Combustion Inhibition of Aluminum–Methane–Air Flames by Fine Nacl Particles

energies

Article Combustion Inhibition of Aluminum–Methane–Air Flames by Fine NaCl Particles

Wu Xu and Yong Jiang *

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230027, China; [email protected] * Correspondence: [email protected]; Tel.: +86-551-63607827

Received: 11 October 2018; Accepted: 12 November 2018; Published: 14 November 2018

Abstract: The effect of NaCl as an extinguishing agent on metal dust fires require further exploration. This paper reports the results of an experimental study on the performance of micron-sized NaCl powders on hybrid aluminum–methane–air flames. NaCl particles with sub-10 µm sizes were newly fabricated via a simple solution/anti-solvent method. The combustion characteristics of aluminum combustion in a methane-air flame were investigated prior to the particle inhibition study to verify the critical aluminum concentration that enables conical aluminum-powder flame formation. To study the inhibition effectiveness, the laminar burning velocity was measured for the established aluminum–methane–air flames with the added NaCl using a modified nozzle burner over a range of dust concentrations. The results were also compared to flames with quartz sand and SiC particles. It is shown that the inhibition performance of NaCl considerably outperformed the sand and SiC particles by more rapidly decreasing the burning velocity. The improved performance can be attributed to contributions from both dilution and thermal effects. In addition, the dynamic behavior of the NaCl particles in the laminar aluminum–methane–air flame was investigated based on experimental observations. The experimental data provided quantified the capabilities of NaCl for metal fire suppression on a fundamental level.

Keywords: aluminum–methane flame; sodium chloride; burning velocity; flame inhibition; conical flames

1. Introduction Prevention of dust explosions is of great scientific and practical interest because micron- or nano-sized particles when dispersed in the form of a cloud, present a real threat to processing industries and to humans [1,2]. Sometimes, these particles are entrained by a combustible gas, resulting in two-phase hybrid mixture combustion. Generally, it has been experimentally confirmed that a hybrid of solid and gaseous fuel has a lower flammability limit, a higher energy density and can be easily ignited by a very small ignition energy compared with that of dust–air systems [3,4]. Earlier studies relating to explosions of hybrid mixtures and their suppression have mainly focused on the mining industry with coal dust mixing with methane gas as the research focus [5,6]. Since metal dust is encountered in numerous modern technological applications [7,8], as well as in numerous fatal explosions, specific research on the suppression of metal dust fires is required and is the topic of the current study. Laminar burning velocity (LBV) is an intrinsic property of any premixed reacting mixture, and it is a measure of the mixture’s exothermicity, diffusivity and overall reactivity in combustion [9,10]. Studies on the propagation and mitigation of aluminum dust flames that are available in the literature have mostly been conducted in closed standard spherical vessels where the combustion characteristics, such as the ignition energy and explosion pressure rise, have been investigated. Denkevits and

Energies 2018, 11, 3147; doi:10.3390/en11113147 www.mdpi.com/journal/energies Energies 2018, 11, 3147 2 of 12

Hoess [11] used a 20 L spherical vessel to study the explosion behavior of hybrid mixtures of 1 µm aluminum dust/hydrogen/air. Recently, Jiang et al. [12]. studied the suppression performance of NaHCO3 on an aluminum dust explosion in a similar spherical chamber and reported that the minimum inerting concentration (MIC) for the explosion of 5 µm aluminum is higher than that of 30 µm. The size distribution of the commercial NaHCO3 powder is in the range of 53–212 µm in Reference [10]. Taveau et al. [13] reviewed suppression experiments on metal dusts on different scales. To our knowledge, few studies of LBV measurements have been performed for metal dust and, particularly, aluminum combustion has not been studied until very recently. Sikes et al. [14] attempted to measure the LBV of oxygen-enriched nano-aluminum/methane hybrid mixtures using a combustion vessel apparatus with a refined injection unit to obtain a uniformly dispersed aerosol. In their study, however, the seeding amount of aluminum was on the trace level, which was lower than ~3.5 g/m3. Julien et al. [15] built a counterflow particulate-fuel burner and used it to measure the burning velocities of aluminum/air mixtures but did not eliminate the flame stretch. In contrast to gaseous or liquid fuels, this revealed that there are challenges in LBV measurements of mixtures in the presence of solid particulates via conventional techniques, such as using counterflow flames [16], spherically expanding flames [17], or flat flames [18]. To determine the burning velocity of particulate fuels, a conical Bunsen flame technique was used by WPI researchers [5,19–21]. This common approach has proven to be adequate if the deviations caused by the stretch effects are fully assessed [22]. To continue this effort, this paper introduces a new hybrid flame burner for LBV measurements of gas and gas–solid fuel premixed flames. The suppression of metal fires using inert dusts, such as sand, has been commonly used for a very long time [23]. Sand particles, however, have practical application limitations associated with particle delivery and storage issues. Currently, although some new agents have been identified as effective fire suppressants (e.g., halon replacements and water mist), the incorrect application of inhibitors for suppressing metal fires can cause an enhancement effect on the system due to exothermic reactions of metal with either or both the inhibitor and products. Sodium chloride (NaCl), which now serves as the main effective substance in portable fire extinguishers for class D (combustible metals) fires, has many advantages as it is not rare, it is environmentally benign, and has a relatively high heat capacity [24,25]. In terms of the inhibition mechanism, sodium chloride particles generally act as an inert inhibitor via the two following physical inhibition mechanisms: (1) a thermal effect associated with heat absorption and the latent heat of vaporization and (2) a dilution effect induced by the decrease in the reactant concentration. For a specific application, detailed quantitative information understanding the performance of NaCl particles on metal fires is useful for designing and optimizing these particles. Fundamentally, this requires an investigation of the burning velocity retardation by NaCl particles; however, no study available in the literature has attempted to explore its impact on flame propagation to quantify the effectiveness of NaCl for flame inhibition. In this work, NaCl particles were prepared using a solution/anti-solvent method in our laboratory. Improvements in the hydrophobicity and dispersivity were achieved by introducing hydrophobic nano-SiO2 (silicon dioxide) attached to the surface of the NaCl particles. The objectives of this study were to estimate the flame propagation velocity inhibition by adding NaCl in a hybrid mixture of aluminum and methane and to study the performance of the newly fabricated micron-sized NaCl particles. The arrangement of this paper is as follows: the methods for preparing the micron-sized NaCl powders and for the characterization of aluminum and NaCl are presented first. The laminar burning velocities of the aluminum–methane–NaCl–air mixtures were determined using conical flame experiments. Finally, the results are concluded with a few comments. Energies 2018, 11, 3147 3 of 12

