Explosion temperatures and pressures of metals and other 

elemental dust clouds 

Kenneth L. Cashdollar , Isaac A. Zlochower 

Piti&& Rwch Wrarory, NN~I fmnrti#uc for Qcmpafio~I Safrty ad Hwlrk, F.O. Box 18070, Cockram Mill Rwd, Pirrsbgh, PA 15236, USA 

The Pittsburgh Research Laboratory of thc National Institute for hptioml Safety and Health INIOSH) oonducted a study of the 

enplosibiiity or various mctals and ocher clemntal dusts, with a focus an the exgerinrenkd explosion temperetures. Tht data arc useful 

for understanding the bash of dust cloud combustioa, as we11 ar for evaluating ~xplosion hazards in the minerale and metala proassing 

industries. The dusts studied included boron, carbon, msgrmium, aluminum, silicon, sulfur, titanium. chromium, imn, nickel, copper, 

zinc, niobyum, molybdenum, tin, hafnium, tantalum, tungsten, and lead. The dusts were cboscn to WVK a wide rangc of phyPical 

propmie+from the more volatile marerials such as magnesium, aluminum, sulfur, and zinc to the highly "refractory" elements such as 

carbon, niobium, molybdenum, tantalum, and tungsten. These ftammability studies were conducted in a 20.1, chamber, using strong 

pyrotaEbniE ignitors. A unique multiwavetwgtb infrared pyrometer war used to measure thc temperatures. For the elnnenfal dusts 

studied, all ignited and burned as airdispersed dupz clouds except for nickel, copper, molykknum, and lead, The measured maximum 

explosion temperatures ranged from -155OK for tin and tungaten powders to --2800K for aluminum. magobum, and titanium 

pow-. The measured temperatures arc compared to the caIcdated, Itdiabatic ffanae temperatm. 

KeywP.. Dust; MeM; Explosion; Tunperatwe 

This paper will concentrate on the explosion characteristics of mctal and nonmetal elmental dusts. These dusts 

am important from both the practical and fundamental 

points of view. These mctal 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 physiwchemica1 processes occurring during 

combustion huse they arc pure, uniform substances with 

wcllcharacrcriad phw transitions and thernodynamic 

propertits. This is in marked contrast so the complex 

structures and heterogeneous chemistry associated with the 

cornbugtion of carbonaceous materials such as coal and 

grain dusts. The elemental dusts also display a mark& 

variation of reaction exothermicities in air and an 

enonnous range in vapor pressures at their respctivc 

flame temperatures. 

In the 1- and earlier, the US Bureau of Mines at 

Pittsburgh studied the explosibility of metal dusts wing a 

cylindrical 1.2-L Hartmann chamber (Jooobon, Coop, & 

Nagy, 1W} with an electric spark ignition source. More 

reccnt Pittsburgh Research taboratory (PRL) studia used 

a nearly-spherical 20-L chamber and much stronger 

pyrotechnic ignitors to study the explosibility of mctal 

and ohr elemental dusts (Chhdollrr, 1994; Hcrtzberg, 

Poehower, & Cashdollor, 1991,1992), The current paw is 

a continuation of these ~~~htudies, with rn emphasis on 

experimental explosion temperatures measured with a sixchannel infrared (IR) pyrometer. The current paper is also 

complcmeatary to an earlier experimental study of coal 

dust ntplosion temperatures (Carbdollrr & Hertrbcrg, 

'rite majonty of Lhis match was conducted in a dy 1- whw 

the PRL waa part of thc US Bureau of Minw bdorc itr transfer to thc 

National fnrtitute Tor Oaupaticlnd Safety and Hcnlth (NIOSH) an lB6. 
(lean flammable limits), maximum explosion pressures, and

rates of pressure rise. In this 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 IR 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,

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

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 3

4-in (19-mm)

diameter bolts which are not shown in the drawings. Two

PRL optical dust probes (Cashdollar, Liebman, & Conti,

1981; Liebman, Conti, & Cashdollar, 1977) 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 38-mm 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.

