Explosion Temperatures of Metals and Other Elemental Dust Clouds 

Kenneth L. cashdollar' and Isaac A. Zlochower - 

Pittsburgh Research Laboratory, 

National Institute for Occupational Safety and Health, 

Pittsburgh, PA, U. S .A. 

Abstract 

The Pittsburgh Research Laboratory of the National Institute for Occupational Safety and 

Health conducted a study of the explosibility of various metals and other elemental dusts 

dispersed in air, with a focus on the experimental explosion temperatures. The data are useful 

for understanding the basics of dust cloud combustion, as well as for evaluating the explosion 

hazards in the minerals and metals processing industries. The dusts studied included boron, 

carbon, magnesium, aluminum, silicon, sulfur, titanium, chromium, iron, nickel, copper, zinc, 

niobium, molybdenum, tin, hafhium, tantalum, tungsten, and lead. The dusts were chosen to 

cover a wide range of physical properties - from the more volatile materials such as magnesium, 

aluminum, sulfur, and zinc to the highly "refractory" (very low volatility) elements such as 

carbon, niobium, molybdenum, tantalum, and tungsten. These flammability studies were 

conducted in a 20-L chamber, using strong pyrotechc ignitors. The experimental data obtained 

included the minimum explosible concentrations, maximum explosion pressures, and maximum 

explosion temperatures. A unique multiwavelength infrared pyrometer was used to measure the 

temperatures. For the elemental dusts studied, all ignited and burned as air-dispersed dust clouds 

except for nickel, copper, molybdenum, and lead. The measured maximum explosion 

temperatures ranged from -1 550 K for tin and tungsten powders to -2800 K for aluminum, 

magnesium, and titanium powders. The measured temperatures are compared to the calculated, 

adiabatic flame temperatures. In general, the dusts whose experimental temperatures were closer 

. to the adiabatic values were those with the finer particle sizes, those that were more easily 

vaporized, and/or those that were intrinsically more reactive. 

Disclaimer: The findings and conclusions in this report are those of the authors and do not necessarily 

represent the yiews of the National Institute for Occupational Safety and Health. 

1, Introduction 

This paper will concentrate on the explosion characteristics of metal and nonmetal 

elemental dusts. These dusts are important from both the practical and fundamental points of 

view. These metal and nonmetal elements are often used or occur in industry as powders or . 

dusts, and an evaluation of their possible explosion hazards is important. In addition, the 

elemental dusts are ideal for studying the fundamental physicochemical processes occurring 

during combustion because they are pure, uniform substances with well-characterized phase 

transitions and thermodynamic properties. This is in marked contrast to the complex structures 

and heterogeneous chemistry associated with the combustion of carbonaceous materials such as 

coal and grain dusts. The elemental dusts also display a marked variation of reaction 

'contact author: Kenneth L. Cashdollar, NOSH, Pittsburgh Research Laboratory, P.O. Box 18070, Cochrans Mill 

Road, Pittsburgh, PA 15236, phone 4 12-3 86-6753, fax 4 12-3 86-6595, e-mail KCashdollar@cdc.gov. 
exothemicities in air and an enormous range in vapor pressures at their respective flame 

temperatures. 

In the 1960's and earlier, the U.S. Bureau of Mines at Pittsburgh studied the explosibility 

of metal dusts using a cylindrical 1.2-L Hartrnann chamber (Jacobson, Cooper, & Nagy, 1964) 

with an electric spark ignition source. More recent Pittsburgh Research Laboratory (PRL) 

studies used a nearly-spherical 20-L chamber and much stronger pyrotechnic ignitors to study 

the explosibility of metal and other elemental dusts (Hertzberg, Zlochower, & Cashdollar, 1991 ; 

Hertzberg, Zlochower, & Cashdollar, 1992; Cashdollar, 1994). The current paper is a 

continuation of these pRL2 studies, with an emphasis on experimental explosion temperatures 

measured with a six-channel ineared (IR) pyrometer. The current paper is also complementary 

to an earlier experimental study of coal dust explosion temperatures (Cashdollar & Hertzberg, 

1983), using the same IR pyrometer. Other explosion characteristics measured for the metal and 

other elemental dusts included minimum explosible concentrations (lean flammable limits), 

maximum explosion pressures, and rates of pressure rise. In hs paper, the terms "flammability" 

and "explosibility" are used interchangeably to refer to the ability of an airborne dust cloud to 

propagate an explosion after it has been initiated by a sufficiently strong ignition source. The 

explosions observed were rapid deflagrations, not detonations. The infrared temperatures and 

other experimental explosion data provide evidence to evaluate the general mechanisms of 

homogeneous versus heterogeneous combustion of these elemental dusts. Some preliminary 

metal explosion temperature data for iron dust are in Cashdollar (1994; 2000). 

2. Experimental Equipment and Test Procedures 

The dust flammability experiments in this paper were conducted in the PRL 20-L 

laboratory chamber (Cashdollar & Hertzberg, 1982a; Cashdollar, 1994), which has been used 

extensively to study the explosibility of various dusts. The chamber (Fig. 1) is near-spherical in 

shape and made of stainless steel, with a pressure rating of 21 bar. The vertical cross section is 

merely a schematic that is meant to show the chamber itself and the vertical positioning of the 

instrumentation; the positions of the instrumentation around the circumference are shown in the 

horizontal cross section. The chamber top is hinged and opens across the full chamber diameter. 