2. Experimental Methods

2.1. Experimental Apparatus and Procedures

EnergiesThe 2018 hybrid, 11, x FOR fuel PEER combustion REVIEW experiments were conducted in an axisymmetric stainless-steel3 of 12 Energies 2018, 11, x FOR PEER REVIEW 3 of 12 burner. An overview of the newly constructed experimental setup is shown in Figure1. The design of of the burner implemented several technical solutions considered previously in the building of nozzle theof the burner burner implemented implemented several several technical technical solutions solutions considered considered previously previously in in the the buildingbuilding ofof nozzle burners [26,27]. In particular, it used a profiled converging nozzle to reduce the boundary layer burners [[26,27].26,27]. In In particular, particular, it it used a profiled profiled converging nozzle to reduce the boundaryboundary layer thickness and to yield a top-hat (plug-flow) velocity profile at the exit plane. Specifically, the axial thickness and to yield a top-hattop-hat (plug-flow)(plug-flow) velocity profileprofile at the exit plane.plane. Specifically, Specifically, the axial velocity of the cold flow that was measured using a hot-wire anemometer (Kanomax 6162, KANO velocity of the coldcold flowflow thatthat waswas measuredmeasured usingusing aa hot-wirehot-wire anemometeranemometer (Kanomax(Kanomax 6162, KANO Co., Ltd, Shenyang, China), is shown in Figure 2, indicating a quite flat velocity profile across over Co., Ltd, Shenyang, China), is shown in Figure 22,, indicatingindicating aa quitequite flatflat velocityvelocity profileprofile acrossacross overover 70% of the exit plane. The contoured nozzle had an outlet diameter of d = 10 mm, and the contraction 70% of the exit plane. The contoured nozzle had an outlet diameter ofof dd = 10 mm, and the contraction ratio was σ = (D/d)2 =2 16. High purity nitrogen was supplied surrounding the nozzle to shield the ratio was σ = ( (DD//d)d2) = =16. 16. High High purity purity nitrogen nitrogen was was supplied supplied surrounding surrounding the thenozzle nozzle to shield to shield the combustible mixture from the ambient environment. The burner inner face was finely polished to thecombustible combustible mixture mixture from from the theambient ambient environment. environment. The The burner burner inner inner face face wa wass finely finely polished polished to minimize disturbances to the flow due to irregularities. To prevent overheating, the nozzle was tominimize minimize disturbances disturbances to tothe the flow flow due due to to irregu irregularities.larities. To To prevent prevent over overheating,heating, the the nozzle was water-cooled to keep the outlet mixture at the ambient temperature. water-cooled toto keepkeep thethe outletoutlet mixturemixture atat thethe ambientambient temperature.temperature.

Figure 1. Schematic of the experimental setup for the premixed hybrid fuel combustion. Figure 1. Schematic of the experimental setup forfor the premixed hybrid fuel combustion.

Figure 2. Normalized axial velocityvelocity u/uu/umax (u(umaxmax = =1.5 1.5 m/s) m/s) as as a afunction function of of normalized normalized radial radial distance distance Figure 2. Normalized axial velocity u/umax (umax = 1.5 m/s) as a function of normalized radial distance r/Rr/R measured measured at at variousaxial variousaxial di distancestance (z (z = = 3, 3, 6, 6, and and 9 9 mm). mm). r/R measured at variousaxial distance (z = 3, 6, and 9 mm).

Methane (purity > 99.99%) 99.99%) and synthetic air (21% O2 inin N N22),), provided provided by by the the Nanjing Nanjing Special Special Gas Gas Methane (purity > 99.99%) and synthetic air (21% O2 in N2), provided by the Nanjing Special Gas Factory (Nanjing,(Nanjing, China)China) were were employed employed as as the the fuel fu gasel gas and and oxidizer, oxidizer, respectively. respectively. The ﬂowThe flow rates rates were Factory (Nanjing, China) were employed as the fuel gas and oxidizer, respectively. The flow rates were metered by MKS mass flow controllers that were periodically calibrated against a digital soap were metered by MKS mass flow controllers that were periodically calibrated against a digital soap film flowmeter (stated accuracy of ±0.5% for the reading). film flowmeter (stated accuracy of ±0.5% for the reading). The methane/air mixture flow acts as the aluminum particle carrier gas. After a group of test The methane/air mixture flow acts as the aluminum particle carrier gas. After a group of test runs, the volumetric flow rate of the carrier gas was fixed at 5.5 standard liters per minute (SLPM, at runs, the volumetric flow rate of the carrier gas was fixed at 5.5 standard liters per minute (SLPM, at 0 °C and 1 atm). During the experiments, the burner assembly and pipelines were heated using a 0 °C and 1 atm). During the experiments, the burner assembly and pipelines were heated using a ceramic heating jacket and heating types. A K-type thermocouple was placed immediately ceramic heating jacket and heating types. A K-type thermocouple was placed immediately downstream of the burner exit to monitor the Tu of the combustible mixture, which was maintained downstream of the burner exit to monitor the Tu of the combustible mixture, which was maintained

