metric standard deviation, o,. For a monodisperse size
distribution, o, = 1.0. The larger @, is, the broader the
size distribution. The sixth row lists the mass median
particle diameter, Dyeg, Which is the 50% point on the
cumulative mass distribution. This is perhaps the most
commonly reported size characteristic of a dust. The
next two rows list the arithmetic mass or volume mean
particle diameter, Dy,, and the arithmetic surface mean
diameter, Ds. The next two rows list Ds as calculated
from the optical dust probe transmission (%7T) data as
the dust is dispersed in the 20 litre chamber, as described
in the next paragraph. One purpose for listing the various
size parameters in Tables 2 and 3 is to show that no
one parameter is sufficient to characterize a dust. The
explosibility data in the remaining rows will be dis-
cussed later in the paper.

The 20 litre chamber test data for the PPC dust (Pgh-
3 in Table 2) with a 2500 J ignitor are shown in Figure
7 as a function of dust concentration. These data were
collected during many series of tests over a period of
six years. At the top of the figure, the transmission data
measured by the optical dust probes are shown. As
described in reference 23, the transmission T is related
to the mass concentration C,, by Bouguer’s law:

T = exp(-30C,,/2pDs),

where Q is a dimensionless extinction coefficient, / is
the path length, p is the density of a particle, and Dg is
the surface mean particle diameter. The data in Figure

8

%
8

8

TRANSMISSION,

3 bar-m/s

(apvat) vTM

PRESSURE, bar, a

° 200 “400 00800-1000
DUST CONCENTRATION, g/m

Figure 7 Explosibility data for Pittsburgh _(high-volatile
bituminous) pulverized coal dust in the 20 litre chamber

7A generally follow the expected linear relationship on
this semi-logarithmic plot. At the highest dust concen-
trations, there is some upward curvature, probably due
to increased agglomeration. The scatter in the data is
probably due to variations in the agglomerated particle
size of the air-dispersed dust. The surface mean particle
diameter can be calculated from the measured trans-
mission data (Figure 7A and similar graphs), using the
above equation. The data are reported as Ds(%7) in
Tables 2 and 3. In general, Ds measured by the particle
size analysers is within the range of Dg(%T) measured
by the dust probes.

In Figure 7B, (dP/dt)V'" is the volume-normalized
maximum rate of pressure rise, Note again that the turbu-
lence level is lower in the USBM 20 litre chamber, and
therefore the (dP/dt)V' data are not recommended for
the sizing of vents. The data are, however, useful as a
relative measure of explosion hazard. At the higher tur-
bulence level recommended in ASTM Standard E1226,
the maximum (dP/dr)V'% data for this Pittsburgh coal
would be roughly three times higher. The maximum
absolute explosion pressures (with the pressure rise of
the ignitor subtracted) are shown in Figure 7C. Because
there are small variations from test to test in the chamber
pressure at the time of ignition, these data were nor-
malized to a starting pressure of 1.0 bar (a). The data in
Figure 7 show that, below a certain dust concentration,
explosions are not observed. This is the minimum
explosible concentration (MEC) or lean flammable limit
(LFL). For the PPC, the measured MEC in the 20 litre
chamber is ~80 g mTM. This laboratory MEC value is the
same as the ~80 g mTM> MEC value'':'” measured for the
PPC in a 1 m* chamber using a 10 kJ ignitor and is com-
parable to the ~60 g mTM value!®!” measured for the PPC
in the USBM Lake Lynn Experimental Mine, using a
152 m long dusted zone and a methane ignition zone. At
higher dust concentrations in Figure 7, the maximum
pressures and rates of pressure rise level off as all of
the oxygen in the chamber is consumed, but there is no
evidence of a rich limit for the PPC dust.

A summary of the 20 litre chamber pressure versus
concentration data for the PPC is shown in Figure 8,

T T T

T
IN
ch,
ao Polyethylene
my L£.—.—.—_4
ty _
toy
eb i Ye
o |i 4
Eel i / |
elif fi
§ ue ft
Bab uy H 4
ra ef 4
ela]
3b t 4
H
H
2b t 4
1
1
1 1 1 \ 1
0 100 200 300 400 500

CONCENTRATION, g/m?

Figure 8 Explosibility of Pittsburgh high-volatile bituminous
coal dust, compared to polyethylene dust and methane gas


the dust becomes more easily ignited as more CH, is
added. Therefore, the curvature is more likely an effect
of ignitability rather than an effect of flammability. Ide-
ally, the true mixing relationship would be determined in
a much larger chamber, such as a 1 m3 chamber, where a
very strong ignition source could be used for the dusts
without overdriving the CH, gas. For most practical situ-
ations for mixtures of hydrocarbon dusts and gases, the
linear mixing law of Le Chatelier would be sufficient.
The approximately linear relationship for the lean limits
of dust and gas mixtures was also observed by Amyotte
and others**° using 5 kJ ignitors in a 26 litre chamber.

In the mining industry, coal dust explosions are pre-
vented by the addition of inert limestone rock dust to
the deposited coal dust. Therefore, the inerting of coal
dust by the addition of limestone rock dust was also
studied in the laboratory chamber, and the results were
compared to those from full-scale experimental mine
tests. Figure 16 shows data on the amount of limestone
rock dust required to inert various sizes of Pittsburgh
(hvb) and Pocahontas (Ivb) coals from Tables 2 and 3.
The vertical axis shows the amount of rock dust in the
coal and rock dust mixture. The horizontal axis is the
mass median particle diameter of the coals. For these
tests, regular limestone rock dust was used instead of the
fluidized rock dust used for previous USBM laboratory
studies'*"'5'7-19. The limestone rock dust had 75%
minus 200 mesh and Dineg = 25 pm. The measurements
were made in the 20 litre chamber using 5000 J ignitors.
The stronger ignitors were used for the inerting tests
because the heavily inhibited, high dust concentrations
were intrinsically more difficult to ignite. At each rock
dust percentage, tests were run over a wide range of coal
dust concentrations (usually 200-800 g mTM*) to deter-
mine the worst case. Tests were made in rock dust
increments of 5% and the final amount to inert was
interpolated for the values reported in the last row of
Tables 2 and 3 and the data shown in Figure 16. For
each size of coal dust, more rock dust is required to inert
the high-volatile Pittsburgh coal than is required for the
low-volatile Pocahontas coal. As with other aspects of
explosibility, the finer sized dusts are the more hazard-
ous, requiring more rock dust in the mixture to be
inerted. In some cases, the effect of particle size domi-
nates over volatility as the finer sizes of Pocahontas coal
require more rock dust than the larger sizes of Pittsburgh
coal. The data reported here are consistent with those

90 T T T T T T T

Nonexplosible

Coat, tub
Coal, vb:

Explosible

ROCK DUST TO INERT, %
3

sol
Oo 10 20 30 40 80 60 70 80

PARTICLE DIAMETER, pm

Figure 16 Inerting of high-volatile Pittsburgh and low-volatile
Pocahontas bituminous coals by limestone rock dust as a func-
tion of coal particle size

reported by Amyotte and others** from tests in a
26 litre chamber.