The hinged top is attached by six : -in (1 9-mm) diameter bolts which are not shown on the 

drawings. Two PRL optical dust probes (Liebman, Conti, & Cashdollar, 1977; Cashdollar, 

Liebman, & Conti, 198 1) were used to measure the uniformity of the dust dispersion at the 

positions shown in Fig. 1. The optical probes measured the transmission through the dust cloud. 

Thin jets of air kept the probe windows dust-free. For the majority of the tests, one dust probe 

with a 3 8-rnm path length and a second dust probe with a 95-mm path length were used. The 

longer path length is more suitable for dusts with higher densities and larger particle sizes. The 

strain gauge pressure transducer measured the absolute explosion pressure and rate of pressure 

rise. 

The patented six-channel 1R pyrometer (Cashdollar, Hertzberg, & Litton, 1979; 

Cashdollar & Hertzberg, 1982b) measured the explosion temperatures by observing the flame 

radiation through the sapphire window in the top of the 20-L chamber. The pyrometer uses room 

2 The majority of this research was conducted in the early 1990s when the PRL was part of the U.S. Bureau of 

Mines before its transfer to the National Institute for Occupational Safety and Health (NIOSH) in 1996. 
dgnition point 

(Pressure transducer 

Dispersion 83 011 

Fig 1. Vertical and horizontal cross sections of the 20-L explosibility test chamber. 

temperature lead selenide photoconductive detectors and infrared interference filters to measure 

flame radiation simultaneously at six wavelengths: 1.57,2.30,3.84, 4.42, 4.57, and 5.00 pm. 

The pyrometer was calibrated over the temperature range 1200 to 1450 K using a large 

blackbody furnace. The linearity of the detectors was confirmed at even higher radiation levels, 

allowing the extrapolation to higher temperatures. The pyrometer measured the continuum 

radiation from the particles in the flame. The experimental explosion temperature was calculated 

by fitting the measured flame radiation data to the Planck equation as modified for nonblackbodies (Menzel, 1955; Cashdollar & Hertzberg 1982b): 

where His the observed radiation, a is an adjustable scale factor, E is the gray emissivity, h is 

Planck=s constant, c is the speed of light, h is the wavelength, k is Boltzmann's constant, and 

T is the absolute temperature. The combined scale factor a& is dependent on the flame 

emissivity, the fraction of the view field filled by the flame, the transmission through any 

unbumed dust on the sapphire window, and the detector sensitivity which varies with ambient 

temperature. Additional details on the infrared pyrometer and the Planck curve fitting procedure 

are in Cashdollar & Hertzberg (1 9 82b). The advantages of this rnultiwavelength p yrometer are 

that temperatures can be calculated without knowing the flame emissivity and that observations 

can be made through windows that are partially obscured by unburned dust. This pyrometer has 

been previously used to measure explosion temperatures of coal dust clouds in an 8-L chamber at , 

PRL (Cashdollar & Hertzberg, 1983), and it was also used to measure temperatures of coal dust 

flames on a flat-flame burner at Brigham Young University (Smoot & Horton, 1978). A similar 
pyrometer has been used by Mackowski, Allenkirch, Peck, & Tong (1983) to measure 

temperatures of coal dust flames on a burner. 

For each 20-L chamber explosion test, the data from the various instruments were 

collected by a custom, high-speed personal computer (PC) based data acquisition system (DAS) 

that has the capability of smoothing the data and searching for peaks. The DAS is programmed 

to conduct an iterative, nonlinear, least square fit of the pyrometer radiation data to the Planck 

equation in order to calculate the explosion temperature versus time. The number of pyrometer 

wavelengths used for the temperature calculation can be varied fiom three to six. After each 

explosion test, the pressure, dust probe transmission, and IR temperature data were displayed 

versus time. 

The test procedures for the 20-L chamber are briefly described here. Additional details 

of the 20-L chamber and test procedures are in Cashdollar & Hertzberg (1 982a) and Cashdollar 

(1 994). For most of the tests, the dust was placed on top of the dispersion nozzle rather than in 

the dust reservoir. After the dust and ignitor were placed in the chamber, the hinged top was 

closed and bolted, and then the chamber was partially evacuated to an absolute pressure of 

0.14 bar,a. Then a 0.3-s blast of dry air (fiom a 16-L reservoir at 8 to 9 bar pressure) dispersed 

the dust and raised the chamber pressure to about 1 bar,a at ignition. The experimental dust 

concentration reported for the 20-L chamber is the mass of dust divided by the chamber volume. 

The ibgn.ition sources used for the 20-L tests were electrically activated, pyrotechnic ignitors 

manufactured by Fr. Sobbe3 of Germany. For the 20-L tests reported in this paper, 2500-J 

ignitors were used. This is the energy recommended in ASTM El 5 15 (2005b) for the 

measurement of minimum explosible dust concentration. The energy is the nominal calorimetric 

value based on the mass of pyrotechnic powder in the ignitor. The 2500-J ignitor by itself 

produces a pressure rise of about 0.3 bar in the 20-L chamber. 