Energies 2018, 11, 3147 4 of 12 metered by MKS mass flow controllers that were periodically calibrated against a digital soap film flowmeter (stated accuracy of ±0.5% for the reading). The methane/air mixture flow acts as the aluminum particle carrier gas. After a group of test runs, the volumetric flow rate of the carrier gas was fixed at 5.5 standard liters per minute (SLPM, at 0 ◦C and 1 atm). During the experiments, the burner assembly and pipelines were heated using a ceramic Energiesheating 2018 jacket, 11, x and FOR heatingPEER REVIEW types. A K-type thermocouple was placed immediately downstream4 of of12 the burner exit to monitor the Tu of the combustible mixture, which was maintained at ambient attemperature ambient temperature (Tu =298 ± 3(T K).u =298 The ± particle 3 K). The samples particle were samples continuously were continuously fed to the main fed jet to using the main a screw jet usingfeeder, a whichscrew isfeeder, an assembly which is of an the assembly following of components:the following acomponents: homemade helicala homemade screw, ahelical feed hopper screw, witha feed a hopper vibrator with and a an vibrator adjustable and speedan adjustable motor withspeed a motor digital with speed a digital controller. speed As controller. a result of As the a resultcontinuity of the of continuity the flow, of the the dust flow, entrained the dust intoentrained the methane-air into the methane-air stream, creating stream, acreating hybrid a mixture. hybrid Amixture. 100 mesh A 100 (aperture mesh (aperture size of 0.154 size mm) of 0.154 screen mm) that screen was installed that was at installed the bottom at the of thebottom converging of the convergingnozzle was utilizednozzle towas dampen utilized the turbulenceto dampen of thethe hybridturbulence mixture of andthe to hybrid homogeneously mixture disperseand to homogeneouslythe particles. As thedisperse flow continuouslythe particles. passesAs the through flow continuously the screen, dustpasses particles through may the deposit screen, on dust the particlesscreen mesh, may causingdeposit aon local the blockagescreen mesh, and causing hence a a flow local instability blockage mayand behence transmitted a flow instability to the burner may beoutlet. transmitted Therefore, to the additional burner outlet care was. Therefore, taken to additional prevent this care and was the ta dustken to deposition prevent this was and neglected the dust if depositioneach experiment was neglected could be finishedif each experiment in 2 min. Figure could3 displaysbe finished images in 2 ofmin. the Figure aluminum 3 displays particles images as they of theexited aluminum the burner particles illuminated as they with exited a 1.5 the mm-thick burner laserilluminated sheet at with different a 1.5 aluminum mm-thick concentrations. laser sheet at Thedifferent powder aluminum feed rate concentrations. was determined The by powder a direct calibrationfeed rate was procedure. determined A vacuum by a direct pump calibration was used procedure.to aspirate theA vacuum dusty flow pump through was aused probe to thataspirate contained the dusty quartz flow wool through for a certain a probe flow that volume; contained the quartzdeposited wool dust for wasa certain then weightedflow volume; and the a calibration deposited curve dust was was then obtained weighted based and on thea calibration variation ofcurve the wasrotary obtained screw speed. based Theon particlethe variation mass loadingof the rotary was determined screw speed. by dividingThe particle the totalmass volume loading of was the determinedsolid–gas mixture by dividing by the the mass total of volume the particles of the that solid–gas passed mixture through by the the mesh mass screen of the during particles a given that passedtime interval. through the mesh screen during a given time interval.

Figure 3. PhotographsPhotographs of of laser-illuminated laser-illuminated Al Al particles particles at at different different concentrations: concentrations: (a ()a 12) 12 g/m g/m3, (b3,() 55b) g/m55 g/m3, and3, and(c) 152 (c) 152g/m g/m3. The3. exposure The exposure time timeof the of camera the camera was 1/8000 was 1/8000 s. s.

Visualization ofof the the flame flame was was achieved achieved using using a transmission a transmission Schlieren Schlieren apparatus. apparatus. A high-intensity A high- intensitydischarge discharge mercury arcmercury lamp (powerarc lamp of (power 100 W, wavelengthof 100 W, wavelength of 200–2500 of nm) 200–2500 was employed nm) was asemployed the light assource the light for the source illumination. for the illumination. A collimated A lightcollimat beamed waslight formed beam was using formed a pinhole using (diameter a pinhole of (diameter 200 mm) placedof 200 mm) at the placed focal pointat the offocal a plano-convex point of a plano-convex lens attached lens to theattached lamp to housing the lamp and housing an achromatic and an achromaticFourier transform Fourier lens transform (diameter lens of (diameter 100 mm, focalof 100 length mm, focal of 400 length mm). of After 400 travellingmm). After through travelling the throughflame region the flame and aregion second and identical a second achromatic identical ac Fourierhromatic transform Fourier lenstransform (diameter lens of(diameter 100 mm, of focal 100 mm,length focal of 400 length mm), of the400 collimatedmm), the collimated light beam light was beam focused was on focused an adjustable on an adjustable iris diaphragm iris diaphragm (aperture (apertureof 0.8–7.5 of mm). 0.8–7.5 Schlieren mm). imagesSchlieren were images recorded were via recorded a high-resolution via a high-resolution CCD camera CCD at 10camera frames at per 10 framessecond. per The second. visual The flame visual chemiluminescence flame chemiluminescence images were images taken were using taken a using digital a digital single single lens reflex lens (DSLR)reflex (DSLR) camera. camera. Laminar burning velocity measurements of the burner stabilized flame flame were performed based on the law of massmass conservation.conservation. The well-established flame flame cone angle [28–31] [28–31] and flameflame surface 0 area [32,33] approaches have been developed to determine the laminar burning velocity Su . In the first technique, the velocity component normal to the flame front is locally equal to the propagation velocity of the flame front, which is interpreted as the laminar burning velocity relative to the unburnt 0 mixture; therefore, the experimental Su can be calculated by Equation (1): 0 =⋅α Suu 0 sin (1) where u0 is the bulk velocity of the unburned mixture, and α is the half cone angle of the flame. Conical flames are known to be affected by a series of inherent problems, such as a strong curvature at the flame tip, strain along the flame sides and heat losses to the burner that cause local