A comparison of USBM experimental mine and
20 litre laboratory limestone rock dust inerting data for
various coal dusts is shown in Figure 17. The vertical
axis shows the amount of rock dust in the mixture neces-
sary to inert the coals. The horizontal axis is the moist-
ure-ash-free volatility of the coals. The data are for vari-
ous bituminous coal dusts from low to high volatility.
The dashed line is a summary of previous inerting
tests*!46 of various pulverized coals in the USBM Bru-
ceton Experimental Mine (BEM). The solid line is a
summary of recent data'S*? for pulverized coals from
the USBM Lake Lynn Experimental Mine (LLEM). The
filled circles are 20 litre laboratory data for coals dusts
with mass median diameters of 40-50 jem, similar to the
sizes of the coals tested in the BEM and the LLEM. The
open circles are 20 litre data for finer sized coals with
Dyes = 20-35 pum. The uncertainties in the mine and
laboratory data are of the order of +3% rock dust con-
tent. The laboratory and experimental mine inerting data
are also consistent with those from the German Experi-
mental Mine Tremonia**°. There is a general trend that
the LLEM tests show a somewhat higher rock dust
inerting requirement than the BEM data or the laboratory
chamber data for the same sized coals. However, the
laboratory data do show sufficiently good agreement
with the mine data such that the 20 litre chamber can be
used for preliminary testing before full-scale mine test-
ing and to study a much wider range of coal sizes and
volatilities than could be done in the experimental mines.

Conclusions

The data reported in this paper show that the USBM
20 litre laboratory chamber is useful for studying a wide
range of explosion characteristics of coals and that the
data show reasonably good agreement with those from
large-scale tests in the experimental mines, Data for vari-
ous coals show that higher volatile coals and finer sized
coals are more hazardous. Because of the importance of
particle size, it is critical that representative samples of
dusts be collected for explosibility evaluation.

90 >

80

70

key

AMOUNT TO INERT, %

coh ew |
~>> BEM
2 Sm soem)
Sa tm | 20-L
so © 2010 354 4
40 - -
10 20 30 40 50

VOLATILITY, %

Figure 17 Rock dust inerting requirements for coal dusts of dif-
ferent volatilities, showing comparison between 20 litre labora-
tory chamber data, Lake Lynn experimental mine (LLEM) data,
and Bruceton experimental mine (BEM) data



88

Pittsburgh coal

MAXIMUM:
(ap/at) v8 bar m/s
6

8

xo

MAXIMUM
PRESSURE, bar, a

MEC, g/m*

io 100 1/000
PARTICLE DIAMETER, pm

Figure 10 Effect of dust particle size on the explosibility of
Pittsburgh coal, « for narrow distributions and x for broad distri-
butions

a combination of narrow distributions, and these data
show that it is the finer particles in a broad distribution
that contribute the most to its hazard. The MEC data
points for the broad size distributions show little differ-
ence from the narrow size distribution data below Djrea
= 100 um. However, the broad size distributions ignite
and propagate at larger Dea Sizes than the narrow size
distributions. The pressure and dP/dt data for the broad
size distributions are somewhat higher than those for the
narrow size distributions, even in the D,,.4 range of 20—
100 ym. This is probably due to the effects of the tail
of fine particles in the broad size distributions. These
fine particles were removed from the narrow size distri-
butions. The main conclusion of Figure 10 is that par-
ticle size has an important effect on the explosibility of
coal dusts.

‘A separate series of tests were made to investigate
possible changes in particle size during dispersion in the
20 litre chamber. That is, do the coal particles break up
during dispersion, resulting in a smaller average size for
the tested dust? Minus 200 mesh samples of Pittsburgh
and Pocahontas coals were dispersed in the 20 litre
chamber. Coulter counter particle-size analyses were
made of samples of the original dust and of the dust that
had settled to the bottom of the 20 litre chamber after
the standard dispersion procedure. For each dust, Dincu
and Dw decreased by about 5% and Ds decreased by
about 10%, relative to the size of the original dust. These
decreases are barely significant, considering the uncer-
tainties in sampling and particle-size analysis. Some other
dusts may fragment significantly during dispersion. In pre-
vious tests with the USBM 20 litre chamber, larger TNT
particles fragmented to less than half their original size
during dispersion..” In Table 2, when comparing Dg from

the particle analysers with Ds(%7) from the dust probes,
there is also some evidence that the -40 mesh Pittsburgh
coal has fragmented somewhat during dispersion.

Examples of scanning electron microscope (SEM)
photomicrographs of coal before and after explosions are
shown in Figure I]. The dust was a narrow size distri-
bution of Pittsburgh coal with Dyea = 23 wm. The orig-
inal unburned particles are shown at two magnifications
on the left side of the figure. They are compared to the
‘bumed’ post-explosion particles in the four frames on
the right side of the figure. The burned particles are
mainly char residues that are often larger than the orig-
inal particles. In the flame, the bituminous coal particles
become molten as shown by the rounded particles on the
right. Some particles form cenospheres. The particles
also devolatilize in the flame. The volatiles are emitted
through the ‘blow holes’ seen in the char residues.
Additional SEM photomicrographs for various post-
explosion residue dusts are shown in reference 38.

One of the ways to prevent a dust explosion is to inert
the atmosphere so that there is insufficient oxygen for a
flame to propagate. One of the most common inerting
gases is nitrogen, which is the main constituent of air.
To determine the limiting oxygen concentration for coal
dust explosions in the 20 litre chamber with 2500 J igni-
tors, the dusts were dispersed with various oxygen—
nitrogen mixtures instead of normal air at 20.9% O,.
Figure 12 is an example of the reduced-oxygen data for
PPC. The explosions are denoted.by the solid circles and
the non-explosions by the open circles. The data for PPC
in air are shown at the top of the figure at 20.9% O,.
At the bottom of the figure, explosions still occur at 14%
down to 12% O,. Based on the data in Figure 6, the
minimum oxygen concentration (MOC) for propagation
of an explosion of this PPC dust is 11 + 0.5% O3. MOC
data for two sizes of Pittsburgh coal are listed in Table
2. The data show only a slight decrease in the MOC
value at the finer size.

