Fig. 9.

Potassium salt with oxygen containing can be used to provideactive substances such as KOH and KxOy in fire extinguishing underthe condition of water vapor. However, for the KCl-water mist,Zheng [4] pointed out the promotion of suppression might fromCl2 radical which produced by Cl radical from KCl at flametemperature.

Clþ ClþM ! Cl2 þM ð17Þ

Then, as an inhibitor, Wilson [47] reported the chemical mech-anism path by reactions (18)–(20).

Cl2 þH ! HClþ Cl ð18Þ

HClþH ! H2 þ Cl ð19Þ

Hþ OHþ Cl ! H2Oþ Cl ð20Þ

Fig. 9. The path of active substances from decomposition of water mist withpotassium additives.

In summary, the extinguishing occurred not only requires theuse of active substances of decomposition, but also consideringthe synchronization of water, the formation of K cations, flame rad-ical, and other anions.

4. Conclusions

The current research enriches the database of minimum fireextinguishing concentration of water mist with some potassiumsalts. The active substances from decomposition in flame temper-ature of water mist with some potassium salts are creativelyobtained by thermoanalysis experimental method which has morereliability and accuracy, and the quantitative analysis of active sub-stances in fire suppression is obtained through thermodynamicsmethod by HSC CHEMISTRY 6. The main conclusions of the presentresearch are:

(1) Potassium salts chosen from current research can improvethe extinguishing performance of pure water, and K2CO3

has the most benefit with the improving rate of 37.6%,47.2% and 64.8% which the mass percent is 1%, 2% and 5%,respectively, followed by K2C2O4, CH3COOK, KNO3, KCl andKH2PO4.

(2) Active substances that participate in extinguishment undergas vapor were consistent and unrelated with the type ofpotassium salts used. Therefore, extinguishing performanceof different potassium salts is due to the activation behaviorsof potassium salts under gasification conditions. Potassiumsalts, which have high extinguishing performance, produceKOH during flame chemical reactions. As the key componentof activation of potassium salts, KOH can further react withfree radicals in flames to produce extinguishing activesubstances.

(3) Higher fire extinguishing efficiency corresponds to moreactive substances. The main potassium salts such as K2CO3,KNO3, KCl and KH2PO4 provides active substances KOH4.85%, 1.3%, 5.13 � 10�13 % and 7.16 � 10�30 % respectively,which indicates the influence of anions on the fire extin-guishing efficiency.

Furthermore, the MEC of some other kinds of potassium addi-tives should be obtained, and for the engineering application ofwater mist with potassium salt additives, physical and chemicalproperties of fire extinguishing agent, mostly the toxicity, forexample, the emission of Cl, P and N species whether be harmfulto firefighters or environment should be the further work.

438 Z. Tianwei et al. / Applied Thermal Engineering 122 (2017) 429–438

Acknowledgments

Financial support of this work by Self-raised Key Research andDevelopment Project of Hebei Province (B2016507017).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.applthermaleng.2017.05.053.

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