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0 area [32,33] approaches have been developed to determine the laminar burning velocity Su. In the first technique, the velocity component normal to the flame front is locally equal to the propagation velocity of the flame front, which is interpreted as the laminar burning velocity relative to the unburnt 0 mixture; therefore, the experimental Su can be calculated by Equation (1):

0 Su = u0 · sin α (1) where u0 is the bulk velocity of the unburned mixture, and α is the half cone angle of the flame. Conical flames are known to be affected by a series of inherent problems, such as a strong curvatureEnergies 2018 at, 11 the, x FOR flame PEER tip, REVIEW strain along the flame sides and heat losses to the burner that5 of cause 12 local quenching at the flame base. Modifications to the burning velocity due to these effects can quenching at the flame base. Modifications to the burning velocity due to these effects can be be minimized by implementing technical solutions to achieve reasonable levels of measurement minimized by implementing technical solutions to achieve reasonable levels of measurement accuracy. In our experimental method, an aerodynamically contoured nozzle was used to generate accuracy. In our experimental method, an aerodynamically contoured nozzle was used to generate fairlyfairly straight-sided straight-sided flame flame cones, cones, whichwhich ensuredensured that the flame flame propagated propagated uniformly uniformly towards towards the the unburnedunburned mixture mixture over over thethe main part part of of the the flame flame zone zone [34,35]. [34,35 The]. heat The losses heat losses at the burner at the burnerrim were rim wereassumed assumed to be to small, be small, considering considering that the that thicknes the thicknesss of the nozzle of the wall nozzle was wall reduced was and reduced only a and small only a smallportion portion of the offlame the border flame borderwas close was to close the burner to the rim. burner Previous rim. Previousdemonstrations demonstrations of this laminar of this laminarburning burning velocity velocity measurement measurement approach approach have been have performed been performed for both forgaseous both [30,35,36] gaseous [ and30,35 hybrid,36] and hybridgas-solid gas-solid mixtures mixtures [19,37], [19 ,demonstrating37], demonstrating that thatthe conical the conical flame flame method method is very is very accurate accurate by by considerablyconsiderably reducing reducing the the effects effects of of flame flame stretch stretch andand curvature.curvature. FigureFigure4 depicts 4 depicts the the conical conical flame flame analysis analysis procedure. procedure. Figure 44aa showsshows a typical visual visual image image of of a hybrida hybrid methane–aluminum–air methane–aluminum–air flame.flame. EachEach available Schlieren Schlieren image image was was averaged averaged from from a set a set of of approximatelyapproximately 30 30 instantaneous instantaneousSchlieren Schlieren imagesimages of the the conical conical flame, flame, as as shown shown in in Figure Figure 4b.4b. Then, Then, thethe edge edge of of the the time-averaged time-averaged flame flame was was derivedderived by an automatic image image pr processingocessing program program based based onon the the Sobel Sobel operator operator in in a a Matlab Matlab environment, environment, as shown by by the the black black symbols symbols in in Figure Figure 4c.4c. AA linear linear fittingfitting algorithm algorithm was was applied applied to to the the continuity continuity pointspoints that characterized characterized the the flame flame edge; edge; then, then, slopes slopes of the fitted lines from the sides of the cone were calculated, denoted as k1 and k2, also shown in Figure of the fitted lines from the sides of the cone were calculated, denoted as k1 and k2, also shown in −−11 4c. The half cone angle of the flame can be obtained from α=ktan ( )+ tan− (1 k ) / 2 . Using−1 this Figure4c. The half cone angle of the flame can be obtained fromα 12= tan (k1) + tan (k2) /2. Usingprocedure, this procedure, the calculated the calculated burning burning velocity velocity matche matchess closely closelywith literature with literature data, data,as discussed as discussed in in Section 33..

FigureFigure 4. 4.Flame Flame imageimage processing.processing. (a ()a )Methane/air Methane/air flame, ﬂame, (b) (bSchlieren) Schlieren photography, photography, and and(c) a (c) a processedprocessed image.image.

AnAn experimental experimental uncertainty uncertainty analysis analysis was was performed performed for the for LBV the measurementLBV measurement using theusing equation the q 22√ 2 δ equation that was outlined by Moffat [38]: δ2 =+(B ) (tS / N ) , where 0 is the overall that was outlined by Moffat [38]: δ 0 = (B 0 ) ss00+ (tS 0 / N) s 0, where δ 0 is the overalls uncertainty su su uusu u su u 0 uncertainty for the measured S , B0 is the total bias uncertainty, S 0 is the standard deviation of u su su N repeated experiments, and t is the Student’s multiplier for 95% confidence and N − 1 degrees of freedom. The relative combined overall uncertainties of the measurements could be associated with each measured component, i.e., initial temperature, flame surface recognition and data analysis. The relative overall uncertainties of φ arise from the measurement of CH4 and the air flow rates were estimated to be 1–2%.