3. Experimental Results and Discussion 

The metal and nonmetal elemental dusts studied are shown in Fig. 2, highlighted at their 

positions within the periodic table. Nineteen elements were studied, including boron (B), 

carbon (C), magnesium (Mg), aluminum (Al), silicon (Si), sulfur (S) titanium (Ti.), 

chromium (Cr), iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), niobium (Nb), molybdenum (Mo), 

tin (Sn), hafnium (Hf), tantalum (Ta), tungsten (W), and lead (Pb). Table 1 lists the pertinent 

physical data for the dusts. The first columns list the atomic number, elemental symbol, and 

name, with a sample number listed for elements that were tested at more than one particle size. 

The next column lists the density for each element. The size data are listed in three ways. 

D(SEM) is an estimate of the size range for each dust as observed with a scanning electron 

microscope. Next is the surface mean diameter as calculated (Cashdollar, Liebman, & Conti, 

1981) from transmission measurements made with the optical dust probes in the 20-L chamber, 

using the following equation: 

3 Mention of any company or product does not imply endorsement by NOSH. 
where z is the transmission, Q is the extinction coefficient, C, is the mass concentration, p is the 

density of a particle, and Ds is the surface mean diameter of the dust particles. The last column 

lists the estimated median diameter from a combination of SEM, dust probe, and other size data 

(sieving and Coulter Counter analyses). 

Fig 2. Periodic table of the elements, highlighting the dusts tested. 

Scanning electron photomicrographs of six of the elemental dusts are shown in Fig. 3. A 

10 pm size marker is shown at bottom center of each photomicrograph. The magnesium (Mg) in 

Fig. 3A is a flake. The aluminurn (Al-3) in Fig. 3B, the iron (Fe-1) in Fig. 3D, and the tin (Sn) in 

Fig. 3F contain rounded particles, with many approximately spherical. The titanium (Ti) in 

Fig. 3C contains elongated or rodlike particles. The niobium (Nb) in Fig. 3E contains particles 

with sharp edges. The iron is the finest in size of these six dusts, as shown in the SEM 

photomicrographs and the listings in Table 1. 

Examples of the pressure and radiation (at 2.3 pm wavelength) traces versus time are 

shown in Fig. 4A and 4B. These are from an iron dust (Fe- 1 in Table 1) explosion in the 20-L 

chamber at a concentration of 600 gh!. The radiation reaches its maximum value before peak 

pressure because the flame front reaches the top wind0.w before combustion is completed in the 

lower parts of the chamber. The calculated temperature and standard deviation in temperature 

are shown in Fig. 4C and 4D. The maximum in the temperature trace (1'740 K) occurs at the 

same time as the maximum in radiation, as expected. The standard deviation in the calculated 

temperature is fairly high at the start of the explosion while the amount of radiation is low. The 

standard deviation drops to a value of 25 K when the temperature reaches its maiimurn value. 
Table 1. - Physical data for elemental dusts. 

Fig. 5 shows the curve fits to the Planck equation for four dust explosions at hgh dust 

concentrations where the flame would be optically thick. The flame radiation, H/as, is plotted 

versus the wavelength. The respective dust concentrations for the explosions were: 850 g/m3 for 

the magnesium (Mg), 800 g/m3 for the niobium (I%- l), 900 g/m3 for the iron (Fe- 1), and 

1200 g/m3 for the tin (Sn). The pyrometer radiation data in Fig. 5 were measured at the time of 

peak flame radiation at the 2.3 pm wavelength for each explosion. All four curves show a good 

fit of the data points to the Planck curve at a particular temperature. As shown in the figure, the 

relative radiation values at the two shortest wavelengths have the most effect on the temperature 

calculation. Of the four dusts, the magnesium explosion had the highest temperature - 2740 K, 

and the niobium had the next highest temperature - 2 1 8 0 K. The iron had a temperature of 

1840 K, and the tin had the lowest temperature - 1590 K. 

Element 

5 B 

6 C 

12 Mg 

13 Al 

14 Si 

16 S 

22 Ti 

24 Cr 

26 Fe 

28 Ni 

29 Cu 

30 Zn 

41 Nb 

42 Mo 

- 50 Sn 

72 Hf 

73 Ta 

74 W 

82 Pb 

Note: For the 

Dust 

Boron 

Carbon-I 

-2 

-3 

Magnesium 

Aluminum-I 

-2 

-3 

-4. 

Silicon 

Sulfur 

Titanium 

Chromium 

Iron-1 

-2 

Nickel-I 

Copper 

Zinc-1 

-2 

Niobium-1 

- 2 

Molybdenum 

Tin 

Hafnium 

Tantalum 

Tungsten-I 

-2 

Lead 

SEM size listing, 

Density, 

glcm3 

2.37 

-2.1 I1 

it 

1.74 

2.70 it 

It 

It 

2.33 

2.07 

4.54 

7.20 

7.87 It 

8.90 

8.96 

7,13 n 

8.57 

A 

10.2 

5.75 

13.31 

16.6 

19.3 I1 

11.3 

f denotes flake or 

D(sEM), 

I.lm 

0.5-1 0 

0.2-2 

4-10 f 

4-30 f 

10-50 f 

3-30 f 

5-1 5 

3-60 

20-100 r 

1-50 

-1 0-50 

- 10-60 r 

4-40 

1-8 

10-80 

3-8 

10-50 

6-50 f 

10-100 r 

4-50 

10-60 

3-6 

2-20 

4-30 

4-30 

#I 

5-25 

20-60 r 

plate-like particles 

Ds(%z), . 