The effect of temperature on the ignitability and
explosibility of the Pittsburgh coal dust is shown in
Figure 13. The dotted curve (from reference 39) shows
the auto-ignition temperature for the PPC as a function
of dust concentration, as measured in the USBM 6.8 litre
furnace®®. The dotted curve is the temperature boundary
between the upper region of the graph where the coal
dust cloud will thermally auto-ignite and the lower
region where the dust may be flammable but does not
thermally auto-ignite. At the higher concentrations in the
figure, the thermal ignition boundary levels off to a value
of ~530°C. This is the minimum auto-ignition tempera-
ture (MAIT) for the Pittsburgh coal as measured in the
6.8 litre furnace. This furnace and test method are listed
in ASTM standard test E1491.

Explosibility tests were also conducted in the 20 litre
chamber at temperatures above ambient but below the
temperature at which the dust would auto-ignite. For
these tests, the 20 litre chamber was wrapped with elec-
trical heater tape and insulated to reach the elevated tem-
perature. The 320 um thermocouple measured the set
temperature of the 20 litre chamber before the test. The
25 wm thermocouple was used during initial tests to
measure the temperature during dispersion of the dust
and air. The solid circle data points show MEC data for


where the data are compared to those for polyethylene
dust and methane (CH,) gas. Only the average curves
are shown and not the data points for individual tests as
in Figure 7. The CH, data are from reference 10, using
500 to 1000 J ignitors because the CH, is much easier
to ignite than the dusts. The two dusts were tested using
2500 J ignitors. The curve for the polyethylene dust is
similar in shape to that for the PPC, except that the poly-
ethylene has a lower MEC and a slightly higher
maximum explosion pressure. This is because the poly-
ethylene has a volatility of 100% compared to 37% vola-
tility for the coal, and it has a higher H:C ratio than the
coal. The methane gas has a LFL or MEC similar to
that of the polyethylene. This shows that the completely
volatilizable polyethylene reacts similarly to the methane
gas at low concentrations'®. For hydrocarbon gases or
dusts, the measured LFL or MEC generally corresponds
to a calculated adiabatic temperature’? of 1300-1500 K.
This is the ‘limit flame temperature’, which is the mini-
mum temperature needed to keep a flame propagating.
Experimentally, the LFLs of most hydrocarbon gases are
easy to measure because the gases have low ignition
energies. Much stronger ignition energies are needed for
dusts!°"", However, if too strong an ignition energy is
used relative to the test chamber volume, the result will
be an overdriven ignition''. A standard method for meas-
uring the MEC of a dust cloud is ASTM E1515, which
is based on the USBM test procedures described here.

In contrast to the two dusts in Figure 8, the methane
gas shows a rich limit. For the dusts, the maximum
pressures level off at concentrations of 200-300 g m=
as all of the oxygen in the chamber is consumed. At
even higher dust concentrations, although the mixtures
are nominally fuel-rich, the pressure nevertheless
remains constant. The normal rich limit observed for
hydrocarbon gases such as CH, is not observed for the
dusts. An explanation of this effect, at least for many
dusts, is that the solid-phase fuel must first devolatilize
before it can mix with the air'?. As soon as sufficient
volatiles are generated to form a stoichiometric concen-
tration of volatiles in air, the flame front propagates
rapidly through the mixture before excess fuel volatiles
can be generated.

Figure 9 shows explosibility data for the high-volatile
PPC dust and for polyethylene dust at very high concen-
trations. This shows that these dusts explode even at con-
centrations beyond 4000 gm. There is, of course, an
increased uncertainty in the dust dispersion effectiveness
at these very high concentrations. The decrease in press-
ure at higher concentrations may be due to the increased
heat sink of the very large dust concentration. The
decrease in dP/dt at higher concentrations may be due
to the increased heat sink effect and to the possible
decrease in turbulence due to the large mass of dust.
Deguingand and Galant had previously observed an
apparent upper limit at ~4 kg mTM for coal dust, but this
may have been only an ignitability limit because they
used an electric spark ignition source that was much
weaker than the 2500 J Sobbe ignitor used here. In early
large-scale USBM tests (reported by Nagy) in the Bru-
ceton Experimental Mine, an upper limit of ~S kg m~*
was observed for a coarser coal dust. Cybulski* reported
an upper limit of ~1 kg mTM for pulverized coals in tests

(aP/at) “3 bar + m/s
8ss ss

°

no

PRESSURE,

o 7900 2000 3000 “4000, 3900
CONCENTRATION, g/m?

Figure 9 Explosibility data for high-voiatile bituminous coal
dust, compared to those for polyethylene dust

at the Polish Experimental Mine Barbara, Mintz**
observed some upper limits under conditions of reduced
oxygen and at large coal particle sizes. The general con-
clusion of all of this research is that, for most practical
purposes, these dusts have no rich limit of explosibility.
Additional MEC, Prax, and (dP/dr)V" data for vari-
ous size distributions of the Pittsburgh and Pocahontas
bituminous coals are found in Tables 2 and 3. In general,
the high-volatile Pittsburgh coals have lower MEC
values, higher explosion pressures, and higher
(dP/dt)V'* values than the low-volatile Pocahontas coals
of similar size. The effect of volatility on the MEC was
also noted by Amyotte and others***° in a 26 litre
chamber. However, the finest sizes of Pocahontas coal
in Table 3 have explosibility data comparable to those
of the larger sizes of the Pittsburgh coal in Table 2. This
shows that particle size is at least as important as vola-
tility in understanding the explosion hazards of coals.
The previous explosibility data were measured using
rather broad size distributions of the coals. The USBM
has also studied the effect of particle size on the explos-
ibility of dusts by using narrow size distributions, such
as those in Figure 6. Figure 10 shows explosibility data
from the 20 litre chamber for Pittsburgh coal dust as a
function of mass median particle diameter, using 2500 J
ignitors. The data for the narrow distributions are shown
as solid circles and a solid curve. These data for narrow
distributions are compared to the data points (x) for the
broad distributions from Table 2. The MEC values in the
bottom section of the figure are relatively independent of
particle size for the finer sizes, At the larger sizes, above
100 jum, the MEC values increase with particle size until
a size is reached that cannot be ignited. The top two
sections of Figure 10 show that the maximum pressures
and rates of pressure rise are found at the finest sizes
tested. The pressures decline slowly and the pressure rise
rates decrease faster with increasing particle size. At
some size above 200 jum, the narrow sizes of Pittsburgh
coal dust can no longer be ignited by the 2500 J ignitor.
These data are typical for narrow size distributions of
carbonaceous fuel dusts. A broad size distribution is just