Fig. 1. Vertical and horizontal cross-sections of the 20-L explosibility test chamber.
flame radiation through the sapphire window in the top of

the 20-L chamber. The pyrometer uses room temperature

lead selenide photoconductive detectors and IR interference filters to measure flame radiation simultaneously at six

wavelengths: 1.57, 2.30, 3.84, 4.42, 4.57, and 5.00 mm. The

pyrometer was calibrated over the temperature range

1200–1450 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 and

therefore the temperature of the particles in the flame. The

temperature of the gases may have been higher. The

experimental particle temperature was calculated by fitting

the measured flame radiation data to the Planck equation

as modified for nonblackbodies (Cashdollar & Hertzberg

1982b; Menzel, 1955):

H 1⁄4 2ahc2

l5 exp hc=lkT   1  1

, (1)

where H is 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, l is the wavelength, k is Boltzmann

constant, and T is the absolute temperature. The combined

scale factor ae is dependent on the flame emissivity, the

fraction of the viewfield filled by the flame, the transmission through any unburned dust on the sapphire window,

and the detector sensitivity which varies with ambient

temperature. Additional details on the IR pyrometer and

the Planck curve fitting procedure are in Cashdollar and

Hertzberg (1982b). The advantages of this multiwavelength

pyrometer 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, Altenkirch, Peck, and 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 from three to six. After each explosion

test, the pressure, dust probe transmission, and IR

temperature data were displayed versus time.

and Hertzberg (1982a). 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 (from

a 16-L reservoir at 8–9 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 ignition sources used for the 20-L tests were

electrically activated, pyrotechnic ignitors manufactured by

Fr. Sobbe2 of Germany. For the 20-L tests reported in this

paper, 2500-J ignitors were used. This is the energy

recommended in ASTM E1515 (2006b) 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). The dusts highlighted by

cross-hatching (Ni, Cu, Mo, and Pb) are the dusts that did

not ignite. 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 et al., 1981) from transmission

measurements made with the optical dust probes in the

20-L chamber, using the following equation:

t 1⁄4 exp

3QCm

2rDS

 , (2)

where t is the transmission, Q is the extinction coefficient,

Cm is the mass concentration, r 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

2

Mention of any company or product does not imply endorsement by

NIOSH.
elemental dusts are shown in Fig. 3. A 10 mm size marker

is shown at bottom center of each photomicrograph.

The magnesium (Mg) in Fig. 3(A) is a flake. The aluminum

(Al-3) in Fig. 3(B), the iron (Fe-1) in Fig. 3(D), and the tin

(Sn) in Fig. 3(F) contain rounded particles, with many

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 mm

wavelength) traces versus time are shown in Fig. 4(A)

and (B). These are from an iron dust (Fe-1 in Table 1)

explosion in the 20-L chamber at a concentration of

600 g/m3

. The radiation reaches its maximum value before

peak pressure because the flame front reaches the top

window before combustion is completed in the lower parts

of the chamber. The calculated temperature and standard

deviation in temperature are shown in Fig. 4(C) and (D).

The maximum in the temperature trace (1740 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 maximum value.

Fig. 5 shows the curve fits to the Planck equation for

four dust explosions at high dust concentrations where

the flame would be optically thick. The flame radiation,

H/ae, 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 (Nb-1),

900 g/m3 for the iron (Fe-1), and 1200 g/m3 for the tin (Sn).