Ci m 

2-6 

#2 

3-6 

5-7 

30-80 

-2-4 

-1 0-1 4 

40-70 

-60-1 80 

3-8 

-60-1 00 

-30- 1 20 

10-1 8 

4-1 1 

-60-1 60 

-30-45 

-30-60 

3-8 

-1 00-200 

-30-55 

-50-1 00 

10-23 

14-30 

-7-1 3 

-20-60 

1-3 

-30-1 00 

-1 00-1 000 

and r denotes 

Drned, 

!Jm 

-3 

# 1 

-1 

-4 

-1 6 

- 1 

-7 

-1 5 

-40 

-4 

-35 

-25 , 

-1 0 

-4 

-45 

-6 

-30 

-4 

-45 

-20 

-30 

-5 

-8 

-1 0 

-1 5 

# 1 

-1 0 

-40 

rod-like particles. 
A. Magnesium 8. Aluminum -3 

C. Titanium D. Iron - 1 

E. Niobium F. Tin 1,400 

100 

bF 

0 50 100 150 200 

TIME, ms 

Fig 3. Scanning electron microscope Fig. 4. Pressure, radiation, temperature, and 

images of metal dust particles. standard deviation in temperature. 

WAVELENGTH, pm 

Fig. 5. Radiation versus wavelength for 

explosions of four metal dusts 

Explosibility data for three aluminum dusts 

(Al-2, A1-3, and A1-4 from Table 1) as a function 

of dust concentration are shown in Fig. 6. The data 

were measured in the 20-L chamber, using 2,500-5 

ignitors. The explosion pressures, rates of pressure 

rise, and measured explosion temperatures are 

shown, as a function of dust concentration. The 

explosion pressure data in Fig. 6C are the maximum 

measured explosion pressures (with the pressure 

rise due to the ignitor subtracted) divided by the 

starting pressure (approximately 1 bar,a). This 

corrects for small variations in the starting pressure. 

Fig. 6B shows the size normalized maximum rate of 

pressure rise, (dPldt)~"~, for each explosion test. 

Note that the turbulence level is lower in the PRL 

20-L chamber than that recornrnended in ASTM 

El 226 (2005a) or NFPA 68 (2002), and therefore 

these (d~ldt)~"~ data are not recommended for 

vent sizing calculations. The rate of pressure rise , 

data are, however, useful as a relative measure 
of dust reactivity. For each dust size in Fig. 6, the explosibility data show that explosions are not 

observed below a certain dust concentration. This is the maurn. explosible concentration 

(MEC) or lean flammable limit (LFL), whch is measured by ASTM El 51 5 (2005b). The 

criterion used to determine the MEC value under these test conditions is an absolute explosion 

pressure of 2 bar,a or, equivalently, a pressure rise of 1 bar. From the data in Fig. 6, the MEC's 

with the 2,500-J ignitors for the A1-2, Al-3, and A1-4 dusts are about 90, 90, and 120 dm3, 

respectively. Additional MEC data at 5,000-5 ignitor energy for these and other dusts are in 

Cashdollar (1 994). For aluminum, the stoichiometric concentration (for formation of A1203) is 

Cstoich = 3 10 g/m3. The explosion pressure and (d~ldt)~'" do not reach their maximum values 

until concentrations well above Cstoich for the aluminum dust. At even higher dust 

concentrations, both P, and (d~ldt),,~'~~ level off as all of the oxygen in the chamber is 

consumed. However, the dusts show no evidence of a "normal" rich limit as would be observed 

for flammable gases. It should be noted that the efficiency of the dust dispersion is more 

uncertain at the higher dust concentrations. The maximum explosion pressure and rate of 

pressure rise are slightly higher for the A1-3 dust than for the larger A1-4 dust. The even finer 

sized Al-2 dust has a significantly hgher maximum explosion pressure and much higher rate of 

pressure rise than the other two aluminum dusts. The data (not shown in the figure) for the finest 

sized Al-1 dust were even higher. 

," zoo t El 

CONCENTRATION, i3/rn3 

Fig. 6. Explosibility data for thee 

aluminum dusts. 

I I 1 

A I 

Titanium - - 

*cc-- *' \ - , ,//-- 

/ 

\ - 

- / 

// 

/ 

,' v 8 

/ 

/ 

,' - /. 