At higher dust concentrations, the maximum
explosion pressure for the Pittsburgh coal was also meas-
ured at elevated temperature in the 20 litre chamber,
using 2500 J ignitors. At near ambient temperature, Prax
for the PPC was 6.6 bar (a). At an elevated temperature
of ~180°C, Pmax Was 4.8 bar (a). This observation of
lower explosion pressures at elevated temperature was
also reported previously by Wiemann‘. The inverse
relationship of explosion pressure with initial tempera-
ture is expected from the perfect gas law because there
are fewer oxygen molecules at elevated temperature to
react with the coal. The ratio of measured maximum
explosion pressure (absolute) at ~180°C (453 K) to Pingg
at near ambient temperature is the same as the ratio of
ambient to elevated temperature in Kelvin. For this com-
parison, the measured value of ~60°C (333 K) has to be
used for the tests at near ambient temperature, as dis-
cussed in the section on ‘Experimental equipment and
test procedures’. This analysis shows that the maximum
explosion pressure would actually be ~13% higher if the
ambient temperature tests were made at ~22°C (295 K).
This may explain, in part, why the measured maximum
explosion pressures in the [m= chamber are ~30%
higher than those measured in 20 litre chambers”!. In the
I-m® procedure, much less air is injected into the
chamber than for the 20 litre procedure. Therefore, the
temperature rise during dispersion would be expected to
be much less in the | m* chamber, so the tests would be
closer to ambient.

The minimum oxygen concentration for the Pittsburgh
coal was also measured at elevated temperature in the
20 litre chamber. The measured MOC value for the PPC
decreased from ~11% at near ambient temperature to
~10% at ~180°C. This observation of lower MOC
values at elevated temperature was also reported pre-
viously by Wiemann“.

The effect of initial chamber pressure'” on the MEC
or LFL is shown in Figure /4. When the methane con-
centration is expressed in vol% in Figure 14A, the LFL.
is shown to be constant as the pressure varies from 0.5
to 3 bar. When the CH, is expressed in mass concen-
tration in Figure 14B, the LFL is shown to vary linearly
with pressure. In Figure 14C, the LFLs of the Pittsburgh
coal and polyethylene dusts also vary linearly with press-
ure.

Another important factor in the explosibility hazard
of a dust is the possible co-presence of a flammable gas.
Hybrid mixtures of a combustible dust (coal) and a
flammable gas (CH,) were also studied in the 20 litre
chamber, using 2500 J ignitors. Data for the low-volatile
Pocahontas coal (Poc-1 in Table 3) are shown in Figure
15A, and the high-volatile Pittsburgh coal (Pgh-3 in
Table 2) data are shown in Figure /5B. The flammable
limits for mixtures of coal and CH, are shown by the
data points and solid curves. The areas above and to the
right of the curves are explosible or flammable and the
areas below and to the left of the curves are non-explos-
ible or non-flammable. The data for mixtures of Pitts-
burgh coal and CH, in Figure 15B show a linear or near-
linear mixing relationship similar to Le Chatelier’s law
for hydrocarbon gases". All of the solid circle data
are for 2500 J ignitors. The repeatability of the measured
dust MEC values is approximately +10 g m- in Figure

LFL, g/m?

MEC or LFL, g/m?

1 L
° 1 2 3

INITIAL PRESSURE, bar

Figure 14 Effect of pressure on the explosibility of Pittsburgh
coal, compared to polyethylene dust and methane gas

8 T T

A. Coal, vb

4 4

3h Explosible 4
z pl
Zeb 4
ez
5 Nonexplosible 1
Ps 5 : :
& B. Coal, bb
z: |
=> 7
g Explosible

2p a

th 4

Nonexplosible
° 30 100 150

COAL DUST CONCENTRATION, g/m>

Figure 15 Minimum explosible concentrations of hybrid mix-
tures of coal dust and methane gas

15. The measured LFL for the pure CH, with this 2500 J
ignitor is 4.4%, but this is an overdriven system as
shown by tests in a larger 120 litre chamber", The more
appropriate LFL for CH, is the 4.9% value measured
with a 1000 J ignitor in the 20 litre chamber and shown
as the symbol x in the figure. The data for Pocahontas
coal and CH, in Figure 15A show some curvature. This
is probably due to the even greater difference in ignit-
ability between the low-volatile coal and the CH,, ie.


Coal dust explosibility

Kenneth L. Cashdollar

Pittsburgh Research Center, Bureau of Mines, US Department of the Interior,
Pittsburgh, PA 15236-0070, USA

This paper reports US Bureau of Mines (USBM) research on the explosibility of coal dusts. The
purpose of this work is to improve safety in mining and other industries that process or use
coal. Most of the tests were conducted in the USBM 20 litre laboratory explosibility chamber.
The laboratory data show relatively good agreement with those from full-scale experimental
mine tests. The parameters measured included minimum explosible concentrations, maximum
explosion pressures, maximum rates of pressure rise, minimum oxygen concentrations, and
amounts of limestone rock dust required to inert the coals. The effects of coal volatility and
Particle size were evaluated, and particle size was determined to be at least as important as
volatility in determining the explosion hazard. For all coals tested, the finest sizes were the most
hazardous. The coal dust explosibility data are compared to those of other hydrocarbons, such
as polyethylene dust and methane gas, in an attempt to understand better the basics of coal

combustion.

Keywords: explosion; coal; dust

In mining and other industries that use or process coals,
an accurate knowledge of the explosion hazards is essen-
tial. Various books such as references 1-7 have been
published since 1980 on the general subject of the
explosion hazards of dusts and powders. A major source
of information on coal dust explosion hazards is refer-
ence 8. The present paper concentrates on the subject of
the explosion hazards of coal dusts, but the concepts are
applicable to many other dusts, and especially to other
carbonaceous dusts. The data on coal are compared to
those for polyethylene dust, a simple hydrocarbon that
is completely volatile, and to methane gas data.