Table 1

Physical data for elemental dusts

Element Dust Density, g/cm3 D(SEM), mm DS (%t), mm Dmed, mm

5 B Boron 2.37 0.5–10 2–6 3

6 C Carbon-1 2.1 0.2–2 p2 p1

Carbon-2 2.1 4–10 f 3–6 1

Carbon-3 2.1 4–30 f 5–7 4

12 Mg Magnesium 1.74 10–50 f 30–80 16

13 Al Aluminum-1 2.70 3–30 f 2–4 1

Aluminum-2 2.70 5–15 10–14 7

Aluminum-3 2.70 3–60 40–70 15

Aluminum-4 2.70 20–100 r 60–180 40

14 Si Silicon 2.33 1–50 3–8 4

16 S Sulfur 2.07 10–50 60–100 35

22 Ti Titanium 4.54 10–60 r 30–120 25

24 Cr Chromium 7.20 4–40 10–18 10

26 Fe Iron-1 7.87 1–8 4–11 4

Iron-2 7.87 10–80 60–160 45

28 Ni Nickel-1 8.90 3–8 30–45 6

29 Cu Copper 8.96 10–50 30–60 30

30 Zn Zinc-1 7.13 6–50 f 3–8 4

Zinc-2 7.13 10–100 r 100–200 45

41 Nb Niobium-1 8.57 4–50 30–55 20

Niobium-2 8.57 10–60 50–100 30

42 Mo Molybdenum 10.2 3–6 10–23 5

50 Sn Tin 5.75 2–20 14–30 8

72 Hf Hafnium 13.31 4–30 7–13 10

73 Ta Tantalum 16.6 4–30 20–60 15

74 W Tungsten-1 19.3 p1 1–3 p1

Tungsten-2 19.3 5–25 30–100 10

82 Pb Lead 11.3 20–60 r 100–1000 40

Note. For the SEM size listing, f denotes flake or plate-like particles and r denotes rod-like particles.

Fig. 2. Periodic table of the elements highlighting the dusts tested.
The pyrometer radiation data in Fig. 5 were measured at

the time of peak flame radiation at the 2.3 mm 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 2180 K. The

iron had a temperature of 1840 K, and the tin had the

lowest temperature 1590 K. It should again be noted that

these are continuum temperatures of the particles in the

explosion flame and that the temperatures of the gases may

be higher.

Explosibility data for three aluminum dusts (Al-2, Al-3,

and Al-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 2500-J ignitors. The explosion pressures,

rates of pressure rise, and measured explosion temperatures

are shown as a function of dust concentration. The

absolute explosion pressure data in Fig. 6(C) are the

maximum measured explosion pressures (with the pressure

rise due to the ignitor subtracted) divided by the starting

Fig. 3. Scanning electron microscope images of metal dust particles.

Fig. 4. Pressure, radiation, temperature, and standard deviation in

temperature.
pressure (approximately 1 bar, a). This corrects for small

variations in the starting pressure. Fig. 6(B) shows the size

normalized maximum rate of pressure rise, (dP/dt)V1/3, for

each explosion test. Note that the turbulence level is lower

in the PRL 20-L chamber than that recommended in

ASTM E1226 (2006a) or NFPA 68 (2002), and therefore

these (dP/dt)V1/3 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 MEC or lean flammable limit (LFL),

which is measured by ASTM E1515 (2006b). 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 MECs with the 2500-J ignitors for the Al-2,

Al-3, and Al-4 dusts are about 90, 90, and 120 g/m3

,

respectively. Additional MEC data at 5000-J ignitor energy

for these and other dusts are in Cashdollar (1994). For

aluminum, the stoichiometric concentration (for formation

of Al2O3) is Cstoich 1⁄4 310 g/m3

. The explosion pressure and

(dP/dt)V1/3 do not reach their maximum values until

concentrations well above Cstoich for the aluminum dust.

At even higher dust concentrations, both Pmax and

(dP/dt)maxV1/3 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 Al-3 dust than for

the larger Al-4 dust. The even finer sized Al-2 dust has a

significantly higher 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.

The explosion temperatures shown in Fig. 6(A) 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. 6(A) 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 front reached

the top sapphire window. The maximum measured

particle temperatures for the Al-3 dust were 2800 K,

well below the maximum calculated adiabatic temperature,

Fig. 5. Radiation versus wavelength for explosions of four metal dusts.

Fig. 6. Explosibility data for three aluminum dusts.
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

higher. There were no experimental temperatures measured

for the two finest sizes of aluminum dust (Al-1 and Al-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 this dust is about 70 g/m3

, using the 2500-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/m3

. The maximum measured

particle temperatures for the titanium dust were

2800–3000 K, well below the maximum calculated

adiabatic temperature, Tad,max 1⁄4 3990 K, for ideal combustion. The maximum experimental explosion temperatures

Table 1) as a function of concentration are shown in Fig. 8.