5 ' - 

I I I 1 

Fig. 7. Explosibility data for titanium dust. 
The explosion temperatures shown in Fig. 6A were measured with the six-wavelength IR 

pyrometer. The pyrometer observed the continuum radiation from the particles, and 

temperatures were calculated from the best Planck curve fit to the IR radiation data. The 

temperature data shown in Fig. 6A were calculated at the time of maximum radiation. This was 

generally also the time when the standard deviation of the fit to the Planck curve was smallest 

and the time when the measured temperature was highest. The time of maximum radiation was 

usually earlier than the time of maximum pressure and probably corresponded to the time when 

the flame fiont reached the top sapphire window. The maximum measured particle temperatures 

for the A1-3 dust were -2,800 K, well below the maximum calculated adiabatic temperature, 

Tad,max = 4,060 K, for ideal combustion at constant volume. The maximum measured particle 

temperatures for the A1-4 dust were even lower, although there were only a limited number of 

measurements for this dust. These experimental temperatures are only those of the particles in 

the explosion, and the gas temperatures may have been different. There were no experimental 

temperatures measured for the two finest sizes of aluminum dust (Al-1 and A1-2) because a 

sapphire window cracked during early testing of the Al-1 dust, and the windows were replaced 

with steel blanks for later tests of both of these dusts. 

Explosibility data from the 20-L chamber for titanium dust as a function of concentration 

are shown in Fig. 7. The MEC for ths dust is about 70 g/m3, using the 2,500-J ignitors. As with 

the aluminum dusts, the maximum explosion pressures and rates of pressure rise for the titanium 

dust are at dust concentrations far above the stoichiometric value of 420 g/rn3. The maxirnurn 

measured particle temperatures for the titanium dust were -2,800 to 3,000 K, well below the 

maximum calculated adiabatic temperature, Tadpax = 3,990 K, for ideal combustion. The 

maximum experimental explosion temperatures for the titanium dust are comparable to those of 

the A1-3 dust in Fig. 6. 

Explosibility data for two iron dusts (Fe-1 and Fe-2 from Table 1) as a function of 

concentration are shown in Fig. 8. The data were measured in the 20-L chamber, using 2,500-5 

ignitors. The explosion pressures, rates of pressure rise, and measured explosion temperatures 

are shown as a function of dust concentration as in the previous two figures. From the data in 

Fig. 8, the MEC's for the Fe-1 and Fe-2 dusts are about 220 and 500 g/m3, respectively, using the 

2,500-5 ignitors. However, there is considerable uncertainty in these values, especially for the 

Fe-2 dust, due to the scatter in the data. For iron, the stoichiometric concentration (for formation 

of FezO3) is (&toich = 650 g/m3. The explosion pressure is close to its maxhum. value at slightly 

above Cstoich for both iron dusts. However, (d~/dt)v'/~ is considerably less than its maximum at 

Cstoich and reaches its maximurn value at almost twice Cstoich for the Fe-1 dust. At the higher dust 

concentrations, the maximum explosion pressure, P, and (d~/dt),,~'/~ level off as all of the 

oxygen in the chamber is consumed and the dusts show no evidence of a "normal" rich limit. It 

should be noted that the efficiency of the dust dispersion is more uncertain at the higher dust 

concentrations, particularly for the dusts with higher density. The explosion temperatures shown 

in Fig. 8A were measured with the six-wavelength IR pyrometer at the time of maximum 

radiation. The maximum measured particle temperatures for the Fe-1 dust were -1,800 K, well 

below the maximum calculated adiabatic temperature, Tad,max = 2,490 K, for ideal combustion at 

constant volume. The maximum measured particle temperatures for the Fe-2 dust were even 

lower - about 1,500 to 1,700 K. Again, these experimental temperatures are only those of the 

particles in the explosion, and the gas temperatures may have been different. For all three 
I 

A I I I I I I 

i ron 

Fe-2 

1 1 

x 

a 

I 1 I I I 

CONCENTRATION, g/m3 

Fig. 8. Explos'ibility data. for two iron dusts 

Niobium 

I 

CONCENTRATION, g/rn3 

Fig. 9. Data for two niobium dusts 

explosion characteristics shown in Fig. 8, the Fe-1 dust has higher values than the Fe-2 dust, 

showing that it is more reactive, due to its finer particle size. 

Explosibility data from the 20-L chamber for the two niobium dusts as a function of 

concentration are shown in Fig. 9. The data were measured in the 20-L chamber, using 2,500-J 

. ignitors. The explosion pressures, rates of pressure rise, and measured explosion temperatures 

are shown as a function of dust concentration as in the previous figures. From the data in Fig. 9, 

the MEC's for the Nb-1 and Nb-2 dusts are about 250 and 420 g/m3, respectively, using the 

2,500-J ignitors. However, there is considerable uncertainty in these values, especially for the 

Nb-2 dust, due to the scatter in the data. For both niobium dusts, the maximum explosion 

pressure and rates of pressure rise are found at dust concentrations at or above the stoichiometric 

value of Csfoich = 650 g/rn3. The explosion temperatures shown in Fig. 9A were measured with 

the six-wavelength IR pyrometer at the time of maximum radiation. The maximum measured 

particle temperatures for the Nb- 1 dust were -2,100 K, well below the maximum calculated 

adiabatic temperature, Tad,max = 3,540 K, for ideal combustion. There were no temperature 

measurements for the Nb-2 dust explosions. As for the other metal dusts, the finer sized niobium 

dust (3%-1) appears to be more reactive than the larger Nb-2 dust. 