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0 for the measured S , B 0 is the total bias uncertainty, S 0 is the standard deviation of N repeated u su su experiments, and t is the Student’s multiplier for 95% confidence and N − 1 degrees of freedom. The relative combined overall uncertainties of the measurements could be associated with each measured component, i.e., initial temperature, flame surface recognition and data analysis. The relative overall uncertainties of φ arise from the measurement of CH4 and the air flow rates were estimated to be 1–2%. Energies 2018, 11, x FOR PEER REVIEW 6 of 12 2.2. Materials and Sample Preparation 2.2. Materials and Sample Preparation A typical experiment for the preparation of NaCl dry powder was as follows. (1) First, 100 g of A typical experiment for the preparation of NaCl dry powder was as follows. (1) First, 100 g of sodium chloride (AP, NaCl, Sinopharm Group Co., Ltd, Shanghai, China) and deionized water were sodium chloride (AP, NaCl, Sinopharm Group Co., Ltd, Shanghai, China) and deionized water were mixed as a saturated solution. The initial NaCl solution pH was adjusted by 1.0 M hydrochloric acid mixed as a saturated solution. The initial NaCl solution pH was adjusted by 1.0 M hydrochloric acid for the desired value of 1.0 during rapid magnetic stirring. Then, a certain amount of polyethylene for the desired value of 1.0 during rapid magnetic stirring. Then, a certain amount of polyethylene glycol (AP, PEG1000, Sinopharm Group Co., Ltd.) was added to the above solution at a temperature of glycol◦ (AP, PEG1000, Sinopharm Group Co., Ltd.) was added to the above solution at a temperature 60 C. (2) We added 0.1 mass% of hydrophobic SiO2 nanoparticles (>99.8% purity, Shanghai Macklin of 60 °C. (2) We added 0.1 mass% of hydrophobic SiO2 nanoparticles (>99.8% purity, Shanghai Biochemical Co., Ltd, Shanghai, China) to 400 mL of anhydrous alcohol in a 3-neck round bottom flask Macklin Biochemical Co., Ltd, Shanghai, China) to 400 mL of anhydrous alcohol in a 3-neck round and conducted ultrasound treatment for 0.5 h. Afterwards, the as-prepared NaCl solution was rapidly bottom flask and conducted ultrasound treatment for 0.5 h. Afterwards, the as-prepared NaCl injected into the dispersed SiO2 sol. Vacuum filtration was implemented for the NaCl slurry and solution was rapidly injected into the dispersed SiO2 sol. Vacuum filtration was implemented for the water was removed by anhydrous alcohol washing. The powder samples were dried under vacuum at NaCl slurry and water was removed by anhydrous alcohol washing. The powder samples were dried 100 ◦C for 4 h prior to further analysis. under vacuum at 100 °C for 4 h prior to further analysis. 2.3. Structural Characterization 2.3. Structural Characterization The particle size distributions of the Al powder and newly fabricated NaCl samples were The particle size distributions of the Al powder and newly fabricated NaCl samples were detected using isopropyl alcohol as the dispersing medium in a Mastersizer 3000E laser particle detected using isopropyl alcohol as the dispersing medium in a Mastersizer 3000E laser particle analyzer (Malvern Panalytical Co., Ltd, Malvern, UK). Phase identifications were determined by analyzer (Malvern Panalytical Co., Ltd, Malvern, UK). Phase identifications were determined by X- X-ray diffraction (XRD, X’Pert Pro, Malvern PANalytical Co., Ltd, Malvern, UK). A scanning electron ray diffraction (XRD, X’Pert Pro, Malvern PANalytical Co., Ltd, Malvern, UK). A scanning electron microscope (SEM, Gemini 500, ZEISS Co., Ltd, Jena, Germany) was used to observe the surface microscope (SEM, Gemini 500, ZEISS Co., Ltd, Jena, Germany) was used to observe the surface morphology of the samples. morphology of the samples. 3. Results and Discussion 3. Results and Discussion 3.1. Particle Characterization 3.1. Particle Characterization The X-ray diffraction (XRD) pattern of the as-prepared NaCl particles, using the The X-ray diffraction (XRD) pattern of the as-prepared NaCl particles, using the solution/anti- solution/anti-solvent method is shown in Figure5. The XRD pattern exhibits sharp diffraction solvent method is shown in Figure 5. The XRD pattern exhibits sharp diffraction peaks, which is in peaks, which is in accordance with the standard NaCl pattern. This indicates the high crystallization accordance with the standard NaCl pattern. This indicates the high crystallization integrity of the integrity of the particles. particles.

FigureFigure 5. 5.XRD XRD ofof sodiumsodium chloridechloride preparedprepared by the solution/anti-solvent crystallization crystallization method. method.

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Figure6 shows representative SEM images of the aluminum and sodium chloride particles. Aluminum submicron particles with a stated purity of 99.9% and a fundamental size of 800 nm, were purchased from Beijing DK Nano Technology Co., Ltd. (Beijing, China). The as-received particles were of a spherical shape and the degree of particle agglomeration was much lower than nano-sized aluminum (80 nm) also produced by the DK Company [39] (Figure6a). Figure6b shows the micromorphology of the NaCl particles, indicating a regular cubic structure with very small quantities Energies 2018, 11, x FOR PEER REVIEW 7 of 12 of SiO2 (d = 20 nm) adhering to the surface of each individual cube. The tiny SiO2 particles improve Energies 2018, 11, x FOR PEER REVIEW 7 of 12 the powderimprove ﬂowability the powder but flowability made no but difference made no differenc to the experimentale to the experimental results. Theresults. Sauter The Sauter mean diameters,mean D32, ofdiameters, the aluminum D32, of the and aluminum NaCl were and 0.87 NaClµ mwere and 0.87 7.6 μµmm, and respectively 7.6 μm, respectively (Figure6 (Figure). 6). improve the powder flowability but made no difference to the experimental results. The Sauter mean diameters, D32, of the aluminum and NaCl were 0.87 μm and 7.6 μm, respectively (Figure 6).