The data were measured in the 20-L chamber, using 2500-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 two figures. From

the data in Fig. 8, the MECs for the Fe-1 and Fe-2 dusts

are about 220 and 500 g/m3

, respectively, using the 2500-J

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 Fe2O3) is Cstoich 1⁄4 650 g/m3

. The explosion pressure is close to its maximum value at slightly

above Cstoich for both iron dusts. However, (dP/dt)V1/3 is

considerably less than its maximum at Cstoich and reaches

its maximum value at almost twice Cstoich for the Fe-1 dust.

At the higher dust concentrations, the maximum explosion

pressure, Pmax, and (dP/dt)maxV1/3 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. 8(A) were measured with the six-wavelength IR

pyrometer at the time of maximum radiation. The

maximum measured particle temperatures for the Fe-1

Fig. 7. Explosibility data for titanium dust. Fig. 8. Explosibility data for two iron dusts.
temperatures for the Fe-2 dust were even lower—about

1500–1700 K. Again, these experimental temperatures are

only those of the particles in the explosion, and the gas

temperatures may have been different. For all the three

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

2500-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 MECs for the Nb-1 and Nb-2

dusts are about 250 and 420 g/m3

, respectively, using the

2500-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

Cstoich 1⁄4 650 g/m3

. The explosion temperatures shown in

Fig. 9(A) 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 2100 K, well below the maximum calculated

adiabatic temperature, Tad,max 1⁄4 3540 K, for ideal combus

than the larger Nb-2 dust.

Table 2 summarizes the explosibility data for the elemental dusts tested, based on the data curves of Figs. 6–9

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 column lists the

MEC as measured in the 20-L chamber using the 2500-J

ignitor. Additional MEC data using a 5000-J ignitor are in

Cashdollar (1994) 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-J

ignitor. The next two columns list the calculated (Pad,max)

and experimental (Pmax) explosion pressures. The calculated maximum adiabatic explosion pressure is expressed as

the ratio of the maximum 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 in Fig. 6, the listed experimental explosion pressure, Pmax, 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 highest three to six data points for each dust. Note that

some of the experimental MEC and Pmax values have been

revised from those reported in Cashdollar (1994) and

Hertzberg et al. (1992), based on additional data.

The sixth column lists the concentration, Cad,max, at

which the calculated adiabatic temperature, Tad,max,

(seventh column) is a maximum. The Cad,max values are

generally somewhat higher than the Cstoich values. The

adiabatic temperature is also calculated from the NASALewis computer code (McBride & Gordon, 1996) for

constant volume combustion. The calculated adiabatic

temperatures listed in Cashdollar (1994) and Hertzberg

et al. (1992) were for constant pressure combustion, which

was more appropriate for studying the MECs (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

Pequilð Þ1⁄4 bar 1:01 exp  DGv  DGc

RT  , (3)

where DGvDGc refers to the free energy difference

between the vapor and condensed phases, R is the gas

Fig. 9. Explosibility data for two niobium dusts.
constant, and T is the calculated adiabatic flame temperature in Kelvin.

The ninth column gives an estimate of b, 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 (Nesmeyanov, 1963). The further assumption of a flame residence

time of 0.01 s gives:

b 1⁄4 2:6

Pequil

dpr

M

T

 1=2

, (4)

where M is the gram-atomic or molecular mass of the

vapor species, T is the flame temperature, dp is the particle

diameter and r is the particle density (Hertzberg et al.,

1992).