Table 2 summarizes the explosibility data for the elemental dusts tested, based on the 

data curves of Figs. 6 through 9 and similar data for the other elemental dusts. The first column 

of the table lists the dust by elemental symbol and sample number. The second column lists the 

median size from the last column of Table 1. The next colurnn lists the MEC as measured in the 

20-L chamber using the 2500-5 ignitor. Additional MEC data using a 5000-J ignitor are in 
Table 2. - Explosibility data for elemental dusts 

Notes: NF means the dust was nonflammable or nonignitable. 

F means the dust was flammable but MEC could not be determined. 

Dust 

B 

C- I 

-2 

-3 

Mg 

Al- I 

- 2 

-3 

-4 

Si 

S 

Ti 

C r 

Fe-I 

-2 

N i 

CU 

Zn-I 

-2 

Nb-? 

-2 

Mo 

Sn 

H f 

Ta 

W- I 

-2 

~b 

Cashdollar (1 994) for most of these dusts. The listings' of NF (for nonflammable) for the Ni, Cu, 

Mo, W-2, and Pb dusts were actually determined using the stronger 5000-5 ignitor. The next two 

columns list the calculated (Pad,max) and experimental (P,) explosion pressures. The calculated 

maximum adiabatic explosion pressure is expressed as the ratio of the maximurn pressure to the 

starting pressure. The values were calculated from the NASA-Lewis computer code (McBride & 

Gordon, 1996) for constant volume combustion, using the thermodynamic properties from the 

JANAF tables (Chase, Davies, Downey, Frurip, & McDonald, 1985) and the Thermodynamic 

Properties of Individual Substances (TPIS) tables (Gurvich, Veyts, & Alcock, 1991). As 

discussed for Fig. 6, the listed experimental explosion pressure, P, is corrected for the pressure 

rise of the ignitor and normalized to a starting pressure of 1 bar,a. For the experimental 

explosion pressure, the values listed in Table 2 are the average of the lxghest three to six data 

points for each dust. Note that some of the experimental MEC and P, values have been revised 

Dmed 

Im 

-3 

#I 

- 1 

-4 

-1 6 

-1 

-7 

-1 5 

-40 

-4 

-35 

-25 

-1 0 

-4 

-45 

-6 

-30 

-4 

-45 

-20 

-30 

-5 

-8 

-8 

-10 

#I 

-1 0 

-40 

Pmax 

bar,a 

7.0 

5.5 

-5 

1.1 

8.5 

9.4 

9.0 

7.5 

6.6 

7.7 

5.0 

5.7 

-3 

4.5 

3.1 

1.0 

1.0 

4.4 

#2 

4.6 

-3.7 

1.0 

4.3 

5.2 

-4 

-3.3 

1.0 

I 

MEC 

glm3 

-1 10 

90 

F 

NF 

55 

85 

90 

90 

120 

200 

100 

70 

-F 

220 

-500 

NF 

NF 

300 

-NF 

F 

-420 

NF 

-450 

-180 

-400 

-700 

NF 

NF 

Cad,max 

gIm3 

130 

120 

A 

A 

530 

330 

A 

A 

A 

300 

280 

520 

610 

1000 

A 

900 

-2200 

1300 

800 

600 

1100 

1600 

1300 

1100 II 

3600 

Pad,max 

ratio 

10.5 

9,4 

A 

A 

15.7 

12.4 

A 

A 

A 

11.0 

7.9 

11.6 

9.0 

6.8 

A 

6.7 

4.2 

8.2 

A 

10.0 

A 

7.7 

7.1 

13.5 

10.6 

8.7 

A 

5.2 

Tad,max 

K 

3300 

2670 II 

I1 

3610 

4060 I1 

If 

11 

3240 

2360 

3990 

3170 

2490 11 

2400 

1520 

2070 If 

3540 !I 

' 2660 

2250 

4850 

3760 

31 00 If 

1830 

Pesuri 

bar 

at Tad,max 

0.027 

7x10-6 

A 

A 

-103 

50 

A 

A 

A 

0.36 

57 

3.4 

2.6 

0.029 

A 

0.010 

I .3~1 0-5 

14 

It 

7x10-4 

A 

4~10-~ 

0.032 

0.73 

2.6~10-4 

2.3~10-7 

A -- 

0.33 

p, fraction 

evaporated 

at Tad,max 

> I 

0.006 

0.006 

0.001 

>> 1 

>>I I1 

I: 

It 

> 1 

>> 1 

>I 

> 1 

> 1 

0.3 

0.8 

0.0002 

. >>I 

II 

0.02 

0.01 

0.0004 

>I 

->I 

0.006 

8x1 0-5 

8x1 0-6 - r > I 

Tpyrometer 

K 

1700V100 

1700V100 

1750V100 

NF 

2800V100 -- 

-- 

2800V1 00 

2400V I 00 

-2300V100 -- 

2850V150 

1850V50 

1800V50 

-1 60OV100 

NF 

NF 

1750V60 

-NF 

21 OOVIOO 

-- 

NF 

1550V50 

2400V60 

2350VlOO 

1550V50 

NF -- 

NF 
from those reported in Hertzberg, Zlochower, & Cashdollar (1992) and Cashdollar (1 994), based 

on additional data. 