FigureFigure 6. SEM 6. SEM photographs photographs and and particle particle size size distributionsdistributions of of (a ()a the) the aluminum aluminum particles particles and and(b) the (b ) the sodium chloride particles. sodiumFigure chloride 6. SEM particles. photographs and particle size distributions of (a) the aluminum particles and (b) the sodium chloride particles. 3.2. Gas3.2. FlamesGas Flames (Validation (Validation Test) Test)

3.2. GasIn thisFlames study, (Validation the laminar Test) burning velocities of pure CH4/air mixtures were first measured to In this study, the laminar burning velocities of pure CH4/air mixtures were first measured to assess the accuracy of the current experimental setup and data analysis procedures. Experimental assess theIn accuracy this study, of the the laminar current burning experimental velocities setup of pure and CH data4/airanalysis mixtures procedures.were first measured Experimental to verification was performed for the range of equivalence ratios of 0.8–1.4. For leaner flames, flame verificationassess the was accuracy performed of the forcurrent the rangeexperimental of equivalence setup and ratios data analysis of 0.8–1.4. procedures. For leaner Experimental flames, flame detachment occurred at even a low flow rate, and the flame could not be stabilized on the burner. detachmentverification occurred was performed at even afor low the flowrange rate, of equivale and thence flame ratios could of 0.8–1.4. not beFor stabilized leaner flames, on theflame burner. Figure 7 compares the current measured LBVs of CH4/air mixtures with the well-documented detachment occurred at even a low flow rate, and the flame could not be stabilized on the burner. Figuredatasets7 compares of previous the current measurements measured from LBVs the of sp CHherically4/air mixturesexpanding with flame the [40–42], well-documented the counterflow datasets Figure 7 compares the current measured LBVs of CH4/air mixtures with the well-documented of previousflame [43] measurements and the heat fromflux method the spherically [44]. The expandingcurrent data flame are in [ 40good–42 ],agreement the counterflow with previous flame [43] datasets of previous measurements from the spherically expanding flame [40–42], the counterflow and theresults heat obtained flux method by these [44 ].independent The current methodolog data are inies good and agreement agree reasonably with previous well with results the GRI-mech obtained by flame [43] and the heat flux method [44]. The current data are in good agreement with previous these3.0 independent model predictions. methodologies The comparison and agree results reasonably show wellthat withthe new the GRI-mechexperimental 3.0 setup model provides predictions. results obtained by these independent methodologies and agree reasonably well with the GRI-mech accurate measurements of burning velocities by using the cone flame method. The comparison3.0 model predictions. results show The thatcomparison the new results experimental show that setup the new provides experimental accurate setup measurements provides of burningaccurate velocities measurements by using of the burning cone flamevelocities method. by using the cone flame method.

Figure 7. Experimental (symbols) and computed (lines) laminar burning velocities of methane/air mixtures at 298 K and atmospheric pressure. FigureFigure 7. Experimental 7. Experimental (symbols) (symbols) and and computed computed (lines) laminar laminar burning burning velocities velocities of methane/air of methane/air mixtures at 298 K and atmospheric pressure. mixtures3.3. Burning at 298 Velocities K and of atmospheric the Aluminum/Methane/Air pressure. Hybrid Mixtures 3.3. BurningThe effect Velocities of the of solidthe Alum aluminuminum/Methane/Air particles Hybridon flame Mixtures propagation in the hybrid mixture of aluminum and methane was investigated for three different aluminum concentration ranges: 10–23, The effect of the solid aluminum particles on flame propagation in the hybrid mixture of 45–68, and 100–120 g/m3. Aluminum dust was added to initially stoichiometric (φ = 1.0), lean (φ = aluminum and methane was investigated for three different aluminum concentration ranges: 10–23, 45–68, and 100–120 g/m3. Aluminum dust was added to initially stoichiometric (φ = 1.0), lean (φ =

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3.3. Burning Velocities of the Aluminum/Methane/Air Hybrid Mixtures The effect of the solid aluminum particles on ﬂame propagation in the hybrid mixture of aluminum