This paper reports US Bureau of Mines (USBM)
research on coal dust explosibility. The laboratory data
were obtained in the USBM 20 litre chamber? which has
been used extensively to study the explosibility of coals
and other carbonaceous dusts. Previous USBM papers
have discussed various aspects of coal dust explosibility.
These included the ignition energy requirements for
accurate measurements of flammability limits of dusts
and gases'®'', a volatility model for coal dust flame
propagation"? the effect of volatility on the explosibility
limits of coals and other carbonaceous dusts'?"'5, the
effects of particle size'®'’, comparisons of data from
20 litre and 1 m* chambers'''’, and comparisons of lab-
oratory and experimental mine data'*!7"'’,

Experimental equipment and test
procedures

The dust explosibility experiments in this paper were
conducted in the USBM 20 litre laboratory chamber?
shown in Figures ] and 2. This is the standard laboratory

test chamber used at the USBM for studying the explos-
ibility and inerting of combustible dusts. There is another
style of 20 litre chamber designed by Siwek!-*?°7! that is
in wide use in Europe and elsewhere. The USBM 20 litre
chamber is near-spherical in shape and made of stainless
steel. It has a pressure rating of 21 bar (g). The chamber
top is hinged and opens across the full chamber diameter
(Figure 1), allowing easy access to the interior for pos-
itioning instruments and for cleaning. The hinged top is
attached with six bolts which are not shown on the draw-
ings. Two optical dust probes?*-** are used to measure
the uniformity of the dust dispersion at the positions
shown in Figures I and 2. The optical probes measure
the transmission through the dust cloud. Thin jets of air
keep the windows of the probe dust-free. References 9
and 18 contain examples of dust probe transmission
measurements showing the dispersion uniformity in the
20 litre chamber. The strain gauge pressure transducer
measures the explosion pressure and rate of pressure rise
(dP/dt). Since it is an absolute pressure gauge, it can
also be used during the partial evacuation of the chamber
Prior to dispersion and for adding gases to the chamber
by partial pressures. For very fast explosions, a piezo-
electric pressure transducer is also used to measure
dP/dt. Multichannel infra-red (IR) pyrometers”*?" can
be used to measure the explosion temperatures by
observing the flame radiation through the sapphire win-
dow. A 25 um (1-mil) Type K (nickel-chromium versus
nickel-aluminium) thermocouple was used to measure
changes in chamber temperature during dispersion of the
dust and air. A 25 zm (1-mil) Type S (platinum versus
platinum-rhodium) thermocouple can be used to meas-



Nomenclature Dy Avithmetic volume or mass mean diameter (tm)
Pid Rate of pressure rise (bar s

D, Geometric mass mean diameter (jm) (aPid)V" Normalized maximum rate of pressure rise (bar ms“)

o Geometric standard deviation Pa ‘Maximum explosion pressure (bar (a)

Dis Mass median diameter (12m) r Transmission (4%)

Ds Arithmetic surface mean diameter (jum) v Vessel volume (im')


Acknowledgements

The author acknowledges the assistance of G. M. Green
in the conduction of the 20 litre chamber tests and size
analyses and C. E, Lucci for the computer program for
data acquisition and analysis. Both are from the Pitts-
burgh Research Center of the USBM. The author also
acknowledges the partial funding of the elevated tem-
perature studies by the Pittsburgh Energy Technology
Center of the US Department of Energy through Interag-
ency Agreement No. DE-AI22-94PC94651.

References

1 Bartknecht, W. ‘Explosions: course, prevention, protection’,
Springer. Berlin, 1981
Field, P. Dust explosions, Vol. 4, ‘Handbook of powder tech-
nology’. Elsevier, Amsterdam, 1982
Nagy, J. and Verakis, H. C. “Development and control of dust-
explosions’, Marvel Dekker, New York, 1983
Cashdollar, K. L. and Hertzberg, M. (eds). “Industrial dust
explosions’, Proceedings of the Symposium on Industrial Dust
Explosions, Pittsburgh, PA, 10-13 June 1986, STP 958, American
Society for Testing and Materials, Philadelphia, PA, 1987
Bartknecht, W. ‘Dust explosions: course, prevention, protection’,
Springer. Berlin, 1989
Eckhoff, R. K. “Dust explosions in the process industries’, But-
terworth Heinemann, Oxford, 1991
Bartknecht, W. ‘Explosions-Schutz", Springer, Berlin, 1993 (in
German)
Cybulski, W. ‘Coal dust explosions and their suppression’
{translated from Polish), TT 73-54001, National Technical Infor~
mation Service, US Department of Commerce, Springfield, VA,
1975
Cashdollar, K. L. and Hertzberg, M. Rev. Sci. Instrum. 1985, 86,
592-602
Hertzberg, M., Cashdollar, K. L. and Zlochower, 1. A. ‘Flamm-
ability limit measurements for dusts and gases: ignition energy
requirements and pressure dependences’, Twenty-first Symposium
(International) on Combustion, The Combustion Institute, Pitts-
burgh, PA, 1988, pp 303-313
Cashdollar, K. L. and Chatrathi, K. Combust. Sci, Technol. 1993,
87, 157-171
Hertzberg, M., Zlochower, 1. A. and Cashdollar, K. L. “Volatility
model for coal dust flame propagation and extinguishment’,
‘Twenty-first Symposium (International) on Combustion, The Com-
bustion Institute, Pittsburgh, PA, 1988, pp 325-333
Cashdollar, K. L., Hertzberg, M. and Zlochower, I. A. “Effect of
volatility on dust flammability limits for coals. gilsonite and poly-
ethylene’. Twenty-second Symposium (International) on Combus-
tion, The Combustion Institute, Pittsburgh, PA, 1989, pp 1757~
1765
Cashdollar, K. L. and Hertzberg, M. “Laboratory study of rock dust
inerting requirements: effects of coal volatility, particle size, and
methane addition’, Proceedings of the 23rd International Confer-
‘ence of Safety in Mines Research Institutes, 11-15 September
1989, Washington, DC, 1989, pp 965-977
Greninger, N. B., Cashdollar, K. L., Weiss, E. S. and Sapko, M. J
“Suppression of dust explosions involving fuels of intermediate and
high volatile content’, Proceedings of the Fourth International Col-
loquium on Dust Explosions, 4-9 November 1990, Porabka-
Kozubnik, Poland, 1991, pp 208-228
Hertzberg, M. and Cashdollar, K. L. “Introduction to dust
explosions’, Industrial Dust. Explosions, STP 958, American
Society for Testing and Materials, Philadelphia, PA, 1987, pp 5-32
Cashdollar, K. L., Weiss, E. S., Greninger, N. B. and Chatrathi,
K. Planw/Opns Progr. 1992, 11, 247-255
Cashdollar, K. L., Sapko, M. J., Weiss, E. S, and Hertzberg, M.
‘Laboratory and mine dust explosion research at the Bureau of
Mines’, Industrial Dust Explosions, STP 958, American Society
for Testing and Materials, Philadelphia, PA, 1987, pp 107-123
Cashdollar, K. L., Weiss, E. S., Greninger, N. B., Hertzberg, M.
and Sapko, M. I. “Laboratory and large-scale explosion research
at the US Bureau of Mines’, Proceedings of The First World Sem-
inar on the Explosion Phenomenon and on the Application of
Explosion Protection Techniques in Practice, Section 1, sponsored
by EuropEx, 17-21 February 1992, Brussels, Belgium
20 Siwek, R. "20-I-Laborapparatur fiir Bestimmung der Explosion-
skenngréssen brennbaer Stiube’ (20-L laboratory apparatus for the
determination of the explosion characteristics of flammable dusts)