The sixth column lists the concentration, Cad,mar, at which the calculated adiabatic 

temperature, Tad,maxi (seventh co~umn) is a maximum. The Cad,,, values are generally somewhat 

higher than the CStoiCh values. The adiabatic temperature is also calculated from the NASA-Lewis 

computer code (McBride & Gordon, 1996) for constant volume combustion. The calculated 

adiabatic temperatures listed in Hertzberg, Zlochower, & Cashdollar (1 992) and Cashdollar 

(1994) were for constant presswe combustion, which was more appropriate for studying the 

minimum explosible concentrations (lean flammability limits). 

The eighth column lists the ideal equilibrium vapor pressures of the elements at the 

maximum adiabatic flame temperatures that are calculated using standard thermodynamic tables 

of the Gibbs free energy of the condensed phases and the product vapors. These values are from 

the JANAF tables (Chase et al., 1985) and the TPIS tables (Gurvich et al., 1991). That vapor 

pressure is given by: 

P,,,, (bar) = 1 . 0 1 exp [- AGvRiAGc 1 , 

where AGv - AGc refers to the free energy difference between the vapor and condensed phases, R 

is the gas constant, and T is the calculated adiabatic flame temperature in kelvins. 

The ninth column gives an estimate of P, the maximum fraction of the element that is 

evaporated in the flame at the calculated maximum temperature. That estimate is based on the 

ideal evaporation rate into an effective vacuum given by the Hertz-Knudsen equation 

(Nesrneyanov, 1963). The further assumption of a flame residence time of 0.01 sec gives: 

where M is the gram-atomic or molecular mass of the vapor species, T is the flame temperature, 

d, is the particle diameter and p is the particle density (Hertzberg et al., 1992). 

The p-values so calculated are idealized estimates where the rate of evaporation in a 

reactive medium is taken to be equivalent to a vacuum in terms of an assumption of no vapor 

recondensation. The calculated flame temperature is also an idealized value which assumes 

perfect mixing, equilibrium conditions, and no heat loss to the walls. Because the assumed flame 

residence time of 0.01 sec is only an estimate, the calculated p-factor should be considered an 

order of magnitude estimate of the relative volatilities of the elements at the ideal maximum 

flame temperature. It is clear from the listed values in Table 2 that most of the elements should 

be totally volatilized at their maximum flame temperatures. The exceptions are carbon, niobium, 

molybdenum, tantalum, and tungsten - the most refractory elements, which have only a small 

fraction of material volatilized. Copper also has a very low volatility, but that is due to its low 

calculated flame temperature. The larger size of the iron (Fe-2) and the nickel are partially 

volatilized. 

The last column in Table 2 lists the experimental IR temperature, Tpyrometer, in kelvins. As 

for the experimental explosion pressure, the experimental temperature value listed in Table 2 is 

the average of the highest three to six data points for each dust. The uncertainty listed for the 

temperature values is based on the scatter in the data for multiple tests. The standard deviation 
for the fit to the Planck equation for an individual test was generally about half that value. There 

may be some additional uncertainty in the experimental temperatures listed in Table 2 due to the 

extrapolation of the blackbody calibration of the pyrometer for the higher temperatures or to the 

possible nongray emissivity of the dust explosion. 

As mentioned in the discussion of Fig. 6, experimental temperatures were not measured 

for the two finest sizes of aluminum dust (Al-1 and A1-2) because a sapphire window had 

cracked during early testing of the Al-1 dust, and the windows were replaced with steel blanks 

for later tests of these dusts. These two aluminum dusts had much faster rates of pressure rise 

than any of the other dusts (Cashdollar, 1994). The listed IR temperatures for the Al-3 and A1-4 

in Table 2 are based on the temperatures of the aluminum metal particles in the flame because 

the combustion product, A1203, is transparent at all of the IR pyrometer wavelengths. For many 

of the metals, the observed infrared temperatures are those of the incompletely volatilized metal 

particles since the metals have higher emissivities than the product oxides, which are often 

transparent in part or all of the infrared spectrum. There is no listed IR temperature for the Nb-2 

dust because there was insufficient material to measure the explosion temperature at hgh dust 

concentrations. Experimental particle temperatures were not measured for the sulfur dust 

because sulfur vaporizes at.a low temperature and the combustion product is a gas. 

Four of the elemental dusts (Ni, Cu, Mo, and Pb) listed in Table 2 could not be ignited, 

even with a 5,000-J ignitor. These four dusts showed almost no pressure rise beyond that of the 

ignitor. It is not surprising that the Cu and Pb did not ignite because their Cad,max values are very 

high and their TadYmau values are probably too low to sustain flame reaction. The Ni and Mo have 

reasonably high Tadymax values, similar to that of Fe and higher than those of zinc and tin, but the 

rate of reaction may be too slow to sustain flame propagation. The larger sizes of several other 

dusts (C-3,Zn-2, and W-2) could not be ignited, although the Zn-2 showed some slight activity 

in some of the tests. 