Energiesand methane 2018, 11, x was FOR PEER investigated REVIEW for three different aluminum concentration ranges: 10–23, 45–68,8 of 12 and 100–120 g/m3. Aluminum dust was added to initially stoichiometric (φ = 1.0), lean (φ = 0.8) 0.8)and and rich rich (φ = (φ 1.2) = 1.2) methane/air methane/air mixtures. mixtures. Figure Figure8 shows8 shows the the laminar laminar burning burning velocity velocity results results forfor thethe Al/CH Al/CH4/air4/air experiments. experiments. The The introduction introduction of ofaluminum aluminum particles particles reduced reduced the the burning burning velocity velocity of methaneof methane for forall allof ofthe the particle particle concentrations concentrations stud studiedied with with a adecreasing decreasing marginal marginal effect effect at at higher higher particle concentrations. For For the the stoichiometric stoichiometric mixture, mixture, the the aluminum aluminum caused caused a a maximum burning velocity retardation of 6.2 cm/s from from the the benchmark benchmark value value of 35.6 35.6 cm/s; cm/s; for for the the lean lean and and rich rich mixtures, mixtures, thethe maximum maximum decrease decrease was slightly lower at ~5.4~5.4 and 2.2 cm/scm/s respectively. respectively. Thus, Thus, with with the the solid combustiblecombustible component suspended in premixed methane/airmethane/air flames, flames, the the mixture mixture became became less less reactive reactive with a steady decrease in the flame flame burning velocity. This This result is consistent with the decreasing behaviorbehavior that that was was experimentally experimentally observed observed by by Sike Sikess et et al. al. [14] [14] and and Soo Soo et et al. al. [45], [45], where aluminum particles were addedadded toto a a similar similar methane/air methane/air mixture mixture but but investigated investigated with with a smaller a smaller (d = (d 100 = 100 nm) nm) and andlarger larger (d = 5.6(d =µ 5.6m) fundamentalμm) fundamental particle particle size. Apparently,size. Apparently, aluminum aluminum combustion combustion in the hybrid in the mixturehybrid mixturewas subjected was subjected to a complicated to a complicated mechanism mechanism due to the du multiplicitye to the multiplicity of the thermal of the physical thermal properties physical propertiesof the solid of fuel. the solid When fuel. an aluminumWhen an aluminum particle reaches particle the reaches combustion the combustion zone, it absorbs zone, it heat absorbs from heat the frommethane the methane flame before flame it burnsbefore in it theburns vapor in the phase. vapor Thus, phase. the Thus, particle the acts particle in a similar acts in way a similar as compared way as comparedto a heat sink, to a resulting heat sink, in aresulting decrease in thea decrease flame temperature. in the flame On temperature. the other hand, On aluminum the other particle hand, aluminumcombustion particle in a methane combustion flame in introduces a methane additional flame introduces heat release, additional which heat would release, likely which increase would the likelyburning increase velocity. the It burning is assumed velocity. that the It aboveis assume twod competing that the above effects two led competing to the burning effects velocity led to trend the burningshown in velocity Figure8 trend. In fact, shown it was in suspected Figure 8. thatIn fact, as the it was aluminum suspected particles that as released the aluminum volatiles, particles it could releasedlead to an volatiles, increase init could the overall lead equivalenceto an increase ratio, in thusthe makingoverall equivalence the mixture becomeratio, thus richer. making However, the mixturedue to the become absence richer. of insufficient However, due oxygen to the after absence the methane of insufficie flame,nt oxygen the aluminum after the particlesmethane mainlyflame, thereacted aluminum with the particles product mainly gases reacted instead with of producing the product full gases combustion. instead of producing full combustion.

FigureFigure 8. Laminar burning velocitiesvelocities ofof Al/CHAl/CH4/air/air mixtures mixtures for for different different aluminum aluminum concentration ranges.ranges. For For clarity, clarity, the dotted line liness only represent the data trends. 3.4. Flame Appearance and Flame Front Formation 3.4. Flame Appearance and Flame Front Formation Figure9 shows images of the aluminum/methane/air flames for two different aluminum Figure 9 shows images of the aluminum/methane/air flames for two different aluminum concentrations and a typical image of an inhibited hybrid flame by NaCl particles. For the methane concentrations and a typical image of an inhibited hybrid flame by NaCl particles. For the methane flame with a low aluminum particle loading, only the hydrocarbon flame front was visible (Figure9a). flame with a low aluminum particle loading, only the hydrocarbon flame front was visible (Figure As more aluminum particles were introduced, the aluminum flame front was formed and was observed 9a). As more aluminum particles were introduced, the aluminum flame front was formed and was observed to be coupled to the methane flame front. The local streamline of the individual aluminum particles is clearly visible (Figure 9b). It was observed that the addition of NaCl particles decreased the flame luminous emissions and suppressed the combustion intensity of each aluminum particle because a diffuse glow appeared instead of the particle burning trajectories. During the experiment, it was also found that at higher NaCl concentrations, the flame showed a propensity to become unstable due to diffusive-thermal instabilities. The tip of the flame cone was observed to split at a

Energies 2018, 11, 3147 9 of 12 to be coupled to the methane flame front. The local streamline of the individual aluminum particles is clearly visible (Figure9b). It was observed that the addition of NaCl particles decreased the flame luminous emissions and suppressed the combustion intensity of each aluminum particle because a diffuse glow appeared instead of the particle burning trajectories. During the experiment, it was alsoEnergies found 2018, 11 that, x FOR at higher PEER REVIEW NaCl concentrations, the flame showed a propensity to become unstable9 of 12 due to diffusive-thermal instabilities. The tip of the flame cone was observed to split at a NaCl concentrationNaCl concentration of approximately of approximately 100 g/m 1003, andg/m a3, smalland a number small number of particles of particles escaped withoutescaped fulfillingwithout theirfulfilling useful their functions. useful functions.

Figure 9. Visual images of of the the hybrid hybrid flames. flames. (a (a) )An An Al/CH Al/CH4/air4/air flame flame with with a low a low Al concentration Al concentration (10 3 3 (10g/m g/m3); (b)); An (b) Al/CH An Al/CH4/air 4flame/airflame with witha medium a medium Al concentration Al concentration (50 g/m (503 g/m); (c) );An (c )Al/CH An Al/CH4/air flame4/air flamewith NaCl with added NaCl added at a medium at a medium concentration concentration (50 g/m (503 Al, g/m 503 g/mAl,3 50 NaCl). g/m3 NaCl).