ea ae

ar

is}

ero}

(in German), Thesis at Winterthur Engineering College, Win-
terthur, Switzerland, available from Ciba-Geigy AG. Basel, Switz-
erland. 1977
21 Siwek, R. ‘Development of a 20 ltr laboratory apparatus and its
application for the investigation of combustible dusts’, Ciba Geigy
AG, Basel, Switzerland, 1985
22 Liebman, I., Conti, R. $. and Cashdollar, K. L. Rev. Sci. Instrum
1977, 48, 1314-1316
23 Cashdollar, K. L., Liebman, 1. and Conti, R. S. “Three Bureau of
Mines optical dust probes’. US Bureau of Mines RI 8542, 1981
24 Conti, R. S., Cashdollar, K. L. and Liebman, 1. Rev. Sei. dnstrum.
1982, 53, 311-313
25 Cashdhollar, K. L. Appl. Optics 1979, 18, 2595-2597
26 Cashdollar, K. L. and Hertzberg, M. Optical Engng 1982, 21,
82-86
27 Cashdollar, K. L. and Hertzberg, M. ‘Infrared temperature
measurements of gas and dust explosions, Temperature: its
measurement and control in science and industry, Vol. 5". Amer-
ican Institute of Physics. New York, 1982, 453-463
Cashdollar, K. L. and Hertzberg, M. Combust. Flume 1983, $1,
23-35
29 Minw, K. J. ‘Commissioning of the 20-1. vessel for studying dust
explosions’, Canada Centre for Mineral and Energy Technology
(CANMET) report MRL 88-32 (TR), October 1988
Lightfoot, P. D. Personal communication, {C1 Explosives. McMas-
terville, Quebec, Canada, 1994
31 Rice, G, S. and Greenwald, H. P. ‘Coal dust explosibility factors
indicated by experimental mine investigations 1911 to 1929", US
Bureau of Mines Tech. Paper 464, 1929
Nagy, J. *The explosion hazard in mining’, US Mine Safety and
Heaith Administration IR 1119, 1981
33 Deguigand, B. and Galant, S. ‘Upper flammability limits of coal
dust-air mixtures’, Eighteenth Symposium (International) on Com-
bustion, The Combustion Institute, Pittsburgh, PA, 1981, pp
705-715
34 Mintz, K. J. Combust. Flame 1993, 94, 125-130
35 Amyotte, P. R., Mintz, K. J., Pegg, M. J.. Sun, Y. H. and Wilkie,
K. LJ. Loss Prev. Process Ind. 1991, 4, 102-109
36 Amyotte, P. R.. Mintz, K. J., Pegg, M. J. and Sun, Y, H. Fuel
1993, 72, 671-679
37 Hertzberg. M., Cashdollar, K. L., Ziochower, I. A. and Green, G. M.
“Explosives dust cloud combustion’, Twenty-fourth Symposium
(International) on Combustion, The Combustion Institute, Pitts-
burgh, PA, 1992, pp 1837-1843
38 Ng, D. L., Cashdollar, K. L., Hertzberg, M. and Lazzara, C. P.
‘Electron microscopy studies of explosion and fire residues’, US
Bureau of Mines IC 8936, 1983
39 Conti, R. S., Cashdollar. K. L. and Thomas, R. A. ‘Improved 6.8-
L furnace for measuring the autoignition temperatures of dust
clouds’, US Bureau of Mines RI 9467, 1993
40 Zabetakis, M. G. “Flammability characteristics of combustible
gases and vapors’, US Bureau of Mines Bulletin 627, 1965
41 Conti, R. $., Cashdollar, K. L., Hertzberg, M. and Liebman, 1
“Thermal and electrical ignitability of dust clouds’, US Bureau of
Mines RI 8798, 1983
42. Coward, H. F. and Jones, G. W. ‘Limits of flammability of gases
and vapors’, US Bureau of Mines Bulletin 503, 1952
43 Wiemann, W. ‘Influence of temperature and pressure on the
explosion’ characteristics of dusVair and dusVait/inert gas mix-
tures’, Industrial Dust Explosions, STP 958, American Society for
Testing and Materials, Philadelphia, PA, 1987, pp 33-44
44 Kuchta, J. M. ‘Investigation of fire and explosion accidents in the
chemical, mining, and fuel-related industries - a manual’, US
Bureau of Mines Bulletin 680, 1985, pp 48-50
45 Amyoite, P. R., Mintz, K. J., Pegg, M. J., Sun, Y. H. and Wilkie,
LK. J. Hazardous Mater. 1991, 27, 187-203
46 Richmond, J. K., Liebman, I. and Miller, L. F. ‘Effect of rock
dust on the explosibility of coal dust’, US Bureau of Mines RI
8077, 1975
47 Weiss, E. S., Greninger, N. B. and Sapko, M. J. ‘Recent results,
of dust explosion studies at the Lake Lynn experimental mine’,
Proceedings of the 23rd International Conference of Safety in
Mines Research Institutes, 11-15 September 1989, Washington,
DC, 1989, pp 843-856
Michelis, J., Margenburg, B., Maller, G. and Kleine, W. ‘Investi-
‘gations into the buildup and development conditions of coal dust
explosions in a 700-m underground gallery’, Industrial Dust
Explosions, STP 958, American Society for Testing and Materials,
Philadelphia, PA, 1987, pp 124-137
49 Cashdollar, K. L. ‘Laboratory explosibility study of U.S., German,
and Polish coal and rock dusts’, Proceedings of Twenty-fourth
International Conference of Safety in Mines Research Institutes,
23-28 September 1991. Donetsk, USSR. Vol. 1, pp 307-316

3

8

a



Burned
C2175 g/m?

Burned
Cmel75 g/m?

Unburned
‘Scola, am

Figure 11 Scanning electron photomicrographs

22 T T ¥
OPYZeeccene coos 6 6
20 4
key
tee @ Explosion
© No explosion
z
a
g's
&
tap
tap °
°
°
° © ° °
10 1 i r L
° Yoo 200. 300 400 S00

COAL CONCENTRATION, g/m?

Figure 12. Effect of oxygen concentration on coal dust explos:
ibility

the PPC from the 20 litre chamber at near ambient tem-
perature (~60°C) and at an elevated temperature of
~180°C. The experimental data points are extrapolated
to even higher temperatures (solid curve) using the
modified Burgess-Wheeler law*®*' for hydrocarbons:

213 +7,
Cr=Cr, Gat) [1-0.000721(7-To)],

°

—o
‘Scola, pm

Burned

Cm 2250 g/m? ae ry

° 100

Burned

Cmt250 g/d Seale

of bituminous coal particles before and after explosions

800

700 Flammable and +

thermally autoignitable

600

Flammable or explosible

‘TEMPERATURE, °C

Coo!