The dust with the lowest Tad,max value that produced explosions was zinc with 

Tad,max = 2070 K. However, the Zn is easily vaporized at flame temperatures. Considering its 

low Tad,max value, it is not surprising that the larger size of Zn could not be reproducibly ignited 

with the 2,500-J ignitor. 

For most of the experimental temperature calculations in Table 2, all six pyrometer 

channels were used (see Fig. 5), but in some cases only channels 1,2, 3, and 6 or channels 1,2, 

and 3 were used. For most of the elemental dusts, the emissivity was high. However, for the Si 

and Ta, the calculated emissivity was low (-0.1). For three dusts (B, Si, and Hf), only the first 

three pyrometer wavelen@&s were used to calculate the temperatures, because of higher . 

emissivity at the three longer wavelengths. Because of this, the temperatures for the boron, 

silicon, and hafnium are more uncertain than those of the other dusts. For the silicon, the 

combustion product Si02 would be transparent at the shorter IR wavelengths and opaque at the 

longer IR wavelengths. Therefore, the temperature measurement from the three shorter 

wavelengths would be the temperature of the silicon metal. The radiation at the longer three 

wavelengths would be a combination of radiation from the Si metal and Si02 metal oxide. 

4. Conclusions 

A summary comparison of the experimentally measured explosion pressures (P,) and 

the calculated adiabatic pressures (Pad,max) from Table 2 is shown in Fig. 10. The data points for 

the dusts are identified by the elemental symbols. The lines. identify values at 40%, 60%, 80%, 
and 100% of the calculated adiabatic pressures, The dusts whose experimental pressures are 

closest to the calculated adiabatic values are B, Al- 1, A-2, Si, and Fe-1 . These are also among 

the fmest sized dusts, with median diameters below 10 pm. 

EXPERIMENTAL PRESSURE RATIO 

Fig. 10. Comparison of calculated and 

experimental explosion pressures, with 

data points shown as element symbols 

1,000 1,500 2,000 , 2,500 3,000 

EXPERIMENTAL TEMPERATURE, K 

Fig. 1 1. Comparison of calculated and 

experimental explosion temperatures, 

with data shown as element symbols 

A summary comparison of the experimentally measured explosion temperatures 

(Tpvomekr) and the calculated adiabatic temperatures (Tad,pax) from Table 2 is shown in Fig. 11. 

The data points for the dusts are identified by the elemental symbols. The lines identify values at 

40%, 60%, 80%, and 100% of the calculated adiabatic temperatures. The two dusts whose 

experimental temperatures are closest to the he&,,, values are Zn-1 and Mg. These are also two 

of the dusts that are the most easily vaporized (see Table 2). 

The aluminum, silicon, titanium, and iron (Fe-1) also have high volatilities at their flame 

temperatures. These dusts have measured explosion temperatures that are at an intermediate 

position in the figure, at about 70% Tad,max. TWO of the dusts with high volatilities, boron and 

hafnium, have experimental temperatures that are only about half of the Tad,max-~al~e~. Thse 

were also dusts that had a larger than normal uncertainty in their temperature measurements, as 
discussed in the previous section. The flammable dusts (C, Nb, Ta, and W) that have very low 

volatilities are also those whose measured explosion temperatures are among the farthest from 

their Tad,max values. 

For dusts that were tested at more than one particle size, the experimental temperatures of 

the finer sizes were closer to the calculated temperatures, as expected. The larger sizes of 

aluminum (Al-4) and iron (Fe-2) had measured temperatures that were farther f?om the adiabatic 

values than the temperatures for the fmer sizes. The two sizes of carbon (C-1 and C-2) that 

produced explosions had about the same measured temperatures, but both were very fine sizes. 

In general, the dusts whose experimental temperatures are closer to the adiabatic values 

are those with the finer particle sizes, those that are more easily vaporized, and/or those that are 

intrinsically more reactive. However, the boron and hafnium appear to be exceptions since they 

are both fine size and easily volatilized. 

The experimental pressure and temperature data presented here will be useful in 

evaluating various models of metal dust combustion. Additional infomation on the mechanisms 

of metal combustion is in Hertzberg, Zlochower, & Cashdollar (1992) which discusses the 

volatility of the metals and in Kanury (1975) which has information on combustion properties of 

carbon and metals. The results reported in this paper provide information on the combustion 

characteristics of nineteen metal and nonmetal elemental dusts. The experimentally measured 

temperatures can help in the understanding of the combustion process. However, because of the 

effects of particle size, these data should not be considered to be representative of all dust 

samples of these elements. Ideally, explosion temperatures would be measured for identical, 

small particle sizes of the various elemental dusts in order to study their intrinsic combustion 

properties, but it would be difficult to obtain these. For the practical use of explosibility data for 

safety planning, it is important to test the particular size of dust that occurs at a particular 

industrial plant, rather than trying to extrapolate from other test data for a different size of the 

same material. 

Acknowledgements 

The authors acknowledge the assistance of G. M. Green of the PRL in the collection of 

the experimental data and C. E. Lucci for writing the software code to calculate the explosion 

temperatures from the pyrometer data. 

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