3.5. Burning Velocities of the Aluminum/Methane/Sodium Chloride/Air Hybrid Mixtures 3.5. Burning Velocities of the Aluminum/Methane/Sodium Chloride/Air Hybrid Mixtures To investigateinvestigate the the suppression suppression effectiveness effectiveness of sodium of sodium chloride chloride in the hybridin the methane/airhybrid methane/air mixture, burning velocity measurements were performed for the Al/CH4/air flames with increasing amounts mixture, burning velocity measurements were performed for the Al/CH4/air flames with increasing ofamounts added NaClof added particles. NaCl Theparticles. results The were results also compared were also to compared data measured to data with measured silicon carbidewith silicon (SiC, d µ d µ carbide= 8 m) (SiC, and d quartz= 8 μm) sand and particlesquartz sand ( = particles 10.5 m). (d The= 10.5 stoichiometric μm). The stoichiometric methane/air methane/air with a medium with concentrationa medium concentration of aluminum of wasaluminum adopted was as aadopted baseline as because a baseline the metalbecause flame the front metal formed flame infront the presenceformed in of the the presence medium concentration,of the medium indicating concentrat thation, a stabilizedindicating metal–powderthat a stabilized conical metal–powder flame was generatedconical flame and was merged generated with the and methane merged flame, with which the wasmethane a prerequisite flame, which to performing was a prerequisite the inhibition to experiments.performing the As inhibition shown in experiments. Figure 10, adding As shown each in inhibitor Figure 10 reduced, adding the each LBV inhibitor data for reduced all particle the concentrations.LBV data for all particle Since the concentrations. specific heat Since of the the quartz specific sand heat is of larger the quartz than that sand of is SiC larger (0.83 than versus that 0.67of SiC J/g (0.83 K) andversus they 0.67 both J/g simplyK) and actthey as both inert simply particles act withas inert a similar particles particle with a size, similar the particle suppression size, effectivenessthe suppression of quartz effectiveness sand is superior of quartz to siliconsand carbide.is superior Furthermore, to silicon the carbide. suppression Furthermore, effectiveness the ofsuppression NaCl is significantly effectiveness better of NaCl than is quartz significantly sand, evenbetter though than quartz they havesand, a even similar though value they of specific have a heatsimilar (0.86 value versus of specific 0.83 J/g heat K). (0.86 In fact, versus in addition 0.83 J/g to K) the. In dilution fact, in addition effect, NaCl to the particles dilution can effect, efficiently NaCl absorbparticles radiation can efficiently emitted absorb from the radiation reaction emitted zone during from the reaction melting process.zone duri Furthermore,ng the melting the process. melting componentFurthermore, was the coatedmelting on component the surface was of coated the aluminum on the surface particles of the excluding aluminum air particles access, which excluding also contributedair access, which to the also fire contributed suppression. to the fire suppression.

Energies 2018, 11, 3147 10 of 12 Energies 2018, 11, x FOR PEER REVIEW 10 of 12

Figure 10. Laminar burning burning velocities velocities of of the the Al/CH Al/CH4/air4/air mixtures mixtures with with inert inert particles particles of NaCl, of NaCl, SiC, SiC, or quartzor quartz sand. sand. For For clarity, clarity, the the dotted dotted lin lineses only only represent represent the the data data trends. trends. 4. Conclusions 4. Conclusions Aluminum dust-entrained fire is highly dangerous and is difficult to extinguish by conventional Aluminum dust-entrained fire is highly dangerous and is difficult to extinguish by conventional fire agents. NaCl is a preferred candidate for handling this specific fire type. In this study, micron-scale fire agents. NaCl is a preferred candidate for handling this specific fire type. In this study, micron- NaCl powder was fabricated via a simple solution/anti-solvent method. A modified nozzle burner scale NaCl powder was fabricated via a simple solution/anti-solvent method. A modified nozzle that was designed to operate with combustible gas–dust hybrid mixtures was used to investigate the burner that was designed to operate with combustible gas–dust hybrid mixtures was used to effect of NaCl on stabilized aluminum/methane/air flames. Combustion tests were performed using investigate the effect of NaCl on stabilized aluminum/methane/air flames. Combustion tests were the Schlieren technique by marking the conical flame edge to determine the laminar burning velocity. performed using the Schlieren technique by marking the conical flame edge to determine the laminar For pure aluminum/methane/air flames, an aluminum combustion front formed above a certain burning velocity. For pure aluminum/methane/air flames, an aluminum combustion front formed Al concentration, which can only then represent a stabilized metallic-involved flame. The results above a certain Al concentration, which can only then represent a stabilized metallic-involved flame. indicate that the addition of aluminum submicron particles influenced the methane flame by reducing The results indicate that the addition of aluminum submicron particles influenced the methane flame the flame burning velocity. When the micron-sized NaCl particles were added, the burning velocity by reducing the flame burning velocity. When the micron-sized NaCl particles were added, the considerably decreased with an increasing concentration of NaCl toward the opened end of the flame burning velocity considerably decreased with an increasing concentration of NaCl toward the opened tip. The suppression effectiveness of NaCl is better than quartz sand and SiC for reducing the burning end of the flame tip. The suppression effectiveness of NaCl is better than quartz sand and SiC for velocity. This is not only due to the rapid melting of NaCl particles that causes a cooling effect on the reducing the burning velocity. This is not only due to the rapid melting of NaCl particles that causes flames but is also due to the molten NaCl coating the metal particles, which excluded air access. a cooling effect on the flames but is also due to the molten NaCl coating the metal particles, which Authorexcluded Contributions: air access. Y.J. managed the project; W.X. designed and built the experiment setup, performed the experiments, analyzed the data, and wrote the paper. AuthorFunding: Contributions:This research wasY.J. fundedmanaged by the the project; National W.X. Natural designed Science an Foundationd built the experiment of China (No. setup, 51576183), performed National the experiments,Key R&D Program analyzed of China the data, (No. and 2016YFC0801505), wrote the paper. the Fundamental Research Funds for the Central Universities of China (No. WK2320000041) and the China Postdoctoral Science Foundation (No. 2018M632550). Funding: This research was funded by the National Natural Science Foundation of China (No. 51576183), NationalConflicts Key of Interest: R&D ProgramThe authors of China declare (No. no 2016YFC0801505) conflicts of interest., the Fundamental Research Funds for the Central Universities of China (No. WK2320000041) and the China Postdoctoral Science Foundation (No. 2018M632550). References Conflicts of Interest: The authors declare no conflicts of interest. 1. Bouillard, J.; Vignes, A.; Dufaud, O.; Perrin, L.; Thomas, D. Ignition and explosion risks of nanopowders. ReferencesJ. Hazard. Mater. 2010, 181, 873–880. 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