1
200

1 1
Too 150 250

CONCENTRATION, g/m®

Figure 13 Effect of temperature on the flammability and ther-
mal ignitability of Pittsburgh coal, compared to methane gas

where Cy is the limit in terms of mass concentration at
temperature T, C,,, is the limit at 7, and the temperatures
are in °C. The concentrations to the right of the solid
curve are flammable or explosible and the region to the
left of the curve is non-flammable. For comparison, the
measured lean flammable limit data for methane gas as
a function of temperature (dashed curve, from reference
42, p43) are also shown.


15) rot tT r 50 : :
Mass or volume:
x _
z Or 7 40
& /
5 Surface”
2 ®
E 5p 4 z* 4
o iN E
v N. 2
° 1 1. 1 ia =
2 5 10 20 50 100 ©=200 B 20 4
DIAMETER, ym °
Figure5 Comparison of surface-w ighted and volume-
weighted distributions of standard Pittsburgh pulverized coal 10 4
(PPC)
‘The majority of data in this paper are for Pittsburgh oa eg a0 «50 100-200-800 1000
seam high-volatile coal and for Pocahontas seam low- DIAMETER, pm

volatile coal. Size data for various broad size distri-
butions of these two coal dusts are listed in Tables 2 and
3. In the tables, the first row identifies the coal by coal
seam and number (Pgh-I, etc.) and by additional descrip-
tive notation (date, mesh size, etc.). The size data were
measured from a combination of sonic sieving and Coul-
ter counter analyses. The second and third rows list the
percentage by weight of each dust less than 20 or 75 jum,
respectively. Since particle size distributions generally

Figure 6 Surface-weighted distributions of several narrow size
distributions of Pittsburgh coal, compared to the dashed curve
for the broad distribution PPC

approximate a log-normal distribution, the geometric
size parameters are listed in rows four and five. The
fourth row lists the geometric mass mean particle diam-
eter, D,, and the fifth row lists the dimensionless geo-

Table 2 Size analyses and explosibility data of Pittsburgh high volatile coal

Pgh-1 Pgh-2 Pgh-3 Pgh-4 Pgh-5 Pgh-6 Pgh-7
40M =70M PPC 200M PPC, fines special fines
< 20 um (%) 4 13 13 26 42 42 83
< 75 um (%) 24 54 80 100 90 94 100
Dj (wm) 132 57 43 29 26 23 af
o, 25 23 19 18 2.6 22 18
Dog (um) 163 68 48 32 28 24 "
(um) 173 76 52 34 37 32 14
Ds (umn) 7 37 34 24 16 7 9
Del%T) (um) @ 100-200 g mvs 35-48 22-27 17-40 16-24 9-10 m1 5-9
DI%T7) (um) @ 300-600 g m-* 48-58 29-42 23-45 27-36 12-19 = 815
MEC (gm) 130 85 80 65 75 ~60 85
Payay (DAN) 6.0 63 66 67 68 - 7
(dPida Vv"? (bar ms) 19 25 39 a 54 - 59
MOC (%0,) = = 1" = = = 105
Amount of rock dust to inert (%) 83 68 74 79 83 83 87
Dashes indicate that no data were available.
Table3 Size analyses and explosibility data of Pocahontas low volatile coal
Poc-1 Poc:2 Poc-3 Poo-4 Poo-5 Poc6 Poc-?
-1200M 78 3/88-4 6/80 3/88-2,3 2188-1 fines
<20 um (%) 10 27 38 40 46 67 83
<75 am (%) 75 65 5 86 86 94 100
D, (um) ~48 39 32 26 24 15 u
o 19 3.0 3.2 2.6 28 24 19
Drea (zm) ~52 a7 30 2 22 4 10
Dy (um) ~68 63 60 41 40 23 13
Ds (um) 39 13 18 ” 6 " 9
Dgl%T) um) @ 100-200 g m=? 17-34 14 =- 6-10 - = =
D.(%T) (um) @ 300-600 g m=? 20-40 14-27 = aa = —
MEC (gm) 120 130 ~100 ~90 80 80 ~80
Prgg (DAN) 6.0 63 - 63 6.2 65 65
(dPidt) V" (bar ms”) 7 ~16 = ~26 = 34 31
Amount of rock dust to inert (%6) 60 64 76 8 7 82 83

Dashes indicate that no data were available.


6

24

2

°

20

3 fe (P76

Bao | 7 (7 genr

a a
TINE, +

Figure 4 Typical data for an explosion of low-volatile bitumin-
‘ous coal at 200 gm” in the 20 litre chamber

an explosion. The pressure trace in Figure 3A starts at
the partially evacuated value of 0.14 bar (a). The blast
of air that disperses the dust starts at 0.1 s and ends at
0.4 s on the pressure-time trace. The ignitor is activated
at 0.5s at a chamber pressure of 1.0 bar (a). The
maximum explosion pressure is about 3 bar (a). In Fig-
ure 3B, the rate of pressure rise, (dP/df),eniur for the
ignitor is larger than (dP/ds),, for the dust explosion
itself. The USBM software program allows the user to
move the peak search starting point beyond the effects
of the ignitor and then determine (dP/dt)... Figure 4
shows data for a larger explosion of the low-volatile coal
dust at a higher concentration of 200gm-*. The
maximum explosion pressure is about 5.5 bar (a). For the
research in this paper, the criteria for significant flame
propagation in the 20 litre chamber were the same as
used previously at the USBM" These criteria are
that the maximum explosion pressure, P., = 2 bar (a)
and that the volume normalized rate of pressure rise,
(dP/d)V' = 1.5 barm s'.

Previously, when the pressure trace for the dust and
air dispersion had been viewed on an expanded scale, a
slight drop-off in pressure after the end of the dispersion
had been observed. Recently, a separate series of disper-
sion tests (no ignitor) were run at ambient and elevated
initial chamber temperature to study this effect. The
25 ym fast response Type K thermocouple (TC) was
positioned near the center of the 20 litre chamber during
these tests to monitor rapid changes in gas temperature.
In air dispersion tests where the 20 litre chamber was
initially at ambient temperature (~22°C), the TC tem-
perature increased during the dispersion process and was
~70°C at the time the ignitor would normally be acti-
vated, decreasing to ~55°C after an additional | s, and
eventually back to ambient. When air and 400 g mTM of
coal dust were dispersed, the TC temperature at the nor-
mal ignition time was ~60°C, decreasing to ~50°C after
1s. This measured temperature increase during disper-
sion and later drop-off explains the observed drop-off in
pressure. The gas has been heated in some manner dur-
ing the dispersion procedure and later cools back to
ambient temperature. This cooling causes the drop-off in
pressure after the end of the dispersion pulse. This press-
ure drop-off effect has also been noted by Mintz” and
Lightfoot’. However, their thermocouples were larger,
and they were not able to completely follow the rapid
temperature changes. The significance of this ~60°C
temperature at ignition on the measured data will be dis-
cused later in the paper.