The b-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 s is only

an estimate, the calculated b-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

particle temperature, Tpyrometer, in Kelvin. 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.

g/m3

ratio

bar, ag/m3Tad,maxevaporated at

Tad,max

B 3 110 10.5 7.0 130 3300 0.027 41 17007100

C-1 p1 90 9.4 5.5 120 2670 7  106 0.006 17007100

C-2 1 F 9.4 5 120 2670 7  106 0.006 17507100

C-3 4 NF 9.4 1.1 120 2670 7  106 0.001 NF

Mg 16 55 15.7 8.5 530 3610 103 b1 28007100

Al-1 1 85 12.4 9.4 330 4060 50 b1 –

Al-2 7 90 12.4 9.0 330 4060 50 b1 –

Al-3 15 90 12.4 7.5 330 4060 50 b1 28007100

Al-4 40 120 12.4 6.6 330 4060 50 b1 24007100

Si 4 200 11.0 7.7 300 3240 0.36 41 23007100

S 35 100 7.9 5.0 280 2360 57 b1 –

Ti 25 70 11.6 5.7 520 3990 3.4 41 28507150

Cr 10 F 9.0 3 610 3170 2.6 41 1850750

Fe-1 4 220 6.8 4.5 1000 2490 0.029 41 1800750

Fe-2 45 500 6.8 3.1 1000 2490 0.029 0.3 16007100

Ni 6 NF 6.7 1.0 900 2400 0.010 0.8 NF

Cu 30 NF 4.2 1.0 2200 1520 1.3  105 0.0002 NF

Zn-1 4 300 8.2 4.4 1300 2070 14 b1 1750760

Zn-2 45 NF 8.2 p2 2070 14 b1 NF

Nb-1 20 F 10.0 4.6 800 3540 7  104 0.02 21007100

Nb-2 30 420 10.0 3.7 3540 7  104 0.01 –

Mo 5 NF 7.7 1.0 600 2660 4  106 0.0004 NF

Sn 8 450 7.1 4.3 1100 2250 0.032 41 1550750

Hf 8 180 13.5 5.2 1600 4850 0.73 41 2400760

Ta 10 400 10.6 4 1300 3760 2.6  104 0.006 23507100

W-1 p1 700 8.7 3.3 1100 3100 2.3  107 8  105 1550750

W-2 10 NF 8.7 1.0 1100 3100 2.3  107 8  106 NF

Pb 40 NF 5.2 1.1 3600 1830 0.33 41 NF

Notes. NF means the dust was nonflammable or nonignitable. F means the dust was flammable but MEC could not be determined.
As mentioned in the discussion of Fig. 6, experimental

temperatures were not measured for the two finest sizes of

aluminum dust (Al-1 and Al-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 Al-4 in Table 2 are based on the temperatures of

the aluminum metal particles in the flame because the

combustion product, Al2O3, is transparent at all of the IR

pyrometer wavelengths. The listed temperature of

28007100 K for the Al-3 is the highest temperature that

can be measured for the aluminum particles because the

boiling point of aluminum is 2740 K (Weast, 1988) to

2800 K (Chase et al., 1985).

For many of the metals, the observed IR 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 IR

spectrum. There is no listed IR temperature for the Nb-2

dust because there was insufficient material to measure the

explosion temperature at high 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 5000-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 Tad,max values are probably too low to sustain flame

reaction. The Ni and Mo have reasonably high Tad,max

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 1⁄4 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 2500-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 wavelengths 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

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 SiO2 metal oxide.

4. Conclusions

A summary comparison of the experimentally measured

explosion pressures (Pmax) 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, Al-2, Si, and Fe-1. These are

also among the finest sized dusts, with median diameters

below 10 mm.

A summary comparison of the experimentally measured

explosion temperatures (Tpyrometer) and the calculated

Fig. 10. Comparison of calculated and experimental explosion pressures,

with data points shown as element symbols.
80%, and 100% of the calculated adiabatic temperatures.

The two dusts whose experimental temperatures are closest

to the Tad,max values are Zn-1 and Mg. These are also two

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

The aluminum (Al-3), silicon, titanium, iron (Fe-1), and

tin 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-values. These 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

than the temperatures for the finer 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 information on the

mechanisms of metal combustion is in Hertzberg et al.

(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 19 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.

Acknowledgment

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.

Disclaimer. The findings and conclusions in this report

are those of the authors and do not necessarily represent

the views of the National Institute for Occupational Safety

and Health.

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