The USBM has historically placed more emphasis on
the measurement of minimum explosible concentrations

(MEC) and rock dust inerting amounts rather than on
the measurement of rates of pressure rise (which are used
for vent sizing) because it is not possible to vent mine
explosions. This is the main reason that the dispersion
time is longer and the turbulence level is lower for the
standard dispersion procedure in the USBM 20 litre
chamber than for the Siwek 20 litre chamber. This
should mainly affect the rates of pressure rise (at high
concentrations) and should not affect MEC measure-
ments''. An advantage of the lower turbulence for the
USBM standard dispersion procedure is that the effects
of the ignitor are more easily separated from the dust
explosion (Figures 3 and 4), which is useful for MEC
testing. The rate of pressure rise data reported in this
paper from the USBM 20 litre chamber should only be
used as relative data for comparison of different dusts
and should not be used for the sizing of vents according
to ASTM Standard E1226, ISO Standard 6184/1, NFPA
Guide 68, and VDI Standard 3673, which are based on
the higher turbulence level of the Siwek 20 litre chamber
and the 1 m* chamber'*.

In this paper, the terms ‘flammability’ and ‘explosibil-
ity’ are used interchangeably to refer to the ability of an
airborne dust cloud and/or gas mixture to propagate a
deflagration after it has been initiated by a sufficiently
strong ignition source. Historically, the term ‘flammabi-
lity’ has been used more for gases and ‘explosibility’
more for dusts.

Experimental data and discussion

The coal that has been used as a standard'*?!*? at the
USBM since the early 1900s is Pittsburgh seam high-
volatile bituminous (hvb) coal. The proximate analyses
and heating values of the Pittsburgh coal and a Poca-
hontas seam low-volatile bituminous (Ivb) coal are listed
in Table J. The standard Pittsburgh pulverized coal
(PPC) dust used for both laboratory and experimental
mine testing'*'*? has 80% minus 200 mesh (< 75 wm)
and a mass median particle diameter of 48 zm. The mass
(volume) and surface weighted particle size distributions
for the PPC are shown in Figure 5. The size distributions
were measured by a combination of sonic sieving and
Coulter counter analyses. The PPC has a fairly broad
size distribution and is meant to represent the float coal
dust carried by the ventilation air in mines**. In order to
determine particle size effects on explosibility, various
narrow size distributions of Pittsburgh coal were also
generated by centrifugal classification and by sieving.
Examples of these narrow size distributions are shown
in Figure 6, where they are compared to the dashed line
showing the broader size distribution of the PPC.

Table 1 Proximate analyses and heating values for coals

Pittsburgh Pocahontas.
Moisture (%) 1 1
Volatility {%) 37 7
Fixed carbon (%) 56 76
Ash (%) 6 6
Heating value (cal gTM") 7720 8050

Proximate analyses measured by ASTM D3172. Heating values
measured by ASTM D1989.


Eo oust reservoir

Figure1 Vertical cross-section of 20|itre explosibility test
chamber

‘Seale,cm

Figure 2. Horizontal cross-section of 20 litre explosibility test
chamber

ure the explosion temperature and compare to the IR
pyrometer temperature data, similar to tests in a previous
8 litre chamber?”8. A 320 pm (12.5-mil) Type K ther-
mocouple was used to measure the chamber set tempera-
ture during tests at an elevated temperature. The data

from the various instruments are collected by a high-
speed personal computer (PC)-based data acquisition
system. It can sample data from 16 channels at a
maximum rate of 9 kHz if all channels are used or at
even faster rates if fewer channels are used. Pressure,
dP/dz, transmission, and calculated temperature data can
be displayed on a colour monitor immediately after each
test. The USBM-designed software program allows for
variable smoothing, rescaling, peak searches, expansion
of the time scale, etc.

The dust to be tested can be placed either in the dust
reservoir or on top of the dispersion nozzle at the bottom
of the chamber (Figure 7). The reported experimental
dust concentration for the 20 litre chamber is the mass
of dust divided by the chamber volume. After the dust
and ignitor (Figure 2) have been placed in the chamber,
the top lid is bolted on and the chamber is partially
evacuated to an absolute pressure of 0.14 bar (a). Then
a short blast of dry air (0.3 s duration at 9 bar (g) from
a 16 litre reserve tank) disperses the dust and raises the
chamber pressure to about | bar (a). The ignitor is acti-
vated after an additional delay of 0.1 s. This results in
a total ignition delay of 0.4 s from the start of dispersion
until ignition for the standard test procedure in the
USBM 20 litre chamber. The standard procedure for the
Siwek 20 litre chamber has a total ignition delay of
~0.1 5 and a reservoir pressure of 20 bar, resulting in a
higher level of turbulence.

The ignition sources used for the 20 litre tests were
electrically activated pyrotechnic ignitors manufactured
by Fr. Sobbe of Germany. These ignitors are composed
of 40% zirconium, 30% barium nitrate, and 30% barium
peroxide. They are activated electrically with an internal
fuse wire and deliver their energy in about 10 ms, When
ignited, they produce a dense cloud of very hot particles
and little gas'®. The Sobbe ignitors are available in vari-
‘ous energies from 250 to 10000 J. For the 20 litre tests
reported in this paper, 2500 and 5000/J ignitors were
used. These energies are nominal calorimetric values
based on the mass of pyrotechnic powder in each ignitor.
The 2500 and 50004 ignitors by themselves produce
pressure rises of about 0.28 and 0.55 bar, respectively,
in the 20 litre chamber. The 2500 J ignitor is comparable
in energy to an entire book of 20 pocket matches, all
ignited at once.

Examples of the pressure data for a weak and a mod-
erate coal dust explosion are shown in Figures 3 and 4.
In both figures, the absolute pressure (A) and rate of
pressure rise (B) are plotted versus time. Figure 3 shows
the data for a 20 litre chamber explosion test of a low
volatile bituminous coal at a dust concentration of
125 g m7, which is just above the minimum required for

en

°
240
3 fe
3 y
2 ao (AP oe
3 (dP/dt,
8 Seas
TIME, 5

Figure 3. Typical data for a weak explosion of low-volatile bitu-
minous coal at 125 g m-? in the 20 litre chamber